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Marine

2006;():1-8. doi:10.1115/GT2006-90114.

Since the early 1970’s, the United States Navy has utilized the General Electric LM2500 gas turbine engine for propulsion aboard its surface combatants including its newest DDG 51 Class Destroyer. These ships have generally operated at a part power operational profile under a COGAG arrangement which has offered system redundancy while exceeding life projections for the gas turbine engines. For its newest ships still in the design phase (LHD 8/LCS/LSC(X)) the Navy intends to continue to utilize gas turbine engines but in different applications including electric drive, high power boost applications in tandem with both diesel engines and electric motor arrangements. Although this paper focuses on the LM2500, its conclusions are meant to apply to a broader scope of future propulsion applications. Specific conclusions are provided describing potential operating profile considerations.

Commentary by Dr. Valentin Fuster
2006;():9-15. doi:10.1115/GT2006-90115.

In December 2000, a 7” long axial crack was visually identified on the compressor rear frame of an LM2500 propulsion gas turbine engine aboard the USS MITSCHER (DDG 57). This was originally thought to be a unique failure mode possibly caused by misaligned brackets external to the engine imparting undo thermal stresses onto the engine casing and flange. Since that time, 17 additional engines in the Navy fleet have been identified with either the large axial crack on the compressor casing or a small craze crack on the CRF flange, which appears to be the origination point prior to crack propagation. This paper discusses the extent of the cracking problem in the US Navy, the engineering investigation undertaken by the OEM and Navy to determine the root cause of the cracks and development of a field repair strategy to mitigate the impact of these cracks. The focus of the paper includes metallurgical analysis of failed compressor rear frame hardware, vibratory evaluation of the engine’s external piping system as a contributory failure mode and results of strain gage testing of the mid flange region.

Commentary by Dr. Valentin Fuster
2006;():17-24. doi:10.1115/GT2006-90252.

Increased shipboard electricity demand, for propulsion, navigational electronics, reduced crew size, and other purposes, has created a need for marinized gas turbine generator sets. The LM500 is a versatile 4 MW class aeroderivative gas turbine with a solid track record. The LM500 gas turbine is now equipped with a dual-redundant 21st century digital control system, which also manages the gas turbine generator set. In addition, to improve combustor life at a rated power, the marinized LM500 was modified for the engineering development model (EDM), and the turbine hot section components proven in the CF34-3B — the life improved model of its aero engine counterpart, may be offered for production. The gas turbine has been fully integrated on a common base with a 4 MW generator and all auxiliary equipment. An epicyclic gearbox was used to reduce power turbine output speed from 7000 RPM to the 1800 RPM required at the four-pole generator. The auxiliary equipment layout was designed to minimize weight and allow for ease of maintenance. Gas turbine engine testing was completed in August 2004. Operational string testing of the integrated gas turbine generator set was completed in January 2005, with full power plant testing scheduled for 2005 at the Land Based Test Site, in Philadelphia, Pennsylvania, by the customer. This paper describes and discusses the gas turbine generator set (genset) design, development, and production plans, as well as the benefits of the packaged power plant and potential applications.

Commentary by Dr. Valentin Fuster
2006;():25-33. doi:10.1115/GT2006-90270.

A recent trend in designing naval ships is to improve performance through using more electric equipment. The reliability and quality of the onboard electric power, therefore, becomes critical as the ship functionality would entirely depend on its availability. This paper investigates the possibility of using Flywheel Energy Storage Systems (FESS), similar to those earlier developed for commercial applications, to address issues related to onboard power supplies. A design of a FESS for onboard power backup and railroad electrical stations is presented. The FESSs power output parameters are 500kWx30sec in high-duty mode and up to 2MW in pulse mode. High power output is one of the main advantages of FESS over commercially available electrochemical batteries. The other advantages include essentially an unlimited number of charge/discharge cycles, observable state of charge and environmental friendliness. Designs of the main FESS components are discussed: low-loss magnetic bearings, an energy-storage hub, a high-efficiency motor/generator and power electronics.

Commentary by Dr. Valentin Fuster
2006;():35-46. doi:10.1115/GT2006-90305.

The US Navy has operated gas turbines (GT’s) on board different types of vessel for several decades. To safe guard these engines against the harsh marine atmosphere several types of air inlet filtration system have been employed, with varying degrees of success. The purpose of this paper is to explore the various filtration technologies which are currently used in all GT inlet filtration applications; point out the benefits of each; and debate how these may be practically applied to the “Modern” Naval GT application. The changing vessel operational requirements; environmental requirements; advances in GT design; and how these impact the design of the GT inlet filtration system will be discussed. The current trends in Naval GT inlet filtration system specifications will also be detailed. The significant divergence of these from the capabilities of current marine GT filtration technology used will be highlighted. Recommendations will be proposed regarding how this situation may be addressed with the use of filtration technologies currently used in other GT applications.

Topics: Gas turbines
Commentary by Dr. Valentin Fuster
2006;():47-53. doi:10.1115/GT2006-90709.

In 1990, GE announced it would begin development of the first-ever gas turbine with output greater than 40MW and a thermal efficiency above 40%. It was designated the LM6000, and was first introduced as the -PA model in December 1992. This turbine used a single annular combustion system with relatively few changes from the successful aircraft engine — the CF6-80C2. At the same time, GE began development of Dry Low Emissions (DLE) combustion technologies, culminating in the LM6000-PB model being introduced in December 1994. As the LM6000 fleet approached the 1 million-hour point, with an installed base of over 100 units, the next step — the development of a turbine with greater power and efficiency — was initiated, creating the LM6000-PC and -PD models. The launch of GE’s LM6000-PC/PD aero-derivative gas turbine was announced in 1996 and the first unit went into commercial operation in a power generation application in late 1997. The mechanical drive version of this gas turbine has been available as a product since early 1998. This machine opens an entirely new market segment, with interest being paced by the development of this segment requiring variable speed drivers with outputs greater than 50,000 shaft horsepower. Although some exploratory interest for mechanical drive applications was generated when the product was first announced, significantly greater interest within both gas pumping, and marine applications has been expressed recently especially considering changes in the global environmental regulations, energy prices, larger ships moving at greater speeds. Typical applications are new designs of large oil and gas production facilities — for gas pumping, processing, and natural gas liquefaction, as well as large marine and naval applications requiring high power-weight densities. GE is currently supporting several ongoing application studies using the LM6000 gas turbine as the driver of choice. This document provides the highlights of the development, testing and qualification of the LM6000 by General Electric as well as the certification program by the American Bureau of Shipping (ABS). Notable engineering accomplishments during this development include part power NOX abatement, auto-throttles, and cubic loading using a generator.

Topics: Gas turbines , Testing
Commentary by Dr. Valentin Fuster
2006;():55-64. doi:10.1115/GT2006-90713.

The YS2000 program is a 73 meter length and 10.5 meter width all composite Corvette class vessel. It displaces 640 metric tons when fully equipped and drafts 2.5 meters. It is to be crewed by 18 officers and 25 enlisted men. It is a CODOG propulsion system supplied by Vericor Power Systems, with two MTU 16V 2000 M90 diesels and four TF50A gas turbine engines. Both the diesels and gas turbines are connected to a pair of MA-107 SBS gearboxes that run two 125 SII KaMeWa waterjets. The Visby is designed to be difficult to detect by enemy using radar, infrared, hydro-acoustic monitoring or any other sensor system. The Visby has been in development In Sweden since 1999. To date, four craft have been constructed and sea trailed out of the five totals. The fifth ship is on schedule to complete construction and sea trials later in the 2006 year. Many refinements to the overall propulsion package and related supporting systems have been incorporated since the first ship “Visby” has been sea trailed and since put in service. This paper will review various areas of the propulsion package, explaining the challenges that had to be overcome. The areas of interest will include: the FADEC digital engine control, the exhaust & inlet systems, the turbine engine and starting system, engine room cooling and turbine engine enclosures. The paper will focus on some of the before and after results and attempts to highlight the specific challenges that had to be overcome.

Commentary by Dr. Valentin Fuster
2006;():65-76. doi:10.1115/GT2006-90715.

GE aero-derivative gas turbines were first introduced into marine operations during the late 1960’s and early 1970’s. GE is now leveraging its many years of proven marine experience and offshore dual-fuel experience to offer dual-fuel gas turbines for LNG Carrier (LNGC) propulsion and electric power. With building of new larger LNGC’s now beginning, the industry is seriously considering a change to gas turbine based systems in order to capitalize on their many advantages. CoGES (combined gas turbine — steam generator electric) plants for LNGC’s consist of dual-fueled gas-turbine-generator (GTG) set(s) and auxiliaries, heatrecovery steam generator (HSRG), a steam-turbine-generator set, feed-water, steam and condensate systems. Leveraging cruise-ship reliability programs, the GTG instrumentation and control systems are single-point fault tolerant. Gas turbine power plants offer many additional advantages, including but not limited to: Use of boil-off gas as a cost-effective and environmentally friendly fuel (slow speed diesel ships require complex on-board reliquifaction of boil-off gas). When installed on deck, CoGES plants provide high power-volume density that translates into increased cargo revenue and deferred capital cost. Gas turbines ease of maintenance and quick changeout. Developed to meet the International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC) and classification society standards for marine applications, GE’s 2 X LM2500 CoGES plant is a very simple and reliable solution. Dry-run capable HRSG’s are used in lieu of exhaust damper and by-pass systems. Outage of any one prime mover leaves the plant with nominally 50% power remaining. Common spares are inherent. Established as having an equivalent level of safety as traditional LNGC propulsion systems via FMECA type studies, the 2 x dual-fueled LM2500 CoGES plant has been “Approved in Principle” by Class for use on LNG Carriers. Alternatively, GE’s 1 X dual-fueled LM6000 or 1 X LM2500+/G4 CoGES plant addresses capital & operating cost pressures via reduced equipment costs and improved fuel economy. Redundancy and simplicity are achieved via a dry-run capable HRSG and an STG, combined with auxiliary diesel generator sets. Both the LM2500 family and LM6000 CoGES plants offer viable alternatives to traditional steam turbine and slow-speed-diesel propulsion. Gas-fuel, liquid-fuel, and bi-fuel operation provide flexibility and redundancy to ship owners who must safely and reliably deliver cargo at the lowest possible cost per MMBTU throughout a fleet life cycle.

Commentary by Dr. Valentin Fuster
2006;():77-84. doi:10.1115/GT2006-90739.

A dynamic model is presented that predicts the transient characteristics of an integrated autothermal fuel reformer and gas turbine engine for extracting a high purity stream of hydrogen from logistical fuels such as F76 marine diesel or JP-5 for use in a shipboard proton exchange membrane (PEM) fuel cell power plant. The model incorporates a water-gas shift reactor for increasing hydrogen yield, and a separation membrane for extracting a pure stream of hydrogen gas from the reformer output. Compressed air supply and energy recovery is achieved by an integrated gas turbine generator. The dynamic model serves as a testing ground for the development of control methodologies and to predict limitations in transient response during electrical load variations.

Commentary by Dr. Valentin Fuster
2006;():85-94. doi:10.1115/GT2006-90751.

Within the marine world gas turbines operate in niche ship types only, but why is this? This paper considers the ship types that have adopted marine gas turbines and the ship characteristics that determine the choice of the propulsion system and prime mover type. Ship determined characteristics include; speed, power density, general arrangements, operating profile, fuel type, fuel consumption, maintenance opportunity, manpower and others. Gas turbine characteristics derived from their parentage — either aero or industrial — influence the characteristics available from the marine gas turbine. Historical marine applications are reviewed and new marine applications, influenced by developments with new marine gas turbines are considered both in the developed but evolving naval markets and within the growing but focused commercial marine sector. Propulsion system selection is also influenced by the prevailing and emerging propulsion system technologies, particularly with transmission systems which influence the choice of the prime mover, their number and specific power, and their disposition in the ship.

Commentary by Dr. Valentin Fuster
2006;():95-99. doi:10.1115/GT2006-90848.

The Navy’s Landing Craft Air Cushion (LCAC) hovercraft vehicles use two Auxiliary Power Units (APUs) to provide bleed air to start the main propulsion TF40B or ETF40B engines. Each main propulsion engine is assigned a normally closed starter shutoff valve to permit APU bleed air to pass to the main propulsion engine air starters. These starter shutoff valves are seizing due to corrosion and prohibit airflow needed to start the main engines. This paper will describe the planned study to identify and resolve starter shutoff valve failures.

Topics: Engines , Corrosion , Valves
Commentary by Dr. Valentin Fuster
2006;():101-108. doi:10.1115/GT2006-90911.

Following the design shift in Naval ship architecture from conventional mechanical through hybrid to Integrated Electric Propulsion (IEP), the starting philosophy of prime movers needs to be rationalised so as to interact and augment the propulsion configuration and electrical distribution whilst remaining cost effective. To fulfil this requirement the UK Ministry of Defence contracted Ultra Electronics (PMES) to work in partnership to develop and demonstrate a gas turbine Electric Start and Generation System (ESGS). The undertaken programme is to demonstrate full control over the gas turbine starting system and associated maintenance features together with generating sufficient electrical power that can be used to supply all the gas turbine alternator auxiliaries. This paper will give an overview of such requirements together with the design of the basic system and the challenges to ‘navalise’ such a product and install it on a marine gas turbine. The paper will continue by reviewing data obtained from factory acceptance testing and on-engine testing with the Marine Trent MT-30. It is hoped to further compare and analyse these results with future planned testing scheduled in late 2005. Conclusions will be drawn from the initial results, the design of the proposed ESGS system and comparisons with existing in-service gas turbine start systems.

Commentary by Dr. Valentin Fuster
2006;():109-115. doi:10.1115/GT2006-90914.

In 2000 the UK Ministry of Defence (MoD) placed a contract on Turbomeca Limited to develop a 1.8 MW advanced cycle gas turbine alternator that could compete directly with comparable diesel generators in terms of performance and cost. The Advanced Cycle Low power Gas Turbine Alternator (ACL GTA) has been designed with simplicity in mind and utilises a revolutionary high-speed alternator and heat recuperator to realise the project aspirations. Development and evaluation testing commenced on this engine in 2004 at Turbomeca’s facility in Pau. This paper will provide a brief overview of the design of the complete gas turbine alternator concentrating on innovative characteristics. It will then continue by presenting in detail the component testing and data obtained followed by system testing results of the gas turbine and gas turbine alternator. Conclusions will be drawn detailing the benefits and suitability of this novel gas turbine alternator within modern Naval platforms and some comparisons with in-service diesel generators.

Commentary by Dr. Valentin Fuster
2006;():117-123. doi:10.1115/GT2006-91075.

With a new perspective on how to conduct business through acquisition reform, the Navy faces infrastructure challenges that are not necessarily in sync with acquisition principles. Historically, the U.S. Navy (Navy hereinafter) has always spearheaded the means of developing design for form, fit, and function of Navy machinery. This leadership role has its roots on the unique requirements that the U.S. Navy has to fulfill its mission. Unfortunately, this process does not always prove to be cost effective since its implementation normally carries heavy restrictions, unique applications, and little competition. This is commonplace for most technology insertion efforts into Navy Ships.

Topics: Navy
Commentary by Dr. Valentin Fuster
2006;():125-129. doi:10.1115/GT2006-91167.

The US Navy Landing Craft, Air Cushion (LCAC) engines, like all marine gas turbines, use water washes to preserve performance and increase reliability by removing salt and other contaminants from the compressor. Due to the severity of the operating environment and unfavorable operations base, water washing can pose risks to the LCAC engines. Galvanic corrosion, crevice corrosion, insufficient contaminant removal and incompatibility among seal materials, contaminants and wash solvents can outweigh the benefits of water wash. The US Navy has incorporated water wash procedures and materials such as silicon rubber seals and glass fiber and Teflon bushings to mitigate these risks.

Commentary by Dr. Valentin Fuster
2006;():131-138. doi:10.1115/GT2006-91256.

Most gas turbine generators rely on an automatic-engaging, free-wheel clutch to connect a starting motor to accelerate the gas turbine generator from zero to some intermediate speed to enable ignition and then provide torque assistance to a higher speed until the gas turbine is self-sustaining. The U.S. Navy has used various designs of starter motors and clutches for its gas turbine fleet. In addition, there has been a requirement to periodically borescope each gas turbine and this has necessitated removal of the starting system and clutch assembly in each instance. This paper examines the U.S. Navy experience with starting clutches and provides details of the development and testing of a synchronous-self-shifting clutch with an additional, stationary, manual turning feature to provide very slow and precise gas turbine rotor rotation for borescope purposes. This paper also gives details of the installation of the first two prototype clutches on the USS Ramage, DDG 61, operating experience for approximately four years, and possible future installations of this type of clutch in U.S Navy gas turbine generator sets.

Commentary by Dr. Valentin Fuster
2006;():139-145. doi:10.1115/GT2006-91301.

This paper looks back at the evolution of the Gas Turbine Generator sets (GTGs) in the U.S. Navy’s DDG 51 Class, reviewing lessons learned, successes and areas where work is still required. Topics are discussed in the context of Mean Time Between Failure (MTBF) Total Ownership Cost (TOC) and maintainability. It reviews changes that resulted in MTBF increasing by a factor of five and TOC dropping by a factor of four. It also looks to the future, identifying potential areas of further improvement.

Commentary by Dr. Valentin Fuster
2006;():147-154. doi:10.1115/GT2006-91313.

The LHD 8 machinery plant represents a significant departure from previous LHD class ships with a propulsion system that combines an LM2500+ and electric motor per shaft. The propulsion system integrates the first US Navy application of this gas turbine engine ultimately with the ship service electric plant. This complex system affords many advantages to the ship designers with respect to reconfigurability, survivability and maneuvering. However the design challenges the ability of the Machinery Control System (MCS) to maximize the effectiveness of this complex machinery plant. The MCS is a software-based distributed system utilizing a high bandwidth network. The system relies on the VME platform for control processing, data acquisition and operator control. The MCS design and architecture is presented. Decisions concerning network capabilities, distribution of control, and processing are considered. The impact of electric plant complexities upon the propulsion system requirements will also be addressed. The MCS architecture solution will describe LM2500+ integration issues with an electric motor.

Commentary by Dr. Valentin Fuster

Microturbines and Small Turbomachinery

2006;():155-164. doi:10.1115/GT2006-90001.

This paper presents a physical solution by eliminating static pressure distortions of impeller exit due to a volute in a centrifugal compressor. The numerical and experimental studies on the circumferential distortion flow characteristics inside the stationary frame of a high-pressure ratio compressor with a large cut back tongue volute. The detailed flow structures and pressure distortions development inside the stationary components are discussed. The numerical results were demonstrated to be in good agreement with the experiments. The volute and diffuser interactions at design and off-design conditions were found to be much smaller for the large cut back volute in comparison with the reported from literature. The study indicated that the large cut back tongue volute design not only benefits the compressor performance but also reduces the impeller exit static pressure non-uniformity caused by discharge volute.

Topics: Pressure
Commentary by Dr. Valentin Fuster
2006;():165-170. doi:10.1115/GT2006-90020.

A prototype of a novel gas turbine concept is being developed to demonstrate the technical feasibility of a gas turbine design based on a straight radial flow with no axial flow turning. The prototype gas turbine consists of only two structural elements—a rotor disk and a stator shroud. The rotor consists of a centrifugal compressor and high impulse radial outward-flow turbine connected to an electric generator. The stator shroud contains the combustor and nozzles. The difference between this novel design and conventional radial gas turbine that the compressor and turbine section are installed on the same side of the rotating wheel, while the combustor and nozzle are mounted on the stationary shroud. Thus, the entire assembly consists of two components. Technical advantages are: • Single Rotating Disk; • Compact Two-Piece Construction; • Ease of Repair and Maintenance; • High Power to Weight Ratio. This paper discusses the combustor development and preparation for design testing of the prototype radial gas turbine.

Commentary by Dr. Valentin Fuster
2006;():171-182. doi:10.1115/GT2006-90068.

HAYNES ® alloy HR-120 ® is being evaluated as a replacement for type 347 stainless steel for use in Microturbine Primary Surface Recuperators. The material has been characterized after being subjected to both steady-state and cyclic engine exposure in a Capstone C60 MicroTurbine™ operating at 100°F above the normal operating temperature. Oxide scale growth and elemental depletion has been analyzed and documented after 1,800 and 2,500 hours of exposure. A preliminary estimate of the remaining usable oxidation life has been made using a simplified parabolic model. Engine test results indicate that HR-120 has improved oxidation resistance and elemental stability.

Commentary by Dr. Valentin Fuster
2006;():183-190. doi:10.1115/GT2006-90095.

A high-temperature regenerator has been designed under MIT and WTPI patents (figure 1). These call for the ceramic-honeycomb regenerator disk to rotate incrementally, in this case through 90 degrees, for each movement. The seals, which can conform to the profile of the sealing surface, are firmly pressed to the regenerator seal faces during the stationary periods, here about fourteen seconds (figure 2). The seals then lift marginally, just enough to allow free disk movement, during the 0.75 second of the indexing period. Thus no sliding wear can take place. In the demonstrator unit tested the measured effectiveness was over 98%, the pressure drops of the hot and cold flows were under 2%, and the leakage was low within experimental error. The demonstrator was tested at a top temperature of 910° C. (The final temperature after development should be well above 1000°C). The paper describes design methods that are substantially different from those in general use, and discusses design choices for materials and actuation systems. This regenerator was designed to be compatible with a high-temperature 5-kW solid-oxide fuel cell. Our design studies indicate that, when applied (in a considerably larger size) to micro turbines it would enable electrical efficiencies of 50% to be exceeded. Other industrial applications that require too high a temperature for currently available recuperators would also become viable.

Commentary by Dr. Valentin Fuster
2006;():191-197. doi:10.1115/GT2006-90121.

Achieving a high heat transfer effectiveness at low pressure losses in narrow channels continue to present a significant challenge for designers of gas turbine components and heat exchangers. The task of low pressure losses often requires that some of these components, particularly heat exchangers, have to operate at a flow rate that corresponds to relatively low Reynolds numbers ranging from 200 to 800. The operation at higher Reynolds numbers permits to improve the recuperator performance, however it leads to unacceptable pressure losses. Introduction of hemispherical dimples for heat transfer augmentation has become recently one of the promising techniques for achieving higher heat transfer performance in narrow channels at an acceptable pressure loss level. A structural support between the primary heat transfer surfaces is usually required for a double walled back-side cooled turbine components and multichannel heat exchangers (recuperators), operating with pressure differential between cold and hot channels. For this purpose in the current study spherical protrusions (reversed dimples) were employed on a panel opposing a dimpled panel. This structural arrangement was expected to result in additional blockage of the channel cross-section and certain increases in a pressure loss. The experimental study has been performed to assess the effect of spherical dimples and protrusions on heat transfer and pressure losses in a formed narrow channel. The airflows in the experiments corresponded to the Reynolds number ranging from 800 to 6,500. A dimple diameter and depth were 10.0 mm and 2.0 mm, correspondingly; the protrusions established the 2.0 mm height of the channels. Both the in-line and staggered dimple arrangements were studied with the x-pitch ranging from 9.0 to 18.0 mm and z-pitch changing from 13.0 to 18.0 mm. The data presented in this paper include results for measurements of average heat transfer coefficients and pressure losses. Reynolds analogy factor and thermal performance of the primary surface were obtained and discussed in the paper. Considering potential application of studied surfaces for gas turbine heat exchangers, the paper provided a comparison between a “pure” dimpled channel, dimpled channel with protrusions against a more traditional channel with sinusoidal corrugated primary surface. As expected, the protrusions in the channel enhanced the heat transfer, but led to increased pressure losses due to the partial destruction of the dimple-generated vortex structures. Nevertheless, it was demonstrated that the Reynolds analogy factor of 0.4 could be achieved in a dimpled channel with protrusions, resulting in overall pressure losses of under 5% for the application in a recuperator core.

Commentary by Dr. Valentin Fuster
2006;():199-206. doi:10.1115/GT2006-90144.

The fabrication of primary surface recuperators for gas turbines and microturbines is a complex process involving a multitude of variables, which are highly interactive. After over 25 years of improvements to recuperator manufacture at Solar Turbines Incorporated, fabrication of the recuperator primary sheet is still prone to scatter of output measures within drawing requirements caused by variations in raw foil properties and changes in environmental conditions. This paper outlines a systematic analysis and improvement of the primary sheet fin folding process using Taguchi methodology. The approach was to break the overall system into subsystems, analyze each subsystem using computer simulation for fin folding and then to analyze the complete system in actual use. The goal was to improve the fabrication process so that multiple drawing requirements for primary recuperator sheet are consistently met with minimum variation and the time needed for subsequent manufacturing steps is reduced. An improved system is more robust, meaning total variability in the primary sheet is minimized and outputs are more insensitive to noise (inputs that are not controllable or are very expensive to control). Improvement is measured by calculating the ratio of desired output to undesired output (noise), i.e. the signal to noise (S/N) ratio. More consistent primary sheet will improve overall recuperator system performance and reduce the cost of this critical gas turbine system component. A secondary objective was to increase the durability of the fin-folding blades, which are subject to wear and breakage. As this paper goes to press, analysis of the final design of experiment on the actual system is in progress.

Topics: Microturbines
Commentary by Dr. Valentin Fuster
2006;():207-216. doi:10.1115/GT2006-90161.

This paper presents a numerical investigation of the heat transfer inside a micro gasturbine and its impact on the performance. The high temperature difference between turbine and compressor in combination with the small dimensions results in a high heat transfer causing a drop in efficiency of both components. Present study aims to quantify this heat transfer and to reveal the different mechanisms that contribute to it. A conjugate heat transfer solver has been developed for this purpose. It combines a 3D conduction calculation inside the rotor and the stator with a 3D flow calculation in the radial compressor, turbine and gap between stator and rotor. The results for micro gasturbines of different size and shape and different material characteristics are presented and the impact on performance is evaluated.

Topics: Heat transfer
Commentary by Dr. Valentin Fuster
2006;():217-224. doi:10.1115/GT2006-90194.

In order to increase the efficiency of advanced microturbines, durable alloy foils are needed for their recuperators to operate at 650°–700°C. Prior work has demonstrated that water vapor in the exhaust gas causes more rapid consumption of Cr from austenitic alloys leading to a reduction in lifetime for the thin-walled components in this application. New commercial alloy foils are being tested in both laboratory tests in humid air and in the exhaust gas of a modified 60kW microturbine. Initial results are presented for a commercial batch of 80μm alloy 120 foil. The Cr consumption rates in laboratory testing were similar to those observed in previous testing. The initial results from the microturbine indicate a faster Cr consumption rate compared to the laboratory test but longer term results are needed to quantify the difference. These results will help to verify a Cr consumption model for predicting lifetimes in this environment based on classical gas transport theory.

Topics: Alloys , Engines , Testing
Commentary by Dr. Valentin Fuster
2006;():225-236. doi:10.1115/GT2006-90195.

The Oak Ridge National Laboratory (ORNL) and ATI Allegheny-Ludlum began a collaborative program in 2004 to produce a wide range of commercial sheets and foils of the new AL20-25+Nb stainless alloy, specifically designed for advanced microturbine recuperator applications. There is a need for cost-effective sheets/foils with more performance and reliability at 650–750°C than 347 stainless steel, particularly for larger 200–250 kW microturbines. Phase I of this collaborative program produced the sheets and foils needed for manufacturing brazed plated-fin (BPF) aircells, while Phase II provided foils for primary surface (PS) aircells, and modified processing to change the microstructure of sheets and foils for improved creep-resistance. Phase I sheets and foils of AL20-25+Nb have much more creep-resistance than 347 steel at 700–750°C, and foils are slightly stronger than HR120 and HR230. Preliminary results for Phase II show nearly double the creep-rupture life of sheets at 750°C/100 MPa, with the first foils tested approaching the creep resistance of alloy 625 foils. AL20-25+Nb alloy foils are also now being tested in the ORNL Recuperator Test Facility.

Topics: Creep , Alloys , Microturbines
Commentary by Dr. Valentin Fuster
2006;():237-245. doi:10.1115/GT2006-90222.

Rolls-Royce has designed, built, and continues to test a high pressure (HP) turbocharging system specifically designed for a hybrid solid oxide fuel cell system being developed by Rolls-Royce Fuel Cell Systems, Ltd. This turbocharger is comprised of a two-spool machine with a shaft speed motor/generator on the low speed spool. Each spool contains a centrifugal compressor driven by a radial inflow turbine. The two spools run independently, but are physically close coupled for a smaller, more efficient system. The spools are mounted into a basic structure to provide structural rigidity, as well as sound and heat isolation. In its current test rig form, the system runs on oil lubricated bearings; is equipped with a liquid fueled slave combustor; and is controlled with a digital control system. Although never intended to be a “stand alone” microturbine — it is simply a high pressure turbocharger; if one wanted to — with a proper fuel and control system installed, it is capable of operation in such a stand alone mode. However, without heat recovery it would be a highly inefficient microturbine and therefore, was not considered in this application. Multiple units have been built and continue developmental testing. Initial matching to a fuel cell stack is scheduled for operation later in 2006.

Commentary by Dr. Valentin Fuster
2006;():247-255. doi:10.1115/GT2006-90237.

In recent years, the development of distributed power generation has resulted in significant reduction in network losses and transmission costs while it has increased reliability. Microturbine is one of these power generators that has the ability of relatively high power generation in spite of its small volume [1], [2]. In this paper, different layouts for advanced microturbine cycles are analyzed. In order to modify cycle characteristics such as power and efficiency, and reduce exergy destruction, different configurations including intercooler, aftercooler, and heat recovery boiler are separately and synthetically analyzed and compared. The effects of various parameters, such as compressor pressure ratio, vapor pressure, and bypass ratio on the cycle performance are studied. For configurations including inter/aftercooling, the possibility of use of heat recovery boiler is surveyed, depending on the temperature of the coolers outlet water and the recuperator outlet gas. Each part of the cycle is analyzed based on first and second laws of thermodynamics. Results show that microturbine with intercooler and recuperator (MTC-IC) is the best option, with regard to efficiency and power criteria, but its steam production is less than simple cycle with recuperator (MTC). MTC-IC has higher total exergetic efficiency in higher pressure ratios, because of the priority of power versus heat. This configuration is also more suitable than MTC with heat recovery boiler for variable thermal loads especially with bypass ratio between 0 and 0.5.

Commentary by Dr. Valentin Fuster
2006;():257-261. doi:10.1115/GT2006-90256.

Compact and efficient recuperator is an important component of a microturbine system. To ascertain the optimum gas cavity configuration of the recuperator in a 100kW-microturbine system, a numerical study of flow performance has been done. The main parameters to change in different cases are cone angle of the gas pipeline, α and depth of the pipeline in the gas cavity, L. By comparing the gas pressure drop, Δp and the gas outlets velocity nonuniformity, Su, we found that the case with α = 5° and L = 370mm is the best configuration. Comparing with the worst case, it may greatly decrease the velocity nonuniformity by 73.3% while the corresponding pressure drop increases only 8%.

Commentary by Dr. Valentin Fuster
2006;():263-268. doi:10.1115/GT2006-90260.

Compact heat exchangers such as tube-fin types and plate-fin types are widely used for gas-liquid or gas-gas applications. Some examples are air-coolers, fan coils, regenerators and recuperators in micro-turbines. In this study, thermal design of fin-and-tube (tube-fin) heat exchanger performance with fins being employed outside and inside tubes was presented, with which designed plate-fin heat exchanger was compared. These designs were performed under identical mass flow rate, inlet temperature and operating pressure on each side for recuperator in 100kW microturbine as well as specified allowable fractions of total pressure drop by means of Log-Mean Temperature Difference (LMTD) method. Heat transfer areas, volumes and weights of designed heat exchangers were evaluated. It is shown that, under identical heat duty, fin-and-tube heat exchanger requires 1.8 times larger heat transfer area outside tubes and volume, 0.6 times smaller heat transfer area inside tubes than plate-fin heat exchanger. Under identical total pressure drop, fin-and-tube heat exchanger requires about 5 times larger volume and heat transfer area in gas-side, 1.6 times larger heat transfer area in air-side than plate-fin heat exchanger. Total weight of fin-and-tube heat exchanger is about 2.7 times higher than plate-fin heat exchanger, however, the heat transfer rate of fin-and-tube heat exchanger is about 1.4 times larger than that of plate-fin heat exchanger. It is indicated that, both-sides finned tube heat exchanger may be used in engineering application where the total pressure drop is severe to a small fraction and the operating pressure is high, and may be adopted for recuperator in microturbine.

Commentary by Dr. Valentin Fuster
2006;():269-276. doi:10.1115/GT2006-90304.

The interest in microturbines and new distributed generation technologies is growing in the entire world because of the many potentially beneficial characteristics they can offer and the developments achieved so far. This paper investigates the performance and degradation effects of microturbines for electric power generation. Diagnostics investigation is also carried out to obtain optimal instrumentation sets for degradation faults. Here the capacity of the gas turbines analyzed is 29kW simple and regenerative cycles. The engine performance is also analyzed operating at constant and variable speed. To simulate the gas turbine performance and carry out the diagnostic analysis the software Pythia and Turbomatch, developed by Cranfield University, were used. In this paper the engines above are simulated at degraded conditions. The effects of the degradation in the compressor, turbine and recuperator on the performance of the engines were investigated. Despite of the improvement on performance achieved with regenerative cycle and variable speed operation the results show that the performance of variable speed microturbines is more sensitive to components degradation than constant speed engines. Also recuperator degradation has greater effect on variable speed than constant speed engines. Due the effects of degradation on each engine different diagnostic approaches are observed.

Topics: Microturbines
Commentary by Dr. Valentin Fuster
2006;():277-285. doi:10.1115/GT2006-90328.

In order to develop a micro gas turbine with high turbine inlet temperature and thermal efficiency, a series of running tests has been carried out. J-850 jet engine (Sophia Precision Co., Ltd.) was chosen as a baseline machine. The turbine nozzle and the rotor are replaced by type SN-01 (Otsuka Ceramics Co., Ltd.) and type SN-235 (Kyocera Corporation) ceramic elements, respectively. By using type 3a engine, we succeeded one-hour running test of the engine without cooling and severe damages. The turbine inlet temperature was higher than 1000 °C. The rotating speed was about 120,000 rpm. Performances of the type 3a engine (with ceramic nozzle and rotor) and the type 1 (with Inconel alloy nozzle and ceramic rotor) were compared as follows: At the same rotation speed, turbine inlet temperature of the type 3a became higher than that of the type 1. Simultaneously, fuel consumption of type 3a was larger than that of the type 1. Thrust of the type 3a was slightly larger than that of the type 1. Those results imply that the thermal efficiency of type 3a is slightly, 2%, lower than that of the type 1. The present sealing configurations between ceramic nozzle-vanes and their holder plate and ceramic rotor-housing and metal combustion chamber were found to work well.

Commentary by Dr. Valentin Fuster
2006;():287-296. doi:10.1115/GT2006-90335.

This paper presents a steady-state on-design and off-design thermodynamic performance investigation of a Humid Air Turbine (HAT) cycle and other competitive gas turbine cycles based on a simple cycle micro turbine. First of all, several stand-alone softwares and programs were integrated to achieve the computational capability of mass and energy balancing and optimization at on-design and off-design conditions. Later on, this paper shows the simulated on-design performance of a micro turbine in three different modified configurations: 1) recuperated cycle, 2) Recuperated Water Injected (RWI) cycle and 3) HAT cycle. The micro turbine is a 90.10 kW single-shaft simple cycle gas turbine with the electrical efficiency of 12.8%. After transformation, the electrical efficiencies of the recuperated, the RWI and the HAT cycle are 26.21%, 29.26% and 29.92%, respectively; the power outputs are 82.52 kW, 106.22 kW and 109.92 kW, respectively. Finally, the effects of load reduction (part-load at ISO conditions) and ambient temperature on the thermodynamic off-design performance of the simple, the recuperated, the RWI and the HAT cycle were investigated and compared. Simulation results indicate that the HAT cycle and the RWI cycle have similar off-design performance. In addition, the two evaporative cycles have more favorable off-design performance compared with the other two cycles with waste heat utilization of converting to power. It is concluded that the adjustment of the water vapor added to the compressed air has beneficial effect on the stability of the off-design performance. However, because of the absence of intercooler, as well as the limited amount of available waste exergy inside the micro gas turbine cycle, the advantages of the on-design and off-design performance of the transformed HAT cycle on the other competitive cycles are not completely displayed.

Commentary by Dr. Valentin Fuster
2006;():297-304. doi:10.1115/GT2006-90366.

Recent years, Genetic Algorithm (GA) technique has been gotten much more attention in solving real-world problems, more successful genetic algorithms applications to engineering optimization have shown the technique has strong ability of global searching and optimizing based on various objectives for their optimal parameters. The technique may be applied to more complicated heat exchangers and is particularly useful for new types. It is important to optimize heat exchanger, for minimum volume/weight to save fabrication cost or for improved effectiveness to save energy consumption, under requirement of allowable pressure drop; simultaneously it is mandatory to optimize geometry parameters of heat plate from the technical and economic standpoints. In this paper, GA is used to optimize the Cross Wavy Primary Surface (CWPS) and Cross Corrugated Primary Surface (CCPS) geometry characteristic of recuperator in 100kW microturbine, in order to get more compactness and minimum volume and weight. Two kinds of fitness assignment methods were considered. Furthermore, the GA parameters were set optimally to yield smoother and faster fitness convergence. The comparison shows the superiority of GA and confirms its potential to solve the objective problem. When the rectangular recuperator core size and corrugated geometries are evaluated, in the CWPS the weight of recuperator decreases 12% or more, the coefficient of compactness increases by up to 19%, with an increase of total pressure drop by a percentage of 0.84 compared to the original design data, the total pressure drop as a percentage of the operating pressure is controlled to be less than 3%. In the CCPS area compactness is increased to 70% the initial design result by decreasing pitch and relatively high height of the passage, the weight decreases by 17% to 36%, depending on different inclination angle (θ). Comparatively the CCPS shows superior performance for use in compact recuperators of the future. The GA technique chooses from a variety of geometry characters, optimizes them and picks out the one which provides the closest fit to the recupertor for microturbine.

Commentary by Dr. Valentin Fuster
2006;():305-313. doi:10.1115/GT2006-90422.

Standard uninterruptible power supply (UPS) systems for radio base stations use lead acid batteries stacks as energy storage device; the fairly short life cycle of such systems, not more than 3 years, together with the presence of highly polluting components make it costly in terms of money and environmental aspects. From these considerations originate the idea to develop an alternative UPS system based on compressed air storage able to replace the battery stacks and to satisfy more restrictive environmental requirements. This work is developed within the European project PNEUMA (PNEumatic Uninterruptible Machine - LIFE 04 ENV/IT/00595) with the final aim to develop some UPS demonstrators for an on site experimentation. In this paper it is described the way followed to set up an air turboalternator with a maximum power of 10.0 kW. Starting from the selection of the turbine, we decided to extract it from an ICE turbocharger because its size and its low cost matches our requirements. First of all, an experimental survey has been realized to test two turbines of different size: expansion ratio, corrected mass flow and efficiency have been measured varying inlet pressure and temperature. This activity led us to select the smaller turbine to develop the first pilot demonstrator; the turbine has been coupled to an high speed alternator (max. 60’000 rpm) by means of an elastic coupling. Some shakedown testing have been performed to verify the mechanical resistance and to test the power conversion system and the control system.

Commentary by Dr. Valentin Fuster
2006;():315-323. doi:10.1115/GT2006-90502.

This study is part of the work performed for the pilot plant of an innovative indirect fired gas turbine cycle for biomass incineration using the Pebble-Heater technology and a microturbine. A Pebble-Heater is a regenerative heat exchanger consisting of an annular bed with radial flow, which implements an unsteady counterflow heat exchanger. It captures the flue gas enthalpy during the atmospheric heating phase and heats compressed air for the turbine during the blowing phase. During the heating phase ash particles from the incinerator deposit on the bulk material and are segregated by periodic exchange and cleaning of the bed. Like this a high degree of de-dusting and cooling of the flue gas is achieved upstream of the emission control filters. In the blowing phase, however, a small amount of ash (< 3mg/mN 3 ) is re-suspended and condensables re-sublimate into the hot air stream, which feeds the turbine. In this paper we develop a simple procedure to estimate how the size distribution and concentration of the dust load decrease the lifetime of the radial turbine due to erosion. It is based on the consideration of single particles following the gas flow. To obtain a lifetime estimate the focus was set on the erosion of the radial turbine blade tips, which are identified as the most critical parts with respect to particle impacts. On the basis of the particle relative velocity and trajectory determined with the flow model the turbine lifetime was calculated with an erosion rate model. The paper discusses the influence of particle size and distribution on turbine lifetime. For the current turbine it was found that equivalent particle diameters above 3–4μm and dust loads greater than 1 mg/mN 3 reduce the nominal turbine lifetime drastically.

Topics: Dust , Erosion , Turbines
Commentary by Dr. Valentin Fuster
2006;():325-332. doi:10.1115/GT2006-90526.

CFD, FEA, and experimental testing have been combined in order to investigate the lifetime limiting design deficiencies of a turbine wheel in a turbo charger. Thermocouples have been applied to the same radial turbine wheel to provide boundary conditions and validation data for the simulations. The tests have been performed on a turbocharger gas-stand. Based on two steady state CHT-calculations for two distinctly different operating points the heating process of the wheel has been simulated in a transient temperature calculation. Since the resulting temperature gradients induce thermal stresses, the temperature distribution serves as a boundary condition for the subsequent structural analysis. To obtain realistic stress distributions, centrifugal forces also need to be accounted for. In this way, the influence of the thermal stress on the overall stress can be evaluated.

Commentary by Dr. Valentin Fuster
2006;():333-341. doi:10.1115/GT2006-90529.

The first phase of the development of an automatic methodology for the design process of small gas turbine recuperators is presented. The different software tools are selected in order to be managed, in a successive step of the research, by means of a design-optimization platform, according to the concept of Multi Disciplinary Optimization (MDO). The methodology has been developed integrating a geometrical parametric model of the heat transfer surfaces, built inside an industrial CAD, a three dimensional meshing tool and a CFD solver. Final objectives of the research will be an optimization process designed to maximize the heat exchange rate and to minimize costs and fluid dynamics losses. The paper deals with the parameterization technique and the numerical model validation.

Topics: Design , Microturbines
Commentary by Dr. Valentin Fuster
2006;():343-352. doi:10.1115/GT2006-90542.

The present work presents a novel approach for the optimised design of small gas turbine combustors, that integrates a 0-D code. CFD analyses and an advanced game theory multi-objective optimization algorithm. The output of the 0-D code is a baseline design of the combustor, given the required fuel characteristics, the basic geometry (tubular or annular) and the combustion concept (i.e. lean premixed primary zone or diffusive processes). For the optimization of the baseline design a parametric CAD/mesher model is then defined and submitted to a CFD code. Free parameters of the optimization process are position and size of the liner holes arrays, their total area and the shape of the exit duct, while different objectives are the minimisation of NOx emissions, pressure losses and combustor exit Pattern Factor. As a first demonstrative example, the integrated design process was applied to a tubular combustion chamber with a lean premixed primary zone for a recuperative methane-fuelled small gas turbine of the 100 kW class.

Commentary by Dr. Valentin Fuster
2006;():353-358. doi:10.1115/GT2006-90555.

The significant reduction in power output of small gas turbines at high ambient temperatures places the technology at a significant disadvantage compared with reciprocating engines. On site power applications in many jurisdictions are experiencing high power costs during summer peak times. A variable speed industrial fan combined with an evaporative cooler has been constructed and operated in the CETC laboratory in Ottawa, Canada to supply supercharged inlet air to a microturbine rated at 70 kW at ISO conditions. The supercharging system can raise the inlet air pressure by 10.5 kPa (42” wc). A mapping of the turbine performance has been done as a function of boost pressure, relative humidity and ambient air temperature. A net power increase has been observed from 57 kW to 70 kW at an ambient air temperature of 33°C (91°F) and RH of 60%, a 23% increase. Supercharging at lower temperatures yields lower net power increases since the microturbine generator rating is the limiting factor; for example an 11% increase in net power was observed at an inlet air temperature of 11°C (52°F) and RH of 60%. Supercharging was shown to decrease net fuel-to-electricity efficiency of this recuperated turbine by about 3%, at an air temperature of 33°C (91°F). An economic analysis using published power prices and weather data from Toronto explores the business case of using supercharging, with the best economies likely for multiple units or larger microturbines, such as 250 kW units. The objective of the project was to demonstrate the concept leading to a field trial in Toronto or in Calgary where the altitude offers a further benefit to the inlet air supercharging concept. Work is underway to design a control system suitable for field deployment for the concept.

Topics: Microturbines
Commentary by Dr. Valentin Fuster
2006;():359-365. doi:10.1115/GT2006-90594.

Prevention of compressor surge is one of the most important tasks in operation of gas turbine engine. The easiest way to see the phenomena is to show the static and dynamic operation characteristic on the map. Its operation zone will be restricted by the surge limit and, static and transient process must have some margin for it. Surge margin of a static regime is normally chosen during its design stage. Safe operation during part load condition without facing the surge is an indispensable task for control system design and a swift response from the engine is required to avoid it. Effect of rotor moment of inertia, air/gas volumes and heat transfer are factors to cause the transition from the static line. In case a large volume such as heat exchanger exists in the system it will exert a substantial influence to dynamic characteristics. In the present paper, influence of air volume bled from the compressor exit on transient process is investigated with an example of an auxiliary power unit micro-turbine engine. A typical compressor characteristic with scaling was used for the calculation. Turbine mass, pressure ratio, rotation speed, power and moment are calculated based on mass and work conservation. Result from the present study can give a guidance to design the control system. A computer program is developed to calculate the dynamic process using the MathCAD commercial software. Air volume is changed from 0.02 to 6 m3 .

Commentary by Dr. Valentin Fuster
2006;():367-373. doi:10.1115/GT2006-90598.

Experiments are presented to quantify the convective heat transfer and the hydrodynamic loss that is obtained by forcing water through blocks of porous carbon foam (PCF) heated from one side. The experiments were conducted in a small-scale water tunnel instrumented to measure the pressure drop and the temperature rise of the water passing through the blocks and the base temperature and heat flux into the foam block. In comparison to similar porosity aluminum foam, the present results indicate that the pressure drop across the porous carbon foam is higher due to the large hydrodynamic loss associated with the cell windows connecting the pores, but the heat transfer performance suggests that there may be a significant advantage to using PCF over aluminum foam for extended surface convection elements in recuperators and electronic cooling devices.

Topics: Carbon
Commentary by Dr. Valentin Fuster
2006;():375-385. doi:10.1115/GT2006-90680.

The efficiency of small and intermediate-size gas turbine engines can be significantly increased by the use of a primary surface recuperator, which uses waste heat from the exhaust gas to preheat the compressor discharge air before it enters the combustor. The result is lower fuel consumption to reach a particular firing temperature. The construction and operation of a primary surface recuperator present numerous challenges in the area of materials selection. Experiences with stainless steels and nickel-base alloys in construction and application in Solar Turbines’ Mercury 50 gas turbine will be presented, along with the results of extensive laboratory creep and oxidation testing and post-test evaluation. Oxidation testing in humidified air has been carried out on a variety of commercially available stainless steel and nickel-base alloy thin foils considered as materials of construction for primary surface recuperators. Two predominant degradation modes have been identified. The active mode generally depends on the exposure conditions and the alloy composition. Alloys which are rich in iron tend to suffer from accelerated oxidation, while alloys with higher chromium and nickel contents tend to exhibit oxide scale evaporation via the formation of volatile chromium-bearing species. The active mechanism is evident in the oxidation kinetics, the oxide scale morphology and composition, and in observations of compositional changes in the metal alloy substrate.

Topics: Alloys
Commentary by Dr. Valentin Fuster
2006;():387-393. doi:10.1115/GT2006-90791.

A 134 Newton thrust class, 120,000 rpm turbojet was redesigned to incorporate a high-temperature compliant foil bearing aft of the turbine rotor and a compliantly mounted ball bearing forward of the centrifugal compressor–cold section. Two rotor-bearing system configurations were evaluated, one for operation above the bending critical speed and one for rigid rotor operation. Required characteristics for the foil bearing and ball bearing equipped with compliant foil damper mount were determined through a series of design tradeoff studies evaluating critical speeds and system stability. Following the design studies, the necessary hardware was fabricated, the engine assembled and operation to full speed achieved. Engine speed, rotor vibrations, compressor discharge pressure, exhaust gas temperature, thrust and fuel consumption were all recorded for both a baseline fluid lubricated ball bearing supported engine and the new turbojet engine using the hybrid foil bearing support system. Issues related to high-speed operation above the bending critical speed are identified and recommendations offered. Engine test data show that approximately 10% less fuel is consumed by the hybrid foil bearing mount system than the baseline conventional design. It is also shown that the foil bearing life was longer than the ball bearing life even though the foil bearing operated in the exhaust gas stream at temperatures exceeding 800°C. The results of this program demonstrate the feasibility of developing a completely oil-free foil bearing gas turbine engine.

Commentary by Dr. Valentin Fuster
2006;():395-400. doi:10.1115/GT2006-90796.

Proton Exchange Membrane, fuel cells intended for automotive applications require widely varying flows of contaminant free air for high efficiency and successful longterm operation. One machine being considered for this application is the oil-free electric turbocharger, which combines a centrifugal compressor, radial inflow turbine, permanent magnet electric motor and air lubricated compliant foil bearings. In this paper the results of a preliminary system design study integrating a motor, foil bearings and aerodynamic wheels, as well as performance testing of a crucial element, the compliant foil thrust bearing, are presented. Several different thrust bearings were tested in sizes suitable for operation to speeds over 250,000 rpm and operating thrust loads greater than 98 Newtons. Additional tradeoff studies are presented for a preliminary design of 12 kW permanent magnet motor driven, 120,000 rpm motorized turbocompressor operating above its bending critical speed.

Commentary by Dr. Valentin Fuster
2006;():401-409. doi:10.1115/GT2006-90861.

Conceptual design and CFD analysis of an inlet to a compressor is undertaken. In this design the inlet transitions pipe flow to annular flow and provides for a mechanical device to be mounted in centerline and in front of the compressor. The solution to this constraint along with the constraints of uniform outlet flow and low weight, size, and total pressure drop consists of: a highly offset transition region, where pipe flow is transitioned into annular flow, and a nozzle region, with the goal of delivering uniform flow to the compressor at minimal pressure drop. The design approach consists of utilizing CFD to guide the overall shape of the inlet toward desirable regions of the design space, then examining specific parameters and optimizing these to meet the constraints. A final analytical inlet design, optimized in the regions of interest, is selected for experimental work.

Commentary by Dr. Valentin Fuster
2006;():411-418. doi:10.1115/GT2006-90893.

Numerical analysis is conducted for the 3-dimensional impeller and vaneless diffuser of a small centrifugal compressor. The influence of impeller tip clearance is investigated. A Navier-Stokes flow solver Finflo has been applied for the simulation. A practical real gas model has been generated for the calculation. Simulations with different sizes of tip clearance at different mass flow rates have been made. The results are compared to experimental results at a certain tip clearance and one operating point. Reasonable agreement has been obtained. The ideal gas model has also been applied to compare with the real gas model. The numerical results show that tip clearance has a significant effect on the performance of a small centrifugal compressor. As the size of tip clearance increases, both the pressure ratio and the efficiency decrease. The decreasing rate of efficiency is higher at higher mass flow rates and lower at lower mass flow rates. The input power of the compressor hardly changes with different sizes of tip clearance, but increases as the mass flow rate increases. The incidence of impeller and flow angle at the exit of the impeller increase as the size of tip clearance increases. Correlations of the size of tip clearance with the efficiency drop and change of flow angle at the exit of impeller are given. The detailed flow distribution shows that as the size of tip clearance increases, the tangential leaking flow at the tip clearance makes the low velocity flow region grow larger and move from the suction-shroud corner to the center of the flow channel. The main flow at the pressure side is compressed and accelerated. Therefore the uniformity of the flow in the whole channel decreases. The detailed flow distribution also shows that the leaking flow is stronger at higher mass flow rates.

Commentary by Dr. Valentin Fuster
2006;():419-428. doi:10.1115/GT2006-90964.

A new foil gas bearing is introduced in this paper. This foil gas bearing uses series of compression springs as a compliant structure instead of expensive corrugated bump foils. The new foil gas bearing is very simple in structure and easy to manufacture. A theoretical model to estimate stiffness and damping of the spring bump was developed. Measured stiffnesses of individual spring bump agree well with predictions. Load capacity was measured up to 62.5 N at 20,000rpm with both cooled and uncooled bearings. Initial selection of spring geometry rendered rather soft supports compared to other bump foil bearings, and allowed only limited load during the test. Developed cooling method using direct air supply holes machined on the bearing sleeve was very effective to cool the test bearing because the spring bumps are not connected along the circumferential direction, and allow very effective circumferential distribution of cooling air. A series of orbit simulation was performed to estimate critical speed and onset speed of instability. Bump dynamics was directly coupled with the orbit simulation. Critical speed was estimated at around 7500 rpm due to relatively soft support structure. Hydrodynamic instability with WFR 0.5 could be predicted at around 15,000 rpm. The rotor instability is predicted even under the equivalent viscous damping extracted from bump dynamics, implying the viscous damping alone within the bump cannot suppress hydrodynamic instability of foil bearings.

Commentary by Dr. Valentin Fuster
2006;():429-438. doi:10.1115/GT2006-91026.

This paper presents an aerodynamic loss prediction and design methodology that has been developed for ducted-fan system of a vertical takeoff and landing Micro Unmanned Air Vehicle (Micro UAV). The method is based on a parametric geometry model and semi-empirical aerodynamic methods. The developed loss correlation method was applied to an initial sizing attempt of a ducted fan system.

Commentary by Dr. Valentin Fuster
2006;():439-448. doi:10.1115/GT2006-91041.

FLOX® — or flameless combustion is characterized by ultra-low NOx emissions. Therefore the potential for its implementation in gas turbine combustors is investigated in recent research activities. The major concern of the present paper is the numerical simulation of flow and combustion in a FLOX® -combustor [1, 2] at high pressure operating conditions with emphasis on the pollutant formation. FLOX® -combustion is a highly turbulent and high-velocity combustion process, which is strongly dominated by turbulent mixing and chemical non-equilibrium effects. By this means the thermal nitric oxide formation is reduced to a minimum, because even in the non-premixed case the maximum combustion temperature does not or rather slightly exceed the adiabatic flame temperature of the global mixture due to almost perfectly mixed reactants prior to combustion. In a turbulent flow the key aspects of a combustion model are twofold, i) chemistry and ii) turbulence/chemistry interaction. In the FLOX® -combustion we find that both physical mechanisms are of equal importance. Throughout our simulations we use the complex finite rate chemistry scheme GRI3.0 for methane and a simple partially stirred reactor (PaSR) model to account for the turbulence effect on the combustion. The computational results agree well with experimental data obtained in DLR test-facilities. For a pressure level of 20 bar, a burner load of 417 kW and an air to fuel ratio of λ = 2.16 computational results are presented and compared with experimental data.

Topics: Combustion , Pollution
Commentary by Dr. Valentin Fuster
2006;():449-458. doi:10.1115/GT2006-91088.

A large number of papers have been published on transient modeling of large industrial and military gas turbines. Few, however, have examined micro turbines. The decrease in size affects the relative rates of change of shaft speed, gas dynamics and heat soak. This paper compares the modeled transient effects of a micro turbojet engine comprised of a single stage of radial compression and a single stage of axial expansion, with a diameter of 12cm. The model was validated with experimental data. Several forms of the model were produced starting with the shaft and fuel transients. Conservation of mass, and then energy, was subsequently added for the compressor, combustor and turbine, and a large inlet plenum that was part of the experimental apparatus. Heat soak to the engine body was incorporated into both the shaft and energy models. Heat soak was considered in the compressor, combustor and turbine. Since the engine diameter appears in the differential equations to different powers, the relative rates of change vary with diameter. The rate of change of shaft speed is very strongly influenced. The responses of the different transient effects are compared. The relative solution times are also discussed, since the relative size of the required time steps changes when compared to a large engine.

Commentary by Dr. Valentin Fuster
2006;():459-466. doi:10.1115/GT2006-91120.

Feasibility of an innovative micro gas turbine with heat recuperation has been studied. The proposed core engine has back-to-back rotor configuration formed by a centrifugal-flow compressor and an axial-flow turbine. A newly Swiss-roll recuperator is wrapped around a can-type combustor to recover the exhaust heat, thus reducing the fuel consumption rate and improving the engine thermal efficiency. From the recuperated cycle analysis, thermodynamic requirements at both inlet and outlet of each component were predetermined to evaluate the engine performance. The conceptual design and preliminary analysis to achieve these requirements were made, component-by-component. The thermal design of the Swiss-roll recuperator was also carried out by theoretical analysis, which gives the thermal characteristics of the recupeator, including the trend of effectiveness, thermal requirement of number-of-transfer-unit (NTU) and the preliminary sizing with the number of turns and the width of the flow channels be determined. The characteristics of Swiss-roll recuperator resemble the counter-flow spiral plate heat exchanger and basically have the excellent performance with high effectiveness and low pressure loss. The performance of the proposed micro gas turbine was investigated, and potentially a compact micro gas turbine with thermal efficiency higher than 20% is possible for the power output less than 10 kW.

Commentary by Dr. Valentin Fuster
2006;():467-476. doi:10.1115/GT2006-91259.

This work presents the modifications in a 30 kW gas micro-turbine speed control model, when it was supplied with castor bean biodiesel in several proportions. The concern about using biodiesel as an alternative fuel is increasing in the Brazilian distributed generation market. For this analytics, a complete study was developed considering the effects of using this new fuel. Characteristics like chemical composition, physical and chemical properties of the different mixtures were analyzed, especially focusing on the kinematic viscosity of the fuel. The tests results performed with the micro-turbine, originally projected for diesel, are shown. Mixtures of 5, 10, 15, 20, 25, 30, 50 e 100% of biodesel were used, and several variables were measured in the whole range of power. The influence of the biodiesel characteristics in the model parameters are commented in the conclusions. The possible application of the proposed model in studies of electrical power network is suggested in the end of the article.

Topics: Turbines , Biodiesel
Commentary by Dr. Valentin Fuster
2006;():477-482. doi:10.1115/GT2006-91264.

Integrated Energy Systems (IES) combine a distributed power generation (DG) system such as a microturbine generator (MTG) or a fuel cell with thermally activated technologies (TAT) such as absorption cooling. This integration maximizes the efficiency of energy use by on-site utilization of most of the waste heat generated by the DG system, and thus reduces harmful emissions to the environment. This study shows experimental results of a real IES commercial unit that has been tested in the Advanced Power and Energy Program (APEP) DG testing facility at the University of California, Irvine. The system consists of an MTG with an internal recuperator that provides a maximum electrical power generation capacity of 28 kW, and a novel absorption cooling cycle. The absorption cycle is a single effect-double effect exhaust fired cycle, which increases the heat exchange from the MTG exhaust gases using two generators at two different temperature levels. With this combination, the maximum cooling capacity of the absorption machine is nominally 14 refrigeration tons (49.2 kW), which represents 25% more cooling capacity than a double effect absorption machine could provide with the same heat source. The results will show the electrical and thermal performance of this system, both at full load and partial load, for different cooling temperatures, and the relationship between the electrical load and the performance of the absorption unit.

Commentary by Dr. Valentin Fuster
2006;():483-490. doi:10.1115/GT2006-91312.

The effects of centrifugal force and operating temperature on the stress and creep deformation of rotating micro-turbo rotors are studied in this work. The thermal-mechanical- creep coupled finite element model provided in the MARC package is employed. The variation of stresses and creep strain distributions in turbo blades made of different materials, e.g. stainless steel (A304L), Ti-alloy (Ti-2411) and two ceramics (SiC and SiN3 ) are analyzed and compared. The effect of rotating speed on the natural frequencies of the micro-turbine with different turbo-blade materials are also evaluated and compared in this study. Numerical results indicate that the strength and dynamic behavior of the micro-turbo rotor is very sensitive to the material of turbo blade.

Topics: Turbines , Blades
Commentary by Dr. Valentin Fuster

Oil and Gas Applications

2006;():491-501. doi:10.1115/GT2006-90004.

Axial compressor deterioration due to removable deposits is a major concern in the operation of gas turbines. It is important to fully understand the flow mechanisms in order to successfully monitor and clean the engine. A test program on the GE J85-13 jet engine quantified the increased surface roughness and the distribution of salt deposits in an axial compressor. The test data showed good agreement with published data for stage performance deterioration. This paper compares the GE J85-13 test data on surface roughness to previously published work on surface roughness in compressors. The effect of surface roughness on the stage characteristics is modeled using theory for frictional losses, blockage and deviation. The results are compared to test data. The most significant effect of increased roughness is found to be the variation in the flow coefficient.

Commentary by Dr. Valentin Fuster
2006;():503-510. doi:10.1115/GT2006-90030.

This paper describes a case study involving chronic plugging of 5μm particulate filters in a gas turbine diesel fuel system in a Red Sea refinery. Rapid plugging of the filters was caused by water-in-diesel emulsions generated in diesel fuel supply tanks. Sludge with a wax-like appearance recovered from the 5 μm filters was, in fact, found to be composed of up to 50 percent water with no significant wax content. X-ray studies of the filter catch solids revealed a variety of iron oxide phases, sodium chloride, and high concentrations of sodium sulfate. Microbial cultures inoculated from storage tank ‘rag’ layers yielded moderate to high counts of general aerobic bacteria (GAB), moderate fungal cultures (yeast and molds) and low sulfate reducing bacteria (SRB). Elemental analysis of water in supply tanks where microbial activity was highest revealed ion concentrations similar to those found in Red Sea water. Sulfur isotope ratios in sulfate from filter catches suggest that much of the sulfate was derived from microbial metathesis of sulfur-bearing hydrocarbons. Frequent contamination of on-shore liquid hydrocarbon fuel tanks with sea water can cause corrosion and create a favorable environment for bacterial growth. Surfactant byproducts of microbial activity are capable of stabilizing emulsions, suspending water soluble salts such as sodium sulfate, and metals such as lead and copper. Copper is well known to promote gum formation, while all of these contaminants are potentially corrosive to gas turbine hot gas path components.

Topics: Fuels , Emulsions
Commentary by Dr. Valentin Fuster
2006;():511-518. doi:10.1115/GT2006-90031.

This paper describes an analytical approach used to identify heavy natural gas liquid (NGL) fuel components and fuel conditioning solutions employed to prevent fouling of the vaporized fuel delivery systems. The discussion includes high pressure vaporized fuel sampling, isolation of C7+ and C14+ hydrocarbon fractions from NGL, and performance validation of fuel processing apparatus. Saudi Aramco operates more than 80 aeroderivative gas turbines (CGT’s), from four manufacturers, to drive crude oil pumps and generate electrical power on the East-West Pipeline that traverses the Arabian Peninsula. Since the pipeline was first commissioned in 1980, the CGT’s have been operated primarily on vaporized C2+ NGL. Although the properties of this C2+ NGL (such as density and heating value) are nearly identical to propane, its use as CGT fuel has presented challenges. Fuel system fouling resulted from the presence of heavy hydrocarbons including residual surface-active compounds derived primarily from corrosion inhibitors and intermittent crude oil carryover. This fouling consisted of hard, epoxy-like deposits coating all manifolds and fuel nozzle passages downstream of the vaporizers. The entire fleet suffered from increased operating and maintenance costs and reduced reliability from plugging of last-chance filters to blocked fuel nozzles. This led to temperature spreads in combustors and hot component damage. High temperature rated coalescing filters were applied successfully in three vaporized NGL fuel system configurations. One fuel system configuration that required even more stringent fuel conditioning was modified to reject approximately 15 percent (heavy ends fraction) of the NGL. Performance tests were conducted to measure the extent to which heavy ends were reduced in the modified fuel vaporizers. Analytical methods were developed to identify and measure heavy hydrocarbons at ppm concentrations. The actual fuel compositions determined analytically agreed with compositions predicted from process simulations.

Topics: Fuels
Commentary by Dr. Valentin Fuster
2006;():519-524. doi:10.1115/GT2006-90037.

Realistic assessment of the remaining serviceable life in turbine hot section components plays an important role in engine condition-based maintenance and overhaul. This paper presents application of an engineering approach that integrates mechanical and performance engineering with metallurgy for turbine blade remaining life on-line assessment. By identifying the life limiting factors in a given application and monitoring the rate of the damage, economical repair, replacement and overhaul intervals can be established. Examples are given to illustrate application of this engineering approach in identifying failure mechanisms and predicting blade remaining serviceable life for TransCanada PipeLines LM1600 fleet.

Commentary by Dr. Valentin Fuster
2006;():525-534. doi:10.1115/GT2006-90069.

Arranging compressor units in series rather than in parallel can offer a number of advantages in many applications. However, especially if the compression ratio is rather high, it is desirable to use different impellers for the low-pressure and the high-pressure compressor. Otherwise, one or both operate off their best efficiency points. This leaves however the problem of the staging of a spare unit. In the paper, a solution to this dilemma is described. To understand the underlying constraints, the relationships between the system resistance, the compressor characteristic, and the power turbine characteristic are analytically derived.

Topics: Compressors
Commentary by Dr. Valentin Fuster
2006;():535-542. doi:10.1115/GT2006-90070.

Compressor stations where both centrifugal and reciprocating compressors operate can exhibit operational problems due to low frequency pulsations. Avoiding these problems requires an understanding of the mechanisms that generate the pulsations, the transmission and attenuation of the pulsation, as well as their effect on the operation of the centrifugal compressor and the accuracy and reliability of flow measurements. The pulsation frequencies in question are typically lower than vortex shedding frequencies and blade passing frequencies. This paper will provide insight into the mechanisms and effects of pulsations and will describe approaches to avoid them.

Topics: Compressors
Commentary by Dr. Valentin Fuster
2006;():543-555. doi:10.1115/GT2006-90117.

In the paper, feed-forward Recurrent Neural Networks with a single hidden layer and trained by using a back-propagation learning algorithm are studied and developed for the simulation of compressor behavior under unsteady conditions. The data used for training and testing the RNNs are both obtained by means of a non-linear physics-based model for compressor dynamic simulation (simulated data) and measured on a multi-stage axial-centrifugal small size compressor (field data). The analysis on simulated data deals with the evaluation of the influence of the number of training patterns and of each RNN input on model response, both for data not corrupted and corrupted with measurement errors, for different RNN configurations and different values of the total delay time. For RNN models trained directly on experimental data, the analysis of the influence of RNN input combination on model response is repeated, as carried out for models trained on simulated data, in order to evaluate real system dynamic behavior. Then, predictor RNNs (i.e., those which do not include among the inputs the exogenous inputs evaluated at the same time step as the output vector) are developed and a discussion about their capabilities is carried out. The analysis on simulated data led to the conclusion that, to improve RNN performance, it is beneficial the adoption of a one-time delayed RNN, with an as low as possible total delay time (in the paper, 0.1 s) and trained with an as high as possible number of training patterns (at least 500). The analysis of the influence of each input on RNN response, conducted for RNN models trained on field data, showed that the single-step-ahead predictor RNN allowed very good performance, comparable with that of RNN models with all inputs (overall error for each single calculation equal to 1.3% and 0.9% for the two test cases considered). Moreover, the analysis of multi-step-ahead predictor capabilities showed that the reduction of the number of RNN calculations is the key factor for improving its performance over a significant time horizon. In fact, when a high test data sampling time is chosen (in the paper, 0.24 s), prediction errors were acceptable (lower than 1.9%).

Commentary by Dr. Valentin Fuster
2006;():557-570. doi:10.1115/GT2006-90134.

The paper deals with the development of a non-linear one-dimensional modular dynamic model for the simulation of transient behavior of compression systems. The model is based on balance equations of mass, momentum and energy, which are derived through a general approach and are written by using the finite difference method. The model also takes rotating mass dynamics into account through a lumped parameter approach. Moreover, it reproduces the behavior of the system in the presence of the surge phenomenon through steady-state performance maps, which represent the compressor operation in the inverse flow region by means of a third degree polynomial curve. The model is implemented through the Matlab Simulink tool, where the system of ordinary differential equations is solved by using a fourth and fifth order Runge-Kutta method. A sensitivity analysis is carried out to evaluate the influence on compressor outlet pressure oscillations of the model parameters, of the supplied torque, of ambient conditions and of the shape of the compressor characteristic curves. The results show that the model proves effective in capturing the physical essence of surge phenomenon without being computationally too heavy.

Commentary by Dr. Valentin Fuster
2006;():571-582. doi:10.1115/GT2006-90557.

Revamping a plant in the oil&gas industry is needed whenever the company’s management seeks improvements on plant availability, production output, energy efficiency or operational life, to quote the most common needs. The achievements of those objectives are reached through business programs usually known as “modernization”, “rejuvenation”, “debottlenecking”. Revamping actions over the compression trains that are within the plants are key for the success of those programs, achieving the business and operational objectives sought by the management. This paper is aimed to show how a revamping program works for turbocompression trains; three real-life examples are shown too. Focus is mainly on debottlenecking and upgrade projects, for which it is more necessary to act extensively on the compression trains.

Commentary by Dr. Valentin Fuster
2006;():583-590. doi:10.1115/GT2006-90601.

Centrifugal compressors are critical for moving large volumes of gas in the natural gas pipeline, hydrocarbon processing, and general energy industries. The operation of centrifugal compressors is limited by a number of factors including the occurrence of surge at low flows. The exact low flow conditions at which surge occurs cannot be determined from external measurements. The precise flow and head at which surge occurs is effected by factors including the installed piping configuration, the dynamic impedance of the piping system, the pulsating pressures in the piping, and the gas properties such as compressibility among other variables. The results of the many factors affected when surge occurs and the sudden nature of surge onset are such that there are no previously identified reliable ways to detect an approaching surge. Current surge control methods rely on external measurements of head or speed and flow to estimate the conditions at which surge will occur and then recycle flow at some margin above the expected surge conditions in order to avoid surge. However, this type of surge control is inefficient and frequently leads to recycling more flow than necessary with the result that fuel and energy are wasted. If a means were available to measure a fundamental pre-cursor to surge then a reliable indication of the operating margin above surge could be developed and the amount of recycle flow and the loss of efficiency could be minimized. Such a pre-cursor has been found and a sensor technique base on drag probe technology has been developed. This paper presents some of the background on pre-surge detection in centrifugal compressors and then describes the known behavior of the impeller inlet outer wall re-circulating flow that develops as surge approaches. In addition, this paper reports on the development of a drag probe sensor to measure these internal flow components. Evidence from direct surge control testing is presented to support the finding that these internal flows are a basic surge pre-cursor and a useful control for centrifugal compressors. The measured flow changes prior to surge are identified and data is shown. The result of this research will lead to an improved surge control system for a significant class of signal stage modern centrifugal compressors and will increase the operating range and overall efficiency of such machines.

Commentary by Dr. Valentin Fuster
2006;():591-606. doi:10.1115/GT2006-90955.

This paper presents the continuation of the work performed during the development of an uncertainty analysis method for estimating error levels in data gathered during factory aero-performance acceptance tests of centrifugal compressors. The previous work incorporated the effects of the variation and uncertainty levels associated with every parameter used in the calculation of centrifugal compressor aero-thermal performance. The work discussed herein focuses on the effects of the variation and uncertainty levels associated with the key measured variables, which are the parameters identified as having the greatest effect on the uncertainty of the performance measurements. Also included in this work is an evaluation of the effects of the correlated bias uncertainty components associated with said key variables, as well as comments on how these effects can be harnessed to reduce the uncertainty of the test data. The evaluation is performed via parametric studies, which present the test uncertainty levels achievable as a function of different correlation levels between the systematic uncertainty components of the measured data. Two different methods are used for the analysis of data measured for several machines. The first method is based on the direct use of the Monte Carlo simulation technique combined with real gas equations of state. The second method employs uncertainty propagation equations and the methodology included in the ASME PTC-19.1(1998) test code. Both approaches use the polytropic compression model and equations for performance evaluation included in the ASME PTC 10 (1997) Power Test Code. Data gathered during an on-site acceptance test of a centrifugal gas compression package are used to illustrate the effects of the uncertainty in the knowledge of the gas composition handled by the compressor over the uncertainty levels that can be obtained with this type of tests.

Commentary by Dr. Valentin Fuster
2006;():607-616. doi:10.1115/GT2006-90960.

The presented paper provides details of the enclosure design validation practice that combines a CFD and experimental study using an engine enclosure subscale model. The main features from the actual engine package were included in geometrically similar experimental model and represented in detail in the CFD model. Performance of ventilation was studied for both cold and heated engine surfaces. A number of experimental techniques, including flow visualization, flow-velocity, temperature measurements, and infrared thermal imaging have been applied in the study. A potential methane leakage was emulated with CO2 and was combined with CO2 sampling and concentration measurements in various locations within the enclosure. Numerical predictions were then compared to experimental data allowing to fine tune the model and applied boundary conditions. Based on presented study certain improvements in design of the enclosure were recommended and described in the paper.

Commentary by Dr. Valentin Fuster
2006;():617-624. doi:10.1115/GT2006-91033.

The compressor polytropic head and efficiency analysis are based on the assumption that the compression process follows the path of a constant polytropic exponent n. Both the ASME PTC10-97 and the ISO 5389 refer to the polytropic analysis by John M. Schultz. The procedure utilizes a head correction factor and two compressibility functions to obtain a solution of the integral Δhp = ∫vdp. Present computer technology renders possible a direct integration of the compression path where the variation in actual gas properties along the path is included. This method eliminates the averaging of gas properties which the Schultz procedure includes. This paper reports deviation in compressor performance using the Schultz procedure with different average gas properties. The implementation of a direct integration procedure, employing actual gas properties from the new GERG-2004 equation of state, is given. The GERG-2004 equation of state has proven to give accurate density values both in the vapour and liquid phases. Depending on how the polytropic compression analysis is implemented, the work has revealed up to 4% deviation in polytropic head and efficiency for some specific compressors. This adds an extra uncertainty in compressor performance verification. Even though the API 617 allows up to 4% deviation, some compressors have to meet a more stringent demand, for instance 2% at the Sno̸hvit LNG plant. Future challenges within oil and natural gas production are related to wet gas compressors. The present paper points out the advantages in using a direct integration method for wet gas performance predictions as this takes phase changes along the compression path into account.

Topics: Compressors
Commentary by Dr. Valentin Fuster
2006;():625-632. doi:10.1115/GT2006-91035.

The growing interest in wet gas compressors calls for accurate methods for performance prediction. Present evaluation methods for compressor and pump performance fail when evaluating the compression of gases containing liquid. Gas compression performance predictions given in ASME PTC-10-97 and ISO 5318 are based on the method John M. Schultz proposed in 1962. This method assumes a polytropic compression path and is based on averaged gas properties of inlet and outlet condition. The polytropic compression path is defined by keeping pvn constant, where n is constant along the compression path. When employing the Schultz method there is a challenge in defining the polytropic constant. This is seen in cases where dry gas compressors are exposed to wet components and compressor efficiency estimates exceed 100%. Today’s computer technology makes a direct integration of the polytropic head (∫vdp) possible where actual fluid properties along the compression path are included. Phase changes along the compression path are included with this method. This enables a detailed prediction to be made of the actual volumetric flow rate for the various compressor stages. This paper reports the implementation of the direct integration procedure for wet gas performance prediction. The procedure enables generic wet gas compression to be studied which forms the foundation for performance analysis with variations in operation at conditions and fluid components and properties.

Commentary by Dr. Valentin Fuster
2006;():633-638. doi:10.1115/GT2006-91130.

The RWE Transgas, a.s., was challenged with the reconstruction of the control system of 6 MW turbosets installed in compressor stations Kralice and Kourim (Czech Republic) in late 1970’s. The control of the turbosets was formerly performed by the compressor station personnel by means of a relay-based control system. The reconstruction has been completed in 2004. In this contribution, some new functions of the modified control system are analyzed in greater detail with special emphasis on the surge identification and protection.

Commentary by Dr. Valentin Fuster
2006;():639-648. doi:10.1115/GT2006-91211.

Given the maturity of the gas turbine user industry, and the quality of the original equipment manufactured, the field experience of these machines has been broadly very good. Nonetheless, there can be difficulties in managing availability and reliability, and the costs of unscheduled shut downs can be extremely large. Advanced gas turbine performance management offers opportunities in improving availability, reliability and productivity, driving down life cycle costs as well as adding to safety and compliance. This paper describes in detail the development of an engine fault diagnostics tool by the authors. The authors focus on structure, functionality as well as possible benefits of its use. The first part describes the structure and various methodologies adopted in the development of the system and the second part of the work describes the system and its salient features. The primary goal at this point of time was to test the utility, functionality and robustness of the presented tool, interesting and relevant results are obtained. Due to space limitations we restrict ourselves to discussing the system and not go into the details of actual simulation runs.

Topics: Engines , Gas turbines
Commentary by Dr. Valentin Fuster
2006;():649-660. doi:10.1115/GT2006-91353.

Diagnosing the root cause of compressor blade failures by high cycle fatigue (HCF) is elusive for gas turbines with a long history of successful operation; long operating times preclude design, fabrication, and material defect issues that are usually associated with short term failures. Long exposure time might implicate erosion or corrosion issues for compressor failures. An investigation of 3rd stage stator vane and 4th stage rotor blade failure in a Frame 7 gas turbine revealed that aerodynamic excitations associated with mild compressor instability (undetected by installed sensors) was the most probable cause. A Blade Vibration Audit (BVA) approach exploits information collected and compared from independent sources: 1) Fracture mode details and expected failure stress levels estimated in metallurgical examination; 2) Aerodynamic excitations due to internal airfoil wakes, rotating stall, and flutter applied to vibratory stress response data obtained by modal testing to estimate relative operating stress levels; 3) Design margins deduced from successful operating experience, which establish a base line for comparison with the excitation sources considered likely. Diagnosis of HCF failure causes must produce results that match observations of the specific failures in three ways: 1) The airfoils must be in resonance for sufficient time for fatigue to occur, 2) The vibratory stress must be greater at the failure location than elsewhere, and 3) The causal effect must significantly increase the stress at the failure location to be more than elsewhere. The analysis showed that the 3rd stator vane failed at 2/3 span, which led to 4th blade failure due to subsequent adverse aerodynamic excitation and impact damage. The most likely cause of the 3rd stator vane failure was a combination of resonant excitation from excessive wakes of the downstream rotor blades and flutter associated with mild intermittent surge.

Commentary by Dr. Valentin Fuster

Structures and Dynamics: General

2006;():661-669. doi:10.1115/GT2006-90120.

The following paper outlines a methodology for accurately predicting the burst of a centrifugal rotor, for the purpose of certifying the tri-hub containment capability of an Auxiliary Power Unit gas turbine engine. The tri hub burst is achieved by introducing three equally spaced slots into a centrifugal rotor. Using 2D and 3D finite element analysis, the slot geometry was optimized to ensure burst of the centrifugal rotor at the desired speed, through spin pit testing, and to account for thermal and centrifugal growth for operation in an engine with proper tip clearances. In order to validate the versatility of this methodology, two centrifugal rotor geometries with different material properties (Ti6Al4V and Ti6Al2Sn4Zr6Mo) and operating conditions were analyzed. The analytical predictions were confirmed with isothermal spit pit tests using temperatures that approximate the bulk average temperature in the high stressed bore for an un-slotted centrifugal rotor. The results of spin pit tests were found to be within 0.4% of predicted values. Burst tests were subsequently conducted on a gas generator rig and a full engine test, where results were found to be within 2% of predicted values.

Commentary by Dr. Valentin Fuster
2006;():671-680. doi:10.1115/GT2006-90454.

Deformation mechanisms and failure modes of FCC (face centered cubic) single crystal components subjected to triaxial states of static and fatigue stress are very complicated to predict, because plasticity precedes fracture in regions of stress concentration, and the evolution of plasticity on the surface and through the thickness is influenced by elastic and plastic anisotropy. The triaxial stress state at regions of stress concentration results in the activation of many slip systems that otherwise would not be activated during uniaxial testing. We recently presented [1] results from a numerical and experimental investigation of evolution of slip systems at the surface of notched FCC single crystal specimens, as a function of secondary crystallographic orientation. Results showed that the slip sector boundaries have complex curved shapes with several slip systems active simultaneously near the notch. We extend our work on slip at the surface to investigating the evolution of slip or plastic deformation through the thickness of the specimen. A single crystal double-edge-notched rectangular specimen of a Ni-base superalloy, under the tensile loading ([001] load orientation and [110] notch direction) is considered. A three dimensional (3-D) finite element model (FEM) including elastic anisotropy is used for the numerical investigation. Results indicate that the stress distribution and slip fields are a strong function of axial location through the thickness. Numerical results are verified by comparing them with experimentally observed slip fields. We demonstrate that inclusion of three dimensional analysis and elastic anisotropy is important for predicting evolution of slip at the surface and through the specimen thickness. The resolved shear stresses (RSS) on the dominant slip systems and the normal stress on the dominant planes are shown to vary significantly from the surface to the midplane of the specimen. Based on the consideration of RSS, normal stress and the number of activated slip systems at each thickness level, it is concluded that fatigue cracks most likely start in the midplane, for the orientation reported here.

Commentary by Dr. Valentin Fuster
2006;():681-690. doi:10.1115/GT2006-90585.

Stress prediction in turbine blades is affected by limitations in the measurement setup as well as by various sources of uncertainty in the model-based inference techniques. The presence of uncertainty diminishes the confidence in the estimated response, whose fidelity lowers further when results are extrapolated to operating conditions or systems other than the tested ones. A procedure is being developed with a twofold objective: maximization, under given computational constraints, of a system’s model accuracy and quantification of any remaining uncertainty associated with the estimation technique. A probabilistic analysis, in which uncertainties are explicitly modeled has been carried out to investigate the effects of modeling and input parameter uncertainties, and to evaluate their contribution to a system’s vibratory response prediction. Results and considerations are herein described and discussed.

Commentary by Dr. Valentin Fuster
2006;():691-700. doi:10.1115/GT2006-90835.

Combustion Turbines operators have recently experienced compressor rubbing related problems during Hot Restart transients. Based on maintenance feedback from in-service combustion turbine, some turbo machinery designers have imposed Hot Restart Restriction and have considered prudent to increase the rotor-to-stator clearances. Combustion Turbines operators have recently experienced compressor rubbing related problems during Hot Restart transients. Based on maintenance feedback from in-service combustion turbine, some turbo machinery designers have imposed Hot Restart Restriction and have considered prudent to increase the rotor-to-stator clearances. Hot Restart has a significant impact on compressor clearances since temperature distribution hasn’t reach its steady-state condition due to heat conduction. When Hot-Restart occurs, thermal distortions of the casing and the bladed rotor are indeed continually evolving. The resulting rotor-to-stator clearances may become extremely low while the rotor expands rapidly due to inertial body forces generated by rotor speed up and vibrates due to mass-unbalance. As a consequence, clearances may dramatically decrease until rubbing occurs. A Finite Element-based analysis has been developed for simulating the time-transient thermo mechanical and the vibratory motion responses of a F-Class combustion turbine. The thermo mechanical responses enable to calculate thermal distortions of the casing and the bladed rotor as well as the rotor expansion due to the inertial body forces generated by rotor speed up. The dynamical responses enable to calculate the amplitude of the shaft line vibration due to mass-unbalance. The approach developed by EDF R&D relies on a 3D thermo mechanical modeling of the compressor rotor/casing and a 1D beam-type modeling of the whole combustion turbine shaft line using Finite Element Code Code_Aster® and Rotor Dynamics Code CADYAC, respectively, developed by EDF R&D. The FE-Models have been validated by comparing compressor tip clearance measurements published by a F-Class manufacturer and FE calculations during a typical start-stop cycle. The FE-predictions obtained with such models are in acceptable agreement with measurements data. The FE models are then used to predict the evolution of the rotor-to-stator clearances and to evaluate the compressor rubbing risk during Hot Restart. The results are then compared with Hot Restart time-restriction imposed by the turbo machinery designer.

Commentary by Dr. Valentin Fuster
2006;():701-709. doi:10.1115/GT2006-91024.

Application of DACE (D esign and A nalysis of C omputer E xperiments) methods for probabilistic design space exploration and optimization to the design of a mechanical component is demonstrated. The key part of the paper is focused on the problem formulation and process flow for performing a probabilistic optimization. The authors have shown that for computationally intensive problems, probabilistic optimization can be carried out efficiently within a DACE framework. For problems that are not costly to compute, direct probabilistic optimization can be carried out by the efficient integration of probabilistic analysis and global optimization (such as Genetic Algorithms). The strategy in the paper proves to be especially beneficial for those organizations that are reluctant to move to probabilistic methods and also for the current practitioners of probabilistics. The methodology is illustrated with examples from both simple and computationally intensive engineering problems.

Topics: Optimization
Commentary by Dr. Valentin Fuster
2006;():711-717. doi:10.1115/GT2006-91136.

In this paper, a new approach for graphical enumeration of epicyclic gear mechanisms is presented using the concept of acyclic graph. A new graphical code has been introduced to specify the priorities of all vertices of associated displacement graph. This graphical code is used to identify open graph, redundant links and isomorphic graphs. A computer program has been developed for automatic enumeration of displacement graphs as well as automatic detection of isomorphic graphs and open graphs and graphs with a redundant link without using adjacency matrices. This simplified methodology has been applied for the enumeration of epicyclic gear mechanisms with up to 12-links having up to 9 coaxial links. An atlas for 11 and 12-link mechanisms has been constructed using the proposed methodology.

Commentary by Dr. Valentin Fuster

Structures and Dynamics: Mechanics and Vibration

2006;():719-728. doi:10.1115/GT2006-90057.

The ability to perform and evaluate the effect of shape changes on the stress, modal and thermal response of components is an important ingredient in the ‘design’ of aircraft engine components. The classical design of experiments (DOE) based approach that is motivated from statistics (for physical experiments) is one of the possible approaches for the evaluation of the component response with respect to design parameters [1]. Since the underlying physical model used for the component response is deterministic and understood through a computer simulation model, one needs to re-think the use of the classical DOE techniques for this class of problems. In this paper, we explore an alternate sensitivity analysis based technique where a deterministic parametric response is constructed using exact derivatives of the complex finite-element (FE) based computer models to design parameters. The method is based on a discrete sensitivity analysis formulation using semi-automatic differentiation [2,3] to compute the Taylor series or its Pade equivalent for finite element based responses. Shape design or optimization in the context of finite element modeling is challenging because the evaluation of the response for different shape requires the need for a meshing consistent with the new geometry. This paper examines the differences in the nature and performance (accuracy and efficiency) of the analytical derivatives approach against other existing approaches with validation on several benchmark structural applications. The use of analytical derivatives for parametric analysis is demonstrated to have accuracy benefits on certain classes of shape applications.

Topics: Design , Shapes
Commentary by Dr. Valentin Fuster
2006;():729-737. doi:10.1115/GT2006-90087.

This paper deals with fundamental aspects of variations in eigenvalues and eigenvectors of a bladed disk due to mistuning. First, the existence of derivatives of repeated eigenvalues and corresponding eigenvectors is thoroughly examined. Next, an algorithm is developed to compute these derivatives. It is shown how a Taylor series expansion can be used to efficiently compute eigenvalues and eigenvectors of a mistuned system. This methodology is developed for perturbations in both repeated and unrepeated eigenvalues of the tuned system. Lastly, numerical examples are presented.

Commentary by Dr. Valentin Fuster
2006;():739-747. doi:10.1115/GT2006-90088.

This paper examines the nature of the statistical distribution of the peak maximum amplitude of the forced response of a mistuned bladed disk. Monte Carlo simulations are performed for a range of values of the structural coupling between blades and the standard deviation of mistuning. First, it is determined if the statistical distribution of a peak maximum amplitude can be called three-parameter Weibull in general. Next, using a neural network, a functional relationship between the statistics of the peak maximum amplitude ;e.g., parameters of Weibull distribution, statistical moments, 99 percentile amplitude, and input parameters (structural coupling between blades and standard deviation of mistuning) is developed.

Commentary by Dr. Valentin Fuster
2006;():749-760. doi:10.1115/GT2006-90102.

A model that predicts the quasi-static behavior of a friction damper that has spherical contacts was developed using Mindlin’s theory. The model was integrated into a dynamic analysis that predicts the vibratory response of frictionally damped blades. The analytical approach was corroborated through a set of benchmark experiments using a blades/damper test fixture. There was good agreement between the theoretical predictions of amplitude and the values that were measured experimentally over a wide range of test conditions. It is concluded that it is possible to predict the vibratory response of frictionally damped vibrating systems using continuum mechanics, provided that the contact geometry is clearly defined and the local nonlinear contact is correctly taken into account.

Topics: Friction , Dampers , Stiffness
Commentary by Dr. Valentin Fuster
2006;():761-767. doi:10.1115/GT2006-90112.

Turbomachinery shrouded rotor blade design has been widely used in fans, compressors, and turbines. By using either tip or mid-span shroud, the blade structural damping and natural frequencies can be increased to reduce flutter or forced response problems. However, shrouded rotor blade design sometimes results in complex system modes with both bending and torsion present at the same time. Since shrouded rotors are rotationally periodic, the bending and torsion combined system mode can be decomposed into a real component and an imaginary component. Using both the real and imaginary mode shapes, coupled with the blade surface flow-field, a complex mode analysis was developed using a modal solution to determine the forced response of the system. This complex mode analysis was applied to both tip and mid-span shrouded rotor blade applications. Bending-dominated or torsion-dominated simple blade modes in general can be analyzed using an existing single mode approach. However, for the bending and torsion combined system modes, the single mode analysis can be misleading. By using the complex mode analysis, it was demonstrated that the combined bending and torsion system mode plays an important role in determining the blade forced response.

Commentary by Dr. Valentin Fuster
2006;():769-778. doi:10.1115/GT2006-90146.

Advanced structural dynamic models for both wedge and split underplatform dampers have been developed. The new damper models take into account inertia forces and the effects of normal load variation on stick-slip transitions at the contact interfaces. The damper models are formulated for the general case of multiharmonic forced response analysis. An approach for using the new damper models in the dynamic analysis of large-scale finite element models of bladed discs is proposed and realised. Numerical investigations of bladed discs are performed to demonstrate the capabilities of the new models and an analysis of the influence of the damper parameters on the forced response of bladed discs is made.

Commentary by Dr. Valentin Fuster
2006;():779-788. doi:10.1115/GT2006-90147.

A method has been developed to calculate directly resonance frequencies and resonance amplitudes as functions of design parameters or as a function of excitation levels. The method provides, for a first time, this capability for analysis of strongly nonlinear periodic vibrations of bladed discs and other structures with nonlinear interaction at contact interfaces. A criterion for determination of major, sub- and superharmonic resonance peaks has been formulated. Analytical expressions have been derived for accurate evaluation of the criterion and for tracing resonance regimes as function of such contact interface parameters as gap and interference values, friction and contact stiffness coefficients, normal stresses. High accuracy and efficiency of the new method have been demonstrated on numerical examples including large-scale nonlinear bladed disc model and major types of contact interfaces including friction contact interfaces, gaps and cubic nonlinearities.

Topics: Resonance , Vibration , Disks
Commentary by Dr. Valentin Fuster
2006;():789-800. doi:10.1115/GT2006-90158.

The effect of multistage coupling on the dynamics of a rotor consisting of eight bladed discs on a solid shaft is considered. Free vibrations are examined using finite element representations of rotating single blades, bladed discs, and the complete rotor. In this study, the global rotating mode shapes of flexible tuned bladed discs-shaft assemblies are calculated. Rotational effects, such as centrifugal stiffening are accounted for. The calculated natural frequencies obtained from the blade, the shaft, the bladed disc, and the complete shaft with discs are carefully examined to discover resonance conditions and the coupling effects. It was found that the flexible modes of the tuned bladed discs affected by shaft motion are those with zero, one and two nodal diameters. In these modes the shaft deflection is clearly visible. Different EO excitation is applied for particular stages in forced vibration analysis.

Topics: Vibration , Disks
Commentary by Dr. Valentin Fuster
2006;():801-807. doi:10.1115/GT2006-90176.

A compressor blade of low aspect ratio is a typical shell-type structure and its natural frequency and vibratory stress are sensitive to a manufacturing tolerance. In designing a blade, the designer should take the variation of the vibration characteristics into account, and evaluate the mechanical strength of the blade. This paper, first, proposes an analysis method for predicting the resonant stress due to the interaction force between the vane and blade, using 3-D CFD and the modal analysis method based on 3-D FEM. Rig tests of the compressor are carried out, and the measured frequencies and vibratory stresses are compared to predicted values, and the validity of the proposed method is verified. In the second place, a practical analysis method for predicting the variation of the natural frequency and vibratory stress of the blade is proposed, where the variation of the vibration characteristics such as a natural frequency and a resonant stress is evaluated by the first order second moment method in addition to 3-D CTD and 3-D EEM. The calculated results are compared with measured ones. From the results, it is confirmed that the variation of the vibration characteristics can be predicted by the simple method proposed in this paper.

Topics: Stress , Blades
Commentary by Dr. Valentin Fuster
2006;():809-819. doi:10.1115/GT2006-90205.

This paper deals with the methods used at Snecma (SAFRAN Group) to simulate the real mistuned dynamic behavior of bladed disks. Many applications of the method on industrial cases are also presented. The dissymmetry of a real bladed disk, which is mainly generated by small deviations in its geometry, leads to different dynamic impedances of the blades. These variations in impedance cause significant amplification of the forced response of the bladed disk. We propose to focus on industrial methods that can be used to simulate the mistuned dynamic behavior of a bladed disk in the case of small “frequency mistuning” or of greater “large mistuning”. This method, based on the modal synthesis technique (Benfield and Hruda family), can be classified as a “first generation method”. The first originality of this method consists in taking into account major mistuning, such as that induced by an FOD event. A second innovation concerns the use of these methods to assess in probabilistic terms the correction factor to apply to the maximum dynamic level measured during engine certification tests. The first part of the paper relates to the proposed method and its validation for large-scale “shape” mistuning on an academic test case. In the second section, we present a number of industrial applications: • An application of “geometric mistuning” is presented on a bladed disk after an FOD event. The same kind of analysis as previously described is carried out and the numerical results are commented. • The mistuning analyses are carried out on an industrial blisk with a local defect. The sensitivity of the response amplification factor is calculated versus the mistuning rate using a probabilistic approach. • Another application of the mistuning strategy is discussed: it concerns the use of the mistuning method to specify the correction factor of partial dynamic strain gauge measurements in order to assess the maximum level of a bladed disk. • The last application is dedicated to the simulation of intentional mistuning to increase the aeroelastic stability of an industrial test case.

Topics: Disks
Commentary by Dr. Valentin Fuster
2006;():821-831. doi:10.1115/GT2006-90315.

The problem of estimating the mutual interaction of the effects of Coriolis forces and of blade mistuning on the vibration characteristics of bladed discs is addressed in this paper. The influence of different degrees of mistuning on forced response and amplification factors are studied in the presence of Coriolis forces and then compared to their non-Coriolis counterparts using a computationally inexpensive, yet representative, model of a bladed disc. The primary objective of the study reported in this paper is to establish whether current mistuned bladed disc analyses should incorporate Coriolis effects in order to represent accurately all the significant factors that affect the forced response levels.

Topics: Coriolis force , Disks
Commentary by Dr. Valentin Fuster
2006;():833-839. doi:10.1115/GT2006-90337.

Anomalies in material condition can arise for Critical Rotating Parts in gas turbines in melt, manufacture and handling. To use a probabilistic approach when designing against burst from such anomalies it is necessary to gather data describing their frequency of occurrence and their size when they do occur. Because of the rare occurrence of such anomalies it is likely that industry data compiled from a wide number of manufacturers will be used to define the exceedance curve. However anomalies may not occur in isolation. Certain causes, such as a worn drill in holemaking, will likely lead to a sequence of damaged holes possibly with increasing depths of distorted material. It is shown that the presence of a batch effect can make a significant difference to the probability of a large anomaly. A method of translating between averaged data gathered from a number of sources and specific component data that recognises the batch effect is described. The examples used here are similar to data gathered from manufacturing anomalies in holes.

Commentary by Dr. Valentin Fuster
2006;():841-853. doi:10.1115/GT2006-90473.

Forced vibrations can lead to a substantial damage of the blading. To reduce the vibrational amplitudes to reasonable magnitudes, several coupling concepts, like shroud coupling or underplatform dampers, have been developed in the past. Mechanical models for the analysis of the forced response of coupled turbine blading are available. These models include an appropriate contact model to account for the contact effects like frictional damping, for example. These contact models demand for a stiffness type parameter, this is the so-called contact stiffness. Unfortunately, there is no straight-forward way for the determination of this parameter. In this paper, simple mechanical models are used for the estimation of the contact stiffness. The estimated contact stiffnesses are then used in forced response computations. The results of these computations are compared to measurements in order to find the most appropriate method for the contact stiffness estimation.

Topics: Turbines , Stiffness
Commentary by Dr. Valentin Fuster
2006;():855-864. doi:10.1115/GT2006-90569.

The results of a complete study of mistuning identification on an industrial blisk are presented. The identification method used here is based on a model-updating technique of a reduced-order model where measured modal data are taken as input. This reduced-order model is build using component mode synthesis and mistuning is introduced as perturbations of the cantilevered-blade modes. The measured modal data are extracted from global measurements of the blisk’s forced response. As we use a one point excitation, this measurement procedure allows the acquisition of the all modes of a given family with a quite simple experimental setup. A selection of the best identified modal data is finally performed. During the mistuning identification procedure, these measured data are regularized using an eigenvector assignment technique which reduces the influence of eventual measurement errors. An inverse problem is defined based on the perturbed (mistuned) modal equation, with measured modes as input and mistuning parameters as unknown. Then, the reduced-order model is updated with the identified mistuning, we first perform a correlation on modal responses (using eigenfrequency deviation criteria and MACs). Finally, correlation results on forced responses are presented and discussed.

Commentary by Dr. Valentin Fuster
2006;():865-872. doi:10.1115/GT2006-90595.

In turbine jet engine, the rotating blades are subjected to cyclic loading, which makes the blades experience the so-called High Cycle Fatigue (HCF). Dry friction is often employed in turbine design to attenuate the blade vibration and increase aeroclastic stability of the turbine. The dry friction dampers are often classified into four types, i.e., blade-to-blade, blade-to-ground, shrouds, and wedge damper, respectively. Compared with the under-platform dampers, shrouds make fan behavior be significantly more complex. It is very difficult to model and predict the nonlinear response of shrouded blades. In the present study, an efficient approach to investigate the nonlinear response of the shrouded blades is suggested using an alternating frequency/time domain (AFT) method. On one hand, the friction force between shrouds is numerically solved in time-domain. The trajectory of relative motion of the moving contact point is traced, and the stick-slip-separation transition for 3-D relative motion of the shroud-contact interface is considered. On the other hand, the response of the shrouded blades is iteratively solved in frequency-domain using Harmonic Balance Method (HBM). In this approach, the influence of high frequency modes of blade, and the coupling of each harmonic component on damping behavior can be taken into account. As an application, the performance of shroud damper is systematically investigated using the AFT method. The influence of shroud-to-shroud preload and contact stiffness on the shroud damping potential is studied. Some valuable results are got to the design of the shroud contact.

Topics: Blades
Commentary by Dr. Valentin Fuster
2006;():873-881. doi:10.1115/GT2006-90599.

A methodology for performing two and three-dimensional fracture analyses in orthotropic materials using ANSYS software (“ANSYS”) is presented. The methodology makes use of analytically known crack tip fields in orthotropic materials and is implemented into a general purpose ANSYS macro. The ANSYS analysis, which takes into account the material orthotropy is performed in a regular manner by including the quarter point elements near the crack front. Then, in the post-processing module, the developed macro is run to associate the crack tip displacements with the orthotropic crack tip displacement fields to compute the mixed-mode stress intensity factors. Numerical examples are also presented that demonstrate application and validation of the procedure. These examples include an edge crack in an orthotropic strip and a surface crack in a transversely isotropic plate. The results show how the orthotropic fracture results may differ from those of isotropic fracture analysis. It is also shown that this difference can be dramatically big when the stress analysis is done using the orthotropic properties, whereas the fracture calculations are performed considering the crack tip fields for a crack in an isotropic material.

Commentary by Dr. Valentin Fuster
2006;():883-891. doi:10.1115/GT2006-90710.

The lifetime distribution of a component subjected to fatigue loading is calculated using a micro-mechanics model for crack initiation and a fracture mechanics model for crack growth. These models are implemented in a computer code which uses the local stress field obtained in a Finite Element analysis as input data. Elemental failure probabilities are defined which allow to identify critical regions and are independent of mesh refinement. An example is given to illustrate the capabilities of the code. Special emphasis is put on the effect of the initiation phase on the lifetime distribution.

Commentary by Dr. Valentin Fuster
2006;():893-902. doi:10.1115/GT2006-90757.

The current paper presents a measurement system for the experimental determination of contact hysteresis cycles at temperatures up to 800° C. A test rig was designed to conduct experiments in a wide range of temperatures, with different combinations of normal and tangential load, frequencies and contacting materials. An induction system supplies the heat for measurements of hysteresis cycles at the required temperatures. Measurements show the dependence of the friction coefficient on temperature.

Commentary by Dr. Valentin Fuster
2006;():903-912. doi:10.1115/GT2006-90774.

Stresses produced by vibrations reduce the life of turbo engine blades. The design of disk assemblies requires the capability to predict their dynamic behavior. In order to achieve this objective, knowledge of damping associated with the contact between blade and disk is fundamental. This paper proposes a technique to measure the influence of blade-disk attachment on the dynamics of turbo engine blades. Moreover, experimental values of damping and resonance frequencies are presented as a function of the amplitudes of vibration and of centrifugal load at the attachment.

Topics: Damping , Blades
Commentary by Dr. Valentin Fuster
2006;():913-919. doi:10.1115/GT2006-90780.

The risk of fracture associated with high energy rotating components in aircraft gas turbine engines can be sensitive to small changes in applied stress values which are often difficult to measure and predict. Although a parametric approach is often used to characterize random variables, it is difficult to apply to multimodal densities. Nonparametric methods provide a direct fit to the data, and can be used to estimate the multimodal densities often associated with rainflow stress data. In this paper, a comparison of parametric and nonparametric methods is presented for density estimation of rainflow stress profiles associated with military aircraft gas turbine engine usages. A nonparametric adaptive kernel density estimator algorithm is illustrated for standard parametric probability density functions and for rainflow stress pairs associated with F-16/F100 engine usages. The kernel estimates are compared to parametric estimates, including a hybrid approach based on separate treatment of maximum stress pairs. The results provide some insight regarding the strengths and weaknesses of parametric and nonparametric density estimation methods for gas turbine engines, and can be used to develop improved stress estimates for probabilistic life predictions.

Commentary by Dr. Valentin Fuster
2006;():921-929. doi:10.1115/GT2006-90785.

In this paper a methodology for forced response calculation of bladed disks with underplatform dampers is described. The FE disk model, supposed to be cyclically symmetric, is reduced by means of Component Mode Synthesis and then DOFs lying at interfaces are further reduced by means of interface modes. Underplatform dampers are modeled as rigid bodies translating both in the radial and in the tangential direction of the engine. Contacts between blade platforms and damper are simulated by means of contact elements characterized by both tangential and normal contact stiffness, allowing partial separation of contact surfaces. Differential equilibrium equations are turned in non-linear algebraic equations by means of the Harmonic Balance Method (HBM). The methodology is implemented in a numerical code for forced response calculation of frictionally damped bladed disks. Numerical calculations are performed to evaluate the effectiveness of both the reduced order model and the underplatform model in simulating the dynamic behavior of bladed disks in presence of underplatform dampers.

Topics: Dampers , Disks
Commentary by Dr. Valentin Fuster
2006;():931-940. doi:10.1115/GT2006-90813.

The empirical models commonly used for probabilistic life prediction do not provide adequate treatment of the physical parameters that characterize fatigue damage development. For these models, probabilistic treatment is limited to statistical analysis of strain-life regression fit parameters. In this paper, a model is proposed for life prediction that is based on separate nucleation and growth phases of total fatigue life. The model was calibrated using existing smooth specimen strain-life data, and has been validated for other geometries. Crack nucleation scatter is estimated based on the variability associated with smooth specimen and fatigue crack growth data, including the influences of correlation among crack nucleation and growth phases. The influences of crack nucleation and growth variability on life and probability of fracture is illustrated for a representative gas turbine engine disk geometry.

Commentary by Dr. Valentin Fuster
2006;():941-950. doi:10.1115/GT2006-90843.

The Aerospace Industries Association (AIA) Rotor Integrity Sub-Committee (RISC) has proposed an enhanced damage tolerance design strategy for critical rotating parts intended to reduce the rate of uncontained rotor events. Building upon the industry committee’s experience in developing a probabilistic relative risk assessment methodology for hard alpha anomalies in titanium rotors, a similar probabilistic approach has been proposed for induced anomalies along machined hole surfaces in engine rotors. Key inputs to this strategy are the development of a surface anomaly distribution for machined holes and the benchmark of design target risk (DTR). The DTR is an FAA/industry agreed upon design target value of relative risk against which the results of the probabilistic risk assessment are compared.

Commentary by Dr. Valentin Fuster
2006;():951-959. doi:10.1115/GT2006-90884.

The proposed method has been demonstrated on the universal slopes equation (Manson 1965) and the modified universal slopes equation (Muralidharan & Manson 1998). New equations take into account independence of the transient strain range from the cycle mean stress, define more precisely the impact of the cycle mean stress upon the durability, take into account the impact of cycle mean strain plastic component upon the durability. The resulted equations have been validated with finite element analyses of smooth samples and full-scale parts, for which the results of cyclic tests in the conditions of asymmetric loading are available. The analyses have been performed employing an elastic-plastic approach using cyclic strain curves taken from original durability equations. The use of new equations ensured a good match between design and experimental durability values. Also, the new equations were used to plot Smith and Hay diagrams for low, mean and high durability. The resulted analytical diagrams represent a high quality illustration of the experimental diagrams found in the publications. The presented approach to the accounting for cycle mean stress and strain will also apply when using experimental cyclic durability curves specific for the material.

Commentary by Dr. Valentin Fuster
2006;():961-971. doi:10.1115/GT2006-90948.

The paper presents the results of a study looking into changes in the forced response levels of bladed disc assemblies subject to both structural and aerodynamic mistuning. A whole annulus FE model, representative of a civil aero-engine fan with 26 blades was used in the calculations. The forced response of all blades of 1000 random mistuned patterns was calculated. The aerodynamic parameters, frequency shifts and damping, were calculated using a three-dimensional Reynolds-averaged Navier-Stokes aero-elasticity code. They were randomly varied for each mistuning pattern, with the assumption that the system would remain stable, i.e. flutter would not occur due to aerodynamic mistuning. The results show the variation of the forced response with different types of mistuning, with structural mistuning only, with aerodynamic mistuning only and with both structural and aerodynamic mistuning.

Commentary by Dr. Valentin Fuster
2006;():973-983. doi:10.1115/GT2006-90951.

Under-platform friction dampers are preferably solutions for minimizing vibrations of rotating turbine blades. Solid dampers, characterized by their compact dimensions, are frequently used in real applications and often appear in patents in different forms. A different type of the friction damper is a thin-walled structure, which has larger dimensions and smaller contact stresses on a wider contact area in relation to the solid damper. The damping performance of a thin-walled damper, mounted under the platforms of two rotating, freestanding high pressure turbine blades is investigated numerically and experimentally in this paper. The tangential and normal contact stiffness, that are crucial parameters in optimal design of each friction damper, are determined from three-dimensional finite element (FE) computations of the contact behaviour of the thin-walled damper on the platform including friction and centrifugal effects. The computed contact stiffness values are applied to non-linear dynamic simulations of the analysed blades with the friction damper of a specified mass. These numerical analyses are performed in the modal frequency domain with a code, which is based on the Harmonic Balance Method (HBM) for the complex linearisation of friction forces. The blade vibrations are characterised by a set of the lowest FE mode shapes of one freestanding blade without damper. The dynamic results of the calculated blades with the damper are in good agreement with the measured data of the real mistuned system. In the analysed excitation range, the numerical performance curve of the thin-walled damper is obtained within the scatter band of the experimental results. For the known friction coefficients and available FE and HBM tools, the described numerical process confirms its usability in the design of under-platform dampers.

Commentary by Dr. Valentin Fuster
2006;():985-994. doi:10.1115/GT2006-91098.

This paper presents a probabilistic model that quantifies the impact of manufacturing variability on combustor liner temperature and low cycle fatigue life. This model is applied to a gas turbine engine combustor for a commercial aircraft and assessed using combustor liner wall temperature and outlet gas temperature measurements. A probabilistic analysis shows that the model estimates cup-to-cup outlet temperature variations and liner wall temperature variations consistent with these measurements. Furthermore, this analysis shows that the typical liner life is 25 percent less than the life estimated using deterministic methods. In addition, approximately 99 percent of the combustors designed using deterministic methods will fail earlier than predicted. A sensitivity analysis shows that the variability in combustor unmixedness is the key driver of liner life.

Commentary by Dr. Valentin Fuster
2006;():995-1002. doi:10.1115/GT2006-91099.

In engine structural life computations, it is common practice to assign a life of certain number of start-stop cycles based on a standard flight or mission. This is done during design through detailed calculations of stresses and temperatures for a standard flight, and the use of material property and failure models. The limitation of the design phase stress and temperature calculations is that they cannot take into account actual operating temperatures and stresses. This limitation results in either very conservative life estimates and subsequent wastage of good components, or in catastrophic damage because of highly aggressive operational conditions which were not accounted for in design. In order to improve significantly the accuracy of the life prediction, the component temperatures and stresses need to be computed for actual operating conditions. However, thermal and stress models are very detailed and complex, and it could take on the order of a few hours to complete a stress and temperature simulation of critical components for a flight. The objective of this work is to develop dynamic neural network models, that would enable us to compute the stresses and temperatures at critical locations, in orders of magnitude less computation time than required by more detailed thermal and stress models. This work expands on the work done previously [1] where a linear system identification approach was developed. The current paper describes the development of a neural network model and the temperature results achieved in comparison with the original models for Honeywell turbine and compressor components. Given certain inputs such as engine speed and gas temperatures for the flight, the models compute the component critical location temperatures for the same flight in a very small fraction of time it would take the original thermal model to compute.

Commentary by Dr. Valentin Fuster
2006;():1003-1016. doi:10.1115/GT2006-91111.

A comprehensive fracture mechanics based life prediction methodology is presented for FCC (Face Centered Cubic) single crystal components, based on computation of stress intensity factors (SIFs), and modeling the crystallographic fatigue crack growth process, under mixed-mode loading conditions. The 3D finite element numerical procedure presented for computing SIFs for anisotropic materials under mixed-mode loading is very general, and not just specific to FCC single crystals. Stress intensity factors for a Brazilian Disc (BD) specimen are presented for the crack on the {111} plane in the 〈101〉 and 〈121〉 directions, which represent the primary and secondary slip directions. Variation of SIFs as a function of thickness is also presented. Modeling of the crystallographic fatigue crack growth (FCG) behavior is performed by using the resolved shear stress intensity coefficient (RSSIC), Krss . This parameter is sensitive to the grain orientation and is based on the resolved shear stresses on the slip planes at the crack tip, which is useful in identifying the active crack plane as well as predicting the crack growth direction. A multiaxial fatigue crack driving force parameter, ΔKrss , was quantified, which can be used to predict the FCG rate and hence life in single crystal components subject to mixed mode fatigue loading.

Commentary by Dr. Valentin Fuster
2006;():1017-1023. doi:10.1115/GT2006-91337.

A number of earlier publications discussed the benefits of probabilistic analysis and probabilistic lifing in application to critical rotating engine components. One of the important variables in both probabilistic and deterministic lifing analysis is the level of residual stress in the component. Near surface residual stresses directly influence the fatigue life of critical engine rotating components. Depending on sign and magnitude a near surface residual stress gradient can either inhibit or accelerate fatigue initiation and crack propagation. A major barrier to introducing subsurface residual stress information into the life calculation process is the necessity to make accurate and reliable nondestructive measurements on as produced hardware. The paper reviews several NDE technologies that could be candidates for both production and in-service non-destructive residual stress measurements. The importance of having accurate residual stress information and its use in the probabilistic design and life management process is illustrated on several examples. A linkage with several ongoing industry R&D programs is discussed.

Commentary by Dr. Valentin Fuster
2006;():1025-1034. doi:10.1115/GT2006-91350.

Lifetime of disks and other aviation engine parts critical for safe operation is currently probabilistically predicted using mainly two methods. One method to predict lifetime is to confirm lifetime to low-cycle fatigue (LCF) cracking of a part without initial defects. The other method to predict lifetime is to confirm lifetime for safe propagation of a crack from initial defects available in a part. Combination of the above stated methods along with usage of margins on cyclic durability that ensure the required nonfracture probability presents another more conservative approach. Confirmation of lifetime to LCF cracking of a part without initial defects is usually performed based on results of cyclic tests of parts. The safe life determination algorithms were developed in Central Institute of Aviation Motors (CIAM) that take into account a difference in test outcome (“failure”–“no failure”). The developed software makes possible to consider the admissible failure probability, use the actual cyclic durability spread characteristics of the parts involving both lognormal and Weibull durability distributions. This paper discusses the examples of application of the elaborated methods and software. To confirm lifetime for safe propagation of a crack from defects available in a part, a program of statistical determination of durability of powder alloy disks with random fields of ceramic inclusions was developed in CIAM. This program allows performing the partition of disks into typical elements, the generation of realizations of initial defects for each typical disk element and calculation of life realizations corresponding to them, the statistical processing of the obtained data made with usage of asymptotic theory of extreme values, and the determination of technical risk functions for a disk as a whole. While developing the software, the problems of crack initiation and its stable propagation from ceramic inclusions were sequentially decided. Example of the calculation made according to the developed program is given in this paper. The probabilistically-justified approaches to determine the safe life shall be supplemented with the methods of economical and ecological risk analysis. In this case these approaches may become a good support for arranging the engine service to Reliability Centered Maintenance.

Topics: Engines , Aviation
Commentary by Dr. Valentin Fuster

Structures and Dynamics: Unsteady Aerodynamics

2006;():1035-1043. doi:10.1115/GT2006-90157.

In this study, numerical simulations of 3D viscous flutter were performed and compared with the available experimental results. The calculations were carried out for bending oscillations of the cascade known as the Eleventh Standard Configuration. The developed numerical algorithm solves the 3D Reynolds-averaged Navier-Stokes equation together with the Baldwin-Lomax turbulence model, using the explicit monotonous second-order accurate Godunov-Kolgan finite-volume scheme and moving hybrid H-O structured grid. Comparison of the calculated and the experimental results for the Eleventh Standard Configurations has shown sufficient quantitative and qualitative agreement for local performances (unsteady pressure amplitude and phase distribution) at off-design conditions. Benchmark solutions are provided for various values of the inter-blade phase angle.

Commentary by Dr. Valentin Fuster
2006;():1045-1058. doi:10.1115/GT2006-90407.

A new simple asymptotic mistuning model (AMM), which constitutes an extension of the well known Fundamental Mistuning Model for groups of modes belonging to a modal family exhibiting a large variation of the tuned vibration characteristics, is used to analyze the effect of mistuning on the stability properties of aerodynamically unstable rotors. The model assumes that both, the aerodynamics and the structural dynamics of the assembly are linear, and retains the first order terms of a fully consistent asymptotic expansion of the tuned system where the small parameter is the blade mistuning. The simplicity of the model allows the optimization of the blade mistuning pattern to achieve maximum rotor stability. The results of the application of this technique to realistic welded-in-pair and interlock low-pressure-turbine rotors are also presented.

Topics: Stability , Rotors
Commentary by Dr. Valentin Fuster
2006;():1059-1069. doi:10.1115/GT2006-90434.

Fan blades of high bypass ratio gas turbine engines are subject to substantial aerodynamic and centrifugal loads, producing the well-known phenomenon of fan blade untwist. The accurate prediction of the running geometry, as opposed to the cold geometry at rest, is crucial in the assessment of aerodynamic performance, vibratory response and noise production of the fan. The situation is further complicated by the fact that some geometric variation is inevitable even for the state-of-the-art manufacturing processes used. The aim of this paper is to investigate the effect of static stagger variability on the dynamic untwist behaviour of fan assemblies. An aeroelastic model was used to show that under certain conditions the stagger pattern changes significantly, both in form and amplitude, relative to the static configuration. At other conditions, a strong correlation between the running and static patterns is demonstrated.

Topics: Gas turbines
Commentary by Dr. Valentin Fuster
2006;():1071-1080. doi:10.1115/GT2006-90541.

This paper documents an investigation into unsteady flow in a three-dimensional oscillating turbine cascade with emphasis on the influence of tip clearance. Systematic experimental measurements were performed on a low-speed turbine cascade rig. The cascade consists of seven prismatic turbine blades, with the middle blade being driven to oscillate in a three-dimensional bending/flapping mode. Blades were instrumented with pressure tappings at six span-wise sections to facilitate three dimensional steady and unsteady pressure measurements on the blade surface. The steady pressure measurements are complemented by CFD simulations. Both are in a good agreement and indicate a marked local pressure suction peak at 70–90% chord on the suction surface resulting from the tip-clearance vortex. The measured unsteady pressure shows that this tip-clearance induced suction peak has a significant destabilising influence on the aerodynamic damping at a large tip-clearance (5% chord). Whilst at a small tip clearance (1.25–2.5% chord), the tip-clearance actually has a stabilising effect. The behaviour is in line with a quasi-steady analysis.

Commentary by Dr. Valentin Fuster
2006;():1081-1089. doi:10.1115/GT2006-90613.

High Cycle Fatigue caused by high vibration levels continues to be a major concern in gas turbine design. The use of Computational Fluid Dynamics methods is becoming more commonplace for calculating the vibration amplitude of turbomachinery blades during the design process. A typical calculation approach would be to calculate the unsteady aerodynamic loads at the resonance condition for each vibration mode of interest. In this paper it is proposed that, for a choked high pressure (HP) turbine, an unsteady flow prediction can be scaled across a wide engine operating range using a few simple parameters. There is a fixed relationship between the turbine inlet pressure and the HP shaft speed (when expressed non-dimensionally) which can be used to scale the flow conditions. The effects of altitude variation in the ratio of shaft speeds, compressor bleed flows and schedule of the variable vanes are secondary, having only a small influence on the behaviour. This paper demonstrates that the steady flow distribution around both stator and rotor is virtually constant across the speed range of the engine and the rotor unsteady surface pressure distribution shows only small differences. Further, the parameter which is of prime interest for vibration assessment, the modal force, can be scaled very well using turbine inlet pressure. For modes of vibration with high amplitudes the errors introduced by scaling are of the order of 6% which is considered acceptable for design predictions.

Commentary by Dr. Valentin Fuster
2006;():1091-1100. doi:10.1115/GT2006-90645.

Possibility of active control on cascade flutter with piezoelectric device was both experimentally and numerically studied under a subsonic condition. A blade on which a piezoelectric device was glued was installed in the test cascade. When AC voltage was provided on the device, the blade could be actively oscillated. The unsteady aerodynamic work induced by the active oscillation was measured on the cascade blades. From the results, the active oscillation of the piezo-blade was found to generate sufficiently large unsteady aerodynamic work for changing instability of blade vibration. The active control was effective for flutter suppression if the phase difference between the unstable blade vibration and the active oscillation was adequately selected. The numerical results also showed effectiveness of the piezoelectric device, and the active oscillation was observed to change the unsteady aerodynamic work distribution on blade surfaces, which change should cause the stabilization effect. By a developed numerical method with flow-structure coupling, a suppression method for instability with a control rule was tested numerically to confirm its effectiveness.

Commentary by Dr. Valentin Fuster
2006;():1101-1113. doi:10.1115/GT2006-90668.

The dimensional analysis of a simple model that represents the vibration of aerodynamically unstable rotor blades is presented. Based on this analysis a methodology to determine the rotor blade vibration amplitude is outlined. The method assumes that the forcing is due solely to the self-excitation of the airfoil and that the vibration amplitude is small enough to represent the unsteady aerodynamics associated to the airfoil motion using the linearized Navier-Stokes equations. The vibration amplitude is saturated due to the non-linearity of fir-tree dry friction which is modelled using a simplified approach. To compensate for the limitations of the friction model several hypotheses need to be done, among them, the geometric similarity of the different configurations and that the aspect ratio of the rotor blades is high. The application of the method to a low-pressure-turbine bladed-disk is discussed in detail. A comparison of the present method against experimental data is presented.

Commentary by Dr. Valentin Fuster
2006;():1115-1122. doi:10.1115/GT2006-90683.

This paper presents a methodology for the modeling of flutter and forced response in axial compressors while taking into account the effect of bleed off-takes. Usually, aeroelasticity analyses are performed assuming smooth solid end walls. This type of analysis has two main shortcomings. Firstly, it does not account for the change in the aerodynamic speed of the stages downstream of the bleed off-take, so that aeroelasticity analyses are not performed at the correct aerodynamic conditions. Secondly, bleed off-takes influence the flow pattern particularly in the stages around or close to the bleed off-take, thus leading to possibility of obtaining the wrong aeroelastic response. Another objective of this paper is to present a methodology for the accurate prediction of the flow in a compressor with bleed off-take, by both including the geometry of the bleed off-take and performing the calculations on the entire compressor, thus eliminating errors resulting from prescribing boundary conditions at inter-blade row boundaries. It is concluded that bleed off-takes could influence significantly the aeroelastic response of the blades.

Commentary by Dr. Valentin Fuster
2006;():1123-1132. doi:10.1115/GT2006-90685.

This paper presents a computational study of the formation and ingestion of ground vortices and resulting fan forced response levels in a large turbofan operating near the ground. The model is based on an integrated aeroelasticity numerical method; the aerodynamic part is based on a 3D unstructured Reynolds-Averaged Naveir-Stokes solver. The mechanical model uses linear modal model for the structure, allowing for the direct computation of the structural response during the unsteady simulations. The analysis shows that under certain fan speed and mass flow rate conditions, for a given fan and intake combination, situated at a fixed distance from the ground, an inlet vortex can form near the ground. This inlet vortex is drawn into the intake causing inlet distortions that could excite several low engine order harmonics of the fan. Predictions are compared with measured data showing good agreement in general. Ability to predict the level of response at the design stage allows for implementing design solutions preventing possible failure due to high cycle fatigue.

Topics: Vortices
Commentary by Dr. Valentin Fuster
2006;():1133-1144. doi:10.1115/GT2006-90829.

In this paper, analytical maps of aerodynamic damping for a two-dimensional compressor cascade (Standard Configuration 10) are presented. The maps are shown as contour plots of the aerodynamic damping as a function of operating condition. The aerodynamic dampings were calculated by a linearized Navier-Stokes flow solver. The flutter boundaries over a wide range of operating conditions are clearly shown on the damping maps and were found to be strongly dependent on the mode frequency and the mode shape. Extremely low values of negative aerodynamic damping were predicted for some off-design operating conditions where flow separation occurred. A damping map was also constructed based on inviscid flow simulations. There were differences in the viscous and inviscid flutter boundaries particularly at off-design inflow angles. The extremely low values of negative aerodynamic damping were only predicted by the viscous simulations and not the inviscid simulations.

Topics: Compressors , Damping
Commentary by Dr. Valentin Fuster
2006;():1145-1150. doi:10.1115/GT2006-90847.

This paper presents the results of a probabilistic flutter study of a mistuned bladed disk using a high fidelity model including both structural and aerodynamic coupling. The approach used in this paper is relatively fast because it does not require any additional information than that required of a tuned flutter analysis, with the exception of the mistuned blade frequencies. The case study shows that the stability of the fleet can be significantly affected by the standard deviation of blade frequencies and the pattern in which they are arranged in the wheel. A method for understanding and identifying the beneficial patterns is presented.

Commentary by Dr. Valentin Fuster
2006;():1151-1163. doi:10.1115/GT2006-91196.

The effect of negative incidence operation on mode shape sensitivity of an oscillating low pressure (LP) turbine rotor blade row has been studied experimentally. An annular sector cascade has been employed in which the middle blade has been made oscillating in controlled three-dimensional rigid-body modes. Unsteady blade surface pressure data were acquired at midspan on the oscillating blade and two pairs of non-oscillating neighbor blades and reduced to aeroelastic stability data. The test program covered variations in reduced frequency, flow velocity and inflow incidence; at each operating point a set of three orthogonal modes was tested such as to allow for generation of stability plots by mode recombination. At nominal incidence it has been found that increasing reduced frequency has a stabilizing effect on all modes. The analysis of mode shape sensitivity yielded that the most stable modes are of bending type with axial to chordwise character whereas high sensitivity has been found for torsion-dominated modes. Negative incidence operation caused the flow to separate on the fore pressure side. This separation was found to have a destabilizing effect on bending modes of chordwise character whereas an increase in stability could be noticed for bending modes of edgewise character. Variations of stability parameter with inflow incidence have hereby found being largely linear within the range of conditions tested. For torsion-dominated modes the influence on aeroelastic stability was close to neutral.

Commentary by Dr. Valentin Fuster

Structures and Dynamics: Rotor Dynamics and Magnetic Bearings

2006;():1165-1177. doi:10.1115/GT2006-90049.

Experimental results obtained for an Inconel compressor blade rubbing a steel casing at engine speed are described. Load cell, strain gauge and accelerometer measurements are discussed and then applied to analyze the metal-on-metal interaction resulting from sudden incursions of varying severity, defined by incursion depths ranging from 13 μm to 762 μm (0.0005-in to 0.030-in). The results presented describe the transient dynamics of rotor and casing vibro-impact response at engine operational speed similar to those experienced in flight. Force components at the blade tip in axial and circumferential directions for a rub of moderate incursion depth (140 μm) are compared to those for a severe rub (406 μm). Similar general trends of variation during the metal-to-metal contact are observed. However, in the nearly three-fold higher incursion the maximum incurred circumferential load increases significantly, while the maximum incurred axial load increases much less, demonstrating the non-linear nature of the rub phenomena. Concurrently, the stress magnification on the rubbing blade at root mid-chord, at tip leading edge, and at tip trailing edge is discussed. The results point to the possibility of failure occurring first at the airfoil trailing edge. Such a failure was in fact observed in the most severe rub obtained to date in the laboratory, consistent with field observations. Computational models to analyze the non-linear dynamic response of a rotating beam with periodic pulse loading at the free-end are currently under development and are noted.

Topics: Engines , Blades
Commentary by Dr. Valentin Fuster
2006;():1179-1186. doi:10.1115/GT2006-90111.

The paper presents the results of evaluation of admissible misalignments of two neuralgic bearings of a great power turbine set. The areas of admissible misalignments have been evaluated by numerical analysis of the discrete model of the machine with the aid of a package of MESWIR computer codes. For this purpose the criteria of “tolerable misalignment” were formulated with regard to permissible bearing vibrations and with regard to permissible static bearing loads. The areas of tolerable misalignments of the two bearings are presented in the graphs as the envelopes of maximum permissible bearing vibration and permissible bearing load, separately. The envelopes limits misalignments of the bearings to any direction regarded as the superposition of simultaneous vertical and horizontal displacements. Dimensions and the shapes of the areas are different for the two analyzed bearings and also depend on the fact, whether the area was calculated with regard to criterion of bearing vibration or criterion of bearing load. Especially interesting and not expected are shapes of the areas calculated with regard to vibration criterion. In this case after reaching the maximum permissible vibration at some bearing misalignment, further misalignment of the bearing may result in decreasing of vibration level. The method and procedure applied in the investigations allow determining how sensitive is a rotating multi-support machine to random misalignment of bearings. Basing on the method it is possible to correct the location of particular bearings to optimize tolerable bearing misalignment areas. Besides the research makes it possible to formulate a system of diagnostic relations connected with the bearing misalignment defects, which can make the knowledge basis for a diagnostic system.

Topics: Machinery , Bearings
Commentary by Dr. Valentin Fuster
2006;():1187-1195. doi:10.1115/GT2006-90212.

A novel method for transient rotor/ active magnetic bearing control using sampled wavelet coefficients is proposed. Control currents are formulated in the wavelet transform domain, prior to signal reconstruction. The wavelet based controller is designed from target transient responses due to step changes in wavelet co-efficients of applied forces. Transient system dynamics are embedded in the controller and evaluated from on-line system identification. Experimental validation is undertaken using a flexible rotor/active magnetic bearing system. Mass loss tests were performed at two critical speeds corresponding to near sudden changes in unbalance that are capable of exciting rotor dynamic modes in a transient manner. The controller is shown to suppress the transient responses within a finite settling time.

Commentary by Dr. Valentin Fuster
2006;():1197-1202. doi:10.1115/GT2006-90280.

Tilting-pad journal bearings (TPJB) dominate as rotor supports in high speed rotating machinery. The paper analyzes frequency effects on the TPJB’s stiffness and damping characteristics based on experimental and theoretical investigations. The experimental investigation has been carried out on a five pad tilting-pad journal bearing of 98 mm in diameter. Time domain and multifrequency excitation has been used to evaluate the dynamic coefficients. The calculated results have been obtained from a three-dimensional computer model of TPJB, which accounts for thermal effects, turbulent oil flow, and elastic effects, including that of pad flexibility. The analyzes of the TPJB’s stiffness and damping properties showed that the frequency effects on the bearing dynamic properties depend on the operating conditions and bearing design. It has been concluded that the pad inertia and pivot flexibility are behind the variations of the stiffness and damping properties with frequency of excitation.

Commentary by Dr. Valentin Fuster
2006;():1203-1210. doi:10.1115/GT2006-90283.

A cryogenic gas expander system that incorporates a high performance, high-speed permanent magnet, direct-drive generator and low loss magnetic bearings is described. Flow loop testing to 30,000 rpm was completed at the system manufacturer’s facility in January 2005, and field installation is scheduled for October 2005. As part of the system testing, the rotor was dropped onto the backup bearings multiple times at an intermediate speed and at 30,000 rpm. Orbit and time-history data from a full speed drop and spin down are presented and discussed in detail. A transient, nonlinear rotordynamic analysis simulation model was developed for the machine to provide insight into the dynamic behavior. The model includes the dead band clearance, the flexible backup bearing support and hard stop. Model predictions are discussed relative to the test data.

Topics: Drops , Rotors , Testing , Generators
Commentary by Dr. Valentin Fuster
2006;():1211-1223. doi:10.1115/GT2006-90374.

An analysis is developed for a compressible bulk-flow model of the leakage path between a centrifugal-compressor impeller’s shroud and its housing along the impeller’s front and back sides. This development is an extension of analyses performed first by Childs [15] for pump impellers. The bulk-flow model is used to predict reaction force and moment coefficients for the impeller shroud. A labyrinth seal code developed by Childs and Scharrer [21] is used to calculate the rotordynamic coefficients developed by the labyrinth seals in the compressor stage and also provides a boundary condition for the shroud calculations. Comparisons between the measured shroud moment coefficients by Yoshida et al. [18] and model predictions show reasonable agreements for the clearance flow and reaction moments. For the conditions considered, low Mach number flow existed in the shroud clearance areas and compressible-flow and incompressible-flow models produced similar predictions. Childs’ model predictions for the direct damping and cross-coupled stiffness coefficients of a pump impeller produced reasonable agreement; hence the present model was validated to the extent possible. A rotor model consisting of an overhung impeller stage supported by a nominally cantilevered rotor was analyzed for stability using the present bulk-flow model and an API standard Wachel-formula model [10]. The bulk-flow model predicted significantly higher onset speeds of instability. Given that some compressors have been predicted to be comfortably stable using API standard Wachel-formula but have been unstable on the test stand, these results suggest that unidentified destabilizing forces and or moments are present in compressors. Seal rub conditions that arise from surge events and increase the seal clearances are simulated, showing that enlarged clearances increase the preswirl at the seals, thus increasing these seal’s destabilizing forces and reducing stability margins. These results are consistent with field experience. Predictions concerning the back shroud indicate that shunt-hole injection mainly acts to enhance stability by changing the flow field of the division wall or balance piston seals, not by influencing the back-shroud’s forces or moments. Effective swirl brakes at these seals also serves this purpose.

Commentary by Dr. Valentin Fuster
2006;():1225-1231. doi:10.1115/GT2006-90435.

The paper presents the dynamic behaviors of a geared rotor-bearing system under the effects of the residual shaft bow, the gear eccentricity and excitation of gear’s transmission error. The coupling effect of lateral and torsional motions is considered in the dynamic analysis of the geared rotor-bearing system. The finite element method is used to model the system and Lagrangian approach is applied to derive the system equations of motion. The dynamic characteristics including system natural frequencies, mode shapes and steady-state response are investigated. The results show that the magnitude of the residual shaft bow, the phase angle between gear eccentricity and residual shaft bow will significantly affect system natural frequencies and steady-state response. When the spin speed closes to the second critical speed, the system steady state response will be dramatically increased by the residual shaft bow for the in-phase case. Moreover the zero response can be obtained when the system is set on special conditions.

Commentary by Dr. Valentin Fuster
2006;():1233-1243. doi:10.1115/GT2006-90481.

A rotordynamic analysis of a large turbo-compressor that models both the casing and supports along with the rotor-bearing system was performed. A three-dimensional (3-D) finite element model of the casing captures the intricate details of the casing and support structure. Two approaches are presented, including development of transfer functions of the casing and foundation, as well as a fully coupled rotor-casing-foundation model. The effect of bearing support compliance is captured, as well as the influence of casing modes on the rotor response. The first approach generates frequency response functions (FRF’s) from the finite element case model at the bearing support locations. A high-order polynomial in numerator-denominator transfer function format is generated from a curve-fit of the FRF. These transfer functions are then incorporated into the rotordynamics model. The second approach is a fully coupled rotor and casing model that is solved together. An unbalance response calculation is performed in both cases to predict the resulting rotor critical speeds and response of the casing modes. The effect of the compressor case and supports caused the second critical speed to drop to a value close to the operating speed and not compliant with API 617 7th edition requirements. A combination of rotor, journal bearing, casing, and support modifications resulted in a satisfactory and API compliant solution. The results of the fully coupled model validated the transfer function approach.

Commentary by Dr. Valentin Fuster
2006;():1245-1249. doi:10.1115/GT2006-90572.

During the start-up and shut-down of a turbomachine supported on compliant foil bearings, before the bearings have full development of the hydrodynamic gas film, sliding occurs between the rotor and the bearing foils. Traditional solid lubricants (e.g., graphite, Teflon®) readily solve this problem at low temperature. High temperature operation, however, has been a key obstacle. Without a suitable high temperature coating, foil air bearing use is limited to about 300°C (570°F). In oil-free gas turbines, a hot section bearing presents a very aggressive environment for these coatings. A NASA developed coating, PS304, represents one tribological approach to this challenge. In this paper, the use of PS304 as a rotor coating operating against a hot foil gas bearing is reviewed and discussed. During the course of several long term, high cycle, engine tests, which included two coating related failures, the PS304 technology evolved and improved. For instance, a post deposition thermal treatment to improve dimensional stability, and improvements to the deposition process to enhance strength resulted from the engine evaluations. Largely because of this work, the bearing/coating combination has been successfully demonstrated at over 500°C (930°F) in an oil-free gas turbine for over 2500 hours and 2900 start-stop cycles without damage or loss of performance when properly applied. Ongoing testing at Glenn Research Center as part of a long term program is over 3500 hours and 150 cycles.

Commentary by Dr. Valentin Fuster
2006;():1251-1259. doi:10.1115/GT2006-90596.

A new-style squeeze film damper with valvular metal rubber squeeze film ring (SFD/VMR) was designed to improve characteristics of the squeeze film force of the SFD. The immobile squeeze film ring of the SFD was replaced by the elastic squeeze film ring with the valvular metal rubber subassembly (VMR). When the unbalance force was smaller, the displacement of the journal changed little, and then the squeeze film force was smaller too, so as to the squeeze film ring of the SFD/VMR was nearly immobile. The working condition was similar with the SFD. When the unbalance force was larger, the displacement of the journal changed bigger, and then the squeeze film force rapidly increased, so as to the VMR deformed, which made the film thickness changed correspondingly, until it reached a balanceable state of the squeeze film force and elastic force of the VMR. Theoretical and experimental investigations showed that the SFD/VMR had optimal effect on reducing vibration, comparing with the SFD, because it could passively adjust the squeeze film clearance by taking advantage of the elastic deformation of the VMR. The SFD/VMR could control the squeeze film clearance in a suitable range, which made the characteristics of the squeeze film force of the SFD/VMR better than the SFD. The SFD/VMR could suppress the occurrence of the nonlinear vibration phenomenon markedly, such as bistable jump up.

Topics: Metals , Rubber , Dampers , Vibration
Commentary by Dr. Valentin Fuster