GT2002-30056 pp. 1-7; (7 pages)

Integrally formed ceramic matrix composite structures are being developed for a range of hot-structure applications involving active cooling. In this paper, some advantages of integral textile approaches are summarised and design possibilities for turbine engine combustors are suggested. Advantages of integral textile structures include: 1) joints between ceramic and other materials in hot zones can be avoided; 2) thin skins (< 1 mm) can be formed that are strong and tough, enabling tolerance of higher heat fluxes; 3) compliant structures can be designed, which can limit the development of thermal mismatch stresses; and 4) fabrication costs can be lowered by reducing part counts and steps in processing. This paper discusses some of the non-traditional design and fabrication challenges that must be met to exploit integral textile ceramic structures and offers a preliminary assessment of their viability for handling the thermomechanical loads of an advanced combustor.

Commentary by Dr. Valentin Fuster
GT2002-30057 pp. 9-14; (6 pages)

Stress-rupture life and residual strength results obtained for a carbon fiber reinforced silicon carbide matrix composite are presented. The material used in this study had a chemical vapor infiltration silicon carbide matrix reinforced by T-300 carbon fibers in a two-dimensional woven cloth, with a plain weave architecture. Tests were conducted in a reduced oxygen environment of 1000 ppm O2 /Ar at 1200 °C. The results from two sets of twenty creep rupture tests performed at a fixed stress are presented and discussed. In the first test set, specimens were run to failure. In the second set, rupture testing was interrupted after a fixed time interval and retained strengths were then measured. The results of this study reveal a simple deterministic relationship between the retained strength and residual life of the ceramic composite when exposed to the oxidative environment. Creep life was also observed to increase for specimens with wider tests section widths. Microstructural examination revealed damage mechanisms in the form of fiber oxidation and this points to the need for developing environmental models for predicting life of a component manufactured with this material.

Commentary by Dr. Valentin Fuster
GT2002-30458 pp. 15-21; (7 pages)

Advanced materials have the potential to improve gas turbine engine durability. One general area of concern for durability is in the hot section components of the engine. Ceramic matrix composites offer improvements in durability at elevated temperatures with a corresponding reduction in weight for nozzles of gas turbine engines. Building on past material efforts, a next generation SiC/SiC composite with a self-sealing matrix has been developed for gas turbine applications. An extensive baseline test characterization has been done that shows the overall material suitability. Prior to ground engine testing, a reduced test matrix was undertaken to aggressively test the material in a long-term hold cycle at elevated temperatures and environments. This tensile low cycle fatigue testing was done in air and a 90% steam environment. While the steam environment aggressively attacked the material, no appreciable debit in material life was noted. Nondestructive testing and post test characterization of this testing were performed. After completion of the aggressive testing effort, two nozzle seals of constant thickness were fabricated and installed in an F100-PW-229 engine for accelerated mission testing. The self sealing CMC seals were tested for over 250 hours in accelerated conditions without damage. The results of the engine testing will be shown and overall conclusions drawn.

Commentary by Dr. Valentin Fuster
GT2002-30459 pp. 23-28; (6 pages)

Combustion tests on SiC/SiC CMC components were performed in an aircraft combustion environment using the R ich-burn, Q uick-quench, L ean-burn (RQL) sector rig. SiC/SiC fasteners were used to attach several of these components to the metallic rig structure. The effect of combustion exposure on the fastener material was characterized via microstructural examination. Fasteners were also destructively tested, after combustion exposure, and the failure loads of fasteners exposed in the sector rig were compared to those of as-manufactured fasteners. Combustion exposure reduced the average fastener failure load by 50% relative to the as-manufactured fasteners for exposure times ranging from 50 to 260 hours. The fasteners exposed in the combustion environment demonstrated failure loads that varied with failure mode. Fasteners that had the highest average failure load, failed in the same manner as the unexposed fasteners.

Commentary by Dr. Valentin Fuster
GT2002-30460 pp. 29-38; (10 pages)

Conventional sintered silicon carbide (SSiC) has been applied in journal bearings of pumps for more than 20 years with the pumped medium itself being the lubricant. High corrosion and wear resistance of SSiC have contributed to this success. The brittle failure of that material quite often is a problem, though, and limits the application of SSiC in highly loaded bearings. In contrast, ceramic matrix composites (CMC) based on C- or SiC-fiber-reinforced SiC-ceramics show strongly improved fracture toughness on the level of cast iron and are applicable in cases where conventional ceramics due to their lack of reliability cannot be used. These CMC-materials have been developed in several programs primarily for space and military applications and are also beeing successfully used in journal bearings for pumps in power plants [1] and for tubular casing pumps [2]. In power plant pumps, low viscosity water of up to 160°C can be the lubricant. In tubular casing pumps quite often water loaded with abrasive sand particles lubricates the bearings. CMC-journal bearings for pumps in cryogenic rocket engines for reusable launch vehicles (RLVs), where the lifetime of mechanical components is a critical issue, are presently tested. Journal bearings of the type introduced in water pumps could replace ball bearings presently in use. Improved stiffness and damping properties, reduced wear, increased reliability and no limitations in speed times diameter would be some of the expected advantages [3]. Journal bearings for hot hinges in re-entry systems are foreseen for the space vehicle CRV (crew rescue vehicle) and have successfully been tested under close to real conditions. They are envisaged to be flight-tested on the experimental NASA vehicle X38. The bearing faces run under dry conditions and temperatures of more than 1600°C in air of about 50 mbar pressure [4]. Presently, only CMCs based on carbon fibers have potential to operate successfully under such conditions.

Commentary by Dr. Valentin Fuster
GT2002-30461 pp. 39-45; (7 pages)

The successful application of ceramic matrix composites as hot-section components in advanced gas turbine engines will require the development of constituent materials and processes that can provide the material systems with the key thermostructural properties required for long-term component service. Much initial progress in identifying these materials and processes was made under the former NASA Enabling Propulsion Materials Program using stoichiometric Sylramic™ silicon-carbide (SiC) fibers, 2D-woven fiber architectures, chemically vapor-infiltrated (CVI) BN fiber coatings (interphases), and SiC-based matrices containing CVI SiC interphase over-coatings, slurry-infiltrated SiC particulate, and melt-infiltrated (MI) silicon. The objective of this paper is to discuss the property benefits of this SiC/SiC composite system for high-temperature engine components and to elaborate on further progress in SiC/SiC development made under the new NASA Ultra Efficient Engine Technology Program. This progress stems from the recent development of advanced constituent materials and manufacturing processes, including specific treatments at NASA that improve the creep, rupture, and environmental resistance of the Sylramic fiber as well as the thermal conductivity and creep resistance of the CVI SiC over-coatings. Also discussed are recent observations concerning the detrimental effects of inadvertent carbon in the fiber-BN interfacial region and the beneficial effects of certain 2D-architectures for thin-walled SiC/SiC panels.

Commentary by Dr. Valentin Fuster
GT2002-30501 pp. 47-58; (12 pages)

Practical limits on number of specimens that can be tested lead to uncertainty in the estimated Weibull parameters. This paper presents an evaluation of four techniques for estimating confidence intervals for size-scaled Weibull parameters of monolithic ceramics. The techniques include normal approximation method, likelihood ratio technique, nonparametric bootstrap, and parametric bootstrap methods. For uncensored fast-fracture data, the confidence intervals for Weibull parameters are compared to the method used in ASTM Standard C1239. A simulation fracture experiment is conducted to evaluate the statistical characteristics, in particular coverage probability, of the four methods. For fast-fracture data with multiple failure modes, the statistical assessment of the confidence interval techniques for size-scaled Weibull parameters complement the existing literature. Overall, it was observed that the likelihood ratio technique and parametric bootstrap method perform very well. These techniques can also be extended for confidence interval estimation using fast-fracture data obtained from various geometry’s of test specimens and/or loading conditions (pooled data).

Commentary by Dr. Valentin Fuster
GT2002-30502 pp. 59-65; (7 pages)

Advanced monolithic ceramics have a good potential to survive under the severe thermal and mechanical loading conditions which are present in stationary gas turbines. Long term volume stability and high creep resistance are hard to achieve from a materials point of view but comparatively easy to verify. Resistance against thermal shock loading, on the other, has to be tested in a test rig, and test results obtained in the lab have to be scaled up to provide a database for a reliable design. This implies that advanced methods of fracture statistics have to be used. In this paper, failure probabilities under thermal shock loading are calculated using a modified version of the Weibull theory which takes the pronounced stress gradients into account. The basic idea is to determine the stress intensity factors of the natural flaws using weight functions. The new approach is used to estimate the failure probability in a thermal shock experiment which is designed to simulate critical gas turbine conditions. The predicted value of the failure probability is lowered by a factor of about 5 if the stress gradients are taken into considerations. The agreement between theory and experiments is very good.

Commentary by Dr. Valentin Fuster
GT2002-30504 pp. 67-76; (10 pages)

The use of silicon nitride for high temperature components in gas turbine engine and other heat engine applications continues to be sought due to the potential for significant improvements in both efficiency and emissions. However, low impact resistance and inadequate resistance to environmental degradation of current materials reduce its reliability for rotating and stationary components. Material improvements are needed to overcome these limitations. Potential and demonstrated methods to achieve improved fracture toughness, strength, and high temperature environmental resistance are reviewed.

Commentary by Dr. Valentin Fuster
GT2002-30505 pp. 77-84; (8 pages)

Impact of foreign or domestic material on components in the hot section of gas turbines with ceramic components is a common cause of catastrophic failure. Several such occurrences were observed during engine testing under the Ceramic Stationary Gas Turbine program sponsored by the U.S. Department of Energy. A limited analysis was carried out at Solar Turbines Incorporated (Solar), which involved modeling of the impact in the hot section. Based on the results of this study an experimental investigation was carried out at the University of Dayton Research Institute Impact Physics Laboratory to establish the conditions leading to significant impact damage in silicon-based ceramics. The experimental set up involved impacting ceramic flexure bars with spherical metal particulates under conditions of elevated temperature and controlled velocity. The results of the study showed a better correlation of impact damage with momentum than with kinetic energy. Increased test specimen mass and fracture toughness were found to improve impact resistance. Continuous fiber-reinforced ceramic composite (CFCC) materials have better impact resistance than monolithics. A threshold velocity was established for impacting particles of a defined mass. Post-impact metallography was carried out at Oak Ridge National Laboratory to further establish the impact mechanism.

Commentary by Dr. Valentin Fuster
GT2002-30506 pp. 85-95; (11 pages)

The life prediction analysis based on an exponential crack velocity formulation was examined using a variety of experimental data on glass and advanced structural ceramics in constant stress-rate (“dynamic fatigue”) and preload testing at ambient and elevated temperatures. The data fit to the strength versus ln (stress rate) relation was to found be very reasonable for most of the materials. It was also found that preloading technique was equally applicable for the case of slow crack growth (SCG) parameter n>30. The major limitation in the exponential crack velocity formulation, however, was that an inert strength of a material must be known priori to evaluate the important SCG parameter n, a significant drawback as compared to the conventional power-law crack velocity formulation.

Commentary by Dr. Valentin Fuster
GT2002-30507 pp. 97-102; (6 pages)

Bars loaded by opposite concentrated forces via rollers are appropriate test specimens for the determination of the fracture toughness, KIc , and the crack resistance curve (R-curve) of ceramic materials. In this paper stress solutions for the proposed test specimens are provided, as well as the stress intensity factor and the T-stress solutions. As practical applications, R-curves are determined for a soft PZT ceramic and several alumina ceramics.

Commentary by Dr. Valentin Fuster
GT2002-30508 pp. 103-107; (5 pages)

A novel technique for estimating the critical size of the frontal process zone in ceramics was proposed using a single-edge V-notched beam (SEVNB) test method. A three-point flexure test was carried out on a silicon nitride and two kinds of graphite test specimens with sharp V-shaped notches whose depths varied from 20 to 1,500 μm. The critical local stress was calculated at a critical distance from the notch tip. The critical size of the frontal process zone was determined as the distance between the notch tip and the point where the critical local stress had the same value as the flexural strength of test specimens without notches. The relationship among the critical size of the frontal process zone, the fracture toughness, and the flexural strength was discussed.

Topics: Ceramics
Commentary by Dr. Valentin Fuster
GT2002-30585 pp. 109-118; (10 pages)

Under the Ceramic Stationary Gas Turbine (CSGT) Program and the Advanced Materials Program, sponsored by the U.S. Department of Energy (DOE), several silicon carbide/silicon carbide (SiC/SiC) combustor liners were field tested in a Solar Turbines Centaur 50S gas turbine, which accumulated approximately 40000 hours by the end of 2001. To date, five field tests were completed at Chevron, Bakersfield, CA, and one test at Malden Mills, Lawrence, MA. The evaluation of SiC/SiC liners with an environmental barrier coating (EBC) after the fifth field test at Bakersfield (13937 hours) and the first field test at Malden Mills (7238 hours) is presented in this paper. The work at Oak Ridge National Laboratory (ORNL) in support of the field tests was supported by DOE’s Continuous Fiber-Reinforced Ceramic Composite (CFCC) Program.

Topics: Gas turbines , Testing
Commentary by Dr. Valentin Fuster
GT2002-30625 pp. 119-125; (7 pages)

A new concept of Ceramic Matrix Composite (CMC), mainly based on the use of a self–sealing technology for matrix and the use of a multilayer woven reinforcement, has been developed by Snecma for achieving high performance levels targeted by future jet engines. The driving force for this development has been to increase both lifetime and temperature capability of previous C/SiC and SiC/SiC materials using a monolithic SiC Chemical Vapor Infiltration (CVI) matrix and finishing treatment against oxidation. The first material, which has been developed with this new approach, is CERASEP® A410, using Hi-Nicalon™ fibers from Nippon Carbon. It has been submitted to a comprehensive characterization in order to determine thermo-mechanical properties and to evaluate lifetime duration, using fatigue and creep testing. Further material development is investigating the use of carbon fiber for economical objectives. The combination of such fibers with the new self-sealing matrix is providing promising results for long duration application at high temperature. Such results are permitted by the very high potential of the new matrix.

Commentary by Dr. Valentin Fuster
GT2002-30626 pp. 127-133; (7 pages)

Environmental barrier coatings (EBCs) with a Si bond coat, a yttria-stabilized zirconia (YSZ) top coat, and various intermediate coats were investigated. EBCs were processed by atmospheric pressure plasma spraying. The EBC durability was determined by thermal cycling tests in water vapor at 1300°C and 1400°C, and in air at 1400°C and 1500°C. EBCs with a mullite (3Al2 O3 ·2SiO2 )+BSAS (1-xBaO·xSrO·Al2 O3 ·2SiO2 ) intermediate coat were more durable than EBCs with a mullite intermediate coat, while EBCs with a mullite/BSAS duplex intermediate coat resulted in inferior durability. The improvement with a mullite+BSAS intermediate coat was attributed to enhanced compliance of the intermediate coat due to the addition of a low modulus BSAS second phase. Mullite+BSAS/YSZ and BSAS/YSZ interfaces produced a low melting (<1400 °C) reaction product, which is expected to degrade the EBC performance by increasing the thermal conductivity. EBCs with a mullite+BSAS/graded mullite+YSZ intermediate coat showed the best durability among the EBCs investigated in this study. This improvement was attributed to diffused CTE mismatch stress and improved chemical stability due to the compositionally graded mullite+YSZ layer.

Topics: Coatings
Commentary by Dr. Valentin Fuster
GT2002-30627 pp. 135-140; (6 pages)

An 8000 kW class Hybrid Gas Turbine (HGT) project, administered by the New Energy and Industrial Technology Development Organization (NEDO), has been ongoing since July of 1999 in Japan. Targets of this project are improvement in thermal efficiency and output power by using ceramic components, and early commercialization of the gas turbine system. The ceramic components are used for stationary parts subjected to high temperature, such as combustor liners, transition ducts, and first stage turbine nozzles. Development of the gas turbine is conducted by Kawasaki Heavy Industries, Ltd. (KHI), to achieve the Turbine Inlet Temperature (TIT) of 1250°C, thermal efficiency of 34%, NOx emission less than standard regulation values, and 4,000 h engine durability. Kyocera is in charge of the development and evaluation of the ceramic components. Recently, recession of the Si based ceramic materials under the combustion gas is the focus of attention to improve the reliability of ceramic components for gas turbine. For the HGT project, the silicon nitride material (SN282 : silicon nitride material produced by Kyocera Corporation) is used for the components subjected to high temperature. The SN282 was evaluated under the combustion gas, and clear recession was observed. Our technology of the Environmental Barrier Coating (EBC) is under development to obtain reliable heat resistive SN282 components, against the recession by combustion gas. Reliability of the SN282 with EBC has been evaluated by exposure and hydrothermal corrosion test. Ceramic components made of SN282 with EBC will be also evaluated by a proof engine test of 4,000 h, which starts in the spring of 2002.

Commentary by Dr. Valentin Fuster
GT2002-30628 pp. 141-146; (6 pages)

Environmental barrier coatings (EBCs) are required for applications of silicon nitride (Si3 N4 ) and silicon carbide (SiC) based materials in gas turbine engines because of the accelerated oxidation of Si3 N4 and SiC and subsequent volatilization of silica in the high temperature high-pressure steam environment. EBC systems for silicon carbide fiber reinforced silicon carbide ceramic matrix composites (SiC/SiC CMC’s) were first developed and have been demonstrated via long-term engine tests. Recently, studies have been carried out at United Technologies Research Center (UTRC) to understand the temperature capability of the current celsian-based EBC systems and its suitability for silicon nitride ceramics concerning thermal expansion mismatch between the EBC coating and silicon nitride substrates. This paper will present recent progress in improving the temperature capability of the celsian –based EBC systems and discuss their effectiveness for silicon nitride.

Commentary by Dr. Valentin Fuster
GT2002-30629 pp. 147-154; (8 pages)

This paper provides a review of recent studies undertaken to examine the mechanical and thermal stability of silicon nitride ceramic vanes with and without an oxide-based environmental barrier coating (EBC) after field tests in an industrial gas turbine. Two commercially available silicon nitride vanes (i.e., AS800 and SN282) were evaluated, where the AS800 vanes had an EBC and the SN282 vanes did not. The average temperature and pressure of gas impinging upon the vanes were approximately 1066°C and 8.9 atm, respectively. Both silicon nitride vanes were subjected to exposure time up to 1818h. Scanning electron microscopy was used to provide an insight into the changes in the microstructures of silicon nitrides and EBC arising from the environmental effects. The recession of the airfoils resulting from the volatilization of the normally protective silica layer, and /or EBC, was also measured using a coordinate measuring machine. The long-term chemical as well as structural stability of the secondary phases as well as EBC were characterized using x-ray diffraction. The surface strength of exposed airfoils was evaluated using a miniature biaxial test specimen, which was prepared by a diamond core drilling.

Commentary by Dr. Valentin Fuster
GT2002-30630 pp. 155-162; (8 pages)

SiC/SiC continuous fiber-reinforced ceramic matrix composite (CFCC) combustor liners having protective environmental barrier coatings (EBCs) applied to the liner working surfaces have been field-tested in a Solar Turbines’ Centaur 50S SoLoNOx engine at the Chevron, Bakersfield, CA engine test site. This latest engine test ran for a total of 13,937h. The EBCs significantly increased the lifetime of the in-service liners compared with uncoated CFCC liners used in previous field-tests. The engine test was concluded when a routine borescope inspection revealed the formation of a small hole in the inner liner. Extensive microstructural evaluation of both the inner and outer liners was conducted after removal from the engine. Post-test analysis indicated that numerous degradation mechanisms contributed to the EBC and CFCC damage observed on the liners, including EBC volatilization, sub-surface CFCC oxidation and recession, and processing defects which resulted in localized EBC spallation and accelerated CFCC oxidation. The characterization results obtained from these field-tested liners have been compared with the analyses of similarly-processed CFCC/EBCs that were laboratory-tested in a high-pressure, high temperature exposure facility (the ORNL “Keiser Rig”) for >6000h.

Commentary by Dr. Valentin Fuster
GT2002-30631 pp. 163-169; (7 pages)

The cohesive strengths of environmental barrier coatings applied to silicon carbide substrates were characterized using a compression test containing a strip of coating along a portion of the gage length. The substrate sample design and test fixture are similar to that described in ASTM D 695-96. The theory needed to extract the cohesive/adhesive strengths from the data is presented. The results of the compression tests are compared to the standard test method for the determination of the cohesive/adhesive strengths by the tension-adhesion test (TAT) (ASTM C 633-79). The preliminary results indicate that the onset of failure in the compression tests can be correlated with TAT test results, allowing for the extraction of the cohesive strength of the coating. For this system, this strength was found to be 15–20 MPa. The compression test has the advantage that it can be conducted at elevated temperature without the use of adhesives and, furthermore, is not limited by the adhesive strengths of polymeric adhesives.

Commentary by Dr. Valentin Fuster
GT2002-30632 pp. 171-178; (8 pages)

Thermal barrier and environmental barrier coatings (TBCs and EBCs) will play a crucial role in future advanced gas turbine engines because of their ability to significantly extend the temperature capability of the ceramic matrix composite (CMC) engine components in harsh combustion environments. In order to develop high performance, robust coating systems for effective thermal and environmental protections of the engine components, appropriate test approaches for evaluating the critical coating properties must be established. In this paper, a laser high-heat-flux, thermal gradient approach for testing the coatings will be described. Thermal cyclic behavior of plasma-sprayed coating systems, consisting of ZrO2 -8wt%Y2 O3 thermal barrier and NASA Enabling Propulsion Materials (EPM) Program developed mullite+BSAS/Si type environmental barrier coatings on SiC/SiC ceramic matrix composites, was investigated under thermal gradients using the laser heat-flux rig in conjunction with the furnace thermal cyclic tests in water-vapor environments. The coating sintering and interface damage were assessed by monitoring the real-time thermal conductivity changes during the laser heat-flux tests and by examining the microstructural changes after the tests. The coating failure mechanisms are discussed based on the cyclic test results and are correlated to the sintering, creep, and thermal stress behavior under simulated engine temperature and heat flux conditions.

Commentary by Dr. Valentin Fuster
GT2002-30645 pp. 179-185; (7 pages)

Full-size silicon nitride gas-turbine vanes coated with an environmental barrier coating (EBC) were evaluated in an exploratory way by two nondestructive methods: polarized elastic optical scattering and infrared thermal imaging. Initial test results indicate that the laser scatter data correlate with EBC thickness. A description of the methods and results of recent tests are presented.

Commentary by Dr. Valentin Fuster

Industrial and Cogeneration

GT2002-30248 pp. 187-195; (9 pages)

In the paper, a comprehensive methodology for gas turbine health state determination is applied to a single-shaft Fiat Avio TG 20 gas turbine working in the cogenerative combined cycle power plant of Fiat – Mirafiori (Italy). In order to determine operating state variations from new and clean condition, the following procedures were applied to historical field measurements: • normalization procedure to determine the variations between measured and expected values; • inverse cycle technique to calculate the values of the characteristic parameters that are indices of the machine health state. The application of these techniques to long period operating data allowed measurement validation and the determination of the machine health state. The results showed the good capability of the developed techniques for the determination and the analysis of performance drop due to compressor fouling and to turbine malfunction.

Commentary by Dr. Valentin Fuster
GT2002-30249 pp. 197-204; (8 pages)

This paper outlines a part of the work under way at GE Oil and Gas – Nuovo Pignone to develop advanced diagnostic tools to evaluate gas turbine hot gas path components life on the basis of actual operating data continuously recorded by remote monitoring systems. The system aims at correlating component metal temperatures and stresses as a function of operating performance data measured through standard machine instrumentation. Monitored data is processed by a new inverse-cycle algorithm to evaluate gas-path temperatures and pressures. The generated gas path information needs then to be correlated to metal temperatures and stresses with precision suitable for input to algorithms evaluating creep, oxidation and hot corrosion damage. Typically, calculations of gas path data to metal temperatures and stresses are performed at the design stage for a limited number of critical operating conditions by using complex and sophisticated CFD and structural/thermal analysis computer codes. For applications to diagnostic, direct use of such tools for any monitored sets of data would be impracticable. On the other hand, they represent the most effective means for assessing hot gas path component temperatures with adequate accuracy, particularly on last generation engines with substantial turbine blade and nozzle cooling. The approach chosen and described herein consists in extensively using high level design tools over a wide range of turbine operating conditions and use the results to produce equations and maps linking field monitored data to component temperatures suitable for easy implementation into a life evaluation system. In the paper major aspects of the above work are reviewed and synthesized, and significant steps in the first application to a turbine first stage cooled blade are illustrated.

Commentary by Dr. Valentin Fuster
GT2002-30250 pp. 205-217; (13 pages)

The design, operation and usage of Heat Recovery Steam Generators (HRSG) has undergone considerable changes in the last 30 years. Nowadays, instead of as an option item, HRSGs are a major part of the Combined Cycle Power Plant. This makes it necessary to optimize the design and operation of the HRSG so that it can be integrated with the total plant. However, because of the complexity, it is not always feasible to evaluate all possible configurations for selecting the most optimum one within the given time constraints. An attempt is made here to present the parametric effect of various variables through descriptive graphs. These graphs are developed for general cases but can be applied to specific cases to give the trend rather than the absolute values. Cycle designers can use those to narrow down the cycle HRSG configurations. Plant operators may be able to use these to improve the performance by simple additions or modifications.

Commentary by Dr. Valentin Fuster
GT2002-30251 pp. 219-228; (10 pages)

This paper describes efforts that were implemented in modifying two Steam Turbine Generators (STG) that are presently operating in Watson Cogeneration Company (WCC) Plant. WCC Plant is comprised of four identical GE made Gas Turbine Generators (GTG) and four Heat Recovery Steam Generators (HRSG) designed and fabricated by Vogt. Portion of high pressure steam is expanded inside two Dresser-Rand-made Steam Turbine Generators (STG). The modifications presented in this paper include replacement of six original stages of expansion, introduction of shaft retractable labyrinths/packing and installation of the spill strips around shrouded blades. The modifications of high pressure steam path (except 1st stage blading) were completed in 1992 and modification of rotor steam sealing elements such as shaft labyrinths were completed in April and May 2001. The steam path modification uprated STG from original 34.50MW to present 40MW each. The upgrades of the rotor sealing elements resulted in 2.80% Heat Rate (HR) reduction.

Commentary by Dr. Valentin Fuster
GT2002-30252 pp. 229-234; (6 pages)

Deregulation of the power industry in Europe and the attendant pressures to innovate have led to new approaches to financing and construction of power plants. This paper discusses the salient design, procurement, and project execution considerations of a combined cycle (CC) plant for a deregulated market.

Commentary by Dr. Valentin Fuster
GT2002-30253 pp. 235-241; (7 pages)

ALSTOM Power has launched the GT10C a 30 MW industrial gas turbine (see figure 1) upgraded from the 25 MW GT10B. The thermal efficiency of the new gas turbine is 37.3% (shaft) and 36% electrical at ISO inlet conditions with no losses. The new GT10C has a D ry L ow E mission (DLE) combustor for both natural gas and diesel oil fuel; it has NOx emissions at 15 ppmv on gas and 42 ppmv on oil fuel (15% O2 dry). The first GT10C is now manufactured and assembled, and has been under testing since October 2001. For this purpose a new test rig has been built in Finspong, Sweden, in order to verify performance and reliability. GT10C will be available to the market mid-2002 and manufactured in parallel with GT10B. The general design is based on the GT10B and measures have been taken for maximum reliability and maintenance in order to keep operation costs to a minimum. Improvements for GT10C are mainly derived from GT10B or taken from ALSTOM Power GTX100 (43 MW gas turbine), as described herein.

Commentary by Dr. Valentin Fuster
GT2002-30254 pp. 243-251; (9 pages)

The Cyclone industrial gas turbine was launched in 1997 and entered commercial operation in 2000. Rated at 13.4MW and with a thermal efficiency in excess of 35% (at ISO operating conditions), the Cyclone was configured as a twin-shaft engine derivative of the Tempest Gas turbine, to meet both power generation and mechanical drive applications. This paper describes the design, development and early operational experience of the Cyclone gas turbine. The design aspects include features, which are common with other products within the ALSTOM product range, those that have been developed out of technology programmes, and those scaled from existing parts. Details are presented of the compressor construction, where a “zero” stage has been added to the Tempest rotor, and coupled with an increase in firing temperature, has resulted in the increase in power output. A two stage overhung compressor turbine, includes cooled blading technology to both stages. A separate free power turbine is based on a scale version of the Typhoon twin-shaft power turbine. The Cyclone includes the ALSTOM, Dry Low Emissions combustion system as standard and is able to operate on a wide range of fuels, in single or dual fuel configurations. The combustion system is based on the proven, generic system first introduced into the Typhoon. The result of engine testing has resulted in the release of both the Cyclone, and the Tempest, with sub 10ppmvd NOx (corrected to 15% O2 ). The first Cyclone engines entered service in the autumn of 2000, in a co-generation facility in Australia. Described in this paper are the early operating experiences, and the evaluation of a large amount of site data that has been recorded. Included in this section is information on issues that have had to be addressed during the first 8000 hours of operation.

Topics: Design , Gas turbines
Commentary by Dr. Valentin Fuster
GT2002-30255 pp. 253-259; (7 pages)

The L20A gas turbine is a newly developed 20 MW class single-shaft machine. With its high simple-cycle efficiency and high exhaust gas temperature, it is particularly suited for use in distributed power generation, cogeneration and combined cycle applications. A design philosophy has been adopted for the turbine which includes a high efficiency transonic axial-flow compressor with eight can-type combustors and a high inlet temperature of 1250°C. This results in a thermal efficiency of 35% and an overall thermal efficiency of 80% for cogeneration system. In addition, the NOx emissions from the combustor is low and the L20A has a long service life. These features permit long-term continuous operation under various environmental limitations. Due to the engine’s high efficiency and its low component totals, the lowest life cycle cost is achieved. Development testing has verified that the performance, the mechanical characteristics and the emission have satisfied the initial design goals. The engine has been in operation from November 2001 as the first operating unit in a co-generation system at Kawasaki Akashi Works.

Topics: Gas turbines
Commentary by Dr. Valentin Fuster
GT2002-30256 pp. 261-270; (10 pages)

California State University, Sacramento, has constructed and put into service a stand alone cogeneration laboratory. The major components are a 75 kW gas turbine and generator, a waste heat boiler, and a 10 ton absorption chiller. Initial testing has been completed with efforts concentrating on the gas turbine engine and the absorption chiller. A two part thermodynamic performance analysis procedure has been developed to analyze the cogeneration plant. A first law energy balance around the gas turbine determines the heat into the engine. A Brayton cycle analysis of the gas turbine engine is then compared with the measured performance. While this engine is quite small, this method of analysis gives very consistent results and can be applied to engines of all sizes. Careful attention to details is required to obtain agreement between the calculated and measured outputs; typically they are within 10 to 15 percent. In the second part of the performance analysis experimental operation of the absorption chiller has been compared to that specified by the manufacturer and a theoretical cycle analysis. While the operation is within a few percent of that specified by the manufacturer, there are some interesting differences when it is compared to a theoretical analysis.

Commentary by Dr. Valentin Fuster
GT2002-30258 pp. 271-281; (11 pages)

Many gas turbines simulation codes have been developed to estimate power plant performance both in design and off-design conditions in order to establish the adequate control criteria or the possible cycle improvements; estimation of pollutant emissions would be very important using these codes in order to determine the optimal performance satisfying legal emission restrictions. This paper present the description of a 1-D emission model to simulate different gas turbine combustor typologies, such as conventional diffusion flame combustors, Dry-Low NOx combustors (DLN) based on lean-premixed technology (LPC) or Rich Quench Lean scheme (RQL) and the new catalytic combustors. This code is based on chemical reactor analysis, using detailed kinetics mechanisms, and it is integrated with an existing power plant simulation code (ESMS Energy System Modular Simulator) to analyze the effects of power plant operations and configurations on emissions. The main goal of this job is the study of the interaction between engine control and combustion system. This is a critical issue for all DLN combustors and, in particular, when burning low-LHV fuel. The objective of this study is to evaluate the effectiveness of different control criteria with regard to pollutant emissions and engine performances. In this paper we present several simulations of actual engines comparing the obtained results with the experimental published data.

Commentary by Dr. Valentin Fuster
GT2002-30259 pp. 283-289; (7 pages)

Carbon dioxide (CO2 ) emissions from fossil-fueled power plants contribute to more than one-third of all CO2 emissions in the U.S. [1]. Any effort to curtail greenhouse gases should therefore include the reduction of this emission source. Methods of CO2 reduction include (1) use of alternative fuels with lower CO2 emissions and (2) CO2 scrubbing and sequestration to prevent its release to the atmosphere. The cost of CO2 reduction varies with the selected technology. This paper compares (1) the cost of electricity (COE) without and with CO2 removal/avoidance and (2) the impact of the incremental cost of CO2 reduction on COE for different technology options, including replacing existing coal plants with natural-gas-fired combined cycle (NGCC), integrated gasification combined cycle (IGCC) with and without CO2 removal, pulverized coal (PC) with CO2 scrubber, and nuclear plants. Full and partial compliance with the Kyoto Protocol are addressed.

Commentary by Dr. Valentin Fuster
GT2002-30416 pp. 291-299; (9 pages)

A new methodology is developed to find the optimal steam injection levels in simple and combined cycle gas turbine power plants. When steam injection process is being applied to simple cycle gas turbines, it is shown to offer many benefits, including increased power output and efficiency as well as reduced exhaust emissions. For combined cycle power plants, steam injection in the gas turbine, significantly decreases the amount of flow and energy through the steam turbine and the overall power output of the combined cycle is decreased. This study focuses on finding the maximum power output and efficiency of steam injected simple and combined cycle gas turbines. For that purpose, the thermodynamic cycle analysis and a genetic algorithm are linked within an automated design loop. The multi-parameter objective function is either based on the power output or on the overall thermal efficiency. NOx levels have also been taken into account in a third objective function denoted as steam injection effectiveness. The calculations are done for a wide range of parameters such as compressor pressure ratio, turbine inlet temperature, air and steam mass flow rates. Firstly, 6 widely used simple and combined cycle power plants performance are used as test cases for thermodynamic cycle validation. Secondly, gas turbine main parameters are modified to yield the maximum generator power and thermal efficiency. Finally, the effects of uniform crossover, creep mutation, different random number seeds, population size and the number of children per pair of parents on the performance of the genetic algorithm are studied. Parametric analyses show that application of high turbine inlet temperature, high air mass flow rate and no steam injection lead to high power and high combined cycle thermal efficiency. On the contrary, when NOx reduction is desired, steam injection is necessary. For simple cycle, almost full amount of steam injection is required to increase power and efficiency as well as to reduce NOx. Moreover, it is found that the compressor pressure ratio for high power output is significantly lower than the compressor pressure ratio that drives the high thermal efficiency.

Commentary by Dr. Valentin Fuster
GT2002-30417 pp. 301-312; (12 pages)

This paper investigates energy savings and economic aspects related to the use of microturbine generators in commercial buildings either for cogeneration (electricity+heat) or for trigeneration (electricity, heat and cold). In all calculations, reference is made to a 25 kWel –class commercial micro-turbine generator (MTG), tested by the authors. Various plant schemes are considered, based on one or several MTG sets. The possibility of generating heat and/or cold also by an electrically driven inverse-cycle air-to-water heat pump/chiller system is also considered. Calculations are based on the simulation code TRIGEN developed by the authors. The code provides detailed energy, economic and emission yearly balances. The plant operating mode is optimized in each time interval. The results indicate that, due to large load variations, (i) the optimum turbine nominal output is in the range of about 70% of the electric peak demand, (ii) energy savings are marginal, (iii) advantages related to splitting the overall capacity on more than one unit are marginal and (iv) the addition of an absorption machine improves the plant economics.

Commentary by Dr. Valentin Fuster
GT2002-30418 pp. 313-318; (6 pages)

High efficiency, environmental friendliness, low operation and maintenance (O&M) costs, and lowest possible impact on the surroundings are some requirements of sustainable energy production. In selection of new power generation systems, a number of steps have to be taken into account to meet these requirements. Here the first law analysis has been implemented and investigated followed by a combination of the first and second law analyses (exergy analysis), and thermoeconomics, and finally an Exergetic Life Cycle Assessment (ELCA) is carried out for two different power cycles. The two cycles, investigated here, are a two-pressure level combined cycle, hereafter called (CC), and a Humid Air Turbine or (HAT-cycle). The main goal of this study is to point out the advantages and the difficulties related to the usage of each and every method and their combinations, and to identify the target groups that can gain knowledge and information using these methods. Since the operators of power plants often do not have access to detailed information about component materials, characteristics, etc., of the power cycle, assumptions have to be made when comparing different cycle configuration with each other. This limited type of data and information has also been used here to create a plausible scenario of how different pre-design methods can differ from each other. One major conclusion that has been drawn is that the two cycles investigated here are favorable in different situations and that the results from application of the three methods mentioned above indicate differences in which cycle is the preferable one. However, using a combination of different analysis methods illuminates the plant strengths and limitations during pre-design studies, but conflicting results need to be resolved to obtain the most cost effective and environmentally-friendly power cycle.

Topics: Exergy , Design , Cycles
Commentary by Dr. Valentin Fuster
GT2002-30482 pp. 319-325; (7 pages)

Turnkey and thermal island supply scopes present turbine suppliers with a perfect way to sell their rotating products. The popularity of these plant configurations, along with the recent availability of more holistic test codes, has led to the need for an accurate and reasonable method of determining the thermal performance of the externally-purchased HRSG component. To assess a multiple pressure HRSG, it is advantageous and convenient to have one single criterion for the evaluation of performance, especially when this criterion provides for the compensation of the different outlet energy streams. The so-called Model Steam Turbine method of HRSG evaluation was developed for these reasons. The result of the calculation, a lone performance criterion, is the shaft power of the fictitious Model Steam Turbine.

Commentary by Dr. Valentin Fuster
GT2002-30483 pp. 327-334; (8 pages)

In this paper, a one-dimensional program for evaluating pollutant emissions in combined cycle power plant with supplementary firing is presented. The program uses Chemical Reactors analysis based on a Perfectly Stirred Reactor approach in conjunction with an emission model that simulates a detailed chemical kinetic scheme for combustion process modelling. The program allows the evaluation of the main pollutant emissions deriving from natural gas and oil combustion. In order to simulate combustion systems that can be found in a fired combined cycle power plant, the developed program presents some extended features with respect to programs developed for gas turbine combustors only. In order to reproduce a wide typology of combustors, the combustor geometry is represented using two characteristic dimensions (hydraulic diameter and length) and the considered domain is divided into reactors in series (along the axial direction) and in parallel (along the radial direction). The temperature in each reactor is determined taking into account both the convective and the radiative heat transfer between hot gases and walls. The program has been applied to two cases. In the first the numerical predictions have been compared with available experimental data relative to two gas turbine combustors. In the second case the program has been instead applied to a cylindrical test burner designed in accordance with EN 267 European Standard. The obtained results are acceptable from an engineering point of view and have been considered sufficiently accurate for this preliminary set-up phase of the model.

Commentary by Dr. Valentin Fuster
GT2002-30484 pp. 335-342; (8 pages)

In the commercial sector, heat and power demands peak in the summer daytime because of high space cooling demands, and cogeneration plants are required to produce maximum heat and power to meet their demands. However, gas turbine cogeneration plants have the disadvantage of decreases in maximum power output in the summer daytime, which reduces the availability of gas turbines. One of the ways to avoid the aforementioned disadvantage is to cool inlet air and augment maximum power output. In addition, one of the ways for inlet air cooling is to make ice by driving electric compression refrigerators using off-peak power generated during the nighttime, store it in ice banks, and use its heat for inlet air cooling during the on-peak period. The objective of this paper is to investigate the effect of inlet air cooling by ice storage on the unit sizing and cost of a gas turbine cogeneration plant. An optimal unit sizing method based on the mixed-integer linear programming is used to rationally determine equipment capacities and operational strategies of the plant. A numerical study is conducted, in which the gas turbine cogeneration plants with and without inlet air cooled by ice storage are compared with each other, and the effect of inlet air cooling on the equipment capacities as well as the annual total cost and its items is clarified.

Commentary by Dr. Valentin Fuster
GT2002-30557 pp. 343-352; (10 pages)

The use of gas turbine and combined cycle power plants for thermal and electric power generation is, nowadays, a consolidated technology. Moreover the employment of combined heat and power production, especially for low power requirements, is constantly increasing. In this scenario, Below Ambient pressure discharge Gas Turbine (BAGT) is an innovative and interesting application; the hot gases discharged from a gas turbine may be expanded below ambient pressure to obtain an increase in electric power generation. The gases are then cooled to supply heat to the thermal utility and finally recompressed to the ambient pressure. The power plant cogenerative performance depends on the heat and electric demand that usually varies during the year (for residential heating the heat to electric power ratio may range from 0.3 to 9). In this paper, the thermal load variation influence on the BAGT performance will be investigated and compared with those of gas turbine and combined cycle power plants.

Commentary by Dr. Valentin Fuster
GT2002-30558 pp. 353-370; (18 pages)

In the present paper, a comprehensive and simple in application design methodology to obtain a gas turbine working on recuperated, intercooled and reheat cycle utilizing existing gas turbines is presented. Applications of the proposed design steps have been implemented on the three existing gas turbines, with wide ranging design complexities. The results of evaluated aero-thermodynamic performance for these existing gas turbines with the proposed modifications are presented and compared in this paper. Sample calculations of the analysis procedures discussed, including stage-by-stage analysis of the compressor and turbine sections of the modified gas turbines, have been also included. All the three modified gas turbines were found to have higher performance, with cycle efficiency increase of 9% to 26%, in comparison to their original values.

Topics: Gas turbines , Cycles
Commentary by Dr. Valentin Fuster
GT2002-30559 pp. 371-386; (16 pages)

Inlet fogging of gas turbine engines has attained considerable popularity due to the ease of installation and the relatively low first cost compared to other inlet cooling methods. With increasing demand for power and with shortages envisioned especially during the peak load times during the summers, there is a need to boost gas turbine power. There is a sizable evaporative cooling potential throughout the world when the climatic data is evaluated based on an analysis of coincident wet bulb and dry bulb information. This data is not readily available to plant users. In this paper, a detailed climatic analysis is made of 106 major locations over the world to provide the hours of cooling that can be obtained by direct evaporative cooling. This data will allow gas turbine operators to easily make an assessment of the economics of evaporative fogging. The paper also covers an introduction to direct evaporative cooling and the methodology and data analysis used to derive the cooling potential. Simulation runs have been made for gas turbine simple cycles showing effects of fogging for a GE Frame 7EA and a GE Frame 9FA Gas turbine for 60 and 50 Hz applications.

Commentary by Dr. Valentin Fuster
GT2002-30560 pp. 387-401; (15 pages)

With deregulation in the power generation market and the need for flexibility in terms of power augmentation during periods of high electricity demand, power plant operators all over the world are exploring means to augment power from both existing and new gas turbines. An approach becoming increasingly popular is that of high pressure fogging. In this paper, the results of a comprehensive parametric analysis on the effects of inlet fogging on a wide range of existing gas turbines have been presented. Both evaporative and overspray fogging conditions have been analyzed. The results of this study show that the performance parameters indicative of inlet fogging effects have definitive correlation with the key gas turbine design parameters. In addition, this study indicates that aeroderivative gas turbines, in comparison to the industrial machines, have higher performance improvement due to the inlet fogging effects. Plausible reasons for the observed trends are discussed in this paper. This paper represents the first systematic study on the effects of inlet fogging for a large number (a total of 67) of gas turbine engines available from major gas turbine manufacturers.

Topics: Gas turbines
Commentary by Dr. Valentin Fuster
GT2002-30561 pp. 403-412; (10 pages)

The purpose of this work is to investigate the benefits of some different ambient air conditioning methods for reducing the gas turbine intake air temperature in order to enhance the gas turbine power. As a reference case the combined heat and power plant of the campus area of the Technische Universität München in Garching is considered, which is equipped with an Allison KH501 Cheng Cycle gas turbine. Three novel technical possibilities of ambient air cooling and power augmentation are shown in detail (desiccant dehumidification and evaporative cooling, absorption chiller unit with air cooler, evaporative cooling at increased inlet air pressure). Based on site ambient conditions and measured yearly load lines for heat and electrical power connected with actual cost functions, the potential economic savings are worked out for the different technical modifications using ambient air cooling for power augmentation of the gas turbine plant. The economic operation lines for power and heat, supplied by the modified gas turbine plant, are calculated by a cost optimization system. The results are compared based on investment costs and economic savings by the extended annual electrical and thermal power production of the modified gas turbine plant.

Commentary by Dr. Valentin Fuster
GT2002-30562 pp. 413-428; (16 pages)

The inlet fogging of gas turbine engines for power augmentation has seen increasing application over the past decade yet not a single technical paper treating the physics and engineering of the fogging process, droplet size measurement, droplet kinetics, or the duct behavior of droplets, from a gas turbine perspective, is available. This paper provides the results of extensive experimental and theoretical studies conducted over several years coupled with practical aspects learned in the implementation of nearly 500 inlet fogging systems on gas turbines ranging in power from 5 to 250 MW. Part A of the paper covers the underlying theory of droplet thermodynamics and heat transfer, and provides several practical pointers relating to the implementation and application of inlet fogging to gas turbine engines.

Commentary by Dr. Valentin Fuster
GT2002-30563 pp. 429-441; (13 pages)

The inlet fogging of gas turbine engines for power augmentation has seen increasing application over the past decade yet not a single technical paper treating the physics and engineering of the fogging process, droplet size measurement, droplet kinetics, or the duct behavior of droplets, from a gas turbine perspective, is available. This paper provides the results of extensive experimental and theoretical studies conducted over several years, coupled with practical aspects learned in the implementation of nearly 500 inlet fogging systems on gas turbines ranging in power from 5 to 250 MW. Part B of the paper treats the practical aspects of fog nozzle droplet sizing, measurement and testing presenting the information from a gas turbine fogging perspective. This paper describes the different measurement techniques available, covers design aspects of nozzles, provides experimental data on different nozzles and provides recommendations for a standardized nozzle testing method for gas turbine inlet air fogging.

Commentary by Dr. Valentin Fuster
GT2002-30564 pp. 443-455; (13 pages)

The inlet fogging of gas turbine engines for power augmentation has seen increasing application over the past decade yet not a single technical paper treating the physics and engineering of the fogging process, droplet size measurement, droplet kinetics, or the duct behavior of droplets, from a gas turbine perspective, is available. This paper along with Parts A and B provides the results of extensive experimental and theoretical studies conducted over several years coupled with practical aspects learned in the implementation of nearly 500 inlet fogging systems on gas turbines ranging in power from 5 to 250 MW. In part C of this paper, the complex behavior of fog droplets in the inlet duct is addressed and experimental results from several wind tunnel studies are covered.

Commentary by Dr. Valentin Fuster
GT2002-30565 pp. 457-464; (8 pages)

The majority of fossil units in many countries including the United States were built from 1950 through the 1970s, and these older plants are now approaching the end of their useful operating design life. Faced with continued demand growth and compliance with stringent emissions requirements, the power industry may choose building new replacement units, extending the operating life of existing units, or repowering these existing units. Repowering has been demonstrated to be an attractive alternative that incorporates state-of-the-art technologies into an existing unit to achieve higher performance and thermal efficiency, lower emissions, higher reliability and usefulness, and the potential for a shorter execution permitting schedule. Combined cycle technology has become desirable and has matured for the repowering existing plants because of its high thermal efficiency, low emissions, low installed and operation cost, short installation time, high reliability and availability, excellent cycling capability, and operating flexibility. Various options are available for repowering applications on existing plants with combined cycle technology. The options include hot windbox repowering, feedwater heater repowering, and combustion turbine (CT) with heat recovery steam generator (HRSG) repowering. This paper examines the performance benefits of these combined cycle repowering options and analyzes associated costs.

Commentary by Dr. Valentin Fuster
GT2002-30566 pp. 465-471; (7 pages)

Flow-Accelerated Corrosion (FAC) has become a source of failures in Low pressure units of HRSGs. Research in past 20 years has indicated that FAC is a very complex phenomenon. It is influenced by many variables, some among them are : pressure, temperature, pH of the water, Chrome content of the metal, the dissolved oxygen content in water, the velocity of steam and water etc. The matter of FAC was discussed in detail in a joint publication by Electric Power research Institute (EPRI), Electricité de France (EDF) and Siemens. However the methodologies presented to understand the phenomena and to find the solution are very complex. Considerable expertise is needed to arrive at a good solution. This paper combines the work done both empirically and experimentally and gives a simpler method to estimate the metal loss due to FAC in an HRSG given the design and operating conditions. The user should then be able to decide whether to proceed further with more complex and comprehensive investigations, install a monitoring system or make some changes in the design or the operating methodology. Since the most prevalent occurrence of FAC in an HRSG is in Low pressure (LP) evaporators, the discussion and solutions are focussed on LP evaporators only. At the end of the paper a tentative action plan is also described to handle the FAC related issues in HRSGs.

Commentary by Dr. Valentin Fuster
GT2002-30567 pp. 473-480; (8 pages)

As deregulation of the electric market finds legitimate global acceptance, more efficient alternatives to centralized power production, such as the Combined Heat and Power (CHP), also known as cogeneration, are finding growing reception. Advances in sizing methodologies, selection criteria, and control technologies, as well as development of associated regulatory issues, must accompany this favorable disposition. This paper presents an overview of some important applications of a heat recovery system and discusses a simplified method of sizing a CHP as a part of an early feasibility decision.

Commentary by Dr. Valentin Fuster
GT2002-30589 pp. 481-486; (6 pages)

Silicon carbide cylindrical filter elements have proven to be an effective mean of removing particulates to levels exceeding the new source performance standard. This paper reports a Laser Doppler anemometry (LDA) investigation to study the flow behavior in the vicinity of the cylindrical filter. A single cylindrical filter has based in vertical channel. The flow enters from the bottom of the channel towards the filter element passing through to the outlet at the top of the channel. The particles deposition distribution around the filter element is studied using different face velocities.

Commentary by Dr. Valentin Fuster
GT2002-30590 pp. 487-496; (10 pages)

Thermodynamic model of wet compression process is established in this paper. The topics of ideal wet compression process, actual wet compression process, water droplet evaporative rate, wet compression work, inlet evaporative cooling, concept of wet compression efficiency, aerodynamic breaking of water droplets etc. are investigated and discussed in this paper.

Commentary by Dr. Valentin Fuster
GT2002-30656 pp. 497-505; (9 pages)

This paper describes the efforts of upgrading the original Interstage Injection System (IIS) that was installed in Gas Turbine Unit (GTU) No2, Summer 2000. Paper presented by Ingistov, No 2001-GT-407 at IGTI Conference in New Orleans, June 2001 describes the original IIS design and its operational results. As presented in New Orleans IGTI Conference the main purpose of Interstage Injection (II) is to preserve the Gas Turbine Unit (GTU) power output rather than augment the power output. Some power augmentation however takes place, and with gradual increase of the injected water flow the power gain will have to be quantified. This paper incorporates design changes of the water injection nozzles and the field results on GTU No1 compiled during Summer and Fall of 2001 in Watson Cogeneration Company (WCC) Plant in Carson California.

Commentary by Dr. Valentin Fuster
GT2002-30657 pp. 507-518; (12 pages)

Power providers in Oman encounter the greatest demand for electricity during the summer months. More than 70% of Oman electric power originate from gas turbines. Unfortunately, the power output of gas turbines decreases with increasing ambient temperature. The growth in electricity generation to the summer peak load has been achieved at a very high cost of installing new generators. This paper presents an assessment of using the exhaust heat to run an aqua-ammonia refrigeration system to cool the inlet air. The performance of two General Electric aeroderivative gas turbines, the LM2500+ and the LM1600, with and without absorption refrigeration was investigated. Climate data series from Fahud, central Oman, was used for modeling the variations in ambient temperature during the year to account for the effects of climate condition in the gas turbine performance simulation. Most of the gas turbines in Oman operate on a simple cycle with exhaust heat discharged to the atmosphere. Vapor absorption refrigeration system uses heat from gas turbine exhaust as its source of energy to cool the inlet ambient air to 7°C. It was observed that the available exhaust heat from the gas turbine exceeded the heat required to run the aqua-ammonia absorption refrigeration system. For meteorological conditions existing in the particular site considered in Oman, pre-cooling gas turbine inlet air to 7°C augments power, on a yearly basis, of LM2500+ and LM1600 by 20% and 14% respectively. During the peak hours in summer months, when power is mostly needed, the percentage of power augmented climbed to 39% and 33% for the two gas turbines. It has been concluded that chilling the inlet air of the gas turbine with aqua-ammonia absorption refrigeration is technically feasible and economically appealing.

Commentary by Dr. Valentin Fuster
GT2002-30658 pp. 519-523; (5 pages)

An evaporative cooling system for lowering the inlet temperature of a gas-turbine compressor is described. This system uses the latent enthalpy of evaporation for injected water droplets to decrease the enthalpy of the air. The requirements for compatible operation between this system and the compressor are given.

Commentary by Dr. Valentin Fuster

Structures and Dynamics

GT2002-30284 pp. 525-534; (10 pages)

Many turbomachines perform well in factory testing, but experience difficulties in the field, mainly due to installation problems. The problems encountered range from misalignment and frame distortion to structural resonance and skid leveling. In this paper, a structured vibration analysis procedure is developed to identify installation problems in rotating machinery. Spectral analysis is the main tool for vibration analysis of turbomachinery faults, however many problems appear at a single frequency. In this case phase analysis, time waveform, orbit and operating deflection shape (ODS) analysis become very important. The paper presents a novel methodology for the diagnosis of installation problems in turbomachinery and provides examples of cases with installation faults, and discusses methods of identification and solution.

Commentary by Dr. Valentin Fuster
GT2002-30285 pp. 535-542; (8 pages)

A series of experimental investigations were made in order to determine the set of coefficients of flexibility of construction supporting the rotor. The investigations were made on the test rig for the rotor dynamics investigation. A harmonic force of known amplitude was applied to all supports of the system to induce vibration of the system. Then the system’s reaction in the form of displacement in all connections at the same time was measured. On the bases of the experimental results a square 36-element (6×6) matrix of flexibility coefficients for each frequency of excitation was created. The matrices are the initial material to determine the modal masses, stiffness coefficients and damping coefficients in such form so that they could be the data to calculate the dynamics of the experimental rotor by means of computer programs. Moreover the experimental results are interesting information in the to analysis of the behavior of the test rig’s pedestals.

Topics: Plasticity , Rotors
Commentary by Dr. Valentin Fuster
GT2002-30286 pp. 543-552; (10 pages)

A computational analysis for prediction of the static and dynamic forced performance of gas honeycomb seals at off-centered rotor conditions follows. The bulk-flow analysis, similar to the two-control volume flow model of Kleynhans and Childs [1], is brought without loss of generality into a single-control volume model, thus simplifying the computational process. The formulation accommodates the honeycomb effective cell depth, and existing software for annular pressure seals is easily upgraded for damper seal analysis. An analytical perturbation method for derivation of zeroth- and first-order flow fields renders the seal equilibrium response and frequency-dependent dynamic force impedances, respectively. Numerical predictions for a centered straight-bore honeycomb gas seal show good agreement with experimentally identified impedances, hence validating the model and confirming the paramount influence of excitation frequency on the rotordynamic force coefficients of honeycomb seals. The effect of rotor eccentricity on the static and dynamic forced response of a smooth annular seal and a honeycomb seal is evaluated for characteristic pressure differentials and rotor speeds. Leakage for the two seal types increases slightly as the rotor eccentricity increases. Rotor off-centering does have a pronounced non-linear effect on the predicted (and experimentally verified) dynamic force coefficients for smooth seals. However, in honeycomb gas seals, even large rotor center excursions do not sensibly affect the effective local film thickness, maintaining the flow azimuthal symmetry. The current model and predictions thus increase confidence in honeycomb seal design, operating performance and reliability in actual applications.

Topics: Flow (Dynamics)
Commentary by Dr. Valentin Fuster
GT2002-30287 pp. 553-562; (10 pages)

The paper introduces a bulk-flow model for prediction of the static and dynamic force coefficients of angled injection Lomakin bearings. The analysis accounts for the flow interaction between the injection orifices, the supply circumferential groove, and the thin film lands. A one control-volume model in the groove is coupled to a bulk-flow model within the film lands of the bearing. Bernoulli-type relationships provide closure at the flow interfaces. Flow turbulence is accounted for with shear stress parameters and Moody’s friction factors. The flow equations are solved numerically using a robust computational method. Comparisons between predictions and experimental results for a tangential-against-rotation injection water Lomakin bearing show the novel model predicts well the leakage and direct stiffness and damping coefficients. Computed cross-coupled stiffness coefficients follow the experimental trends for increasing rotor speeds and supply pressures, but quantitative agreement remains poor. A parameter investigation evidences the effects of the groove and land geometries on the Lomakin bearing flowrate and force coefficients. The orifice injection angle does not influence the bearing static performance, although it largely affects its stability characteristics through the evolution of the cross-coupled stiffnesses. The predictions confirm the promising stabilizing effect of the tangential-against-rotation injection configuration. Two design parameters, comprising the feed orifices area and groove geometry, define the static and dynamic performance of Lomakin bearing. The analysis also shows that the film land clearance and length have a larger impact on the Lomakin bearing rotordynamic behavior than its groove depth and length.

Commentary by Dr. Valentin Fuster
GT2002-30288 pp. 563-572; (10 pages)

The study of eccentric squeeze film damped rotor dynamic systems has largely concentrated on rigid rotors. In this paper, a newly developed receptance harmonic balance method is used to efficiently analyze a squeeze film damped flexible rotor test rig. The aim of the study is to investigate the influence of damper static eccentricity and unbalance level on cavitation and its resulting effect on the vibration level. By comparing predictions for the rotor vibration levels obtained respectively with, and without, lower pressure limits for the eccentric squeeze film damper model, it is demonstrated that cavitation is promoted by increasing static eccentricity and/or unbalance level. This, in turn, is found to have a profound effect on the predictions for the critical vibration levels, which such dampers are designed to attenuate. The reported findings are backed by experimental evidence from the test rig.

Topics: Cavitation , Rotors
Commentary by Dr. Valentin Fuster
GT2002-30289 pp. 573-580; (8 pages)

Deterministic micro asperities show potential for enhancement of lubrication in conformal contacts as found in many bearing and seal designs. Several manufacturing methods have been proposed for deterministic micro asperities. Of these, laser texturing has emerged as the most viable option. This paper proposes the LIGA MEMs manufacturing method as an alternative. Using LIGA, surfaces with patterned micron sized surface features of arbitrary cross section (cylindrical, hex, triangular, etc.) can be fabricated from electroplated nickel, gel-cast silicon nitride or plastic. The resulting asperities can be positive (protuberances) or negative (recesses) and can have heights (depths) from 1–1000 microns and be patterned over surface areas up to about 150 mm × 150 mm. In this paper, the LIGA method is used to fabricate a sample thrust bearing surface with a hexagonal array of positive asperities. The resulting asperities are 550 μm in average diameter, 165 μm in edge-to-edge spacing and have heights of 3–100μm. Surface metrology indicates sub-micron accuracy of form and 13 nm Ra roughness on the asperity tops (land). Tribology testing in a non-pressurized oil bath indicates full film conditions and shows a 14–22% reduction in friction coefficient for a thrust surface covered with the micro asperities. A model confirms the experimental trends and indicates the potential to further reduce the friction coefficient by about 60% through optimization of the asperity geometry and layout.

Topics: Bearings
Commentary by Dr. Valentin Fuster
GT2002-30290 pp. 581-587; (7 pages)

Preliminary measurements have been conducted to determine the effect of oil aeration on journal bearing performance. Oil aeration was observed to reduce the bearing load capacity and to increase the bearing stiffness. Also, the bearing damping capacity was improved significantly by oil aeration.

Commentary by Dr. Valentin Fuster
GT2002-30291 pp. 589-598; (10 pages)

Gas turbine engines and other high speed rotating machinery supported by magnetic bearings require some form of backup bearing to ensure reliable and safe operation. To date, this backup capability has been provided by either rolling element bearings or solid lubricated bushings. Both of these solutions have drawbacks — must notably limited life and uncertain dynamic performance. In many cases, the backup bearing system requires substantial maintenance following an activation event. An alternative approach investigated in this work is the use of a compliant foil bearing as a backup bearing. This work discusses tests of this concept on a test rig with a 63 kg rotor. In this application, the foil bearing demonstrated smooth, stable operation during a variety of simulated magnetic bearing failure events, and allowed for continued operation of the rotor following the simulated failures.

Commentary by Dr. Valentin Fuster
GT2002-30292 pp. 599-606; (8 pages)

During the normal operation of rotor/magnetic bearing systems, contacts with auxiliary bearings or bushes are avoided. However, auxiliary bearings are required under abnormal conditions and in malfunctions situations to prevent contact between the rotor and stator laminations. Studies in the open literature deal largely with rotor drop and the requirements of auxiliary bearing design parameters for safe run-down. Rotor drop occurs when the rotor is de-levitated and no further means of magnetic bearing control is available. This paper considers the case when full control is still available and rotor/auxiliary bearing contact has been induced by an abnormal operating condition or temporary fault. It is demonstrated that events leading to contact from a linearly stable rotor orbit can drive the rotor into a non-linear vibratory motion involving persistent contacts. Furthermore, the phase of the measured vibration response may be changed to such an extent that synchronous controllers designed to minimize rotor vibration amplitudes will worsen the rotor response, resulting in higher contact forces. A modified controller design is proposed and demonstrated to be capable of returning a rotor from a contacting to a non-contacting state.

Commentary by Dr. Valentin Fuster
GT2002-30293 pp. 607-614; (8 pages)

Recent developments improving load capacity foretell the practical implementation of Active Magnetic Bearings (AMB) on industrial level, pushed by the advantages of reduced wear and higher speeds that they make available. However, the possibility of an eventual power failure forces the use of back-up (catcher) bearings, which usually are of the ball bearing type. The back-up bearings present a clearance between the shaft and the inner race, such that there is not contact during normal operation. On a power failure or an emergency stop, the rotor is only supported by the catcher bearings. Thus, the rotor motion within the clearance results on a succession of impacts, contact and non-contact intervals producing a non-linear behavior of the system. The complexity of this non-linear behavior prevents the use of traditional methods for the design of the catcher bearings, calling for the need of extensive experimentation and previous experience in their dimensioning process. Here, the response of a rigid rotor, supported by a pivot on the drive side and a magnetic bearing on the other side, is measured during the emergency stop from several operating speeds. Non-linear analysis tools, such as Poincaré Maps and Bifurcation Diagrams, are employed to demonstrate the non-linear characteristics of the motion, which in some conditions is shown to become chaotic with a vibration limit cycle. The rotor motion on the catcher bearings (with the magnetic bearing deactivated) is measured at constant running speeds. The limit cycles and chaotic attractors are described, showing the relation of the non-linear effects to the rotational speed.

Commentary by Dr. Valentin Fuster
GT2002-30294 pp. 615-625; (11 pages)

The use of magnetic bearings in high speed/low friction applications is increasing in industry. Magnetic bearings are sophisticated electromechanical systems, and modeling magnetic bearings using standard techniques is complex and time consuming. In this work a Neural network is designed and trained to emulate the operation of a complete system (magnetic bearing, PID controller and power amplifiers). The neural network is simulated and integrated into a virtual instrument that will be used in the laboratory both as a teaching and a research tool. The main aims in this work are: 1-Determining the minimum amount of artificial neurons required in the neural network to emulate the magnetic bearing system. 2-Determining the more appropriate ANN training method for this application. 3-Determining the errors produced when a neural network trained to emulate system operation with a balanced rotor is used to predict system response when operating with an unbalanced rotor. The neural network is trained using as input the position data from the proximity sensors; neural network outputs are the control signals to the coil amplifiers.

Commentary by Dr. Valentin Fuster
GT2002-30295 pp. 627-633; (7 pages)

The beta distribution is a particularly convenient model for random variables when only the minimum, maximum and most likely values are available. It is also very useful for estimating the mean and standard deviation given this information. In this paper a simple method is proposed to estimate the beta parameters from these three values. The proposed method has advantages over the conventional approach. In the conventional approach, the four parameters of the beta distribution are determined from only three values by assuming a standard deviation that is one-sixth the range. In contrast, the new method assumes a value for one of the beta shape parameters based on an analogy with the normal distribution. This new approach allows for a very simple algebraic solution of the beta shape parameters in contrast to the simultaneous solution required by the conventional method. The results of the proposed method are very similar to the conventional method. However, the proposed method generally gives a slightly higher (more conservative) estimate of the standard deviation when the distribution is skewed. In addition, the new approach allows the standard deviation to vary as the shape or skew of the distribution varies. Both methods were applied to modeling the probability distribution of temperature.

Commentary by Dr. Valentin Fuster
GT2002-30296 pp. 635-641; (7 pages)

Land based gas turbines are utilized to provide electric power on a continuous basis or just to satisfy peak consumer demand for a short operating period during each day. A unit that is operating on a continuous basis will incur few start and stop thermal cycles, and is typically referred to running in a baseload mode (BL). However, a unit that is operating to meet daily peak loads will accumulate an increased number of start and stop thermal cycles, and is typically referred to as operating in a daily start and stop mode (DSS). Gas turbine units are also operated with a weekly start and stop (WSS) cycle due to a different customer power demand. Hot parts in engines running in BL, WSS and DSS accumulate different levels of fatigue damage because thermal cycles vary significantly within the same actual operating hour period; e.g., the DSS engine will accumulate the most fatigue damage during the same operating hour period. Material testing has shown that an interaction of creep and fatigue damage produces a reduction in the fatigue life of a component. Different criteria and maintenance scheduling philosophy are used within the industrial gas turbine industry to account for the interaction of the creep and fatigue damage in engines. The purpose of this paper is to document how a maintenance scheduling philosophy has been developed that takes into account actual differences in engine usage on the capability of gas turbine hot parts and how current maintenance schedule intervals are being revised for Mitsubishi’s customers.

Commentary by Dr. Valentin Fuster
GT2002-30297 pp. 643-649; (7 pages)

The load history that blade/disk contacts in jet engine attachment hardware are subject to can be very complex. Using Finite Element Method (FEM) to track changes in the contact tractions due to changing loads can be computationally very expensive. For 2D plane strain contact problems with friction involving similar/dissimilar isotropic materials, the contact tractions can be related to the initial gap function and the slip function using coupled Cauchy Singular Integral Equations (SIEs). The effect of load history on the contact tractions is illustrated by presenting results for an example fretting “mission”. For the case of dissimilar isotropic materials the misson results show the effect of the coupling between the shear traction and the contact pressure.

Topics: Stress
Commentary by Dr. Valentin Fuster
GT2002-30298 pp. 651-657; (7 pages)

To improve the reliability of compressor stator blades of gas turbines, an analytical method for estimating their fatigue damage has been developed. This method is based on blade-vibratory-stress analysis, stress-peak counting, and use of actual environmental data. The blade-vibratory-stress analysis takes the superposition of multi-peaks of the stress spectrum into account. The numerically simulated stress showed better agreement with measured stress than that from a conventional stress analysis, which is based on frequency-response analysis considering a single peak of the lowest single eigen-vibration-mode. A time-domain stress history was synthesized from the blade-vibratory-stress analysis results. Furthermore, the fatigue damage of the blade under rotating stall was estimated by the linear-damage-rule from the stress-peak counting of the stress and from material data. The estimated fatigue-damage agrees well with the measured results. This agreement means that our new fatigue-damage-estimation method is more accurate than the conventional one.

Commentary by Dr. Valentin Fuster
GT2002-30299 pp. 659-674; (16 pages)

Mating flanges of two rotating discs with dissimilar materials were stress analysed by 3D Finite Elements using a contact algorithm to simulate the relative motion between the flanges. All the hardware associated with the flanges (bolts, nuts, bushings and washers) were also modelled. The difference in radial growths of the two flanges, hereby called “slip”, produces high cyclic stresses in the bolt holes leading to Low Cycle Fatigue (LCF) and premature cracking. Design changes were studied where the slip was minimised by changing the thermal and centrifugal response of the mating flanges. In order to substantiate the LCF life of a rotating part, spin pit testing is carried out. In the spin pits available for testing the temperature remains constant while the speed is cycled. So in order to simulate the engine duty cycle which includes thermal stresses, the spin pit test conditions are modified. The procedure adopted in this case is explained in the paper.

Commentary by Dr. Valentin Fuster
GT2002-30300 pp. 675-682; (8 pages)

Single crystal nickel-base superalloys deform by shearing along <111> planes, sometimes referred to as “octahedral” slip planes. Under fatigue loading, cyclic stress produces alternating slip reversals on the critical slip systems which eventually results in fatigue crack initiation along the ‘critical’ octahedral planes. A ‘critical plane’ fatigue modeling approach was developed in the present study to analyze high cycle fatigue (HCF) failures in single crystal materials. This approach accounted for the effects of crystal orientation and the micromechanics of the deformation and slip mechanisms observed in single crystal materials. Three-dimensional (3-D) stress and strain transformation equations were developed to determine stresses and strains along the crystallographic octahedral planes and corresponding slip systems. These stresses and strains were then used to calculate several multiaxial critical plane parameters to determine the amount of fatigue damage and also the ‘critical planes’ along which HCF failures would initiate. The computed fatigue damage parameters were used along with experimentally measured fatigue lives, at 1100° F, to correlate the data for different loading orientations. Microscopic observations of the fracture surfaces were used to determine the actual octahedral plane (or facet) on which fatigue initiation occurred. X-ray diffraction measurements were then used to uniquely identify this damage initiation facet with respect to the crystal orientation in each specimen. These experimentally determined HCF initiation planes were compared with the analytically predicted ‘critical planes’.

Commentary by Dr. Valentin Fuster
GT2002-30301 pp. 683-690; (8 pages)

Whenever the operations and maintenance economics so dominate the operation of a particular power producer, engineering insight is the plant’s vanguard in maintaining and managing costs. To provide a basis for operators to confront the spiraling parts replacement costs that are so closely tied to the durability of the hot section, EPRI has developed the Gas Turbine Life Management Platform (LMP), an effort which has been directly supported by F-Class turbine operators. Tested and verified on the advanced 7FA and 9FA class of turbine designs, the LMP provides unprecedented detail on temperatures and stresses throughout a selected hot section component. In turn, the platform relies on explicit details of stress and temperature to track the damage due to creep, coating oxidation and TMF (thermal mechanical fatigue). Supplied with operating data, it graphically represents the recent practices of either base load or peaking service. The platform supplies the engineering data needed by operators to make informed decisions regarding the disposition (continued use, repair, or scrap) of these most advanced and expensive parts. In this paper, the procedures of predicting temperature/life and how well these predictions correlated with field observations will be discussed in detail.

Commentary by Dr. Valentin Fuster
GT2002-30302 pp. 691-697; (7 pages)

Expenditure of maintenance cost for gas turbines can be greatly reduced if the parts can be well managed during the service to avoid excessive fallout rate or unplanned forced outage. In order to achieve the optimum part life management plan, the ability to accurately predict the life capabilities and fallout rates of the life limiting components becomes very important. An empirical model for the stage 1 nozzle of F-class frame 9 GE gas turbine has been developed based on Weibull statistic analysis of measured crack dimensions at observed critical locations of the nozzle. The crack measurements were taken during the hot gas path inspections and data from several different machine units which had different operating histories were used to establish relationship between Weibull parameters and elapsed operation histories. The fallout rate as function operation history is predicted by performing a Monte Carlo analysis to account for the combined effects of multiple cracks.

Commentary by Dr. Valentin Fuster
GT2002-30303 pp. 699-704; (6 pages)

This paper summarizes recent enhancements to a probabilistic damage tolerance software code, DARWIN™, that can be used for design certification of aircraft jet engine titanium disks/rotors that may contain melt-related anomalies. Evaluations of DARWIN™ by engine manufacturers are also discussed, including comparisons with existing codes for accuracy and time efficiency. In addition, relevant test results, including various fatigue tests on material containing melt-related anomalies, are summarized.

Topics: Design , Rotors , Turbines , Aircraft
Commentary by Dr. Valentin Fuster
GT2002-30304 pp. 705-712; (8 pages)

Fatigue crack growth prediction methods using three dimensional finite element analyses were investigated to improve the predictability of part-through surface crack growth life. First, a direct analysis method of cyclic finite element analysis was adopted. Fatigue crack growth was predicted on a step by step basis from the Paris’ law using stress intensity factor range (ΔK) calculated by 3D-FEM. This method takes the procedure of cyclic operation of FE-analysis modeled with crack tip elements, crack growth increment calculation and remeshing of FE-model. Second, a method based on influence function method for ΔK calculation directly using 3D-FE-analysis result has been developed and applied. It was found that crack growth prediction based on step by step finite element method and the method based on the influence function method showed good correlation with the experimental results if Paris’ law coefficient C determined by CT specimen was appropriately used for semi-elliptical surface crack.

Commentary by Dr. Valentin Fuster
GT2002-30305 pp. 713-723; (11 pages)

The stress analysis of dovetail attachments presents some challenges. These stem from the high stress gradients at the edges of contact. They also stem from the nonlinearities accompanying conforming contact. Even with two-dimensional analysis, obtaining converged peak stresses is not trivial. With three-dimensional analysis, convergence can be expected to be more difficult to achieve because of the added computational costs of refinement in three dimensions. To meet these challenges, this paper describes a submodeling procedure with finite elements. The submodeling approach features bicubic surface fits to displacements for submodel boundary conditions. The approach also features a means of verifying these boundary conditions have converged: This is crucial to obtaining accurate converged peak stresses. The approach is applied to a three-dimensional test piece used to simulate a dovetail attachment. This application leads to converged three-dimensional stresses. These stresses serve to quantify the sort of increases in contact stresses in attachments due to three-dimensional effects.

Commentary by Dr. Valentin Fuster
GT2002-30306 pp. 725-733; (9 pages)

Optimum placements of the strain gauges assure reliable vibration measurements of structural components such as rotating blades. Within the framework of cyclic vibration theory, a novel approach has been developed for computation of the optimum gauge positions on tuned bladed discs regarding the determined sensitivity, orthogonality, gradient and distance criteria. The utilized genetic algorithm optimization tool allows for an effective numerical search of suitable solutions of the defined optimization function. A rotating impeller disc represented by a cyclic finite element model demonstrates the application of this method. The present technique can be easily applied to other structural components requiring optimal strain gauge instrumentation.

Topics: Strain gages
Commentary by Dr. Valentin Fuster
GT2002-30307 pp. 735-744; (10 pages)

The paper presents the formulation to compute numerically the unsteady aerodynamic forces on the vibrating annular cascade blades. The formulation is based on the finite volume method, the type, and the TVD scheme, following the UPACS code developed by NAL, Japan. By applying the TVD scheme to the linear unsteady calculations, the precise calculation of the peak of unsteady aerodynamic forces at the shock wave location like the delta function singularity becomes possible without empirical constants. As a further feature of the present paper, results of the present numerical calculation are compared with those of the double linearization theory (DLT), which assumes small unsteady and steady disturbances but the unsteady disturbances are much smaller than the steady disturbances. Since DLT requires far less computational resources than the present numerical calculation, the validation of DLT is quite important from the engineering point of view. Under the conditions of small steady disturbances, a good agreement between these two results is observed, so that the two codes are cross-validated. The comparison also reveals the limitation on the applicability of DLT.

Commentary by Dr. Valentin Fuster
GT2002-30308 pp. 745-754; (10 pages)

A combustor liner was computationally simulated and probabilistically evaluated in view of the several uncertainties in the aerodynamic, structural, material and thermal variables that govern the combustor liner. The interconnection between the computational fluid dynamics code and the finite element structural analysis codes was necessary to couple the thermal profiles with structural design. The stresses and their variations were evaluated at critical points on the liner. Cumulative distribution functions and sensitivity factors were computed for stress responses due to the aerodynamic, mechanical and thermal random variables. It was observed that the inlet and exit temperatures have a lot of influence on the hoop stress. For prescribed values of inlet and exit temperatures, the Reynolds number of the flow, coefficient of thermal expansion, gas emissivity and absorptivity and thermal conductivity of the material have about the same impact on the hoop stress. These results can be used to quickly identify the most critical design variables in order to optimize the design and make it cost effective.

Commentary by Dr. Valentin Fuster
GT2002-30310 pp. 755-763; (9 pages)

This paper presents some numerical parametric studies of the multi-row interaction mechanisms for a one and half stage (NGV-rotor-stator) transonic turbine. Firstly both steady and unsteady flows under the nominal operating condition for this turbine have been validated against the experimental data available. The sub-harmonic interaction induced by the two fundamental passing frequencies from the upstream and downstream vanes has been identified in the rotor row. But more significant is an aperiodic unsteady flow pattern characterized by variable amplitudes and inter-blade phase angles in the downstream stator row. Although the time-averaged blade forces only vary by about 5%, the maximum unsteady force can be changed by factor of three among stator blades. The parametric studies have revealed a strong dependence of the aperiodic flow behavior on blade count ratio between the NGV and the stator. The spatial mode of the unsteadiness amplitude variation is shown to correspond exactly to the spatial wavelength due to the NGV-stator interference. The longer the spatial NGV-stator interference wavelength, the larger the aperiodic unsteady loading variation. Given that the spatial mode amplifies the unsteady loading aperiodically on the stator, the present results suggest that the choice of stator-stator relative blade counts may be used to limit the maximum unsteady force on the downstream stator.

Topics: Turbines , Blades
Commentary by Dr. Valentin Fuster
GT2002-30311 pp. 765-774; (10 pages)

Numerical calculations of the 3D transonic flow of an ideal gas through turbomachinery blade rows moving relatively one to another with taking into account the blades oscillations is presented. The approach is based on the solution of the coupled aerodynamic-structure problem for the 3D flow through the turbine stage in which fluid and dynamic equations are integrated simultaneously in time, thus providing the correct formulation of a coupled problem, as the blades oscillations and loads, acting on the blades, are a part of solution. An ideal gas flow through the mutually moving stator and rotor blades with periodicity on the whole annulus is described by the unsteady Euler conservation equations, which are integrated using the explicit monotonous finite-volume difference scheme of Godunov-Kolgan and moving hybrid H-H grid. The structure analysis uses the modal approach and 3D finite element model of a blade. The blade motion is assumed to be constituted as a linear combination of the first natural modes of blade oscillations with the modal coefficients depending on time. The algorithm proposed allows to calculate turbine stages with an arbitrary pitch ratio of stator and rotor blades, taking into account the blade oscillations by action of unsteady loads caused both outer flow nonuniformity and blades motion. There has been performed the calculation for the stage of the turbine with rotor blades of 0.765 m. The numerical results for unsteady aerodynamic forces due to stator-rotor interaction are compared with results obtained with taking into account the blades oscillations.

Commentary by Dr. Valentin Fuster
GT2002-30312 pp. 775-786; (12 pages)

An extensive set of unsteady pressure data was acquired along the midspan of a modern transonic fan blade for simulated flutter conditions. The data set was acquired in a nine-blade linear cascade with an oscillating middle blade to provide a database for the influence coefficient method to calculate instantaneous blade loadings. The cascade was set for an incidence of 10 dg. The data were acquired on three stationary blades on each side of the middle blade that was oscillated at an amplitude of 0.6 dg. The matrix of test conditions covered inlet Mach numbers of 0.5, 0.8, and 1.1 and the oscillation frequencies of 200, 300, 400, and 500 Hz. A simple quasi-unsteady two-dimensional computer simulation was developed to aid in the running of the experimental program. For high Mach number subsonic inlet flows the blade pressures exhibit very strong, low-frequency, self-induced oscillations even without forced blade oscillations, while for low subsonic and supersonic inlet Mach numbers the blade pressure unsteadiness is quite low. The amplitude of forced pressure fluctuations on neighboring stationary blades strongly depends on the inlet Mach number and forcing frequency. The flowfield behavior is believed to be governed by strong nonlinear effects, due to a combination of viscosity, compressibility, and unsteadiness. Therefore, the validity of the quasi-unsteady simplified computer simulation is limited to conditions when the flowfield is behaving in a linear, steady manner. Finally, an extensive set of unsteady pressure data was acquired to help development and verification of computer codes for blade flutter effects.

Commentary by Dr. Valentin Fuster
GT2002-30313 pp. 787-794; (8 pages)

The present study demonstrates the capabilities of a fluid/structure coupled computational approach which consists of an unsteady three-dimensional Navier-Stokes flow solver, TFLO, and a finite element structural analysis package, MSC/NASTRAN. The parallelized flow solver relies on a multi-block cell-centered finite volume discretization and the dual time stepping time integration scheme with multigrid for convergence acceleration. High accuracy is pursued with respect to load transfer, deformation tracking and synchronization between the two disciplines. As a result, the program successfully predicts the aeroelastic responses of a high performance fan, NASA Rotor 67, over a range of operational conditions. The results show that the unsteady pressure generated at the shock may act to damp or excite the blade motion mainly depending on the inter-blade phase angle. It is concluded that the level of sophistication in the individually sophisticated disciplines together with an accurate coupling interface will allow for accurate prediction of flutter boundaries of turbomachinery components.

Commentary by Dr. Valentin Fuster
GT2002-30314 pp. 795-801; (7 pages)

For investigating the dynamic response of a hydrodynamic thrust bearing-mounted flexible rotor, the dynamic characteristic data of thrust bearings for high surface velocities are applied for constructing the equation of motion for the rotor system, which is modeled with the finite element method (FEM). Based on the short bearing approximation and the π film cavitation model of the nonlinear oil-film forces, the dynamic responses are investigated using direct numerical integration with a variable order solver based on the numerical differentiation formulas (NDFs). Harmonic, quasi-periodic and chaotic motions are predicted for a range of spin speeds of the rotor. Poincaré maps of predicted rotor trajectories are also examined. It shows that spin speeds of the rotor and the oil film force coefficient might promote undesirable non-synchronous vibrations.

Commentary by Dr. Valentin Fuster
GT2002-30315 pp. 803-810; (8 pages)

It is very common for aircraft engines to have dual rotor or even triple rotor designs. Due to the complexity of having multiple rotor design, the transfer matrix methods have used in the past to deal with multiple rotor-bearing systems. However, due to transfer matrix method’s assumptions, sometimes resulted in numerical stability problems or root-missing problems. The purpose of this paper is to develop a systematic theoretical analysis of the dynamic characteristics of turbomachinery dual rotor-bearing systems. This dual rotor-bearing system analysis will start with a finite element (FEM) rotor-bearing system dynamic model, then using different methods to verify the analysis results including critical speed map and bearing stiffness. In an inertia coordinate system, a general model of continuous dual rotor-bearing systems is established based on a lagrangian formulation. Gyroscopic moment, rotary inertia, bending and shear deformations have been included in the model. From a point of view of the systematic approach, a solution of the finite element method is used to calculate the critical speeds by several different methods, which in turn can help to verify this dual rotor-bearing system approach. The effects of the speed ratio of dual rotors on the critical speed will be studied, which in turn can be used as one of the dual rotor design parameters. Also, both critical speeds are in effect functions of dual rotor speeds. Finally, the bearing stiffness between high speed and low speed shafts not only affect the critical speeds of the dual rotor system, but also affect the mode shapes of the system. Therefore, the bearing stiffness in between is of even greater importance in turbomachinery dual rotor or multiple rotor design.

Commentary by Dr. Valentin Fuster
GT2002-30316 pp. 811-817; (7 pages)

A microturbine of 12-pound thrust was developed for the Unmanned Aerial Vehicle (UAV) applications. Recent tests of the microturbines reveal problems associated with rear ball bearing integrity after extended run times. The microturbine rotor design originally calls for a critical speed margin of at least 15∼20% to prevent excessive vibrations. However, the microturbine was using an existing turbocharger rotor component with unknown margins. Therefore, the purpose of this paper is to perform both theoretical and experimental analyses of the dynamic characteristics of the 12-pound thrust microturbine rotor-bearing system. This rotor-bearing system analyses will start with a finite element (FEM) rotor-bearing system dynamic model, then using modal testing and dynamic engine test to verify the analysis results including critical speed map and bearing stiffness. In this paper, the rotor-bearing system dynamic model will be established under an inertia coordinate system. Through finite element method, this model can be used to predict natural frequencies, critical speed map, and bearing stiffness. Also, under free-free condition, a modal testing will be performed, and its results are used to compare with the FEM model. Then the gyroscopic moment effects are included in the FEM model to calculate the critical speed map. Finally the critical speed map is used to compare with the results of the dynamic experiments of the 12-pound thrust microturbine engine and the bearing stiffness is estimated through an optimization approach. Examination of the microturbine engine and recent product developments indicate that thrust performance and engine life goals can be improved to upgrade the present design. With the rotor-bearing system analysis, the goal of increasing the current engine life and improved performance is sought as a practical goal for the microturbine design.

Commentary by Dr. Valentin Fuster
GT2002-30317 pp. 819-826; (8 pages)

Squeeze film dampers (SFDs) aid to attenuate vibrations in compressors and turbines while traversing critical speeds. In actual applications, gas ingestion from the environment may lead to the formation of a foamy lubricant that degrades the rotor/bearing system dynamic performance. Impact and imbalance response tests conducted on a rigid rotor supported on SFDs, and aimed to emulate the pervasive effect of air ingestion into the damper film lands, are reported. Two types of squeeze film damper support the test rotor, one is a conventional cylindrical design with a squirrel cage type elastic support, and the other is a compact four-pad damper with integral wire EDM elastic supports. Both dampers have identical diameter and radial clearance. Controlled (air in oil) mixtures ranging from pure oil to all air conditions are supplied to the SFDs, and measurements of the transient rotor response to calibrated impact loads are conducted. System damping coefficients, identified from acceleration/load transfer functions, decrease steadily as the air content in the mixture increases. However, measurements of the rotor synchronous imbalance response conducted with a lubricant bubbly mixture (50% air volume) show little difference with test results obtained with pure lubricant supplied to the dampers. The experimental results show that air entrainment is process and device dependent, and that small amounts of lubricant enable the effective action of SFDs when the rotor traverses a critical speed.

Commentary by Dr. Valentin Fuster
GT2002-30318 pp. 827-836; (10 pages)

The traditional 8-coefficient bearing model, used in linear rotor dynamics, is shown here to be inadequate for the unbalance response calculation of rotor systems supported on hydrodynamic journal bearings placed close to nodal points of excited modes of vibration. In such situations, one cannot neglect the time varying tilt angle between journals and bearings, whose consideration leads to the adoption of a 32-coefficient bearing model. Numerical results indicate that the differences between vibration amplitudes calculated using both bearing models can be greater than 100%, while discrepancies in the predicted stability thresholds are small. The conclusions of the study are coherent with previously published theoretical and experimental results.

Topics: Bearings , Rotors , Vibration
Commentary by Dr. Valentin Fuster
GT2002-30319 pp. 837-844; (8 pages)

This paper describes the calculation of flutter stability characteristics for a transonic forward swept fan configuration using a viscous aeroelastic analysis program. Unsteady Navier-Stokes equations are solved on a dynamically deforming, body fitted, grid to obtain the aeroelastic characteristics using the energy exchange method. The non-zero inter-blade phase angle is modeled using phase-lagged boundary conditions. Results obtained show good correlation with measurements. It is found that the location of shock and variation of shock strength strongly influenced stability. Also, outboard stations primarily contributed to stability characteristics. Results demonstrate that changes in blade shape impact the calculated aerodynamic damping, indicating importance of using accurate blade operating shape under centrifugal and steady aerodynamic loading for flutter prediction. It was found that the calculated aerodynamic damping was relatively insensitive to variation in natural frequency.

Commentary by Dr. Valentin Fuster
GT2002-30320 pp. 845-851; (7 pages)

The ability to predict levels of unsteady forcing on high-pressure turbine blades is critical to avoid high-cycle fatigue failures. In this study, 3D time-resolved computational fluid dynamics is used within the design cycle to predict accurately the levels of unsteady forcing on a single-stage high-pressure turbine blade. Further, nozzle-guide-vane geometry changes including asymmetric circumferential spacing and suction-side modification are considered and rigorously analyzed to reduce levels of unsteady blade forcing. The latter is ultimately implemented in a development engine, and it is shown successfully to reduce resonant stresses on the blade. This investigation builds upon data that was recently obtained in a full-scale, transonic turbine rig to validate a Reynolds-Averaged Navier-Stokes (RANS) flow solver for the prediction of both the magnitude and phase of unsteady forcing in a single-stage HPT and the lessons learned in that study.

Commentary by Dr. Valentin Fuster
GT2002-30321 pp. 853-860; (8 pages)

This work provides an accurate and efficient numerical method for turbomachinery flutter. The unsteady Euler or Reynolds-averaged Navier–Stokes (RANS) equations are solved in integral form, the blade passages being discretised using a background fixed C-grid and a body-fitted C-grid moving with the blade. In the overlapping region data are exchanged between the two grids at every time step, using bilinear interpolation. The method employs Roe’s second-order-accurate flux difference splitting scheme for the inviscid fluxes, a standard second-order discretisation of the viscous terms, and a three-level backward difference formula for the time derivatives. The state-of-the-art second-order accuracy of numerical methods for unsteady compressible flows with shocks is thus carried over, for the first time to the authors knowledge, to flutter computations. The dual time stepping technique is used to evaluate the nonlinear residual at each time step, thus extending to turbomachinery aeroelasticity the state-of-the-art efficiency of unsteady RANS solvers. The code is proven to be accurate and efficient by computing the 4th Aeroelastic Standard Configuration, namely, the subsonic flow through a turbine cascade with flutter instability in the first bending mode, where viscous effect are found practically negligible. Then, the very severe 11th Aeroelastic Standard Configuration is computed, namely, the transonic flow through a turbine cascade at off-design conditions, where the turbulence model is found to be the critical feature of the method.

Commentary by Dr. Valentin Fuster
GT2002-30322 pp. 861-874; (14 pages)

Forced vibrations in turbomachinery components can cause blades to crack or fail due to high-cycle fatigue. Such forced response problems will become more pronounced in newer engines with higher pressure ratios and smaller axial gap between blade rows. An accurate numerical prediction of the unsteady aerodynamics phenomena that cause resonant forced vibrations is increasingly important to designers. Validation of the computational fluid dynamics (CFD) codes used to model the unsteady aerodynamic excitations is necessary before these codes can be used with confidence. Recently published benchmark data, including unsteady pressures and vibratory strains, for a high-pressure turbine stage makes such code validation possible. In the present work, a three dimensional, unsteady, multi blade-row, Reynolds-Averaged Navier Stokes code is applied to a turbine stage that was recently tested in a short duration test facility. Two configurations with three operating conditions corresponding to modes 2, 3, and 4 crossings on the Campbell diagram are analyzed. Unsteady pressures on the rotor surface are compared with data.

Commentary by Dr. Valentin Fuster
GT2002-30323 pp. 875-889; (15 pages)

This paper focuses on the determination and study of the maximum amplification of the steady state forced response of bladed disks due to mistuning. First, an optimization strategy is proposed in which partially mistuned bladed disks are considered as physical approximations of the worst case disk and the mistuned properties are sought to maximize the response of a specific blade. This approach is exemplified on both a reduced order model of a blisk and a single-degree-of-freedom per blade disk model an extensive parametric study of which is conducted with respect to blade-to-blade coupling, damping, and engine order. A mode shape-based formulation of the amplification factor is then developed to clarify the findings of the parametric study in the strong coupling/small damping limit. In this process, the upper bound of Whitehead is recovered for all engine orders and number of blades and the conditions under which this limit is exactly achieved or closely approached are clarified. This process also uncovers a simple yet reliable approximation of the resonant mode shapes and natural frequencies of the worst disk.

Topics: Disks , Blades , Shapes
Commentary by Dr. Valentin Fuster
GT2002-30324 pp. 891-898; (8 pages)

Aim of the present work is the validation against experimental results of the dynamic calculation of a vane segment with two friction contacts. The segment is in contact with two adjacent supports as in the experimental setup [1]. An empirical friction model derived from the experiments is introduced. The calculation has been performed for different interlocking values (different values of force normal to the contact surfaces). An analysis about the dependence of the parameters of the adopted empirical contact model on the hysteresis cycle shapes has been performed.

Commentary by Dr. Valentin Fuster
GT2002-30325 pp. 899-908; (10 pages)

An analytical formulation for the vectors of contact forces and the stiffness matrix of the non-linear friction contact interface is developed for the analysis of multi-harmonic vibrations in the frequency domain. The contact interface elements provided here an exact description of friction and unilateral contact forces at the interacting surfaces, taking into account the influence of the variable normal load on the friction forces, including the extreme cases of separation of the two surfaces. Initial gaps and interferences at the contact nodes, which affect the normal force, as well as the unilateral action of the normal force at the contact surface, are all included in the model. The accurate calculation of the force vector and the tangent stiffness matrix provides a very reliable and fast convergence of the iteration process used in the search for the amplitudes of nonlinear vibrations of bladed discs. Numerical investigations demonstrate excellent performance with respect to speed, accuracy and stability of computation.

Topics: Friction , Vibration , Disks
Commentary by Dr. Valentin Fuster
GT2002-30420 pp. 909-916; (8 pages)

The reliable estimation of a flexible foundation model and the state of unbalance (both amplitude and phase) of a turbogenerator from machine run-down measured vibration data is an active research area. Earlier studies on the estimation of both these quantities used the whole frequency range of the run-down as a single band. However, such an identification may be inaccurate for large flexible foundations having many modes in the run-down frequency range. For reliable identification, the whole frequency range has to be divided into a number of frequency bands and the frequency dependent foundation models have to be estimated together with the unbalance. This paper combines the unbalance estimation with the split frequency range for the foundation model, and the highlights the limitations observed during estimation of foundation models and state of unbalance. Having established the method in simulation, experimental data from a 3 m long test rig, with four journal bearings, is used to test the method. The robustness of the unbalance estimates to a number of errors in the bearing and shaft model is demonstrated. The approach seems to give reliable estimates of the machine unbalance.

Topics: Machinery
Commentary by Dr. Valentin Fuster
GT2002-30421 pp. 917-924; (8 pages)

The dependence of the vibration characteristics of gas turbine engines on the rotor speeds becomes highly complicated in engines with two and three rotors, both because of the simultaneous dynamic action of the multiple rotors and the ambiguous relationships between their speeds. In this paper, the gas turbine engine is analyzed in the context of the theory of non-linear oscillation — as a complex system comprising a large number of non-linear elements and multiple periodical forces of different frequencies (defined by the rotor speeds). This paper presents results, which indicate that the level of vibration can obtain critical values at certain relationships between the rotor speeds. As a practical application of this phenomena it is shown that the number of three-spool engines returns from the aircraft to the engine manufacturer, due to different kinds of malfunctions, for example due to activation of the “intensified vibration” alarm, may be approximately three times that of returns of analogous two-rotor engines.

Commentary by Dr. Valentin Fuster
GT2002-30422 pp. 925-932; (8 pages)

Analysis to predict whirl instability of a whole engine, including flexible bladed disks, due to both linear and nonlinear destabilizing mechanisms at any speed, not only the critical, is advanced. The basis is the previously published, and now extended, equivalent two-dimensional mechanism (E2dM). Proven through physical logic are the principles: (1) a deflection or slope with a phase angle greater than 90 deg lead in response to an external disturbing force or moment respectively signals instability; (2) response of a system absorbing energy is modulated at speeds below onset of instability. Tactics to exploit these principles through fundamental analyses of response are devised.

Topics: Engines , Whirls , Mechanisms
Commentary by Dr. Valentin Fuster
GT2002-30423 pp. 933-942; (10 pages)

The eigenvalue problem of an orthotropy composite blade is formulated by employing the differential quadrature method (DQM). The Euler-Bernoulli beam model is used to characterize the orthotropy composite blade. The differential quadrature method is used to transform the partial differential equations of an orthotropy composite blade into a discrete eigenvalue problem. The Chebyshev-Gauss-Lobatto sample point equation is used to select the sample points. In this study, the effects of the fiber orientation, internal damping, external damping, inclined angle and the rotation speed on the eigen solutions for an orthotropy composite blade are investigated. The effect of the number of sample points on the accuracy of the calculated natural frequencies is also discussed. The integrity and computational efficiency of the DQM in this problem will be demonstrated through a number of case studies. Numerical results indicated that the differential quadrature method is valid and efficient for an orthotropy composite blade formulation.

Commentary by Dr. Valentin Fuster
GT2002-30424 pp. 943-952; (10 pages)

It is currently a major challenge for aeroengines manufacturer to be able to predict early in the design process the dynamic response of bladed disk. To guaranty a good accuracy of prediction, it is necessary to define properly the excitation (unsteady aerodynamics) and to take into account some phenomenon such as mistuning. This paper proposes an application of Snecma prediction method for mistune forced response on an experimental test case. The method used is a component modes synthesis method similar to the one proposed by Castanier and Pierre in 1997 [1] and validated against experiment in [2]. Some improvement have been performed to take into account more accurately the centrifugal forces effects in the projection basis and to couple the method with unsteady Computational Fluids Dynamic (CFD) codes. It is now possible to use this method in an industrial process. The method is applied to a HP turbine representative case, for which experimental results are available. These experimental results have been obtained in a European Community funded project dedicated to forced response study [3]. Mistuning effects have been measured. Moreover, a full characterization, of unsteady aerodynamics, aeroelastic and structural dynamics aspects have been performed. The results obtained with the proposed method are then compared to the experimental one. This application shows the consistency of the method and its efficiency.

Topics: Turbines
Commentary by Dr. Valentin Fuster
GT2002-30425 pp. 953-964; (12 pages)

A new reduced order model of mistuned bladed disk vibration is presented. This new approach is shown to accurately represent the response of real turbine geometries when only a single family of modes is excited. Yet its mathematical form is even simpler than that of a mass-spring model. Because it requires only minimal input data, this model is much easier to use than previous reduced order methods. Furthermore, its simplicity allows the fundamental parameters that control mistuning to be readily identified.

Commentary by Dr. Valentin Fuster
GT2002-30426 pp. 965-980; (16 pages)

A method is presented for obtaining maximum bladed disk forced response from distortion of a structural mode. It is shown that maximum response from mode distortion in a bladed disk occurs when the harmonic components of a distorted mode superimpose in a certain manner, causing localization of the mode and strong response in a particular blade. In addition, it is shown that the response of an intentionally mistuned system with maximum response does not change significantly when small random mistuning is added to the system. A method is described for calculating the structural mistuning necessary to obtain the distorted mode that gives maximum response. The theory is validated numerically.

Topics: Disks
Commentary by Dr. Valentin Fuster
GT2002-30427 pp. 981-989; (9 pages)

In turbomachinery one major problem is still the calculation and the optimization of the spatial vibrations of mistuned bladed disk assemblies with friction contacts. Friction contacts are widely used to reduce dynamic stresses in turbine blades. Due to dry friction and the relative motion of the contact planes energy is dissipated. This effect results in a reduction of blade vibration amplitudes. In the case of a tuned bladed disk cyclic boundary conditions can be used for the calculation of the dynamic response. For a mistuned bladed disk the complete system has to be modeled and simulated. To reduce the computation time the so-called substructure method is applied. This method is based on the modal description of each substructure, especially disk and blades, combined with a reduction of the degrees of freedom, to describe the dynamics of each component. The spatial dynamical behavior of each component is considered and described by the mode shapes, natural frequencies and modal damping ratios. Using the Harmonic Balance Method the nonlinear friction forces can be linearized. From here it is possible to calculate the frequency response functions of a mistuned bladed disk assembly with friction contacts. In many cases Monte-Carlo simulations are used to find regions, where the system response is sensitive to parameter uncertainties like the natural frequencies of the blades. These simulations require a large computation time. Therefore, an approximate method is developed to calculate the envelopes of the frequency response functions for statistically varying natural frequencies of the blades. This method is based on a sensitivity analysis and the Weibull-distribution of the vibration amplitudes. From here, a measure for the strength of localization for mistuned cyclic systems is derived. Regions, where localization can occur with a high probability, can be calculated by this method. The mean value and the standard deviation of the vibration amplitudes are calculated by simulation and by the approximate method. The comparisons between the approximate method and the Monte-Carlo simulations show a good agreement. Therefore, applying this method leads to remarkable reduction of computation time and gives a quick insight into the system behavior. The approximate method can also be applied to systems, that include the elasticity of the disk and/or the coupling by shrouds or other friction devices.

Topics: Friction , Disks
Commentary by Dr. Valentin Fuster
GT2002-30429 pp. 991-1002; (12 pages)

The blades of rotating compressor or turbine disks are subjected to fluctuating fluid forces that cause blade vibrations. To avoid high resonance stresses, in many applications additional damping is introduced into the bladed disk assembly by means of friction damping devices such as underplatform dampers. These are mounted between adjacent turbine blades and pressed onto the platforms due to centrifugal forces to dissipate energy by the generated friction forces due to relative motions between the damper and the neighboring blades. In real turbomachinery applications, the rotating blades are subjected to spatial vibrations caused by a complex blade geometry and distributed excitation forces acting on the airfoil. Therefore, a spatial model is presented including an appropriate spatial contact model to predict the generalized contact forces acting between the damper and the blades accurately. Six degrees of freedom are considered for each contact between the damper and the respective neighboring blades. Roughness effects are considered that determine the real contact area with respect to the nominal contact area. Different spatial blade vibration modes are investigated with regard to the friction damping that is provided by the underplatform damper. To gain the maximum damping effect, the damper mass is optimized at different working conditions of the assembly like the excitation amplitudes and the engine order. Furthermore, the influence of the contact geometry upon the damping potential is investigated in detail including the damper as well as the blade platform geometry. In practice, different damper geometries are in operation. Studies will be presented that prove the capability of the developed model to compare the effectiveness of different damper and blade platform geometries. Asymmetric platform angles leading to different contact conditions at the left and right damper contact, respectively, are studied in detail to improve the damping effect.

Commentary by Dr. Valentin Fuster
GT2002-30430 pp. 1003-1014; (12 pages)

In this paper, responses of a mistuned bladed disk assembly are examined and compared for three types of excitations: uncorrelated narrow band random excitations, correlated narrow band random excitations and sinusoidal excitations with unknown (time-invariant and random) amplitudes. Analytical techniques are also developed to compute the statistics of responses for these types of excitations. Effects of correlations of narrow band excitations are investigated in details. It has been found that the response statistics for correlated narrow band random excitations can be viewed in terms of the concepts related to the response to a deterministic engine order excitation.

Topics: Manufacturing , Disks
Commentary by Dr. Valentin Fuster
GT2002-30431 pp. 1015-1024; (10 pages)

This paper addresses the topic of machinery management by describing what it is, how it is performed, and what benefits those who employ machinery management practices can expect to derive. The practice has evolved since its inception in the 1960s, and a historical perspective is provided showing the roots of machinery management practices, its growth to currently adopted practices, and the limitations of such current practices. A new method of machinery management is presented, using currently available off-the-shelf products, and the advantages of this new method are shown. Machinery management is also placed within the broader context of Plant Asset Management, an emerging focus in the process industries to obtain competitive advantage through an integration of condition monitoring, maintenance management, and reliability management practices applied to all plant production assets, not just machinery.

Topics: Machinery
Commentary by Dr. Valentin Fuster
GT2002-30450 pp. 1025-1033; (9 pages)

This paper presents a study of the blade pressure perturbation levels and the resulting rotor blade force in three high-pressure transonic turbine stages, based on three-dimensional unsteady viscous computations. The aim is to identify stage characteristics that correlate with the perturbation strength and degree of force realization on the rotor blades. To address the effects of off-design operation, the computations were performed at high subsonic, design and higher vane exit Mach number operating conditions. Furthermore spanwise variations in pressure levels and blade force are addressed. In our investigation the RMS of the pressure perturbations integrated in both time and along the blade surface is utilized as a global measure of the blade pressure perturbation strength on the rotor blade surface. The relative strength of the different pressure perturbation events on the rotor blade surface is also investigated. To obtain information about the relative strength of events related to the blade passing frequency the pressure field is Fourier decomposed in time at different radial positions along the blade arc-length. With the help of the observations and results from the blade pressure study, the radial variations of the unsteady blade force are addressed.

Topics: Turbines , Blades , Mechanisms
Commentary by Dr. Valentin Fuster
GT2002-30451 pp. 1035-1046; (12 pages)

The effect of the finite extent of linear cascades on the unsteady pressure distribution of vibrating blades is assessed by means of a numerical study. The span of a reference cascade made up of flat plates has been changed to investigate its influence on the computed influence coefficients. It is concluded that the number of passages required to match a solution obtained with a traveling-wave mode strongly depends on the inter-blade phase angle under consideration and that existing linear vibrating cascade facilities have a marginal resolution to accurately match CFD analysis that assume that the blade is vibrating in a traveling-wave mode.

Commentary by Dr. Valentin Fuster
GT2002-30452 pp. 1047-1056; (10 pages)

The excitability of single rotor blade mode shapes due to the excitations by upstream stators in high-pressure turbine stages is subject of the present work. An evaluation of unsteady aerodynamic analyses of the stator-rotor interaction towards their sensitivity to the rotor blade mode shape is presented and applied. The unsteady aerodynamic analyses were performed at midspan sections with a well validated 2D/Q3D hybrid Euler/Navier Stokes non-linear flow solver (UNSFLO). The mode shape is parametrized by a torsion axis location in the plane of the blade section, which allows the construction of excitability maps as a function of 2D rigid body mode shapes. Excitability itself is derived from a generalized force analysis. The evaluation demonstrates the high sensitivity of excitability to the mode shape, which suggests that only small modifications in mode shape can significantly change the risk of blade mode excitation. It also highlights the central importance of the relative phase of unsteady blade pressure harmonic. Changes in axial gap can significantly modify the excitability and transform highly excited modes to less excited modes and vice versa. The variation of rotational speed (−5% to +10%) did not show remarkable changes in the mode excitability of the investigated rotor.

Commentary by Dr. Valentin Fuster
GT2002-30453 pp. 1057-1064; (8 pages)

An experimental investigation was conducted on a single stage high pressure turbine in order to gain a deeper unterstanding of turbine blade forced response. In particular the main objective of this experiment was to obtain good quality validation data for the prediction methods used by major engine manufacturers. The stage investigated consists of an uncooled nozzle guide vane (NGV) and a rotor with 64 blades. To study the complete forced response problem a so called Flexible Rotor was designed and manufactured. This rotor has three modes of interest in the operating range of the stage: first torsion, second flap and second edge. The design of the experiment was supported by detailed CFD and structural analysis. The mechanical behavior of the Flexible Rotor is well known. In order to identify all interesting modes all blades are equipped with strain gauges individually calibrated. To check the unsteady pressures 18 unsteady pressure transducers were mounted at midspan. This paper deals with experiments only with the Flexible Rotor. Forced response results are presented for the first torsion mode at two different pressure ratios. The results obtained show a large scatter for the maximum response amplitudes at each pressure ratio. The distribution of the amplitudes around the disk is controlled by the mechanical properties of the rotor.

Commentary by Dr. Valentin Fuster
GT2002-30454 pp. 1065-1075; (11 pages)

This paper focuses on the formulation and validation of an automatic strategy for the selection of the locations and directions of strain gauges to capture at best the modal response of a blade in a series of modes. These locations and directions are selected to render the strain measurements as robust as possible with respect to random mispositioning of the gauges and gauge failures. The approach relies on the evaluation of the signal-to-noise ratios of the gauge measurements from finite element strain data and includes the effects of gauge size. A genetic algorithm is used to find the strain gauge locations-directions that lead to the largest possible value of the smallest modal strain signal-to-noise ratio, in the absence of gauge failure, or of its expected value when gauge failure is possible. A fan blade is used to exemplify the applicability of the proposed methodology and to demonstrate the effects of the essential parameters of the problem, i.e. the mispositioning level, the probability of gauge failure, and the number of gauges.

Topics: Blades , Strain gages
Commentary by Dr. Valentin Fuster
GT2002-30488 pp. 1077-1091; (15 pages)

This paper presents a first principles-based model of the fluid-induced forces acting on the rotor of an axial compressor. These forces are primarily associated with the presence of a nonuniform flow field around the rotor, such as that produced by a rotor tip clearance asymmetry. Simple, analytical expressions for the forces as functions of basic flow field quantities are obtained. These expressions allow an intuitive understanding of the nature of the forces and—when combined with a rudimentary model of an axial compressor flow field (the Moore-Greitzer model)—enable computation of the forces as a function of compressor geometry, torque and pressure-rise characteristics, and operating point. The forces predicted by the model are also compared to recently published measurements and more complex analytical models, and are found to be in reasonable agreement. The model elucidates that the fluid-induced forces comprise three main contributions: fluid turning in the rotor blades, pressure distribution around the rotor, and unsteady momentum storage within the rotor. The model also confirms recent efforts in that the orientation of fluid-induced forces is locked to the flow nonuniformity, not to tip clearance asymmetry as is traditionally assumed. The turning and pressure force contributions are shown to be of comparable magnitudes—and therefore of equal importance—for operating points between the design point and the peak of the compressor characteristic. Within this operating range, both “forward” and “backward” rotor whirl tendencies are shown to be possible. This work extends recent efforts by developing a more complete, yet compact, description of fluid-induced forces in that it accounts for all relevant force contributions, both tangential and radial, that may influence the dynamics of the rotor. Hence it constitutes an essential element of a consistent treatment of rotordynamic stability under the action of fluid-induced forces, which is the subject of Part II of this paper.

Commentary by Dr. Valentin Fuster
GT2002-30489 pp. 1093-1105; (13 pages)

This paper presents an integrated treatment of the dynamic coupling between the flow field (aerodynamics) and rotor structural vibration (rotordynamics) in axial compression systems. This work is motivated by documented observations of tip clearance effects on axial compressor flow field stability, the destabilizing effect of fluid-induced aerodynamic forces on rotordynamics, and their potential interaction. This investigation is aimed at identifying the main nondimensional design parameters governing this interaction, and assessing its impact on overall stability of the coupled system. The model developed in this work employs a reduced-order Moore-Greitzer model for the flow field, and a Jeffcott-type model for the rotordynamics. The coupling between the fluid and structural dynamics is captured by incorporating a compressor pressure rise sensitivity to tip clearance, together with a momentum based model for the aerodynamic forces on the rotor (presented in Part I of this paper). The resulting dynamic model suggests that the interaction is largely governed by two nondimensional parameters: the sensitivity of the compressor to tip clearance and the ratio of fluid mass to rotor mass. The aerodynamic-rotordynamic coupling is shown to generally have an adverse effect on system stability. For a supercritical rotor and a typical value of the coupling parameter, the stability margin to the left of the design point is shown to decrease by about 5% in flow coefficient (from 20% for the uncoupled case). Doubling the value of the coupling parameter not only produces a reduction of about 8% in the stability margin at low flow coefficients, but also gives rise to a rotordynamic instability at flow coefficients 7% higher than the design point.

Commentary by Dr. Valentin Fuster
GT2002-30595 pp. 1107-1116; (10 pages)

This paper contributes to the magnetic bearing literature in two distinct areas: high temperature and redundant actuation. Design considerations and test results are given for the first published combined 538°C (1000°F)-high speed rotating test performance of a magnetic bearing. Secondly, a significant extension of the flux isolation based, redundant actuator control algorithm is proposed to eliminate the prior deficiency of changing position stiffness after failure. The benefit of the novel extension was not experimentally demonstrated due to a high active stiffness requirement. In addition, test results are given for actuator failure tests at 399°C (750°F), 12,500rpm. Finally, simulation results are presented confirming the experimental data and validating the redundant control algorithm.

Commentary by Dr. Valentin Fuster
GT2002-30633 pp. 1117-1124; (8 pages)

The traditional balancing methods using trial or calibration weights are quite effective, yet too many trials may result in a lengthy balancing process. It had been suggested in the literature that it is possible to balance flexible rotors without the use of trial weights, if a rotor model is available. A procedure is developed in this paper to balance flexible rotors using complex modes and complex vibration measurements. It is shown that a complex rotor model is essential for the success of the technique. Moreover, careful calibration of the rotor model is the major cornerstone of the procedure. Experimental results illustrate the success of the procedure.

Commentary by Dr. Valentin Fuster
GT2002-30634 pp. 1125-1138; (14 pages)

A slotted pocket gas damper seal has been experimentally evaluated on a flexible rotor test rig for rotor vibration reduction properties. For unbalance response tests, the experiments were conducted at several seal inlet pressures out to 14.5 bar (210 psia), and the rotor speed ranged up to 8,000 rpm, which was well above the damped first critical speed of the flexible rotor. The influence of gas pre-swirl and seal eccentricity on the seal dynamic performance was investigated in the tests. Experimental results showed that the slotted pocket damper seal provided high positive damping and small positive stiffness in the tested pressure range. Comparisons of seal dynamic characteristics and leakage were made between the slotted pocket damper seal and a conventional pocket damper seal tested previously on the same test stand. The stability characteristics of the slotted pocket gas damper seal were also experimentally evaluated at 12,000 rpm, the maximum operating speed of this test rig. Test conditions involved three inlet pressures up to 20.7 bar (300 psia) and intentional pre-swirl gas flows. Experimental results demonstrated that the slotted pocket damper seal was rotordynamically stable at high speeds without any subsynchronous vibration in the frequency spectrum measured using proximity probes for the test rotor.

Topics: Dampers
Commentary by Dr. Valentin Fuster
GT2002-30635 pp. 1139-1151; (13 pages)

Reported in this paper are the field rotor instability and vibration elimination experienced recently on a compressor train installed on an offshore platform for natural gas service. The compressor train consists of a low-pressure compressor and a high-pressure compressor, which is driven by a gas turbine through a gearbox. The compressor train is rated at a maximum continuous speed of 12,054 rpm. During its first commissioning, a high subsynchronous vibration showed up on the high-pressure compressor when it was put on load at full speed. The high-pressure compressor has nine stages, which are arranged back-to-back in two sections. The high-pressure compressor is rated at a discharge pressure of 176.5 bar (2,560 psia). Field vibration data were analyzed and compared to the rotordynamic results from the lateral vibration model of the rotor. The root of the subsynchronous vibration was identified to be the destabilizing aerodynamic excitation generated mainly by the intermediate seal and the impellers. To eliminate the subsynchronous vibration, a gas pocket damper seal was designed specially to replace the existing intermediate labyrinth seal. Meanwhile, the existing tilt-pad bearings were replaced with the deflection pivot bearings to further improve the rotordynamic performance. The compressor was tested with the new center seal and journal bearings at full load, full pressure, and full speed. The subsynchronous vibration was eliminated. Now the compressor train operates smoothly at its design conditions and the vibration readings remain low and stable.

Commentary by Dr. Valentin Fuster
GT2002-30636 pp. 1153-1174; (22 pages)

There are many nonlinear factors in the optimization process. The objective of this study is to determine some effects of these nonlinear factors on the sensitivity of optimum design. A rotor-bearing system is composed of rigid disks, shaft elements with distributed mass and stiffness, and magnetic bearings. To obtain an optimum design, the objectives including shaft weight and amplitude of shaft lateral response as well as the constraints including critical speeds and bending stresses are considered. The inner radii of shaft elements, the bias currents of magnetic bearings, and the axial positions of bearings and disks are chosen as the design variables. The single objective method, the multi-objective method and the multilevel technique are employed to deal with the problem of optimality. Optimum results obtained by applying these methods are compared and discussed in this study. Finally, the sensitivity of optimum design is analyzed to determine what nonlinear effects the position of magnetic bearings have on different initial designs.

Commentary by Dr. Valentin Fuster
GT2002-30637 pp. 1175-1186; (12 pages)

Most closed form solutions of Reynolds’ equation assume either a short bearing approximation or a long bearing approximation. These closed form approximations are used in rotordynamic simulation applications, otherwise a Finite Difference solution of Reynolds’ equation would be prohibitively time consuming. Recently, there have been proposed series solutions for Reynolds’ equation for special cases. In this paper, a perturbation solution to the governing equations is proposed to obtain a closed form solution of Reynolds’ equation for a finite squeeze film damper executing a circular centered orbit. The pressure field and velocity profiles are obtained. It is shown that in the limit the finite damper solution approaches either the appropriate short or long damper. This perturbation solution can be used with appropriate boundary conditions, for various damper sealing configurations, and provides insight into the damper performance.

Topics: Dampers , Modeling
Commentary by Dr. Valentin Fuster
GT2002-30638 pp. 1187-1195; (9 pages)

The experimental investigation into pressure field in the shroud clearance and rotor trajectory were performed on an one-stage air model turbine of impulse type. Basing on the pressure distribution, the aerodynamic forces and moments were investigated as a function of rotor eccentricity, axial gap, rotor-stator misalignment, rotor speed and turbine load. We obtained the following results: 1. confirmation of the linear correlation between the rotor eccentricity (or rotor-stator misalignment angle) and aerodynamic forces, 2. proof that radial eccentricity influences the average location of the rotor, while the general shape of rotor trajectory remains unchanged, 3. proof that although the axial component of the pressure force is relatively low, the moments exerted by this force should be take into account when total aerodynamic moments are determined.

Commentary by Dr. Valentin Fuster

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