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Aircraft Engine

2002;():1-8. doi:10.1115/GT2002-30001.

This paper describes the teaming strategy between the U. S. Air Force’s two major propulsion test centers, Arnold Engineering Development Center (AEDC) and the Air Force Flight Test Center (AFFTC), to employ new modeling and simulation (M&S) techniques to reduce test cost and cycle time. With the long-term goal of developing joint ground/flight modeling and simulation capabilities, AEDC and AFFTC have teamed to apply these data analysis tools to two major propulsion flight test programs, the F-22 and the Joint Strike Fighter (JSF). The emphasis of this paper is on the development and application of a non-linear aerothermodynamic component-level model that serves as the basis for the model-based analysis and fault identification process. Model-to-data comparisons and model-based fault detection and analysis results for F-22/F119 propulsion ground and flight test are also presented for a variety of flight conditions.

Commentary by Dr. Valentin Fuster
2002;():9-20. doi:10.1115/GT2002-30002.

The aerodynamic stability of aero engine compressors must be assured by active control systems in all operating conditions when the design surge margin is reduced in order to improve efficiency. While this has been investigated only on compressor rigs and single-spool engines in the past, this study focuses on the active control of the LARZAC 04 twin-spool turbofan. The objective is to demonstrate potential benefits, problems and solutions and also to provide a data base for numerical modeling and simulation of the capabilities of active control. Three different control strategies have been employed each of which refers to a specific operating condition and instability inception of the engine: The attenuation of disturbances travelling at rotor speed by modulated air injection into the LPC in the high speed range, the recovery of fully developed LPC stall at low speeds with a minimized amount of air and finally a constant air recirculation (HPC exit to LPC inlet) for stabilizing the compression system at different speeds based on the monitoring of a stability parameter. The injector is mounted upstream of the LPC and has ten circumferentially distributed nozzles for air injection into the tip region of the first rotor. The injected air which is either taken from an external source or from bleed air ports at the HPC exit is controlled by high-bandwidth direct-drive-valves. Disturbances travelling at rotor speed can be detected and attenuated with modulated air injection leading to a delay of stall onset. Fully developed rotating stall in the LPC was eliminated by asymmetric injection based on modal control strategies with less air than needed with constant air injection. By using online-stability-monitoring it is possible to initiate constant air recirculation when approaching the surge line, though the current design of the injector does not allow for large extension of the operating range for all spool speeds.

Topics: Engines , Turbofans
Commentary by Dr. Valentin Fuster
2002;():21-31. doi:10.1115/GT2002-30003.

Testing of a gas turbine engine for aircraft propulsion applications may be conducted in the actual aircraft or in a ground-test environment. Ground test facilities simulate flight conditions by providing airflow at pressures and temperatures experienced during flight. Flight-testing of the full aircraft system provides the best means of obtaining the exact environment that the propulsion system must operate in but must deal with limitations in the amount and type of instrumentation that can be put on-board the aircraft. Due to this limitation, engine performance may not be fully characterized. On the other hand, ground-test simulation provides the ability to enhance the instrumentation set such that engine performance can be fully quantified. However, the current ground-test methodology only simulates the flight environment thus placing limitations on obtaining system performance in the real environment. Generally, a combination of ground and flight tests is necessary to quantify the propulsion system performance over the entire envelop of aircraft operation. To alleviate some of the dependence on flight-testing to obtain engine performance during maneuvers or transients that are not currently done during ground testing, a planned enhancement to ground-test facilities was investigated and reported in this paper that will allow certain categories of flight maneuvers to be conducted. Ground-test facility performance is simulated via a numerical model that duplicates the current facility capabilities and with proper modifications represents planned improvements that allow certain aircraft maneuvers. The vision presented in this paper includes using an aircraft simulator that uses pilot inputs to maneuver the aircraft engine. The aircraft simulator then drives the facility to provide the correct engine environmental conditions represented by the flight maneuver.

Commentary by Dr. Valentin Fuster
2002;():33-38. doi:10.1115/GT2002-30004.

A comprehensive test program was performed in the Propulsion Systems Laboratory at the NASA Glenn Research Center, Cleveland Ohio using a highly instrumented Pratt and Whitney Canada PW 545 turbofan engine. A key objective of this program was the development of a high-altitude database on small, high-bypass ratio engine performance and operability. In particular, the program documents the impact of altitude (Reynolds number) on the aero-performance of the low-pressure turbine (fan turbine). A second objective was to assess the ability of a state-of-the-art CFD code to predict the effect of Reynolds number on the efficiency of the low-pressure turbine. CFD simulation performed prior and after the engine tests will be presented and discussed. Key findings are the ability of a state-of-the art CFD code to accurately predict the impact of Reynolds number on the efficiency and flow capacity of the low-pressure turbine. In addition the CFD simulations showed the turbulence intensity exiting the low-pressure turbine to be high (9%). The level is consistent with measurements taken within an engine.

Topics: Pressure , Engines , Turbines
Commentary by Dr. Valentin Fuster
2002;():39-48. doi:10.1115/GT2002-30005.

A multi-purpose rotor-bearing dynamic simulator was designed and fabricated for the purpose of experimentally evaluating and validating performance of advanced oil-free and back-up bearings under realistic dynamic conditions. The rotor-bearing dynamic test rig is capable of operation to 25,000 RPM, has a 54 kg test rotor, is designed to simulate a medium size aero gas turbine engine rotor, and incorporates an electromagnetic loader/shaker capable of applying both static and dynamic loads to the rotating shaft. Testing was completed with the rotor fully supported by magnetic bearings, compliant foil bearings, hybrid foil/magnetic and Zero Clearance Auxiliary Bearings. These tests demonstrated numerous advances in oil-free bearing technology. The first ever achievements include: operation of a rotor with a mass in excess of 50 kg supported solely by foil bearings, operation of hybrid foil/magnetic bearings to high speed, continued operation following simulated magnetic bearing failures for a fully hybrid foil/magnetic bearing support system, and operation of a rotor supported solely by Zero Clearance Auxiliary Bearings. Data from several tests of the bearing systems are presented.

Commentary by Dr. Valentin Fuster
2002;():49-55. doi:10.1115/GT2002-30006.

On-line calculation methods are currently used to evaluate stress and temperature of engine components in order to assess fatigue damage accumulation and residual life. On-line temperature calculation algorithms are necessary because temperature affects fatigue damage curves. Since it is neither possible nor necessary to compute on-line temperature on the whole component, a number of critical nodes are selected and their temperatures are evaluated with simplified algorithms. A well-known technique used to reduce the degrees of freedom of dynamic structural FE models is the component modes synthesis (CMS). By this technique the nodal degrees of freedom are divided into two sets: active and omitted. Active degrees of freedom are translated into the reduced model, while omitted ones are replaced by the most important modal shapes, in order to evaluate the dynamic behavior of the system. In the present work CMS has been applied to thermal transient analyses, in order to compute temperatures in low-pressure turbine discs critical areas. Due to the complexity of the geometry, the disc has been sub-structured into super-elements. The methodology has been tested on axi-symmetric FE model of a low-pressure turbine disc, comparing thermal transients performed by complete FE model with those evaluated by the sub-structured model.

Commentary by Dr. Valentin Fuster
2002;():57-65. doi:10.1115/GT2002-30007.

Computational Fluid Dynamics analysis for a novel pulsed-ejector showed that the pumping effectiveness (ratio of secondary (entrainment) to primary mass flow rate) was significantly increased relative to the unpulsed ejector, and that this was optimized by pulsing at off-resonance (3250 Hz) to the fundamental (2000 Hz) of the ejector. Testing of a novel model pulsed-ejector confirmed strong response for pulse frequencies less than half of the fundamental frequency of about 746 Hz. Acoustic driver frequency response limitations only allowed testing at less than the experimental fundamental frequency. Specifically pumping effectiveness was increased by up to five times, and even more in suction performance, for only 150 W electrical power at the acoustic driver input. Optimum performance was at a frequency of about 250 Hz, and the acoustic drive produced a small but significant increase in the flow stagnation pressure. The results demonstrate important research potential for the novel pulsed-ejector.

Commentary by Dr. Valentin Fuster
2002;():67-74. doi:10.1115/GT2002-30008.

The main contributor to the high-cycle fatigue of compressor blades is the response to aerodynamic forcing functions generated by an upstream row of stators or inlet guide vanes. Resonant response to engine order excitation at certain rotor speeds is especially damaging. Studies have shown that flow control by trailing edge blowing (TEB) can reduce stator wake strength and the amplitude of the downstream rotor blade vibrations generated by the unsteady stator-rotor interaction. In the present study, the effectiveness of TEB to reduce forced blade vibrations was evaluated in a modern transonic compressor rig. A row of wake generator (WG) vanes with TEB capability was installed upstream of the rotor, which was instrumented with strain gages. Data was collected with and without TEB at various rotor speeds involving resonance crossings. Using 0.8% of the compressor core flow for TEB along the full WG-span, rotor blade strain was reduced by 66% at the first torsional resonance crossing. Substantial reductions were also achieved with only partial span TEB. The results demonstrate the effectiveness of the TEB technique for reducing rotor vibrations in the complex flow environment of a closely-spaced transonic stage row. Moderate increases in stage performance were also measured.

Commentary by Dr. Valentin Fuster
2002;():75-84. doi:10.1115/GT2002-30496.

A prototype preliminary design task for gas turbines is set up to outline the four major requirements a preliminary design program must typically meet: assessment of all major engine components and their interrelations; inclusion of all relevant disciplines; designing over several operating points; and model fidelity zooming at least for individual components. It is described how the “MOdular Performance and Engine Design System” (MOPEDS) — MTU Aero Engines’ software package for the preliminary design of airborne and stationary gas turbines — fulfills these requirements. The program structure, the graphical user interface, and the physical models are briefly presented. A typical design example is carried out emphasizing the necessity for a numerical procedure to find a solution to the many variables and constraints that the design problem comprises. Finally, some dominating multidisciplinary effects are discussed.

Topics: Design , Gas turbines
Commentary by Dr. Valentin Fuster
2002;():85-92. doi:10.1115/GT2002-30497.

Quoting numbers for the efficiency of a turbine is ambiguous if it is not known how this efficiency is defined. This is especially true for a heavily cooled turbine where for the same machine the efficiency may be quoted as 88% or 91%, for example. In aero-engine industry, several different turbine efficiency bookkeeping systems are in use. Since nearly always a consortium of two or more companies is involved into any new engine project it is important to understand the various bookkeeping systems. Turbine designers and performance specialists should not quote a number for the efficiency of a cooled turbine without clearly stating on which methodology and control volume it is based. In this paper, the commonly used turbine performance bookkeeping methods are compared. The pros and cons for the different methodologies are discussed and correlations between the definitions are shown for single and multistage turbines with various amounts of cooling air. Furthermore it is pointed out, that simulating a cooled multistage turbine with an equivalent single-stage model requires the use of an equivalent Stator Outlet Temperature SOTeqv which differs from the true SOT. It depends on the bookkeeping system used what the calculated impact on turbine performance is when the amount of cooling air changes. Without carefully adhering to a unique bookkeeping system with a clearly defined control volume the probability of misunderstandings in collaborative engine development projects is not to be underestimated.

Topics: Modeling , Turbines
Commentary by Dr. Valentin Fuster
2002;():93-101. doi:10.1115/GT2002-30498.

Many variants of designs of different engine components have to be analysed in detail during the design and subsequent optimisation of modern aero engines. This often involves repetitive tasks and even today this process still contains a considerable amount of manual work for the majority of the tasks in the design process. Experts from different technical disciplines are involved and several different analysis tools are used. An automation of this process not only saves a lot of time during the design phase; it also increases the quality of the design since many more design variants can be screened. In the present paper the integration of different analysis codes and optimisation tools into an automated process using off-the-shelf software is described. A mix of commercial and in-house codes is integrated in a loose coupling way. Several applications from different areas of aero engine design are described. It is shown that in all cases the computer based optimisation and the process automation yields results of equal or better technical quality compared to the original hand optimised ones or improves the understanding of the design space. In addition, the necessary wall-clock time to reach the results was in all cases a fraction of that of the manual process.

Commentary by Dr. Valentin Fuster
2002;():103-109. doi:10.1115/GT2002-30499.

When highly non-symmetric exhaust ducts are installed on a gas turbine engine, the asymmetries result in a non-uniform circumferential total pressure condition at the inlet of the duct. When testing these ducts experimentally or computationally the correct inlet conditions are often not known or cannot be reproduced. To study the sensitivity of duct performance to inlet conditions, an experimental and computational study of a non-symmetric gas turbine exhaust duct that includes a 160° turn with an annular to rectangular transition, has been carried out over a range of inlet conditions. The inlet conditions varied include circumferential total pressure profiles and swirl. The experimental studies have been carried out in cold flow with several non-uniform total pressure inlet conditions. Computational fluid dynamic (CFD) techniques validated against the experimental results, have been used to extend the range of inlet conditions beyond the range that could be obtained experimentally to those typical of an engine installation. Results show that the total pressure inlet conditions have a significant effect on the flow structure in the exhaust duct and that the performance of the exhaust duct degrades as the level of circumferential non-uniformities increase. However, trends in geometric optimization identified experimentally using cold flow and uniform total pressure inlet conditions are confirmed computationally with circumferential non-uniformities typical of actual engine operations. This suggests that although inlet conditions are important for determining the level of performance, the configuration of the optimized geometry is somewhat independent of the inlet conditions.

Commentary by Dr. Valentin Fuster
2002;():111-118. doi:10.1115/GT2002-30500.

Reliable engine-weight estimation at the conceptual design stage is critical to the development of new aircraft engines. It helps to identify the best engine concept amongst several candidates. In this paper, the major enhancements to NASA’s engine-weight estimate computer code (WATE) are described. These enhancements include the incorporation of improved weight-calculation routines for the compressor and turbine disks using the finite-difference technique. Furthermore, the stress distribution for various disk geometries was also incorporated, for a life-prediction module to calculate disk life. A material database, consisting of the material data of most of the commonly-used aerospace materials, has also been incorporated into WATE. Collectively, these enhancements provide a more realistic and systematic way to calculate the engine weight. They also provide additional insight into the design trade-off between engine life and engine weight. To demonstrate the new capabilities, the enhanced WATE code is used to perform an engine weight/life trade-off assessment on a production aircraft engine.

Commentary by Dr. Valentin Fuster
2002;():119-128. doi:10.1115/GT2002-30605.

Meeting the test needs of future aircraft systems is the driving goal behind Arnold Engineering Development Center’s (AEDC) ongoing programs of turbine engine test facility modernization, improvement, consolidation, and streamlining. This paper will discuss the evolution of the AEDC J2 turbine engine test cell, its conditioned air supply system, and other related systems with emphasis on how modeling and simulation played a vital role throughout the evolutionary process. Modeling and simulation was used for the analysis of potential design changes, the coding and checkout of control system changes, as well as to improve control system performance and increase overall facility capabilities. These changes began under the Test Operations Modernization and Improvement Program (TOMIP) and continue today in the Propulsion Consolidation and Streamlining Program (PC&S). Already significant improvements have been made in facility capability, and these along with the steps still to come in the near future will be discussed as a turbine engine test facility continues to evolve to meet the test needs of tomorrow’s aircraft systems.

Commentary by Dr. Valentin Fuster
2002;():129-137. doi:10.1115/GT2002-30606.

Whether the intended application of a gas turbine engine is aero-propulsion or power production, the engine exhaust gas emissions must comply with all applicable air quality regulations. For an aircraft gas turbine engine, the current range of certifiable operating conditions includes the landing and takeoff cycle. For a stationary, power-producing engine the range of operating conditions is narrow, but there is increasing interest in measurements of speciated hydrocarbons and other species that pose health and environmental hazards. Existing emission measurement methodologies are time consuming and expensive. An engine emissions survey was performed on a JT-12 engine at Middle Tennessee State University using a new integrated rapid sampling, measurement, and analysis system. Improved methodologies should be considered for turbine engine emissions certification.

Commentary by Dr. Valentin Fuster
2002;():139-146. doi:10.1115/GT2002-30622.

The principles on which an adaptive engine performance model is based are first discussed. The mathematical ways of matching given performance data are presented and their implications for practical implementation are discussed. The consequences of using a number of measurements equal or fewer than the number of adapting parameters are analysed. The numerical behaviour of an adaptive model is also discussed. Solution methods based on non-linear system solvers are compared to methods using optimisation techniques. Execution time requirements are also discussed for the different approaches and implications for possible on-line or off-line applications are evaluated.

Commentary by Dr. Valentin Fuster
2002;():147-152. doi:10.1115/GT2002-30623.

The objective of this paper is to describe a method for selecting optimal engine technology solution sets while simultaneously accounting for the presence of technology risk. This method uses a genetic algorithm in conjunction with Technology Identification, Evaluation, and Selection methods to find optimal combinations of technologies. The unique feature of this method is that the technology evaluation itself is probabilistic in nature. This allows the performance impact and associated risk of each technology to be quantified in terms of a distribution on key engine technology metrics. The resulting method can best be characterized as a concurrent genetic algorithm/Monte Carlo analysis that yields a performance- and risk-optimal technology solution set. This solution set is inherently a robust solution because the method will naturally strive to find those technologies representing the best compromise between performance improvement and technology risk. Finally, a practical demonstration of the method and accompanying results is given for a typical commercial aircraft engine technology selection problem.

Commentary by Dr. Valentin Fuster
2002;():153-160. doi:10.1115/GT2002-30624.

This paper gives a brief overview of the status of high temperature electronics and sensor development at NASA Glenn Research Center supported in part or in whole by the Ultra Efficient Engine Technology Program. These activities contribute to the long-term development of an intelligent engine by providing information on engine conditions even in high temperature, harsh environments. The technology areas discussed are: 1) High temperature electronics, 2) Sensor technology development (Pressure sensor and High temperature electronic nose), 3) Packaging of harsh environment devices and sensors, and 4) Improved Silicon Carbide electronic materials. A description of the state-of-the-art and technology challenges is given for each area. It is concluded that the realization of a future intelligent engine depends on the development of both hardware and software including electronics and sensors to make smart components. When such smart components become available, an intelligent engine composed of smart components may become a reality.

Commentary by Dr. Valentin Fuster

Coal, Biomass and Alternative Fuels

2002;():161-170. doi:10.1115/GT2002-30009.

In early studies addressing national energy/environmental (EE) problems we concluded that co-utilization of domestic fuels can significantly reduce national reliance on imported fuels, mitigate NOx, SOx, CO2 and other undesirable emissions and provide valuable waste disposal services. Co-firing of coal and biomass for steam turbine power generation is a near-term co-utilization approach that can make use of existing facilities with relatively minor modifications. However, co-gasification by providing fuel for more efficient combustion turbines and fuel cells and co-liquification to produce transportation fuels have greater long-term EE potential. The development of optimum thermo-chemical co-conversion systems can be fostered by developing a common systematics for the pyrolysis of biomass and coal. Towards this goal we have used the large data bases from ASTM standard ultimate and proximate analyses for all fuels along natures coalification path from biomass to peat, lignite, bituminous and anthracite coal. With this composite data we find systematics in the weight percentages of carbon, hydrogen, total volatiles, fixed carbon and feedstock HHVs vs the weight percentage of oxygen. To meet the need for knowledge of the volatile constituents we have used sparsely available slow pyrolysis data in the literature and our own data to further develop a plausible semi-empirical model (SEM) that relates feedstock and product compositions. We here extend these analytic correlations to lower temperatures with the help of CCTL measurements of yields from the pyrolysis of rice hulls. We have recently applied this SEM to exam the systematic yields of a short list (SL) of products (five gases and five liquids) vs [O], the weight percentage of oxygen in the feedstock. Here anchored to the rice hull data we use our analytical relationships to estimate the yields of a long list (LL) of products including many organic compounds that are known to be slow pyrolysis products of coals and biomass. These relations are put forth as a heuristic challenge to ourselves and to specialists in biomass and coal pyrolysis to obtain more and better data and to seek improved engineering formulas that are needed to advanced co-utilization technology. Then energy debtor nations could utilize all of their available domestic fuels, including opportunity fuels, to mitigate their national EE problems. These preliminary results point to a path towards the development of a co-utilization science and technology for optimizing feedstock blends in many co-firing, co-gasifying or co-liquifying applications.

Topics: Fuels , Feedstock
Commentary by Dr. Valentin Fuster
2002;():171-182. doi:10.1115/GT2002-30010.

The potential use of MBM (Meat and Bone Meal) as fuel in a power plant has been recently originated by the mad cow disease, affecting not only Europe (the origin of the disease) but also other continents. MBM manufacturing companies have been forced to change their traditional ways of distribution due to the current ban of using MBM as cattle feed, therefore using a dumping site or an incinerator. To be considered as a fuel, several studies should be carried out. Preliminary characterisation of MBM showed a heating value higher than existing in coal, and a grain size acceptable to be mixed with regular fuel, hence appropriate to be brought into a boiler or a gasifier. Additionally, an expected advantage of using MBM in a gasification process was the possibility of using it as adequate slag/ash fusion agent (instead of traditional limestone), due to the high presence of Ca compounds. Related to environmental issues, the conventional thermal oxidation process (like incineration) shows several inconveniences, associated to the presence of hazardous compounds (like furans and dioxins) expected in organic matter combustion. There are few references of the existence of this kind of compounds in gasification process, but it is known that the existing reducing environment in a gasifier does not benefit its formation at all. Some of these issues were analysed in short duration full-scale tests developed in Puertollano IGCC Power Plant, owned by ELCOGAS, in which several MBM/regular fuel mixtures were tested. This paper describes the methodology used in these tests, fuel characteristics, main systems performance, and general conclusions about the viability of IGCC co-gasification using alternative fuels.

Commentary by Dr. Valentin Fuster
2002;():183-190. doi:10.1115/GT2002-30011.

The continuous grow of world population has led to a substantial increase in energy demand. On the other hand, it has given rise to generating large amounts of wastes, which need to be disposed of without any damage to the environment. Such scenario has led to the idea of studying the application of gasification technology to mixtures of coal and wastes. The results obtained so far are encouraging, as they have shown that it is possible to co-gasify coal mixed with either pine or polyethylene wastes to values up to 40% (w/w) of wastes, being even possible to substitute one waste type by the other, whenever their availability are seasonal affected. However, the presence of PE wastes favoured the release of hydrocarbons, which may be reduced by either an increase in gasification temperature or in air flow.

Commentary by Dr. Valentin Fuster
2002;():191-196. doi:10.1115/GT2002-30012.

High NH3 concentrations were measured in the fuel gas produced by a pilot scale, air blown gasifier that was operated by British Coal. A laboratory scale gasifier has subsequently been developed to investigate the reactions that produce these potentially high concentrations. It has been found that in addition to the NH3 formed through pyrolytic processes, the introduction of steam (or H2 produced by its decomposition) increases the amount formed. The latter reaction produced the higher proportion of the total NH3 . The effect of the gasifier operating conditions on the amount of NH3 formed has been studied. The main control options to minimise the NH3 formed are using an alternative method of bed temperature control (i.e. avoid the use of steam), operating with higher bed temperatures and operation at lower pressures.

Commentary by Dr. Valentin Fuster
2002;():197-202. doi:10.1115/GT2002-30013.

A series of tests have been done in a pilot scale air blown gasifier, to assess the performance of sewage sludge pellets and sewage sludge pellet/coal mixtures. The aim has been to compare the performances with that achieved with coal alone and to assess the suitability of the sewage containing fuel as a candidate fuel for the Air Blown Gasification Cycle (ABGC). The co-gasification of sewage sludge with coal raised both the CV of the fuel gas and the fuel conversion compared with values achieved with coal alone. The mixtures were operated under very similar conditions to those needed with coal and no adverse operational problems were encountered. A lower fluidising velocity was needed with the neat pellets to enable a stable bed height to be achieved. However, the conversion of the pellets to gas was very high and the fuel gas CV was higher than that achieved during the co-gasification tests. Overall, the results suggest that sewage based materials are suitable for use in the ABGC and that their use can improve the process performance.

Commentary by Dr. Valentin Fuster
2002;():203-208. doi:10.1115/GT2002-30014.

ANSALDO ENERGIA S.p.A. has been commissioned by ELETTRA GLT S.p.A, a company located in Trieste, Italy for the realisation of a combined cycle plant where all the main components (gas turbine, steam turbine, generator and heat recovery steam generator) are provided by ANSALDO ENERGIA. The total power output of the plant is 180 MW. The gas turbine is a V94.2 K model gas turbine dual fuel (natural gas and steelworks process gas), where the fuel used as main fuel is composed by a mixture of natural gas, blast furnace gas and coke oven gas in variable proportions according to the different working conditions of the steel work plant. The main features adopted to burn such a kind of variability of fuels are reported below: • fuel as by product of steel making factory gas (coke oven gas “COG”, blast furnace gas “BFG”) with natural gas integration; • modified compressor from standard V94.2, since no air extraction is foreseen; • dual fuel burner realised based on Siemens design. This paper describes the operating experience achieved on the gas turbine, focusing on the main critical aspect to be overcome and on to the test results during the commissioning and the early operating phase. The successful performances carried out have been showing a high flexibility in burning with stable combustion a very different fuel compositions with low emissions measured all operating conditions.

Commentary by Dr. Valentin Fuster
2002;():209-216. doi:10.1115/GT2002-30015.

When a gas turbine is operated on low calorific value (LCV) gas instead of natural gas, the operating point of the compressor will shift towards the surge line. The compressor pressure ratio can rise to a level where stall or surge can occur. Premixing LCV gas with air inside the compressor of a gas turbine can solve this problem. With all fuel premixed, no fuel needs to be injected through the normal fuel inlet. The mass flow balance between turbine and compressor is restored and matching problems will not occur. From calculations with two LCV gases it could be concluded that all LCV gas could be premixed with compressor air when a low percentage of hydrogen gas was present in the LCV gas. The LCV gas could not be fully premixed in case of a high amount of hydrogen. The calculations show that an OPRA OD 500 gas turbine operated on premixed LCV gas with a low amount of hydrogen can maintain its original efficiency and a loss of 14 efficiency points can be prevented.

Commentary by Dr. Valentin Fuster
2002;():217-225. doi:10.1115/GT2002-30016.

The results of a study of a novel gas turbine configuration is being presented. In this power plant, an Indirectly Fired Gas Turbine (IFGT), is being fueled with very wet biomass. The exhaust gas is being used to dry the biomass, but instead of striving to recover as much as possible of the thermal energy, which has been the practice up to now, the low temperature exhaust gases after having served as drying agent, are lead out into the environment; a simple change of process integration that has a profound effect on the performance. Four different cycles have been studied. These are the Simple IFGT fueled by dry biomass assuming negligible pressure loss in the heat exchanger and the combustion chamber, the IFGT fueled with wet biomass (Wet IFGT) assuming no pressure losses, and finally both the Simple and the Wet IFGT incorporating typical data for pressure losses of commercially available micro turbines. The study shows that the novel configuration, in which an IFGT and a drying unit have been combined, has considerable merit, in that its performance exceeds that of the currently available methods converting wet biomass to electric power by a factor of five. The configuration also has clear advantages with respect to corrosion and to the environmental friendliness and the quantity of the waste products and their usefulness.

Commentary by Dr. Valentin Fuster
2002;():227-238. doi:10.1115/GT2002-30017.

Gas Turbines accept a wide range of alternative fuels in connection with the most diverse economy branches, including industry (coal; oil and gas; refining; petrochemistry; steel and mining activities) and, more recently, agriculture (biofuels). This fuel flexibility enhances the other qualities demonstrated by Gas Turbines among which the prominent ones are: energy effectiveness, operational reliability and emission compliance. Therefore, Gas Turbines using local fuel resources and deployed in simple or combined cycles or in cogeneration plants, enable the concept of cost-effective and environmentally-conscious power projects and can make a valuable contribution to the sustainable, regional development. However, in order to benefit from the fuel flexibility of Gas Turbines, some basic technical considerations are necessary. The paper intends to provide the power community with comprehensive information about alternative GT fuels. It offers a review of the main alternative fuel candidates and sets out the primary technical/engineering considerations that underlie their safe and reliable utilization. Special emphasis is placed on: (i) volatile fuels (naphtha, NLG, condensates); (iii) weak gas fuels from the coal/iron industry (coal-bed; coke-oven, blast furnace gas); (iv) paraffin-rich and hydrogen-rich by-products from refineries (‘fuel gas’; LPG) and (iv) ash-forming oils (residuals; heavy crude’s).

Topics: Fuels , Gas turbines
Commentary by Dr. Valentin Fuster
2002;():239-256. doi:10.1115/GT2002-30666.

In order to improve the thermal efficiency of the oxygen-blown IGCC (Integrated Gasification Combined Cycle) for stricter environmental standards and cost-effective option, it is necessary to adopt the hot/dry gas cleaning system. In this system, the flame temperature of medium-btu gasified fuel is higher and so NOx production from nitrogen fixation is expected to increase significantly. Also the gasified fuel contains fuel nitrogen, such as ammonia, in the case of employing the hot/dry gas cleaning system. This ammonia is easily oxidized into fuel-NOx in the combustor. For contribution to the protection of the environment and low cost operations of all kinds of oxygen-blown IGCC, low NOx combustion technology for reducing both the fuel-NOx and thermal-NOx emission has to be developed. In this paper, we clarified effectiveness of applying both the two-stage combustion and the nitrogen injection, and the useful engineering guidelines for the low-NOx combustor design of oxygen-blown gasified, medium-btu fuels. Main results obtained are as follows: (1) Based on the fundamental combustion tests using the small diffusion burner, we clarified that equivalence ratio at the primary combustion zone has to be adjusted due to the fuel conditions, such as methane concentration, CO/H2 molar ratio, and calorific values of gasified fuels in the case of the two-stage combustion method for reducing fuel-NOx emission. (2) From the combustion tests of the medium-btu fueled combustor the two-stage combustion with nitrogen direct injection into the combustor results in reduction of NOx emission to 80ppm (corrected at 16% O2) or less, the conversion rate of ammonia to NOx was 35% under the gas turbine operational conditions for IGCC in the case where fuel contains 3% of methane and 2135ppm of ammonia. By means of nitrogen direct injection, the thermal efficiency of the plant improved by approximately 0.3 percent (absolute), compared with a case where nitrogen is premixed with gasified fuel. The CO emission concentration decreased drastically, as low as 20ppm, or combustion efficiency was kept higher than 99.9%. Furthermore, based on the fundamental combustion tests’ results, the ammonia conversion rate is expected to decrease to 16% and NOx emission to 26ppm in the case of gasified fuel that contains 0.1% methane and 500ppm of ammonia. From the above results, it is clarified that two-stage combustion method with nitrogen injection is very effective for reducing both the fuel-NOx and thermal-NOx emissions at once in IGCC and it shows the bright prospects for low NOx and stable combustion technology of the medium-btu fuel.

Commentary by Dr. Valentin Fuster
2002;():257-265. doi:10.1115/GT2002-30672.

Gasification has attracted considerable interest from water utilities as a sewage sludge disposal option, with the advantages of waste volume reduction, pathogen destruction and energy recovery. Co-gasification with coal in a larger plant (>10 MWt ) employing a gas turbine for energy recovery may reduce the risk and cost of this option. However, controlling the release of trace elements such as Pb and Zn in the gas produced may be necessary to avoid corrosion, and to meet environmental requirements. A thermodynamic equilibrium model has been used to make predictions of the speciation of trace elements in the fuel gas from co-gasification of sewage sludge with coal. Experimental data from a pilot scale 2 MWt sewage sludge/coal co-gasification plant with a hot gas filter was used to test the validity of these predictions. No significant amount of Be, Co, Cu, V and Zn was predicted to be in the form of gaseous phase species, and this was confirmed by the experimental data. On the other hand, Hg and Se were predicted to be only present in gas phase species, and this was also confirmed experimentally. The elements As, B, Cd, Pb, Sb and Sn were all predicted to form a larger amount of gaseous species than was observed in the experimental measurements. Refinement of the predictions for As and B by inclusion of specific minor/trace element interactions with Ni and Ca respectively gave a better agreement with the experimental data. Whilst the experimentally-observed lowering of Pb emissions by reduction of the gas cleaning temperature from 580 °C to 450 °C was qualitatively predicted, the concentration of Pb in the fine dust removed by the hot gas filter indicates condensation at higher temperatures than predicted. The absence of thermodynamic data for the more complex minerals and adsorbed species that may be formed is thought to account for some of these differences.

Commentary by Dr. Valentin Fuster

Combustion and Fuels

2002;():267-274. doi:10.1115/GT2002-30059.

DLR investigated forced combustion oscillations of two liquid fuel burners in a research combustion chamber at elevated pressures simulating idle conditions of aircraft engine combustors. The work was performed in collaboration with MTU Munich. An existing combustion chamber with optical access, capable to operate up to 20 bar, was upgraded with an air flow pulsator, that bypasses air from the combustor plenum to the exhaust with a sinusoidal massflow variation up to 700 Hz. Pressure transducers in the plenum and the flame tube monitored the forced disturbances. A photomultiplier recorded the OH* chemiluminescence of the flame. For the agreed operating conditions frequency scans of these values were registered. Additionally images of the OH* chemiluminescence were taken at selected frequencies and evaluated in a statistical manner, to separate turbulent and periodic behaviour. From the analysis of the pressure data, it can be concluded, that serious thermo-acoustic feedback was not observed for both burners. However burner 2 with the flame detached from the wall exhibited a higher fluctuation level as burner 1 with the wall attached flame. A resonant behaviour was observed near the characteristic frequency of the sound room comprised of plenum, flame tube and burner nozzle as connecting passage. The chemiluminescence images show different modes of spatial fluctuation for the burners and for burner 2 they also vary with the operating condition.

Commentary by Dr. Valentin Fuster
2002;():275-283. doi:10.1115/GT2002-30060.

Experimental evidence correlating equivalence ratio fluctuations with combustion instabilities and NOX emissions in a jet-A fueled lean premixed prevaporized (LPP) combustor utilizing a non-proprietary ‘generic’ fuel injector is presented. Real-time laser absorption measurements of equivalence ratio, together with dynamic combustor pressure, flame luminosity and fuel pressure were obtained at inlet air conditions up to 16.7 atm and 817 K. From this data, an extensive database of real-time variables was obtained for the purposes of providing validation data for future studies of LPP combustion modeling. In addition, time and frequency space analysis of the data revealed measurable levels of acoustic coupling between all variables. Equivalence ratio and dynamic pressure cross-correlations were found to predict the level of combustion instability. Furthermore, NOX production was found to follow the root-mean-square (RMS) flame luminosity and RMS combustor dynamic pressure. However, the unmixedness of the fuel-air mixture was not found to predict NOX production in this combustor. The generic LPP injector, although not optimized for low-emissions or combustion stability, provides some of the essential features of real injectors for the purposes of studying the relationship between fluctuations in equivalence ratios and combustion instability. In particular, the fuel premixer advection time was found to have a significant and direct impact on the level of combustion instability. The results of this work support the time-lag concept for avoiding combustion instability when designing injector/premixers in LPP combustors.

Commentary by Dr. Valentin Fuster
2002;():285-297. doi:10.1115/GT2002-30061.

With the advent of lean premixed gas turbine combustors, research in the area of thermo-acoustic instabilities and active combustion control came into the limelight. To be able to predict and control these instabilities, it is required that both the acoustics of the system, and a frequency-resolved response of the combustion process to velocity perturbations be understood. Experimental techniques developed by the Virginia Active Combustion Control Group at Virginia Tech, to obtain an open loop flame transfer function were applied to both fully and partially premixed swirl stabilized turbulent gaseous flames using commercial grade methane as fuel. A frequency-resolved fluctuating velocity was applied at the inlet of the combustor within the frequency range of 20–400 Hz, and the OH* chemiluminescence was used as a measure of the fluctuating heat release rate within the flame. Experiments were conducted at atmospheric pressure for two swirl numbers of 0.79 and 1.19, and three equivalence ratios of 0.55, 0.60 and 0.65. The flow rates studied resulted in Reynolds numbers of 14,866 and 19,821. The results show that for the linear range, the magnitude of the FRF is primarily dependent on the premixing quality and the mean energy content of the mixture, while the phase of the FRF is quite sensitive to Φ′ oscillations and to the variations in the species concentration across the cross-section of the flow.

Commentary by Dr. Valentin Fuster
2002;():299-308. doi:10.1115/GT2002-30062.

This paper considers the effects of background turbulent fluctuations upon a combustor’s stability boundaries. Inherent turbulent fluctuations act as both additive and parametric (also called multiplicative) excitation sources to acoustic waves in combustors. While additive noise sources exert primarily quantitative effects upon combustor oscillations, parametric noise sources can exert qualitative impacts upon its dynamics; particularly of interest here is their ability to destabilize a “nominally” stable system. The significance of these parametric noise sources increases with increased background noise levels and, thus, may play more of a role in realistic, high Reynolds number systems than experiments on simplified, lab scale combustors might suggest. The objective of this paper is to determine whether and/or when these effects might be significant. The analysis considers the effects of fluctuations in damping rate, frequency and combustion response. It is found that the effects of noisy damping and frequency upon the combustor’s stability limits is quite small, at least for the fluctuation intensities estimated here. The effects of a noisy combustion response, particularly of a fluctuating time delay between flow and heat release perturbations, can be quite significant, however, in some cases for turbulence intensities as low as <(u′/ū)2 >1/2 ∼5–10%. These results suggest that deterministic stability models calibrated on low turbulence intensity, lab scale combustors may not adequately describe the stability limits of realistic, highly turbulent combustors.

Commentary by Dr. Valentin Fuster
2002;():309-320. doi:10.1115/GT2002-30063.

A novel method for the simulation of combustion instabilities in annular combustors is presented. It is based on the idea to solve the equations governing the acoustics in the time domain and couple them to a model for the heat release in the flames. The linear wave equation describing the temporal and spatial evolution of the pressure fluctuations is implemented in a finite element code. Providing high flexibility, this code in principle allows both the computational domain to be of arbitrary shape and the mean flow to be included. This yields applicability to realistic technical combustors. The fluctuating heat release acting as a volume source appears as a source term in the equation to be solved. Employing a time-lag model, the heat release rate at each individual burner is related to the velocity in the corresponding burner at an earlier time. As saturation also is considered, a non-linearity is introduced into the system. Starting the simulation from a random initial perturbation with suitable values for the parameters of the heat release model, a self-excited instability is induced, leading to a finite-amplitude limit cycle oscillation. The feasiblity of the approach is demonstrated with 3D-simulations of a simple model annular combustor. The effect of the model parameters and of axial mean flow on the stability and the shape of the excited modes is shown.

Commentary by Dr. Valentin Fuster
2002;():321-331. doi:10.1115/GT2002-30064.

A two-dimensional low-order model of a generic annular premixed combustor, comprising an annular combustion chamber connected to an annular plenum via a finite number of burners, is developed and validated. The shapes and frequencies of the eigenmodes as well as the stability of the combustor against self-excited oscillations can be predicted with such a model. The dynamical characteristics of each burner is described mathematically in terms of its transfer matrix. The case where the transfer matrices of individual burners differ from each other can be handled by the model formulation presented. This is important in situations where non-identical burners are used in an annular combustor as a means of passive control, or where nonlinear effects lead to non-identical burner behaviour. The resulting loss of axisymmetry enhances the coupling between nonplane acoustic modes of different order. This modal coupling is accounted for by the model. The eigenmode shapes and frequencies predicted by the low-order model are validated by comparison with the results of a three-dimensional finite element acoustic model of a generic annular combustor configuration.

Commentary by Dr. Valentin Fuster
2002;():333-344. doi:10.1115/GT2002-30065.

The operating range of heavy duty gas turbines featuring lean premix combustion to achieve low Nox emissions may be limited by thermoacoustic oscillations. The most promising way to extend the operational envelope of the gas turbine is to modify the burner outlet conditions which itself strongly affect the flame response on acoustic perturbations. The objective of the present paper is the analysis and prediction of the flame response of premixed swirl flames which are typical for gas turbine combustion. The flame response has been determined experimentally by measuring the velocity fluctuations of a forced pulsated burner flow with hot wire probes and the resulting heat release fluctuations OH radiation. The experimentally determined flame response function for the swirl premixed flame follows almost a time lag law. Hence, reasonable agreement has been found between measurements and calculations using a time lag model.

Topics: Flames
Commentary by Dr. Valentin Fuster
2002;():345-353. doi:10.1115/GT2002-30068.

The present paper describes an active control system consisting of a fast-acting actuator valve, coupled with a control algorithm capable of adaptive phase and amplitude control for pressure oscillation suppression. Experiments were conducted using two separate combustion test rigs: a small, lean premixed, tubular combustor (75 kW) and a larger premixed annular DLE system (4 MW). Active control of pressure oscillations at frequencies of approximately 90 Hz and 300 Hz was demonstrated on the 75 kW rig. Up to 90% reduction in single-frequency dynamic pressure amplitude and 70% reduction in peak-mean was achieved. Simultaneous suppression of these two distinct modes was also demonstrated. The system was also demonstrated on a full-scale 4 MW combustion rig, with peak-mean dynamic pressure reductions between 33–45%. Instability reductions were achieved by pulsing fuel supplied to either a diffusion or a premixed flame. The response of the flame was notably better for the premixed case. For the premixed flame, combustor pressure pulses were realizable up to a frequency of about 400Hz, while the diffusion flame could only induce combustor pressure pulses up to approximately 50Hz. Adaptive frequency, phase and amplitude logic were developed, allowing automatic selection of the optimal values of these parameters to maximize suppression efficacy to suit the particular operating condition.

Commentary by Dr. Valentin Fuster
2002;():355-366. doi:10.1115/GT2002-30069.

Modern industrial gas-turbine spray combustors feature multiple swirlers and distributed fuel injection for rapid mixing and stabilization. The present paper is the first of a sequence of papers that aim to study the flow field of such combustors, the related combustion characteristics and their control. The present paper focuses on an experimental investigation of the velocity flow field downstream of a Triple Annular Swirler (TAS) and CFD description of the flow field inside the TAS. Multiple combinations of swirlers of various swirl level and rotational direction were tested in cold flow under atmospheric conditions without a confining combustion chamber. The experiments showed that a Central Toroidal Recirculation Zone (CTRZ) and an annular jet with internal and external shear layers characterize the flow field downstream of TAS. The CTRZ is axisymmetric but the jet contains imprints of the internal flow and has some nonaxisymmetric features. Numerical RANS results confirmed these observations and showed that the asymmetry relates to the effect of the internal swirling vanes on the jet flow.

Commentary by Dr. Valentin Fuster
2002;():367-376. doi:10.1115/GT2002-30070.

Fuel-cooled thermal management, including endothermic cracking and reforming of hydrocarbon fuels, is an enabling technology for advanced aero engines and offers potential for cycle improvements and pollutant emissions control in gas-turbine engine applications. The successful implementation of this technology is, however, predicated on the use of conventional multi-component hydrocarbon fuels and an understanding of the combustion characteristics of the reformed fuel mixture. The objective of this research is to develop and demonstrate the technologies necessary for utilizing conventional multi-component hydrocarbon fuels for fuel-cooled thermal management, including the development of the endothermic potential of JP-7 and JP-8+100, a demonstration of the combustion of supercritical/endothermic fuel mixtures, and conceptual design of a fuel-air heat exchanger. The ability to achieve high heat sinks with existing jet fuels (e.g., JP-7 and JP-8+100) was demonstrated with a bench-scale test rig operating under flow conditions and passage geometries simulative of practical heat exchangers for aircraft and missile applications. Key measurements included fuel heat sink, reaction products, and extent of conversion. Full-scale sector rig tests were conducted to characterize the combustion and emissions of supercritical jet fuel, and demonstrate the safety and operability of the fuel system, including a fuel-air heat exchanger.

Commentary by Dr. Valentin Fuster
2002;():377-383. doi:10.1115/GT2002-30071.

Fuel deoxygenation is being developed as a means for suppressing autoxidative coke formation in aircraft fuel systems, thereby increasing the exploitable cooling capacity of the fuel, enabling major increases in engine operating temperature and cycle efficiency. Reduced maintenance is an added benefit. A prototype membrane filter module for on-line removal of dissolved oxygen, which would otherwise react to form coke precursors, was constructed and successfully demonstrated. The fuel flows over the membrane, while oxygen diffuses through it at a rate that is proportional to the difference in oxygen partial pressures across the surface. Tests were conducted over a range of fuel flow rates (residence times) and temperatures. The filter was operated with air-saturated jet fuel for several hours at a steady-state condition, verifying the capability to remove essentially all of the dissolved oxygen (to <1 ppm) and proving the viability of the concept. A convincing demonstration of coke suppression was performed when air-saturated (normal) and deoxygenated jet fuels were tested in a standard ASTM heated tube apparatus at wall temperatures as high as 850 F. With deoxygenated fuel, there was a dramatic reduction (more than an order of magnitude) in coke deposition relative to air-saturated Jet A, which will allow the maximum fuel temperature to be increased by more than 200 F, doubling the available heat sink. Moreover, deoxygenated Jet A was shown to perform as well as JP-7, the Air Force’s highest thermal stability fuel. An analytical model for oxygen permeation through the membrane was formulated, and used in conjunction with the test data to estimate the filter size required for a practical (i.e., low-volume/high-flowrate) deoxygenator.

Topics: Fuels , Coke
Commentary by Dr. Valentin Fuster
2002;():385-394. doi:10.1115/GT2002-30072.

An LPP (Lean Pre-mixed Pre-vaporized) combustor is one of the most promising systems to make it possible to reduce NOx emission drastically. To realize low NOx combustors using liquid fuel, uniformity and fine atomization of fuel droplets are essential requirements. Droplet diameters of a fuel nozzle designed for LPP combustor as determined by PDPA measurement system are presented in this paper. An annulus pre-mixing duct was employed for the LPP fuel nozzle test model. Spray tests were conducted at pressures from 0.18MPa to 0.53MPa. Pre-mixing air velocity was also varied. Data show that the test nozzle produces a fine spray. In this paper, fuel droplet size distribution and velocity are presented and effects of air pressure and velocity on atomization are discussed. SMD of fuel droplets increases with the increases of ambient pressure. This is inconsistent with the trend determined by other works. But when the effect of fuel flow rate (or fuel film thickness) is considered, these inconsistencies can be resolved.

Commentary by Dr. Valentin Fuster
2002;():395-404. doi:10.1115/GT2002-30073.

Results from ignition and cross-ignition tests performed on an atmospheric 60°-sector test rig equipped with three EV-type burners are presented. Based on these results a model was developed for an annular combustor, which calculates the primary ignition and burner-burner cross-ignition limits for the combustor in terms of burner operation variables (equivalence ratio and pilot fuel ratio) using a generally applicable methodology described in the paper. Key ingredients of the model are the description of mixture flammability and a mixing model representing the ignition relevant mixing behaviour of the burners in the annular combustor. Ignition and cross-ignition are observed to occur, if the mixture equivalence ratio determined from the mixing model is above the flammability limits calculated for the particular operating conditions. Even in the case of cross iginition across an externally piloted or switched-off burner, the model reproduces the experimental cross-ignition limits, confirming that the basic physics have been captured.

Commentary by Dr. Valentin Fuster
2002;():405-412. doi:10.1115/GT2002-30074.

The present paper describes a new burner for a micro gas turbine utilizing the lean premixed prevaporized (LPP) combustion. The major objective of the new combustor concept is to achieve low pollutant emissions, in particular carbon monoxide (CO) and nitrogen oxide (NOx ). Therefore, a homogeneous air fuel mixture is imperative for a lean combustion. Due to the thermodynamic cycle conditions of the micro gas turbine, the combustion air temperature is too low for an intense evaporation of a liquid fuel droplet spray. The new combustor concept therefore, is based on fuel film evaporation on the hot inner surface of a premix tube. The heat required for fuel film evaporation is transferred from the hot combustion gases, flowing along the outer surface of the tube, through the tube wall. The combustor wall is a multi-layered assembly consisting of a ceramic inner liner, a compliant layer, and the outer metal casing. This design allows almost adiabatic combustion to be established. The design process of the combustor is assisted by comprehensive numerical studies of droplet and fuel film evaporation. The commercial CFD code “CFD-RC” has been utilized to investigate the isothermal flow of the combustor. The vortex flow of the burner, which provides for flame stabilization, is described in detail. First experimental tests have been conducted. Measured pollutant concentrations of the exhaust gases meet international standards and demonstrate the great potential of the new combustor.

Commentary by Dr. Valentin Fuster
2002;():413-422. doi:10.1115/GT2002-30075.

Flame flashback from the combustion chamber into the mixing zone limits the reliability of swirl stabilized lean premixed combustion in gas turbines. In a former study, the combustion induced vortex breakdown (CIVB) has been identified as a prevailing flashback mechanism of swirl burners. The present study has been performed to determine the flash-back limits of a swirl burner with cylindrical premixing tube without centerbody at atmospheric conditions. The flashback limits, herein defined as the upstream flame propagation through the entire mixing tube, have been detected by a special optical flame sensor with a high temporal resolution. In order to study the effect of the relevant parameters on the flashback limits, the burning velocity of the fuel has been varied using four different natural gas-hydrogen-mixtures with a volume fraction of up to 60% hydrogen. A simple approach for the calculation of the laminar flame speeds of these mixtures is proposed which is used in the next step to correlate the experimental results. In the study, the preheat temperature of the fuel mixture was varied from 100 °C to 450 °C in order to investigate influence of the burning velocity as well as the density ratio over the flame front. Moreover, the mass flow rate has been modified in a wide range as an additional parameter of technical importance. It was found that the quenching of the chemical reaction is the governing factor for the flashback limit. A Peclet number model was successfully applied to correlate the flashback limits as a function of the mixing tube diameter, the flow rate and the laminar burning velocity. Using this model, a quench factor can be determined for the burner, which is a criterion for the flashback resistance of the swirler and which allows to calculate the flashback limit for all operating conditions on the basis of a limited number of flashback tests.

Topics: Combustion , Vortices
Commentary by Dr. Valentin Fuster
2002;():423-428. doi:10.1115/GT2002-30076.

Most of the processes using wood fuels in gas turbine applications that are presently being studied are based on gasification of the wood fuel and operating the gas turbine with the product gas. An alternative is running the gas turbine with the hot gas from a wood combustor — the directly wood particle fired gas turbine. This technique offers the possibility to realise efficient and cost effective small scale power generation systems in the low power range (1–2 MWe). For realizing a directly wood particle fired gas turbine, the Institute of Thermal Turbomachines and Powerplants at the Vienna University of Technology developed a two stage combustor. Solid and liquid fuels require relatively long residence times and good mixing with the oxidant to be completely burned. This can be achieved in the primary stage designed as a cyclone combustor/gasifier. In the cyclone chamber, burning fuel particles are suspended, according to their size, caused by centrifugal and drag forces. This cyclone effect of the flow offers the possibility that big particles remain in the cyclone combustor until they have been completely burned. Using a two stage combustor, the combustion process can be divided into two zones: A primary zone for fuel-rich pyrolysed-gasified-combustion and a secondary zone where the gasification products from the primary zone are oxidized with excess air. Staged combustion has the potential to reduce NOx (NO, NO2 and N2 O), CO and total hydrocarbons Cn Hm concentrations in the exhaust. A large series of test runs was carried out with 3 different fuels, numerous fuel feed rates and equivalence ratios in the cyclone combustor resulting in stable operating conditions and almost total carbon burn-out. The main purpose of the test runs was to investigate the effect of air staging and temperature on the emissions of CO, Cn Hm and NOx .

Commentary by Dr. Valentin Fuster
2002;():429-439. doi:10.1115/GT2002-30077.

Demand for greater engine efficiency and thrust-to-weight ratio has driven the production of aircraft engines with higher core temperatures and pressures. Such engines operate at higher fuel–air ratios, resulting in the potential for significant heat release through the turbine if energetic species emitted from the combustor are further oxidized. This paper outlines the magnitude and potential for turbine heat release for current and future engines. The analysis indicates that in the future, high fuel-air ratio designs may have to consider changes to cooling strategies to accommodate turbine heat release. A characteristic time methodology is developed to evaluate the chemical and fluid mechanical conditions that lead to combustion within the turbine. The local concentration of energetic emissions partly determines the potential for energy release. An energy release parameter, here defined as a maximum increase in total temperature (ΔTt ), is used to specify an upper limit on the magnitude of impact. The likelihood of such impacts relies on the convective, mixing, and chemical processes that determine the fate and transport of energetic species through the turbine. Appropriately defined Damköhler numbers (Da)—the comparative ratio of a characteristic flow time (τflow ) to a characteristic chemical time (τchem )—are employed to capture the macroscopic physical features controlling the flow-chemistry interactions that lead to heat release in the turbine.

Commentary by Dr. Valentin Fuster
2002;():441-447. doi:10.1115/GT2002-30078.

To meet increasingly tight regulations on emission control appropriate combustor designs need to be developed. With different combustion concepts like RQL (Rich Quench Lean) and LPP (Lean Premixed Prevaporized) it has been proven that it is possible to reach the objective of a significant reduction of the NOX emissions. To gain further insight into the real combustion process it is of importance to be able to “look into” the flame without interfering with the actual combustion process. At the combustion laboratory of the Institute of Flight Propulsion at Munich University of Technology a combustion test facility is set up to study combustion characteristics under pressure up to 6 bar and inlet airflow temperature up to 650 K. A newly designed LPP concept was adapted into an optically accessible model combustion chamber. The objective of the study was to operate the LPP combustor under semi-realistic conditions and to obtain more knowledge on the influence of pressure on the combustion process. With suitable non-intrusive laser-spectroscopic measuring techniques like LIF (Laser Induced Fluorescence) the fuel spray, the nitric oxides and the hydroxyl radical were detected in several planes parallel to the combustor axis at different combustor pressures. As expected the pressure has a strong effect on droplet distribution and evaporation. Also with increasing pressure it was possible to operate the combustor under leaner conditions. A strong dependence on pressure of the formation of nitric oxides was detected. To quantify these results samples with a water-cooled probe were taken, analyzed and compared with the non intrusive measurements.

Commentary by Dr. Valentin Fuster
2002;():449-458. doi:10.1115/GT2002-30080.

In the area of stationary power generation, there exists a growing interest in understanding the role that gaseous fuel composition plays on the performance of natural gas-fired gas turbine systems. In this study, an atmospherically fired model gas turbine combustor with a fuel flexible fuel/air premixer is employed to investigate the impact of significant amounts of ethane and propane addition into a baseline natural gas fuel supply. The impacts of these various fuel compositions, in terms of the emissions of NOX and CO, and the coupled impact of the degree of fuel/air mixing, are captured explicitly for the present system by means of a statistically oriented testing methodology. These explicit expressions are also compared to emissions maps that encompass and expand beyond the statistically based test matrix to verify the validity of the employed statistical approach.

Commentary by Dr. Valentin Fuster
2002;():459-468. doi:10.1115/GT2002-30081.

This paper describes a cycle analysis study on the use of the staged prevaporizer-premixer injector (SPP) in high-pressure gas turbine systems fired with liquid fuel. A review of the SPP is given, including discussions of its operational concepts and previous research. The main portions of the paper consist of analyzing the use of the SPP in three different gas turbine systems: a steam-injected gas turbine (STIG) engine, a Frame H gas turbine in combined cycle, and a reheat gas turbine in combined cycle. Focus is placed on determining the effect of the SPP on cycle efficiency. In addition, SPP use in an engine conventionally recuperated by heat exchange from the exhaust gas stream to the compressor discharge air is examined. The SPP offers the potential of low NOx emissions for liquid-fired gas turbines. Because water injection is a method currently practiced for the reduction of NOx, simulations of engines without the SPP but with water injection into the combustor are also performed and comparisons are made. The simulation process is described, as are methods of how the SPP is implemented into the various engines. Results of the study are given, showing the effect of SPP use on cycle efficiency. In general, except for application to the conventionally recuperated engine, use of the SPP causes a decrease in cycle efficiency of around 1–3 percent (relative). The impact of water injection is somewhat greater, causing a 2.5–4 percent (relative) decrease in cycle efficiency. Further, the water injection does not provide as much NOx control as the lean prevaporized-premixed combustion.

Topics: Gas turbines , Cycles
Commentary by Dr. Valentin Fuster
2002;():469-481. doi:10.1115/GT2002-30082.

As part of an effort to develop a micro-scale gas turbine engine for power generation and micro-propulsion applications, this paper presents the design, fabrication, experimental testing, and modeling of the combustion system. Two radial inflow combustor designs were examined; a single-zone arrangement and a primary and dilution-zone configuration. Both combustors were micro-machined from silicon using Deep Reactive Ion Etching (DRIE) and aligned fusion wafer bonding. Hydrogen-air and hydrocarbon-air combustion was stabilized in both devices, each with chamber volumes of 191 mm3 . Exit gas temperatures as high as 1800 K and power densities in excess of 1100 MW/m3 were achieved. For the same equivalence ratio and overall efficiency, the dual-zone combustor reached power densities nearly double that of the single-zone design. Because diagnostics in micro-scale devices are often highly intrusive, numerical simulations were used to gain insight into the fluid and combustion physics. Unlike large-scale combustors, the performance of the micro-combustors was found to be more severely limited by heat transfer and chemical kinetics constraints. Important design trades are identified and recommendations for micro-combustor design are presented.

Commentary by Dr. Valentin Fuster
2002;():483-490. doi:10.1115/GT2002-30083.

This paper describes the design and testing of a catalytically-stabilized pilot burner for current and advanced Dry Low NOx (DLN) gas turbine combustors. In this paper, application of the catalytic pilot technology to industrial engines is described using Solar Turbines’ Taurus 70 engine. The objective of the work described is to develop the catalytic pilot technology and document the emission benefits of catalytic pilot technology when compared to higher, NOx producing pilots. The catalytic pilot was designed to replace the existing pilot in the existing DLN injector without major modification to the injector. During high pressure testing, the catalytic pilot showed no incidence of flashback or autoignition while operating over wide range of combustion temperatures. The catalytic reactor lit off at a temperature of approximately 598K (325°C/617°F) and operated at simulated 100% and 50% load conditions without a preburner. At high pressure, the maximum catalyst surface temperature was similar to that observed during atmospheric pressure testing and considerably lower than the surface temperature expected in lean-burn catalytic devices. In single injector rig testing, the integrated assembly of the catalytic pilot and Taurus 70 injector demonstrated NOx and CO emission less than 5 ppm @ 15% O2 for 100% and 50% load conditions along with low acoustics. The results demonstrate that a catalytic pilot burner replacing a diffusion flame or partially-premixed pilot in an otherwise DLN combustor can enable operation at conditions with substantially reduced NOx emissions.

Commentary by Dr. Valentin Fuster
2002;():491-500. doi:10.1115/GT2002-30084.

The research and development of a unique combustion engine is presented. The engine converts the thrust from ramjet modules located on the rim of a disk into shaft torque, which in turn can be used for electrical power generation or mechanical drive applications. A test program was undertaken that included evaluation of the pre-prototype engine and incorporation of improvements to the thrust modules and supporting systems. Fuel mixing studies with vortex generators and bluff body flame holders demonstrated the importance of increasing the shear-layer area and spreading angle to augment flame volume. Evaluation of flame-holding configurations (with variable fuel injection methods) concluded that the heat release zone, and therefore combustion efficiency, could be manipulated by judicious selection of bluff body geometry, and is less influenced by fuel injection distribution. Finally, successful operation of novel fuel and cooling air delivery systems have resolved issues of gas (fuel and air) delivery to the individual rotor segments. The lessons learned from the pre-prototype engine are currently being applied to the development of a 2.8MW engine.

Commentary by Dr. Valentin Fuster
2002;():501-511. doi:10.1115/GT2002-30085.

A wave rotor is proposed for use as a constant volume combustor. A novel design feature is investigated as a remedy for hot gas leakage, premature ignition and pollutant emissions that are possible in this class of unsteady machines. The base geometry involves fuel injection partitions that allow stratification of fuel/oxidizer mixtures in the wave rotor channel radially, enabling pilot ignition of overall lean mixture for low NOx combustion. In this study, available turbulent combustion models are applied to simulate approximately constant volume combustion of propane and resulting transient compressible flow. Thermal NO production histories are predicted by simulations of the STAR-CD code. Passage inlet/outlet/wall boundary conditions are time-dependent, enabling the representation of a typical deflagrative internal combustor wave rotor cycle. Some practical design improvements are anticipated from the computational results. For a large number of derivative design configurations, fuel burn rate, two-dimensional flow and emission levels are evaluated. The sensitivity of channel combustion to initial turbulence levels is evaluated.

Commentary by Dr. Valentin Fuster
2002;():513-521. doi:10.1115/GT2002-30087.

A general efficient strategy for the design optimization of duct-burners for combined-cycle plants is presented. This methodology combines a widely employed commercial code, used for the flow analysis, with a progressive optimization strategy, whose efficiency relies on the simultaneous convergence of both the flow solution and the optimization process, as well as on the use of progressively finer grid levels. The proposed strategy has been initially tested versus two inverse design examples with known solutions; then, it has been employed to flatten the outlet thermal profile of a new enhanced-mixing after-burner. The presented results show that the overall optimization process requires a computational time compared to that required by 5 ÷ 14 flow analyses.

Commentary by Dr. Valentin Fuster
2002;():523-531. doi:10.1115/GT2002-30088.

A combustor has been developed to burn a low calorific gas mixture reflecting a composition typically available from a bio-mass gasification plant. This reference composition contained (by volume) 11,7% H2 , 15,4% CO, 5,9% CH4 , 13,3% CO2 , 46,7% N2 and 7% H2 O. The combustor was subsequently tested with gas compositions having varying amounts of NH3 , H2 O and CO/H2 content. It was also tested with three compositions rich in CO, but lacking H2 ; these are typically available from blast furnace, or other metallurgical processes. The combustor is designed to be stoichiometric/lean and is suitable for up to 2,1 MW thermal input. The flame tube walls are predominantly effusion-cooled. A natural gas pilot is provided for ignition and operation up to 20% load. UHC emissions were only seen when operating on the reference LCV composition below 40% load. CO emissions were less than 20 ppmv between 40% and 100% load regardless of gas composition tested. Combined thermal and prompt NOX , when operating without ammonia addition, was found to be less than 9 ppmv at full load for the reference gas. When ammonia was introduced to the gas composition the molar ammonia conversion to NOX was approximately 60% for 2500 ppmv NH3 fuel concentration. This was seen to increase to 90% as the ammonia concentration was reduced to 500 ppmv. The combustor showed acceptable temperatures while operating on the reference composition. The compositions having higher net calorific value produced excessive flame tube temperatures. The combustor had excellent combustion stability regardless of gas composition and operating condition.

Commentary by Dr. Valentin Fuster
2002;():533-544. doi:10.1115/GT2002-30089.

Results of a low-NOx combustor test with a 15° sector are presented. A multipoint, lean-direct injection concept is used. The configuration tested has 36 fuel injectors and fuel-air mixers in place of a dual annular arrangement of two conventional fuel injectors. An integrated-module approach is used for the construction where chemically etched laminates that are diffusion bonded, combine the fuel injectors, air swirlers and fuel manifold into a single element. Test conditions include inlet temperatures up to 866K, and inlet pressures up to 4825 kPa. The fuel used was Jet A. A correlation is developed relating the NOx emissions to the inlet temperature, inlet pressure, and fuel-air ratio. Using a hypothetical 55:1 pressure-ratio engine, cycle NOx emissions are estimated to be less than 40% of the 1996 ICAO standard.

Commentary by Dr. Valentin Fuster
2002;():545-552. doi:10.1115/GT2002-30090.

Since 1998, the Honeywell Engines & Systems, Combustion & Emissions Group has been developing an advanced, CFD-based, parametric, detailed design-by-analysis tool for gas turbine combustors called A dvanced C ombustion T ools (ACT). ACT solves the entire flow regime from the compressor deswirl exit to the turbine stator inlet, and can be used for combustor diagnostics, design, and development. ACT is applicable to can, through-flow, and reverse-flow combustors, and accommodates features unique to different combustor designs. The main features of ACT are: 1. Reduction of Analysis Cycle Time : Geometry modeling and grid generation are fully parametric and modular, using an inhouse feature-based technology. Each geometrical feature can be deleted, replaced, added, and modified easily, quickly, and efficiently. 2. Elimination of Inter-Feature Boundary Assumptions : All the complex combustor features, such as wall cooling configuration, details of the air swirler assemblies and fuel atomizer systems, dome-shroud/cowl wall, and splash cooling plate, are considered and fully coupled into the CFD calculations. This allows the plenum and annulus aerodynamics to interact directly with the combustor internal flow. 3. Ease of Use : To reduce setup time and errors and to facilitate parametric studies, ACT is highly customized for engineers. 4. Accurate and Efficient CFD Solutions : Advanced physical submodels of combustion and spray have been implemented. This paper provides an overview and development experiences of ACT. Application of ACT to a through-flow combustor system is presented to illustrate the approach as applied to real-world combustors. Validation of the ACT system, by comparison to test cell data, is in-progress and will be the subject of a future paper.

Commentary by Dr. Valentin Fuster
2002;():553-562. doi:10.1115/GT2002-30091.

A prediction methodology based on Large-Eddy Simulation (LES) has been used to study turbulence-chemistry interactions in spray combustion. The unsteady interactions between spray dispersion and vaporization, fuel-air mixing and heat release has been investigated using a Stochastic Separated Flow model for spray within the LES formulation. The effects of swirl intensity and heat release are investigated here. Results show that the central toroidal recirculation zone (CTRZ), which is a manifestation of the vortex breakdown process, occurs only under high swirl conditions. Under non-reacting condition, droplets tend to concentrate in regions of low vorticity and increase in swirl increases the dispersion of the droplets. Mixing efficiency is enhanced and the size of the corner recirculation zone is decreased with increase in swirl. Increase in swirl also enhances the combustion processes for cases with heat release.

Commentary by Dr. Valentin Fuster
2002;():563-572. doi:10.1115/GT2002-30092.

It is well recognised that many important combustion phenomena are kinetically controlled. Whether it be the burning velocity of a premixed flame, the formation of pollutants in an exhaust stack or the conversion of NO to NO2 in a gas turbine combustor, it is important that a detailed chemical kinetic approach be undertaken in order to fully understand the chemical processes taking place. This study uses a genetic algorithm to determine new reaction rate parameters (A’s, β’s and Ea ’s in the Arrhenius expressions) for the combustion of both a hydrogen/air and methane/air mixture in a perfectly stirred reactor. In both cases, output species profiles obtained from an original set of rate constants are reproduced by a new different set obtained using a genetic algorithm inversion process. The new set of rate constants lie between predefined boundaries (±25% of the original values) which in future work can be extended to represent the uncertainty associated with experimental findings. In addition, this powerful technique may be used in developing reaction mechanisms whose newly optimised rate constants reproduce all the experimental data available, enabling a greater confidence in their predictive capabilities. The results of this study therefore demonstrate that the genetic algorithm inversion process promises the ability to assess combustion behaviour for fuels where the reaction rate coefficients are not known with any confidence and, subsequently, accurately predict emission characteristics, stable species concentrations and flame characterisation. Such predictive capabilities will be of paramount importance within the gas turbine industry.

Commentary by Dr. Valentin Fuster
2002;():573-580. doi:10.1115/GT2002-30094.

This paper presents the premixed Conditional Moment Closure (CMC) method as a new tool for modeling turbulent premixed combustion with detailed chemistry. By using conditional averages the CMC method can more accurately model the affects of the turbulent fluctuations of the temperature on the reaction rates. This provides an improved means of solving a major problem with traditional turbulent reacting flow models, namely how to close the reaction rate source term. Combined with a commercial CFD code this model provides insight into the emission formation pathways with reasonable runtimes. Results using the full GRI2.11 methane kinetic mechanism are compared to experimental data for a backward facing step burning premixed methane. This model holds promise as a design tool for lean premixed gas turbine combustors.

Commentary by Dr. Valentin Fuster
2002;():581-590. doi:10.1115/GT2002-30095.

Using a consistent coupling of the ILDM-method for the chemical kinetics with the G-equation approach for describing the flame front propagation, the kinetic effects in the post-flame region of turbulent premixed flames are investigated. Turbulent flow and mixing fields are described by means of first order moment closures, whilst a multidimensional presumed PDF approach is applied to capture the turbulence-chemistry interaction. Mean scalar quantities of both side of the premixed flame front are calculated based on reaction progress variables conditioned on the G-evolution equation. The model derived, very well applicable to premixed and partially premixed flames, is used to simulate a turbulent premixed methane/air flame. A fair agreement with experimental data for major concentration fields and minor species could be achieved. Apparently, the coupling to the G-equation does not provide more additional accuracy in a case where the consideration of piloting plays a non negligible role.

Commentary by Dr. Valentin Fuster
2002;():591-600. doi:10.1115/GT2002-30096.

Over the past two decades Computational Fluid Dynamics (CFD) has become increasingly popular with the gas turbine industry as a design tool. By applying CFD techniques during the early stages of designing a product, engineers can establish the key parameters and dimensions of a system before any experimental trial and error tests are made, thus reducing the product cycle time and costs. This study compares CFD predictions with a comprehensive set of experimental measurements made at QinetiQ on the combustion of aviation fuel within a modem airspray combustor. The performances of two separate models describing the chemical interactions are compared. First, an equilibrium model was employed and linked to the 3D commercial solver, FLUENT 5.5, through a mixture fraction/PDF lookup table approach. Similarly a flamelet model was implemented using a recently developed detailed chemical reaction mechanism describing aviation fuel combustion which has previously received rigorous testing with regard to its predictive performance over a wide range of combustion conditions (Patterson et al., 2001). Both cases predicted heat transfer through a new non-adiabatic PDF lookup table generator developed within the department. This allowed the implementation of a discrete phase model that treats the fuel entering the combustor as a fine liquid spray before evaporating and arriving in the gaseous phase. Two turbulence models (k-ε and Reynolds Stress models) were also used and the results of each compared.

Commentary by Dr. Valentin Fuster
2002;():601-608. doi:10.1115/GT2002-30097.

Highly unsteady flow fields are generated in recent low-emissions gas turbine combustors. Numerical simulation of such flows using conventional numerical code using a time-averaged turbulence model is difficult and time-accurate LES (Large Eddy Simulation) is expected to predict them. Calculation of turbulent combusting and non-combusting flow field in a staged combustor were conducted using LES. To validate the LES calculation, a prediction of time-averaged velocity field is compared with those by an experiment and a conventional numerical method based on RANS model. Turbulence intensity affects flame speed so much that velocity fluctuations were measured to obtain turbulence intensity in the non-combustion test. Strongly turbulent regions between the pilot and main stages, which are important for the flame propagation, were simulated. The combustion was calculated using a laminar flamelet model and the flame propagating phenomenon was simulated properly, which is impractical by the conventional simulations using time-averaged turbulence models. The feasibility of the LES calculation is discussed.

Commentary by Dr. Valentin Fuster
2002;():609-617. doi:10.1115/GT2002-30098.

This paper describes a fuel and air mixer being developed for an 11 MW gas turbine catalytic combustion system. The fuel is natural gas. The mixer is based loosely on the radial-swirler design used in the XONON™-2.0 combustor, but has an axial swirler and inlet. Features have been incorporated in the design to make it resistant to flameholding. A combination of atmospheric testing and advanced CFD analysis have resulted in a design that is close to meeting its design targets of +/− 5% fuel to air uniformity, +/− 10°C thermal uniformity and 0.5% pressure loss. Modal and thermal stress finite element analyses have been incorporated in the development from its beginning phases to assure that the final design will meet life targets. This mixer is one of two alternative designs being considered for the production version of the catalytic combustion system.

Commentary by Dr. Valentin Fuster
2002;():619-628. doi:10.1115/GT2002-30099.

This paper describes a test programme undertaken by Rolls-Royce plc and QinetiQ Ltd. where a series staged Dry Low Emissions (DLE) combustor was internally traversed in 3 dimensions. The primary motivation of the exercise was to generate gaseous composition data for Combustion CFD validation and to gain greater insight into the changes in the detail of chemical composition along the length of the combustor. The combustor was run at baseload conditions corrected for 10 bar. An RB211 DLE combustion system was modified so that it could be installed into the QinetiQ internal traversing facility. Within this facility a position-calibrated gas-sampling probe was traversed within the combustor. Measurements of gaseous concentrations were made of CO, CO2 , NOx , NO2 , HC and O2 for over 400 points in 6 axial planes in both primary and secondary zones. Data within this paper, however, is reported for the four axial locations of most interest. The combustion temperature, AFR and efficiency were calculated from the gas analysis. A selected summary of the traverse results in terms of the gaseous compositions (NO and NOx ), efficiency and AFR is included. Combustor performance in terms of efficiency, mixing and emissions has also been evaluated. The results show that although the combustion contains a great deal of structure at the inlets to both the primary and secondary zones, efficiency is close to 100% by the exit of each zone. Peak NOx emissions identified in the primary zone reached acceptable values by the combustor exit plane. Although a large number of points were measured, recommendations include further work with different operating conditions and better coverage of each axial plane.

Commentary by Dr. Valentin Fuster
2002;():629-638. doi:10.1115/GT2002-30100.

An experimental and numerical characterization of a macrolaminate pressure atomizer, placed perpendicularly to a high-velocity, turbulent air stream, is presented in this work. The purpose of the study was to compare detailed spray measurements with computations using a commercial CFD code. This study was part of the development of the premixing section of a midsize gas turbine, redesigned to meet low emissions and dual fuel market requirements. First, the spray characteristics were determined by injecting into a quiescent environment at ambient conditions. This data provided input for CFD calculations. Then the fuel injector was placed in a test section, at ambient conditions as well, that simulated the cross flow position of the atomizer in the prototype combustor. Droplet size and velocity were measured downstream of the injector nozzle, using a one-dimensional Phase Doppler Particle Analyzer. Measurements were done in two measuring planes. Flow field measurements were made to establish a common base for the computations. 2D computations were made of these experiments, using a k-ε turbulence model. The droplet trajectories were calculated with a Lagrangian ‘random walk’ technique, including drop break-up. The computed droplet size and velocity show agreement with the measurements. Drop break-up was also well represented by the model. The computed dispersion of the injected mass is not in agreement with the measured profile. This discrepancy in droplet dispersion is possibly due to high turbulence levels in the flow field, which were not well captured in the model.

Commentary by Dr. Valentin Fuster
2002;():639-647. doi:10.1115/GT2002-30101.

An aerodynamic study for the premixing device of an industrial turbine gas combustor is discussed. The present work is based on a joint application of numerical CFD and experimental investigation tools in order to verify and optimize the combustor gaseous fuel injection system. The objective is the retrofit of an old generation gas turbine combustion chamber that is carried out considering new targets of NOx emission keeping the same CO and combustion stability performances. CFD has been used to compare different premixing duct configurations for improved mixing features. Experimental test has been carried out in order to assess the pollutant emissions, flame stability and pattern factor characteristics of the full combustion chamber retrofitted with the modified injection system.

Topics: Gas turbines
Commentary by Dr. Valentin Fuster
2002;():649-657. doi:10.1115/GT2002-30102.

Fuel placement and air-fuel mixing in a generic aeroengine premix module employing plain jet liquid fuel injection into a counter-swirling double-annular crossflow were investigated at different values of air inlet pressure (6 bar, 700 K and 12 bar, 700 K) and liquid-to-air momentum flux ratio, both parameters being a function of engine power. Kerosene Jet A-1 was used as liquid fuel. Measurement techniques included LDA for investigation of the airflow and Mie-scattering laser light sheets and PDA for investigation of the two-phase flow. Measurements were taken at various axial distances from the fuel nozzle equivalent to mean residence times of up to 0.47 ms. It was found that the initial fuel placement reacts very sensitively to a variation of liquid-to-air momentum flux ratio. Susceptibility of the spray to dispersion due to centrifugal forces and to turbulent mixing is primarily a function of the fuel droplet diameters, which in turn depend on operating pressure. The data are interpreted by evaluation of the corresponding Stokes numbers.

Topics: Fuels
Commentary by Dr. Valentin Fuster
2002;():659-663. doi:10.1115/GT2002-30103.

Cofiring of biogas in existing gas turbines is a feasible option to reduce the consumption of natural gas. However, admixing of biogas will have an effect on the combustion process. As a consequence, the burning velocity and, therefore, the flame stability may be affected when a significant amount of biogas is mixed with the natural gas. The effect of admixing natural gas with biogas on the stability of the combustion process in lean premixed gas turbines is insufficiently known. In the present paper, a Computational Fluid Dynamics (CFD) methodology will be presented for the assessment of the safe limit of biogas cofiring in a gasturbine. An advanced Flamelet/Flamefront combustion model [1, 2, 3] and the Coherent Flame Model [4] are utilized. In both models, the detailed GRI 3.0 reaction mechanism [5] has been used to describe the combustion chemistry. The degree of mixing of fuel and air in the lean premix-burners of the gasturbine has been determined with a separate CFD model of the burners.

Commentary by Dr. Valentin Fuster
2002;():665-671. doi:10.1115/GT2002-30104.

This contribution describes the systematic refinement of the hybrid burner used in Siemens Vx4.3A gas turbines for lean premix combustion of various liquid fuels such as Distillate fuel No. 2, Naphtha and Condensate. Additionally to the dry premix operation fuel/water emulsions are used in premix mode for a further reduction of NOx emissions or power augmentation. NOx emissions of less than 72 ppm are already achieved with the HR3 hybrid burner in dry premix mode. These can be reduced to values below of 42 ppm NOx in emulsion mode.

Commentary by Dr. Valentin Fuster
2002;():673-687. doi:10.1115/GT2002-30105.

A multi-species/reacting combustion study was performed. The focus of the study was to quantify the effects of variation in air extraction and power rates on flame/outlet temperatures of a General Electric (GE), Frame 5 gas turbine. The environmental contamination level due to generation of carbon monoxide was also reported. GE, Frame 5 gas turbine has been widely used around the world for power generation, and as mechanical drives. The combustion products were examined throughout a range of air extraction rates, upon which it was determined that the combustion liners were susceptible to damage at air extraction rates above 10%, and the environmental contamination level due to carbon monoxide was increased. Furthermore, the gas flow exiting the combustion liner became non-homogeneous (i.e. a pocket of relatively hot gas formed in the middle of the flow path), which would cause damage to the downstream components. In conclusion, the short-term monetary gains from using compressed air from a gas turbine do not justify the costs of down time for repairs and the replacement of expensive hot-gas-path components.

Commentary by Dr. Valentin Fuster
2002;():689-695. doi:10.1115/GT2002-30106.

From the very first beginning of the V64.3A development the HR3 burner was selected as standard design for this frame. The HR3 burner was originally developed for the Vx4.2 and Vx4.3 fleet featuring silo combustors in order to mitigate the risk of flashback and to improve the NOx-emissions (Prade, Streb, 1996). Due to its favourable performance characteristics in the Vx4.3 family the advanced HR3 burner was adapted to the Vx4.3A series with annular combustor (hybrid burner ring – HBR). This paper reports about the burner development for V64.3A gas turbines to reach NOx emissions below 25 ppmvd and CO emissions below 10 ppmvd. It is described how performance and NOx emissions have been optimised by implementation of fuel system and burner modifications. The development approach, emission results and commercial operation experiences as well are described. The modifications of the combustion system were successfully and reliably demonstrated on commercially running units. NOx emissions considerably below 25ppmvd were achieved at and above design baseload. An outlook to further steps of V64.3A burner development in the near future will be given in this paper.

Commentary by Dr. Valentin Fuster
2002;():697-703. doi:10.1115/GT2002-30107.

During the last few years OPRA has been working intensively on the development of an ultra low emissions combustor for the OP16 gas turbine. The main focus has been on the combustion of liquid fuels (diesel fuel #2), but a natural gas and a dual fuel system has also been developed. The most important aspect of the development has been the patented Controlled Fuel Air Ratio (COFAR) system incorporating the venturi premixer, the air valve and the fuel injection nozzle. The original diesel fuel injection nozzle of the OP16 was a hybrid design, comprising a pressure swirl central injector surrounded by a classic air-blast atomizer. While the emissions with this fuel nozzle were quite good (30 ppm up to 85% load), subsequent natural gas tests demonstrating single digit emissions, while running at a higher average flame temperature indicated that there was scope for improvement of the fuel preparation system. It was clear that atomization, evaporation and mixing of the diesel fuel could be further improved. For better understanding of the combustion of diesel fuel, an atomization and mixing model was developed, to study the quality of the fuel/air mixture leaving the pre-mixer. Based on the results of this study, a fuel nozzle system, using multipoint injection with small pressure swirl nozzles was selected. Three different sets of atomizers have been evaluated and a nozzle arrangement comprising five identical pressure swirl nozzles showed the best results. The emissions on diesel fuel with the new injector proved very satisfactory. The NOx concentration was kept below 25 ppm from 50% load up to 90% load and below 30 ppm at full load. CO and UHC were well below 10 ppm. These low emissions were achieved by running at a low flame temperature (below 1820K). Furthermore, no combustion dynamics or flame instability was observed.

Commentary by Dr. Valentin Fuster
2002;():705-712. doi:10.1115/GT2002-30108.

ALSTOM Power’s GT13E2 gas turbine has been successfully commissioned in a refinery residual oil gasification process (api Energia, Italy) operating on Medium Btu gas (GT13E2-MBtu). The modification of the standard GT13E2 to operate with MBtu fuel has resulted in an improvement in the performance of the GT13E2 to exceed 192 MW and 38% efficiency (simple cycle) at ISO conditions. The GT compressor has been upgraded to incorporate an extra-end stage to boost the pressure ratio to 17:1 and improve performance. Syngas from residual oil gasification has a typical volumetric composition of 45% H2 , 48% CO and 7% CO2 and a lower heating value of 13.9 MJ/kg. This syngas has been diluted with N2 to reduce the heating value to 7 MJ/kg lowering reactivity and allowing partially premixed operation. In order to operate with syngas a redesign of the standard EV burners was necessary to deal with the associated high flame velocities and volume fluxes. The fuel injection for syngas operation was placed at the burner end and the gas injected radially inward to obtain inherently safe operation. The gas turbine demonstrated successful operation with both syngas and oil No. 2 fuels. At the standard dilution of 7MJ/kg NOx emissions are in the 20–25 vppm range and the CO emissions are below 5 vppm independent of load. The modified burners demonstrated safe operation on syngas with and without dilution of nitrogen in a tested LHV range from 6.8 to 14 MJ/kg. This behavior allows high flexibility of the entire power plant. Changeover from oil no. 2 to syngas and vice versa can be done between 50 and 100% load. The gas turbine components have been inspected several times during the commissioning period and shown to be in good condition.

Topics: Gas turbines
Commentary by Dr. Valentin Fuster
2002;():713-720. doi:10.1115/GT2002-30409.

CFD Research Corporation has developed a promising lean direct fuel injector for application to all sizes of aero gas turbine engines. The patented injector design utilizes a bifurcated flow pattern structure that produces low NOx emissions at full power conditions and low CO, UHC and smoke emissions at low power. The design consists of three swirlers and two fuel circuits, and a flow splitter that divides the airflow into two airstreams. Two concentric fuel circuits fuel the two airstreams, producing two distinct flames: a pilot flame and a main flame. This unique flame structure allows separate control of two flame regions, but still allows them to interact. A large percent of combustor airflow enters through the injector. At low power (idle and approach), only the pilot circuit is fueled. The pilot fuel flows into the pilot airstream, and a flame is anchored in the bifurcated recirculation zone. Lean blowouts at idle conditions as low as an injector equivalence ratio of 0.04 have been realized. Low CO, UHC and smoke emissions have been demonstrated at these conditions. For power settings above approach, the fuel flow through the pilot is reduced, and the main is fueled. At cruise to max power conditions, only 10% of the fuel goes through the pilot and 90% goes through the main to achieve both low NOx and good stability. At max power conditions, the injector equivalence ratio is approximately 0.60–0.65, and the flame is blue and non-luminous, resulting in reduced heat loads to the liner. NOx emission levels have been measured in single-injector tests that show this injector has the potential to reduce the Landing/Takeoff (LTO) NOx levels by 60–70% compared with 1996 ICAO standards. This paper presents an overview of how the fuel injector works and how overall performance was achieved. The most difficult aspect of injector performance, i.e. achieving low lean blowout (LBO) fuel-air ratio, is described at the end of the paper, including LBO data for a range of configurations.

Commentary by Dr. Valentin Fuster
2002;():721-730. doi:10.1115/GT2002-30462.

Flame stabilization in a swirl-stabilized combustor occurs in an aerodynamically generated recirculation region which is a result of vortex breakdown. The characteristics of the recirculating flow are dependent on the swirl number and on axial pressure gradients. Coupling to downstream pressure pulsations is also possible. In order to fix the position of the recirculation zone, an extended fuel lance was inserted into the burner. An additional benefit of the extended lance was to enable secondary fuel injection directly into the recirculation zone where the flame is stabilized. Tests were conducted with and without secondary fuel injection. The measurements included optimization of the location of the extended lance in the mixing chamber and variation of the amount of secondary fuel injection at different equivalence ratios and output powers. Flow visualizations showed that stabilization of the recirculation zone was achieved. The effect of the extended lance on pressure and heat release oscillations and on emissions of NOx , UHC and CO was investigated. The results were confirmed in high pressure single burner pressure tests and in a full scale land-based test gas-turbine. The lance has been successfully implemented in engines with sufficient stability margins and good operational flexibility. This paper shows the careful development process from lab scale tests to full scale engine tests until the implementation into the field engines.

Topics: Combustion , Fuels
Commentary by Dr. Valentin Fuster
2002;():731-737. doi:10.1115/GT2002-30463.

For the micro gas turbine combustor, low NOx emission, high stability and complete combustion are requested. The objective of this study is to construct the flame to establish the above targets. The concepts of the combustor are (1) to use the circulation zone by swirl flow to ensure the flame stability and complete combustion and (2) to induce lean premixed combustion by mixing fuel and air at the inlet of combustor to ensure low NOx emission and prevention of flashback. Town gas is used as the fuel. We conduct experiments using three types of combustor design to investigate the characteristics of NOx and CO emission together with the investigation of the flame stability and visualization of the flame configuration. By improving the premixing of the fuel and air the NOx emission was minimized to get 3ppm (at 0% O2 ) with sufficiently low CO emission.

Commentary by Dr. Valentin Fuster
2002;():739-748. doi:10.1115/GT2002-30464.

The non-reacting fluid flow fields generated by multi-swirler arrays with two different port configurations were investigated experimentally. These two cases include a co-swirling array, where all swirlers act in the same direction, and a counter-swirling array, where all swirlers alternate between clockwise and counter-clockwise rotations. Each configuration consists of a three by three arrangement of swirlers, which use analogous geometries to generate the swirling flows, i.e. eight discrete jets. For each array, the mean and turbulence velocities were measured at approximately 25,000 discrete spatial locations by a two-component LDV system. Furthermore, the third velocity component was derived based on the symmetric geometry of the arrays. In both cases, the coalescence distance, distance for eight discrete jet streams to form uniform swirling flows, is very short; and each swirler within the arrangement possesses a central recirculation zone. These nine zones have varying strengths depending on the individual swirler location, and some rotate with the swirler direction. In comparison, the co-swirler configuration provides a large unified rotational flow in the same direction as the individual swirlers, while a counter-rotating flow pattern is formed by the other case. Lastly, interactions between the unified flow with that of each swirler in the array are discussed and further comparisons are made between the two cases.

Commentary by Dr. Valentin Fuster
2002;():749-757. doi:10.1115/GT2002-30465.

Legislation controlling the permitted levels of pollutant emissions from aircraft gas turbines has been an increasingly important design driver for the combustion system for some time, particularly with respect to oxides of nitrogen. This has lead to many suggestions for radical departures from the geometry of the classical combustor configuration involving, for example, lean premixed module technology, or staging (axially or radially) of combustor pilot and main zones. The optimum operation of any combustor also requires, however, appropriate and efficient distribution of compressor delivery air to the various flametube features (fuel injectors, dilution ports, for cooling and for air bleed purposes). Radial staging, leading to double annular combustor configurations, poses a particularly difficult challenge. The radial depth of the combustor increases to a level where the external aerodynamics of the combustor involves large flow turning after the pre-diffuser. Careful design is then needed to achieve acceptable levels of loss coefficient in the outer annulus. If these aspects are not properly addressed then inadequate penetration and mixing in the combustor interior can result, rendering low emissions performance impossible. This paper will report on the design, instrumentation and operation of a fully annular isothermal test facility, which has been developed specifically to enable this important issue of external flow quality in double annular combustor systems to be assessed. Representative inlet conditions to the combustion system are generated using a single stage axial compressor; modular construction enables quick and inexpensive changes to components of the combustor (pre-diffuser, cowl shape, liner port locations and geometrical details). Computerised rig control and data acquisition allows the collection of large amounts of high quality data. In addition to the calculation of overall system performance, it is then possible to identify flow mechanisms and loss-producing features in various zones and suggest appropriate modifications.

Commentary by Dr. Valentin Fuster
2002;():759-763. doi:10.1115/GT2002-30586.

Small and inexpensive jet engines are usually equipped with vaporizing fuel supply systems. This is in order to deliver low fuel flow-rates from relatively low-pressure fuel supply systems and the need for simple configuration. The difficulties associated with small engines are mainly during ignition or at high altitude re-lights, when the combustor is cold, air supply is poor, and fuel demand and pressure are low. Such conditions lead to poor atomization within the vaporizer resulting in very large droplets at its exit tip or even to a pool of liquid fuel within the combustor. Thus, there is no fuel vapor for ignition. Ignition is very difficult or even impossible under such conditions. Therefore, small engines are commonly equipped with dual fuel supply systems, either in the form of gaseous fuel for the ignition stage or with an additional higher-pressure supply line to the dedicated fuel nozzles for the purpose of ignition. Additional solutions involve the use of a large glow plug or high-energy pyrotechnic cartridges in the kilo-Joule range, to heat the combustor casing prior to ignition. The present work is concerned with the development of alternative and novel atomization systems, which would improve atomization at low pressures and consequently facilitate the ignition process, thus minimizing the need for supporting systems. The work refers to an alternative design for an existing vaporizer system of a small jet engine with 400 Nt of thrust. It focuses on an alternative design for the fuel injection within the vaporizer housing while maintaining all external dimensions and operating conditions unchanged. Three types of fuel nozzles were investigated: • a special impact atomizer, • a miniature pressure swirl atomizer, • a doublet atomizer involving two swirling nozzles (preliminary study only). Droplet size distribution under various nozzle pressure drops and air velocities were measured with Phase Doppler Particle Anemometry (PDPA ) and global spray characteristics were obtained by photography. All modified atomization systems demonstrated improved performance and better atomization than the existing system. Initially, water was used as a liquid. At a later stage, the modified impact atomizer was tested and successful spark ignition was demonstrated.

Topics: Fuels , Jet engines
Commentary by Dr. Valentin Fuster
2002;():765-771. doi:10.1115/GT2002-30587.

Research conducted at the supercritical (SC) facility of MIT’s Energy Laboratory provided visual confirmation of a single phase, homogeneous water/fuel mixture near the critical temperature and pressure of water. This mixture was subsequently burned under atmospheric spray conditions with very low NOx , smoke, CO, and HC. Larger supercritical fuel systems were developed and tested in a 300 hp combustion turbine combustor simulator, and a 30 kW microturbine. Early results comparing combustion of this water/fuel mixture with standard #2 fuel operation in the 30 kW turbine generator were uniformly positive. At full load conditions, levels of under 2.5 parts per million (ppmv) of NOx were achieved, over 90% lower than the baseline emissions, with CO emissions under 5 ppmv. These emission levels were in fact lower than those recorded at full load with a similar model microturbine, fueled with natural gas. These low levels were achieved without the increase in CO that is typically observed with water injection. It is suspected that these results are caused not only by thermal effects of the steam but by altering reactions in the flame.

Commentary by Dr. Valentin Fuster
2002;():773-780. doi:10.1115/GT2002-30607.

Fluidic modulation of liquid fuel flow has potential as an actuator for active control of gas turbine engine combustors, improving thrust and emissions performance. This paper describes an experimental investigation into the feasibility of liquid fuel modulation using a bi-stable fluidic diverter for active combustion control. The results demonstrate that a practical bi-stable fluidic valve can be designed for the gas turbine combustor application, satisfying requirements for flowrate, pressure drop and tolerance to downstream pressure variations. A limiting frequency of approximately 100 Hz was obtained in this investigation, demonstrating that the flow capacity requirement sets the upper limit of the fluidic valve response. A suggestion for improving the frequency response is discussed.

Topics: Combustion , Fuels , Valves
Commentary by Dr. Valentin Fuster
2002;():781-789. doi:10.1115/GT2002-30608.

At present, large-eddy simulations (LES) of turbulent flames with multi-species finite-rate kinetics is computationally infeasible due to the enormous cost associated with computation of reaction kinetics in 3D flows. In a recent study, In-Situ Adaptive Tabulation (ISAT) and Artificial Neural Network (ANN) methodologies were developed for computing finite-rate kinetics in a cost effective manner. Although ISAT reduces the cost of direct integration considerably, the ISAT tables require significant on-line storage in memory and can continue to grow over multiple flow-through times (an essential feature in LES). Hence, direct use of ISAT in LES may not be practical, especially in parallel solvers. In this study, a storage-efficient Artificial Neural Network (ANN) is investigated for LES application. Preliminary studies using ANN to predict freely propagating turbulent premixed flames over a range of operational parameters are described and issues regarding the implementation of such ANNs for engineering LES are discussed.

Commentary by Dr. Valentin Fuster
2002;():791-797. doi:10.1115/GT2002-30609.

Diffusion flame combustor test results are presented for methane firing in steam/air mixtures containing up to 20% steam. The tests were conducted at atmospheric pressure with combustor inlet temperatures up to 700K. Steam and air were fully premixed before combustion. Combustion efficiency and NOX levels were measured. The well-known Θ loading parameter was modified by replacing the combustor inlet temperature with the flame temperature. The flame temperature was defined as the stoichiometric temperature of the steam/air mixture. The combustion efficiency obtained with and without steam correlated nicely with this modified loading parameter. Calculated NOX levels agreed well with the measurements, where NOX was predicted using the flamelet technique. This approach makes it possible to predict combustor efficiencies with steam by using combustor performance data taken without steam. Preliminary design analyses of gas turbine cycles with significant steam addition can now easily include the impact of the steam on combustor performance.

Commentary by Dr. Valentin Fuster
2002;():799-805. doi:10.1115/GT2002-30610.

Combustion of wood powder may be applied in a two-stage multi-inlet combustion chamber. The primary stage of the combustion chamber has tangential air inlets to provide high swirl flow. The wood powder and its conveying air enter the gasification chamber axially through a center inlet in the bottom. The aim of the investigation was the analysis of the combustion flow of the primary stage of the combustion chamber. The calculation grid was three-dimensional and unstructured. Turbulence was modelled with the Reynolds-Stress-Model, species with mixture fraction/pdf-approach, radiation with the P1-model. Postprocessing has been done for particle tracks, the temperature distribution and tangential velocity distribution and for the species distributions of solid carbon, carbon monoxide, carbon dioxide and oxygen as well.

Commentary by Dr. Valentin Fuster
2002;():807-815. doi:10.1115/GT2002-30646.

Combustion instabilities are a major challenge in the development of low-emissions premixed gas turbine combustors. The development and demonstration of predictive capabilities for instabilities has progressed considerably. One of the major fundamental mechanisms demonstrated in several instances is the convection of fuel concentration fluctuations from the fuel injector to the reaction zone. A one-dimensional model has been developed which captures this mechanism coupled to solutions for standing acoustic waves. Since many real combustion systems include multiple flow paths for mixing and/or staged fuel injection, the model has been extended to include a parallel acoustic path and two fuel injection locations. Splitting of fuel between two injection positions is a common method to influence combustion dynamics toward a more operable system. A relatively simple model which only partially couples acoustics and heat release was applied to an axially staged combustor and the predictions are compared with the experimental behavior. The results from this model successfully predict the overall dynamics behavior as a function of the fuel split between the two injection locations.

Commentary by Dr. Valentin Fuster
2002;():817-822. doi:10.1115/GT2002-30668.

The United States Department of Energy’s National Energy Technology Laboratory (US DOE NETL) through the Strategic Center for Natural Gas (SCNG) recognizes the potential and benefits offered by the hybrid power system. In order to pursue this opportunity NETL manages a fuel cell turbine hybrid program. This program is designed to address the reduction of system costs through the resolution of technical issues and technology demonstrations. This paper reports on the status of the NETL fuel cell turbine hybrid program and the market opportunities for these systems.

Commentary by Dr. Valentin Fuster
2002;():823-829. doi:10.1115/GT2002-30670.

Pressurized solid oxide fuel cell (PSOFC)/micro gas turbine generator (MTG) hybrid power systems have the potential to generate electric power at high efficiency [circa 60% (net AC/LHV)] at multi-hundred kWe and multi-MWe capacities. Thus, good fuel economy and low CO2 emissions are positive system attributes, as are low NOx and SOx emissions due to the propensity of the SOFC for low NOx generation, the need for no firing of the gas turbine combustor during normal hybrid system power operations, and the use of desulfurized fuel. Exhaust temperatures are sufficiently high to enable the recovery of heat for steam/hot-water production, and system energy efficiencies of at least 80% are feasible. Work is ongoing at Siemens Westinghouse on three PSOFC/MTG power systems. Two, with 220 kWe and 300 kWe capacities, are proof-of-concept demonstration units. The 220 kWe PSOFC/MTG power system is in test at the National Fuel Cell Research Center, University of California-Irvine, and the 300 kWe system, which is currently being designed, will be demonstrated in two tests to be performed in Europe. The status of work on the 220 kWe and 300 kWe power systems is reviewed. The third system is to have capacity of at least 500 kWe. This system, which will be demonstrated also, is viewed as a prototype commercial product. The 500 kWe-class PSOFC/MTG concept is described and performance estimates are presented.

Commentary by Dr. Valentin Fuster
2002;():831-844. doi:10.1115/GT2002-30671.

Under the sponsorship of the U.S. Department of Energy/National Energy Technology Laboratory, a multi-disciplinary team led by the Advanced Power and Energy Program of the University of California at Irvine is defining the system engineering issues associated with the integration of key components and subsystems into power plant systems that meet performance and emission goals of the Vision 21 program. The myriad of fuel processing, power generation, and emission control technologies are narrowed down to selected scenarios in order to identify those combinations that have the potential to achieve the Vision 21 program goals of high efficiency and minimized environmental impact while using fossil fuels. The technology levels considered are based on projected technical and manufacturing advances being made in industry and on advances identified in current and future government supported research. Examples of systems included in these advanced cycles are solid oxide and molten carbonate fuel cells, advanced gas turbines, ion transport membrane separation and hydrogen-oxygen combustion.

Topics: Power stations
Commentary by Dr. Valentin Fuster

Education

2002;():845-851. doi:10.1115/GT2002-30153.

The mission of the United States Military Academy (USMA) is “To educate, train, and inspire the Corps of Cadets so that each graduate is a commissioned leader of character committed to the values of Duty, Honor, Country; professional growth throughout a career as an officer in the United States Army; and a lifetime of selfless service to the nation.” [1] In order to accomplish this mission, USMA puts their cadets through a 47-month program that includes a variety of military training, and college courses totaling about 150 credit-hours. Upon completion of the program, cadets receive a Bachelor of Science degree and become Second Lieutenants in the United States Army. A very unique aspect of the academic program at USMA is that each cadet is required to take a minimum of five engineering classes regardless of their major or field of study. This means that about 500 cadets will have taken the one-semester course in thermodynamics. The thermodynamics course taught at USMA is different from others throughout the country because within every class there is a mixture of cadets majoring in engineering and those that are in other majors, i.e. languages, history [2]. Topics on gas turbine machinery have been integrated into this unique thermodynamics course. Because the cadets will encounter gas turbines throughout their service in the Army, we feel that it is important for all of the students, not just engineering majors, to learn about gas turbines, their operation, and their applications. This is accomplished by four methods. The first is in a classroom environment. Cadets learn how actual gas turbines work, how to model them, and learn how to solve problems. Thermodynamics instructors have access to several actual gas turbines used in military applications to aid in cadet learning. The second method occurs in the laboratory where cadets take measurements and analyze an operational auxiliary power unit (APU) from an Army helicopter. The third method occurs in the form of a design project. The engineering majors redesign the cogeneration plant that exists here at West Point. Many of them use a topping cycle in this design. The final method is a capstone design project. During the 2001–02 academic year, three cadets are improving the thermodynamic laboratories. Among their tasks are designing a new test stand for the APU, increasing the benefit of the gas turbine laboratory through more student interaction, and designing a web-based gas turbine pre-laboratory instruction to compliment the actual laboratory exercise.

Commentary by Dr. Valentin Fuster
2002;():853-857. doi:10.1115/GT2002-30154.

This paper describes an undergraduate course on rotating machinery that has been developed at Virginia Tech with the close collaboration of an industry partner, the Bently Nevada Corporation. The course is offered as a technical elective in the Department of Mechanical Engineering for undergraduate students at the senior level. The motivation for the development of the course was to give students the experience of applying some of their basic courses to real engineering scenarios while educating students in additional specialized issues associated with the operation, design, and maintenance of rotating machinery. The development of this course is novel in that multiple academic, industrial, and institutional resources are utilized to create real engineering experiences appropriate for the undergraduate classroom. Creating this type of learning experience at the senior level helps students translate their basic engineering skills into practical engineering skills in order to successfully address real engineering scenarios that are not always straightforward problems based on straightforward information. This paper describes some of the course details and how a diversity of resources is incorporated into an undergraduate course. It is hoped that the information presented here will facilitate more industry and academic collaborations for the benefits of future engineers and the discipline itself.

Topics: Machinery
Commentary by Dr. Valentin Fuster
2002;():859-866. doi:10.1115/GT2002-30155.

One analysis that is often overlooked in the gas turbine classroom is that of component matching. Some “student friendly” analyses have been published for gas turbines with propulsion applications, including modeling of the components for computer analyses. To complement the propulsion analyses, a method of matching gas turbine components for an aeroderivative power generation gas turbine is presented in this paper. Matching is the process by which components are integrated to allow predictions of overall gas turbine performance. The fundamental method of matching components with generalized characteristic maps is first described. A single shaft power generation gas turbine (inlet/gas generator/regenerator/exhaust/shaft/load) is used. Matching is accomplished by simultaneously solving the matching closure equations along with the component maps (either mathematical models or graphical data). The method is demonstrated for an example moderate sized power generation unit with given inlet, compressor, burner, turbine, regenerator, exhaust, load, and shaft maps. Improvements with the inclusion of the regeneration unit are included. For the example, a range of fuel ratios was used and the unit was shown to increase in rotational speed and the compressor eventually surged as the fuel ratio was increased. Overall thermodynamic efficiency, power output, mass flow rate, rotational speed, and other overall and component parameters are shown as functions of the fuel flow rate. The technique lends itself well in the classroom so that students can appreciate the interdependence of the component performances. The technique is a tool in which a student can select components to optimize the overall performance and can predict off-design performance of a power generation unit.

Commentary by Dr. Valentin Fuster
2002;():867-876. doi:10.1115/GT2002-30588.

This paper describes an undergraduate program at the USAF Academy that uses the threads of engine design and research to teach and reinforce the fundamentals of gas turbine engines. Each year approximately twelve cadets who have chosen to specialize in the propulsion track of the Aeronautical Engineering major enter a five-course sequence that includes 1.) engineering thermodynamics, 2.) intro to propulsion (advanced cycle analysis), 3.) advanced propulsion (focus on engine component performance), 4.) intro to aircraft and propulsion system design, and 5.) aircraft engine design. During the fifth course, the cadets perform a preliminary engine design to meet various specified performance requirements unique to that semester. The cadets must select the engine cycle, size the engine considering installation effects, design the major components, select the materials, and consider safety, reliability, maintainability, and cost issues. The cadets then make an oral and written design presentation to a group from government and industry. In addition to the course work and the detailed design project, the cadets take a course in experimental methodology that is centered around an actual ongoing research project. The cadets are teamed up with experienced faculty researchers who lead them through each step in the research process. Examples of past research projects include: boundary layer separation on linear cascade blades, enhanced heat transfer in internal blade passages, and high-cycle fatigue forcing functions. Facilities that are available for cadet propulsion research include: a linear cascade wind tunnel, a Garrett F109 turbofan engine, a Continental J69 turbojet engine, and an Allison T63 turboshaft engine. Cadets are also encouraged to participate in a six-week research experience at one of several government, industry, or university laboratories. Finally, cadets can continue their participation with an independent study project. The design and research threads that are woven through the course work equip the cadets for success in graduate school or for making immediate contributions in the USAF propulsion community.

Commentary by Dr. Valentin Fuster

Electric Power

2002;():877-884. doi:10.1115/GT2002-30159.

Industrial gas turbines utilize numerous design features that cannot be implemented in jet aircraft turbines for weight reasons, but because of their straight-forward and robust nature trim costs and reduce both maintenance effort and operating risks. Regardless of manufacturer, the following design features, for example, have become the established industry standard: • single-shaft rotor; • 2 bearings at atmospheric pressure; • Journal bearing instead of ball bearings; • steel blading in the compressor. For the key components compressor, turbine, rotor, and combustion chamber of its 3A family (Fig. 1 ), Siemens has developed and tested additional features that reduce wear further and improve operational reliability. Operating experience gathered to date has shown that these features enable achievements of very high reliability and availability. Some of the measures described also contribute to enhanced output and efficiency.

Topics: Design , Gas turbines
Commentary by Dr. Valentin Fuster
2002;():885-899. doi:10.1115/GT2002-30160.

Repairs of gas turbine buckets (blades) are currently limited to the upper tip region of the airfoil where operating stresses are commonly low. This limitation is predicated upon the use of low strength welding filler materials and the use of high energy welding processes such as gas tungsten arc welding (GTAW). When bucket damage is located within parts of the airfoil subjected to higher stress levels, buckets are often scrapped or replaced, costing utilities/independent power producers millions of dollars. Advanced repair techniques which allow repairs at higher stress regions of the bucket airfoil will significantly reduce the need and costs of scrapping buckets. “Structural” repairs to higher stress areas of the airfoil will require the use of welding fillers with similar composition and strength to that of the bucket base alloy and improvements in repair processing.

Topics: Lasers , Maintenance
Commentary by Dr. Valentin Fuster
2002;():901-908. doi:10.1115/GT2002-30161.

The proliferation of new codes & standards for power generation equipment procurement, and their increased frequency of revision, contributes to an atmosphere of increasingly rapid change in global trade considerations. This dynamic environment has amplified intensely with each year, to an extent that the life cycle of a given standard is in many instances appreciably less than the delivery cycles of heavy machinery. Other issues are created by the slower pace of harmonization of codes & standards in the European Union (EU), US and elsewhere. These codes & standards cover requirements that include emissions, acoustics, and safety that exert pronounced effects on the design, manufacture, and integration of power plant components. Conformity assessment partnering and the importance of other expert interpretation services are a key component to successfully meeting evolving compliance requirements. Delivering Customer Fulfillment for the Order to Remittance (OTR) phase of a project must be circled back to the Inquiry to Order (ITO) front end of the business cycle for new proposals. Another interesting arena is the relationship of advanced prime mover design balanced with the need for standardization to meet these regulatory challenges in the face of high production volume. The typical power generation project cycle, measured in terms of years, coupled with the present high demand worldwide results in orders for equipment that in many cases can’t foresee regulatory requirements 2 to 3 years into the future. Examples include projects in the EU where the Pressure Equipment Directive (PED) and Atmospheres Explosive (ATEX) Directive have mandatory compliance dates of May 2002 and June 2003 respectively. Electric power generation Original Equipment Manufacturers (OEMs) and their suppliers must plan for and price into contracts compliance with these laws years before the equipment is built and shipped. This is further complicated by the interpretation of specific requirements and the definition of the OEM conformity assessment strategy. To rectify this situation, it is recommended that steps be initiated to accelerate the worldwide harmonization of technical standards. In addition, consideration for the delivery cycles and commissioning of new power plants must be included in the regulatory process and in setting the dates for mandatory compliance with regional law.

Commentary by Dr. Valentin Fuster
2002;():909-915. doi:10.1115/GT2002-30162.

Large combined cycle power plants utilizing advanced gas turbine technology are in demand worldwide due to attractive $/kw installation and operating cost advantages. A combined cycle plant has been operating since 1997 to determine the long-term reliability and hot parts durability of 1,500 degree C class M501G gas turbine technology that utilizes steam cooling of the combustor hardware. The verification is being conducted at MHI’s in-house combined cycle verification power plant known as T-Point. The verification is conducted while dispatching power to a local utility to augment the summer peak demand period. The gas turbine has accumulated over 12,000 actual operating hours and 650 start/stop cycles since it is primarily applied under Daily Start and Stop (DSS) duty. To date the availability has been 98.6 per cent, where Availability is defined as the actual power supply hours over the demanded power supply hours. The DSS duty imposes severe thermal-mechanical conditions that also facilitate in the accelerated assessment of the long-term reliability and parts durability. During the initial period of verification nearly 1,800 items were checked with special instrumentation, and about 1,000 items continue to be monitored in order to better quantify the physics. This has been supplemented by annual detailed overhaul inspections of the hardware to compare the accuracy of the predictions versus actual condition. Such inspections also included the rotor after approx. 10,000 operating hours to verify the integrity of all the parts in the rotor train. The knowledge and experience from the long-term verification has enabled several improvements because of valuable quantified data. (e.g enhancements, steam cooling effectiveness, etc.) Such verification data is critical for being able to introduce steam-cooled technology in new land based advanced gas technologies such as “G” and “H” class. Those are also important steps in commercial introductions of the M501G and M701G steam cooled combustor technologies. This paper describes results from the verification of the new technology with respect to operation, and design enhancements focused at reliability and hot parts life durability improvement.

Commentary by Dr. Valentin Fuster
2002;():917-925. doi:10.1115/GT2002-30163.

An experimental investigation was carried out at DOE NETL on the humid air combustion process using liquid fuel to determine the effects of humidity on pollutant emissions and flame stability. Tests were conducted at pressures of up to 100 psia (690 kPa), and a typical inlet air temperature of 860 °F (733 K). The emissions and RMS pressures were documented for a relatively wide range of flame temperature from 2440–3090 °F (1610 − 1970 K) with and without added humidity. The results show more than 90 percent reduction of NOx through 10 percent humidity addition to the compressed air compared with the dry case at the same flame temperature. The substantial reduction of NOx is due to a shift in the chemical mechanisms and cannot be explained by flame temperature reduction due to added moisture since the comparison was made for the same flame temperature.

Commentary by Dr. Valentin Fuster
2002;():927-934. doi:10.1115/GT2002-30164.

This study deals with a new internal combustion power system with a condensing steam-gas turbine (CSGT) and a circulating fluidized bed coal boiler (CFB) to produce electricity. Heat is delivered to the system by burning gaseous fuel in the CSGT combustor and by using coal combustion heat to generate the superheated steam for injection into the CSGT combustor. The optimization analysis of this novel dual-fuel energy system addresses the thermal efficiency of electric power generation and electric power generated by the system while offering the possibility of premium gaseous fuel saving. The ecological advantages are also identified.

Commentary by Dr. Valentin Fuster
2002;():935-943. doi:10.1115/GT2002-30466.

Methods for correcting data from gas turbine acceptance testing are discussed, focusing on matters which are not sufficiently covered by existing standards. First a brief outline is presented of the reasoning on which correction curves are based. Typical performance correction curves are shown together with the method of calculating mass flow rate and turbine inlet temperature from test data. A procedure for verifying guarantee data at a specific operating point is then given. Operation with water injection is then considered. Ways of correcting performance data are proposed, and the reasoning of following such a procedure is discussed. Corrections for water amount as well as power and efficiency are discussed. Data from actual gas turbine testing are used to demonstrate how the proposed procedure can be applied in actual cases of acceptance testing.

Commentary by Dr. Valentin Fuster
2002;():945-952. doi:10.1115/GT2002-30467.

The turbine inlet gas temperature ( Toso ) is an important parameter in design and performance analysis of gas turbine cycles. By increasing Toso , air bleeding for blade cooling increases and it can be about 25 percent of compressor inlet air mass flow rate for Toso equal to 1600 K. Therefore air bleeding has an important impact on thermal efficiency, specific power output and the optimum compressor pressure ratio at which maximum efficiency occurs. For the gas turbine part of a combined cycle, these performance curves are obtained and shown using a developed simulation program (GTE). Also for heat recovery steam generator (HRSG) part of a combined cycle plant, HRSG simulates the transient and steady state temperature distribution of hot gases, steam and tube metal at different parts of HRSG. Any number of pressure levels (high, intermediate and low) and heating elements (superheater, evaporator and economizer) including desuperheater and deaerator can be included. GTE outputs show less than two percent difference from reported measured values. This difference was less than six percent for HRSG model.

Topics: Simulation , Cycles
Commentary by Dr. Valentin Fuster
2002;():953-957. doi:10.1115/GT2002-30468.

The gas turbine efficiency drops quickly at part load as it is very dependant on turbine firing temperature. Therefore in combined cycle power plants the inlet guide vane is adjusted to maintain the high combustion chamber exhaust temperature. Simulations on influence of the inlet guide vane position control on combined cycle power plant transients have been carried out.

Commentary by Dr. Valentin Fuster
2002;():959-966. doi:10.1115/GT2002-30469.

Gas turbine driven power plants are subject to a number of new European Directives. The paper refers to the ATEX Directives and contains recommendations from a UK perspective of particular relevance to suppliers and users of gas turbines. It describes specifically the application of demonstrably effective dilution ventilation as a basis of safety and includes CFD methodology recommendations and a leak size determination method. It includes detail on the development and progress of the new ISO/European gas turbine safety standard intended to give further guidance and form a basis of compliance with Directives. It includes details of an extensive Industry funded research project intended to provide a better founded ventilation design basis. Its scope includes micro turbines as well as the full range of industrial and aero-derivatives. This paper follows from ASME 98-GT-215 [1].

Topics: Safety , Gas turbines
Commentary by Dr. Valentin Fuster
2002;():967-974. doi:10.1115/GT2002-30654.

This paper summarises achievements in the Siemens Westinghouse Advanced Turbine Systems (ATS) Program. The ATS Program, co-funded by the U.S. Department of Energy, Office of Fossil Energy, was a very successful multi-year (from 1992 to 2001) collaborative effort between government, industry and participating universities. The program goals were to develop technologies necessary for achieving significant gains in natural gas-fired power generation plant efficiency, a reduction in emissions, and a decrease in cost of electricity, while maintaining current state-of-the-art electricity generation systems’ reliability, availability, and maintainability levels. Siemens Westinghouse technology development concentrated on the following areas: aerodynamic design, combustion, heat transfer/cooling design, engine mechanical design, advanced alloys, advanced coating systems, and single crystal (SC) alloy casting development. Success was achieved in designing and full scale verification testing of a high pressure high efficiency compressor, airfoil clocking concept verification on a two stage turbine rig test, high temperature bond coat/TBC system development, and demonstrating feasibility of large SC turbine airfoil castings. The ATS program included successful completion of W501G engine development testing. This engine is the first step in the W501ATS engine introduction and incorporates many ATS technologies, such as closed-loop steam cooling, advanced compressor design, advanced sealing and high temperature materials and coatings.

Topics: Turbines
Commentary by Dr. Valentin Fuster
2002;():975-985. doi:10.1115/GT2002-30655.

The South Carolina Institute for Energy Studies (SCIES), administratively housed at Clemson University, has participated in the advancement of combustion turbine technology for nearly a decade. The Advanced Gas Turbine Systems Research (AGTSR) program has been administered by SCIES for the U. S. DOE. Under the supervision of the DOE National Energy Technology Laboratory (NETL), the AGTSR has brought together the engineering departments at the leading U.S. universities and U.S. combustion turbine developers to assist in providing a solid base of knowledge for the future generations of gas turbines. In the AGTSR program, an Industrial Review Board (IRB) of gas turbine companies and related organizations defines needed gas turbine research. SCIES prepares yearly requests for university proposals that address the research needs identified by the IRB organizations. IRB technical representatives evaluate the university proposals and review progress reports from the awarded university projects. Seventy-five (75) AGTSR university projects have been awarded in the areas of gas turbine combustion, aerodynamics/heat transfer, and materials. An overview of recent AGTSR university projects is given in this paper and research results from several of the projects are described in greater detail.

Topics: Gas turbines
Commentary by Dr. Valentin Fuster
2002;():987-1002. doi:10.1115/GT2002-30663.

All major manufacturers of large size gas turbines are developing new techniques aimed at achieving net electric efficiency higher than 60% in combined cycle applications. An essential factor for this goal is the effective cooling of the hottest rows of the gas turbine. The present work investigates three different approaches to this problem: (i) the most conventional open-loop air cooling; (ii) the closed-loop steam cooling for vanes and rotor blades; (iii) the use of two independent closed-loop circuits: steam for stator vanes and air for rotor blades. Reference is made uniquely to large size, single shaft units and performance is estimated through an updated release of the thermodynamic code GS, developed at the Energy Dept. of Politecnico di Milano. A detailed presentation of the calculation method is given in the paper. Although many aspects (such as reliability, capital cost, environmental issues) which can affect gas turbine design were neglected, thermodynamic analysis showed that efficiency higher than 61% can be achieved in the frame of current, available technology.

Commentary by Dr. Valentin Fuster

Vehicular and Small Turbomachines

2002;():1003-1007. doi:10.1115/GT2002-30402.

Rekuperator Svenska AB owned by VOLVO Technology Transfer Corporation and Avesta Polarit, has successfully developed a completely laser welded recuperator for micro-gas turbine applications. Tests have shown that the thermal performance is very competitive. The recuperator was installed in a 100 kW(e) micro-gas turbine power plant for combined electricity and heat generation by a customer. The recuperator is a primary surface counter flow heat exchanger with cross corrugated duct configuration. The primary heat transfer surface plate patterns are stamped and a pair of the plates are laser welded to form an air cell. The air cells are then stacked and laser welded together to form the recuperator core which is tied between two end beams. Manifolds for air inlet and outlet as well as piping system are welded to the core. Through varying the number of air cells the recuperator core can easily be adapted for micro-gas turbine applications with different output rates of electrical power. The key manufacturing technologies are stamping of the air cell plates and laser welding of the air cells. These processes can be fully automated for mass production at low costs.

Topics: Turbines
Commentary by Dr. Valentin Fuster
2002;():1009-1016. doi:10.1115/GT2002-30403.

During the last few years, a number of small microturbines (<100kW) have been tested in commercial markets. These microturbines have demonstrated low emissions, increased fuel flexibility, and reasonable durability. However, if these microturbines are to compete economically with larger gas turbines and reciprocating engines, manufacturing costs will need to be significantly reduced and thermal efficiencies will need to be increased. A preliminary study has been completed that evaluated larger and more efficient microturbines (∼300 kW) that operate at higher pressure ratios based on an intercooled and recuperated cycle. The thermal efficiency of the proposed concept increases to 34–37% and is competitive with larger gas turbines and similarly rated diesel engines. Two-stage turbocharger compressors and intercoolers that were developed by the automotive industry for high volume manufacturing will further improve the specific fuel consumption and specific power of this proposed microturbine concept. An additional benefit of the higher pressure, intercooled cycle is that the temperature of the exhaust gases exiting the turbine and entering the recuperator is significantly lower facilitating the use of lower cost materials in the recuperator.

Commentary by Dr. Valentin Fuster
2002;():1017-1023. doi:10.1115/GT2002-30404.

Capstone Turbine Corporation has produced thousands of recuperated microturbines since 1998. This paper will discuss the manufacturing and field operating experience with these recuperators for microturbines less than 100kW.

Commentary by Dr. Valentin Fuster
2002;():1025-1031. doi:10.1115/GT2002-30405.

A new recuperator is now being introduced for small and micro-turbine applications. These applications are characterized by low pressure ratios, exhaust gas temperatures that are suitable for use with stainless steel, and a limited allowance for leakage that does not impair the performance. However high reliability is required, because microturbines typically experience 4000 start/stop cycles during their life, and have to be totally reliable when supplying electricity for applications that are independent of the grid. Recuperated gas turbines have a history of expensive installations, prone to cracking under thermal shocks. The ACTE recuperator has been specifically designed for reliablity, compactness and low cost. This paper shows how these seemingly conflicting requirements are met in the ACTE design.

Topics: Gas turbines
Commentary by Dr. Valentin Fuster
2002;():1033-1044. doi:10.1115/GT2002-30406.

Microturbines are becoming increasingly important in the distributed power generation market. These machines are typically low pressure ratio gas turbines that require a recuperator to achieve the high, 30% or more, efficiency needed to compete in this market. However, the additional efficiency gained by a recuperator can easily be offset by its high initial cost. In response to this challenge, Proe Power Systems has developed, and has a U.S. patent pending on, the Proe 90™ gas turbine recuperator. The principal feature of the Proe 90™ recuperator is that it allows a high performance (high temperature, high effectiveness, low pressure drop) gas turbine recuperator to be manufactured by simply welding, brazing, or otherwise joining standard commercial tubing without the need for special tooling or manufacturing processes. The objective in developing the Proe 90™ recuperator was to provide a recuperator for gas turbine and related applications that can attain a minimum of 90% effectiveness with reasonable size and minimal cost. It meets those objectives by: having linear, counterflow, annular flow paths that avoid any thermal “short circuits”; by having sufficient margin to accommodate potential exhaust gas fouling of the low pressure flow passages; by having all surfaces either curved or stayed by flow tubes so that they can be made from commercially available tube and sheet stock while maintaining high margins of strength and creep resistance; and by avoiding thermal gradient stresses by having all non-isothermal portions of the recuperator able to freely expand and contract. The simple manufacturing process, design modeling techniques and predicted performance of the Proe 90™ recuperator are presented. Effects of tube length, diameter, and numbers of tubes on effectiveness and pressure losses are quantified. Additional parametric data show the effectiveness losses caused by axial conduction, flow misdistribution, manufacturing tolerances, and insulation losses. The Proe 90™ recuperator is ideally suited for microturbine distributed power applications in the 20–50 kW range. With properly sized tubes, the flow regime is laminar and results in a very small pressure loss while still producing very high heat exchanger effectiveness in a low cost, compact package.

Topics: Microturbines
Commentary by Dr. Valentin Fuster
2002;():1045-1051. doi:10.1115/GT2002-30543.

The use of a recuperator to recover waste heat from the exhaust gases is one method for improving a microturbine’s energy efficiency. This study looked at the effect of water vapor in the exhaust gas on the oxidation resistance of a current technology stainless steel and several high performance replacement alloys. Alloys of interest are high-Cr, Ni-base superalloys such as alloy 625 and aluminum-containing alloys such as Haynes alloy 214 and Plansee alloy PM2000, which is an oxide-dispersed FeCrAl. The latter two alloys form a protective external alumina scale which is more resistant to water vapor environments than chromia scales. Scanning and transmission electron microscopy characterization of the specimen surface oxides after laboratory exposures showed only minor effects of the addition of water vapor to the environment, which is consistent with the excellent corrosion resistance of these high performance alloys.

Commentary by Dr. Valentin Fuster
2002;():1053-1060. doi:10.1115/GT2002-30544.

A highly effective recuperator with low-pressure drop is a key enabling technology to increase the efficiency of a micro turbine. To achieve a cycle efficiency above 40%, the average micro turbine would require a recuperator that achieves close to 99% effectiveness with a pressure drop as low as 2%. This paper presents the design and analysis of a microchannel heat exchanger that could be used as a recuperator in a micro turbine as an alternative to the currently used metallic primary surface recuperators that achieve an effectiveness of 90% at most. In the proposed design, the recuperator will be a counter flow, multi-layer design of parallel ducts with wall thickness of 50 μm and will be constructed with SiCN or a similar polymer derived ceramics, and fabricated using micro-stereolithography technology. Two designs, one having ducts with square cross-sections and the other with equilateral triangle cross-sections, are proposed and compared. For each design, the geometric parameters are optimized to provide the highest overall cycle efficiency while the volume of the heat exchanger is kept limited to 0.125 m3 and other cycle parameters are kept constant at typical values. For the square cross section design, the optimization process provides a design with an effectiveness of 0.961 and pressure loss of 2.86% that correspond to a cycle efficiency of 39.4%. The corresponding values for the triangle cross-section design are 0.983, 2.4% and 42.2%, respectively. Both designs are expected to withstand temperatures up to 1300°C in combustion gases. Special strategy is needed to fabricate any of these two microchannel designs by existing or proposed micro-stereolithography systems. One possible option is to make the complete heat exchanger as a bundle of identical smaller segments so that overall performance is not affected.

Commentary by Dr. Valentin Fuster
2002;():1061-1066. doi:10.1115/GT2002-30545.

Since 1990, by the order of the Joint-Stock Company “GAZPROM” the Research-Engineering Center “Ceramic Heat Engines” has been involved in the development of the stationary ceramic gas-turbine engine (CGTE) of 2.5 MW, efficiency 42% which is notably in excess of the characteristics of the serial-production engines of the similar power. Alongside the application of the ceramic materials, a goal to increase the fire and explosion safety of CGTE through use of journal and thrust oil beatings was set up. The refinement of the embodied design of the CGTE gas-static bearings was carried out on specially designed test beds, initially on simplified and then on baseline test beds. On all stages, the rotors were “rigid”, the balancing accuracy ensured the residual imbalance not exceeding 5–7 g*mm, the air cleaning provided the moisture separation of at least 90% with the filtration being not worse than 80μm. The tests were carried out by two stages: initially the journal beatings without the axial load applied and then the thrust bearing component with variation of the axial load within the 0.2 ≤ Presi /Pbase ≤ 1.2 were developed. The maximum speed was 30,000 rpm (noper = 23,000–24,000 rpm). Thus, the performed complex of the computation-experimental operations allowed the development of the gas-static thrust-journal beatings for the 250 N mass CGTE rotor.

Commentary by Dr. Valentin Fuster
2002;():1067-1072. doi:10.1115/GT2002-30547.

Progress in the preliminary design of an axial vane ring and axial turbine rotor composed of silicon nitride structural ceramics is presented. The silicon nitride components allow for significantly increased turbine inlet temperature in the ST5+, an advanced version of the Pratt & Whitney Canada’s ST5, which powers the DTE Technologies ENT400 Distributed Energy Generator. Preliminary aerodynamic and mechanical design, steady state thermal and stress analysis, and life prediction results will be reviewed. Foreign object damage resistance will be compared between ceramic versions of the baseline rotor blade geometry and that developed for the ST5+ engine.

Commentary by Dr. Valentin Fuster
2002;():1073-1080. doi:10.1115/GT2002-30548.

Efficient micro turbines are expected to play a major role in power generation in the coming years. One of the biggest challenges in significantly increasing the system efficiency from the currently achievable values is the availability of high temperature materials that can be micro-fabricated with a low value of relative tolerance. This paper suggests a possible solution for both the material and the fabrication technique by which this goal can be achieved. Thermodynamic analysis shows that high turbine inlet temperatures, high isentropic efficiencies, and a recuperator with high effectiveness and low pressure losses are imperative to improve system efficiency, particularly in a micro turbine. An excellent relative tolerance with a high temperature ceramics is one way to achieve these improvements. The paper introduces new polymer-derived-ceramics (PDC), which could be used in turbine and recuperator designs. These materials can be used in micro-fabrication techniques to produce absolute tolerances of a few microns, and some of them are thermally stable up to 1800°C in air. The paper presents an enhancement to the typical micro-stereolithography technique, by which PDC can be micro-fabricated to form parts up to a few centimeters in overall linear dimensions and yet to have a relative tolerance that is comparable to or better than that in large-sized conventional parts. It is projected that for a turbine with 10 cm rotor outside diameter and 5 mm blade height, a tip-gap-to-blade-height ratio of better than 0.28% can be achieved by this proposed enhancement. This technique appears to be quite promising for next generation micro turbines, and hence requires further investigation and development.

Commentary by Dr. Valentin Fuster
2002;():1081-1085. doi:10.1115/GT2002-30549.

Use of ceramic components in the hot section of gas-fired microturbines would allow higher operating temperatures and thus better operating efficiencies. However, the cost of such ceramic components is an issue for commercial-scale production. Costs can be reduced, in part, through improving fabrication yields. Use of nondestructive evaluation (NDE) methods in early stages of fabrication will support the development process to improve yields and subsequently reduce costs by rejecting flawed components prior to final processing and proof testing. An NDE approach using high-speed 3D X-ray tomographic imaging has been investigated. A large (40 × 40 cm), flat-panel, amorphous silicon X-ray detector, together with fast image processing, has been shown to allow full-volume X-ray imaging with detection of internal features in full-size as-cast parts. Gelcast radial-flow microturbine rotors, ≈23 cm in diameter, have been studied for internal defects with this 3D X-ray imaging method. Internal cracks, voids, and other variations in density within the rotors have been detected. Data acquisition speeds of 3 full frames per second have been achieved with reconstruction times of individual cross-sections of less than 1 second. This paper presents details of the 3D X-ray imaging method and results achieved on full-size microturbine rotors.

Commentary by Dr. Valentin Fuster
2002;():1087-1095. doi:10.1115/GT2002-30576.

In this paper, the performance of the twin-entry radial flow turbine under steady state and partial admission conditions is modeled. The method, which is developed here, is based on one-dimensional performance prediction. In one-dimensional modeling, the flow properties are assumed constant on a plane normal to the flow direction. This assumption is in contrast with the flow at the rotor entry of a twin-entry turbine under partial admission condition. In this study the one-dimensional performance prediction method for single-entry turbine is modified to analyze the twin-entry turbine. In particular, the loss coefficients due to friction, clearance and blade loading, which are already developed for single-entry turbines, are modified. Also additional losses in the rotor are considered because of twin-entry rotor inlet conditions and the rotor-mixing losses. Indeed in a single-entry turbine with symmetric volute the flow tends to move toward the shroud. A correlation for the radial velocity profile at the rotor entry for this case is obtained and is considered to be optimum. Then the rotor mixing loss is estimated. Finally a model based on the above mentioned matters is developed. The results obtained from the model are compared with experimental results and good agreements are obtained. In this paper, special behaviors of the flow in the twin-entry turbine are also investigated and some physical interpretations are presented.

Topics: Turbines
Commentary by Dr. Valentin Fuster
2002;():1097-1104. doi:10.1115/GT2002-30577.

An investigation was conducted to perform study of small turbojet engine testing and analysis. This program was mainly to investigate the current steady and dynamic performance of a 12-pound thrust turbojet engine. As a result, we can establish the basis for future small turbojet engine design guidelines. This program involved the bench testing of the small turbojet engine at both design and off-design conditions, in order to establish the baseline performance of the engine. Using the PC based data acquisition system, we will be able to test the engine for both steady and dynamic performance and record data for different operating conditions, especially, during very rapid acceleration and deceleration of the engine at different rates. Also, a compressor map testing was conducted to provide the compressor map data to be used by the performance analysis. With the above testing, this program will establish a useful database for the 12-pound thrust turbojet engine. A performance analysis using GASTURB cycle analysis software was performed for performance prediction of the small turbojet engine for both steady state conditions and dynamic transient conditions. Both performance predictions agreed reasonably well with actual performance of the engine.

Topics: Engines , Turbojets
Commentary by Dr. Valentin Fuster
2002;():1105-1117. doi:10.1115/GT2002-30578.

Radial flow turbo machines have been used for a long time in a variety of applications such as turbochargers, cryogenics, auxiliary power units, and air conditioning of aircraft cabins. Hence numerous papers have been written on the design and performance of these machines. The only justification for yet another paper is that it would describe a unified approach for designing a single stage inward flow radial turbine comprising a rotor and the casing. The current turbine is designed to drive a direct-coupled permanent magnet high-speed alternator running at 60000 rpm and developing a maximum of 60 kW electrical power. The freedom of choice of the tip diameter and the tip width of the rotor that would be necessary for optimum isentropic efficiency of the turbine stage was restricted by the specified rotational speed and power output. Hence, an optimisation procedure was developed to determine the principal dimension of the rotor. The mean relative velocity in the rotor passages in the direction of the flow would be accelerated but flow velocity on the blade surfaces experiences a significant space rate of deceleration. The rate of deceleration can be controlled by means of a proper choice of the axial length of the rotor. A prescribed mean stream velocity distribution procedure was used to spread the rate of deceleration of the mean flow velocity along the meridional length of the flow passages. The nozzle-less volute casing was designed to satisfy the mass flow rate, energy and angular momentum equations simultaneously. This paper describes the work undertaken to design both the rotor and the casing. The work was motivated by the growing interest in developing gas turbine based hybrid power plant for road vehicles. The authors believe that the paper would lead to a stimulating discussion.

Commentary by Dr. Valentin Fuster
2002;():1119-1125. doi:10.1115/GT2002-30579.

The demand for high power density, reliable, low maintenance, oil-free turbomachinery imposes significant demands on the bearing system. The full benefits of high speed, permanent magnet driven machines, for example are realized at speeds exceeding the capabilities of rolling element bearings. The high speeds, and a desire for oil-free operation also make conventional liquid lubricated bearings an undesirable alternative. The modern, oil-free foil bearing provides an excellent alternative, providing low power loss, adequate damping for supercritical operation, tolerance of elevated temperatures and long life. In this paper, the application of modern foil bearings to a high speed, oil-free turbo-compressor is discussed. In this demanding application, foil bearings support a 24 pound, multi-component rotor operating at 70,000 RPM with a bending critical speed of approximately 43,000 RPM. Stable and reliable operation over the full speed range has been demonstrated. This application also required low bearing start-up torque for compatibility with the constant torque characteristic of the integral permanent magnet motor. This work discusses the rotor bearing system design, the development program approach, and the results of testing to date. Data for both a turbine driven configuration, as well as a high speed integral motor driven configuration are presented.

Commentary by Dr. Valentin Fuster
2002;():1127-1134. doi:10.1115/GT2002-30580.

A microscale gas turbine is under development at Tohoku University in Japan. Current objective of the project is to reveal the performance of the gas turbine at microscale with optimum aerodynamic shape. Therefore the engine to be tested will be fabricated by machining using a micro-5-axis end mill to realize three-dimensional modeling. The first step of the development has been split into the development of microturbocharger and microcombustor, to prevent the problem of the heat flow effect pointed out in the previous study [1]. The heat flow from the combustor to compressor will become relatively large at microscale, and this will degrade the performance of the compressor. The goal of the first step of the development is to achieve the required performance of the components to realize the gas turbine cycle, without the heat effect. Those are, 62% compressor efficiency, 870,000 rpm shaft rotating speed, and the self sustained combustion. A microscale turbocharger has been designed. The compressor impeller of diameter 10mm is expected to produce a pressure ratio of 3, and 68% compressor adiabatic efficiency. The bearings to realize the design rotational speed are hydrodynamic type gas bearing. Fabrication of the herring-bone grooves have been attempted, and successfully formed on a cylindrical surface by new etching procedure. A technique to fabricate three-dimensional turbine impellers at microscale by powder sintering of ceramics has been demonstrated. A semi-microcombustor has been fabricated and shown successful performance by burning hydrogen fuel.

Commentary by Dr. Valentin Fuster
2002;():1135-1141. doi:10.1115/GT2002-30581.

A test facility for screening and evaluating candidate materials for advanced microturbine recuperators is described. The central piece of the test facility is a modified 60 kW Capstone microturbine that serves as a test bed for subjecting test specimens to conditions of stress, environment and temperature that are representative of those experienced by the recuperator during microturbine operation. Special provisions have been incorporated into the design of this test facility for controlling the magnitude of the applied mechanical stress and the surface temperature of the test specimens with the objective of carrying out accelerated testing. Candidate materials for evaluation in this test facility are identified.

Commentary by Dr. Valentin Fuster

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