Showing papers in "Journal of Engineering for Gas Turbines and Power-transactions of The Asme in 2007"

Reduction of Numerical Parameter Dependencies in Diesel Spray Models

Abstract: Numerical grid and time-step-dependencies of Discrete Droplet Lagrangian spray models are identified. The two main sources of grid-dependency are due to errors in predicting the droplet-gas relative velocity, and errors in describing droplet-droplet collision and coalescence processes. For reducing grid-dependency due to the relative velocity effects, gas jet theory is proposed and applied to model diesel sprays. For the time-step dependency, it is identified that the collision sub-model results in drop size variation in the standard spray model. A proposed spray model based on the gas-jet theory is found to improve the time-step independency also along with the mesh independency. The use of both Eulerian (collision mesh) and Lagrangian (radius of influence) collision models along with gas-jet theory is found to provide mesh-independent results.Copyright © 2007 by ASME

FLOX® Combustion at High Pressure With Different Fuel Compositions

Abstract: In Flameless Oxidation (FLOX® ) the combustion is distributed over a large volume by a high internal flue gas recirculation. This technology has been successfully used for many years in technical furnaces under atmospheric conditions with very low NOx emissions. In the work presented here, FLOX® combustion was for the first time investigated at high pressure in order to assess its applicability for gas turbine combustors. A FLOX® burner was equipped with a combustion chamber with quartz windows and installed into a high pressure test rig with optical access. The burner was operated under typical gas turbine conditions at pressure of 20 bar with thermal powers up to 475 kW. Natural gas as well as mixtures of natural gas and H2 were used as fuel. The NOx and CO emissions were recorded for the different operating conditions. OH* chemiluminescence imaging and planar laser-induced fluorescence of OH were applied in order to characterize the flame zone and the relative temperature distributions. The combustion behaviour was investigated as a function of equivalence ratio and fuel composition, and the influence of the gas inlet velocity on mixing and emissions was studied. For various operating conditions the lean extinction limits were determined.Copyright © 2007 by ASME

Ignition of Lean Methane-Based Fuel Blends at Gas Turbine Pressures

Abstract: Shock-tube experiments and chemical kinetics modeling were performed to further understand the ignition and oxidation kinetics of lean methane-based fuel blends at gas turbine pressures. Such data are required because the likelihood of gas turbine engines operating on CH 4 -based fuel blends with significant (>10%) amounts of hydrogen, ethane, and other hydrocarbons is very high. Ignition delay times were obtained behind reflected shock waves for fuel mixtures consisting of CH 4 , CH 4 /H 2 , CH 4 /C 2 H 6 , and CH 4 /C 3 H 8 in ratios ranging from 90/10% to 60/40%. Lean fuel/air equivalence ratios (Φ=0.5) were utilized, and the test pressures ranged from 0.54 to 30.0 atm. The test temperatures were from 1090 K to 2001 K. Significant reductions in ignition delay time were seen with the fuel blends relative to the CH 4 -only mixtures at all conditions. However, the temperature dependence (i.e., activation energy) of the ignition times was little affected by the additives for the range of mixtures and temperatures of this study. In general, the activation energy of ignition for all mixtures except the CH 4 /C 3 H 8 one was smaller at temperatures below approximately 1300 K (∼27 kcal/mol) than at temperatures above this value (∼41 kcal/mol). A methane/hydrocarbon-oxidation chemical kinetics mechanism developed in a recent study was able to reproduce the high-pressure, fuel-lean data for the fuel/air mixtures. The results herein extend the ignition delay time database for lean methane blends to higher pressures (30 atm) and lower temperatures (1100 K) than considered previously and represent a major step toward understanding the oxidation chemistry of such mixtures at gas turbine pressures. Extrapolation of the results to gas turbine premixer conditions at temperatures less than 800 K should be avoided however because the temperature dependence of the ignition time may change dramatically from that obtained herein.

A New Bump-Type Foil Bearing Structure Analytical Model

Abstract: A gas bearing of bump foil type comprises an underlying structure made of one or several strips of corrugated sheet metal covered by a top foil surface. The fluid film pressure needs to be coupled with the behavior of the structure for obtaining the whole bearing characteristics. Unlike in classical elasto-aerodynamic models, a foil bearing (FB) structure has a very particular behavior due to friction interfaces, bump interactions, and nonisotropic stiffness. Some authors have studied this complex behavior with the help of three-dimensional finite element simulations. These simulations evidenced a lack of reliable analytical models that can be easily implemented in a FB prediction code. The models found in the literature tend to overestimate the foil flexibility because most of them do not consider the interactions between bumps that are highly important. The present work then develops a model that describes the FB structure as a multidegree of freedom system of interacting bumps. Each bump includes three degrees of freedom linked with elementary springs. The stiffnesses of these springs are analytically expressed so that the model can be adjusted for any dimensions and material properties. Once the stiffness matrix of the whole FB structure is obtained, the entire static system is solved taking friction into account. Despite its relative simplicity, comparisons with finite elements simulations for various static load distributions and friction coefficients show a good correlation. This analytical model has been integrated into a foil bearing prediction code. The load capacity of a first generation foil bearing was then calculated using this structure model as well as other simplified theoretical approaches. Significant differences were observed, revealing the paramount influence of the structure on the static and dynamic characteristics of the foil bearing. Some experimental investigations of the static stiffness of the structure were also realized for complete foil bearings. The structure reaction force was calculated for a shaft displacement with zero rotation speed, using either the multidegree of freedom model or the usual stiffness formulas. The comparisons between theoretical and experimental results also tend to confirm the importance of taking into account the bump interactions in determining the response of the structure.

Nonlinear flame transfer function characteristics in a swirl- stabilized combustor

Abstract: An understanding of the amplitude dependence of the flame response to acoustic excitation is required in order to predict and/or correlate combustion instability amplitudes. This paper describes an experimental investigation of the nonlinear response of a lean, premixed flame to imposed acoustic oscillations. Detailed measurements of the amplitude dependence of the flame response were obtained at approximately 100 test points, corresponding to different flow rates and forcing frequencies. It is observed that the nonlinear flame response can exhibit a variety of behaviors, both in the shape of the response curve and the forcing amplitude at which nonlinearity is first observed. The phase between the flow oscillation and heat release is also seen to have substantial amplitude dependence. The nonlinear flame dynamics appear to be governed by different mechanisms in different frequency and flowrate regimes. These mechanisms were investigated using phase-locked, two- dimensional OH Planar laser-induced fluorescence imaging. From these images, two mechanisms, vortex rollup and unsteady flame liftoff, are identified as important in the saturation of the flame’s response to large velocity oscillations. Both mechanisms appear to reduce the flame’s area and thus its response at these high levels of driving. DOI: 10.1115/1.2720545

Advancements in Gas Turbine Fuels From 1943 to 2005

Abstract: The first provisional jet fuel specifications were published in 1943 in England (RDE/F/ KER/210) and 1944 in the U.S. (AN-F-32a). Jet fuel has undergone many changes in subsequent years, with current specifications for JP-5 and JP-8 for the military in the U.S. and Jet A/Jet A-1 for commercial use worldwide. Jet fuel specifications are subject to constant tension between performance requirements and availability/cost considerations. In this paper we will discuss how jet fuels have evolved over the years from the first engines to current gas turbine engines. Jet fuels derived from nonpetroleum sources will also be discussed.

Hybrid Kalman Filter Approach for Aircraft Engine In-Flight Diagnostics: Sensor Fault Detection Case

Abstract: In this paper, a diagnostic system based on a uniquely structured Kalman filter is developed for its application to in-flight fault detection of aircraft engine sensors. The Kalman filter is a hybrid of a nonlinear on-board engine model (OBEM) and piecewise linear models. The utilization of the nonlinear OBEM allows the reference health baseline of the diagnostic system to be updated, through a relatively simple process, to the health condition of degraded engines. Through this health baseline update, the diagnostic effectiveness of the in-flight sensor fault detection system is maintained as the health of the engine degrades over time. The performance of the sensor fault detection system is evaluated in a simulation environment at several operating conditions during the cruise phase of flight.

Structural Stiffness, Dry Friction Coefficient, and Equivalent Viscous Damping in a Bump-Type Foil Gas Bearing

Abstract: High performance oil-free turbomachinery implements gas foil bearings (FBs) to improve mechanical efficiency in compact units. FB design, however, is still largely empirical due to its mechanical complexity. The paper provides test results for the structural parameters in a bump-type foil bearing. The stiffness and damping (Coulomb or viscous type) coefficients characterize the bearing compliant structure. The test bearing, 38.1 mm in diameter and length, consists of a thin top foil supported on bump-foil strips. A prior investigation identified the stiffness due to static loads. Presently, the test FB is mounted on a non-rotating stiff shaft and a shaker exerts single frequency loads on the bearing. The dynamic tests are conducted at shaft surface temperatures from 25 to 75°C. Time and frequency domain methods are implemented to determine the FB parameters from the recorded periodic load and bearing motions. Both methods deliver identical parameters. The dry friction coefficient ranges from 0.05 to 0.20, increasing as the amplitude of load increases. The recorded motions evidence a resonance at the system natural frequency, i.e., null damping. The test derived equivalent viscous damping is inversely proportional to the motion amplitude and excitation frequency. The characteristic stick-slip of dry friction is dominant at small amplitude dynamic loads leading to a hardening effect (stiffening) of the FB structure. The operating temperature produces shaft growth generating a bearing preload. However, the temperature does not significantly affect the identified FB parameters, albeit the experimental range was too small considering the bearings intended use in industry.

Analysis of combustion induced vortex breakdown driven flame flashback in a premix burner with cylindrical mixing zone

Abstract: In earlier experimental studies of the authors a previously unknown mechanism leading to flame flashback-combustion induced vortex breakdown (CIVB)-was discovered in premixed swirl burners. It exhibits the sudden formation of a recirculation bubble in vortical flows, which propagates upstream into the mixing zone after the equivalence ratio has exceeded a critical value. This bubble then stabilizes the chemical reaction and causes overheat with subsequent damage to the combustion system. Although it was shown earlier that the sudden change of the macroscopic character of the vortex flow leading to flashback can be qualitatively computed with three-dimensional as well as axisymmetric two-dimensional URANS-codes, the proper prediction of the flashback limits could not be achieved with this approach. For the first time, the paper shows quantitative predictions using a modified code with a combustion model, which covers the interaction of chemistry with vortex dynamics properly. Since the root cause for the macroscopic breakdown of the flow could not be explained on the basis of experiments or CFD results in the past, the vorticity transport equation is employed in the paper for the analysis of the source terms of the azimuthal component using the data delivered by the URANS-model. The analysis reveals that CIVB is initiated by the baroclinic torque in the flame and it is shown that CIVB is essentially a two-dimensional effect. As the most critical zone, the upstream part of the bubble was identified. The location and distribution of the heat release in this zone governs whether or not a flow field is prone to CIVB.

25Years of BBC/ABB/Alstom Lean Premix Combustion Technologies

Abstract: The paper will show the development of lean premix combustion technologies in BBC, ABB, and Alstom gas turbines. Different technologies have been developed and applied in Brown Boveri Company (BBC) before 1990. Considerable improvements with respect to NOx emissions as compared to gas turbines with a single combustor and a single diffusion burner for liquid and gaseous fuel have been achieved with burners with extended premixing sections and with multi-injection burners for annular combustors. Between 1990 and 2005, burners with short but effective premixing zones (EV burners: environmentally friendly V-shaped burners) have been implemented in all new gas turbines of the ABB (and later Alstom) fleet with NOx levels well below 25 vppmd (@15% O2). In addition to this, three variants of premix technologies have been successfully developed and deployed into Alstom GT engines: the sequential EV burners—a technology that allows premixing of natural gas and oil into a hot exhaust stream to reheat the exhaust gases of a first high-pressure turbine; the MBtu EV burners that are used to burn syngas in a premix flame with low NOx emissions; and the advanced EV burners (AEV) that are capable to prevaporize and premix liquid fuel prior to combustion and burn it with very low NOx emissions without water injection. The paper will give an overview of these technologies and their usage in Alstom gas turbines over the last 25years.

High-Pressure Turbine Deposition in Land-Based Gas Turbines From Various Synfuels

Abstract: Ash deposits from four candidate power turbine synfuels were studied in an accelerated deposition test facility. The facility matches the gas temperature and velocity of modern first stage high pressure turbine vanes. A natural-gas combustor was seeded with finely-ground fuel ash particulate from four different fuels: straw, sawdust, coal, and petroleum coke. The entrained ash particles were accelerated to a combustor exit flow Mach number of 0.31 before impinging on a thermal barrier coating (TBC) target coupon at 1150°C. Post exposure analyses included surface topography, scanning electron microscopy, and x-ray spectroscopy. Due to significant differences in the chemical composition of the various fuel ash samples, deposit thickness and structure vary considerably for each fuel. Biomass products (e.g. sawdust and straw) are significantly less prone to deposition than coal and petcoke for the same particle loading conditions. In a test simulating one turbine operating year at a moderate particulate loading of 0.02 parts per million by weight, deposit thickness from coal and petcoke ash exceeded 1 mm and 2 mm respectively. These large deposits from coal and petcoke were found to detach readily from the turbine material with thermal cycling and handling. The smaller biomass deposit samples showed greater tenacity in adhering to the TBC surface. In all cases, corrosive elements (e.g. Na, K, V, Cl, S) were found to penetrate the TBC layer during the accelerated deposition test. Implications for the power generation goal of fuel flexibility are discussed. [Keywords: deposition, syngas, turbines] NOMENCLATURE ESEM environmental scanning electron microscope TBC thermal barrier coating

Qualitative and Quantitative Comparison of Two Promising Oxy-Fuel Power Cycles for CO2 Capture

Abstract: Since the Kyoto conference there is a broad consensus that the human emission of greenhouse gases, mainly CO2 , has to be reduced. In the power generation sector there are three main alternatives which are currently studied world wide. Among them oxy-fuel cycles with internal combustion with pure oxygen are a very promising technology. Within the European project ENCAP — ENhanced CO2 CAPture — the benchmarking of a number of novel power cycles with CO2 capture was carried out [1]. Within the category oxy-fuel cycles the Graz Cycle and the Semi-Closed Oxy-Fuel Combustion Combined Cycle (SCOC-CC) both achieved a net efficiency of nearly 50%. In a second step a qualitative comparison of the critical components was performed according to their technical maturity. In contrast to the Graz Cycle the study authors claimed that no major technical barriers would exist for the SCOC-CC. In this work the ENCAP study is repeated for the SCOC-CC and for a modified Graz Cycle variant as presented at the ASME IGTI conference 2006 [2]. Both oxy-fuel cycles are thermodynamically investigated based on common assumptions agreed with industry in previous work. The calculations showed that the high-temperature turbine of the SCOC-CC plant needs a much higher cooling flow supply due to the less favorable properties of the working fluid. A layout of the main components of both cycles is further presented which shows that both cycles rely on the new designs of the high-temperature turbine and the compressors. The SCOC-CC compressor needs more stages due to a lower rotational speed but has a more favorable operating temperature. In general, all turbomachines of both cycles show similar technical challenges and are regarded as feasible.Copyright © 2007 by ASME

Lean Blowout Limits and NOx Emissions of Turbulent, Lean Premixed, Hydrogen-Enriched Methane/Air Flames at High Pressure

Abstract: Flame stability is a crucial issue in low NO x combustion systems operating at extremely lean conditions. Hydrogen enrichment seems to be a promising option to extend lean blowout limits (LBO) of natural gas combustion. This experimental study addresses flame stability enhancement and NO x reduction in turbulent, high-pressure, lean premixed methane/air flames in a generic combustor capable of a wide range of operating conditions. Lean blowout limits and NO x emissions are presented for pressures up to 14 bar, bulk velocities in the range of 32-80 m/s, two different preheating temperatures (673 K, 773 K), and a range of fuel mixtures from pure methane to 20% H 2 /80% CH 4 by volume. The influence of turbulence on LBO limits is also discussed. In addition to the investigation of perfectly premixed H 2 -enriched flames, LBO and NO x are also discussed for hydrogen piloting. Experiments have revealed that a mixture of 20% hydrogen and 80% methane, by volume, can typically extend the lean blowout limit by ∼10% compared to pure methane. The flame temperature at LBO is ∼60 K lower resulting in the reduction of NO x concentration by ≈35% (0.5→0.3 ppm/15% O 2 ).

CO2 Sequestration From IGCC Power Plants by Means of Metallic Membranes

Abstract: This paper investigates novel IGCC plants that employ hydrogen separation membranes in order to capture carbon dioxide for long-term storage. The thermodynamic performance of these membrane-based plants are compared with similar IGCCs that capture CO{sub 2} using conventional (i.e., solvent absorption) technology. The basic plant configuration employs an entrained-flow, oxygen-blown coal gasifier with quench cooling, followed by an adiabatic water gas shaft (WGS) reactor that converts most of CO contained in the syngas into CO{sub 2} and H{sub 2}. The syngas then enters a WGS membrane reactor where the syngas undergoes further shifting; simultaneously, H{sub 2} in the syngas permeates through the hydrogen-selective, dense metal membrane into a counter-current nitrogen 'sweep' flow. The permeated H{sub 2}, diluted by N{sub 2}, constitutes a decarbonized fuel for the combined cycle power plant whose exhaust is CO{sub 2} free. Exiting the membrane reactor is a hot, high pressure 'raffinate' stream composed primarily of CO{sub 2} and steam, but also containing 'fuel species' such as H{sub 2}S, unconverted CO, and unpermeated H{sub 2}. Two different schemes (oxygen catalytic combustion and cryogenic separation) have been investigated to both exploit the heating value of the fuel species and produce a CO{sub 2}-rich stream for long termmore » storage. Our calculations indicate that, when 85 vol % of the H{sub 2}+CO in the original syngas is extracted as H{sub 2} by the membrane reactor, the membrane-based IGCC systems are more efficient by about to 1.7 percentage points than the reference IGCC with CO{sub 2} capture based on commercially ready technology.« less

Experimental Investigation of the Effect of Biodiesel Utilization on Lubricating Oil Degradation and Wear of a Transportation CIDI Engine

Abstract: Increased environmental awareness and depletion of fossil petroleum resources are driving industry to develop alternative fuels that are environmentally more acceptable. Biodiesel is an alternative fuel derived from vegetable oils by modifying its molecular structure. In the present experimental research work, methyl ester of rice-bran oil (ROME) is derived through transesterification of rice-bran oil using methanol in presence of sodium hydroxide (NaOH) catalyst. On the basis of previous research for performance, emission and combustion characteristics, a 20% blend of ROME (B20) was selected as optimum biodiesel blend. In the present research, the experimental investigation was aimed to investigate the effect of biodiesel on wear of in-cylinder components. Endurance tests were conducted on a medium duty direct injection transportation diesel engine with 20% blend of the ROME with mineral diesel. Tests were conducted under predetermined loading cycles in two phases: engine operating on mineral diesel and engine fuelled with 20% biodiesel blend. After completion of the tests, engines were dismantled for observing the physical condition of the various parts, e.g. piston rings, bearings, cylinder liner, cylinder head etc. Physical measurements of various vital parts were also carried out to assess the wear of the parts of engine. The physical wear of various parts except big end bearings were found to be lower in case of 20% biodiesel fuelled engine. Wear metals in the lubricating oil samples drawn from the engines at regular intervals were investigated. Relatively lower wear concentration of all wear metals except lead were found in the lubricating oil of B20 fuelled engine. Two quantify the wear of cylinder liners, surface parameters at different locations in the liner (TDC, BDC and mid-stroke) were measured and investigated. A qualitative analysis was also carried out by taking surface profiles and conducting scanning electron microscopy at same locations.Copyright © 2007 by ASME

Rotordynamic Performance of Flexure Pivot Hydrostatic Gas Bearings for Oil-Free Turbomachinery

Abstract: Micro-turbomachinery demands gas bearings to ensure compactness, light weight, and extreme temperature operation. Gas bearings with large stiffness and damping, and preferably of low cost, will enable successful commercial applications. Presently, tests conducted on a small rotor supported on flexure pivot hydrostatic pad gas bearings (FPTPBs) demonstrate stable rotordynamic responses up to 100,000 rpm (limit of the drive motor). Test rotor responses show the feed pressure raises the system critical speed (increase in bearing direct stiffness) while the viscous damping ratio decreases. Predictions correlate favorably with experimentally identified (synchronous) direct stiffness bearing force coefficients. Identified experimental gas bearing synchronous damping coefficients are 50% or less of the predicted magnitudes, though remaining relatively constant as the rotor speed increases. Tests without feed pressure show the rotor becomes unstable at ∼81 krpm with a whirl frequency ratio of 20%. FPTPBs are mechanically complex and more expensive than cylindrical plain bearings. However, their enhanced stability characteristics and predictable rotordynamic performance makes them desirable for the envisioned oil-free applications in high speed micro-turbomachinery.

Characterization of Fuel Composition Effects in H2∕CO∕CH4 Mixtures Upon Lean Blowout

Abstract: This paper describes measurements of the dependence of lean blowout limits upon fuel composition for H 2 /CO/CH 4 mixtures Blowout limits were obtained at fixed approach flow velocity, reactant temperature, and combustor pressure at several conditions Consistent with prior studies, these results indicate that the percentage of H 2 in the fuel dominates the mixture blowout characteristics That is, flames can be stabilized at lower equivalence ratios, adiabatic flame temperatures, and laminar flame speeds with increasing H 2 percentage In addition, the blowoff phenomenology qualitatively changes with hydrogen levels in the fuel, being very different for mixtures with H 2 levels above and below about 50% It is shown that standard well stirred reactor based correlations, based upon a Damkohler number with a diffusivity ratio correction, can capture the effects of fuel composition variability on blowoff limits

Fundamental Mechanism of Entropy Noise in Aero-Engines: Experimental Investigation

Abstract: Entropy noise caused by combustors increases rapidly with rising Mach number in the nozzle downstream of the combustion chamber. This is experimentally shown with a dedicated test facility, in which entropy waves are generated in a controlled way by unsteady electrical heating of fine platinum wires immersed in the flow. Downstream of the heating module called entropy wave generator (EWG), the pipe flow is accelerated through a convergent-divergent nozzle with a maximum Mach number of 1.2 downstream of the nozzle throat. Parameters like mass flux of the flow, nozzle Mach number, amount of heating energy, excitation mode (periodic, pulsed, or continuously), and propagation length between EWG and nozzle have been varied for the analysis of the generated entropy noise. The results are compared with the results of a one-dimensional theory found in early literature.

Development and Application of an Eight-Step Global Mechanism for CFD and CRN Simulations of Lean-Premixed Combustors

Abstract: In this paper, the development of an eight-step global chemical kinetic mechanism for methane oxidation with nitric oxide formation in lean-premixed combustion at elevated pressures is described and applied. In particular, the mechanism has been developed for use in computational fluid dynamics (CFD) and chemical reactor network (CRN) simulations of combustion in lean-premixed gas turbine engines. Special attention is focused on the ability of the mechanism to predict NOx and CO exhaust emissions. Applications of the eight-step mechanism are reported in the paper, all for high-pressure, lean-premixed, methane-air (or natural gas-air) combustion. The eight steps of the mechanism are as follows: 1. Oxidation of the methane fuel to CO and H2 O. 2. Oxidation of the CO to CO2 . 3. Dissociation of the CO2 to CO. 4. Flame NO formation by the Zeldovich and nitrous oxide mechanisms. 5. Flame NO formation by the prompt and NNH mechanisms. 6. Post-flame NO formation by equilibrium H-atom attack on equilibrium N2 O. 7. Post-flame NO formation by equilibrium O-atom attack on equilibrium N2 O. 8. Post-flame Zeldovich NO formation by equilibrium O-atom attack on N2 .Copyright © 2007 by ASME

Experimental Investigations on a Hydrogen-Diesel Dual Fuel Engine at Different Compression Ratios

Abstract: An experimental investigation was carried out on a computer interfaced single cylinder variable compression ratio, compression ignition engine to optimize the performance characteristics and to find the useful higher compression ratio (UHCR) with hydrogen-diesel dual fuel mode. Experimentations were conducted on five different compression ratios and the performance characteristics were calculated. The effect of blending on NO x , HC, CO, and particulate matter were measured and reported. The rate of heat release and speed of combustion with increase in compression ratio with simultaneous increase in hydrogen substitution were measured. Intake temperature of air was increased and for three different temperatures, the effect of increase in temperature of air-hydrogen mixture on NO x were studied and found that there was a sharp increase in the NO x value as the inlet temperature was increased from 65 to 85°C.

Gas Turbine Fogging Technology: A State-Of-The-Art Review, Part I: Inlet Evaporative Fogging- Analytical And Experimental Aspects

Abstract: Ambient temperature strongly influences gas turbine power output causing a reduction of around 0.50% to 0.90% for every 1°C of temperature rise. There is also a significant increase in the gas turbine heat rate as the ambient temperature rises, resulting in an increased operating cost. As the increase in power demand is usually coincident with high ambient temperature, power augmentation during the hot part of the day becomes important for independent power producers, cogenerators, and electric utilities. Evaporative and overspray fogging are simple, proven, and cost effective approaches for recovering lost gas turbine performance. A comprehensive review of the current understanding of the analytical, experimental, and practical aspects including climatic and psychrometric aspects of high-pressure inlet evaporative fogging technology is provided. A discussion of analytical and experimental results relating to droplets dynamics, factors affecting droplets size, and inlet duct configuration effects on inlet evaporative fogging is covered in this paper. Characteristics of commonly used fogging nozzles are also described and experimental findings presented.

Rotordynamic Performance of a Rotor Supported on Bump Type Foil Gas Bearings: Experiments and Predictions

Abstract: Gas foil bearings (GFBs) satisfy the requirements for oil-free turbomachinery, i.e., simple construction and ensuring low drag friction and reliable high speed operation. However, GFBs have a limited load capacity and minimal damping, as well as frequency and amplitude dependent stiffness and damping characteristics. This paper provides experimental results of the rotordynamic performance of a small rotor supported on two bump-type GFBs of length and diameter equal to 38.10 mm. Coast down rotor responses from 25 krpm to rest are recorded for various imbalance conditions and increasing air feed pressures. The peak amplitudes of rotor synchronous motion at the system critical speed are not proportional to the imbalance introduced. Furthermore, for the largest imbalance, the test system shows subsynchronous motions from 20.5 krpm to 15 krpm with a whirl frequency at ∼50% of shaft speed. Rotor imbalance exacerbates the severity of subsynchronous motions, thus denoting a forced nonlinearity in the GFBs. The rotor dynamic analysis with calculated GFB force coefficients predicts a critical speed at 8.5 krpm, as in the experiments; and importantly enough, unstable operation in the same speed range as the test results for the largest imbalance. Predicted imbalance responses do not agree with the rotor measurements while crossing the critical speed, except for the lowest imbalance case. Gas pressurization through the bearings' side ameliorates rotor subsynchronous motions and reduces the peak amplitudes at the critical speed. Posttest inspection reveal wear spots on the top foils and rotor surface.

Ceramic Matrix Composite Combustor Liners: A Summary of Field Evaluations

Abstract: Solar Turbines Incorporated, under U.S. government sponsored programs, has been evaluating ceramic matrix composite combustor liners in test rigs and Solar's Centaur® 50S gas turbine engines since 1992. The objective is to evaluate and improve the performance and durability of CMCs as high-temperature materials for advanced low emissions combustors. Field testing of CMC combustor liners started in May of 1997 and by the end of 2004, over 67,000 operating hours had been accumulated on SiC/SiC and oxide/oxide CMC liners. NO X and CO emissions have been consistently <15 ppmv and <10 ppmv, respectively. Maximum test durations of 15,144 h and 13,937 h have been logged for SiC/SiC liners with protective environmental barrier coatings. An oxide/oxide CMC liner with a Friable Graded Insulation coating has been tested for 12,582 h. EBCs significantly improve SiC/SiC CMC liner life. The basic three-layer EBC consists of consecutive layers of Si, mullite, and BSAS. The durability of the baseline EBC can be improved by mixing BSAS with mullite in the intermediate coating layer. The efficacy of replacing BSAS with SAS has not been demonstrated yet. Heavy degradation was observed for two-layer Si/BSAS and Si/SAS EBCs, indicating that the elimination of the intermediate layer is detrimental to EBC durability. Equivalent performance was observed when the Hi-Nicalon fiber reinforcement was replaced with Tyranno ZM or ZMI fiber. Melt infiltrated SiC/SiC CMCs have improved durability compared to SiC/SiC CMCs fabricated by Chemical Vapor Infiltration of the matrix, in the absence of an EBC. However, the presence of an EBC results in roughly equivalent service life for Ml and CVI CMCs. Results to date indicate that oxide/oxide CMCs with protective FG1 show minor degradation under Centaur® 50S gas turbine engine operating conditions. The results of, and lessons learned from CMC combustor liner engine field testing, conducted through 2004, have been summarized.

Dynamic properties of tilting-pad journal bearings : Experimental and theoretical investigation of frequency effects due to pivot flexibility

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

Development of an Improved High Cycle Fatigue Criterion

Abstract: An integrated computational-experimental approach for prediction of total fatigue life applied to a uniaxial stress state is developed. The approach consists of the following elements: (1) development of a vibration based fatigue testing procedure to achieve low cost bending fatigue experiments and (2) development of a life prediction and estimation implementation scheme for calculating effective fatigue cycles. A series of fully reversed bending fatigue tests were carried out using a vibration-based testing procedure to investigate the effects of bending stress on fatigue limit. The results indicate that the fatigue limit for 6061-T6 aluminum is approximately 20% higher than the respective limit in fully reversed tension-compression (axial). To validate the experimental observations and further evaluate the possibility of prediction of fatigue life, an improved high cycle fatigue criterion has been developed, which allows one to systematically determine the fatigue life based on the amount of energy loss per fatigue cycle. A comparison between the prediction and the experimental results was conducted and shows that the criterion is capable of providing accurate fatigue life prediction.

Artificial Intelligence for the Diagnostics of Gas Turbines—Part I: Neural Network Approach

Abstract: In the paper, neural network (NN) models for gas turbine diagnostics are studied and developed. The analyses carried out are aimed at the selection of the most appropriate NN structure for gas turbine diagnostics, in terms of computational time of the NN training phase, accuracy, and robustness with respect to measurement uncertainty. In particular, feed-forward NNs with a single hidden layer trained by using a back-propagation learning algorithm are considered and tested. Moreover, multi-input/ multioutput NN architectures (i.e., NNs calculating all the system outputs) are compared to multi-input/single-output NNs, each of them calculating a single output of the system. The results obtained show that NNs are sufficiently robust with respect to measurement uncertainty, if a sufficient number of training patterns are used. Moreover, multi-input/ multioutput NNs trained with data corrupted with measurement errors seem to be the best compromise between the computational time required for NN training phase and the NN accuracy in performing gas turbine diagnostics.

Experimental and Kinetic Modeling of Kerosene-Type Fuels at Gas Turbine Operating Conditions

Abstract: Experimental and kinetic modeling of kerosene-type fuels is reported in the present work with special emphasis on the low-temperature oxidation phenomenon relevant to gas turbine premixing conditions. Experiments were performed in an atmospheric pressure, tubular flow reactor to measure ignition delay time of kerosene (fuel-oil No. 1) in order to study the premature autoignition of liquid fuels at gas turbine premixing conditions. The experimental results indicate that the ignition delay time decreases exponentially with the equivalence ratio at fuel-lean conditions. However, for very high equivalence ratios (>2), the ignition delay time approaches an asymptotic value. Equivalence ratio fluctuations in the premixer can create conditions conducive for autoignition of fuel in the premixer, as the gas turbines generally operate under lean conditions during premixed prevaporized combustion. Ignition delay time measurements of stoichiometric fuel-oil No. 1/air mixture at 1 atm were comparable with that of kerosene type Jet-A fuel available in the literature. A detailed kerosene mechanism with approximately 1400 reactions of 550 species is developed using a surrogate mixture of n-decane, n-propylcyclohexane, n-propylbenzene, and decene to represent the major chemical constituents of kerosene, namely n-alkanes, cyclo-alkanes, aromatics, and olefins, respectively. As the major portion of kerosene-type fuels consists of alkanes, which are relatively more reactive at low temperatures, a detailed kinetic mechanism is developed for n-decane oxidation including low temperature reaction kinetics. With the objective of achieving a more comprehensive kinetic model for n-decane, the mechanism is validated against target data for a wide range of experimental conditions available in the literature. The data include shock tube ignition delay time measurements, jet-stirred reactor reactivity profiles, and plug-flow reactor species time-history profiles. The kerosene model predictions agree fairly well with the ignition delay time measurements obtained in the present work as well as the data available in the literature for Jet A. The kerosene model was able to reproduce the low-temperature preignition reactivity profile of JP-8 obtained in a flow reactor at 12 atm. Also, the kerosene mechanism predicts the species reactivity profiles of Jet A-1 obtained in a jet-stirred reactor fairly well.

Modeling Cyclic Variability in Spark-Assisted HCCI

Abstract: Spark assist appears to offer considerable potential for increasing the speed and load range over which homogeneous charge compression ignition (HCCI) is possible in gasoline engines. Numerous experimental studies of the transition between conventional spark-ignited (SI) propagating-flame combustion and HCCI combustion in gasoline engines with spark assist have demonstrated a high degree of deterministic coupling between successive combustion events. Analysis of this coupling suggests that the transition between SI and HCCI can be described as a sequence of bifurcations in a low-dimensional dynamic map. In this paper we describe methods for utilizing the deterministic relationship between cycles to extract global kinetic rate parameters that can be used to discriminate multiple distinct combustion states and develop a more quantitative understanding of the SI-HCCI transition. We demonstrate the application of these methods for indolene-containing fuels and point out an apparent HCCI mode switching not previously reported. Our results have specific implications for developing dynamic combustion models and feedback control strategies that utilize spark-assist to expand the operating range of HCCI combustion.

Catalytic Combustion Systems for Microscale Gas Turbine Engines

Abstract: As part of an ongoing effort to develop a microscale gas turbine engine for power generation and micropropulsion applications, this paper presents the design, modeling, and experimental assessment of a catalytic combustion system. Previous work has indicated that homogenous gas-phase microcombustors are severely limited by chemical reaction timescales. Storable hydrocarbon fuels, such as propane, have been shown to blow out well below the desired mass flow rate per unit volume. Heterogeneous catalytic combustion has been identified as a possible improvement. Surface catalysis can increase hydrocarbon-air reaction rates, improve ignition characteristics, and broaden stability limits. Several radial inflow combustors were micromachined from silicon wafers using deep reactive ion etching and aligned fusion wafer bonding. The 191 mm 3 combustion chambers were filled with platinum-coated foam materials of various porosity and surface area. For near stoichiometric propane-air mixtures, exit gas temperatures of 1100 K were achieved at mass flow rates in excess of 0.35 g/s. This corresponds to a power density of ∼1200 MW/m 3 ; an 8.5-fold increase over the maximum power density achieved for gas-phase propane-air combustion in a similar geometry. Low-order models, including time-scale analyses and a one-dimensional steady-state plug-flow reactor model, were developed to elucidate the underlying physics and to identify important design parameters. High power density catalytic microcombustors were found to be limited by the diffusion of fuel species to the active surface, while substrate porosity and surface area-to-volume ratio were the dominant design variables.

Nonlinear Rotordynamics of Automotive Turbochargers: Predictions and Comparisons to Test Data

Abstract: Passenger vehicle turbochargers (TCs) offer increased engine power and efficiency in an ever-competitive marketplace. Turbochargers operate at high rotational speeds and use engine oil to lubricate fluid-film-bearing supports (radial and axial). However, TCs are prone to large amplitudes of subsynchronous shaft motion over wide ranges of their operating speed. Linear rotordynamic tools cannot predict the amplitudes and multiple frequency shaft motions. A comprehensive nonlinear rotordynamics model coupled to a complete fluid-film-bearing model solves in real time the dynamics of automotive turbochargers. The computational design tool predicts the limit cycle response for several inner and outer film clearances and operating conditions including rotor speed and lubricant feed pressure. Substantial savings in product development and prototype testing are the benefits of the present development. The paper presents predictions of the linear and nonlinear shaft motion of an automotive turbocharger supported on a semi-floating ring bearing. The shaft motion predictions are compared to measurements of shaft motion at the compressor nose for speeds up to 240 krpm, and for lubricant inlet pressure of 4 bar at 150°C. Linear and nonlinear rotordynamic models reproduce very well the test data for synchronous response to imbalance. The nonlinear results show two subsynchronous whirl frequencies whose large magnitudes agree well with the measurements. A large side load predicted for this turbocharger must be considered for accurate prediction of the rotordynamic response.