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Research Papers: Gas Turbines: Aircraft Engine

J. Eng. Gas Turbines Power. 2014;136(7):071201-071201-9. doi:10.1115/1.4026541.

To reduce the size and weight of power generation machines for portable devices, several systems to replace the currently used heavy batteries are being investigated worldwide. As micro gas turbines are expected to offer the highest power density, several research groups launched programs to develop ultra micro gas turbines: IHI firm (Japan), PowerMEMS Consortium (Belgium). At Onera, a research program called DecaWatt is under development in order to realize a demonstrator of a micro gas turbine engine in the 50 to 100 Watts electrical power range. A single-stage gas turbine is currently being studied. First of all, a calculation of the overall efficiency of the micro gas turbine engine has been carried out according to the pressure ratio, the turbine inlet temperature, and the compressor and turbine efficiencies. With realistic hypotheses, we could obtain an overall efficiency of about 5% to 10%, which leads to around 200 W/kg when taking into account the mass of the micro gas turbine engine, its electronics, fuel and packaging. Moreover, the specific energy could be in the range 300 to 600 Wh/kg, which largely exceeds the performance of secondary batteries. To develop such a micro gas turbine engine, experimental and computational work focused on: (1) a 10-mm diameter centrifugal compressor, with the objective to obtain a pressure ratio of about 2.5; (2) a radial inflow turbine; (3) journal and thrust gas bearings (lobe bearings and spiral grooves) and their manufacturing; (4) a small combustor working with hydrogen or hydrocarbon gaseous fuel (propane); (5) a high rotation speed microgenerator; and (6) the choice of materials. Components of this tiny engine were tested prior to the test with all the parts assembled together. Tests of the generator at 700,000 rpm showed a very good efficiency of this component. In the same way, compressor testing was performed up to 500,000 rpm and showed that the nominal compression rate at the 840,000 rpm nominal speed should nearly be reached.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(7):071202-071202-8. doi:10.1115/1.4026548.

Gas turbine off design performance prediction is strictly dependent on the accuracy of compressor and turbine map characteristics. Experimental data regarding component maps are very difficult to find in literature, since it is undisclosed proprietary information of the engine manufacturers. To overcome this limitation, gas turbine engineers use available generic component maps and modify them to reach the maximum adherence with the experimental measures. Different scaling and adaptation techniques have been employed to this aim; these methodologies are usually based upon analytic regression models which minimize the deviation from experimental data. However, since these models are built mainly for a specific compressor or turbine map, their generalization is quite difficult: in fact, regression is highly shape-dependent and, therefore, requires a different model for each different specific component. This paper proposes a solution to the problem stated above: a new method for map adaptation is investigated to improve steady-state off design prediction accuracy of a generic gas turbine component. The methodology does not employ analytical regression models; its main principle relies in performing map modifications in an appropriate neighborhood of the multiple experimental points used for the adaptation. When using gas turbine simulation codes, component maps are usually stored in a data matrix and are ordered in a format suitable for 2D interpolation. A perturbation of the values contained in the matrix results in component map morphing. An optimization algorithm varies the perturbation intensity vector in order to minimize the deviation between experimental and predicted points. The adaptation method is integrated inside TSHAFT, the gas turbine prediction code developed at the University of Padova. The assessment of this methodology will be exposed by illustrating a case study carried out upon a turbojet engine.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Combustion, Fuels, and Emissions

J. Eng. Gas Turbines Power. 2014;136(7):071501-071501-8. doi:10.1115/1.4026529.

Combustion tests with bioethanol and diesel as a reference have been performed in OPRA's 2 MWe class OP16 gas turbine combustor. The main purposes of this work are to investigate the combustion quality of ethanol with respect to diesel and to validate the developed CFD model for ethanol spray combustion. The experimental investigation has been conducted in a modified OP16 gas turbine combustor, which is a reverse-flow tubular combustor of the diffusion type. Bioethanol and diesel burning experiments have been performed at atmospheric pressure with a thermal input ranging from 29 to 59 kW. Exhaust gas temperature and emissions (CO, CO2, O2, NOx) were measured at various fuel flow rates while keeping the air flow rate and air temperature constant. In addition, the temperature profile of the combustor liner has been determined by applying thermochromic paint. CFD simulations have been performed with ethanol for five different operating conditions using ANSYS FLUENT. The simulations are based on a 3D RANS code. Fuel droplets representing the fuel spray are tracked throughout the domain while they interact with the gas phase. A liner temperature measurement has been used to account for heat transfer through the flame tube wall. Detailed combustion chemistry is included by using the steady laminar flamelet model. Comparison between diesel and bioethanol burning tests show similar CO emissions, but NOx concentrations are lower for bioethanol. The CFD results for CO2 and O2 are in good agreement, proving the overall integrity of the model. NOx concentrations were found to be in fair agreement, but the model failed to predict CO levels in the exhaust gas. Simulations of the fuel spray suggest that some liner wetting might have occurred. However, this finding could not be clearly confirmed by the test data.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(7):071502-071502-9. doi:10.1115/1.4026530.

Amplitude-dependent flame transfer functions, also denoted as flame describing functions, are valuable tools for the prediction of limit-cycle amplitudes of thermoacoustic instabilities. However, the effects that govern the transfer function magnitude at low and high amplitudes are not yet fully understood. It is shown in the present work that the flame response at perfectly premixed conditions is strongly influenced by the growth rate of vortical structures in the shear layers. An experimental study in a generic swirl-stabilized combustor was conducted in order to measure the amplitude-dependent flame transfer function and the corresponding flow fields subjected to acoustic forcing. The applied measurement techniques included the multi-microphone-method, high-speed OH*-chemiluminescence measurements, and high-speed particle image velocimetry. The flame response and the corresponding flow fields are assessed for three different swirl numbers at 196 Hz forcing frequency. The results show that forcing leads to significant changes in the time-averaged reacting flow fields and flame shapes. A triple decomposition is applied to the time-resolved data, which reveals that coherent velocity fluctuations at the forcing frequency are amplified considerably stronger in the shear layers at low forcing amplitudes than at high amplitudes, which is an indicator for a nonlinear saturation process. The strongest saturation is found for the lowest swirl number, where the forcing additionally detached the flame. For the highest swirl number, the saturation of the vortex amplitude is weaker. Overall, the amplitude-dependent vortex amplification resembles the characteristics of the flame response very well. An application of a linear stability analysis to the time-averaged flow fields at increasing forcing amplitudes yields the decreasing growth rates of shear flow instabilities at the forcing frequency. It therefore successfully predicts a saturation at high forcing amplitudes and demonstrates that the mean flow field and its modifications are of utmost importance for the growth of vortices in the shear layers. Moreover, the results clearly show that the amplification of vortices in the shear layers is an important driver for heat release fluctuations and their saturation.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(7):071503-071503-10. doi:10.1115/1.4026531.

Practical aero-engine fuel injection systems are highly complicated, combining complex fuel atomizer and air swirling elements to achieve good fuel-air mixing and long residence time in order to enhance both the combustion efficiency and stability. While a detailed understanding of the multiphase flow processes occurring in a realistic injector has been limited due to the complex geometries and the challenges in near-field measurements, high fidelity, first principles simulation offers, for the first time, the potential for a comprehensive physics-based understanding. In this work, such simulations have been performed to investigate the spray atomization and subsequent droplet transport in a swirling air stream generated by a complex multinozzle/swirler combination. A coupled level set and volume of fluid (CLSVOF) approach is used to directly capture the liquid-gas interface and an embedded boundary (EB) method is applied to flexibly handle the complex injector geometry. The ghost fluid (GF) method is also used to facilitate simulations at a realistic fuel-air density ratio. Adaptive mesh refinement (AMR) and Lagrangian droplet models are used to efficiently resolve the multiscale processes. To alleviate the global constraint on the time step imposed by the locally activated AMR near liquid jets, a separate AMR simulation focusing on jet atomization was performed for a relatively short physical time and the resulting Lagrangian droplets are coupled into another simulation on a uniform grid at larger time-steps. The high cost simulations were performed at the U.S. Department of Defense high performance computing facilities using over 5000 processors. Experiments at the same flow conditions were conducted at the United Technologies Research Center (UTRC). The simulation details of flow velocity and vorticity due to the interaction of the fuel jet and swirling air are presented. The velocity magnitude is compared with the experimental measurement at two downstream planes. The two-phase spray spreading is compared with experimental images and the flow details are further analyzed to enhance the understanding of the complex physics.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(7):071504-071504-5. doi:10.1115/1.4026549.

Atomization of fuel is a key integral part for efficient combustion in gas turbines. This demands a thorough investigation of the spray characteristics using innovative and useful spray diagnostics techniques. In this work, an experimental study is carried out on a commercial hollow cone nozzle (Lechler) using laser diagnostics techniques. A hollow cone spray is useful in many applications because of its ability to produce fine droplets. But apart from the droplet diameter, the velocity field in the spray is also an important parameter to monitor and has been addressed in this work. Kerosene is used as the test fuel, which is recycled using a plunger pump providing a variation in the injection pressure from 100 to 300 psi. An innovative diagnostic technique used in this study is through illumination of the spray with a continuous laser sheet and capturing the same with a high speed camera. A ray of a laser beam is converted to a planer sheet using a lens combination which is used to illuminate a cross section of the hollow cone spray. This provides a continuous planar light source which allows capturing high speed images at 285 fps. The high speed images thus obtained are processed to understand the nonlinearity associated with disintegration of the spray into fine droplets. The images are shown to follow a fractal representation and the fractal dimension is found to increase with rise in injection pressure. Also, using PDPA, the droplet diameter distribution is calculated at different spatial and radial locations at a wide range of pressure.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(7):071505-071505-7. doi:10.1115/1.4026657.

There are currently numerous efforts to create renewable fuels that have similar properties to conventional diesel fuels. One major future challenge is evaluating how these new fuels will function in older legacy diesel engines. It is desired to have physically based modeling tools that will predict new fuel performance without extensive full scale engine testing. This study evaluates two modeling tools that are used together to predict ignition delay in a military diesel engine running n-hexadecane as a fuel across the engine's speed-load range. AVL-FIRE® is used to predict the physical delay of the fuel from the start of injection until the formation of a combustible mixture. Then a detailed Lawrence Livermore National Laboratory (LLNL) chemical kinetic mechanism is used to predict the chemical ignition delay. This total model predicted ignition delay is then compared to the experimental engine data. The combined model predicted results show good agreement to that of the experimental data across the engine operating range with the chemical delay being a larger fraction of the total ignition delay. This study shows that predictive tools have the potential to evaluate new fuel combustion performance.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Controls, Diagnostics, and Instrumentation

J. Eng. Gas Turbines Power. 2014;136(7):071601-071601-17. doi:10.1115/1.4026598.

The supervision of performance in gas turbine applications is crucial in order to achieve: (i) reliable operations, (ii) low heat stress in components, (iii) low fuel consumption, and (iv) efficient overhaul and maintenance. To obtain a good diagnosis of performance it is important to have tests which are based on models with high accuracy. A main contribution is a systematic design procedure to construct a fault detection and isolation (FDI) system for complex nonlinear models. To fulfill the requirement of an automated design procedure, a thermodynamic gas turbine package (GTLib) is developed. Using the GTLib framework, a gas turbine diagnosis model is constructed where component deterioration is introduced. In the design of the test quantities, equations from the developed diagnosis model are carefully selected. These equations are then used to implement a constant gain extended Kalman filter (CGEKF)-based test quantity. The test quantity is used in the FDI-system to supervise the performance and in the controller to estimate the flame temperature. An evaluation is performed using experimental data from a gas turbine site. The case study shows that the designed FDI-system can be used when the decision about a compressor wash is taken. Thus, the proposed model-based design procedure can be considered when an FDI-system of an industrial gas turbine is constructed.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(7):071602-071602-10. doi:10.1115/1.4026547.

This paper introduces the equipment for generating an excitation signal during the process of thermocouple dynamic calibration in air medium, with the objective of dealing with the problems existing extensively in the excitation signal of incapable assessment of the rising time and inaccuracy in the traceability for the step amplitude. Based on the shock-tube theory, the step-temperature excitation signal that is suitable for the dynamic calibration of the thermocouple with appreciable rising time, wide-frequency bandwidth, and traceable step amplitude can be generated by means of the improvement in the structure of the traditional shock tube as well as the compensation of the shock-tube parameters. Through on-site assessment experimentation and dynamic modeling of the thermocouple, the time constant of the thermocouple can be obtained and the dynamic response of the thermocouple can be modified and compensated, the calibration result of which shows that the dynamic calibration method of the thermocouple proposed in this paper can be implemented for different ranges of frequencies and different phases of the temperature sensor with high reliability; moreover, the calibration equipment is miniaturized.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(7):071603-071603-8. doi:10.1115/1.4026600.

Current trends for advanced automotive engines focusing on downsizing, better fuel efficiency, and lower emissions have led to several changes in turbocharger bearing systems design, and technology. Automotive turbochargers are running faster under high engine vibration level. Vibration control is becoming a real critical issue and turbocharger manufacturers are focusing more and more on new and improved balancing technology. This paper deals with turbocharger synchronous vibration control on high speed balancers. In a first step the synchronous rotordynamics behavior is identified. The developed fluid bearing code predicts bearing rotational speed (in case of fully floating design), operating inner and outer bearing film clearances and bearing force coefficients. A rotordynamics code uses this input to predict the synchronous lateral dynamic response of the rotor-bearing system by converging with bearing eccentricity ratio. The rotor-bearing system model is validated by shaft motion test data on high speed balancer (HSB). It shows that only one of the peaks seen on the synchronous G level plot collected in a high speed balancer can be explained by rotordynamics physics. A step-by-step structural dynamics model and analysis validated by experimental frequency response functions provides robust explanations for the other G level peaks. The synchronous vibration response of the system “turbocharger-HSB fixture” is predicted by integrating the predicted rotordynamics rotational bearing loads on the structural dynamics model. Numerous test data show very good correlation with the prediction, which validates the developed analytical model. The “rotordynamics—structural dynamics model” allows deep understanding of turbocharger synchronous vibration control, as well as optimization of the high speed balancer tooling.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Cycle Innovations

J. Eng. Gas Turbines Power. 2014;136(7):071701-071701-6. doi:10.1115/1.4026539.

Bechtel Marine Propulsion Corporation (BMPC) is testing a supercritical carbon dioxide (S-CO2) Brayton system at the Bettis Atomic Power Laboratory. The 100 kWe integrated system test (IST) is a two shaft recuperated closed Brayton cycle with a variable speed turbine driven compressor and a constant speed turbine driven generator using S-CO2 as the working fluid. The IST was designed to demonstrate operational, control, and performance characteristics of an S-CO2 Brayton power cycle over a wide range of conditions. Initial operation of the IST has proven a reliable method for startup of the Brayton loop and heatup to normal operating temperature (570 °F). An overview of the startup process, including initial loop fill and charging, and heatup to normal operating temperature is presented. Additionally, aspects of the IST startup process which are related to the loop size and component design which may be different for larger systems are discussed.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(7):071702-071702-5. doi:10.1115/1.4026599.

This paper investigates on a gas-to-liquids (GTL) plant with ATR syngas production and proposes a new process to use a gas turbine and waste heat recovery gas/steam streams preheater to replace the fired heater. The new process features cascade utilization of fuel gas energy, as fuel gas is firstly used in a gas turbine (GT) at very high temperature and then lower-temperature GT exhaust gas is further used for preheating. Large exergy loss of heat transfer in the fired heater is eliminated. The improved process has an equivalent power generation efficiency of 80% which is significantly higher than conventional technology. Economic analysis indicates 129.8 M$ revenue would be produced over the lifetime if the extra power from a 15,000 bbl/d GTL plant can be exported to the grid at the price of cost of electricity for a conventional natural gas fired combined cycle plant.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(7):071703-071703-11. doi:10.1115/1.4026612.

Because molten fluoride salts can deliver heat at temperatures above 600 °C, they can be used to couple nuclear and concentrating solar power heat sources to reheat air combined cycles (RACC). With the open-air configuration used in RACC power conversion, the ability to also inject natural gas or other fuel to boost power at times of high demand provides the electric grid with contingency and flexible capacity while also increasing revenues for the operator. This combination provides several distinct benefits over conventional stand-alone nuclear power plants and natural gas combined cycle and peaking plants. A companion paper discusses the necessary modifications and issues for coupling an external heat source to a conventional gas turbine and provides two baseline designs (derived from the GE 7FB and Alstom GT24). This paper discusses off-nominal operation, transient response, and start-up and shutdown using the GE 7FB gas turbine as the reference design.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2014;136(7):072501-072501-9. doi:10.1115/1.4026535.

Many high speed turbochargers are known to operate with limit cycle vibration as a result of fluid-film instability. The goal of this research was to achieve a stable synchronous response with a minimum of self-excited nonsynchronous contribution. Those vibration components excited by the engine harmonics and exhaust pressure pulsations were not the target of this research. This paper will review the experimental results of the fixed geometry fluid-film bearing designs selected to replace the standard stock floating-ring design. In addition, the paper documents a novel radial tilting pad bearing concept that was designed to replace the fixed geometry bearings, with a minimum of modification to the stock bearing housing. A summary of the on-engine testing over the past seven years is documented in this paper.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(7):072502-072502-12. doi:10.1115/1.4026543.

Rotary screw type positive displacement (RSPD) pumps are commonly used in oil and gas industry for pumping of mineral lube oil in services where they can be mechanically driven by gears coupled to a train driver. Installation of these pumps is critical and should be designed jointly by vendors and users according to project specific restrictions (i.e., the arrangement of the entire oil circulation system). This paper describes a real case in which restrictions due to lube oil system arrangement have produced low pump suction head and have amplified the influence of air bubbles that remained entrained in oil despite lube oil tank degassing. The investigations have been directed toward the mathematical modeling of the aeration phenomenon coupled with experimental measurements of critical parameters taken on the shop plant. Among corrective actions identified and considered there are reduction of quantity of air entering the lube oil system and revamping of the entire lube oil system with changes in piping, tank and also in pump model together with special modifications of internal path to enhance air handling capabilities. In order to validate pump behavior with reference to resistance to aeration (monitoring noise and vibration) a special simulation setup was jointly developed by end user and manufacturer on a pilot test bench to carry out the various performance tests. The numerical data collected during shop aeration test have confirmed that the pump was able to handle the expected amount of entrained air with noise and vibrations within industrial limits. The pumps tested in the pilot bench were installed at user's site and the effectiveness of the synergic corrective actions listed above was successfully verified. The study concludes that an early estimation of entrained air in the lube oil system is critical for design and development of either the RSPD pump or the entire lube oil circuit of a motor compressor train. When a critical quantity of entrained air is likely to be reached at pump suction (near 10% in volume), pump manufacturers and end users should apply some basic rules related to “design for aeration” of the pump and agree on a nonroutine test to be performed at manufacturer's shop before pump installation at site. This will serve as a reliable prediction of pump air handling capabilities, without which effective operation, reliability and durability of the pump could be jeopardized.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(7):072503-072503-4. doi:10.1115/1.4026802.

The blade tip timing (BTT) method uses the differential arrival timings of the blades at case-mounted sensors to effectively characterize the vibrations of all blades in a rotor. This paper studies the use of the BTT method for pre-emptive prediction of rotor blade damage; through a careful monitoring of blade natural frequencies in conjunction with the blade tip position during an engine test. In the current study, the low pressure turbine stage of a developmental aero engine is instrumented with a combination of eddy current and optical sensors located circumferentially on the casing. This instrumentation effectively captures the engine order resonances of interest for the blade bending mode. During one of the normal engine tests, one of the blades in the LPT stage suddenly showed a drop in natural frequency beyond the allowable scatter and an abrupt change in the blade tip position. As the engine test was continued further, this drop in blade natural frequency and change in blade tip position progressively increased towards blade failure limits. Suspecting a propagating crack in the particular blade, the test was aborted and the engine was withdrawn for detailed inspection. Inspection of the rotor blades confirmed the presence of significant aerofoil crack in the suspect blade.

Topics: Engines , Blades , Rotors
Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2014;136(7):072504-072504-8. doi:10.1115/1.4026656.

Future coal-fired steam turbines promise increased efficiency and low emissions. However, this comes at the expense of increased thermal load from higher inlet steam temperatures and pressures leading to severe creep that significantly influences the sealing behavior and high temperature strength of bolted flange-seal couplings. Flanges with different thicknesses were employed for a comparative study. The important stress/creep values in the flanges and U-type seals had been obtained for variations in flange thickness and bolt relaxation while maintaining other leading parameters constant. The variation of contact stresses due to creep deformation plays an important role in achieving a leak proof sealing. In this paper, a two-dimensional finite element analysis of bolted flange-seal couplings has been carried out by taking the relaxation of bolt stress under full-loading turbine service. The creep strength of flanges and U-type seals are investigated by Cocks–Ashby (C–A) equivalent strain method. The multiaxial state of stresses is considered in this method by using C–A multiaxial coefficient. According to ASME allowable creep limit, the C–A equivalent strains of three flange-seal couplings are evaluated and compared. Furthermore, based on the results of contact stresses, the creep behavior of U-type seals is analyzed varying flange thickness. Finally, analysis shows that the thinner flange-seal coupling has larger long-term contact stress, while the U-type seal with the thicker flange has the least creep strength.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Turbomachinery

J. Eng. Gas Turbines Power. 2014;136(7):072601-072601-7. doi:10.1115/1.4026611.

Heavy EGR required on diesel engines for future emission regulation compliance has posed a big challenge to conventional turbocharger technology for high efficiency and wide operation range. This study, as part of the U.S. Department of Energy sponsored research program, is focused on advanced turbocharger technologies that can improve turbocharger efficiency on customer driving cycles while extending the operation range significantly, compared to a production turbocharger. The production turbocharger for a medium-duty truck application was selected as a donor turbo. Design optimizations were focused on the compressor impeller and turbine wheel. On the compressor side, advanced impeller design with arbitrary surface can improve the efficiency and surge margin at the low end while extending the flow capacity, while a so-called active casing treatment can provide additional operation range extension without compromising compressor efficiency. On the turbine side, mixed flow turbine technology was revisited with renewed interest due to its performance characteristics, i.e., high efficiency at low-speed ratio, relative to the base conventional radial flow turbine, which is relevant to heavy EGR operation for future diesel applications. The engine dynamometer test shows that the advanced turbocharger technology enables over 3% BSFC improvement at part-load as well as full-load condition, in addition to an increase in rated power. The performance improvement demonstrated on an engine dynamometer seems to be more than what would typically be translated from the turbocharger flow bench data, indicating that mixed flow turbine may provide additional performance benefits under pulsed exhaust flow on an internal combustion engine and in the low-speed ratio areas that are typically not covered by steady state flow bench tests.

Commentary by Dr. Valentin Fuster

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