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TECHNICAL PAPERS: Gas Turbines: Controls, Diagnostics, and Instrumentation

J. Eng. Gas Turbines Power. 2002;124(2):256-262. doi:10.1115/1.1426407.

A method for defining which parts of a combined cycle gas turbine (CCGT) power plant are responsible for performance deviations is presented. When the overall performances deviate from their baseline values, application of the method allows the determination of the component(s) of the plant, responsible for this deviation. It is shown that simple differentiation approaches may lead to erroneous conclusions, because they do not reveal the nature of deviations for individual components. Contributions of individual components are then assessed by separating deviations due to permanent changes and deviations due to change of operating conditions. A generalized formulation is presented together with the way of implementing it. Test cases are given, to make clearer the ideas put forward in the proposed method.

Commentary by Dr. Valentin Fuster

TECHNICAL PAPERS: Gas Turbines: Oil and Gas Applications

J. Eng. Gas Turbines Power. 2002;124(2):276-283. doi:10.1115/1.1413769.

CFD simulations of the combustion process and formation of emissions in an industrial GE LM1600 gas turbine have been performed over a range of unit loads. Two combustion models were considered here to characterize the combustion process, the chemical equilibrium model and the nonequilibrium laminar flamelet model. The flamelet model predictions of nitric oxide concentration demonstrated much closer agreement with field measurements of continuous emission monitoring systems, due to accurate modeling of the oxygen radial concentration. The predictions made with this model are within 15 percent at maximum load and considerably better at lower loads. Field measurements also showed that nitric oxide constitutes about 95 percent of the total NOx measured. Unburnt hydrocarbons and carbon monoxide emissions are significantly overpredicted, however, arguably as a result of neglecting their oxidation in the high temperature, fuel-lean environment of the turbine and exhaust stack downstream of the combustor.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2002;124(2):284-297. doi:10.1115/1.1414130.

This paper presents a successful demonstration of application of neural networks to perform various data mining functions on an RB211 gas-turbine-driven compressor station. Radial basis function networks were optimized and were capable of performing the following functions: (a) backup of critical parameters, (b) detection of sensor faults, (c) prediction of complete engine operating health with few variables, and (d) estimation of parameters that cannot be measured. A Kohonen SOM technique has also been applied to recognize the correctness and validity of any data once the network is trained on a good set of data. This was achieved by examining the activation levels of the winning unit on the output layer of the network. Additionally, it would also be possible to determine the suspicious, faulty or corrupted parameter(s) in the cases which are not recognized by the network by simply examining the activation levels of the input neurons.

Commentary by Dr. Valentin Fuster

TECHNICAL PAPERS: Internal Combustion Engines

J. Eng. Gas Turbines Power. 2002;124(2):404-411. doi:10.1115/1.1445438.

The influence of variations in the composition of natural gas on the ignition and combustion processes in engines is investigated. Particular attention is given to changes in the relatively small concentrations of high molar mass alkanes that may be present in the fuel. A detailed chemical kinetic scheme for the oxidation of the higher hydrocarbon components of up to n-heptane was used to investigate analytically the combustion reactions of different fuel mixtures under constant volume adiabatic conditions with initial states that are similar to those during the ignition delay of a typical internal combustion engine. These comprehensive simulation calculations require much computing capacity and time that would preclude their incorporation in full simulation models of engine processes. A simplification is introduced based on replacing artificially the small concentrations of any higher hydrocarbons that may be present in the natural gas by a kinetically equivalent amount of propane in the fuel mixture. This is done such that the resulting equivalent fuel has the same ignition delay as the original fuel under constant volume engine T.D.C. conditions. This “propane equivalent” concept was used in full engine simulation models while employing a relatively short scheme of 150 steps for the oxidation of fuel mixtures of propane, ethane, and methane in air.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2002;124(2):412-420. doi:10.1115/1.1445439.

Extensive flow visualization in an automotive fuel filler pipe made visible by introducing dyes and smoke in water and air, respectively, were conducted for nominal flow rates of 4–18 liters per minute. Video and still cameras were used for imaging. Features of the flow such as laminar-to-turbulent transition, progressive development of strong swirl along filler pipe axis, air entrainment, and mixing with the liquid were observed in the experiments. The experimental observations were supported by computational fluid dynamics (CFD) simulations of the flow which also showed features such as swirl and air entrainment.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2002;124(2):421-428. doi:10.1115/1.1424295.

This paper describes the development of a comprehensive mathematical and numerical model for simulating the performance of automotive three-way catalytic converters, which are employed to reduce engine exhaust emissions. The model simulates the emission system behavior by using an exhaust system heat conservation and catalyst chemical kinetic submodel. The resulting governing equations are solved numerically. Good agreements were found between the numerical predictions and experimental measurements under both steady-state and transient conditions. The developed model will be used to facilitate the converter design improvement efforts, which are necessary in order to meet the increasingly stricter emission requirements.

Commentary by Dr. Valentin Fuster

TECHNICAL PAPERS: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2002;124(2):298-302. doi:10.1115/1.1445440.

This paper will discuss a study of an innovative design for an advanced turbine rotor that could have a great impact on future engines. The design challenge is to provide a minimum weight turbine rotor system that can withstand beyond state-of-the-art levels of AN2 (turbine annulus area multiplied by speed squared). An AN2 limit has been reached for high-pressure turbine (HPT) disks configured in conventional (single web) geometry with state-of-the-art nickel alloys. The problem has reached the point where increased AN2 has been declared a “break-through” technology. The twin-web disk has the potential to provide this break through. This paper will present the history of this turbine rotor design, analytical results, material/component processing, and concept validation results. All work was performed under an Air Force sponsored program entitled “Composite Ring Reinforced Turbine” (CRRT).

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2002;124(2):303-310. doi:10.1115/1.1447235.

An improved transfer matrix method is developed to analyze nonlinear rotor-bearing systems. The rotating shaft is described by the Timoshenko beam theory which considers the effect of the rotary inertia and shear deformation. A typical roller bearing model is assumed which has cubic nonlinear spring and linear damping characteristics. Transfer matrices for the Timoshenko shaft element, disk element, and nonlinear bearing element are derived and the global transfer matrix is formed. The steady-state response of synchronous, subharmonic, and superharmonic whirls is determined using the harmonic balance method. Two numerical examples are presented to demonstrate the effectiveness of this approach.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2002;124(2):311-324. doi:10.1115/1.1447236.

This paper explores the effects of random blade mistuning on the dynamics of an advanced industrial compressor rotor, using a component-mode-based reduced-order model formulation for tuned and mistuned bladed disks. The technique uses modal data obtained from finite element models to create computationally inexpensive models of mistuned bladed disks in a systematic manner. Both free and forced responses of the rotor are considered, and the obtained results are compared with “benchmark” finite element solutions. A brief statistical study is presented, in which Weibull distributions are shown to yield reliable estimates of forced response statistics. Moreover, a simple method is presented for computing natural frequencies of noninteger harmonics, using conventional cyclic symmetry finite element analysis. This procedure enables quantification of frequency veering data relevant to the assessment of mistuning sensitivity (e.g., veering curvatures), and it may provide a tool for quantifying structural interblade coupling in finite element rotor models of arbitrary complexity and size. The mistuned forced response amplitudes and stresses are found to vary considerably with mistuning strength and the degree of structural coupling between the blades. In general, this work demonstrates how reduced order modeling and Weibull estimates of the forced response statistics combine to facilitate thorough investigations of the mistuning sensitivity of industrial turbomachinery rotors.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2002;124(2):325-331. doi:10.1115/1.1415740.

Simple physical models for the stresses in dovetail attachments are developed. These models address: to slip or not to slip, nominal stresses during loading up, peak contact and shear stresses during loading up, hoop stresses during loading up, peak contact and shear stresses during unloading, and hoop stresses during unloading. Comparisons are made with a previous paper on companion finite element modeling. Generally there is good agreement between the simple physical models and the finite element analysis. Together the two identify a pinching mechanism as leading to large fluctuations in hoop stresses at the edges of contact. These fluctuating hoop stresses can be expected to be a major contributor to the fatigue of dovetail attachments.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2002;124(2):332-339. doi:10.1115/1.1416152.

A model order reduction technique that yields low-order models of blade row unsteady aerodynamics is introduced. The technique is applied to linearized unsteady Euler CFD solutions in such a way that the resulting blade row models can be linked to their surroundings through their boundary conditions. The technique is applied to a transonic compressor aeroelastic analysis, in which the high-fidelity CFD forced-response results are better captured than with models that use single-frequency influence coefficients. A low-speed compressor stage is also modeled to demonstrate the multistage capability of the method. These examples demonstrate how model order reduction can be used to systematically improve the versatility, fidelity, and range of applicability of the low-order aerodynamic models typically used for incorporation of CFD results into aeroelastic analyses.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2002;124(2):340-350. doi:10.1115/1.1416691.

Rotor/seal full annular rub, including synchronous (forward) and reverse (backward) precessions, has been investigated both experimentally and analytically. Of particular interest is the finding of reverse precessional full annular rub (dry whip) that occurs repeatedly in small clearance cases without any outside disturbance. The experimental results include rub triggering mechanism, mass unbalance, and rotative speed effects. A simplified mathematical model is used to interpret experimental results. Nonlinear solutions for both synchronous and reverse precessions are obtained along with instability zones. Mass unbalance effect on shifting from synchronous response to reverse rub and destabilizing factors such as dry friction, rotor damping, and seal stiffness, are discussed.

Topics: Rotors , Stiffness
Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2002;124(2):351-356. doi:10.1115/1.1416692.

A computationally efficient strategy is presented for adjusting analytic rotordynamic models to make them consistent with experimental data. The approach permits use of conventional rotordynamic models derived using finite element methods in conjunction with conventional plant identification models derived from impact or sine sweep testing in a transfer function or influence coefficient format. The underlying assumption is that the predominant uncertainties in engineered models occur at discrete points as effects like shrink fits, seal coefficients or foundation interactions. Further, it is assumed that these unmodeled or poorly modeled effects are essentially linear (at least within the testing and expected operating domains). Matching is accomplished by deriving a dynamic model for these uncertain effects such that the resulting composite model has a transfer function which matches that obtained experimentally. The derived augmentations are computationally compatible with the original rotor model and valid for stability or forced response predictions. Further, computation of this augmentation is accomplished using well developed and widely disseminated tools for modern control. Background theory and a complete recipe for the solution are supported by a number of examples.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2002;124(2):357-362. doi:10.1115/1.1417482.

Synchronous response estimation attempts to determine the forced response (displacement) of a rotor at critical points which cannot be measured directly. This type of prediction, if accurate and reliable, has broad potential use in the rotating machinery industry. Many machines have close clearance points on their shafts, such as seals, which can easily be damaged by excess vibration. Accurate estimates of the actual level of vibration at these points could usefully assist machine operators in troubleshooting and in protecting the equipment from expensive damage. This type of response information can be used both to generate less conservative alarm limits and, if magnetic bearings are available, to directly guide the bearing controllers in restricting the rotor motion at these critical points. It is assumed that the disturbance forces acting upon the rotor are predominantly synchronous. The response estimate is constructed using the measurable response in conjunction with an estimator gain matrix derived from a model of the transmissibilities of the rotor system. A fundamental performance bound is established based on the single-speed set of measurements by bounding the response to the unmeasurable component of the disturbance force. Acknowledging that some model uncertainty will always exist, a robust performance analysis is developed using structured singular value (μ) analysis techniques. Assuming some reasonable levels of uncertainty for the model parameters (natural frequencies, modal dampings, mode shapes, bearing stiffnesses, and dampings) the results of the estimator construction and analysis establish feasibility of the proposed estimation. Two reference rotor models that are representative of industrially sized machines are used to demonstrate and evaluate the estimation. The unmeasurable response estimation errors consistently lie below 25 μm for the examples examined.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2002;124(2):363-368. doi:10.1115/1.1403461.

Test results are presented for the rotordynamic coefficients of a hybrid bearing that is representative of bearings for liquid-rocket-engine turbopump applications. The bearing is tested in the following two degraded conditions: (a) one of five orifices plugged, and (b) a locally enlarged clearance to simulate a worn condition. Test data are presented at 24,600 rpm, with supply pressures of 4.0, 5.5, and 7.0 MPa, and eccentricity ratios from 0.1 to 0.5 in 0.1 increments. Overall, the results suggest that neither a single plugged orifice nor significant wear on the bearing land will “disable” a well-designed hybrid bearing. These results do not speak to multiple plugged orifices and are not an endorsement for operations without filters to prevent plugging orifices.

Topics: Bearings , Wear
Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2002;124(2):369-374. doi:10.1115/1.1426085.

A laboratory rotor, representing a scaled-down model of a three-stage compressor supported by fluid film bearings on anisotropic flexible supports was analyzed. The support characteristics were measured at the bearing locations by exciting the bearing housings with electromechanical shakers and measuring the acceleration. Direct, cross-coupled, and cross-talk accelerance between supports were measured. Unbalance response and stability analyses of the rotor were performed using polynomial transfer functions extracted from the measured accelarance data. The predicted critical speeds and instability threshold agree with measured data. Predictions using other support models are included to show the effectiveness of this method.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2002;124(2):375-382. doi:10.1115/1.1417485.

To meet the advanced bearing needs of modern turbomachinery, a hybrid foil-magnetic hybrid bearing system was designed, fabricated, and tested in a test rig designed to simulate the rotor dynamics of a small gas turbine engine (31 kN to 53 kN thrust class). This oil-free bearing system combines the excellent low and zero-speed capabilities of the magnetic bearing with the high-load capacity and high-speed performance of the compliant foil bearing. An experimental program is described which documents the capabilities of the bearing system for sharing load during operation at up to 30,000 rpm and the foil bearing component’s ability to function as a backup in case of magnetic bearing failure. At an operating speed of 22,000 rpm, loads exceeding 5300 N were carried by the system. This load sharing could be manipulated by an especially designed electronic control algorithm. In all tests, rotor excursions were small and stable. During deliberately staged magnetic bearing malfunctions, the foil bearing proved capable of supporting the rotor during continued operation at full load and speed, as well as allowing a safe rotor coastdown. The hybrid system tripled the load capacity of the magnetic bearing alone and can offer a significant reduction in total bearing weight compared to a comparable magnetic bearing.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2002;124(2):383-387. doi:10.1115/1.1417982.

The primary objective of this report involves studying and developing various experimental techniques for accurate measurement of the mean stress effect in thermoelastic stress analysis (TSA, also recognized as SPATE: stress pattern analysis by thermal emission). The analysis of cyclic mean stresses at the coupon level directly relates to the measurement of residual stresses in structures. In a previous study by the authors, it was shown that cyclic mean stresses significantly influenced the TSA results for titanium and nickel-based alloys, although, difficulties were encountered concerning the quantification of the mean stress effect because of large test-to-test variations. This study continues the effort of accurate direct measurements of the mean stress effect by implementing various experimental modifications. In addition, a more in-depth analysis is included which involves analyzing the second harmonic of the temperature response. By obtaining the amplitudes of the first and second harmonics, the stress amplitude and the mean stress at a given point on a structure subjected to a cyclic load can be simultaneously obtained. The rather complex analysis of the temperature response involves obtaining the first and second harmonic amplitudes for 16384 infrared detectors (128×128 focal plane array). Upon establishing a protocol for mean stress measurements in the laboratory using the TSA technique, the next step is to utilize the method to assess residual stress states in complex structures during manufacturing and life.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2002;124(2):388-394. doi:10.1115/1.1419015.

The paper describes the development of an instrumented test rig suitable to carry out vibration testing of the bladed segments of a turbine (belonging to a stator low pressure stage) in which the friction occurring at the contact of neighboring segments is used in service to damp the vibrations. The system is able to assign the desired preload to the segment and to measure the forces exerted in the contact and the relative displacements due to slipping. The main results that can be obtained concern the type of motion that takes place (stick/slip) and the effect of the interlocking on it, the friction coefficients, the contact tangential stiffness, and, more generally, the frequency response.

Commentary by Dr. Valentin Fuster

TECHNICAL PAPERS: Gas Turbines: Coal, Biomass, and Alternative Fuels

J. Eng. Gas Turbines Power. 2002;124(2):235-238. doi:10.1115/1.1335478.

There is an increasing interest in catalytic combustors fuelled by low-heating value (LHV) gases, with a LHV of 5–7 MJ/Nm3 . This is because catalytic combustion could be advantageous compared to flame combustion with respect to stable combustion of LHV-gases and low conversions of fuel-N (mainly NH3) to NOX. In the present project, funded by the EU Joule Program, catalytic combustion of gasified wood for gas turbine applications is studied. A synthetic gas mixture of H2, CO, CO2,H2O,CH4,N2, and NH3, that resembles the output from a fluidized bed gasifier using biomass as raw material, is used. The gas mixture is mixed with air at atmospheric pressure and combusted over washcoated cordierite monoliths in a bench-scale laboratory quartz-reactor. The objectives of the work described here are twofold. To begin with, improvement of the thermal stability of hexaaluminate washcoats by substitutions of rare earth or transition metal compounds is being studied. Secondly, catalytic combustion of gasified biomass over these washcoats has been studied in a bench-scale unit. In this on-going project, obtained result show that it is possible to improve the surface area of hexaaluminate compounds up to 17 m2 /g after careful synthesis and calcination up to 1400°C for four hours. The selectivity of NH3-conversion to N2 is at present at 60 percent, but varies strongly with temperature. Fuel components such as H2, CO, C2H4, and NH3 ignite at temperatures close to compressor outlet temperatures. This means that a pilot-flame may not be needed for ignition of the fuel. A comparison between a Pd-impregnated lanthanum hexaaluminate and a Mn-substituted lanthanum hexaaluminate showed that the ignition temperature and the NOX-formation varied strongly over the two different catalysts.

Commentary by Dr. Valentin Fuster

TECHNICAL PAPERS: Gas Turbines: Electric Power

J. Eng. Gas Turbines Power. 2002;124(2):263-269. doi:10.1115/1.1448325.

The recuperation by means of external waste heat sources, as opposed to the recuperation of the turbine exhaust gases (to preheat the compressed air), allows one to utilize the hot exhaust gases of the gas turbine in the bottoming steam cycle to produce steam in order to generate additional power. Such a combined gas/steam energy system, closely integrated with the industrial process, can produce electric power (and useful heat) with high efficiency and very low atmospheric air pollution. In the present paper two examples of applications of this new technology have been analyzed from the economic and ecological viewpoint.

Topics: Heat , Gas turbines , Cycles , Steam
Commentary by Dr. Valentin Fuster

TECHNICAL PAPERS: Advanced Energy Systems

J. Eng. Gas Turbines Power. 2002;124(2):395-403. doi:10.1115/1.1423320.

In small Rankine cycle power plants, it is advantageous to use organic media as the working fluid. A low-cost single-stage turbine design together with the high molecular weight of the fluid leads to high Mach numbers in the turbine. Turbine efficiency can be improved significantly by using an iterative design procedure based on an accurate CFD simulation of the flow. For this purpose, an existing Navier-Stokes solver is tailored for real gas, because the expansion of an organic fluid cannot be described with ideal gas equations. The proposed simulation method is applied for the calculation of supersonic flow in a turbine stator. The main contribution of the paper is to demonstrate how a typical ideal-gas CFD code can be adapted for real gases in a very general, fast, and robust manner.

Commentary by Dr. Valentin Fuster

TECHNICAL PAPERS: Internal Combustion Engines: Fuels and combustion technology

J. Eng. Gas Turbines Power. 2002;124(2):429-436. doi:10.1115/1.1423639.

The purpose of this study is to find a maximum work output from various combinations of thermodynamic cycles from a viewpoint of the cycle systems. Three systems were discussed in this study: a fundamental combined cycle and two other cycles evolved from the fundamental dual combined cycle: series-type and parallel-type triple cycles. In each system, parametric studies were carried out in order to find optimal configurations of the cycle combinations based on the influences of tested parameters on the systems. The study shows that the series-type triple cycle exhibits no significant difference as compared with the combined cycle. On the other hand, the efficiency of the parallel-type triple cycle can be raised, especially in the application of recovering low-enthalpy-content waste heat. Therefore, by properly combining with a steam Rankine cycle, the organic Rankine cycle is expected to efficiently utilize residual yet available energy to an optimal extent. The present study has pointed out a conceptual design in multiple-cycle energy conversion systems.

Commentary by Dr. Valentin Fuster

TECHNICAL PAPERS: Gas Turbines: Manufacturing, Materials, and Metallurgy

J. Eng. Gas Turbines Power. 2002;124(2):270-275. doi:10.1115/1.1423914.

The objective of this study is aluminide overlay coatings of MCrAlY sprayed by a vacuum plasma spraying (VPS) process for the protection against high-temperature corrosion and oxidation of gas turbine components. Diffusion coating processes have been applied for many years to improve similarly the environmental resistance by enriching the surface of nickel-based superalloys with chromium, aluminum, or silicon element. Recently, aluminizing of MCrAlY coatings is used for improving further the high-temperature oxidation resistance. However, the aluminizing properties of plasma-sprayed MCrAlY coatings, which have an important effect on the coating performance, have not been clarified. In this study, five kinds of plasma-sprayed MCrAlY (CoCrAlY, CoNiCrAlY, CoNiCrAlY+Ta, NiCrAlY, and NiCoCrAlY) coating were selected for pack-aluminizing tests. The as sprayed and the heat-treated (1393 K, 2 h, argon cooled and 1116 K, 24 h, argon cooled) MCrAlY specimens were Al–Cr–Al2O3–NH4Cl pack-aluminized at 1173, 1223, and 1273 K for 5, 10, and 20 h, respectively. The experimental results showed that the aluminizing process formed the aluminum rich layers of NiAl or CoAl phase. It also indicated that the thickness of the aluminum rich layer showed a parabolic time-dependence in all MCrAlY coatings. The order of reaction diffusion rate was NiCoCrAlY=NiCrAlY>CoNiCrAlY>CoNiCrAlY+Ta>CoCrAlY. There was a tendency that the reaction diffusion rate by aluminizing increased with increasing nickel content in the MCrAlY coatings and the reaction diffusion rate of as sprayed MCrAlY coatings is faster than that of the heat-treated MCrAlY coatings.

Commentary by Dr. Valentin Fuster

TECHNICAL PAPERS: Gas Turbines: Ceramics, and Manufacturing Materials and Metallurgy

J. Eng. Gas Turbines Power. 2002;124(2):229-234. doi:10.1115/1.1447238.

The paper addresses the effect of processing parameters on microstructure and lifetime of electron beam physical vapor deposition, partially yttria-stabilized zirconia (EB-PVD PYSZ) coatings deposited onto NiCoCrAlY-coated Ni-base superalloys. In particular, the formation of a thermally grown oxide layer, an equi-axed zone, and various columnar arrangements of the highly textured PYSZ layers are discussed with respect to processing conditions. Three different microstructures were cyclically tested at 1100°C. The intermediate columnar structure was superior with respect to cyclic life times to a fine and to a coarse columnar structure which was mainly attributed to differences in the elastic properties. The effect of PYSZ microstructure on hot corrosion behavior of the thermal barrier coating (TBC) system at 950°C is briefly discussed.

Commentary by Dr. Valentin Fuster

TECHNICAL PAPERS: Gas Turbines: Combustion and Fuel

J. Eng. Gas Turbines Power. 2002;124(2):239-247. doi:10.1115/1.1383255.

An experimental method to determine the thermoacoustic properties of a gas turbine combustor using a lean-premixed low emission swirl stabilized burner is presented. To model thermoacoustic oscillations, a combustion system can be described as a network of acoustic elements, representing for example fuel and air supply, burner and flame, combustor, cooling channels, suitable terminations, etc. For most of these elements, simple analytical models provide an adequate description of their thermoacoustic properties. However, the complex response of burner and flame (involving a three-dimensional flow field, recirculation zones, flow instabilities, and heat release) to acoustic perturbations has—at least in a first step—to be determined by experiment. In our approach, we describe the burner as an active acoustical two-port, where the state variables pressure and velocity at the inlet and the outlet of the two port are coupled via a four element transfer matrix. This approach is similar to the “black box” theory in communication engineering. To determine all four transfer matrix coefficients, two test states, which are independent in the state vectors, have to be created. This is achieved by using acoustic excitation by loudspeakers upstream and downstream of the burner, respectively. In addition, the burner might act as an acoustic source, emitting acoustic waves due to an unsteady combustion process. The source characteristics were determined by using a third test state, which again must be independent from the two other state vectors. In application to a full size gas turbine burner, the method’s accuracy was tested in a first step without combustion and the results were compared to an analytical model for the burner’s acoustic properties. Then the method was used to determine the burner transfer matrix with combustion. An experimental swirl stabilized premixed gas turbine burner was used for this purpose. The treatment of burners as acoustic two-ports with feedback including a source term and the experimental determination of the burner transfer matrix is novel.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2002;124(2):248-255. doi:10.1115/1.1423640.

This paper deals with the multicomponent nature of gas turbine fuels under high-pressure conditions. The study is motivated by the consideration that the droplet submodels that are currently employed in spray codes for predicting gas turbine combustor flows do not adequately incorporate the multicomponent fuel and high-pressure effects. The quasi-steady multicomponent droplet model has been employed to investigate conditions under which the vaporization behavior of a multicomponent fuel droplet can be represented by a surrogate pure fuel droplet. The physical system considered is that of a multicomponent fuel droplet undergoing quasi-steady vaporization in an environment characterized by its temperature, pressure, and composition. Using different vaporization models, such as infinite-diffusion and diffusion-limit models, the predicted vaporization history and other relevant properties of a bicomponent droplet are compared with those of a surrogate single-component fuel droplet over a range of parameters relevant to gas turbine combustors. Results indicate that for moderate and high-power operation, a suitably selected single-component (50 percent boiling point) fuel can be used to represent the vaporization behavior of a bicomponent fuel, provided one employs the diffusion-limit or effective-diffusivity model. Simulation of the bicomponent fuel by a surrogate fuel becomes increasingly better at higher pressures. In fact, the droplet vaporization behavior at higher pressures is observed to be more sensitive to droplet heating models rather than to liquid fuel composition. This can be attributed to increase in the droplet heatup time and reduction in the volatility differential between the constituent fuels at higher pressures. For ignition, lean blowout and idle operations, characterized by low pressure and temperature ambient, the multicomponent fuel evaporation cannot be simulated by a single-component fuel. The validity of a quasi-steady high-pressure droplet vaporization model has also been examined. The model includes the nonideal gas behavior, liquid-phase solubility of gases, and variable thermo-transport properties including their dependence on pressure. Predictions of the high-pressure droplet model show good agreement with the available experimental data over a wide range of pressures, implying that quasi-steady vaporization model can be used at pressures up to the fuel critical pressure.

Commentary by Dr. Valentin Fuster

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