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

J. Eng. Gas Turbines Power. 2013;135(12):121501-121501-9. doi:10.1115/1.4025261.

The stability of the combustion process in a V-gutter stabilized combustor is numerically investigated. To this end, 3D compressible turbulent and unsteady reacting flow calculations have been carried out using LES. The time history of the pressure at several locations is used to determine the frequency and amplitude of the oscillations along with the mode shapes. A shift in the dominant mode of the frequency spectra from the acoustic mode to the hydrodynamic mode is observed. A POD analysis of pressure time histories on the symmetry plane also corroborates this trend. The computational domain is divided into several subvolumes in the wake region of the V-gutter and the time histories of pressure, temperature, and heat release are collected in the individual volumes. It is seen that the fluctuation of pressure and heat release tend to oscillate from being in phase to out of phase over a time period. Unstable regions predicted by the Rayleigh index across a plane are shown to be different from those predicted in a volume owing to the three dimensionality of the flame. Quite interestingly, the calculated values of the indices show the combustor to be most unstable for an equivalence ratio of 0.1665 which is not the leanest one considered here. The global Rayleigh index is shown to correlate well with the amplitude of the dominant mode.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;135(12):121502-121502-9. doi:10.1115/1.4025262.

Rifled fillisters were milled on cannular frustums to modulate flow behavior and to increase the turbulence intensity (TI). The TI and combustion intensity were compared in four configurations of frustums—unrifled, inner-rifled, outer-rifled, and two-faced rifled. The flame patterns and flame lengths were observed and measured by direct-color photography. The temperature profiles and (total) combustion intensity were detected and calculated with an R-type thermocouple. Three flame patterns (jet, flickering, and lifted flames) were defined behind the pure-jet nozzle. Four flame patterns (jet, flickering, bubble, and turbulent flames) were observed behind the unrifled frustum. The bluff-body frustum changes the lifted flame to turbulent flame due to a high T.I at high central-fuel velocity (uc). The experimental data showed that the grooved rifles improved the air-propane mixing, which then improved the combustion intensity. The rifled mechanism intensified the swirling effect and then the flame-temperature profiles were more uniform than those behind the pure-jet nozzle. The increased TI also resulted in the shortest flame length behind the two-faced rifled frustum and increased the total combustion intensity.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;135(12):121503-121503-9. doi:10.1115/1.4025147.

An increasing demand is being put on the fuel as a heat sink in modern aircraft. In the end, the fuel flows through the atomizer, which is both the hottest part in the thermal history of the fuel and the most critical for resisting deposition. Most studies have concentrated on the chemistry of deposition and in recent years there have been modeling efforts. Deposition is really the end product of a coupling between heat transfer to the fuel, chemical reactions to form insoluble gums, followed by the transport of these gums to the surface to form deposits. There is conflicting evidence and theory in the literature concerning the effect of turbulence on deposition, i.e., whether deposition increases or decreases with increasing Reynolds number. This paper demonstrates, through a heat transfer analysis, that the effect of the Reynolds number depends upon the boundary/initial conditions. If the flow is heated from the surface, deposition decreases with increasing Reynolds number; however, for isothermal flows, i.e., preheated, deposition can increase with the Reynolds number.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;135(12):121504-121504-10. doi:10.1115/1.4025129.

Flashback is a key challenge for low NOx premixed combustion of high hydrogen content fuels. Previous work on jet burner configurations has systematically investigated the impact of fuel composition on flashback propensity, and noted that burner tip temperature played an important role on flashback, yet did not quantify any specific effect (Shaffer, B., Duan, Z., and McDonell, V., 2013, “Study of Fuel Composition Effects on Flashback Using a Confined Jet Flame Burner,” ASME J. Eng. Gas Turb. Power, 135(1), p. 011502). The present work further investigates the coupling of flashback with burner tip temperature and leads to models for flashback propensity as a function of parameters studied. To achieve this, a jet burner configuration with interchangeable burner materials was developed along with automated flashback detection and rim temperature monitoring. An inline heater provides preheated air up to 810 K. Key observations include that for a given condition, tip temperature of a quartz burner at flashback is higher than that of a stainless burner. As a result, the flashback propensity of a quartz tube is about double of that of a stainless tube. A polynomial model based on analysis of variance is presented and shows that, if the tip temperature is introduced as a parameter, better correlations result. A physical model is developed and illustrates that the critical velocity gradient is proportional to the laminar flame speed computed using the measured tip temperature. The addition of multiple parameters further refined the prediction of the flashback propensity, and the effects of materials are discussed qualitatively using a simple heat transfer analysis.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;135(12):121505-121505-7. doi:10.1115/1.4025238.

Understanding the effects of inlet velocity and inlet equivalence ratio fluctuations on heat release rate fluctuations in lean premixed gas turbine combustors is essential for predicting combustor instability characteristics. This information is typically obtained from independent velocity-forced and fuel-forced flame transfer function measurements, where the global chemiluminescence intensity is used as a measure of the flame's overall rate of heat release. Current lean premixed combustors operate in a technically premixed mode where the flame is exposed to both velocity and equivalence ratio fluctuations and, as a result, the chemiluminescence intensity does not provide an accurate measure of the flame's rate of heat release. The objective of this work is to experimentally assess the validity of a technique for measuring heat release rate fluctuations in technically premixed flames based on the linear superposition of fuel-forced and velocity-forced flame transfer function measurements. In the absence of a technique for directly measuring heat release rate fluctuations in technically premixed flames, the heat release rate reconstruction is validated indirectly by comparing measured and reconstructed chemiluminescence intensity fluctuations. The results are reported for a range of operating conditions and forcing frequencies which demonstrate the capabilities and limitations of the heat release rate reconstruction technique. A variation of this technique, referred to as a reverse reconstruction, is also proposed, which does not require a measurement of the fuel-forced flame transfer function. The results obtained using the reverse reconstruction technique are presented and compared to the results from the direct reconstruction technique.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;135(12):121506-121506-9. doi:10.1115/1.4025130.

Linear stability analysis is applied to a swirl-stabilized combustor flow with the aim to understand how the flame shape and associated density field affects the manifestation of self-excited flow instabilities. In isothermal swirling jets, self-excited flow oscillations typically manifest in a precessing vortex core and synchronized growth of large-scale spiral-shaped vortical structures. Recent theoretical studies relate these dynamics to a hydrodynamic global instability. These global modes also emerge in reacting flows, thereby crucially affecting the mixing characteristics and the flame dynamics. It is, however, observed that these self-excited flow oscillations are often suppressed in the reacting flow, while they are clearly present at isothermal conditions. This study provides strong evidence that the suppression of the precessing vortex core is caused by density inhomogeneities created by the flame. This mechanism is revealed by considering two reacting flow configurations: The first configuration represents a perfectly premixed steam-diluted detached flame featuring a strong precessing vortex core. The second represents a perfectly premixed dry flame anchoring near the combustor inlet, which does not exhibit self-excited oscillations. Experiments are conducted in a generic combustor test rig and the flow dynamics are captured using PIV and LDA. The corresponding density fields are approximated from the seeding density using a quantitative light sheet technique. The experimental results are compared to the global instability properties derived from hydrodynamic linear stability theory. Excellent agreement between the theoretically derived global mode frequency and measured precession frequency provide sufficient evidence to conclude that the self-excited oscillations are, indeed, driven by a global hydrodynamic instability. The effect of the density field on the global instability is studied explicitly by performing the analysis with and without density stratification. It turns out that the significant change in instability is caused by the radial density gradients in the inner recirculation zone and not by the change of the mean velocity field. The present work provides a theoretical framework to analyze the global hydrodynamic instability of realistic combustion configurations. It allows for relating the flame position and the resulting density field to the emergence of a precessing vortex core.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;135(12):121507-121507-8. doi:10.1115/1.4025131.

A design for thermo-acoustic stability (DeTAS) procedure is presented, that aims at selecting a most stable burner geometry for a given combustor. It is based on the premise that a thermo-acoustic stability model of the combustor can be formulated and that a burner design exists, which has geometric design parameters that sufficiently influence the dynamics of the flame. Describing the burner and flame dynamics in dependence of the geometrical parameters an optimization procedure involving a linear stability model of the target combustor maximizes the damping and thereby yields the optimal geometrical parameters. To demonstrate the procedure on an existing annular combustor a generic burner design was developed that features two geometrical parameters that can easily be adjusted. To provide the database for the DeTAS procedure static and dynamical properties of burner and flame were measured for three by three configurations at a fixed operation point. The data is presented and discussed. It is found that the chosen design exhibits a significant variability of the flame dynamics in dependence of the geometrical parameters indicating that a DeTAS should be possible for the targeted annular combustor.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;135(12):121508-121508-7. doi:10.1115/1.4025148.

For lean burn combustor development in low emission aero-engines, the pilot stage of the fuel injector plays a key role with respect to stability, operability, NOx emissions, and smoke production. Therefore it is of considerable interest to characterize the pilot module in terms of pilot zone mixing, fuel placement, flow field, and interaction with the main stage. This contribution focuses on the investigation of soot formation during pilot-only operation. Optical test methods were applied in an optically accessible single sector rig at engine idle conditions. Using planar laser-induced incandescence (LII), the distribution of soot and its dependence on air/fuel ratio, as well as geometric injector parameters, was studied. The data shows that below a certain air/fuel ratio, an increase of soot production occurs. This is in agreement with smoke number measurements in a standard single sector flame tube rig without optical access. Reaction zones were identified using chemiluminescence of OH radicals. In addition, the injector flow field was investigated with PIV. A hypothesis regarding the mechanism of pilot smoke formation was made based on these findings. This along with further investigations will form the basis for developing strategies for smoke improvement at elevated pilot-only conditions.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2013;135(12):121601-121601-8. doi:10.1115/1.4025299.

Models for the analysis of thermoacoustic instabilities are conveniently formulated in the frequency domain. In this case one often faces the difficulty that the response behavior of some elements of the system is only known at real-valued frequencies, although the transfer behavior at complex-valued frequencies is required for the quantification of the growth rates of instabilities. The present paper discusses various methods for extrapolation of frequency response data at real-valued frequencies into the complex plane. Some methods have been used previously in thermoacoustic stability analysis; others are newly proposed. First the pertinent mathematical background is reviewed, then the sensitivity of predicted growth rates on the extrapolation scheme is explored. This is done by applying different methods to a simple thermoacoustic system, i.e., a ducted premixed flame, for which an analytical solution is known. A short analysis determining the region of confidence of the extrapolated transfer function is carried out to link the present study to practical applications. The present study can be seen as a practical guideline for using frequency response data collected for a set of real-valued frequencies in quantitative linear stability analysis.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2013;135(12):122501-122501-13. doi:10.1115/1.4025309.

Gas turbines are generally used in power generation, the oil and gas industries, and as jet engines in aircrafts. Fault tolerance and reliability is important in such applications. Thus, accurate modeling and control system design is necessary. In this paper, first a nonlinear hybrid fuzzy model was developed for an industrial gas turbine, and then this model was used as the core of a fault tolerant control (FTC) system. The aforementioned model was trained by use of three months of operational data of a GE MS 5002 D gas turbine that is used for gas injection application, then it was fine tuned using expert knowledge and physical principles. A graphical user interface (GUI) was also developed to run various realistic operational scenarios of the gas turbine. The main point of the present work consists in introducing nonlinear fuzzy model schemes as the core of an adaptive unscented Kalman filter (AUKF) for fault diagnostic purposes. Analysis of the simulation results discloses that this FTC approach alleviates the effects of faults in two different scenarios such as sequential drift and bias in sensors/actuators and also in simultaneous faults that are a disastrous situation.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;135(12):122502-122502-7. doi:10.1115/1.4025315.

The energy-based lifing method is based on the theory that the cumulative energy in all hysteresis loops of a specimens' lifetime is equal to the energy in a monotonic tension test. Based on this theory, fatigue life can be calculated by dividing monotonic tensile energy by a hysteresis energy model, which is a function of stress amplitude. Due to variations in the empirically measured hysteresis loops and monotonic fracture area, fatigue life prediction with the energy-based method shows some variation as well. In order to account for these variations, a robust design optimization technique is employed. The robust optimization procedure uses an interval uncertainty technique, eliminating the need to know an exact probability density function for the uncertain parameters. The robust optimization framework ensures that the difference between the predicted lifetime at a given stress amplitude and the corresponding experimental fatigue data point is minimized and within a specified tolerance range while accounting for variations in hysteresis loop energy and fracture diameter measurements. Accounting for these experimental variations will boost confidence in the energy-based fatigue life prediction method despite a limited number of test specimens.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;135(12):122503-122503-8. doi:10.1115/1.4025146.

Proven low-cost gas bearing technologies are sought to enable more compact rotating machinery products with extended maintenance intervals. The paper presents an analysis for predicting the static and dynamic forced performance characteristics of metal mesh foil bearings (MMFBs) which comprise a top foil supported on a layer of metal mesh of a certain compactness. The analysis couples a finite element model of the top foil and underspring support with the gas film Reynolds equation. A comparison of the predictions against laboratory measurements with two bearings aims to validate the analysis. The predicted drag friction factor in one bearing (L = D = 28.00 mm) during full film operation is just f ∼ 0.03 at ∼50,000 rpm, in good agreement with measurements at increasing applied loads. The predictions further elucidate the effect of the applied load and rotor speed on the bearing minimum film thickness, journal eccentricity, and attitude angle. For a second bearing (L = 38.0 mm, D = 36.5 mm), predicted bearing force coefficients show magnitudes comparable with the measurements, with less than a 20% difference, in the 250–350 Hz excitation frequency range. While the predicted direct stiffness coefficients are rather constant, the experimental force coefficients increase with frequency (maximum 400 Hz), due mainly to the increasing amplitudes of dynamic force applied to excite the bearing with a set amplitude of motion. The analysis underpredicts the direct damping coefficients at high frequencies (>300 Hz). The cross-coupled stiffness and damping coefficients are typically lower (<40%) than the direct ones. The bearings operated stably at all speeds without any subsynchronous whirl. The reasonable agreement of the predictions with the available test data promote the better design and further development of MMFB supported rotating machinery.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;135(12):122504-122504-8. doi:10.1115/1.4025076.

Linear dynamic finite element analysis can be considered very reliable today for the design of aircraft engine components. Unfortunately, when these individual components are built into assemblies, the level of confidence in the results is reduced since the joints in the real structure introduce nonlinearity that cannot be reproduced with a linear model. Certain types of nonlinear joints in an aircraft engine, such as underplatform dampers and blade roots, have been investigated in great detail in the past, and their design and impact on the dynamic response of the engine is now well understood. With this increased confidence in the nonlinear analysis, the focus of research now moves towards other joint types of the engine that must be included in an analysis to allow an accurate prediction of the engine behavior. One such joint is the bolted flange, which is present in many forms on an aircraft engine. Its main use is the connection of different casing components to provide the structural support and gas tightness to the engine. This flange type is known to have a strong influence on the dynamics of the engine carcase. A detailed understanding of the nonlinear mechanisms at the contact is required to generate reliable models and this has been achieved through a combination of an existing nonlinear analysis capability and an experimental technique to accurately measure the nonlinear damping behavior of the flange. Initial results showed that the model could reproduce the correct characteristics of flange behavior, but the quantitative comparison was poor. From further experimental and analytical investigations it was identified that the quality of the flange model is critically dependent on two aspects: the steady stress/load distribution across the joint and the number and distribution of nonlinear elements. An improved modeling approach was developed that led to a good correlation with the experimental results and a good understanding of the underlying nonlinear mechanisms at the flange interface.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;135(12):122505-122505-8. doi:10.1115/1.4025234.

This paper presents an investigation on dynamic characteristics of a rod-fastened rotor. Based on the framework of a traditional Riccati transfer matrix method (TMM), an improved Riccati TMM considering contact effects brought by a face tooth is developed. A correction coefficient for equivalent stiffness imported from a three-dimensional (3D) finite element contact case analysis is defined to evaluate the contact effects, and then the dynamic model of the rod-fastened rotor including bearing support is established. A computer program is further developed to obtain the dynamic characteristics such as critical speeds of lateral vibration, mode shapes, and an unbalance response. The improved TMM is applied to investigate the dynamic characteristics of a real central tie rod rotor of the class-F gas turbine for verification of its effectiveness, and the calculated critical speeds are in good agreement with test measurement results, implying that the method is accurate and the dynamic model is reliable. This approach can also be applied to analyze other combined rotors with a homogeneous structure.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;135(12):122506-122506-11. doi:10.1115/1.4025236.

A brush-labyrinth sealing configuration consisting of two labyrinth fins upstream and one brush seal downstream is studied experimentally and theoretically. Two slightly different brush seal designs with zero cold radial clearance are considered. The sealing configurations are tested on the no-whirl and dynamic test rigs to obtain leakage performance and rotordynamic stiffness and damping coefficients. The no-whirl tests allow identification of the local rotordynamic direct and cross-coupled stiffness coefficients for a wide range of operating conditions, while the dynamic test rig is used to obtain both global stiffness and damping coefficients but for a narrower operating range limited by the capabilities of a magnetic actuator. Modeling of the brush-labyrinth seals is performed using computational fluid dynamics. The experimental global rotordynamic coefficients consist of an aerodynamic component due to the gas flow and a mechanical component due to the contact between the bristle tips and rotor surface. The computational fluid dynamics (CFD)–based calculations of rotordynamic coefficients provide, however, only the aerodynamic component. A simple mechanical model is used to estimate the theoretical value of the mechanical stiffness of the bristle pack during the contact. The results obtained for the sealing configurations with zero cold radial clearance brush seals are compared with available data on three-tooth-on-stator labyrinth seals and a brush seal with positive cold radial clearance. Results show that the sealing arrangement with a line-on-line welded brush seal has the best performance overall with the lowest leakage and cross-coupled stiffness. The predictions are generally in agreement with the measurements for leakage and stiffness coefficients. The seal-damping capability is noticeably underpredicted.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;135(12):122507-122507-10. doi:10.1115/1.4025237.

Gas bearings in power generation microturbomachinery (MTM) and for automotive turbocharger applications must demonstrate adequate thermal management without performance degradation while operating in a harsh environment. The paper presents rotor surface temperatures and rotordynamic measurements of a rigid rotor supported on a pair of metal mesh foil bearings (MMFBs) (L = 38.1 mm, D = 36.6 mm). In the tests, to a maximum rotor speed of 50 krpm, an electric cartridge heats the hollow rotor over several hours while a steady inlet air flow rate at ∼160 L/min cools the bearings. In the tests with the heater set to a high temperature (max. 200 °C), the rotor and bearing OD temperatures increase by 70 °C and 25 °C, respectively. Most rotor dynamic responses do not show a marked difference for operation under cold (ambient temperature) or hot rotor conditions. A linear rotordynamics structural model with predicted MMFB force coefficients delivers rotor response amplitudes in agreement with the measured ones for operation with the rotor at ambient temperature. There are marked differences in the peak amplitudes when the rotor crosses its (rigid body) critical speeds. The test bearings provide lesser damping than predictions otherwise indicate. Waterfalls of rotor motion show no sub synchronous whirl frequency motions; the rotor-bearing system being stable for all operating conditions. The measurements demonstrate that MMFBs can survive operation with severe thermal gradients, radial and axial, and with little rotordynamic performance changes when the rotor is either cold or hot. The experimental results, accompanied by acceptable predictions of the bearings dynamic forced performance, promote further MMFBs as an inexpensive reliable technology for MTM.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;135(12):122508-122508-8. doi:10.1115/1.4025239.

The micro hybrid spiral-grooved thrust bearing is a promising candidate to support the rotating elements in power MEMS devices such as micro gas turbine engines. However, the realization of hybrid thrust bearings has encountered a number of technical challenges due to the very high rotating speed and DN number (the product of the inner diameter and the rotational speed of the bearing, mm · rpm) to achieve high power density, the super thin gas film between rotors and thrust pad, and the relative large fabrication uncertainties according to the imperfection of the fabrication technology. In this paper, the configuration of a micro hybrid spiral-grooved thrust bearing for power MEMS is designed, and the steady and dynamic characteristics of this kind of bearing are then analyzed comprehensively, with the consideration of both the rarefaction effects and the influence of potential microfabrication defects. The nonlinear equations of molecular gas-film lubrication describing the gas rarefaction effects in a micro hybrid bearing are discretized by the finite volume method and solved by the Newton–Raphson techniques. The small perturbation technique is employed to study the dynamic behavior of a micro hybrid bearing. The results show that the micro hybrid thrust bearing exhibits better steady-state and dynamic performance than the existing micro hydrodynamic and hydrostatic bearings and that the hybrid bearings are likelier to be stable than their hydrodynamic counterparts, especially when the frequency number is high. The load capacity of the micro hybrid bearing increases slightly with the number of orifices and gradually with the diameter ratio of the orifice. The microfabrication defects of clogged orifices could lessen the load capacity and the dynamic coefficients of the hybrid thrust bearing. The model developed in this paper can serve as a useful tool to provide insight into micro hybrid gas thrust bearing-rotor systems.

Commentary by Dr. Valentin Fuster

Technical Briefs

J. Eng. Gas Turbines Power. 2013;135(12):124501-124501-4. doi:10.1115/1.4025260.

A supercritical steam bottoming cycle has been proposed as a performance enhancement option for gas turbine combined cycle power plants. The technology has been widely used in coal-fired steam turbine power plants since the 1950s and can be considered a mature technology. Its application to the gas-fired combined cycle systems presents unique design challenges due to the much lower gas temperatures (i.e., 650 °C at the gas turbine exhaust vis-à-vis 2000 °C in fossil fuel-fired steam boilers). Thus, the potential impact of the supercritical steam conditions is hampered to the point of economic infeasibility. This technical brief draws upon the second-law based exergy concept to rigorously quantify the performance entitlement of a supercritical high-pressure boiler section in a heat recovery steam generator utilizing the exhaust of a gas turbine to generate steam for power generation in a steam turbine.

Commentary by Dr. Valentin Fuster


J. Eng. Gas Turbines Power. 2013;135(12):125501-125501-1. doi:10.1115/1.4025233.

The reader appreciates the experimental data obtained by the authors for the completely sealed squeeze field damper (SFD) performing small amplitude vibrations and the detailed presentation of the test conditions. This would enable further comparisons with different theoretical models.

Commentary by Dr. Valentin Fuster


J. Eng. Gas Turbines Power. 2013;135(12):125502-125502-1. doi:10.1115/1.4025266.

The authors thank Professor Mihai Arghir for having performed the computational fluid dynamics (CFD) simulations of the flow fields for the two test damper configurations. The predictions demonstrate, as expected, the flow field in the deep feed groove and lubricated through feed holes to be extremely complicated. The brief discussion on the evolution of the flow patterns (vortices) along the various planes of observation is most enlightening.

Commentary by Dr. Valentin Fuster

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