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### Research Papers: Gas Turbines: Aircraft Engine

J. Eng. Gas Turbines Power. 2013;136(2):021201-021201-12. doi:10.1115/1.4025485.

The main goal of current engine development is to increase power density and efficiency and to minimize engine emissions. The idea is to obtain the desired power output with a highly charged combustion engine in combination with exhaust gas turbocharging and a very small engine displacement, which is known as downsizing. The selection of a turbocharger is based on the maps of the turbine and compressor, which are usually measured on a test bench. They also provide important boundary conditions on the engine process simulation of a supercharged engine with this turbocharger. In general, a very accurate measurement of the characteristic maps is desired to ensure the best possible matching. However, random and systematic errors have an impact on the measurement results. In order to assess the quality of the measured and calculated values, it is necessary to determine the uncertainties of the measurement variables as accurately as possible; particularly, the error propagation in calculating the efficiencies. The uncertainties are based on a systematic uncertainty component of the sensor and the confidence interval. In this way, the measurement uncertainty is estimated by linear and geometric combination of the calculated random and systematic uncertainties. After that, the respective uncertainty contributions and the identification of the relevant parameters that influence the resulting measurement uncertainty are evaluated. Knowing the measurement uncertainties of the characteristic maps of a turbocharger, the influence on engine operation will be determined with a one-dimensional engine process simulation model. Consequently, the determined measurement uncertainty will be applied as a deviation on the efficiencies and will be investigated in a GT POWER simulation. The impact of the measurement uncertainty on the engine performance is shown on the basis of load steps.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Coal, Biomass, and Alternative Fuels

J. Eng. Gas Turbines Power. 2013;136(2):021401-021401-12. doi:10.1115/1.4025479.

Amid various methods available to reduce pollutant emissions and to improve performance and combustion characteristics of a diesel engine, emulsified fuel seems to be promising. However, because of its different properties from diesel, a biodiesel emulsion is incompetent to provide standard diesel performance. Once combusted in a diesel engine; the proper adjustment of engine operating parameters with the presence of “micro-explosion” may amend the performance of a biodiesel emulsion run engine. In order to realize this fact, a comprehensive study has been carried out in a variable compression ratio diesel engine running with two-phase water in a palm biodiesel emulsion. The engine operating parameters studied and optimized are compression ratio (CR), injection timing (IT), and load. The water emulsions of palm oil methyl ester (WIP) with various specifications have been prepared by commercially available surfactants with appropriate HLB values. Water quantity (5% and 10%), surfactant quantity (1%, 2%, and 3%), and HLB values (4.3, 5, and 6) are the parameters optimized to attain the stable WIP by means of mean droplet diameter measurement and stability study. The optimized WIP of 5% water, 3% surfactant of 6 HLB is then tested in a diesel engine at varying CR (17, 17.5, and 18) and IT (20, 23, and 28 deg BTDC). For each of the combinations of CR and IT, the load has been varied from idling conditions to full load (12 kg) with an increment of 20% (2.4 kg) and 110% (13.2 kg) of full load. The results are analyzed in the form of performance, combustion, and emission parameters with respect to the baseline diesel run (CR = 17.5 and IT = 23 deg BTDC).

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2013;136(2):021501-021501-8. doi:10.1115/1.4025321.

As is well known, the increasing energy demand requires an efficient use of conventional energy sources, as well as the development of renewable technologies. The distributed generation systems entail significant benefits in terms of efficiency, emission reduction, availability and economy consequences. Renewable energy technologies are fed by intermittent resources. This feature makes the energy storage an important issue in order to improve the management or to enlarge annual operation of the facility. The use of hydrogen as an energy vector may satisfy this requirement and; at the same time, it introduces additional advantages in terms of energy efficiency and emissions reduction. This work presents an analysis based on the first and second thermodynamics law to investigate the efficiency of a hydrogen/oxygen-fueled gas turbine, which produces both electrical and thermal energy (cogeneration). A 20 kWe, microgas turbine is proposed to supply the base load demand of a residential area. The results show that the proposed facility is appropriate when the thermal energy demand is significant. We obtain an exergy efficiency of 45.7% and an energy efficiency of 89.4% regarding the lower heating value (LHV) of hydrogen. This high energy efficiency remains on the use of the liquid water effluent and the condensation heat. The main sources of irreversibility are analyzed and the effect of the design parameters on the energy and exergy efficiencies is discussed.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;136(2):021502-021502-10. doi:10.1115/1.4025359.

Flashback is a key operability issue for low emission premixed combustion systems operated on high hydrogen content fuels. Previous work investigated fuel composition impacts on flashback propensity and found that burner tip temperature was important in correlating flashback data in premixed jet flames. An enclosure around the jet flame was found to enhance the flame–burner rim interaction. The present study further addresses these issues using a jet burner with various geometric configurations and interchangeable materials. Systematic studies addressing the quantitative influence of various parameters such as tip temperature, burner material, enclosure size, and burner diameter on flashback propensity were carried out. A comprehensive overview of the flashback limits for all conditions tested in the current study as well as those published previously is given. The collective results indicate that the burner materials, tip temperature, and flame confinement play significant roles for flashback propensity and thus help explain previous scatter in flashback data. Furthermore, the present work indicates that the upstream flame propagation during flashback is affected by the burner material. The material with lower thermal conductivity yields larger flashback propensity but slower flame regression inside the tube. These observations can be potentially exploited to minimize the negative impacts of flashback in practical applications.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;136(2):021503-021503-11. doi:10.1115/1.4025361.

Flame response to imposed velocity fluctuations is experimentally measured in a single-nozzle turbulent swirling fully-premixed combustor. The flame transfer function is used to quantify the flame's response to imposed velocity fluctuations. Both the gain and phase of the flame transfer function are qualitatively similar for all operating conditions tested. The flame transfer function gain exhibits alternating regions of decreasing gain with increasing forcing frequency followed by regions of increasing gain with increasing forcing frequency. This alternating behavior gives rise to gain extrema. The flame transfer function phase magnitude increases quasi-linearly with increasing forcing frequency. Deviations from the linear behavior occur in the form of inflection points. Within the field, the current understanding is that the flame transfer function gain extrema are caused by the constructive/destructive interference of swirl number fluctuations and vortex shedding. Phase-synchronized images of forced flames are acquired to investigate the presence/importance of swirl number fluctuations, which manifest as fluctuations in the mean flame position and vortex shedding in this combustor. An analysis of phase-synchronized flame images reveals that mean flame position fluctuations are present at forcing frequencies corresponding to flame transfer function gain minima but not at forcing frequencies corresponding to flame transfer function gain maxima. This observation contradicts the understanding that flame transfer function gain maxima are caused by the constructive interference of mean flame position fluctuations and vortex shedding, since mean flame position fluctuations are shown not to exist at flame transfer function gain maxima. Further analysis of phase-synchronized flame images shows that the variation of the mean flame position fluctuation magnitude with forcing frequency follows an inverse trend to the variation of flame transfer function gain with forcing frequency, i.e., when the mean flame position fluctuation magnitude increases, the flame transfer function gain decreases and vice versa. Based on these observations it is concluded that mean flame position fluctuations are a subtractive effect. The physical mechanism through which mean flame position fluctuations decrease flame response is through the interaction of the flame with the Kelvin–Helmholtz instability of the mixing layer in the combustor. When mean flame position fluctuations are large the flame moves closer to the mixing layer and damps the Kelvin–Helmholtz instability due to the increased kinematic viscosity, fluid dilatation, and baroclinic production of vorticity with the opposite sign associated with the high temperature reaction zone.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;136(2):021504-021504-12. doi:10.1115/1.4025373.

The objective of this study is to investigate the sensitivity and accuracy of the reaction flow-field prediction for the LIMOUSINE combustor with regard to choices in computational mesh and turbulent combustion model. The LIMOUSINE combustor is a partially premixed, bluff body-stabilized natural gas combustor designed to operate at 40–80 kW and atmospheric pressure and used to study combustion instabilities. The transient simulation of a turbulent combusting flow with the purpose to study thermoacoustic instabilities is a very time-consuming process. For that reason, the meshing approach leading to accurate numerical prediction, known sensitivity, and minimized amount of mesh elements is important. Since the numerical dissipation (and dispersion) is highly dependent on, and affected by, the geometrical mesh quality, it is of high importance to control the mesh distribution and element size across the computational domain. Typically, the structural mesh topology allows using much fewer grid elements compared to the unstructured grid; however, an unstructured mesh is favorable for flows in complex geometries. To explore computational stability and accuracy, the numerical dissipation of the cold flow with mixing of fuel and air is studied first in the absence of the combustion process. Thereafter, the studies are extended to combustible flows using standard available ansys-cfx combustion models. To validate the predicted variable fields of the combustor's transient reactive flows, the numerical results for dynamic pressure and temperature variations, resolved under structured and unstructured mesh conditions, are compared with experimental data. The obtained results show minor dependence on the used mesh in the velocity and pressure profiles of the investigated grids under nonreacting conditions. More significant differences are observed in the mixing behavior of air and fuel flows. Here, the numerical dissipation of the (unstructured) tetrahedral mesh topology is higher than in the case of the (structured) hexahedral mesh. For that reason, the combusting flow, resolved with the use of the hexahedral mesh, presents better agreement with experimental data and demands less computational effort. Finally, in the paper, the performance of the combustion model for reacting flow is presented and the main issues of the applied combustion modeling are reviewed.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;136(2):021505-021505-7. doi:10.1115/1.4025374.

Swirl-stabilized, nonpremixed ethylene/air flames were investigated at pressures up to 5 bar to study the effect of different operating parameters on soot formation and oxidation. Focus of the experiments was the establishment of a database describing well-defined flames, serving for validation of numerical simulation. Good optical access via pressure chamber windows and combustion chamber windows enables application of laser-induced incandescence to derive soot volume fractions after suitable calibration. This results in ensemble averaged, as well as instantaneous soot distributions. Beyond pressure, parameters under study were the equivalence ratio, thermal power, and amount of oxidation air. The latter could be injected radially into the combustor downstream of the main reaction zone through holes in the combustion chamber posts. Combustion air was introduced through a dual swirl injector whose two flow rates were controlled separately. The split of those air flows provided an additional parameter variation. Nominal power of the operating points was approximately 10 kW/bar leading to a maximum power of roughly 50 kW, not including oxidation air.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;136(2):021506-021506-8. doi:10.1115/1.4025375.

Equivalence ratio fluctuations are known to be one of the key factors controlling thermoacoustic stability in lean premixed gas turbine combustors. The mixing and thus the spatiotemporal evolution of these perturbations in the combustor flow is, however, difficult to account for in present low-order modeling approaches. To investigate this mechanism, experiments in an atmospheric combustion test rig are conducted. To assess the importance of equivalence ratio fluctuations in the present case, flame transfer functions for different injection positions are measured. By adding known perturbations in the fuel flow using a solenoid valve, the influence of equivalence ratio oscillations on the heat release rate is investigated. The equivalence ratio fluctuations in the reaction zone are measured spatially and temporally resolved using two optical chemiluminescence signals, captured with an intensified camera. A steady calibration measurement allows for the quantitative assessment of the equivalence ratio fluctuations in the flame. This information is used to obtain a mixing transfer function, which relates fluctuations in the fuel flow to corresponding fluctuations in the equivalence ratio of the flame. The current study focuses on the measurement of the global, spatially integrated, transfer function for equivalence ratio fluctuations and the corresponding modeling. In addition, the spatially resolved mixing transfer function is shown and discussed. The global mixing transfer function reveals that, despite the good spatial mixing quality of the investigated generic burner, the ability to damp temporal fluctuations at low frequencies is rather poor. It is shown that the equivalence ratio fluctuations are the governing heat release rate oscillation response mechanism for this burner in the low-frequency regime. The global transfer function for equivalence ratio fluctuations derived from the measurements is characterized by a pronounced low-pass characteristic, which is in good agreement with the presented convection–diffusion mixing model.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2013;136(2):021601-021601-8. doi:10.1115/1.4025235.

It is widely recognized that the fuel-air mixing process is a critical factor in improving combustion efficiency and in minimizing pollutants such as NOx. Enhancement of fuel-air mixing can lead to lower pollutant emissions and greater efficiency. However, swirling flows in lean combustors play the role of fuel-air mixing and flame stability. The complex fluid dynamic phenomena encountered in swirling two-phase flow contribute to the difficulty in complete understanding of the different processes occurring in combustors. Fortunately, optical and laser-based visualization techniques available in our lab are important nonintrusive tools for visualizing flow process, especially for fuel injection and fuel-air mixing. To provide for a better understanding of effects of counter-rotating flow on droplets in atomization process, this study is a detailed characterization of the spray generated by an airblast atomizer by planar laser sheet imaging method. Optical facility for spray diagnostics with fuel planar laser induced fluorescence (fuel-PLIF) method for fuel distribution and particle image velocimetry (PIV) method for the velocity of droplets is used to evaluate the performance of an airblast atomizer. The results show that the performance of secondary atomization is influenced by swirling flow and primary atomization simultaneously; the swirling flow exhibits significant influence on the droplet size and space distribution relative to that of primary atomization. The primary swirling air reopens the spray cone generated by pressure-swirl atomizer, and the secondary swirling air affects the fuel distribution by forming the recirculation zone. The results provide critical information for the design and development of combustion chambers.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;136(2):021602-021602-10. doi:10.1115/1.4025556.

The steam turbine cooldown has a significant impact on the cyclic fatigue life. A lower initial metal temperature after standstill results in a higher temperature difference to be overcome during the next start-up. Generally, lower initial metal temperatures result in higher start-up stress. In order to optimize steam turbines for cyclic operation, it is essential to fully understand natural cooling, which is especially challenging for rotors. This paper presents a first-in-time application of a 2D numerical procedure for the assessment of the thermal regime during natural cooling, including the rotors, casings, valves, and main pipes. The concept of the cooling calculation is to replace the fluid gross buoyancy during natural cooling by an equivalent fluid conductivity that gives the same thermal effect on the metal parts. The fluid equivalent conductivity is calculated based on experimental data. The turbine temperature was measured with pyrometric probes on the rotor and with standard thermocouples on inner and outer casings. The pyrometric probes were calibrated with standard temperature measurements on a thermo well, where the steam transmittance and the rotor metal transmissivity were measured.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Industrial & Cogeneration

J. Eng. Gas Turbines Power. 2013;136(2):022001-022001-10. doi:10.1115/1.4025474.

The liquefaction of natural gas is an energy intensive process and accounts for a considerable portion of the costs in the liquefied natural gas (LNG) value chain. Within this, the selection of the driver for running the gas compressor is one of the most important decisions and indeed the plant may well be designed around the driver, so one can appreciate the importance of driver selection. This paper forms part of a series of papers focusing on the research collaboration between Shell Global Solutions and Cranfield University, looking at the equipment selection of gas turbines in LNG service. The paper is a broad summary of the LNG Technoeconomic and Environmental Risk Analysis (TERA) tool created for equipment selection and looks at all the important factors affecting selection, including thermodynamic performance simulation of the gas turbines, lifing of hot gas path components, risk analysis, emissions, maintenance scheduling, and economic aspects. Moreover, the paper looks at comparisons between heavy duty industrial frame engines and two artificial design variants representing potential engine uprates. The focus is to provide a quantitative and multidisciplinary approach to equipment selection. The paper is not aimed to produce absolute accurate results (e.g., in terms of engine life prediction or emissions), but useful and realistic trends for the comparison of different driver solutions. The process technology is simulated based on the Shell DMR technology and single isolated trains are simulated with two engines in each train. The final analysis is normalized per tonne of LNG produced to better compare the technologies.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Manufacturing, Materials, and Metallurgy

J. Eng. Gas Turbines Power. 2013;136(2):022101-022101-7. doi:10.1115/1.4025358.

Compliant plate seals are being developed for various turbomachinery sealing applications including gas turbines, steam turbines, aircraft engines, and oil and gas compressors. These seals consist of compliant plates attached to a stator in a circumferential fashion around a rotor. The compliant plates have a slot that extends radially inward from the seal outer diameter and an intermediate plate extends inward into this slot from the stator. This design is capable of providing passive hydrostatic feedback forces acting on the compliant plates that balance at a small tip clearance. Due to this self-correcting behavior, this seal is capable of providing high differential pressure capability and low leakage within a limited axial span, and robust noncontact operation even in the presence of large rotor transients. Manufacturing of compliant plate seals is a challenging problem and in this paper we describe the development of a novel manufacturing technique called side weld and bend (SWAB). The compliant plates are tightly packed with alternating spacer shims in a straight line fixture and welded to a top plate from the side along a straight line. After removal of the spacer shims, the welded assembly is bent to form an arcuate seal of a desired diameter. The side weld and bend (SWAB) manufacturing method reduces distortion, deformity, differential shrinkage, and other associated problems with welding across gaps between adjacent compliant plate seals as is typical in current manufacturing processes.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Marine

J. Eng. Gas Turbines Power. 2013;136(2):022201-022201-8. doi:10.1115/1.4025486.

The increasingly stringent NOx emission regulations of the International Marine Organization (IMO) have demanded new design concepts and architectures for diesel engines. The Miller cycle, which reduces the in-cylinder combustion temperature by reducing the effective compression ratio, is the principal measure used for reducing NOx specific emissions; however, this is at the cost of volumetric efficiency and engine power. Therefore, it is essential to combine the Miller cycle with a highly boosted turbocharging system, two-stage turbocharging for example, to recover the power. While much work has been done in the development of Miller-cycle regulatable two stage turbocharging system for marine diesel engines, there are nonetheless few, if any, thorough discussions on system optimization and performance comparison. This study presents a theoretical optimization design process for a Miller-cycle regulatable, two-stage turbocharging system for marine diesel engines. First, the different scenarios and regulation methods of two-stage turbocharging systems are compared according to the system efficiency and equivalent turbine flow characteristics. Then, a multizone combustion model based on a one-dimensional cycle simulation model is established and used for the optimization of valve timings according to the IMO NOx emission limits and fuel efficiencies. The high- and low-stage turbochargers are selected by an iterative matching method. Then, the control strategies for the boost air and high-stage turbine bypass valves are also studied. As an example, a Miller-cycle regulatable, two-stage turbocharging system is designed for a highly boosted high-speed marine diesel engine. The results show that NOx emissions can be reduced by 30% and brake specific fuel consumption (BSFC) can also be improved by a moderate Miller cycle combined with regulatable two-stage turbocharging.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2013;136(2):022501-022501-9. doi:10.1115/1.4025360.

The current centrifugal compressor design for the oil & gas market is more and more challenging, and the presence of many competitors is pushing technology towards both a casing size reduction and a rotational speed increase. The first point is leading to an increase in the number of wheels per rotor (to do the same service), and the second point is forcing to cross two or even three rotor modes (hence a higher control of rotor damping is necessary). The two points together are leading to increase the rotor “flexibility ratio” (defined as the ratio between the maximum continuous speed and the first critical speed at infinite support stiffness according to API standard, and finally the rotordynamic stability is very much challenged. The centrifugal compressor's rotordynamic stability is strongly related to the internal seals' dynamic behavior, and for this reason, the authors' company decided several years ago to develop internally a high pressure seal test rig to measure internal seals stiffness and damping. The rig is now in operation, and in a previous paper the authors described its main capabilities, the applied identification procedures, and the preliminary test results captured for a long labyrinth seal (smooth rotor, straight toothed stator) tested up to 200 bar. This paper is intended to show more data for the same long Laby with special focus on some peculiar test as negative preswirl test, single frequency versus multifrequency test, offset versus centered seal test. The negative preswirl test shows a drastic change in the effective damping (from destabilizing to stabilizing) and provides a support in favor of the selection of swirl reversal devices at seals upstream. The multifrequency excitation test approach (based on the concurrent presence of several frequencies not multiples at each other) is compared with a single frequency excitation providing similar results and thus confirming the soundness of the multiple effects linear superimposition assumption. The effect of a static offset (simulating the real position of a rotor inside an annular seal) is also investigated proving that the relevant impact is negligible within the range of eccentricity explored (10% of seal clearance). Moreover, a pocket damper seal (PDS) with the same nominal diameter, clearance, and effective length has been tested (up to 300 bar) and compared with the Laby. As expected, the PDS shows both a higher effective stiffness and damping at the same test conditions, so the promising results already collected in a previous test campaign which was performed on a smaller scale and lower pressure test rig were mostly confirmed with the only exception for the effective damping crossover frequency which was lower than expected.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;136(2):022502-022502-6. doi:10.1115/1.4025497.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;136(2):022503-022503-11. doi:10.1115/1.4025537.

The main objective of the current work is to determine a relationship between the top and bump foil's geometry and load-carrying capacity in a journal compliant generation I air foil bearing, as well as determining the effect of the thermohydrodynamic phenomena in the performance of the air foil bearing (AFB). Static and steady-state operation is assumed throughout the analysis. A finite element model is adopted in order to investigate the operational characteristics of the specific bearing. Bump foil's elastic behavior is modeled using two node linear link spring elements. During the hydrodynamic analysis, incompressible viscous steady state Navier–Stokes equations are numerically solved, due to the low bearing compressibility number. During the thermohydrodynamic analysis, compressible, viscous, steady-state Navier–Stokes equations were solved, coupled with the energy equation. The material used during the structural analysis is Inconel X750, and it is assumed that it has linear and elastic behavior. Constant ambient pressure is applied at the free faces of the fluid as well as no slip condition at the surface of the fluid that faces the top foil. Computational fluid dynamics (CFD) and structural models are solved separately. At the beginning of the analysis, the CFD problem is solved with the assumption that the top foil has not yet been deformed. After the solution of the CFD problem, the pressure distribution at the surface of the fluid that faces the top foil is applied at the top foil and then the structural problem is solved. Consequently, the deflections of the top foil are applied on the corresponding surface of the CFD model and the algorithm continues until convergence is obtained. As soon as the converged solution for the pressure distribution is obtained, numerical integration is performed along the surface of the bearing in order to calculate its load-carrying capacity. Static bearing performance characteristics, such as pressure distribution, bump foil deflection, and load capacity are calculated and presented. Furthermore, fluid film thickness, top foil deflection, and fluid pressure are investigated as functions of the bearing angle as well as load-carrying capacity as a function of the bump and top foil stiffness. The same procedure is repeated for the thermohydrodynamic analysis. Moreover, in order to estimate the heat flux from the top foil to the bump foil channel as a function of the top foil temperature, a simple finite element model of the bump foil–cooling channel is constructed.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;136(2):022504-022504-11. doi:10.1115/1.4025483.

A simplified nonlinear transient analysis method for gas bearings was recently published by the authors (Hassini, M. A., and Arghir, M., 2012, “Simplified Nonlinear Transient Analysis Method for Gas Bearings,” J. Tribol., 134(1), 011704). The method uses the fact that linearized dynamic characteristics of gas bearings, namely the impedances, can be approximated by rational transfer functions. The method gave good results if the rational transfer function approach approximated the linearized dynamic characteristics well. Indeed, each of the four complex impedances $Zαβ,α,β={x,y}$ had one or two poles depending on the order of the rational function that were used. These poles appear as supplementary eigenvalues of the extended matrix of the homogeneous system of first order differential equations describing the model of the rotor. They govern the stability of the dynamic model in the same way as the original eigenvalues do and therefore they impose non-negligible constraints on the rational function approximation of the impedances of gas bearings. The present improvement of the method overrides this problem. The basic idea is to impose the same set of poles for $Zxx$, $Zxy$, $Zyx$, and $Zyy$. By imposing this constraint, the poles are stable and the introduction of artificial instability or erratic eigenvalues is avoided. Campbell and stability diagrams naturally taking into account the variation of the dynamic coefficients with the excitation frequency can now be easily plotted. For example, the method is used for analyzing the stability of rigid and flexible rotors supported by two identical gas bearings modeled with second order rational transfer functions. The method can be applied to any bearing or seal whose impedance is approximated by rational transfer functions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;136(2):022505-022505-8. doi:10.1115/1.4025555.

Advanced Ni-based gas turbine disks are expected to operate at higher service temperatures in aggressive environments for longer time durations. Exposures of Ni-based alloys to alkaline-metal salts and sulfur compounds at elevated temperatures can lead to hot corrosion fatigue crack growth in engine disks. Type II hot corrosion involves the formation and growth of corrosion pits in Ni-based alloys at a temperature range of 650 °C to 750 °C. Once formed, these corrosion pits can serve as stress concentration sites where fatigue cracks can initiate and propagate to failure under subsequent cyclic loading. In this paper, a probabilistic methodology is developed for predicting the corrosion fatigue crack growth life of gas turbine engine disks made from a powder-metallurgy Ni-based superalloy (ME3). The key features of the approach include: (1) a pit growth model that describes the depth and width of corrosion pits as a function of exposure time, (2) a cycle-dependent crack growth model for treating fatigue, and (3) a time-dependent crack growth model for treating corrosion. This set of deterministic models is implemented into a probabilistic life-prediction code called DARWIN. Application of this approach is demonstrated for predicting corrosion fatigue crack growth life in a gas turbine disk based on the ME3 properties from the literature. The results of this study are used to assess the conditions that control the transition of a corrosion pit to a fatigue crack and to identify the pertinent material parameters influencing corrosion fatigue life and disk reliability.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;136(2):022506-022506-6. doi:10.1115/1.4025484.

A structural change quantification methodology is proposed in which the magnitude and location of a structural alteration is identified experimentally in a rotor system. The resonance and antiresonance frequencies are captured from multiple frequency response functions and are compared with baseline data to extract frequency shifts due to these features. The resulting expression contains sufficient information to identify the dynamic characteristics of the rotor in both the frequency and spatial domains. A finite element model with carefully selected tunable parameters is iteratively adjusted using a numerical optimization algorithm to determine the source of the structural change. The methodology is experimentally demonstrated on a test rig with a laterally damaged rotor and the frequency response functions are acquired through utilization of magnetic actuators positioned near the ball bearings.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;136(2):022507-022507-7. doi:10.1115/1.4025536.

To reduce manufacturing cost and time, a new larger-diameter hole-pattern seal incorporating hole diameters of 12.27 mm, versus prior hole diameters of 3.175 mm has been proposed. The 12.27 mm hole-diameter seal had substantially better stability performance with higher effective damping and a markedly lower crossover frequency. It had negative direct stiffness coefficients at low frequency, while the 3.175 mm hole-diameter seal did not. Predictions for the rotordynamic coefficients of this new seal were made based on a two-control-volume model developed by Kleynhans and Childs in 1997. The two control volumes consisted of a through-flow control-volume and a control-volume B that extended from the surface of the stator at the top of the holes to the bottom of holes. Predictions agreed poorly with measured results, because the model used, assumes gas flows only radially within control-volume B. With the large hole-diameters axial and circumferential flow is readily accomplished. Compared to the prior 3.175 mm hole-diameter seals, the 12.27 mm hole-diameter seal design leaked approximately 37.5% more which probably precludes its commercial application. Leakage for the seal was well predicted. Although the larger hole diameters were initially proposed to reduce costs, the fabrication was more challenging than originally thought. The larger holes could not be manufactured with a single pass. Hence, manufacturing costs and time were not reduced.

Commentary by Dr. Valentin Fuster

### Technical Briefs

J. Eng. Gas Turbines Power. 2013;136(2):024501-024501-4. doi:10.1115/1.4025480.

The present work investigated the effects of fuel components on particulate matter (PM) from a natural gas-fueled micro gas turbine engine. A variety of fuel compositions were prepared considering atomic ratio of hydrogen to carbon (H/C ratio) and sulfur level. In the first test, controlled amounts of propane were injected into natural gas to establish H/C ratios between 3.23 and 3.99. In the second test, fuel-bound sulfur was scrubbed and controlled amounts of methyl mercaptan were injected into natural gas to establish sulfur levels between 0.0 ppm and 12.9 ppm. Sonic orifices were used for H/C ratio and fuel sulfur management. In each test, PM was collected from engine exhaust and analyzed. In the second test, total gaseous sulfur in the exhaust was also measured to establish the ratio of PM and gaseous sulfur formed from fuel sulfur. Test result showed no correlation between H/C ratio and PM, and strong correlation between fuel sulfur and PM. 82.4% of fuel sulfur contributed to form gaseous sulfur and 17.6% contributed to form PM in the exhaust. An increase of 1.0 ppm fuel sulfur produced an increase of approximately 4.7μg/m3 PM. By removing fuel-bound sulfur, PM levels from micro gas turbine engine exhaust are comparable to ambient levels of PM.

Commentary by Dr. Valentin Fuster