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

J. Eng. Gas Turbines Power. 2017;139(12):121501-121501-9. doi:10.1115/1.4037325.

The present study investigates the response of recent primary breakup models in the presence of an oscillating air flow and compares them to an experiment realized by Müller (2015, “Experimentelle Untersuchung des Zerstäubungsverhaltens Luftgestützter Brennstoffdüsen bei Oszillierenden Strömungen,” Ph.D. thesis, Karlsruhe Institute of Technology, Karlsruhe, Germany). The experiment showed that the oscillating flow field has a significant influence on the Sauter mean diameter (SMD) up to a given frequency. This observation highlights the low-pass filter character of the prefilming airblast atomization phenomenon, which also introduces a significant phase shift on the dynamics of SMD of the generated spray. The models are tested in their original formulations without any calibration in order to assess their robustness versus different experiments in terms of SMD and time-response to an oscillating flow field. Special emphasis is put to identify the advantages and weaknesses of theses models, in order to facilitate their future implementation in computational fluid dynamics (CFD) codes. It is observed that some models need an additional calibration of the time constant in order to match the time shift observed in the experiment, whereas some others show a good agreement with the experiment without any modification. Finally, it is demonstrated that the low-pass filter character of the breakup phenomenon can be retrieved by considering the history of the local gas velocity, instead of the instantaneous velocity. This might result in a higher simulation fidelity within CFD codes.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(12):121502-121502-10. doi:10.1115/1.4037451.

In order to reduce NOx emissions, modern gas turbines are often equipped with lean-burn combustion systems, where the high-velocity fuel-lean conditions that limit NOx formation in combustors also inhibit the ability of ignition, high altitude relight, and lean combustion stability. To face these issues, internally staged scheme of fuel injection is proposed. Primary and main fuel staging enable fuel distribution control, and multi-injections of main fuel lead to a fast and efficient mixing. A fuel-staged low emission combustor in the framework of lean-burn combustion is developed in the present study, i.e., the central pilot stage for low power conditions is swirl-cup prefilming atomization, the main stage is jet-in-crossflow multi-injection, and a combination of primary and main stage injection is provided for higher power output conditions. In lean-burn combustors, the swirling main air naturally tends to entrain the pilot flame and quench it at low power conditions, which is adverse to the operability specifications, such as ignition, lean blow-out (LBO), and high-altitude relight. In order to investigate the effects of the main swirl angle on combustion performances, the ignition and LBO performances were evaluated in a single dome rectangular combustor. Furthermore, the spray patterns and flow field are characterized by kerosene-planar laser induced fluorescence and particle image velocimetry (PIV) to provide insight into spray and combustion performances. Flow–flow interactions between pilot and main air streams, spray–flow interactions between pilot spray and main air streams, and flame–flow interactions between pilot flame and main air streams are comprehensively analyzed. The entrainment of recirculating main air streams on pilot air streams enhances with the increase of main swirl angle, because of the upward motion and increasing width of main recirculation zone. A small part of droplets are entrained by the recirculating main air streams at periphery of combustor and a majority of droplets concentrate near the centerline of combustor, making that entrainment of recirculating main air streams on pilot spray and quenching effects of recirculating main air streams on pilot flame are slight, and the extinguishing effects can be ignored. The contributions of main swirl strength to improvement of ignition and LBO performances are due to enhancement of air/fuel mixing by strengthening turbulence level in pilot zone.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(12):121503-121503-7. doi:10.1115/1.4037458.

In this study, syngas combustion was investigated behind reflected shock waves in order to gain insight into the behavior of ignition delay times and effects of the CO2 dilution. Pressure and light emissions time-histories measurements were taken at a 2 cm axial location away from the end wall. High-speed visualization of the experiments from the end wall was also conducted. Oxy-syngas mixtures that were tested in the shock tube were diluted with CO2 fractions ranging from 60% to 85% by volume. A 10% fuel concentration was consistently used throughout the experiments. This study looked at the effects of changing the equivalence ratios (ϕ), between 0.33, 0.5, and 1.0 as well as changing the fuel ratio (θ), hydrogen to carbon monoxide, from 0.25, 1.0, and 4.0. The study was performed at 1.61–1.77 atm and a temperature range of 1006–1162 K. The high-speed imaging was performed through a quartz end wall with a Phantom V710 camera operated at 67,065 frames per second. From the experiments, when increasing the equivalence ratio, it resulted in a longer ignition delay time. In addition, when increasing the fuel ratio, a lower ignition delay time was observed. These trends are generally expected with this combustion reaction system. The high-speed imaging showed nonhomogeneous combustion in the system; however, most of the light emissions were outside the visible light range where the camera is designed for. The results were compared to predictions of two combustion chemical kinetic mechanisms: GRI v3.0 and AramcoMech v2.0 mechanisms. In general, both mechanisms did not accurately predict the experimental data. The results showed that current models are inaccurate in predicting CO2 diluted environments for syngas combustion.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(12):121504-121504-10. doi:10.1115/1.4037461.

Fuel staging is a commonly used strategy in the operation of gas turbine engines. In multinozzle combustor configurations, this is achieved by varying fuel flow rate to different nozzles. The effect of fuel staging on flame structure and self-excited instabilities is investigated in a research can combustor employing five swirl-stabilized, lean-premixed nozzles. At an operating condition where all nozzles are fueled equally and the combustor undergoes a self-excited instability, fuel staging successfully suppresses the instability: both when overall equivalence ratio is increased by staging as well as when overall equivalence ratio is kept constant while staging. Increased fuel staging changes the distribution of time-averaged heat release rate in the regions where adjacent flames interact and reduces the amplitudes of heat release rate fluctuations in those regions. Increased fuel staging also causes a breakup in the monotonic phase behavior that is characteristic of convective disturbances that travel along a flame. In particular, heat release rate fluctuations in the middle flame and flame–flame interaction region are out-of-phase with those in the outer flames, resulting in a cancelation of the global heat release rate oscillations. The Rayleigh integral distribution within the combustor shows that during a self-excited instability, the regions of highest heat release rate fluctuation are in phase-with the combustor pressure fluctuation. When staging fuel is introduced, these regions fluctuate out-of-phase with the pressure fluctuation, further illustrating that fuel staging suppresses instabilities through a phase cancelation mechanism.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(12):121505-121505-14. doi:10.1115/1.4037579.

The direct-fired supercritical CO2 (sCO2) cycle is currently considered as a zero-emission power generation concept. It is of interest to know how to optimize various components of this cycle using computational tools; however, a comprehensive effort in this area is currently lacking. In this work, the behavior of thermal properties of sCO2 combustion at various reaction stages has been investigated by coupling real gas CHEMKIN (CHEMKIN-RG) (Schmitt et al., 1994, Chemkin Real Gas: A Fortran Package for Analysis of Thermodynamic Properties and Chemical Kinetics in Nonideal Systems, University of Iowa, Iowa City, IA) with an in-house premixed conditional moment closure code (Martin, 2003, “The Conditional Moment Closure Method for Modeling Lean Premixed Turbulent Combustion,” Ph.D. thesis, University of Washington, Seattle, WA) and the high-pressure Aramco 2.0 kinetic mechanism. Also, the necessary fundamental information for sCO2 combustion modeling is reviewed. The Soave–Redlich–Kwong equation of state (SRK EOS) is identified as the most accurate EOS to predict the thermal states at all turbulence levels. Also, a model for the compression factor Z is proposed for sCO2 combustors, which is a function of mixture inlet conditions and the reaction progress variable. This empirical model is validated between the operating conditions 250–300 bar, inlet temperatures of 800–1200 K, and within the currently designed inlet mole fractions, and the accuracy is estimated to be less than 0.5% different from the exact relation. For sCO2 operating conditions, the compression factor Z always decreases as the reaction progresses, and this leads to the static pressure loss between inlet and exit of the sCO2 combustor. Further, the Lucas et al. and Stiel and Thodos methods are identified as best suitable models for predicting the viscosity and thermal conductivity of the sCO2 combustion mixtures.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(12):121506-121506-6. doi:10.1115/1.4037580.

This paper reports an investigation of soot formation in ethylene–air partially premixed flames (PPFs) over a wide range of premixedness. An axisymmetric co-flow configuration is chosen to establish PPFs from the fully nonpremixed to fully premixed conditions. Reducing the fuel flow rate as a percentage of the maximum from the core stream and supplying the same to the annular stream leads to stratification of the reactant concentrations. The thermal power, overall equivalence ratio, and the average velocity in both the streams are maintained constant under all conditions. The soot volume fraction is estimated by light attenuation method, and laser-induced incandescence (LII) is performed to map the soot distribution in the flow field. The soot volume fraction is observed to exhibit an “S”-type trend as the conditions are traversed from near the premixed to the nonpremixed regimes. That is, when traversing from the nonpremixed to near-premixed regime, below 60% fuel flow rate in core, the soot volume fraction drops drastically. The onset of sooting in the PPFs is clearly seen to be at the tip of the rich-premixed flame (RPF) branch of their triple flame structure, which advances upstream toward the base of the flame as the premixing is reduced. The S-type variation is clearly the effect of partial premixing, more specifically due to the presence of the lean premixed flame (LPF) branch of the triple flame. LII intensities are insufficient to capture the upstream advance of the soot onset with decreased premixedness. So, a quick and inexpensive technique to isolate soot luminescence through flame imaging is presented in the paper involving quasi-simultaneous imaging with a 650 nm and a BG-3 filter using a normal color camera.

Topics: Flames , Soot , Lasers , Fuels
Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(12):121507-121507-8. doi:10.1115/1.4037602.

Methane and ethane are the two main components of natural gas and typically constitute more than 95% of it. In this study, a mixture of 90% CH4/10% C2H6 diluted in 99% Ar was studied at fuel lean (equiv. ratio = 0.5) conditions, for pressures around 1, 4, and 10 atm. Using laser absorption diagnostics, the time histories of CO and H2O were recorded between 1400 and 1800 K. Water is a final product from combustion, and its formation is a good marker of the completion of the combustion process. Carbon monoxide is an intermediate combustion species, a good marker of incomplete/inefficient combustion, as well as a regulated pollutant for the gas turbine industry. Measurements such as these species time histories are important for validating and assessing chemical kinetics models beyond just ignition delay times and laminar flame speeds. Time-history profiles for these two molecules were compared to a state-of-the-art detailed kinetics mechanism as well as to the well-established GRI 3.0 mechanism. Results show that the H2O profile is accurately reproduced by both models. However, discrepancies are observed for the CO profiles. Under the conditions of this study, the CO profiles typically increase rapidly after an induction time, reach a maximum, and then decrease. This maximum CO mole fraction is often largely over-predicted by the models, whereas the depletion rate of CO past this peak is often over-estimated for pressures above 1 atm.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2017;139(12):121601-121601-12. doi:10.1115/1.4037336.

The use of multishaft industrial gas turbines is expanding in various industries because of variation in their structure, flexibility, and their appropriate power generation range. In this study, a semi-simplified black-box dynamic modeling has been done for the three-shaft gas turbine MGT-30. Modeling is done in such a way that all the important variables can be calculated and evaluated. One of the important parameters in dynamic modeling of gas turbine is the time lag relevant to the performance properties of sensors and actuators of the system. In this study, in order to measure the transfer function, physical and actual characteristics of the system were applied. Depending on the type of thermocouples (TCs) used, their activation time was eliminated using a lead compensator. In modeling of the system, the functions were related to the implementation of off-design conditions for compliance with the outputs of a real system model, and outputs were presented proportional to the rate and type of changes for each variable. Finally, validation was done by comparing the power-turbine generated power, exhaust gas temperatures downstream of low pressure (LP) turbine, and speeds of LP and high-pressure (HP) turbines with the real values of Qeshm turbogenerator power plant.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Cycle Innovations

J. Eng. Gas Turbines Power. 2017;139(12):121701-121701-10. doi:10.1115/1.4037323.

Postcombustion CO2 capture from natural gas combined cycle (NGCC) power plants is challenging due to the large flow of flue gas with low CO2 content (∼3–4 vol %) that needs to be processed in the capture stage. A number of alternatives have been proposed to solve this issue and reduce the costs of the associated CO2 capture plant. This work focuses on the selective exhaust gas recirculation (S-EGR) configuration, which uses a membrane to selectively recirculate CO2 back to the inlet of the compressor of the turbine, thereby greatly increasing the CO2 content of the flue gas sent to the capture system. For this purpose, a parallel S-EGR NGCC system (53% S-EGR ratio) coupled to an amine capture plant (ACP) using monoethanolamine (MEA) 30 wt % was simulated using gCCS (gPROMS). It was benchmarked against an unabated NGCC system, a conventional NGCC coupled with an ACP (NGCC + carbon capture and storage (CCS)), and an EGR NGCC power plant (39% EGR ratio) using amine scrubbing as the downstream capture technology. The results obtained indicate that the net power efficiency of the parallel S-EGR system can be up to 49.3% depending on the specific consumption of the auxiliary S-EGR systems, compared to the 49.0% and 49.8% values obtained for the NGCC + CCS and EGR systems, respectively. A preliminary economic study was also carried out to quantify the potential of the parallel S-EGR configuration. This high-level analysis shows that the cost of electricity (COE) for the parallel S-EGR system varies from 82.1 to 90.0 $/MWhe for the scenarios considered, with the cost of CO2 avoided (COA) being in the range of 79.7–105.1 $/ton CO2. The results obtained indicate that there are potential advantages of the parallel S-EGR system in comparison to the NGCC + CCS configuration in some scenarios. However, further benefits with respect to the EGR configuration will depend on future advancements and cost reductions achieved on membrane-based systems.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2017;139(12):122101-122101-6. doi:10.1115/1.4037587.

Thermal barrier coatings (TBCs) are ceramic coatings used in gas turbines to lower the base metal temperature. During operation, the TBC may fail through, for example, fatigue. In this study, a TBC system deposited on a Ni-base alloy was tested in tensile bending fatigue. The TBC system was tested as-sprayed and oxidized, and two load levels were used. After interrupting the tests, at 10,000–50,000 cycles, the TBC tested at the lower load had extensive delamination damage, whereas the TBC tested at the higher load was relatively undamaged. At the higher load, the TBC formed vertical cracks which relieved the stresses in the TBC and retarded delamination damage. A finite element (FE) analysis was used to establish a likely vertical crack configuration (spacing and depth), and it could be confirmed that the corresponding stress drop in the TBC should prohibit delamination damage at the higher load.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2017;139(12):122501-122501-11. doi:10.1115/1.4037315.

A high-speed gas bearing test rig was developed to characterize rotordynamic, thermal, and thrust load performance of gas bearings being developed for an oil-free turboexpander. The radial bearings (RBs) tested in this paper were tilting pad journal bearings with radial compliance features that allow the bearing bore to increase to accommodate shaft growth, and the thrust bearings (TBs) were a spiral groove type with axial compliance features. The TB accounts for over 90% of the combined bearing power consumption, which has a cubic relationship with speed and increases with case pressure. RB circumferential pad temperatures increased approximately with speed to the fourth or fifth power, with slightly higher temperature rise for lower case pressure. Maximum steady-state bearing pad temperatures increase with increasing speed for similar cooling mass flow rates; however, only the TB showed a significant increase in temperature with higher case pressure. The TBs were stable at all speeds, but the load capacity was found to be lower than anticipated, apparently due to pad deformations caused by radial temperature gradients in the stator. More advanced modeling approaches have been proposed to better understand the TB thermal behavior and to improve the TB design. Finally, the RBs tested were demonstrated to be stable up to the design speed of 130 krpm, which represents the highest surface speed for tilting pad gas bearings tested in the literature.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(12):122502-122502-13. doi:10.1115/1.4036969.

Static and thermal characteristics (measured and predicted) are presented for a four-pad, spherical-seat, tilting-pad journal bearing (TPJB) with 0.5 pivot offset, 0.6 L/D, 101.6 mm nominal diameter, and 0.3 preload in the load-between-pivots orientation. One bearing is tested four separate times in the following four different lubrication configurations: (1) flooded single-orifice (SO) at the bearing shell, (2) evacuated leading edge groove (LEG), (3) evacuated spray-bar blocker (SBB), and (4) evacuated spray-bar (SB). The LEG, SBB, and SB are all considered methods of “directed lubrication.” These methods rely on lubrication injected directly to the pad/rotor interface. The same set of pads is used for every test to maintain clearance and preload; each method of lubrication is added as an assembly to the bearing. Test conditions include surface speeds and unit loads up to 85 m/s and 2.9 MPa, respectively. Static data include rotor–bearing eccentricities and attitude angles. Thermal data include measured temperatures from 16 bearing thermocouples. Twelve of the bearing thermocouples are embedded in the babbitt layer of the pads, while the remaining four are oriented at the leading and trailing edge of the loaded pads exposed to the lubricant. Bearing thermocouples provide a circumferential and axial temperature gradient. The pivot stiffness (pad and pivot in series) is measured and incorporated into predictions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(12):122503-122503-13. doi:10.1115/1.4036970.

Measured and predicted rotordynamic characteristics are presented for a four-pad, spherical-seat, tilting-pad journal bearing (TPJB) with 0.5 pivot offset, 0.6 L/D, 101.6 mm nominal diameter, and 0.3 preload in the load-between-pivots orientation. One bearing is tested four separate times in the following four different lubrication configurations: (1) flooded single-orifice (SO) at the bearing shell, (2) evacuated leading edge groove (LEG), (3) evacuated spray-bar blocker (SBB), and (4) evacuated spray-bar (SB). The same set of pads is used for every test to maintain clearance and preload; each method of lubrication is added as an assembly to the bearing. Test conditions include surface speeds and unit loads up to 85 m/s and 2.9 MPa, respectively. Dynamic data includes four sets (one set for each bearing configuration) of direct and cross-coupled rotordynamic coefficients derived from measurements and fit to a frequency-independent stiffness-damping-mass (KCM) matrix model. The pivot stiffness (pad and pivot in series) is measured and incorporated into predictions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(12):122504-122504-11. doi:10.1115/1.4037607.

A two-phase annular seal stand (2PASS) has been developed at the Turbomachinery Laboratory of Texas A&M University to measure the leakage and rotordynamic coefficients of division wall or balance-piston annular seals in centrifugal compressors. 2PASS was modified from an existing pure-air annular seal test rig. A special mixer has been designed to inject the oil into the compressed air, aiming to make a homogenous air-rich mixture. Test results are presented for a smooth seal with an inner diameter D of 89.306 mm, a radial clearance Cr of 0.188 mm, and a length-to-diameter ratio (L/D) of 0.65. The test fluid is a mixture of air and silicone oil (PSF-5cSt). Tests are conducted with inlet liquid volume fraction (LVF) = 0%, 2%, 5%, and 8%, shaft speed ω = 10, 15, and 20 krpm, and pressure ratio (PR) = 0.43, 0.5, and 0.57. The test seal is concentric with the shaft (centered), and the inlet pressure is 62.1 bar. Complex dynamic-stiffness coefficients are measured for the seal. The real parts are generally too dependent on excitation frequency Ω to be modeled by constant stiffness and virtual-mass coefficients. The direct real dynamic-stiffness coefficients are denoted as K; the cross-coupled real dynamic-stiffness coefficients are denoted as k. The imaginary parts of the dynamic-stiffness coefficients are modeled by frequency-independent direct C and cross-coupled c damping coefficients. Test results show that the leakage and rotordynamic coefficients are remarkable impacted by changes in inlet LVF. Leakage mass flow rate m˙ drops slightly as inlet LVF increases from zero to 2% and then increases with further increasing inlet LVF to 8%. As inlet LVF increases from zero to 8%, K generally decreases except it increases as inlet LVF increases from zero to 2% when PR = 0.43. k increases virtually with increasing inlet LVF from zero to 2%. As inlet LVF further increases to 8%, k decreases or remains unchanged. C increases as inlet LVF increases; however, its rate of increase drops significantly at inlet LVF = 2%. Effective damping Ceff combines the stabilizing impact of C and the destabilizing impact of k. Ceff is negative (destabilizing) for lower Ω values and becomes more destabilizing as inlet LVF increases from zero to 2%. It then becomes less destabilizing as inlet LVF is further increased to 8%. Measured m˙ and rotordynamic coefficients are compared with predictions from XLHseal_mix, a program developed by San Andrés (2011, “Rotordynamic Force Coefficients of Bubbly Mixture Annular Pressure Seals,” ASME J. Eng. Gas Turbines Power, 134(2), p. 022503) based on a bulk-flow model, using the Moody wall-friction model while assuming constant temperature and a homogenous mixture. Predicted m˙ values are close to measurements when inlet LVF = 0% and 2% and are smaller than test results by about 17% when inlet LVF = 5% and 8%. As with measurements, predicted m˙ drops slightly as inlet LVF increases from zero to 2% and then increases with increasing inlet LVF further to 8%. However, in the inlet LVF range of 2–8%, the predicted effects of inlet LVF on m˙ are weaker than measurements. XLHseal_mix poorly predicts K in most test cases. For all test cases, predicted K decreases as inlet LVF increases from zero to 8%. The increase of K induced by increasing inlet LVF from zero to 2% at PR = 0.43 is not predicted. C is reasonably predicted, and predicted C values are consistently smaller than measured results by 14–34%. Both predicted and measured C increase as inlet LVF increases. k and Ceff are predicted adequately at pure-air conditions, but not at most mainly air conditions. The significant increase of k induced by changing inlet LVF from zero to 2% is predicted. As inlet LVF increases from 2% to 8%, predicted k continues increasing versus that measured k typically decreases. As with measurements, increasing inlet LVF from zero to 2% decreases the predicted negative values of Ceff, making the test seal more destabilizing. However, as inlet LVF increases further to 8%, the predicted negative values of Ceff drop versus measured values increase. For high inlet LVF values (5% and 8%), the predicted negative values of Ceff are smaller than measurements. So, the seal is more stabilizing than predicted for high inlet LVF cases.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Turbomachinery

J. Eng. Gas Turbines Power. 2017;139(12):122601-122601-11. doi:10.1115/1.4037456.

Optimization problems in many engineering applications are usually considered as complex subjects. Researchers are often obliged to solve a multi-objective optimization problem. Several methodologies such as genetic algorithm (GA) and artificial neural network (ANN) are proposed to optimize multi-objective optimization problems. In the present study, various levels of sweep and lean were exerted to blades of an existing transonic rotor, the well-known NASA rotor-67. Afterward, an ANN optimization method was used to find the most appropriate settings to achieve the maximum stage pressure ratio, efficiency, and operating range. At first, the study of the impact of sweep and lean on aerodynamic and performance parameters of the transonic axial flow compressor rotors was undertaken using a systematic step-by-step procedure. This was done by employing a three-dimensional (3D) compressible turbulent model. The results were then used as the input data to the optimization computer code. It was found that the optimized sweep angles can increase the safe operating range up to 30% and simultaneously increase the pressure ratio and subsequently the efficiency by 1% and 2%. Moreover, it was found that the optimized leaned blades, according to their target function, had positive (forward (FW)) or negative (backward (BW)) optimized angles. Leaning the blade at the optimum point can increase the safe operating range up to 12% and simultaneously increase the pressure ratio and subsequently the efficiency by 4% and 5%.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(12):122602-122602-11. doi:10.1115/1.4037453.

This paper presents a well-researched subject area within academia, with a high degree of application in the industry. Compressor fouling effect is one of the commonest degradations associated with gas turbine operations. The aim of this review is to broadly communicate some of the current knowledge while identifying some gaps in understanding, in an effort to present some industry/operational interest for academic research. Likewise, highlight some studies from academia that present the current state of research, with their corresponding methods (experimental, numerical, actual operations, and analytical methods). The merits and limitations of the individual method and their approaches are discussed, thereby providing industry practitioners with a view to appreciating academic research outputs. The review shows opportunities for improving compressor washing effectiveness through computational fluid dynamics (CFD). This is presented in the form of addressing the factors influencing compressor washing efficiency. Pertinent questions from academic research and operational experiences are posed, on the basis of this review.

Commentary by Dr. Valentin Fuster

Research Papers: Internal Combustion Engines

J. Eng. Gas Turbines Power. 2017;139(12):122801-122801-10. doi:10.1115/1.4037207.

In-cylinder reforming of injected fuel during a negative valve overlap (NVO) recompression period can be used to optimize main-cycle combustion phasing for low-load low-temperature gasoline combustion (LTGC). The objective of this work is to examine the effects of reformate composition on main-cycle engine performance. An alternate-fire sequence was used to generate a common exhaust temperature and composition boundary condition for a cycle-of-interest, with performance metrics measured for these custom cycles. NVO reformate was also separately collected using a dump-valve apparatus and characterized by both gas chromatography (GC) and photoionization mass spectroscopy (PIMS). To facilitate gas sample analysis, sampling experiments were conducted using a five-component gasoline surrogate (iso-octane, n-heptane, ethanol, 1-hexene, and toluene) that matched the molecular composition, 50% boiling point, and ignition characteristics of the research gasoline. For the gasoline, it was found that an advance of the NVO start-of-injection (SOI) led to a corresponding advance in main-period combustion phasing as the combination of longer residence times and lower amounts of liquid spray piston impingement led to a greater degree of fuel decomposition. The effect was more pronounced as the fraction of total fuel injected in the NVO period increased. Main-period combustion phasing was also found to advance as the main-period fueling decreased. Slower kinetics for leaner mixtures were offset by a combination of increased bulk-gas temperature from higher charge specific heat ratios and increased fuel reactivity due to higher charge reformate fractions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(12):122802-122802-12. doi:10.1115/1.4036968.

Natural gas direct injection (DI) and glow plug ignition assist technologies were implemented in a single-cylinder, compression-ignition optical research engine. Initial experiments studied the effects of injector and glow plug shield geometry on ignition quality. Injector and shield geometric effects were found to be significant, with only two of 20 tested geometric combinations resulting in reproducible ignition. Of the two successful combinations, the combination with 0 deg injector angle and 60 deg shield angle was found to result in shorter ignition delay and was selected for further testing. Further experiments explored the effects of the overall equivalence ratio (controlled by injection duration) and intake pressure on ignition delay and combustion performance. Ignition delay was measured to be in the range of 1.6–2.0 ms. Equivalence ratio was found to have little to no effect on the ignition delay. Higher intake pressure was shown to increase ignition delay due to the effect of swirl momentum on fuel jet development, air entrainment, and jet deflection away from optimal contact with the glow plug ignition source. Analysis of combustion was carried out by examination of the rate of heat release (ROHR) profiles. ROHR profiles were consistent with two distinct modes of combustion: premixed mode at all test conditions, and a mixing-controlled mode that only appeared at higher equivalence ratios following premixed combustion.

Commentary by Dr. Valentin Fuster

Expert View

J. Eng. Gas Turbines Power. 2017;139(12):124701-124701-7. doi:10.1115/1.4037577.

A purposeful approach has been taken to match teaching pedagogies (techniques), learning experiences, and assessment methods to various types of students learning in undergraduate aerospace propulsion courses at the junior-level at the United States Air Force Academy (USAFA) and senior-level at Oklahoma State University (OSU), Stillwater, OK. Prior studies in the scholarship of teaching and learning have shown the benefits of matching assessment methods, as well as teaching pedagogies and learning experiences, to the types of students learning associated with desired educational outcomes. Literature suggests the best method for teaching and assessing student’ cognitive learning is through explanation and presentation. Oral assessments have been implemented at the Air Force Academy and Oklahoma State University to evaluate students' cognitive learning in undergraduate aerospace propulsion and power courses. An oral midterm exam was performed to assess students' acquisition knowledge and understanding of fundamental concepts, the type of learning occurring early in course lesson sequences. End-of-semester design project poster sessions and presentations served as summative oral assessments of students' creative thinking, decision making, and professional judgment. Conversely, two written midterm exams and a final exam primarily focused on assessing students' problem solving skills and less on comprehensive knowledge. Oral assessments also served as reflective thinking experiences that reinforced student learning. Student feedback on oral assessment methods was collected through surveys conducted after each assessment. Survey results not only revealed the effectiveness of using oral assessments but also on how to improve their design and implementation, including the use of information technology (IT) and broader curricular employment.

Commentary by Dr. Valentin Fuster

Design Innovation Paper: Design Innovation Paper

J. Eng. Gas Turbines Power. 2017;139(12):125001-125001-7. doi:10.1115/1.4037316.

Cruise specific fuel consumption (SFC) of turbofan engines is a key metric for increasing airline profitability and for reducing CO2 emissions. Although increasing design bypass ratio (BPR) of separate exhaust turbofan configurations improves cruise SFC, further improvements can be obtained with online control actuated variable geometry modulations of bypass nozzle throat area, core nozzle throat area, and compressor variable vanes (CVV/CVG). The scope of this paper is to show only the benefits possible, and the process used in determining those benefits, and not to suggest any particular control algorithm for searching the best combination of the control effectors. A parametric cycle study indicated that the effector modulations could increase the cruise BPR, core efficiency, transmission efficiency, propulsive efficiency, and ideal velocity ratio resulting in a cruise SFC improvement of as much as 2.6% depending upon the engine configuration. The changes in these metrics with control effector variations will be presented. Scheduling of CVV is already possible in legacy digital controls; perturbation to this schedule and modulation of nozzle areas should be explored in light of the low bandwidth requirements at steady-state cruise conditions.

Commentary by Dr. Valentin Fuster

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