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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

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: 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

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

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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