Research Papers: Gas Turbines: Aircraft Engine

J. Eng. Gas Turbines Power. 2017;139(11):111201-111201-14. doi:10.1115/1.4036954.

Current engine condition monitoring (ECM) systems for jet engines include the analysis of on-wing gas path data using steady-state performance models. Such data, which are also referred to as performance snapshots, usually are taken during cruise flight and during takeoff. Using steady-state analysis, it is assumed that these snapshots have been taken under stabilized operating conditions. However, this assumption is reasonable only for cruise snapshots. During takeoff, jet engines operate in highly transient conditions with significant heat transfer occurring between the fluid and the engine structure. Hence, steady-state analysis of takeoff snapshots is subject to high uncertainty. Because of this, takeoff snapshots are not used for performance analysis in current ECM systems. We quantify the analysis uncertainty by transient simulation of a generic takeoff maneuver using a performance model of a medium size two-shaft turbofan engine with high bypass ratio. Taking into account the influence of the preceding operating regimes on the transient heat transfer effects, this takeoff maneuver is extended backward in time to cover the aircraft turnaround as well as the end of the last flight mission. We present a hybrid approach for thermal calculation of both the fired engine and the shutdown engine. The simulation results show that takeoff derate, ambient temperature, taxi-out (XO) duration and the duration of the preceding aircraft turnaround have a major influence on the transient effects occurring during takeoff. The analysis uncertainty caused by the transient effects is significant. Based on the simulation results, we propose a method for correction of takeoff snapshots to steady-state operating conditions. Furthermore, we show that the simultaneous analysis of cruise and corrected takeoff snapshots leads to significant improvements in observability.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(11):111202-111202-13. doi:10.1115/1.4037155.

This paper presents the design and full-scale ground-test demonstration of an engine air-brake (EAB) nozzle that uses a deployable swirl vane mechanism to switch the operation of a turbofan's exhaust stream from thrust generation to drag generation during the approach and/or descent phase of flight. The EAB generates a swirling outflow from the turbofan exhaust nozzle, allowing an aircraft to generate equivalent drag in the form of thrust reduction at a fixed fan rotor speed. The drag generated by the swirling exhaust flow is sustained by the strong radial pressure gradient created by the EAB swirl vanes. Such drag-on-demand is an enabler to operational benefits such as slower, steeper, and/or aeroacoustically cleaner flight on approach, addressing the aviation community's need for active and passive control of aeroacoustic noise sources and access to confined airports. Using NASA's technology readiness level (TRL) definitions, the EAB technology has been matured to a level of six, i.e., a fully functional prototype. The TRL-maturation effort involved design, fabrication, assembly, and ground-testing of the EAB's deployable mechanism on a full-scale, mixed-exhaust, medium-bypass-ratio business jet engine (Williams International FJ44-4A) operating at the upper end of typical approach throttle settings. The final prototype design satisfied a set of critical technology demonstration requirements that included (1) aerodynamic equivalent drag production equal to 15% of nominal thrust in a high-powered approach throttle setting (called dirty approach), (2) excess nozzle flow capacity and fuel burn reduction in the fully deployed configuration, (3) acceptable engine operability during dynamic deployment and stowing, (4) deployment time of 3–5 s, (5) stowing time under 0.5 s, and (6) packaging of the mechanism within a notional engine cowl. For a typical twin-jet aircraft application, a constant-speed, steep approach analysis suggests that the EAB drag could be used without additional external airframe drag to increase the conventional glideslope from 3 deg to 4.3 deg, with about 3 dB noise reduction at a fixed observer location.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2017;139(11):111501-111501-7. doi:10.1115/1.4036621.

Market demands for lower fueling costs and higher specific powers in stationary natural gas engines have engine designs trending toward higher in-cylinder pressures and leaner combustion operation. However, ignition remains as the main limiting factor in achieving further performance improvements in these engines. Addressing this concern, while incorporating various recent advances in optics and laser technologies, laser igniters were designed and developed through numerous iterations. Final designs incorporated water-cooled, passively Q-switched, Nd:YAG microlasers that were optimized for stable operation under harsh engine conditions. Subsequently, the microlasers were installed in the individual cylinders of a lean-burn, 350 kW, inline six-cylinder, open-chamber, spark ignited engine, and tests were conducted. The engine was operated at high-load (298 kW) and rated speed (1800 rpm) conditions. Ignition timing (IT) sweeps and excess-air ratio (λ) sweeps were performed while keeping the NOx emissions below the United States Environmental Protection Agency (USEPA) regulated value (brake-specific NOx (BSNOx) < 1.34 g/kW h), and while maintaining ignition stability at industry acceptable values (coefficient of variation of integrated mean effective pressure (COV_IMEP) < 5%). Through such engine tests, the relative merits of (i) standard electrical ignition system and (ii) laser ignition system were determined. A rigorous combustion data analysis was performed and the main reasons leading to improved performance in the case of laser ignition were identified.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(11):111502-111502-13. doi:10.1115/1.4036945.

Simulations and exhaust measurements of temperature and pollutants in a syngas-fired model trapped vortex combustor for stationary power generation applications are reported. Numerical simulations employing Reynolds-averaged Navier–Stokes (RANS) and large eddy simulations (LES) with presumed probability distribution function (PPDF) model were also carried out. Mixture fraction profiles in the trapped vortex combustor (TVC) cavity for nonreacting conditions show that LES simulations are able to capture the mean mixing field better than the RANS-based approach. This is attributed to the prediction of the jet decay rate and is reflected on the mean velocity magnitude fields, which reinforce this observation at different sections in the cavity. Both RANS and LES simulations show close agreement with the experimentally measured OH concentration; however, the RANS approach does not perform satisfactorily in capturing the trend of velocity magnitude. LES simulations satisfactorily capture the trend observed in exhaust measurements which is primarily attributed to the flame stabilization mechanism. In the exhaust measurements, mixing enhancement struts were employed, and their effect was evaluated. The exhaust temperature pattern factor was found to be poor for baseline cases, but improved with the introduction of struts. NO emissions were steadily below 3 ppm across various flow conditions, whereas CO emissions tended to increase with increasing momentum flux ratios (MFRs) and mainstream fuel addition. Combustion efficiencies ∼96% were observed for all conditions. The performance characteristics were found to be favorable at higher MFRs with low pattern factors and high combustion efficiencies.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(11):111503-111503-11. doi:10.1115/1.4035819.

Fuel composition has a strong influence on the turbulent flame speed, even at very high turbulence intensities. An important implication of this result is that the turbulent flame speed cannot be extrapolated from one fuel to the next using only the laminar flame speed and turbulence intensity as scaling variables. This paper presents curvature and tangential strain rate statistics of premixed turbulent flames for high hydrogen content (HHC) fuels. Global (unconditioned) stretch statistics are presented as well as measurements conditioned on the leading points of the flame front. These measurements are motivated by previous experimental and theoretical work that suggests the turbulent flame speed is controlled by the flame front characteristics at these points. The data were acquired with high-speed particle image velocimetry (PIV) in a low-swirl burner (LSB). We attained measurements for several H2:CO mixtures over a range of mean flow velocities and turbulence intensities. The results show that fuel composition has a systematic, yet weak effect on curvatures and tangential strain rates at the leading points. Instead, stretch statistics at the leading points are more strongly influenced by mean flow velocity and turbulence level. It has been argued that the increased turbulent flame speeds seen with increasing hydrogen content are the result of increasing flame stretch rates, and therefore, SL,max values, at the flame leading points. However, the differences observed with changing fuel compositions are not significant enough to support this hypothesis. Additional analysis is needed to understand the physical mechanisms through which the turbulent flame speed is altered by fuel composition effects.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(11):111504-111504-9. doi:10.1115/1.4036577.

In-cylinder surface temperature has significant impacts on the thermo-kinetics governing the homogeneous charge compression ignition (HCCI) process. Thermal barrier coatings (TBCs) enable selective manipulation of combustion chamber surface temperature profiles throughout a fired cycle. In this way, TBCs enable a dynamic surface temperature swing, which prevents charge heating during intake while minimizing heat rejection during combustion. This preserves volumetric efficiency while fostering more complete combustion and reducing emissions. This study investigates the effect of a yttria-stabilized zirconia (YSZ) coating on low temperature combustion (LTC), efficiency, and emissions. This is an initial step in a systematic effort to engineer coatings best suited for LTC concepts. A YSZ coating was applied to the top of the aluminum piston using a powder air plasma spray (APS) process; final thickness of the YSZ was approximately 150 μm. The coated piston was subsequently evaluated in the single-cylinder HCCI engine with exhaust re-induction. Engine tests indicated significant advancement of the autoignition point and reduced combustion durations with the YSZ coating. Hydrocarbon and carbon monoxide emissions were reduced, thereby increasing combustion efficiency. The combination of higher combustion efficiency and decreased heat loss during combustion produced tangible improvements in thermal efficiency. When the effects of combustion advance were removed, the overall improvements in emissions and efficiency were lower, but still significant. Overall, the results encourage continued efforts to devise novel coatings for LTC.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Cycle Innovations

J. Eng. Gas Turbines Power. 2017;139(11):111701-111701-10. doi:10.1115/1.4036685.

Water is a scarce natural resource fundamental for human life. Power plant architects, engineers, and power utilities owners must do everything within their hands and technical capabilities to decrease the usage of water in power plants. This paper illustrates the research carried out by Pöyry Switzerland to reduce the water consumption on power and desalination combined cycle power plants, on which there are gas turbine evaporative cooling systems in operation. The present study analyzed the potential re-utilization and integration of the heat recovery steam generator (HRSG) blowdown into the evaporative cooling system. Relatively clean demineralized water, coming from the HRSG blowdown, is routed to a large water tank, where it is blended with distillate water to achieve the required water quality, before being used on the gas turbine evaporative cooling system. To prove the feasibility of the HRSG blowdown recycling concept, the Ras Al Khair Power and Desalination Plant owned and operated by the Saline Water Conversion Corporation (SWCC), located in the Eastern Province of the Kingdom of Saudi Arabia, was used as case study. Nevertheless, it is important to mention that the principles and methodology presented on this paper are applicable to every power and desalination combined cycle power plant making use of evaporative cooling. Sea water desalination is the primary source for potable water production on Saudi Arabia, with secondary sources being surface water and groundwater extracted from deep wells and aquifers. Saving water is of utmost importance for power plants located in locations where water is scarce, and as such, this paper aims to demonstrate that it is possible to decrease the water consumption of power and desalination combined cycle plants, on which evaporative cooling is used as gas turbine power booster, without having to curtail power production. The outcome of the study indicates that during the summer season, recycling the HRSG water blowdown into the gas turbine evaporative cooling systems would result on the internal water consumption for the gas turbine evaporative coolers decreasing by 545 ton/day, or 23.79%, compared with the original plant design which does not contemplate blowdown re-use. Using evaporative cooling results on an overall gain of 186 MW, or 10.27%, on gross power output, while CO2 emissions decrease by 46.8 ton CO2/h, which represents a 13.8% reduction compared with the case on which the evaporative cooling system is not in operation. A brief cost analysis demonstrated that implementation of the changes would result in a negligible increase of the operational expenses (OPEX) of the plant, i.e., implementation of the suggested modification has an unnoticeable impact on the cost of electricity (CoE). The payback of the project, due to limited operating hours on evaporative cooling every year, is of 12 years for a 30 year plant lifetime, while 2.22 M USD of extra-revenue on potable water sales are generated as a result of implementing the proposed solution. Although in principle this value is modest, the effect of government subsidies on water tariffs as well as political and strategic cost of water is not included on the calculations. In conclusion, the study results indicate that water recycling, and reduction of plant's water footprint for power and desalination combined cycle plants using evaporative cooling, is not only technically possible but commercially feasible.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2017;139(11):112501-112501-10. doi:10.1115/1.4036946.

Hybrid bearings are getting more and more attention because of their ability to provide both hydrodynamic support for high-speed rotors and hydrostatic lift in low-speed conditions such as during startup. Hybrid bearings are typically designed with recess grooves to modify the pressure profile and as a result to enable the lift capacity of the bearing under various operating conditions. The literature has shown that the size and shape of the recesses have not been systematically and quantitatively studied in detail. The goal of this study is to build a 3D analytical model for a hybrid-recessed bearing with five pockets and provide a comprehensive analysis for the effect of recess geometry on the overall performance of the bearing. In this study, a baseline model selected from the literature is constructed and validated using the ANSYS cfx computational fluid dynamics software package. A sensitivity analysis of the design variables on the performance of the bearing has been performed using design expert software. The length, width, and depth of the recess as well as the diameter and location of the five inlet ports have been selected as design variables. A multivariable and multi-objective genetic algorithm has also been solved using isight software with the goal of optimizing the geometry of the recess to maximize load capacity while minimizing bearing power loss from friction torque. The results of the baseline model show reasonable agreement with the experimental data published in the literature. The regression models for lift force and friction torque were both found to be statistically significant and accurate. It has been shown that friction torque decreases as the length of recess in the circumferential direction increases. The results showed that the load capacity is highly correlated to the diameter of the orifice, d. These results provide a deeper understanding of the relationship between the shape of the recess and bearing performance and are expected to be useful in practical hybrid-bearing design.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(11):112502-112502-15. doi:10.1115/1.4036953.

The aim of this study was to investigate the cyclic creep–fatigue interaction behavior in a steam turbine inlet valve under cyclic thermomechanical loading conditions. Three years and nine iterations of idealized startup–steady-state operation–shutdown process were chosen. The Ramberg–Osgood model, the Norton–Bailey law, and continuum damage mechanics were applied to describe the stress–strain behavior and calculate the damage. The strength of the steam valve revealed that significant stress variation mainly occurred at the joint parts between the valve diffuser and the adjust valve body, due to the combination of the enhanced turbulent flow and assembly force at these areas. The contact stress at the region of component assembly was sensitive to the cyclic loading at the initial iterations. The maximum decrease amplitude in the normalized contact stress between the second and the fourth iterations reached 0.12. The damage analysis disclosed that the notch of the deflector in the adjust valve had the maximum damage due to the stress concentration.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Turbomachinery

J. Eng. Gas Turbines Power. 2017;139(11):112601-112601-13. doi:10.1115/1.4037027.

The effect of the purge flow, engine-like blade pressure field, and mainstream flow coefficient are studied experimentally for a single and double lip rim seal. Compared to the single lip, the double lip seal requires less purge flow for similar levels of cavity seal effectiveness. Unlike the double lip seal, the single lip seal is sensitive to overall Reynolds number, the addition of a simulated blade pressure field, and large-scale nonuniform ingestion. In the case of both seals, unsteady pressure variations attributed to shear layer interaction between the mainstream and rim seal flows appear to be important for ingestion at off-design flow coefficients. The double lip seal has both a weaker vane pressure field in the rim seal cavity and a smaller difference in seal effectiveness across the lower lip than the single lip seal. As a result, the double lip seal is less sensitive in the rotor–stator cavity to changes in shear layer interaction and the effects of large-scale circumferentially nonuniform ingestion. However, the reduced flow rate through the double lip seal means that the outer lip has increased sensitivity to shear layer interactions. Overall, it is shown that seal performance is driven by both the vane/blade pressure field and the gradient in seal effectiveness across the inner lip. This implies that accurate representation of both, the pressure field and the mixing due to shear layer interaction, would be necessary for more reliable modeling.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(11):112602-112602-9. doi:10.1115/1.4037193.

Compared to other engines, turbine-based combined cycle (TBCC) engine is one of the most suitable propulsion systems for hypersonic vehicle. Because of its fine reusability, wide flight envelope, and safety margins, TBCC engine is becoming a more and more important hotspot of research. In this paper, a three-dimensional (3D) over–under TBCC exhaust system is designed, simulated, and the results are discussed, wherein the ramjet flowpath is designed by the quasi-two-dimensional method of characteristics (MOC). A new scheme of rotating around the rear shaft is proposed to regulate the throat area of turbine flowpath. Cold flow experiments are conducted to gain a thorough and fundamental understanding of TBCC exhaust system at on and off-design conditions. To characterize the flow regimes, static pressure taps and schlieren apparatus are employed to obtain the wall pressure distributions and flowfield structures during the experiments. Detailed flow features, as well as the thrust performance, are simulated by the computational fluid dynamics (CFD) method. Both the numerical and experimental results show that the TBCC exhaust nozzle in this study can provide sufficient thrust during the whole flight envelop despite a little deterioration at the beginning of the mode transition. The research provides a new and effective scheme for the exhaust system of TBCC engine.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(11):112603-112603-12. doi:10.1115/1.4037097.

This paper studies the origin and applicability of the traditional Stodola ellipse law and demonstrates its deficiencies when applied in certain conditions. It extends the equation by Cooke and Traupel through the definition of a semi-ellipse law. This new law produces more accurate results as compared to the ellipse law (EL), especially for turbines with a low number of stages. It does, however, require knowledge of the choking behavior of the turbine, as well as an appropriate pressure ratio exponent. Through numerical studies and careful application of nozzle flow equations, correlations were developed to predict the critical pressure ratio of a multistage turbine, taking nozzle and blade efficiency into account. Correlations are also presented to obtain an appropriate pressure ratio exponent to use in the semi-ellipse law. A methodology is proposed through which the necessary semi-ellipse law terms can be calculated using only design base conditions and estimates of efficiencies. This was successfully validated on a steam turbine. The semi-ellipse law is believed to be the most accurate way of modeling an axial-flow multistage steam or gas turbine from design base conditions, without requiring a stage-by-stage analysis.

Commentary by Dr. Valentin Fuster

Research Papers: Internal Combustion Engines

J. Eng. Gas Turbines Power. 2017;139(11):112801-112801-9. doi:10.1115/1.4036766.

Modern diesel engines are charged with the difficult problem of balancing emissions and efficiency. For this work, a variant of the artificial bee colony (ABC) algorithm was applied for the first time to the experimental optimization of diesel engine combustion and emissions. In this study, the employed and onlooker bee phases were modified to balance both the exploration and exploitation of the algorithm. The improved algorithm was successfully trialed against particle swarm optimization (PSO), genetic algorithm (GA), and a recently proposed PSO-GA hybrid with three standard benchmark functions. For the engine experiments, six variables were changed throughout the optimization process, including exhaust gas recirculation (EGR) rate, intake temperature, quantity and timing of pilot fuel injections, main injection timing, and fuel pressure. Low sulfur diesel fuel was used for all the tests. In total, 65 engine runs were completed in order to reduce a five-dimensional objective function. In order to reduce nitrogen oxide (NOx) emissions while keeping particulate matter (PM) below 0.09 g/kW h, solutions call for 43% exhaust gas recirculation, with a late main fuel injection near top-dead center. Results show that early pilot injections can be used with high exhaust gas recirculation to improve the combustion process without a large nitrogen oxide penalty when main injection is timed near top-dead center. The emission reductions in this work show the improved ABC algorithm presented here to be an effective new tool in engine optimization.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(11):112802-112802-17. doi:10.1115/1.4036575.

Increasingly stringent fuel economy and CO2 emission regulations provide a strong impetus for development of high-efficiency engine technologies. Diesel engines dominate the heavy duty market and significant segments of the global light duty market due to their intrinsically higher thermal efficiency compared to spark-ignited (SI) engine counterparts. Predictive simulation tools can significantly reduce the time and cost associated with optimization of engine injection strategies, and enable investigation over a broad operating space unconstrained by availability of prototype hardware. In comparison with 0D/1D and 3D simulations, Quasi-Dimensional (quasi-D) models offer a balance between predictiveness and computational effort, thus making them very suitable for enhancing the fidelity of engine system simulation tools. A most widely used approach for diesel engine applications is a multizone spray and combustion model pioneered by Hiroyasu and his group. It divides diesel spray into packets and tracks fuel evaporation, air entrainment, gas properties, and ignition delay (induction time) individually during the injection and combustion event. However, original submodels are not well suited for modern diesel engines, and the main objective of this work is to develop a multizonal simulation capable of capturing the impact of high-injection pressures and exhaust gas recirculation (EGR). In particular, a new spray tip penetration submodel is developed based on measurements obtained in a high-pressure, high-temperature constant volume combustion vessel for pressures as high as 1450 bar. Next, ignition delay correlation is modified to capture the effect of reduced oxygen concentration in engines with EGR, and an algorithm considering the chemical reaction rate of hydrocarbon–oxygen mixture improves prediction of the heat release rates. Spray and combustion predictions were validated with experiments on a single-cylinder diesel engine with common rail fuel injection, charge boosting, and EGR.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(11):112803-112803-11. doi:10.1115/1.4036622.

The need for cost-effective fuel economy improvements has driven the introduction of automatic transmissions with an increasing number of gear ratios. Incorporation of interlocking dog clutches in these transmissions decreases package space and increases efficiency, as compared to conventional dry or wet clutches. Unlike friction-based clutches, interlocking dog clutches require very precise rotational speed matching prior to engagement. Precise engine speed control is, therefore, critical to maintaining high shift quality. This research focuses on controlling the engine speed during a gearshift period by manipulating throttle position and combustion phasing. Model predictive control (MPC) is advantageous in this application since the speed profile of a future prediction horizon is known with relatively high confidence. The MPC can find the optimal control actions to achieve the designated speed target without invoking unnecessary actuator manipulation and violating hardware and combustion constraints. This research utilizes linear parameter varying (LPV) MPC to control the engine speed during the gearshift period. Combustion stability constraints are considered with a control-oriented covariance of indicated mean effective pressure model (COV of IMEP). The proposed MPC engine speed controller is validated with a high-fidelity zero-dimensional engine model with crank angle resolution. Four case studies, based on simulation, investigate the impact of different MPC design parameters. They also demonstrate that the proposed MPC engine controller successfully achieves the speed reference tracking objective while considering combustion variation constraints.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(11):112804-112804-9. doi:10.1115/1.4036955.

A combined organic Rankine cycle (ORC) was proposed for both engine coolant energy recovery (CER) and exhaust energy recovery (EER), and it was applied to a gasoline direct injection (GDI) engine to verify its waste heat recovery (WHR) potential. After several kinds of organic working medium were compared, R123 was selected as the working fluid of this ORC. Two cycle modes, low-temperature cycle and high-temperature cycle, were designed according to the evaporation way of working fluid. The working fluid is evaporated by coolant heat in low-temperature cycle but by exhaust heat in high-temperature cycle. The influence factors of cycle performance and recovery potential of engine waste heat energy were investigated by cycle simulation and parametric analysis. The results show that recovery efficiency of waste heat energy is influenced by both engine operating conditions and cycle parameters. At 2000 r/min, the maximum recovery efficiency of waste heat energy is 7.3% under 0.2 MPa brake mean effective pressure (BMEP) but 10.7% under 1.4 MPa BMEP. With the combined ORC employed, the fuel efficiency improvement of engine comes up to 4.7% points under the operations of 2000 r/min and 0.2 MPa BMEP, while it further increases to 5.8% points under the operations of 2000 r/min and 1.4 MPa BMEP. All these indicate that the combined ORC is suitable for internal combustion (IC) engine WHR.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(11):112805-112805-9. doi:10.1115/1.4037128.

Recent experiments have shown that the lateral motion of a high pressure injector needle can lead to significant asymmetrical flow in the sac and asymmetric spray pattern in the combustor, which in turn degrades the combustion efficiency and results in spray hole damage. However, the underlying cause of the lateral needle motion is not understood. In this paper, we numerically studied the complex transient flow in a high pressure diesel injector using the detached eddy simulation to understand the cause of the lateral needle motion. The flow field was described by solving the compressible Navier–Stokes equations. The mass transfer between the liquid and vapor phases of the fuel was modeled using the Zwart–Gerber–Belamri equations. Our study revealed that the vortical flow structures in the sac are responsible for the lateral needle motion and the hole-to-hole flow variation. The transient motion of the vortical structure also affected vapor formation variations in spray holes. Further analysis showed that the rotational speed of the vortical flow structure is proportional to the lateral force magnitude on the lower needle surfaces.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(11):112806-112806-13. doi:10.1115/1.4036967.

This paper presents the development and performance measurements of a beta-type free-piston Stirling engine (FPSE) along with dynamic model predictions. The FPSE is modeled as a two degrees-of-freedom (2DOF) vibration system with the equations of motion for displacer and piston masses, which are connected to the spring and damping elements and coupled by working pressure. A test FPSE is designed from root locus analyses and developed with flexure springs and a dashpot load. The stiffness of the test springs and the damping characteristics of the dashpot are identified through experiments. An experimental test rig is developed with an electric heater and a water cooler, operating under the atmospheric air. The piston dynamic behaviors, including the operating frequency, piston stroke, and phase angle, and engine output performance are measured at various heater temperatures and external loads. The experimental results are compared to dynamic model predictions. The test FPSE is also compared to a conventional kinematic engine in terms of engine output performance and dynamic adaptation to environments. Incidentally, nonlinear dynamic behaviors are observed during the experiments and discussed in detail.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Eng. Gas Turbines Power. 2017;139(11):114501-114501-6. doi:10.1115/1.4036947.

Multiple-spool gas turbines are usually utilized for power supply in aircrafts, ships, and terrestrial electric utility plants. As a result, having a reliable model of them can aid with the control design process and stability analysis. Since several interconnected components are coupled both thermodynamically and through shafts, these engines cannot be modeled linearly as single shaft gas turbines. In this paper, intercomponent volume method (ICV) has been implemented for turbine modeling. A switched feedback control system incorporating bump-less transfer and antiwindup functionality is employed as governor for the engine. Validation with test results from a three spool gas turbine highlights high accuracy of turbine-governor model in various maneuvers. Results show that over-speed after load rejection is considerable due to the fact that in this arrangement, the power turbine (PT) is not coupled with the compressor which acts like a damper for single shaft gas turbines. To address this problem, bleed valves (mainly before combustion chamber) are used to arrest the over-speed by 20%. In addition, a switch is employed into the governor system to rapidly shift fuel to permissible minimum flow.

Commentary by Dr. Valentin Fuster

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