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

J. Eng. Gas Turbines Power. 2017;139(9):091501-091501-11. doi:10.1115/1.4036010.

Modern combustion chambers of gas turbines for power generation and aero-engines suffer of thermo-acoustic combustion instabilities generated by the coupling of heat release rate fluctuations with pressure oscillations. The present article reports a numerical analysis of limit cycles arising in a longitudinal combustor. This corresponds to experiments carried out on the longitudinal rig for instability analysis (LRIA) test facility equipped with a full-scale lean-premixed burner. Heat release rate fluctuations are modeled considering a distributed flame describing function (DFDF), since the flame under analysis is not compact with respect to the wavelengths of the unstable modes recorded experimentally. For each point of the flame, a saturation model is assumed for the gain and the phase of the DFDF with increasing amplitude of velocity fluctuations. A weakly nonlinear stability analysis is performed by combining the DFDF with a Helmholtz solver to determine the limit cycle condition. The numerical approach is used to study two configurations of the rig characterized by different lengths of the combustion chamber. In each configuration, a good match has been found between numerical predictions and experiments in terms of frequency and wave shape of the unstable mode. Time-resolved pressure fluctuations in the system plenum and chamber are reconstructed and compared with measurements. A suitable estimate of the limit cycle oscillation is found.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(9):091502-091502-11. doi:10.1115/1.4035906.

This paper presents an experimental procedure developed to simulate the behavior of ceramic matrix composites (CMCs) under the cyclic thermal stresses of a gas turbine combustion chamber. An experimental apparatus was assembled that produces a temperature gradient across the thickness of a CMC specimen while holding the specimen at its two extremities, which simulates the bending stress that would be observed at the center of a combustor panel. Preliminary validation tests were performed in which A-N720 oxide–oxide CMC specimens were heated to a surface temperature of up to 1160 °C using an infrared heater, which allowed for the calibration of heat losses and material thermal conductivity. The specimen test conditions were compared with predicted conditions in generic annular combustor panels made of the same material. Provided that a more powerful heat source is made available to reach sufficiently high temperatures and through-thickness temperature gradients simultaneously, the proposed experiment promises to allow laboratory observation of representative deterioration modes of a CMC inside an actual combustion chamber.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2017;139(9):091601-091601-11. doi:10.1115/1.4036011.

The largest share of electricity production worldwide belongs to steam turbines. However, the increase of renewable energy production has led steam turbines to operate under part load conditions and increase in size. As a consequence, long rotor blades will generate a relative supersonic flow field at the inlet of the last rotor. This paper presents a unique experiment work that focuses at the top 30% of stator exit in the last stage of an low pressure (LP) steam turbine test facility with coarse droplets and high wetness mass fraction under different operating conditions. The measurements were performed with two novel fast response probes: a fast response probe for three-dimensional flow field wet steam measurements and an optical backscatter probe for coarse water droplet measurements ranging from 30 μm up to 110 μm in diameter. This study has shown that the attached bow shock at the rotor leading edge is the main source of interblade row interactions between the stator and rotor of the last stage. In addition, the measurements showed that coarse droplets are present in the entire stator pitch with larger droplets located at the vicinity of the stator's suction side. Unsteady droplet measurements showed that the coarse water droplets are modulated with the downstream rotor blade-passing period. This set of time-resolved data will be used for in-house computational fluid dynamics (CFD) code development and validation.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Cycle Innovations

J. Eng. Gas Turbines Power. 2017;139(9):091701-091701-9. doi:10.1115/1.4036093.

In this work, we apply a sequence of concepts for mechanism reduction on one reaction mechanism including novel quality control. We introduce a moment-based accuracy rating method for species profiles. The concept is used for a necessity-based mechanism reduction utilizing 0D reactors. Thereafter a stochastic reactor model for internal combustion engines is applied to control the quality of the reduced reaction mechanism during the expansion phase of the engine. This phase is sensitive on engine out emissions, and is often not considered in mechanism reduction work. The proposed process allows to compile highly reduced reaction schemes for computational fluid dynamics application for internal combustion engine simulations. It is demonstrated that the resulting reduced mechanisms predict combustion and emission formation in engines with accuracies comparable to the original detailed scheme.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2017;139(9):092501-092501-9. doi:10.1115/1.4036058.

In the application of film-riding sealing technology, there are various groove features that can be used to induce hydrodynamic lift. However, there is little guidance in selecting the relative parameter settings in order to maximize hydrodynamic load and fluid stiffness. In this study, two groove types are investigated—Rayleigh step and inclined groove. The study uses a design of experiments approach and a Reynolds equation solver to explore the design space. Key parameters have been identified that can be used to optimize a seal design. The results indicate that the relationship between parameters is not a simple linear relationship. It was also found that higher pressure drops hinder the hydrodynamic load and stiffness of the seal suggesting an advantage for using hydrostatic load support in such conditions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(9):092502-092502-9. doi:10.1115/1.4036188.

Textured thrust bearings are capable of providing higher load capacity and lower friction torque compared to nontextured bearings. However, most previous optimization efforts for texturing geometry were focused on rectangular dimples and employed Reynolds equation. Limited studies have been done to investigate the effects of partially textured thrust bearings with elliptical dimples. This study proposes a new optimization approach to find the optimal partially texture geometry with elliptical dimples, which maximize the loading capacity and minimize the friction torque. In this study, a 3D computational fluid dynamics (CFD) model for a parallel sector-pad thrust bearing is built using ANSYS cfx. Mass conserving cavitation model is used to simulate the cavitation regions. Energy equation for Newtonian flow is also solved. The results of the model are validated by the experimental data from the literature. Based on this model, the flow pattern and pressure distribution inside the dimples are analyzed. The geometry of elliptical dimple is parameterized and analyzed using design of experiments (DOE). The selected geometry parameters include the length of major and minor axes, dimple depth, radial and circumferential space between two dimples, and the radial and circumferential extend. A multi-objective optimization scheme is used to find the optimal texture structure with the load force and friction torque set as objective functions. The results show that the shape of dimples has a crucial effect on the performance of the textured thrust bearings. Searching the design space for a proper combination among the design variables satisfying the constraints has the advantage of capturing the codependence among design variables and leads to a surface patterning of the bearing, which showed a 42.7% improvement on the load capacity.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(9):092503-092503-9. doi:10.1115/1.4035912.

This paper presents an efficient approach for stabilizing solution and accelerating convergence of a harmonic balance equation system for an efficient analysis of turbomachinery unsteady flows due to flutter and blade row interaction. The proposed approach combines the Runge–Kutta method with the lower upper symmetric Gauss Seidel (LU-SGS) method and the block Jacobi method. The LU-SGS method, different from its original application as an implicit time marching scheme, is used as an implicit residual smoother with under-relaxation, allowing big Courant–Friedrichs–Lewy (CFL) numbers (in the order of hundreds), leading to significant convergence speedup. The block Jacobi method is introduced to implicitly integrate the time spectral source term of a harmonic balance equation system, in order to reduce the complexity of the direct implicit time integration by the LU-SGS method. The implicit treatment of the time spectral source term thus greatly augments the stability region of a harmonic balance equation system in the case of grid-reduced frequency well above ten. Validation of the harmonic balance flow solver was carried out using linear cascade test data. Flutter analysis of a transonic rotor and blade row interaction analyses for a transonic compressor stage were presented to demonstrate the stabilization and acceleration effect by the combination of the LU-SGS and the block Jacobi methods. The influence of the number of Jacobi iterations on solution stabilization is also investigated, showing that two Jacobi iterations are sufficient for stability purpose, which is much more efficient than existing methods of its kind in the open literature.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(9):092504-092504-12. doi:10.1115/1.4036063.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Turbomachinery

J. Eng. Gas Turbines Power. 2017;139(9):092601-092601-10. doi:10.1115/1.4036059.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(9):092602-092602-9. doi:10.1115/1.4036060.

Current maintenance, having a great impact on the safety, reliability and economics of a gas turbine, becomes the major obstacle for the application of gas turbines in energy field. An effective solution is to process condition based maintenance (CBM) thoroughly for gas turbines. Maintenance of high temperature blade, accounting for the most of the maintenance costs and time, is the crucial section of gas turbine maintenance. The suggested life of high temperature blade by original equipment manufacturer (OEM) is based on several certain operating conditions, which is used for time based maintenance (TBM). Thus, for the requirement of gas turbine CBM, a damage evaluation model is demanded to estimate the life consumption online. A physics-based model is built, consisting of thermodynamic performance simulation model, stress estimation model, thermal estimation model, and interactive damage analysis model. Unmeasured parameters are simulated by the thermodynamic performance simulation model, as the input of the stress estimation model and the thermal estimation model. Due to the ability to analyze online data, this model can be used to calculate online damage and support CBM decision. Then the stress and temperature distribution of blades will become as the input of the creep damage analysis model and the fatigue damage analysis model. The interactive damage of blades will be evaluated based on the creep and fatigue analysis results. To validate this physics-based model, it is used to calculate the lifes of high temperature blade under several certain operating conditions. And the results are compared to the suggestion value of OEM. An application case is designed to evaluate the application effect of this model. The result shows that the relative error of this model is less than 10.4% in selected cases. And it can cut overhaul costs and increase the availability of gas turbines significantly. Finally, a simple application of this model is proposed to show its functions. The physical-based damage evaluation model proposed in this paper is found to be a useful tool to tracing the online life consumption of a high temperature blade, to support the implementation of CBM for gas turbines, and to guarantee the reliability of gas turbines with lowest maintenance costs.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(9):092603-092603-12. doi:10.1115/1.4036061.

The transient heat transfer facility (THTF) was developed to test full-scale high pressure compressor and turbine casing air systems using gas turbine engine representative secondary air system conditions. Transient casing response together with blade and disk responses governs achievable tip clearances in both compressors and turbines. This paper investigates the use of air impingement as a means to speed up the casing response. The thermal growth of the casing was characterized by surface temperature rise over a given period to assess achievable dynamic response. The experimental setup resembles a typical aircraft engine with features that can lead to circumferential temperature nonuniformities, as evident from the experimental results. The experimental data were compared against numerical predictions from a conjugate heat transfer (CHT) model. The studies show the significance of analyzing the full annulus, at engine representative conditions and the benefit of an impingement array to potentially speed up casing response for future engines.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(9):092604-092604-9. doi:10.1115/1.4036062.

Finite element models (FEMs) are extensively used in the design optimization of utility scale steam turbines. As an example, by simulating multiple startup scenarios of steam power plants, engineers can obtain turbine designs that minimize material utilization, and at the same time, avoid the damaging effects of large thermal stresses or rubs between rotating and stationary parts. Unfortunately, FEMs are computationally expensive and only a limited amount of simulations can be afforded to get the final design. For this reason, numerous model reduction techniques have been developed to reduce the size of the original model without a significant loss of accuracy. When the models are nonlinear, as is the case for steam turbine FEMs, model reduction techniques are relatively scarce and their effectiveness becomes application dependent. Although there is an abundant literature on model reduction for nonlinear systems, many of these techniques become impractical when applied to a realistic industrial problem. This paper focuses on a class of nonlinear FEM characteristic of thermo-elastic problems with large temperature excursions. A brief overview of popular model reduction techniques is presented along with a detailed description of the computational challenges faced when applying them to a realistic problem. The main contribution of this work is a set of modifications to existing methods to increase their computational efficiency. The methodology is demonstrated on a steam turbine model, achieving a model size reduction by four orders of magnitude with only 4% loss of accuracy with respect to the full order FEMs.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(9):092605-092605-17. doi:10.1115/1.4036284.

In large modern turbochargers, transonic compressors often constitute the main source of noise, with a frequency spectrum typically dominated by tonal noise at the blade passing frequency (BPF) and its harmonics. Inflow BPF noise is mainly generated by rotor locked shock fronts. Outflow noise, while also dominated by BPF tones, is linked to more complex source mechanisms. Its modal structure and the relationships between sources and modal sound pressure levels (SPL) are less well understood, and its numerical analysis is, in general, significantly more complex than for compressor inflows. To shed some light on the outflow acoustic characteristics of radial machines, transient simulations of a 360 deg model of a radial compressor stage, including its vaned diffuser and volute, were carried out. Four increasingly finer grids were used for this purpose. On all grids, numerical damping had detrimental effects on prediction quality. A simple and mathematically sound method is proposed to account for this damping. With it, the global outflow acoustic power level ($PWLg$) is predicted to within an accuracy of 2 dB of the experimental result on the finest grid. This shows that satisfactory accuracy can be obtained with state-of-the-art computational fluid dynamics (CFD) codes if care is taken with the simulation setup. The simulations are further validated with experimental data from 17 transient wall pressure sensors.

Commentary by Dr. Valentin Fuster

### Research Papers: Internal Combustion Engines

J. Eng. Gas Turbines Power. 2017;139(9):092801-092801-9. doi:10.1115/1.4035910.

Sasol isomerized paraffinic kerosene (IPK) is a coal-derived synthetic fuel under consideration as a blending stock with jet propellant 8 (JP-8) for use in military equipment. However, Sasol IPK is a low ignition quality fuel with derived cetane number (DCN) of 31. The proper use of such alternative fuels in internal combustion engines (ICEs) requires the modification in control strategies to operate engines efficiently. With computational cycle simulation coupled with surrogate fuel mechanism, the engine development process is proved to be very effective. Therefore, a methodology to formulate Sasol IPK surrogate fuels for diesel engine application using ignition quality tester (IQT) is developed. An in-house developed matlab code is used to formulate the appropriate mixture blends, also known as surrogate fuel. And aspen hysys is used to emulate the distillation curve of the surrogate fuels. The properties of the surrogate fuels are compared to those of the target Sasol IPK fuel. The DCNs of surrogate fuels are measured in the IQT and compared with the target Sasol IPK fuel at the standard condition. Furthermore, the ignition delay, combustion gas pressure, and rate of heat release (RHR) of Sasol IPK and its formulated surrogate fuels are analyzed and compared at five different charge temperatures. In addition, the apparent activation energies derived from chemical ignition delay of the surrogate fuel and Sasol IPK are determined and compared.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(9):092802-092802-8. doi:10.1115/1.4036189.

Energetic nanoparticles are promising fuel additives due to their high specific surface area, high energy content, and catalytic capability. Novel amorphous reactive mixed-metal nanopowders (RMNPs) containing Ti, Al, and B, synthesized via a sonochemical reaction, have been developed at the Naval Research Laboratory. These materials have higher energy content than commercial nano-aluminum (nano-Al), making them potentially useful as energy-boosting fuel components. This work examines combustion of RMNPs in a single-cylinder diesel engine (Yanmar L48V). Fuel formulations included up to 4 wt % RMNPs suspended in JP-5, and equivalent nano-Al suspensions for comparison. Although the effects were small, both nano-Al and RMNPs resulted in shorter ignition delays, retarded peak pressure locations, decreased maximum heat release rates, and increased burn durations. A similar but larger engine (Yanmar L100V) was used to examine fuel consumption and emissions for a suspension of 8 wt % RMNPs in JP-5 (and 8 wt % nano-Al for comparison). The engine was operated as a genset under constant load with nominal gross indicated mean effective pressure of 6.5 bar. Unfortunately, the RMNP suspension led to deposits on the injector tip around the orifices, while nano-Al suspensions led to clogging in the fuel reservoir and subsequent engine stall. Nevertheless, fuel consumption rate was 17% lower for the nano-Al suspension compared to baseline JP-5 for the time period prior to stall, which demonstrates the potential value of reactive metal powder additives in boosting volumetric energy density of hydrocarbon fuels.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(9):092803-092803-8. doi:10.1115/1.4036102.

During the last several decades, investigations of the operation of internal combustion engines utilizing exhaust gas recirculation (EGR) have increased. This increased interest has been driven by the advantages of the use of EGR with respect to emissions and, in some cases, thermal efficiency. The current study uses a thermodynamic engine cycle simulation to explore the fundamental reasons for the changes of thermal efficiency as functions of EGR. EGR with various levels of cooling is studied. Both a conventional (throttled) operating condition and a high efficiency (HE) operating condition are examined. With no EGR, the net indicated thermal efficiencies were 32.1% and 44.6% for the conventional and high efficiency engines, respectively. For the conditions examined, the cylinder heat transfer is a function of the gas temperatures and convective heat transfer coefficient. For increasing EGR, the gas temperatures generally decrease due to the lower combustion temperatures. For increasing EGR, however, the convective heat transfer coefficient generally increases due to increasing cylinder pressures and decreasing gas temperatures. Whether the cylinder heat transfer increases or decreases with increasing EGR is the net result of the gas temperature decreases and the heat transfer coefficient increases. For significantly cooled EGR, the efficiency increases partly due to decreases of the heat transfer. On the other hand, for less cooled EGR, the efficiency decreases due at least partly to the increasing heat transfer. Two other considerations to explain the efficiency changes include the changes of the pumping work and the specific heats during combustion.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(9):092804-092804-7. doi:10.1115/1.4036285.

Lower engine emissions like CO2, particulate matter (PM), and NOx have recently become more necessary in automobile engines to protect the earth's environment. Keeping uniformity of air/fuel mixture and decreasing fuel adhesion on walls of cylinder and piston are effective in order to reduce the engine emissions. In order to achieve the target fuel-spray, fuel injectors for gasoline direct injection engines need to be designed to deal with multiple injections with high speed of opening and closing of valves. One of the difficulties in the multiple injections is to control fuel-spray behaviors during opening and closing of valve; flow rate and spray penetration which are changed due to slow velocity of fluid during opening and closing of valve cause nonuniformity of air/fuel mixture that results in the increase of PM. Fuel-spray behaviors are controlled by the valve-lifts of fuel injectors; therefore, air/fuel mixture simulations that integrate with inner flow simulations in fuel injectors during the opening and closing of valves are essential for studying the effects of valve motions on air/fuel mixtures. In this study, we developed an air/fuel mixture simulation that is connected with an inner-flow simulation with a valve opening and closing function. The simulation results were validated by comparing the simulated fuel breakup near the nozzle outlets and the air/fuel mixtures in the air region with the measured ones, revealing good agreement between them. The effects of opening and closing the valve on the air/fuel mixtures were also studied; the opening and closing of the valve affected the front and rear behaviors of the air/fuel mixture and also affected spray penetrations. The developed simulation was found to be an effective tool for studying the effects of valve motions on the air/fuel mixtures. It was also found that the magnetic circuit with the solenoid needs to be designed to achieve high-speed valve motion and also keeps same valve motion in each injection, especially during opening and closing of valve.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(9):092805-092805-14. doi:10.1115/1.4036101.

To meet the increasingly stringent emissions standards, diesel engines need to include more active technologies with their associated control systems. Hardware-in-the-loop (HiL) approaches are becoming popular where the engine system is represented as a real-time capable model to allow development of the controller hardware and software without the need for the real engine system. This paper focusses on the engine model required in such approaches. A number of semi-physical, zero-dimensional combustion modeling techniques are enhanced and combined into a complete model, these include—ignition delay, premixed and diffusion combustion and wall impingement. In addition, a fuel injection model was used to provide fuel injection rate from solenoid energizing signals. The model was parameterized using a small set of experimental data from an engine dynamometer test facility and validated against a complete data set covering the full engine speed and torque range. The model was shown to characterize the rate of heat release (RoHR) well over the engine speed and load range. Critically, the wall impingement model improved R2 value for maximum RoHR from 0.89 to 0.96. This was reflected in the model's ability to match both pilot and main combustion phasing, and peak heat release rates derived from measured data. The model predicted indicated mean effective pressure and maximum pressure with R2 values of 0.99 across the engine map. The worst prediction was for the angle of maximum pressure which had an R2 of 0.74. The results demonstrate the predictive ability of the model, with only a small set of empirical data for training—this is a key advantage over conventional methods. The fuel injection model yielded good results for predicted injection quantity (R2 = 0.99) and enabled the use of the RoHR model without the need for measured rate of injection.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(9):092806-092806-11. doi:10.1115/1.4036293.

This paper presents a numerical study on fuel injection, ignition, and combustion in a direct-injection natural gas (DING) engine with ignition assisted by a shielded glow plug (GP). The shield geometry is investigated by employing different sizes of elliptical shield opening and changing the position of the shield opening. The results simulated by KIVA-3V indicated that fuel ignition and combustion is very sensitive to the relative angle between the fuel injection and the shield opening, and the use of an elliptical opening for the glow plug shield can reduce ignition delay by 0.1–0.2 ms for several specific combinations of the injection angle and shield opening size, compared to a circular shield opening. In addition, the numerical results also revealed that the natural gas ignition and flame propagation will be delayed by lowering a circular shield opening from the fuel jet center plane, due to the blocking effect of the shield to the fuel mixture, and hence, it will reduce the DING engine performance by causing a longer ignition delay.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Eng. Gas Turbines Power. 2017;139(9):094501-094501-9. doi:10.1115/1.4036283.

To improve engine power at high altitude, the regulated two-stage turbocharger (RTST) which was applied to different altitudes was developed by the authors. The working process model of heavy-duty common-rail diesel engine matched with RTST was built to study the regulating characteristic of variable geometry turbocharger (VGT) vane and both turbine bypass valves and also matching performance of RTST with engine at different altitudes. The control scheme of RTST at different altitudes and engine operating conditions was first put forward, and the optimal opening maps of VGT vane and both turbine bypass valves at different altitudes and engine operating conditions were obtained. The results show that the optimal openings of VGT vane and both turbine bypass valves decrease with increase of altitude, and the optimal opening range of VGT vane becomes narrower with increase of altitude. The operating points of both high-pressure (HP) and low-pressure (LP) compressors locate at high-efficiency region of each compressor map, respectively, and compressor efficiency exceeds 70% at altitude of 5500 m. The total boost pressure ratio increases with altitude and reaches the maximum value of 5.1 at altitude of 5500 m. Compared with single-stage turbocharged engine, the rated power, maximum torque, and torques at lower engine speed at altitude of 5500 m increase by 48.2%, 51%, and 65–121% separately, and the minimum fuel consumption decreases by 12.6%.

Commentary by Dr. Valentin Fuster