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Research Papers: Gas Turbines: Aircraft Engine

J. Eng. Gas Turbines Power. 2018;140(8):081201-081201-10. doi:10.1115/1.4038364.

An exergy framework was developed taking into consideration a detailed analysis of the heat exchanger (HEX) (intercooler (IC)) component irreversibilities. Moreover, it was further extended to include an adequate formulation for closed systems, e.g., a secondary cycle (SC), moving with the aircraft. Afterward, the proposed framework was employed to study two radical intercooling concepts. The first proposed concept uses already available wetted surfaces, i.e., nacelle surfaces, to reject the core heat and contributes to an overall drag reduction. The second concept uses the rejected core heat to power a secondary organic Rankine cycle and produces useful power to the aircraft-engine system. Both radical concepts are integrated into a high bypass ratio (BPR) turbofan engine, with technology levels assumed to be available by year 2025. A reference intercooled cycle incorporating a HEX in the bypass (BP) duct is established for comparison. Results indicate that the radical intercooling concepts studied in this paper show similar performance levels to the reference cycle. This is mainly due to higher irreversibility rates created during the heat exchange process. A detailed assessment of the irreversibility contributors, including the considered HEXs and SC, is made. A striking strength of the present analysis is the assessment of the component-level irreversibility rate and its contribution to the overall aero-engine losses.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2018;140(8):081501-081501-11. doi:10.1115/1.4038798.

In-cylinder pressure-based combustion descriptors have been widely used for engine combustion control and spark advance scheduling. Although these combustion descriptors have been extensively studied for gasoline-fueled spark ignition (SI) engines, adequate literature is not available on use of alternative fuels in SI engines. In an attempt to partially address this gap, present work focuses on spark advance modeling of hydrogen-fueled SI engines based on combustion descriptors. In this study, two such combustion descriptors, namely, position of the pressure peak (PPP) and 50% mass fraction burned (MFB) have been used to evaluate the efficiency of the combustion. With a view to achieve this objective, numerical simulation of engine processes was carried out in computational fluid dynamics (CFD) software ANSYS fluent and simulation data were subsequently validated with the experimental results. In view of typical combustion characteristics of hydrogen fuel, spark advance plays a very crucial role in the system development. Based on these numerical simulation results, it was observed that the empirical rules used for combustion descriptors (PPP and 50% MFB) for the best spark advance in conventional gasoline fueled engines do not hold good for hydrogen engines. This work suggests revised empirical rules as: PPP is 8–9 deg after piston top dead center (ATDC) and position of 50% MFB is 0–1 deg ATDC for the maximum brake torque (MBT) conditions. This range may vary slightly with engine design but remains almost constant for a particular engine configuration. Furthermore, using these empirical rules, spark advance timings for the engine are presented for its working range.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(8):081502-081502-9. doi:10.1115/1.4038617.

The stabilization of premixed flames within a swirling flow produced by an axial-plus-tangential swirler is investigated in an atmospheric test rig. In this system, flames are stabilized aerodynamically away from the solid components of the combustor without the help of any solid anchoring device. Experiments are reported for lean CH4/air mixtures, eventually also diluted with N2, with injection Reynolds numbers varying from 8500 to 25,000. Changes of the flame shape are examined with OH* chemiluminescence and OH laser-induced fluorescence measurements as a function of the operating conditions. Particle image velocimetry (PIV) measurements are used to reveal the structure of the velocity field in nonreacting and reacting conditions. It is shown that the axial-plus-tangential swirler allows to easily control the flame shape and the position of the flame leading edge with respect to the injector outlet. The ratio of the bulk injection velocity over the laminar burning velocity Ub/SL, the adiabatic flame temperature Tad, and the swirl number S0 are shown to control the flame shape and its position inside the combustion chamber. It is then shown that the axial velocity field produced by the axial-plus-tangential swirler is different from those produced by purely axial or radial devices. It takes here a W-shape profile with three local maxima and two minima. The mean turbulent flame front also takes this W-shape in an axial plane, with two lower positions located slightly off-axis and corresponding to the positions where the axial flow velocity is the lowest. It is finally shown that these positions can be inferred from axial flow velocity profiles under nonreacting conditions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(8):081503-081503-10. doi:10.1115/1.4039178.

The effects of effusion and film cooling momenta on combustor flow fields are investigated. Steady, compressible three-dimensional (3D) simulations are performed on a single-swirler combustor using Reynolds-averaged Navier–Stokes (RANS) with flamelet generated manifold and Lagrangian–Eulerian multiphase spray, while accounting for dome and liner cooling. Two simulations are performed on the same mesh. One simulation is conducted using a parallelized, automated, predictive, imprint cooling (PAPRICO) model with dynamic flux boundary conditions and downstream pressure probing (DFBC-DPP). PAPRICO involves removing the cooling jet geometry from the dome and liner while retaining the cooling hole imprints. The PAPRICO model does not require a priori knowledge of the cooling flow rates through various combustor liner regions nor specific mesh partitioning. The other simulation is conducted using the homogenously patched cooling (HPC) model, which involves removing all the cooling jets. The HPC model applies volumetric sources adjacent to the combustor wall regions where cooling jets are present. The momentum source, however, becomes negligible. The HPC model is not predictive and requires tedious ex situ mass flow measurements from an auxiliary flowbench experiment, afflicted with systematic errors. Hence, the actual in situ air flow splits through the several combustor regions is not known with absolute certainty. The numerical results are compared with measurements of mass flow rates, static pressure drops, and path-integrated temperatures. The results demonstrate that it is critical to account for the discrete dome and liner cooling momentum to better emulate the reacting flow in a combustor.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(8):081504-081504-9. doi:10.1115/1.4038882.

The Dry-Low-NOx (DLN) Micromix combustion technology has been developed as low emission combustion principle for industrial gas turbines fueled with hydrogen or syngas. The combustion process is based on the phenomenon of jet-in-crossflow-mixing (JICF). Fuel is injected perpendicular into the air-cross-flow and burned in a multitude of miniaturized, diffusion-like flames. The miniaturization of the flames leads to a significant reduction of NOx emissions due to the very short residence time of reactants in the flame. In the Micromix research approach, computational fluid dynamics (CFD) analyses are validated toward experimental results. The combination of numerical and experimental methods allows an efficient design and optimization of DLN Micromix combustors concerning combustion stability and low NOx emissions. The paper presents a comparison of several numerical combustion models for hydrogen and hydrogen-rich syngas. They differ in the complexity of the underlying reaction mechanism and the associated computational effort. The performance of a hybrid eddy-break-up (EBU) model with a one-step global reaction is compared to a complex chemistry model and a flamelet generated manifolds (FGM) model, both using detailed reaction schemes for hydrogen or syngas combustion. Validation of numerical results is based on exhaust gas compositions available from experimental investigation on DLN Micromix combustors. The conducted evaluation confirms that the applied detailed combustion mechanisms are able to predict the general physics of the DLN-Micromix combustion process accurately. The FGM method proved to be generally suitable to reduce the computational effort while maintaining the accuracy of detailed chemistry.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(8):081505-081505-9. doi:10.1115/1.4038915.

A staged injector developed by JAXA and fueled with kerosene is studied in a high-pressure combustion experiment. With a stable pilot fuel flow rate, the fuel flow rate in the main stage is progressively increased. A high-speed OH-planar laser-induced fluorescence (PLIF) system is used to record the flame motion at 10,000 fps. In the beginning of the recording, the flame behavior is dominated by relatively low-frequency rotation due to the swirling motion of the flow. These rotational motions then coexist with a thermo-acoustic instability around 475 Hz which increases the amplitude of the pressure fluctuations inside the chamber. Dynamic mode decomposition (DMD) analyses indicate that this instability is associated with a widening of the flame occurring when the pressure fluctuations are the highest, giving the instability a positive feedback. The instability frequency then abruptly switches to 500 Hz, while the mode shape remains the same. This frequency change is studied using time–frequency analysis to highlight a change in the feedback mechanism characterized by a modification of the time delay between pressure and heat release fluctuations.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Cycle Innovations

J. Eng. Gas Turbines Power. 2018;140(8):081701-081701-6. doi:10.1115/1.4038459.

The Allam Cycle is a high-performance oxy-fuel, supercritical CO2 power cycle that offers significant benefits over traditional fossil and hydrocarbon fuel-based power generation systems. A major benefit arises in the elimination of costly precombustion acid gas removal (AGR) for sulfur- (SOx) and nitrogen-based (NOx) impurities by utilizing a novel downstream cleanup process that utilizes NOx first as a gas phase catalyst to effect SOx oxidation, followed by NOx removal. The basic reactions required for this process, which have been well demonstrated in several facilities for the cleanup of exhaust gasses, ultimately convert SOx and NOx species to sulfuric, nitric, and nitrous acids for removal from the supercritical CO2 stream. The process results in simplified and significantly lower cost removal of these species and utilizes conditions inherent to the Allam Cycle that are ideally suited to facilitate this process. 8 Rivers Capital and the Energy & Environmental Research Center (EERC), supported by the state of North Dakota, the U.S. Department of Energy and an Industrial consortium from the State of North Dakota, are currently working together to test and optimize this novel impurity removal process for pressurized, semi-closed supercritical CO2 cycles, such as the Allam Cycle. Both reaction kinetic modeling and on-site testing have been completed. Initial results show that both SOx and NOx can be substantially removed from CO2-rich exhaust gas containing excess oxygen under 20 bar operating pressure utilizing a simple packed spray column. Sensitivity of the removal rate to the concentration of oxygen and NOx was investigated. Follow-on work will focus on system optimization to improve removal efficiency and removal control, to minimize metallurgy and corrosion risks from handling concentrated acids, and to reduce overall capital cost and operating cost of the system.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2018;140(8):082101-082101-8. doi:10.1115/1.4038607.

Durability of thermal barrier coating (TBC) systems is important because of recent rising of turbine inlet temperature (TIT) for improved efficiency of industrial gas turbine engines. However, high-temperature environment accelerates the degradation of the TBC as well as causes spalling of the top coat. Spalling of the top coat may be attributed to several factors, but evidently the growth of thermally grown oxide (TGO) should be considered as an important factor. One method for reducing the growth rate of TGO is to provide a dense α-Al2O3 layer at the boundary of the bond coat and top coat. This α-Al2O3 layer will suppress the diffusion of oxygen to the bond coat and consumption of aluminum of the bond coat is suppressed. In this study, we focused on thermal pre-oxidation of the bond coat as a means for forming an α-Al2O3 barrier layer that would be effective at reducing the growth rate of TGO, and we studied the suitable pre-oxidation conditions. In the primary stage, we analyzed the oxidation behavior of the bond coat surface during pre-oxidation heat treatment by means of in situ synchrotron X-ray diffraction (XRD) analysis. As a result, we learned that during oxidation in ambient air environment, in the initial stage of oxidation metastable alumina is produced in addition to α-Al2O3, but if the thermal treatment is conducted under some specific low oxygen partial pressure condition, unlike in the ambient air environment, only α-Al2O3 is formed with suppressing formation of metastable alumina. We also conducted transmission electron microscope (TEM) and XRD analysis of oxide scale formed after pre-oxidation heat treatment of the bond coat. As a result, we learned that if pre-oxidation is performed under specific oxygen partial pressure conditions, a monolithic α-Al2O3 layer is formed on the bond coat. We performed a durability evaluation test of TBC with the monolithic α-Al2O3 layer formed by pre-oxidation of the bond coat. An isothermal oxidation test confirmed that the growth of TGO in the TBC that had undergone pre-oxidation was suppressed more thoroughly than that in the TBC that had not undergone pre-oxidation. Cyclic thermal shock test by hydrogen burner rig was also carried out. TBC with the monolithic α-Al2O3 layer has resistance to >2000 cycle thermal shock at a load equivalent to that of actual gas turbine.

Topics: oxidation , Durability
Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2018;140(8):082501-082501-9. doi:10.1115/1.4038755.

Significant dynamic forces can be generated by annular seals in rotordynamics and can under certain conditions destabilize the system leading to a machine failure. Mathematical modeling of dynamic seal forces are still challenging, especially for multiphase fluids and for seals with complex geometries. This results in much uncertainty in the estimation of the dynamic seal forces, which often leads to unexpected system behavior. This paper presents the results of a method suitable for on-site identification of uncertain dynamic annular seal forces in rotordynamic systems supported by active magnetic bearings (AMB). An excitation current is applied through the AMBs to obtain perturbation forces and a system response, from which the seal coefficients are extracted by utilizing optimization and a priori information about the mathematical model structure and its known system dynamics. As a study case, the method is applied to a full-scale test facility supported by two radial AMBs interacting with one annular center-mounted test seal. Specifically, the dynamic behavior of a smooth annular seal with high preswirl and large clearance (worn seal) is investigated in this study for different excitation frequencies and differential pressures across the seal. The seal coefficients are extracted and a global model on reduced state-space modal form is obtained using the identification process. The global model can be used to update the model-based controller to improve the performance of the overall system. This could potentially be implemented in all rotordynamic systems supported by AMBs and subjected to seal forces or other fluid film forces.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(8):082502-082502-9. doi:10.1115/1.4038693.

Gas foil bearings can operate in extreme conditions such as high temperature and high rotating speed, compared to traditional bearings. They also provide better damping and stability characteristics and have larger tolerance to debris and rotor misalignment. Gas foil bearings have been successfully applied to micro- and small-sized turbomachinery, such as microgas turbine and cryogenic turbo expander. In the last decades, a lot of theoretical and experimental work has been conducted to investigate the properties of gas foil bearings. However, very little work has been done to study the influence of the foil bearing pad configuration. This study proposes a robust approach to analyze the effect of the foil geometry on the performance of a six-pad thrust foil bearing. In this study, a three-dimensional (3D) computational fluid dynamics (CFD) model for a parallel six-pad thrust foil bearing is created. In order to predict the thermal property, the total energy with viscous dissipation is used. Based on this model, the geometry of the thrust foil bearing is parameterized and analyzed using the design of experiments (DOE) methodology. In this paper, the selected geometry parameters of the foil structure include minimum film thickness, inlet film thickness, the ramp extent on the inner circle, the ramp extent on the outer circle, the arc extent of the pad, and the orientation of the leading edge. The objectives in the sensitivity study are load capacity and maximal temperature. An optimal foil geometry is derived based on the results of the DOE process by using a goal-driven optimization technique to maximize the load capacity and minimize the maximal temperature. The results show that the geometry of the foil structure is a key factor for foil bearing performance. The numerical approach proposed in this study is expected to be useful from the thrust foil bearing design perspective.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(8):082503-082503-9. doi:10.1115/1.4038914.

In noncontact annular labyrinth seals used in turbomachinery, fluid prerotation in the direction of shaft rotation effectively increases fluid velocity in the circumferential direction and generates fluid forces with potential destabilizing effects to be exerted on the rotor. Swirl brakes are typically employed to reduce the fluid prerotation at the inlet of the seal. The inlet flow separates as it follows the swirl brakes, and the ratio between tangential component of the velocity at the seal, and the velocity of the rotor surface varies consequently. Effective swirl brakes can significantly suppress the destabilizing fluid forces as it is effectively reducing the tangential velocity. The literature shows that leakage rate can also be reduced by using swirl brakes with “negative-swirl.” In this study, a labyrinth seal with inlet swirl brakes is selected from the literature and considered the baseline design. The seal performance is evaluated using ANSYS-cfx. The design of experiments (DOEs) approach is used to investigate the effects of various design variables on the seal performance. The design space consists of the swirl brake's length, width, curvature at the ends, the tilt angle, as well as the number of swirl brakes in the circumferential direction. Simple random sampling method with Euclidean distances for the design matrix is used to generate the design points. Steady-state computational fluid dynamics simulations are then performed for each design point to analyze the performance of the swirl brakes. Quadratic polynomial fitting is used to evaluate the sensitivity of the average circumferential velocity with respect to the design variables, which gives a qualitative estimation for the performance of the swirl brakes. These results assist in creating a better understanding of which design variables are critical and more effective in reduction of the destabilizing forces acting on the rotor, and thus will support the swirl brake design for annular pressure seals.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(8):082504-082504-8. doi:10.1115/1.4038994.

Developments in brush seal analyses tools have been covering advanced flow and structural analyses since brush seals are applied at elevated pressure loads, temperatures, surface speeds, and transients. Brush seals have dynamic flow and structural behaviors that need to be investigated in detail in order to estimate final leakage output and service life. Bristles move, bend, and form a grift matrix depending on pressure load. The level of pressure load determines the tightness of the bristle pack, and thus, the leakage. In the computational fluid dynamics (CFD) analyses of this work, the bristle pack is treated as a porous medium. Based on brush seal test data, the flow resistance coefficients (FRC) for the porous bristle pack are calibrated as a function of pressure load. A circular seal is tested in a static test rig under various pressure loads at room temperature. The FRC calibration is based on test leakage and literature-based axial pressure distribution on the rotor surface and radial pressure distribution over the backing plate. The anisotropic FRC are treated as spatial dependent in axisymmetrical coordinates. The fence height region and the upper region of bristle pack have different FRC since the upper region is supported by backing plate, while bristles are free to move and bend at the fence height region. The FRC are found to be almost linearly dependent on the pressure load for investigated conditions. The blow-down is also calculated by incorporating test leakage and calibrated FRC.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(8):082505-082505-9. doi:10.1115/1.4038857.

Oil-lubricated bearings are widely used in high-speed rotating machines such as those used in the aerospace and automotive industries that often require this type of lubrication. However, environmental issues and risk-adverse operations have made water-lubricated bearings increasingly popular. Due to different viscosity properties between oil and water, the low viscosity of water increases Reynolds numbers drastically and therefore makes water-lubricated bearings prone to turbulence effects. The turbulence model is affected by eddy viscosity, while eddy viscosity depends on wall shear stress. Therefore, effective wall shear stress modeling is necessary in producing an accurate turbulence model. Improving the accuracy and efficiency of methodologies of modeling eddy viscosity in the turbulence model is important, especially considering the increasingly popular application of water-lubricated bearings and also the traditional oil-lubricated bearings in high-speed machinery. This purpose of this paper is to study the sensitivity of using different methodologies of solving eddy viscosity for turbulence modeling. Eddy viscosity together with flow viscosity forms the effective viscosity, which is the coefficient of the shear stress in the film. The turbulence model and Reynolds equation are bound together to solve when hydrodynamic analysis is performed, therefore improving the accuracy of the turbulence model is also vital to improving a bearing model's ability to predict film pressure values, which will determine the velocity and velocity gradients in the film. The velocity gradients in the film are the other term determining the shear stress. In this paper, three approaches applying Reichardt's formula were used to model eddy viscosity in the fluid film. These methods are for determining where one wall's effects begin and the other wall's effects end. Trying to find a suitable model to capture the wall's effects of these bearings, with an aim to improve the accuracy of the turbulence model, would be of high value to the bearing industry. The results of this study could aid in improving future designs and models of both oil- and water-lubricated bearings.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(8):082507-082507-9. doi:10.1115/1.4039083.

An experimental study is carried out investigating hydraulic performance for various designs of dual-mechanical-seal cartridges with integral pumping-rings. The tested devices are classified into two main families: radial-flow and axial-flow. The radial-flow family utilizes the modified-paddle-wheel (MPW) pumping ring with either a radial or a tangential oriented cartridge outlet port, while the axial-flow family utilizes the pumping scroll (PS) pumping ring in the following cartridge-geometries: single-pumping-scroll (SS) and double-pumping-scroll (DS); as both types can be of either a radial or a tangential outlet port; and of an internal clearance value complying with American Petroleum Institute (API) 682 standard or smaller clearance. An experimental setup is constructed, and appropriate instrumentation is employed to measure inlet pressure, outlet pressure, rotational speed, and barrier fluid flow rate acquiring flow-head characteristic curves. Moreover, empirical generalized characteristic curves are deduced from experimental observations obtained from the present study and previous companion work. The generalized curves can be employed in estimating pumping ring performance for a different size or other operation conditions such as varying rotational speed or fluid type. They can be also utilized to validate numerical simulations for geometrically similar designs.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Turbomachinery

J. Eng. Gas Turbines Power. 2018;140(8):082601-082601-15. doi:10.1115/1.4038608.

Solid particle ingestion is one of the principal degradation mechanisms in the compressor and turbine sections of gas turbines. In particular, in industrial applications, the microparticles not captured by the air filtration system can cause deposits on blading and, consequently, result in a decrease in compressor performance. In the literature, there are some studies related to the fouling phenomena in transonic compressors, but in industrial applications (heavy-duty compressors, pump stations, etc.), the subsonic compressors are widespread. It is highly important for the manufacturer to gather information about the fouling phenomenon related to this type of compressor. This paper presents three-dimensional (3D) numerical simulations of the microparticle ingestion (0.15–1.50 μm) in a multistage (i.e., eight stage) subsonic axial compressor, carried out by means of a commercial computational fluid dynamic (CFD) code. Particles of this size can follow the main air flow with relatively little slip, while being impacted by flow turbulence. It is of great interest to the industry to determine which zones of the compressor blades are impacted by these small particles. Particle trajectory simulations use a stochastic Lagrangian tracking method that solves the equations of motion separately from the continuous phase. The adopted computational strategy allows the evaluation of particle deposition in a multistage axial compressor thanks to the use of a mixing plane approach to model the rotor/stator interaction. The compressor numerical model and the discrete phase model are set up and validated against the experimental and numerical data available in the literature. The number of particles and sizes is specified in order to perform a quantitative analysis of the particle impacts on the blade surface. The blade zones affected by particle impacts and the kinematic characteristics (velocity and angle) of the impact of micrometric and submicrometric particles with the blade surface are shown. Both blade zones affected by particle impact and deposition are analyzed. The particle deposition is established by using the quantity called sticking probability (SP), adopted from the literature. The SP links the kinematic characteristics of particle impact on the blade with the fouling phenomenon. The results show that microparticles tend to follow the flow by impacting on the compressor blades at full span. The suction side (SS) of the blade is only affected by the impacts of the smallest particles. Particular fluid dynamic phenomena, such as corner separations and clearance vortices, strongly influence the impact location of the particles. The impact and deposition trends decrease according to the stages. The front stages appear more affected by particle impact and deposition than the rear ones.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(8):082602-082602-12. doi:10.1115/1.4038793.

Serpentine nozzles have been used in stealth fighters to increase their survivability. For real turbofan aero-engines, the existence of the double ducts (bypass and core flow), the tail cone, the struts, the lobed mixers, and the swirl flows from the engine turbine, could lead to complex flow features of serpentine nozzle. The aim of this paper is to ascertain the effect of different inlet configurations on the flow characteristics of a double serpentine convergent nozzle. The detailed flow features of the double serpentine convergent nozzle including/excluding the tail cone and the struts are investigated. The effects of inlet swirl angles and strut setting angles on the flow field and performance of the serpentine nozzle are also computed. The results show that the vortices, which inherently exist at the corners, are not affected by the existence of the bypass, the tail cone, and the struts. The existence of the tail cone and the struts leads to differences in the high-vorticity regions of the core flow. The static temperature contours are dependent on the distributions of the x-streamwise vorticity around the core flow. The high static temperature region is decreased with the increase of the inlet swirl angle and the setting angle of the struts. The performance loss of the serpentine nozzle is mostly caused by its inherent losses such as the friction loss and the shock loss. The performance of the serpentine nozzle is decreased as the inlet swirl angle and the setting angle of the struts increase.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(8):082603-082603-8. doi:10.1115/1.4039425.

The prominence of highly integrated engine/airframe architectures in modern commercial aircraft design concepts has led to significant research efforts investigating the use of conventional turbofan engines in unconventional installations where severe inlet distortions can arise. In order to determine fan rotor capabilities for reducing or eliminating a complex inlet swirl distortion, an experimental investigation using a StreamVaneTM swirl distortion generator was conducted in a turbofan engine research platform. Three-dimensional (3D) flow data collected at two discrete planes surrounding the fan rotor indicated that the intensity of the swirl distortion was decreased by the fan rotor; however, substantial swirl distortion effects remained in the fan exit flow. Flow angle magnitudes and swirl intensity (SI) decreased by approximately 30–40% across the fan rotor, while the presence of large-scale features within the distortion profile was nearly eliminated. Secondary flow streamlines indicated that small-scale features of the distortion were less affected by the rotating component and remained coherent at the fan rotor outlet plane. These results led to the conclusion that swirl distortion survived interactions with the fan rotor, leading to off-design conditions cascading through downstream engine components.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(8):082604-082604-9. doi:10.1115/1.4039050.

Flow disturbances are generated inside a duct via pulsed injection of helium into a flow of air. This leads to the generation of an acoustic pulse (direct noise), as well as the production of entropic and compositional inhomogeneities, which are convected with the mean flow. As these inhomogeneities are convected through a choked nozzle, they generate indirect noise. The resulting acoustic pressure fluctuations are measured experimentally using pressure transducers upstream of the nozzle. Insight obtained from theoretical models and a time-delay analysis can be used to isolate and extract the contributions of direct and indirect noise in the experimental signal. These results are directly compared to existing one-dimensional (1D) direct and indirect noise models. The experimental measurement of indirect noise is found to be in good agreement with the theoretical models for entropy noise and compositional noise for a compact 1D isentropic nozzle.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(8):082605-082605-7. doi:10.1115/1.4038913.

The swirl recovery vane (SRV) oriented in the slipstream of the propeller can in principle recover the swirl effect and thus would improve the propulsion performance in terms of thrust production and propulsive efficiency. The present study employs the design of experiments (DoEs) method to optimize the geometry of the specific SRV for Fokker 29 propeller for the sake of further enhancing the thrust generation and swirling recovery. First, orthogonal experiment was employed to identify the most significant factors, which directly influence the thrust production. Second, steepest ascent method was used to search the optimum range of target factors through climbing and factorial experiments. The resulting optimal solution was evaluated by the center composite experiment. Results show that the thrust generated by the SRV has been increased significantly (11.78%) after optimization at the design point, and a 0.66% increment in the total efficiency of the propeller–SRV system has been obtained. For the off-design point, an increment of the total efficiency (2.10%) can be observed at low rotating speed. Additionally, the optimized SRV is able to correct the out-flow behavior at the tip region of the vane, where the tip vortex and swirl kinetic energy loss is weaken, and the thrust distribution along the spanwise direction tends to be more uniform.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Vehicular and Small Turbomachines

J. Eng. Gas Turbines Power. 2018;140(8):082701-082701-7. doi:10.1115/1.4038366.

To meet the challenging demands for high performance, affordable compliant foil bearings (CFBs), a novel compliant support element has been developed. This recently patented, novel support element uses a multidimensional array of multiple, formed, cantilever “wing foil” tabs. The wing foil bearing has all the features required to achieve state-of-the-art performance (Gen III for radial bearings). This paper describes two radial foil bearings using the wing foil and the unique design features. Test data for a 31.75 mm diameter bearing operating in air and in steam up to 42 krpm are presented to demonstrate the performance of this bearing. It is shown to have low subsynchronous vibration and reasonable damping through rigid shaft critical speeds.

Commentary by Dr. Valentin Fuster

Research Papers: Internal Combustion Engines

J. Eng. Gas Turbines Power. 2018;140(8):082801-082801-11. doi:10.1115/1.4039805.

Surrogates development is important to extensively investigate the combustion behavior of fuels. Development of comprehensive surrogates has been focusing on matching chemical and physical properties of their target fuel to mimic its atomization, evaporation, mixing, and auto-ignition behavior. More focus has been given to matching the derived cetane number (DCN) as a measure of the auto-ignition quality. In this investigation, we carried out experimental validation of a three-component surrogate for Sasol-Isoparaffinic Kerosene (IPK) in ignition quality tester (IQT) and in an actual diesel engine. The surrogate fuel is composed of three components (46% iso-cetane, 44% decalin, and 10% n-nonane on a volume basis). The IQT experiments were conducted as per ASTM D6890-10a. The engine experiments were conducted at 1500 rpm, two engine loads, and two injection timings. Analysis of ignition delay (ID), peak pressure, peak rate of heat release (RHR), and other combustion phasing parameters showed a closer match in the IQT than in the diesel engine. Comparison between the surrogate combustion behavior in the diesel engine and IQT revealed that matching the DCN of the surrogate to its respective target fuel did not result in the same negative temperature coefficient (NTC) profile—which led to unmatched combustion characteristics in the high temperature combustion (HTC) regimes, despite the same auto-ignition and low temperature combustion (LTC) profiles. Moreover, a comparison between the combustion behaviors of the two fuels in the IQT is not consistent with the comparison in the diesel engine, which suggests that the surrogate validation in a single-cylinder diesel engine should be part of the surrogate development methodology, particularly for low ignition quality fuels.

Commentary by Dr. Valentin Fuster

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