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

J. Eng. Gas Turbines Power. 2017;139(7):071501-071501-9. doi:10.1115/1.4035591.

This paper presents the experimental approach for determination and validation of noncompact flame transfer functions of high-frequency, transverse combustion instabilities observed in a generic lean premixed gas turbine combustor. The established noncompact transfer functions describe the interaction of the flame's heat release with the acoustics locally, which is necessary due to the respective length scales being of the same order of magnitude. Spatiotemporal dynamics of the flame are measured by imaging the OH chemiluminescence signal, phase-locked to the dynamic pressure at the combustor's front plate. Radon transforms provide a local insight into the flame's modulated reaction zone. Applied to different burner configurations, the impact of the unsteady heat release distribution on the thermoacoustic driving potential, as well as distinct flame regions that exhibit high modulation intensity, is revealed. Utilizing these spatially distributed transfer functions within thermoacoustic analysis tools (addressed in this joint publication's Part II) allows then to predict transverse linear stability of gas turbine combustors.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(7):071502-071502-10. doi:10.1115/1.4035592.

This paper deals with high-frequency (HF) thermoacoustic instabilities in swirl-stabilized gas turbine combustors. Driving mechanisms associated with periodic flame displacement and flame shape deformations are theoretically discussed, and corresponding flame transfer functions (FTF) are derived from first principles. These linear feedback models are then evaluated by means of a lab-scale swirl-stabilized combustor in combination with part one of this joint publication. For this purpose, the models are used to thermoacoustically characterize a complete set of operation points of this combustor facility. Specifically, growth rates of the first transversal modes are computed, and compared against experimentally obtained pressure amplitudes as an indicator for thermoacoustic stability. The characterization is based on a hybrid analysis approach relying on a frequency domain formulation of acoustic conservation equations, in which nonuniform temperature fields and distributed thermoacoustic source terms/flame transfer functions can be straightforwardly considered. The relative contribution of flame displacement and deformation driving mechanisms–i.e., their significance with respect to the total driving–is identified. Furthermore, promoting/inhibiting conditions for the occurrence of high frequency, transversal acoustic instabilities within swirl-stabilized gas turbine combustors are revealed.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(7):071503-071503-11. doi:10.1115/1.4035529.

This paper analyzes transversal thermoacoustic oscillations in an experimental gas turbine combustor utilizing dynamical system theory. Limit-cycle acoustic motions related to the first linearly unstable transversal mode of a given 3D combustor configuration are modeled and reconstructed by means of a low-order dynamical system simulation. The source of nonlinearity is solely allocated to flame dynamics, saturating the growth of acoustic amplitudes, while the oscillation amplitudes are assumed to always remain within the linearity limit. First, a reduced order model (ROM) which reproduces the combustor's modal distribution and damping of acoustic oscillations is derived. The ROM is a low-order state-space system, which results from a projection of the linearized Euler equations (LEE) into their truncated eigenspace. Second, flame dynamics are modeled as a function of acoustic perturbations by means of a nonlinear transfer function. This function has a linear and a nonlinear contribution. The linear part is modeled analytically from first principles, while the nonlinear part is mathematically cast into a cubic saturation functional form. Additionally, the impact of stochastic forcing due to broadband combustion noise is included by additive white noise sources. Then, the acoustic and the flame system is interconnected, where thermoacoustic noncompactness due to the transversal modes' high frequency (HF) is accounted for by a distributed source term framework. The resulting nonlinear thermoacoustic system is solved in frequency and time domain. Linear growth rates predict linear stability, while envelope plots and probability density diagrams of the resulting pressure traces characterize the thermoacoustic performance of the combustor from a dynamical systems theory perspective. Comparisons against experimental data are conducted, which allow the rating of the flame modes in terms of their capability to reproduce the observed combustor dynamics. Ultimately, insight into the physics of high-frequency, transversal thermoacoustic systems is created.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(7):071504-071504-10. doi:10.1115/1.4035739.

Trapped vortex combustor (TVC) is a relatively new concept, having potential application in gas turbine engines. In this work, an attempt has been made to characterize the 2D twin cavity TVC experimentally in terms of its visible flame length, pollutant emission level, and exit temperature profile. Besides this, numerical results are also discussed to explain certain intricacies in flow and flame characteristics. Experimental results reveal that visible flame length value is sensitive to mainstream Reynolds number (Rems), primary (cavity) air velocity (Vp), and cavity equivalence ratio (Φc). For a particular Rems and Φc, an increase in Vp results in longer flame length; whereas, flame length gets shortened at higher mainstream Reynolds number cases. Numerical studies indicate that shortening of flame length at higher Rems cases is caused due to quenching of flame at the shear layer by the incoming flow. An attempt has been made to correlate flame length data with the operating parameters and Damkohler number (Da); Da takes care of flame quenching effects. Moreover, it is also brought out that the emission profile at the combustor exit is dependent on primary air velocity, mainstream Reynolds number, and cavity equivalence ratio. Emission studies indicate that higher primary air velocity cases make the carbon monoxide (CO) and unburned hydrocarbon (UHC) emission levels to lower values. Reduction in emission level is caused mainly due to the flame merging effects. Besides this, the influence of cavity flame merging on the exit temperature profile uniformity is also brought out. This study reveals that merging of cavity flames is essential for the optimized operation of a 2D trapped vortex combustor.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Heat Transfer

J. Eng. Gas Turbines Power. 2017;139(7):071901-071901-16. doi:10.1115/1.4035659.

The present study aims to investigate the heat transfer and pressure loss characteristics for multiple rows of jets impinging on a curved surface in the presence of rib turbulators. The target plate contains a straight section downstream of the impingement section. The rib turbulators are added only over the straight section, in an attempt to enhance the heat transfer while minimizing the pressure loss. The jet plate configuration in this study has fixed jet hole diameters and hole spacing. For the curved plate, the radius of the target plate is 32 times the diameter of the impingement holes. Impingement array configuration was chosen such that validation and comparison can be made with the open literature. For all the configurations, crossflow air is drawn out in the streamwise direction. Average jet Reynolds numbers ranging from 55,000 to 125,000 were tested. Heat transfer characteristics are measured using steady-state temperature-sensitive paint (TSP) to obtain local heat transfer distribution. The experimental results are compared with computational fluid dynamics (CFD) simulations. CFD results show that CFD simulations predict the heat transfer distribution well in the postimpingement area with turbulators.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2017;139(7):072501-072501-8. doi:10.1115/1.4035401.

An energy-based framework is developed for welded steel and AL6061-T6 for assessment of nonlinear evolution of fatigue damage accumulation along fatigue life. The framework involves interrogation at continuum using a newly developed experimental procedure to determine the cyclic damaging energy to reveal that the accumulated fatigue damage evolves nonlinearly along cycle in case of low cycle fatigue but has somewhat linear relationship with cycle in case of high cycle fatigue. The accumulated fatigue damage is defined as the ratio of the accumulated cyclic damaging energy to the fatigue toughness, a material property and hence remains the same at all applied stress ranges. Based on the experimental data, a model is developed in order to predict cyclic damaging energy history at any applied stress range. The predicted fatigue damage evolution from the energy-based model are found to agree well with the experimental data.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(7):072502-072502-9. doi:10.1115/1.4035594.

Impulse mistuning is an alternative approach for the reduction of vibration stresses of blades and vanes. In contrast to most other approaches, it is not a direct energy dissipation approach but a mistuning based one. However, the approach is not aimed at making use of the geometrical mistuning of the structure (e.g., a blade or a vane stage). Mistuners, specially designed small bodies are placed at specific locations inside of the component, e.g., of a blade or of a vane. They do not directly dissipate enough energy to cause relevant damping like a friction or friction-impact damper, because of the small mass involved, but rather mistune the eigen frequencies of the structure using impulses (impacts). As a result, the structure absorbs less energy at the original resonance and hence answers with lower vibration amplitude. In fact, impulse mistuning is a special case of absorption—the so-called targeted energy transfer (TET) with “vibro-impact nonlinear energy sinks” (VI-NES)—with very small impact mass involved, and thus, a negligible role of dissipation while experiencing a significant amount of absorption. The energy will be transferred (or “pumped”) to other resonances, sometimes outside of the primary resonance crossing and partially dissipated. We use the names “impulse mistuning” or “mistuners” instead of TET or VI-NES because (in our opinion) it better describes the physics of this special kind of absorption. In the paper, the design and validation of two impulse mistuning systems, for a blade stage and a vane cluster of a lower power turbine, are presented.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(7):072503-072503-8. doi:10.1115/1.4035601.

Over the past two decades, significant efforts have been made to introduce film riding sealing technology on large industrial or aerospace gas turbines. The main challenge comes from the high surface speeds and high temperatures, which lead to large thermal distortions. One approach to tackle the effect of thermally induced distortion is to design a seal to operate at a large film to limit the viscous heat generation. To design a seal pad that maximizes force at relatively high film heights, it is important to select the seal groove type that looks the most promising to deliver this characteristic. Several groove types have been assessed as part of this study. The most promising groove type is the Rayleigh step, which gives the strongest level of combined hydrostatic and hydrodynamic load support while also being easier to tessellate on individual seal segments. The results generated using a uniform grid Reynolds equation method show reasonable agreement with computational fluid dynamics (CFD) calculations. This provides confidence in the validity of the method, approach, and results.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(7):072504-072504-8. doi:10.1115/1.4035595.

Aero-engine gas turbine performance and efficiency can be improved through the application of compliant shaft seal types to certain sealing locations within the secondary air system. Leaf seals offer better performance than traditional labyrinth seals, giving lower leakage flows at design duties. However, for aero-engine applications, seal designs must be able to cope with relatively large off-design seal closures and closure uncertainties. The two-way coupling between temperatures of seal components and seal closures, through the frictional heat generated at the leaf–rotor interface when in contact, represents an important challenge for leaf seal analysis and design. This coupling can lead to leaf wear and loss, rotor overheating, and possibly to unstable sealing system behavior (thermal runaway). In this paper, we use computational fluid dynamics (CFD), finite element (FE) thermal analysis, and experimental data to characterize the thermal behavior of leaf seals. This sets the basis for a study of the coupled thermomechanical behavior. CFD is used to understand the fluid-mechanics of a leaf pack. The leaf seal tested at the Oxford Osney Laboratory is used for the study. Simulations for four seal axial Reynolds number are conducted; for each value of the Reynolds number, leaf tip-rotor contact, and clearance are considered. Distribution of mass flow within the leaf pack, distribution of heat transfer coefficient (HTC) at the leaf surface, and swirl velocity pick-up across the pack predicted using CFD are discussed. The experimental data obtained from the Oxford rig is used to develop a set of thermal boundary conditions for the leaf pack. An FE thermal model of the rig is devised, informed by the aforementioned CFD study. Four experiments are simulated; thermal boundary conditions are calibrated to match the predicted metal temperatures to those measured on the rig. A sensitivity analysis of the rotor temperature predictions to the heat transfer assumptions is carried out. The calibrated set of thermal boundary conditions is shown to accurately predict the measured rotor temperatures.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(7):072505-072505-7. doi:10.1115/1.4035625.

HAYNES® NS-163® alloy was developed by Haynes International Inc., Kokomo, IN, for high-temperature structural applications by pursuing a dual manufacturing approach: the fabrication of components in the readily weldable and formable mill-annealed condition, and their subsequent strengthening by means of a gas nitriding process.ff2 The latter process results in dispersion-strengthening by virtue of formation of internal nitrides. Since this process is diffusion-controlled, component section thicknesses are limited to approximately 2.0 mm (0.080 in.). Microstructures and mechanical properties of nitrided sheet samples are presented. Oxidation resistance and the need for coatings at temperatures exceeding 980 °C (1800 °F) are addressed as well.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(7):072506-072506-12. doi:10.1115/1.4035657.

In this paper, labyrinth seal leakage is numerically quantified for an acute trapezoidal rub-groove accompanied with a rounded tooth, as a function of rub-groove sizes and tooth-groove axial positions. Analyses parameters include clearance, pressure ratio, number of teeth, and rotor speed. Labyrinth seals wear during engine transients. Radial incursion and axial movement of the rotor–stator pair cause the labyrinth teeth to rub against the unworn stator surface. The labyrinth teeth and/or stator wear depending on their material hardness. Wear damage in the form of material loss or deformation permanently increases seal clearance, and thus, leakage. This leakage is known to be dependent on the shape and geometry of the worn tooth and the stator rub groove. There are two types of reported tooth tip wear. These can be approximated as a mushroom shape and a round shape. The stator rub-groove shapes can be approximately simulated in five forms: rectangle, trapezoid (isosceles and acute), triangle, and ellipse. In this paper, the acute trapezoidal rub-groove shape is specifically chosen, since it is the most similar to the most commonly observed rub-groove form. The tooth tip is considered to be rounded, because the tooth tip wears smoothly and a round shape forms during rub-groove formation. To compare the unworn tooth, the flat stator is also analyzed as a reference case. All analyzed parameters for geometric dimensions (groove width, depth, wall angle, and tooth-groove axial position) and operating conditions (flow direction, clearance, pressure ratio, number of teeth, and rotor speed) are analyzed in their practical ranges. Computational fluid dynamics (CFD) analyses are carried out by employing a compressible turbulent flow solver in a 2D axisymmetrical coordinate system. CFD analyses show that the rounded tooth leaks more than an unworn sharp-edged tooth, due to the formation of a smooth and streamlined flow around the rounded geometry. This smooth flow yields less flow separation, flow disturbance, and less of vena contracta effect. The geometric dimensions of the acute trapezoidal rub-groove (width, depth, wall angle) significantly affect leakage. The effects of clearance, pressure ratio, number of teeth, and rotor speed on the leakage are also quantified. Analyses results are separately evaluated for each parameter.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Turbomachinery

J. Eng. Gas Turbines Power. 2017;139(7):072601-072601-8. doi:10.1115/1.4035527.

The performance of the radial diffuser of a low pressure (LP) steam turbine is important to the power output of the turbine. A reliable and robust prediction and optimization tool is desirable in industry for preliminary design and performance evaluation. This is particularly critical during the tendering phase of retrofit projects, which typically cover a wide range of original equipment manufacturer and other original equipment manufacturers designs. This work describes a fast and reliable numerical approach for the simulation of flow in the last stage and radial diffuser coupled with the exhaust hood. The numerical solver is based on a streamline curvature throughflow method and a geometry-modification treatment has been developed for off-design conditions, at which large-scale flow separation may occur in the diffuser domain causing convergence difficulty. To take into account the effect of tip leakage jet flow, a boundary layer solver is coupled with the throughflow calculation to predict flow separation on the diffuser lip. The performance of the downstream exhaust hood is modeled by a hood loss model (HLM) that accounts for various loss generations along the flow paths. Furthermore, the solver is implemented in an optimization process. Both the diffuser lip and hub profiles can be quickly optimized, together or separately, to improve the design in the early tender phase. 3D computational fluid dynamics (CFD) simulations are used to validate the solver and the optimization process. The results show that the current method predicts the diffuser/exhaust hood performance within good agreement with the CFD calculation and the optimized diffuser profile improves the diffuser recovery over the datum design. The tool provides General Electric the capability to rapidly optimize and customize retrofit diffusers for each customer considering different constraints.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(7):072602-072602-10. doi:10.1115/1.4035593.

A high-order numerical method is employed to investigate flow in a rotor/stator cavity without heat transfer and buoyant flow in a rotor/rotor cavity. The numerical tool used employs a spectral element discretization in two dimensions and a Fourier expansion in the remaining direction, which is periodic and corresponds to the azimuthal coordinate in cylindrical coordinates. The spectral element approximation uses a Galerkin method to discretize the governing equations, but employs high-order polynomials within each element to obtain spectral accuracy. A second-order, semi-implicit, stiffly stable algorithm is used for the time discretization. Numerical results obtained for the rotor/stator cavity compare favorably with experimental results for Reynolds numbers up to Re1 = 106 in terms of velocities and Reynolds stresses. The buoyancy-driven flow is simulated using the Boussinesq approximation. Predictions are compared with previous computational and experimental results. Analysis of the present results shows close correspondence to natural convection in a gravitational field and consistency with experimentally observed flow structures in a water-filled rotating annulus. Predicted mean heat transfer levels are higher than the available measurements for an air-filled rotating annulus, but in agreement with correlations for natural convection under gravity.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(7):072603-072603-9. doi:10.1115/1.4035288.

At present, it is a common practice to expose engine components to main annulus air temperatures exceeding the thermal material limit in order to increase the overall engine performance and to minimize the engine specific fuel consumption. To prevent overheating of the materials and thus the reduction of component life, an internal flow system is required to cool and protect the critical engine parts. Previous studies have shown that the insertion of a deflector plate in turbine cavities leads to a more effective use of reduced cooling air, since the coolant is fed more effectively into the disk boundary layer. This paper describes a flexible design parameterization of an engine representative turbine stator well geometry with stationary deflector plate and its implementation within an automated design optimization process using automatic meshing and steady-state computational fluid dynamics (CFD). Special attention and effort is turned to the flexibility of the parameterization method in order to reduce the number of design variables to a minimum on the one hand, but increasing the design space flexibility and generality on the other. Finally, the optimized design is evaluated using a previously validated conjugate heat transfer method (by coupling a finite element analysis (FEA) to CFD) and compared against both the nonoptimized deflector design and a reference baseline design without a deflector plate.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(7):072604-072604-13. doi:10.1115/1.4035599.

Increased computing power has enabled designers to efficiently perform robust design analyses of engine systems. Traditional, filtered Monte Carlo methods involve creating surrogate model representations of a physics-based model in order to rapidly generate tens of thousands of model responses as design and technology input parameters are randomly varied within user-defined distributions. The downside to this approach is that the designer is often faced with a large design space, requiring significant postprocessing to arrive at probabilities of meeting design requirements. This research enhances the traditional, filtered Monte Carlo robust design approach by regressing surrogate responses of joint confidence intervals for metric responses of interest. Fitting surrogate responses of probabilistic confidence intervals rather than the raw response data changes the problem the engineer is able to answer. Using the new approach, the question can be better phrased in terms of the probability of meeting certain requirements. A more traditional approach does not have the ability to include confidence in the process without significant postprocessing. The process is demonstrated using a turboshaft engine modeled using the numerical propulsion system simulation (NPSS) program. The new robust design process enables the designer to account for probabilistic impacts of both technology and design variables, resulting in the selection of an engine cycle that is robust to requirements and technology uncertainty.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(7):072605-072605-8. doi:10.1115/1.4035626.

The subject of this paper is a statistical method for the evaluation of the uncertainties for pneumatic multihole probe measurements. The method can be applied to different types of evaluation algorithms and is suitable for steady flow-field measurements in compressible flows. The evaluation of uncertainties is performed by a Monte Carlo method (MCM). Each calibration and measurement input quantity are randomly varied on the basis of its corresponding probability density function (PDF) and propagated through the deterministic parameter evaluation algorithm. Other than linear Taylor series based uncertainty evaluation methods, the MCM features several advantages: it does not suffer from lower-order expansion errors and can therefore reproduce nonlinearity effects. Furthermore, different types of PDFs can be assumed for the input quantities, and the corresponding coverage intervals can be calculated for any coverage probability. To demonstrate the uncertainty evaluation, a calibration and subsequent measurements in the wake of an airfoil with a five-hole probe are performed. The MCM is applied to different parameter evaluation algorithms. It is found that the MCM cannot be applied to polynomial curve fits, if the differences between the calibration data and the polynomial curve fits are of the same order of magnitude compared to the calibration uncertainty. Since this method has not yet been used for the evaluation of measurement uncertainties for pneumatic multihole probes, the aim of this paper is to present a highly accurate and easy-to-implement uncertainty evaluation method.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(7):072606-072606-7. doi:10.1115/1.4035630.

In a turbine stage for a vehicular turbocharger or a pulse detonation engine (PDE) system, the power extraction process is inherently unsteady due to a highly pulsating flow delivered from the upstream combustor. Characterizing the operating performance of such a turbine stage would call for defining unsteady efficiency on a physically rigorous basis. Since the instantaneous efficiency can be calculated as the fraction of the actual power to the unsteady ideal power, an expression for the unsteady ideal power from the turbine stage is first derived by applying mass conservation and the first/second law of thermodynamics for the turbine stage. The newly derived expression elucidates the distinction from the quasi-steady situation in that the storage effect of mass/energy/entropy over the turbine stage is no longer negligible compared to the flux of mass/energy/entropy at the inlet and outlet. The storage effect resolves the previously reported physical inconsistency that the instantaneous efficiency can be a value of above unity or below zero; an erroneous result associated with defining the efficiency based on a quasi-steady basis. As the reduced frequency of the inlet pulsation of the turbine stage becomes larger than unity, the mass/energy/entropy accumulation rate over the turbine stage becomes significant compared to the mass/energy/entropy influx rate. Then, the definition of the efficiency based on a quasi-steady assumption loses its applicability. In this paper, the role of mass/energy/entropy storage rate in the unsteady ideal power is assessed in order to underpin the inconsistency in the previous quasi-steady approach. The utility of the unsteady efficiency definition is elucidated for the case of a turbocharger turbine stage subjected to high inlet flow pulsation.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Vehicular and Small Turbomachines

J. Eng. Gas Turbines Power. 2017;139(7):072701-072701-9. doi:10.1115/1.4035629.

Unsteady three-dimensional computations have been implemented on a turbocharger twin-scroll turbine system (volute–turbine wheel–diffuser). The flow unsteadiness in a turbocharger turbine system is essentially driven by a highly pulsating flow from the upstream combustor which causes a pulsating stagnation pressure boundary condition at the inlet to the turbine system. Computed results have been postprocessed and interrogated in depth in order to infer the significance of the induced flow unsteadiness on performance. The induced flow unsteadiness could be deemed important, since the reduced frequency of the turbine system (based on the time scale of the inlet flow fluctuation and the flow through time) is higher than unity. Thus, the computed time-accurate pressure field and the loss generation process have been assessed to establish the causal link to the induced flow unsteadiness in the turbine system. To do this consistently both for the individual subcomponents and the system, a framework of characterizing the operation of the turbine system linked to the fluctuating inlet stagnation pressure is proposed. The framework effectively categorizes the operation of the unsteady turbine system in both spatial and temporal dimensions; such a framework would facilitate determining whether the loss generation process in a subcomponent can be approximated as unsteady (e.g., volute) or as locally quasi-steady (LQS) (e.g., turbine wheel) in response to the unsteady inlet pulsation in the inlet-to-outlet stagnation pressure ratios of the two inlets. The notion that a specific subcomponent can be approximated as locally quasi-steady while the entire turbine system in itself is unsteady is of interest as it suggests a strategy for an appropriate flow modeling and scaling as well as for the turbine system performance improvement. Also, computed results are used to determine situations where the flow effects in a specific subcomponent can be approximated as quasi-one-dimensional; thus, for instance, the flow mechanisms in the volute can reasonably be approximated on an unsteady one-dimensional basis. For a turbine stage with sudden-expansion type diffuser, the framework for integrating subcomponent models into a turbine system is formulated. The effectiveness and generality of the proposed framework are demonstrated by applying it to three distinctly different turbocharger operating conditions. The estimated power from the integrated turbine system model is in good agreement with the full unsteady computational fluid dynamics (CFD) results for all three situations. The formulated framework will be generally applicable for assessing the new design configurations as long as the corresponding high-fidelity steady CFD results are utilized to determine the quasi-steady (or acoustically compact) behavior of each new subcomponent.

Commentary by Dr. Valentin Fuster

Research Papers: Internal Combustion Engines

J. Eng. Gas Turbines Power. 2017;139(7):072801-072801-12. doi:10.1115/1.4035600.

For an internal combustion engine, a large quantity of fuel energy (accounting for approximately 30% of the total combustion energy) is expelled through the exhaust without being converted into useful work. Various technologies including turbocompounding and the pressurized Brayton bottoming cycle have been developed to recover the exhaust heat and thus reduce the fuel consumption and CO2 emission. However, the application of these approaches in small automotive power plants has been relatively less explored because of the inherent difficulties, such as the detrimental backpressure and higher complexity imposed by the additional devices. Therefore, research has been conducted, in which modifications were made to the traditional arrangement aiming to minimize the weaknesses. The turbocharger of the baseline series turbocompounding was eliminated from the system so that the power turbine became the only heat recovery device on the exhaust side of the engine, and operated at a higher expansion ratio. The compressor was separated from the turbine shaft and mechanically connected to the engine via continuous variable transmission (CVT). According to the results, the backpressure of the novel system is significantly reduced comparing with the series turbocompounding model. The power output at lower engine speed was also promoted. For the pressurized Brayton bottoming cycle, rather than transferring the thermal energy from the exhaust to the working fluid, the exhaust gas was directly utilized as the working medium and was simply cooled by ambient coolant before the compressor. This arrangement, which is known as the inverted Brayton cycle (IBC) was simpler to implement. Besides, it allowed the exhaust gasses to be expanded below the ambient pressure. Thereby, the primary cycle was less compromised by the bottoming cycle. The potential of recovering energy from the exhaust was increased as well. This paper analyzed and optimized the parameters (including CVT ratio, turbine and compressor speed and the inlet pressure to the bottoming cycle) that are sensitive to the performance of the small vehicle engine equipped with inverted Brayton cycle and novel turbocompounding system, respectively. The performance evaluation was given in terms of brake power output and specific fuel consumption. Two working conditions, full and partial load (10 and 2 bar brake mean effective pressure (BMEP)) were investigated. Evaluation of the transient performance was also carried out. Simulated results of these two designs were compared with each other as well as the performance from the corresponding baseline models. The system models in this paper were built in GT-Power which is a one dimension (1D) engine simulation code. All the waste heat recovery systems were combined with a 2.0 L gasoline engine.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(7):072802-072802-8. doi:10.1115/1.4035627.

Intake port flow performance plays a substantial role in determining the volumetric efficiency and in-cylinder charge motion of a spark-ignited engine. Steady-state flow bench and motored engine flow computational fluid dynamics (CFD) simulations were carried out to bridge these two approaches for the evaluation of port flow and charge motion (such as discharge coefficient, swirl/tumble ratios (SR/TR)). The intake port polar velocity profile and polar physical clearance profile were generated to evaluate the port performance based on local flow velocity and physical clearance in the valve-seat region. The measured data were taken from standard steady-state flow bench tests of an intake port for validation of CFD simulations. It was reconfirmed that the predicted discharge coefficients and swirl/tumble index (SI/TI) of steady flow bench simulations have a good correlation with those of motored engine flow simulations. Polar velocity profile is strongly affected by polar physical clearance profile. The polar velocity inhomogeneity factor (IHF) correlates well with the port discharge coefficient, swirl/tumble index. Useful information can be extracted from local polar physical clearance and velocity, which can help for intake port design.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(7):072803-072803-10. doi:10.1115/1.4035628.

It has been observed that the swirl characteristics of in-cylinder air flow in a spark ignition direct injection (SIDI) engine affect the fuel spray dispersion and flame propagation speed, impacting the fuel mixture formation and combustion process under high swirl conditions. In addition, the cycle-to-cycle variations (CCVs) of swirl flow often degrade the air–fuel mixing and combustion quality in the cylinder. In this study, the 2D flow structure along a swirl plane at 30 mm below the injector tip was recorded using high-speed particle image velocimetry (PIV) in a four-valve optical SIDI engine under high swirl condition. Quadruple proper orthogonal decomposition (POD) was used to investigate the cycle-to-cycle variations of 200 consecutive cycles. The flow fields were analyzed by dividing the swirl plane into four zones along the measured swirl plane according to the positions of intake and exhaust valves in the cylinder head. Experimental results revealed that the coefficient of variation (COV) of the quadruple POD mode coefficients could be used to estimate the cycle-to-cycle variations at a specific crank angle. The dominant structure was represented by the first POD mode in which its kinetic energy could be correlated with the motions of the intake valves. Moreover, higher order flow variations were closely related to the flow stability at different zones. In summary, quadruple POD provides another meaningful way to understand the intake swirl impact on the cycle-to-cycle variations of the in-cylinder flow characteristics in SIDI engine.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(7):072804-072804-8. doi:10.1115/1.4035736.

Variable valve timing technologies for internal combustion engines are used to improve power, torque, and increase fuel efficiency. Details of a new solution are presented in this paper for optimizing valve motions of a full variable valve actuation (FVVA) system. The optimization is conducted at different speeds by varying full variable valve motion (variable exhaust open angle, intake close angle, velocity of opening and closing, overlap, dwell duration, and lift) parameters simultaneously; the final optimized valve motions of CY4102 diesel engine are given. The CY4102 diesel engine with standard cam drives is used in large quantities in Asia. An optimized electrohydraulic actuation motion used for the FVVA system is presented. The electrohydraulic actuation and optimized valve motions were applied to the CY4102 diesel engine and modeled using gt-power engine simulation software. Advantages in terms of volumetric efficiency, maximum power, brake efficiency, and fuel consumption are compared with baseline results. Simulation results show that brake power is improved between 12.8% and 19.5% and torque is improved by 10%. Brake thermal efficiency and volumetric efficiency also show improvement. Modeling and simulation results show significant advantages of the full variable valve motion over standard cam drives.

Commentary by Dr. Valentin Fuster

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