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

J. Eng. Gas Turbines Power. 2017;140(4):041501-041501-10. doi:10.1115/1.4038026.

The reduction of pollution and noise emissions of modern aero engines represents a key concept to meet the requirements of the future air traffic. This requires an improvement in the understanding of combustion noise and its sources, as well as the development of accurate predictive tools. This is the major goal of the current study where the low-order thermo-acoustic network (LOTAN) solver and a hybrid computational fluid dynamics/computational aeroacoustics approach are applied on a generic premixed and pressurized combustor to evaluate their capabilities for combustion noise predictions. LOTAN solves the linearized Euler equations (LEE) whereas the hybrid approach consists of Reynolds-averaged Navier–Stokes (RANS) mean flow and frequency-domain simulations based on linearized Navier–Stokes equations (LNSE). Both solvers are fed in turn by three different combustion noise source terms which are obtained from the application of a statistical noise model on the RANS simulations and a post-processing of incompressible and compressible large eddy simulations (LES). In this way, the influence of the source model and acoustic solver is identified. The numerical results are compared with experimental data. In general, good agreement with the experiment is found for both the LOTAN and LNSE solvers. The LES source models deliver better results than the statistical noise model with respect to the amplitude and shape of the heat release spectrum. Beyond this, it is demonstrated that the phase relation of the source term does not affect the noise spectrum. Finally, a second simulation based on the inhomogeneous Helmholtz equation indicates the minor importance of the aerodynamic mean flow on the broadband noise spectrum.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;140(4):041502-041502-11. doi:10.1115/1.4038038.

Owing to the increasing consumption of fossil fuels and emission of greenhouse gases, interests in highly efficient and low carbon emitting power systems are growing fast. Several research groups have been suggesting advanced systems based on fuel cells and have also been applying carbon capture and storage technology to satisfy the demand for clean energy. In this study, the performance of a hybrid system, which is a combination of a molten carbonate fuel cell (MCFC) with oxy-combustion carbon capture and an indirectly fired micro gas turbine (MGT), was predicted. A 2.5 MW MCFC system that is used in commercial applications was used as the reference system so that the results of the study could be applied to practical situations. The ambient pressure type hybrid system was modeled by referring to the design parameters of an MGT that is currently being developed. A semi-closed type design characterized by flow recirculation was adopted for this hybrid system. A part of the recirculating gas is converted into liquefied carbon dioxide and captured for storage at the carbon separation unit (CSU). Almost 100% carbon dioxide capture is possible with this system. In these systems, the output power of the fuel cell is larger than in the normal hybrid system without carbon capture because the partial pressure of carbon dioxide increases. The increased cell power partially compensates for the power loss due to the carbon capture and MGT power reduction. The dependence of net system efficiency of the oxy-hybrid on compressor pressure ratio is marginal, especially beyond an optimal value.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;140(4):041503-041503-10. doi:10.1115/1.4038039.

The precessing vortex core (PVC) represents a helical-shaped coherent flow structure typically occurring in both reacting and nonreacting swirling flows. Until now, the fundamental impact of the PVC on flame dynamics, thermoacoustic instabilities, and pollutant emissions is still unclear. In order to identify and investigate these mechanisms, the PVC needs to be controlled effectively with a feedback control system. A previous study successfully applied feedback control in a generic swirling jet setup. The next step is to transfer this approach into a swirl-stabilized combustor, which poses big challenges on the actuator and sensor design and placement. In this paper, different actuator designs are investigated with the goal of controlling the PVC dynamics. The actuation strategy aims to force the flow near the origin of the instability—the so-called wavemaker. To monitor the PVC dynamics, arrays of pressure sensors are flush-mounted at the combustor inlet and the combustion chamber walls. The best sensor placement is evaluated with respect to the prediction of the PVC dynamics. Particle image velocimetry (PIV) is used to evaluate the passive impact of the actuator shape on the mean flow field. The performance of each actuator design is evaluated from lock-in experiments showing excellent control authority for two out of seven actuators. All measurements are conducted at isothermal conditions in a prototype of a swirl-stabilized combustor.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;140(4):041504-041504-7. doi:10.1115/1.4038041.

A change in the combustion concept of gas turbines from conventional isobaric to constant volume combustion, such as in pulse detonation combustion (PDC), promises a significant increase in gas turbine efficiency. Current research focuses on the realization of reliable PDC operation and its challenging integration into a gas turbine. The topic of pollutant emissions from such systems has so far received very little attention. Few rare studies indicate that the extreme combustion conditions in PDC systems can lead to high emissions of nitrogen oxides (NOx). Therefore, it is essential already at this stage of development to begin working on primary measures for NOx emissions reduction if commercialization is to be feasible. The present study evaluates the potential of different primary methods for reducing NOx emissions produced during PDC of hydrogen. The considered primary methods involve utilization of lean combustion mixtures or its dilution by steam injection or exhaust gas recirculation. The influence of such measures on the detonability of the combustion mixture has been evaluated based on detonation cell sizes modeled with detailed chemistry. For the mixtures and operating conditions featuring promising detonability, NOx formation in the detonation wave has been simulated by solving the one-dimensional (1D) reacting Euler equations. The study enables an insight into the potential and limitations of considered measures for NOx emissions reduction and lays the groundwork for optimized operation of PDC systems.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;140(4):041505-041505-9. doi:10.1115/1.4038016.

The increasing amount of volatile renewable energy sources drives the necessity of flexible conventional power plants to compensate for fluctuations of the power supply. Gas turbines in a combined cycle power plant (CCPP) adjust the power output quickly but a sudden increase of CO and unburned hydrocarbons emissions limits their turn-down ratio. To extend the turn-down ratio, part of the fuel can be processed to syngas, which exerts a higher reactivity. An autothermal on-board syngas generator in combination with two different burner concepts for natural gas (NG) and syngas mixtures is presented in this study. A mixture of NG, water vapor, and air reacts catalytically in an autothermal reactor test rig to form syngas. At atmospheric pressure, the fuel processor generates syngas with a hydrogen content of −30 vol % and a temperature of 800 K within a residence time of 200 ms. One concept for the combustion of NG and syngas mixtures comprises a generic swirl stage with a central lance injector for the syngas. The second concept includes a central swirl stage with an outer ring of jets. The combustion is analyzed for both concepts by OH*-chemiluminescence, lean blow out (LBO) limit, and gaseous emissions. The central lance concept with syngas injection exhibits an LBO adiabatic flame temperature that is 150 K lower than in premixed NG operation. For the second concept, an extension of almost 200 K with low CO emission levels can be reached. This study shows that autothermal on-board syngas generation is feasible and efficient in terms of turn-down ratio extension and CO burn-out.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;140(4):041506-041506-9. doi:10.1115/1.4038042.

The dynamic characteristics of foil bearings operating at high rotation speeds depend very much on the mechanical characteristics of the foil structure. For this reason, the stiffness and damping of the structure of foil bearings are problems that are the focus of many analyses. The mechanical characteristics of the foil structure (top and bump foil) are analyzed either by using a simple approach obtained for an isolated bump modeled as a beam or with more elaborate ones taking into account the three-dimensional nature of the bumps and their mutual interactions. These two kinds of models give different foil structure stiffness, with lower values for the simplified model. However, the published experimental results of the foil bearing structure tend to validate the simplified model. The present paper explains the differences between the simplified and the elaborate models by taking into account the manufacturing errors of the foil structure. A three-dimensional model based on the nonlinear theory of elasticity is developed. The model offers a unique insight into the way the bearing structure deforms when the rotor is incrementally pushed into the foil structure. Three realistic manufacturing errors, bump height, bump length, and radius of the bump foil, are analyzed. Bump height and length vary following a normal distribution with a given standard deviation while the radius of the bump foil is given a waviness form with an imposed peak-to-peak amplitude. Three to five cases were calculated for each kind of error. Results show that only the manufacturing errors of the bump height affect the stiffness of the foil structure by diminishing its values. Height errors of 20 μm standard deviation (4% of the average bump height and 60% of the radial clearance) may induce a 40–50% reduction of the stiffness of the foil structure, i.e., in the range of the predictions of the simplified model.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;140(4):041507-041507-10. doi:10.1115/1.4038036.

This paper presents the experimental investigation of pulsation-amplitude-dependent flame dynamics associated with transverse thermoacoustic oscillations at screech level frequencies in a generic gas turbine combustor. Specifically, the flame behavior at different levels of pulsation amplitudes is assessed and interpreted. Spatial dynamics of the flame are measured by imaging the OH chemiluminescence (CL) signal synchronously to the dynamic pressure at the combustor's face plate. First, linear thermoacoustic stability states, modal dynamics, and flame-acoustic phase relations are evaluated. It is found that the unstable acoustic modes converge into a predominantly rotating character in the direction of the mean flow swirl. Furthermore, the flame modulation is observed to be in phase with the acoustic pressure at all levels of the oscillation amplitude. Second, distributed flame dynamics are investigated by means of visualizing the mean and oscillating heat release distribution at different pulsation amplitudes. The observed flame dynamics are then compared against numerical evaluations of the respective amplitude-dependent thermoacoustic growth rates, which are computed using analytical models in the fashion of a noncompact flame-describing function. While results show a nonlinear contribution for the individual growth rates, the superposition of flame deformation and displacement balances out to a constant flame driving. This latter observation contradicts the state-of-the-art perception of root-causes for limit-cycle oscillations in thermoacoustic gas turbine systems, for which the heat release saturates with increasing amplitudes. Consequently, the systematic observations and analysis of amplitude-dependent flame modulation shows alternative paths to the explanation of mechanisms that might cause thermoacoustic saturation in high frequency systems.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;140(4):041508-041508-10. doi:10.1115/1.4038126.

A model FLOX® combustor, featuring a single high momentum premixed jet flame, has been investigated using laser diagnostics in an optically accessible combustion chamber at a pressure of 8 bar. The model combustor was designed as a large single eccentric nozzle main burner (Ø 40 mm) together with an adjoining pilot burner and was operated with natural gas. To gain insight into the flame stabilization mechanisms with and without piloting, simultaneous particle image velocimetry (PIV) and OH laser-induced fluorescence (LIF) measurements have been performed at numerous two-dimensional (2D) sections of the flame. Additional OH-LIF measurements without PIV particles were analyzed quantitatively resulting in absolute OH concentrations and temperature fields. The flow field looks rather similar for both the unpiloted and the piloted cases, featuring a large recirculation zone next to the high momentum jet. However, flame shape and position change drastically. For the unpiloted case, the flame is lifted and widely distributed. Isolated flame kernels are found at the flame root in the vicinity of small-scale vortices. For the piloted flame, on the other hand, both pilot and main flame are attached to the burner base plate, and flame stabilization seems to take place on much smaller spatial scales with a connected flame front and no isolated flame kernels. The single-shot analysis gives rise to the assumption that for the unpiloted case, small-scale vortices act like the pilot burner flow in the opposed case and constantly impinge and ignite the high momentum jet at its root.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;140(4):041509-041509-9. doi:10.1115/1.4038079.

Nitric oxide (NO) produced during combustion will be present in vitiated air used in many devices. An experimental and modeling investigation of the effect of NO on the ignition of C1–C3 hydrocarbon fuels, namely, CH4, C2H4, C2H6, and C3H6, is presented. These molecules are important intermediate species generated during the decomposition of long-chain hydrocarbon fuel components typically present in jet fuels. Moreover, CH4 and C2H6 are major components of natural gas fuels. Although the interaction between NOx and CH4 has been studied extensively, limited experimental work is reported on C2H4, C2H6, and C3H6. As a continuation of previous work with C3H8, ignition delay time (IDT) measurements were obtained using a flow reactor facility with the alkanes (CH4 and C2H6) and olefins (C2H4 and C3H6) at 900 K and 950 K temperatures with 15 mole% and 21 mole% O2. Based on the experimental data, the overall effectiveness of NO in promoting ignition is found to be: CH4 > C3H6 > C3H8 > C2H6 > C2H4. A detailed kinetic mechanism is used for model predictions as well as for reaction path analysis. The reaction between HO2 and NO plays a critical role in promoting the ignition by generating the OH radical. In addition, various important fuel-dependent reaction pathways also promote the ignition. H-atom abstraction by NO2 has significant contribution to the ignition of C2H4 and C2H6, whereas the reaction between NO2 and allyl radical (aC3H5) is an important route for the ignition of C3H6.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;140(4):041510-041510-9. doi:10.1115/1.4038083.

Spectral distributions of the sound pressure level (SPL) observed in a premixed, swirl stabilized combustion test rig are scrutinized. Spectral peaks in the SPL for stable as well as unstable cases are interpreted with the help of a novel criterion for the resonance frequencies of the intrinsic thermo-acoustic (ITA) feedback loop. This criterion takes into the account the flow inertia of the burner and indicates that in the limit of very large flow inertia, ITA resonance should appear at frequencies where the phase of the flame transfer function (FTF) approaches $−π/2$. Conversely, in the limiting case of vanishing flow inertia, the new criterion agrees with previous results, which state that ITA modes may arise when the phase of the FTF is close to $−π$. Relying on the novel criterion, peaks in the SPL spectra are identified to correspond to either ITA or acoustic modes. Various combustor configurations are investigated over a range of operating conditions. It is found that in this particular combustor, ITA modes are prevalent and dominate the unstable cases. Remarkably, the ITA frequencies change significantly with the bulk flow velocity and the position of the swirler but are almost insensitive to changes in the length of the combustion chamber (CC). These observations imply that the resonance frequencies of the ITA feedback loop are governed by convective time scales. A scaling rule for ITA frequencies that relies on a model for the overall convective flame time lag shows good consistency for all operating conditions considered in this study.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;140(4):041511-041511-8. doi:10.1115/1.4038084.

In order to understand the reasons for the apparent benefits of using a flow-blurring (FB) atomizer in a combustion system, it is necessary to first examine fundamental spray characteristics under nonreacting conditions. Previous work on FB atomizers, however, has mostly involved only water and a relatively narrow range of parameters. In this study, a phase Doppler anemometry (PDA) instrument was used to characterize FB atomizer sprays and determine the effects of varying surface tension and viscosity of the liquid. Operating at room pressure and temperature (i.e., a “cold spray”), droplet sizes and velocities were measured for water, a water/surfactant mixture (lower surface tension), a water/glycerol mixture (higher viscosity), and glycerol (much higher viscosity). For all of the tested fluids, with the exception of pure glycerol, the FB atomizer produced small droplets (below 50 μm) whose size did not vary significantly in the radial or axial direction, particularly above a characteristic distance from the atomizer exit. Results show that the spray is essentially unaffected by a 4.5× decrease in surface tension or a 7× increase in viscosity, and that Sauter mean diameter (SMD) only increased by approximately a factor of three when substituting glycerol (750× higher viscosity) for water. The results suggest that the FB atomizer can effectively atomize a wide range of liquids, making it a useful fuel-flexible atomizer for combustion applications.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Cycle Innovations

J. Eng. Gas Turbines Power. 2017;140(4):041701-041701-11. doi:10.1115/1.4038082.

Carbon capture and storage could significantly reduce carbon dioxide (CO2) emissions. One of the major limitations of this technology is the energy penalty for the compression of CO2 to supercritical conditions. To reduce the power requirements, supercritical carbon dioxide compressors must operate near saturation where phase change effects are important. Nonequilibrium condensation can occur at the leading edge of the compressor, causing performance and stability issues. The characterization of the fluid at these conditions is vital to enable advanced compressor designs at enhanced efficiency levels but the analysis is challenging due to the lack of data on metastable fluid properties. In this paper, we assess the behavior and nucleation characteristics of high-pressure subcooled CO2 during the expansion in a de Laval nozzle. The assessment is conducted with numerical calculations and corroborated by experimental measurements. The Wilson line is determined via optical measurements in the range of 41–82 bar. The state of the metastable fluid is characterized through pressure and density measurements, with the latter obtained in a first-of-its-kind laser interferometry setup. The inlet conditions of the nozzle are moved close to the critical point to allow for reduced margins to condensation. The analysis suggests that direct extrapolation using the Span and Wagner equation of state (S–W EOS) model yields results within 2% of the experimental data. The results are applied to define inlet conditions for a supercritical carbon dioxide compressor. Full-scale compressor experiments demonstrate that the reduced inlet temperature can decrease the shaft power input by 16%.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2017;140(4):042501-042501-11. doi:10.1115/1.4038040.

Slender turbine blades are susceptible to excitation. Resulting vibrations stress the blade's fixture to the rotor or stator. In this paper, high cycle fatigue at the edge of contact (EOC) between blade and rotor/stator of such fixtures is investigated both experimentally and numerically. Plasticity in the contact zone and its effects on, e.g., contact tractions, fatigue determinative quantities, and fatigue itself are shown to be of considerable relevance. The accuracy of the finite element analysis (FEA) is demonstrated by comparing the predicted utilizations and slip region widths with data gained from tests. For the evaluation of EOC fatigue, tests on simple notched specimens provide the limit data. Predictions on the utilization are made for the EOC of a dovetail setup. Tests with this setup provide the experimental fatigue limit to be compared to. The comparisons carried out show a good agreement between the experimental results and the plasticity-based calculations of the demonstrated approach.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;140(4):042502-042502-11. doi:10.1115/1.4037872.

This study presents the dynamic motion of a ball bearing cage submerged in a cryogenic fluid under high-speed conditions. The dynamic motion of the cage has been studied as a function of the race land-cage and ball-cage pocket clearances for different inner race rotation speeds under light load conditions. In addition, this study conducted computational fluid dynamics (CFD) analysis using commercial software to analyze the fluid dynamic forces on the cage. The hydraulic force obtained from the CFD analysis was coded in commercial ball bearing analysis software as a function of the eccentricity ratio and rotation speed of the cage. Finally, the dynamic motion of the ball bearing cage considering the effects of fluid dynamic forces has been studied. The results include the cage whirling amplitude, fluctuation of cage whirling speed, and cage wear for various cage clearances and rotation speeds. The cage whirling amplitude decreases as the outer guidance clearance decreases, and it decreases as the rotation speed increases up to 11,000 rpm because of the increasing hydrodynamic force of the liquid nitrogen (LN2). However, the probability density function curves indicate that an increase in the rotor speed increases the standard deviation in the cage whirling frequency. The wear loss of the cage was greatest for the largest race land-cage and the smallest ball-cage pocket clearances. Consequently, the analysis results for various operating conditions (inner race rotation speeds, cage clearances, traction coefficients, etc.) are in good agreement with the reference results.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;140(4):042503-042503-16. doi:10.1115/1.4038081.

Annular gas seals for compressors and turbines are designed to operate in a nominally centered position in which the rotor and stator are at concentric condition, but due to the rotor–stator misalignment or flexible rotor deflection, many seals usually are suffering from high eccentricity. The centering force (represented by static stiffness) of an annular gas seal at eccentricity plays a pronounced effect on the rotordynamic and static stability behavior of rotating machines. The paper deals with the leakage and static stability behavior of a fully partitioned pocket damper seal (FPDS) at high eccentricity ratios. The present work introduces a novel mesh generation method for the full 360 deg mesh of annular gas seals with eccentric rotor, based on the mesh deformation technique. The leakage flow rates, static fluid-induced response forces, and static stiffness coefficients were solved for the FPDS at high eccentricity ratios, using the steady Reynolds-averaged Navier–Stokes solution approach. The calculations were performed at typical operating conditions including seven rotor eccentricity ratios up to 0.9 for four rotational speeds (0 rpm, 7000 rpm, 11,000 rpm, and 15,000 rpm) including the nonrotating condition, three pressure ratios (0.17, 0.35, and 0.50) including the choked exit flow condition, two inlet preswirl velocities (0 m/s, 60 m/s). The numerical method was validated by comparisons to the experiment data of static stiffness coefficients at choked exit flow conditions. The static direct and cross-coupling stiffness coefficients are in reasonable agreement with the experiment data. An interesting observation stemming from these numerical results is that the FPDS has a positive direct stiffness as long as it operates at subsonic exit flow conditions; no matter the eccentricity ratio and rotational speed are high or low. For the choked exit condition, the FPDS shows negative direct stiffness at low eccentricity ratio and then crosses over to positive value at the crossover eccentricity ratio (0.5–0.7) following a trend indicative of a parabola. Therefore, the negative static direct stiffness is limited to the specific operating conditions: choked exit flow condition and low eccentricity ratio less than the crossover eccentricity ratio, where the pocket damper seal (PDS) would be statically unstable.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Turbomachinery

J. Eng. Gas Turbines Power. 2017;140(4):042601-042601-15. doi:10.1115/1.4038076.

A turbocharger is a key enabler for lowering CO2 emission of an internal combustion engine (ICE) through the reutilization of the exhaust gas energy that would otherwise have been released to the ambient. In its actual operating conditions, a turbocharger turbine operates under highly pulsating flow due to the reciprocating nature of the ICE. Despite this, the turbocharger turbines are still designed using the standard steady-state approach due to the lack of understanding of the complex unsteady pressure and mass propagation within the stage. The application of guide vanes in a turbocharger turbine stage has increased the complexity of flow interactions regardless of whether the vanes are fixed or variable. Although it is enticing to assume that the performance of the vaned turbine is better than the one without (vaneless), there are currently no tangible evidences to support this claim, particularly during the actual pulsating flow operations. Therefore, this research looks into comparing the differences between the two turbine arrangements in terms of their performance at flow field level. For this purpose, a three-dimensional (3D) “full-stage” unsteady turbine computational fluid dynamics (CFD) models for both volutes are constructed and validated against the experimental data. These models are subject to identical instantaneous inlet pressure profile of 60 Hz, which is equivalent to an actual three-cylinder four-stroke engine rotating at 2400 rpm. A similar 95.14 mm diameter mixed-flow turbine rotor rotating at 48,000 rpm is used for both models to enable direct comparison. The complete validation exercises for both steady and unsteady flow conditions are also presented. Results have indicated that neither vaned nor vaneless turbine is capable of maintaining constant efficiency throughout the pulse cycle. Despite that, the vaneless turbine indicated better performance during peak power instances. This work also showed that the pulsating pressure at the turbine inlet affected the vaned and vaneless turbines differently at the flow field level. Furthermore, results also indicated that both the turbines matched its optimum incidence angle for only a fraction of pulse cycle, which is unfavorable.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;140(4):042602-042602-12. doi:10.1115/1.4038021.

Engine downsizing is a modern solution for the reduction of CO2 emissions from internal combustion engines. This technology has been gaining increasing attention from industry. In order to enable a downsized engine to operate properly at low speed conditions, it is essential to have a compressor stage with very good surge margin. The ported shroud, also known as the casing treatment, is a conventional way used in turbochargers to widen the working range. However, the ported shroud works effectively only at pressure ratios higher than 3:1. At lower pressure ratio, its advantages for surge margin enhancements are very limited. The variable inlet guide vanes are also a solution to this problem. By adjusting the setting angles of variable inlet guide vanes, it is possible to shift the compressor map toward the smaller flow rates. However, this would also undermine the stage efficiency, require extra space for installing the inlet guide vanes, and add costs. The best solution is therefore to improve the design of impeller blade itself to attain high aerodynamic performances and wide operating ranges. This paper reports a recent study of using inverse design method for the redesign of a centrifugal compressor stage used in an electric supercharger, including the impeller blade and volute. The main requirements were to substantially increase the stable operating range of the compressor in order to meet the demands of the downsized engine. The three-dimensional (3D) inverse design method was used to optimize the impeller geometry and achieve higher efficiency and stable operating range. The predicted performance map shows great advantages when compared with the existing design. To validate the computational fluid dynamics (CFD) results, this new compressor stage has also been prototyped and tested. It will be shown that the CFD predictions have very good agreement with experiments and the redesigned compressor stage has improved the pressure ratio, aerodynamic efficiency, choke, and surge margins considerably.

Commentary by Dr. Valentin Fuster

### Research Papers: Internal Combustion Engines

J. Eng. Gas Turbines Power. 2017;140(4):042801-042801-13. doi:10.1115/1.4038015.

Light-end fuels have recently garnered interest as potential fuel for advanced compression ignition (CI) engines. This next generation of engines, which aim to combine the high efficiency of diesel engines with the relative simplicity of gasoline engines, may allow engine manufacturers to continue improving efficiency and reducing emissions without a large increase in engine and aftertreatment system complexity. In this work, a 1D heavy-duty engine model was validated with measured data and then used to generate boundary conditions for the detailed chemical kinetic simulation corresponding to various combustion modes and operating points. Using these boundary conditions, homogeneous simulations were conducted for 242 fuels with research octane number (RON) from 40 to 100 and sensitivity (S) from 0 to 12. Combustion phasing (CA50) was most dependent on RON and less dependent on S under all conditions. Both RON and S had a greater effect on combustion phasing under partially premixed compression ignition (PPCI) conditions (19.3 deg) than under mixing-controlled combustion (MCC) conditions (5.8 deg). The effect of RON and S were also greatest for the lowest reactivity (RON > 90) fuels and under low-load conditions. The results for CA50 reflect the relative ignition delay for the various fuels at the start-of-injection (SOI) temperature. At higher SOI temperatures (>950K), CA50 was found to be less dependent on fuel sensitivity due to the convergence of ignition delay behavior of different fuels in the high-temperature region. Combustion of light-end fuels in CI engines can be an important opportunity for regulators, consumers, and engine-makers alike. However, selection of the right fuel specifications will be critical in development of the combustion strategy. This work, therefore, provides a first look at quantifying the effect of light-end fuel chemistry on advanced CI engine combustion across the entire light-end fuel reactivity space and provides a comparison of the trends for different combustion modes.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;140(4):042802-042802-11. doi:10.1115/1.4038078.

Particulate matters (PM) accumulation through a low-pressure exhaust gas recirculation (LP-EGR) path may hinder to obtain the desired LP-EGR rate and thus causes an increase of nitrogen oxides (NOx). The degree of lack of the LP-EGR rate should be detected, i.e., an LP-EGR fault, and a remedy to compensate for the lack of LP-EGR rate should be a mandate to suppress NOx emission, i.e., a fault management. In order to accomplish those objectives, this paper proposes an LP-EGR fault management system, which consists of a fault diagnosis algorithm, fault-tolerant control algorithm, and an LP-EGR rate model. The model applies a combustion parameter derived from in-cylinder pressure information to the conventional orifice valve model. Consequently, the LP-EGR rate estimation was improved to the maximum error of 2.38% and root-mean-square-error (RMSE) of 1.34% at various operating conditions even under the fault condition compared to that of the conventional model with the maximum error of 7.46% and RMSE of 5.39%. Using this LP-EGR rate model as a virtual sensor, the fault diagnosis algorithm determines an LP-EGR fault state. Based on the state, the fault-tolerant control determines whether or not to generate the offset of the exhaust throttle valve (ETV) position. This offset combines with the look-up table (LUT)-based feedforward controller to control an LP-EGR rate. As a result of real-time verification of the fault management system in the fault condition, the NOx emission decreased by up to about 15%.

Commentary by Dr. Valentin Fuster