Research Papers

J. Eng. Gas Turbines Power. 2018;140(12):121001-121001-13. doi:10.1115/1.4040682.

Biodiesel engines are found to have improved soot, hydrocarbon (HC), and carbon monoxide (CO) emissions, with modestly increased nitrogen oxides (NOx) emissions. Exhaust gas recirculation (EGR) could be used for the NOx emissions control, especially in the fuel-kinetics-dominated engine combustion concepts. A detailed chemical kinetic model of methyl decanoate (MD), a biodiesel surrogate fuel, was used here to simulate the two-stage auto-ignition process of biodiesel with EGR addition. The effects of EGR constituents, including carbon dioxide (CO2), water vapor (H2O), CO and H2, were identified in a constant-pressure ignition process and in a variable pressure, variable volume process. Firstly, numerical methods were used to isolate the dilution, thermal, and chemical effects of CO2 and H2O at a constant pressure. It was found that in the biodiesel auto-ignition processes, the dilution effects of CO2 and H2O always played the primary role. Their thermal and chemical effects mainly influenced the second-stage ignition, and the chemical effect of H2O was more significant than CO2. The triple effects of CO and H2 were also analyzed at the same temperature and pressure conditions. Additionally, the sensitivity analysis and reaction pathway analysis were conducted to elucidate the chemical effects of CO and H2 on the ignition processes at different temperatures. Finally, based on a variable pressure, variable volume model simulating the engine compression stroke, the effects of CO2, H2O, CO and H2 addition under the engine operational conditions were studied and compared to those under the constant pressure conditions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(12):121002-121002-10. doi:10.1115/1.4040770.

This work describes a methodology used for counter-rotating (CR) propellers performance estimation. The method is implemented in an in-house program for gas turbine performance prediction, making possible the simulation of the counter-rotating open rotor (CROR) architecture. The methodology is used together with a variable geometry compressor control strategy to avoid surge conditions. Two cases are simulated under transient operation for both fixed and variable geometry compressor. The influence of the variable geometry control on the transient performance of CROR engines is evaluated and a comprehensive understanding on the transient behavior of this type of engine could be obtained. It is shown that the use of the variable geometry compressor control does not significantly affect the overall engine performance, while avoiding the surge conditions, thus ensuring the engine operation safety.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(12):121003-121003-8. doi:10.1115/1.4040711.

A numerical second law analysis is performed to determine the entropy production due to irreversibilities in condensing steam flows. In the present work, the classical approach to calculate entropy production rates in turbulent flows based on velocity and temperature gradients is extended to two-phase condensing flows modeled within an Eulerian–Eulerian framework. This requires some modifications of the general approach and the inclusion of additional models to account for thermodynamic and kinematic relaxation processes. With this approach, the entropy production within each mesh element is obtained. In addition to the quantification of thermodynamic and kinematic wetness losses, a breakdown of aerodynamic losses is possible to allow for a detailed loss analysis. The aerodynamic losses are classified into wake mixing, boundary layer, and shock losses. The application of the method is demonstrated by means of the flow through a well-known steam turbine cascade test case. Predicted variations of loss coefficients for different operating conditions can be confirmed by experimental observations. For the investigated test cases, the thermodynamic relaxation contributes the most to the total losses and the losses due to droplet inertia are only of minor importance. The variation of the predicted aerodynamic losses for different operating conditions is as expected and demonstrates the suitability of the approach.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2018;140(12):121501-121501-10. doi:10.1115/1.4040090.

The present paper represents a small piece of an extensive experimental effort investigating the dual-fuel operation of a light-duty spark ignited engine. Natural gas (NG) was directly injected into the cylinder and gasoline was injected into the intake-port. Direct injection (DI) of NG was used in order to overcome the power density loss usually experienced with NG port-fuel injection (PFI) as it allows an injection after intake valve closing. Having two separate fuel systems allows for a continuum of in-cylinder blend levels from pure gasoline to pure NG operation. The huge benefit of gasoline is its availability and energy density, whereas NG allows efficient operation at high load due to improved combustion phasing enabled by its higher knock resistance. Furthermore, using NG allowed a reduction of carbon dioxide emissions across the entire engine map due to the higher hydrogen-to-carbon ratio. Exhaust gas recirculation (EGR) was used to (a) increase efficiency at low and part-load operation and (b) reduce the propensity of knock at higher compression ratios (CRs) thereby enabling blend levels with greater amount of gasoline across a wider operating range. Two integral engine parameters, CR and in-cylinder turbulence levels, were varied in order to study their influence on efficiency, emissions, and performance over a specific speed and load range. Increasing the CR from 10.5 to 14.5 allowed an absolute increase in indicated thermal efficiency of more than 3% for 75% NG (25% gasoline) operation at 8 bar net indicated mean effective pressure (IMEP) and 2500 rpm. However, as anticipated, the achievable peak load at CR 14.5 with 100% gasoline was greatly reduced due to its lower knock resistance. The in-cylinder turbulence level was varied by means of tumble plates (TPs) as well as an insert for the NG injector that guides the injection “spray” to augment the tumble motion. The usage of TPs showed a significant increase in EGR dilution tolerance for pure gasoline operation, however, no such impact was found for blended operation of gasoline and NG.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(12):121502-121502-11. doi:10.1115/1.4040175.

Pilot flames have been widely used for flame stabilization in low-emission gas turbine combustors. Effects of pilot flame on dynamic instabilities, however, are not well understood. In this work, the dynamic interactions between main and pilot flames are studied by perturbing both flames simultaneously, i.e., with a dual-input forcing. A burner is used to generate a premixed axisymmetric V-shaped methane flame stabilized by a central pilot flame. Servo valve and sirens are used to produce forcing in the pilot and main flames, respectively. A diagnostic system is applied to measure the flame structure and heat release rate. The effects of forcing frequency, forcing amplitude, phase difference between the two forcing signals as well as the Reynolds number are studied. Both the flame transfer function (FTF) and the flame dynamic position are measured and analyzed. It is found that the total flame response can be modified by the perturbation in the pilot flame. The mechanism can be attributed to the effect of pilot flame on the velocity field of the burnt side. Vortex is found to be able to amplify the pilot–main dynamic interactions under certain conditions. An analytical model is developed based on the linearized G-equation, to further understand the flame interactions through the velocity perturbations in the burnt side. Good agreements were found between the prediction and the experiment results.

Topics: Flames
Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(12):121503-121503-18. doi:10.1115/1.4040659.

Turbulent spray combustion of n-dodecane was modeled at relevant engine conditions using two combustion models (direct integration of chemistry (DIC) and flamelet generated manifolds (FGM)) and multifidelity turbulence models (dynamic structure large eddy simulation (LES) and renormalization group (RNG) Reynolds-averaged Naiver–Stokes (RANS)). The main objective of this work is to study the effect of various combustion and turbulence models on spray behavior and quantify these effects. To reach these objectives, a recently developed kinetic mechanism and well-established spray models were utilized for the three-dimensional turbulent spray simulation at various combustion chamber initial gas temperature and pressure conditions. Fine mesh with a size of 31 μm was utilized to resolve small eddies in the periphery of the spray. In addition, a new methodology for mesh generation was proposed and investigated to simulate the measured data fluctuation in the CFD domain. The pressure-based ignition delay, flame lift-off length (LOL), species concentrations, spray, and jet penetrations were modeled and compared with measured data. Differences were observed between various combustion and turbulence models in predicting the spray characteristics. However, these differences are within the uncertainties, error, and variations of the measured data.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(12):121504-121504-10. doi:10.1115/1.4040737.

The work in this paper investigates on how a fuel flexible microgas turbine (MGT) combustion chamber, developed by ANSALDO ENERGIA and installed in a Turbec T100 P MGT, can operate when transferring from natural gas (NG) to a hydrogen-rich syngas. A syngas composition, which satisfies the fuel supply system specifications, is identified. Such syngas contains (by volume) 45% of hydrogen, 50% of carbon dioxide, and 5% of methane. The transfer procedure from NG to syngas is defined and modeled. A series of nonreactive and reactive Reynolds-averaged numerical simulations (RANS) on a full-scale three-dimensional (3D) model of the combustion chamber is then performed. The thermo-fluid dynamics inside its casing, the combustion regimes, the heat transfer across the liner walls as well as NOx emissions are evaluated. Results provide useful information on the operational problems associated with the fuel change and on how to define a successful fuel transfer procedure.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(12):121505-121505-11. doi:10.1115/1.4040765.

The side-wall cooling liner in a gas turbine combustor serves main purposes—heat transfer and emission control. Additionally, it functions as a passive damper to attenuate thermoacoustic instabilities. The perforations in the liner mainly convert acoustic energy into kinetic energy through vortex shedding at the orifice rims. In the previous decades, several analytical and semi-empirical models have been proposed to predict the acoustic damping of the perforated liner. In the current study, a few of the models are considered to embody the transfer matrix method (TMM) for analyzing the acoustic dissipation in a concentric tube resonator with a perforated element and validated against experimental data in the literature. All models are shown to quantitatively appropriately predict the acoustic behavior under high bias flow velocity conditions. Then, the models are applied to maximize the damping performance in a realistic gas turbine combustor, which is under development. It is found that the ratio of the bias flow Mach number to the porosity can be used as a design guideline in choosing the optimal combination of the number and diameter of perforations in terms of acoustic damping.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(12):121506-121506-12. doi:10.1115/1.4040516.

Large eddy simulation (LES) with three-dimensional conditional moment closure (CMC) subgrid model for combustion is applied to simulate a swirl-stabilized nonpremixed methane flame with localized extinction, with special focus on the effects of heat loss to the burner surface. The convective wall heat loss is modeled through introducing a source term in the conditionally filtered total enthalpy equation for the CMC cells adjacent to the wall. The mean heat flux is high on the middle surface of the bluff body, but relatively low near its edges. The turbulent heat flux based on the gradient of the resolved temperature is relatively low compared to the laminar counterpart, but increases with the turbulent intensity. The heat loss facilitates the occurrences of extinction and re-ignition for the CMC cells immediately adjacent to the wall, evidenced by comparing flame structures in the near-wall CMC cells. This can be directly linked to the increase of the mean conditional scalar dissipation near the wall in the heat loss case. Furthermore, the degree of local extinction near the bluff body measured by conditional reactedness at stoichiometry is intensified due to the wall heat loss. However, the results also show that there is negligible influence of wall heat loss on the probability density function (PDF) of the lift-off height, demonstrating the dominance of aerodynamic effects on flame stabilization. The results are in reasonable agreement with experimental measurements.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(12):121507-121507-7. doi:10.1115/1.4038766.

Fractal analysis is undertaken to characterize flame surface fluctuations on an unconfined turbulent premixed flame and the resulting far-field acoustics fluctuations. Results indicate that combustion noise is monofractal and is characterized by an anticorrelated structure with a Hurst exponent less than 0.5. The anticorrelated nature was identified in the pressure fluctuations as well as flame surface fluctuations for small time-scales. Additionally, results suggest that flame surface fluctuations are multifractal for large time scales. The calculated Hurst exponent increases noticeably with the equivalence ratio and decreases slightly with Reynolds number for the investigated operating conditions. Variation in the Hurst exponent for combustion noise data is compared with a case study of synthetic fluctuations comprised of linear combinations of white and 1/f2 noise. These results provide a more detailed characterization of the temporal structure of flame surface fluctuations and resulting noise emission from turbulent premixed flames than is presently known.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2018;140(12):122501-122501-11. doi:10.1115/1.4040418.

Limaçon machine, of which the relative motion between the rotor and housing follows the limaçon curve, belongs to a class of rotary positive displacement machines. The profiles of rotors and housings of those machines can be constructed of either limaçon or circular curves, hence the names: limaçon-to-limaçon, circolimaçon, and limaçon-to-circular machines. This paper presents the investigation into the thermodynamic performance of the limaçon-to-circular machines with the presence of apex seals and inlet valve. This paper sets out by briefly introducing the limaçon technology and the construction of the limaçon-to-circular machine working volume. The mathematical descriptions of ports' positions and areas have also been introduced. The paper then discusses the flow and phase composition of working fluid through the working chambers as well as how the fluid velocity is modeled and calculated. Then the seal dynamic model and response of inlet valve are presented followed by the machine thermodynamic model. A case study has also been presented to show the responses of seals and inlet valve during the machine operation.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(12):122502-122502-10. doi:10.1115/1.4040813.

Reduced oil supply flow rates in fluid film bearings can cause cavitation, or lack of a fully developed film layer, over one or more of the pads due to starvation. Reduced oil flow has the well-documented effects of higher bearing operating temperatures and decreased power losses; however, little experimental data are available on its effects on system stability and dynamic performance. The study looks at the effects of oil supply flow rate on dynamic bearing performance by comparing experimentally identified damped natural frequencies and damping ratios to predictive models. A test rig consisting of a flexible rotor and supported by two tilting pad bearings in flooded housings is utilized in this study. Tests are conducted over a range of supercritical operating speeds and bearing loads while systematically reducing the oil supply flow rates provided to the bearings. Shaft response measured as a magnetic actuator is used to perform sine sweep excitations of the rotor. A single-input, multiple-output system identification technique is then used to obtain frequency response functions (FRFs) and modal parameters. All experimental results are compared to predicted results obtained from bearing models based on thermoelastohydrodynamic (TEHD) lubrication theory. Both flooded and starved model flow assumptions are considered and compared to the data. Differences in the predicted trends of the models and the experimental data across varying operating conditions are examined. Predicted pressure profiles and dynamic coefficients from the models are presented to help explain any differences in trends.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(12):122503-122503-9. doi:10.1115/1.4040767.

This paper presents the methodology and results of the optimization of a straight-through labyrinth seal with two inclined fins against smooth-land. The optimization was performed using commercial tools implemented in the ANSYS environment, such as goal-driven optimization. The response surfaces were created based on Latin hypercube samples found from computational fluid dynamics (CFD) calculations. The CFD solver, using a steady-state scheme with the k–ω shear stress transport (SST) turbulence model, was applied. A screening algorithm was used to find the best candidates on the response surfaces. The objective function adopted in the labyrinth seal optimization was the minimization of the discharge coefficient value. A wide range of parameters of the fins position and shape were taken into account, with physically justified degrees-of-freedom. The optimization results were supported by the results of an in-house experiment performed on a stationary, linear test rig. The test rig was fed by a high-capacity vacuum air blower with high-precision hot-wire anemometry mass flow evaluation. The reductions in the leakage significantly exceed the uncertainties of the CFD model and the test rig accuracy. The factors that had the most substantial impact on the leakage reduction were the location, inclination, and thickness of the fins. The experimental results were compared with the calculations and the optimization effects, highlighting some tendencies in the labyrinth seal flow behavior. Good agreement was obtained between the optimization results and the experimental data, proving that the presented methodology is sufficient for the labyrinth seal optimization.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Turbomachinery

J. Eng. Gas Turbines Power. 2018;140(12):122601-122601-9. doi:10.1115/1.4039935.

The evolution of the wake of a wind turbine contributes significantly to its operation and performance, as well as to those of machines installed in the vicinity. The inherent unsteady and three-dimensional (3D) aerodynamics of vertical axis wind turbines (VAWT) have hitherto limited the research on wake evolution. In this paper, the wakes of both a troposkien and a H-type VAWT rotor are investigated by comparing experiments and calculations. Experiments were carried out in the large-scale wind tunnel of the Politecnico di Milano, where unsteady velocity measurements in the wake were performed by means of hot wire anemometry. The geometry of the rotors was reconstructed in the open-source wind-turbine software QBlade, developed at the TU Berlin. The aerodynamic model makes use of a lifting line free-vortex wake (LLFVW) formulation, including an adapted Beddoes-Leishman unsteady aerodynamic model; airfoil polars are introduced to assign sectional lift and drag coefficients. A wake sensitivity analysis was carried out to maximize the reliability of wake predictions. The calculations are shown to reproduce several wake features observed in the experiments, including blade-tip vortex, dominant and minor vortical structures, and periodic unsteadiness caused by sectional dynamic stall. The experimental assessment of the simulations illustrates that the LLFVW model is capable of predicting the unsteady wake development with very limited computational cost, thus making the model ideal for the design and optimization of VAWTs.

Topics: Wakes , Blades , Rotors , Vortices , Turbines
Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(12):122602-122602-9. doi:10.1115/1.4040285.

Steady-state stagnation temperature probes are used during gas turbine engine testing as a means of characterizing turbomachinery component performance. The probes are located in the high-velocity gas-path, where temperature recovery and heat transfer effects cause a shortfall between the measured temperature and the flow stagnation temperature. To improve accuracy, the measurement shortfall is corrected post-test using data acquired at representative Mach numbers in a steady aerodynamic calibration facility. However, probes installed in engines are typically subject to unsteady flows, which are characterized by periodic variations in Mach number and temperature caused by the wakes shed from upstream blades. The present work examines the impact of this periodic unsteadiness on stagnation temperature measurements by translating probes between jets with dissimilar Mach numbers. For conventional Kiel probes in unsteady flows, a greater temperature measurement shortfall is recorded compared to equivalent steady flows, which is related to greater conductive heat loss from the temperature sensor. This result is important for the application of post-test corrections, since an incorrect value will be applied using steady calibration data. A new probe design with low susceptibility to conductive heat losses is therefore developed, which is shown to deliver the same performance in both steady and unsteady flows. Measurements from this device can successfully be corrected using steady aerodynamic calibration data, resulting in improved stagnation temperature accuracy compared to conventional probe designs. This is essential for resolving in-engine component performance to better than ±0.5% across all component pressure ratios.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(12):122603-122603-11. doi:10.1115/1.4040681.

A volute is one of the key components in a centrifugal compressor. The aerodynamic stability of the compressor deteriorates remarkably when a volute is employed. This paper investigates the influence of volute-induced circumferential flow distortion on aerodynamic stability of a centrifugal compressor via experimentally validated three-dimensional (3D) numerical simulation method. First, the compressor performance is analyzed based on a newly developed stability parameter. The impeller is confirmed to be the main contributor to the instability of the investigated compressor. Next, the influence of volute on impeller performance is studied by circumferentially distorted boundary conditions at the impeller exit which are extracted from flow field at the volute inlet. Results show that the performance of an impeller passage is determined by not only the back pressure but also the local gradient of pressure distribution in the circumferential direction. Moreover, these passages confronted with pressure reduction in the rotational direction are most unstable, while those confronted with pressure rise have better performance. Consequently, the circumferentially distorted distribution at impeller exit results in a loop of passage performance encapsulating the performance of uniform case. The size of the loop is enhanced by the distortion amplitude. Moreover, the influence of volute-induced distortion on the impeller performance is concluded into two main reasons: the imbalance of the force on flow and the imbalance of tip clearance flow taken by passages. The force imbalance influences the accumulation of secondary flow, while the imbalance of the tip clearance flow results in discrepancies of the low momentum flow in passages.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(12):122604-122604-7. doi:10.1115/1.4040577.

Centrifugal compressors are one of the best choices among compressors in supercritical Brayton cycles. A supercritical CO2 centrifugal compressor increases the pressure of the fluid which state is initially very close to the critical point. When the supercritical fluid is compressed near the critical point, wide variations of fluid properties occur. The density of carbon dioxide at its critical point is close to the liquid density which leads to reduction in the compression work. This paper explains a method to overcome the simulation instabilities and challenges near the critical point in which the thermophysical properties change sharply. The investigated compressor is a centrifugal compressor tested in the Sandia supercritical CO2 test loop. In order to get results with the high accuracy and take into account the nonlinear variation of the properties near the critical point, the computational fluid dynamics (CFD) flow solver is coupled with a look-up table of properties of fluid. Behavior of real gas close to its critical point and the effect of the accuracy of the real gas model on the compressor performance are studied in this paper, and the results are compared with the experimental data from the Sandia compression facility.

Commentary by Dr. Valentin Fuster

Research Papers: Internal Combustion Engines

J. Eng. Gas Turbines Power. 2018;140(12):122801-122801-8. doi:10.1115/1.4040012.

Electrically assisted engine boosting systems lend themselves to better throttle response, wider effective operating ranges, and can provide the ability to extract excess energy during deceleration and high-load events (and store it in a vehicle's onboard batteries). This can lead to better overall vehicle performance, emissions, and efficiency while allowing for further engine downsizing and increased power density. In this research effort, a hybrid-electric turbocharger, variable-frequency drive (VFD), and novel sensorless control algorithm were developed. An 11 kW permanent-magnet (PM) machine was coupled to a commercial turbocharger via an in-line, bolt-on housing attached to the compressor inlet. A high-efficiency, high-temperature VFD, consisting of custom control and power electronics, was also developed. The VFD uses SiC MOSFETS to achieve high-switching frequency and can be cooled using an existing engine coolant loop operating at up to 105 °C at an efficiency greater than 98%. A digital sliding mode-observer sensorless speed control algorithm was created to command and regulate speed and achieved ramp rates of over 68,000 rpm/s. A two-machine benchtop motor/generator test stand was constructed for initial testing and tuning of the VFD and sensorless control algorithm. A gas blow-down test stand was constructed to test the mechanical operation of the hybrid-electric turbocharger and speed control using the VFD. In addition, a liquid-pump cart was assembled for high-temperature testing of the VFD. Initial on-engine testing is planned for later this year. This paper intends to present a design overview of the in-line, hybrid-electric device, VFD, and performance characterization of the electronics and sensorless control algorithm.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(12):122802-122802-6. doi:10.1115/1.4040179.

The North American oil and gas industry has experienced a market pull for dual fuel (DF) engines that can run on any ratio of fuels ranging from 100% diesel to a high proportion of field gas relative to diesel, while also meeting the U.S. Tier 4 Nonroad emissions standards. A DF engine must meet complex and at times competing requirements in terms of performance, fuel tolerance, and emissions. The challenges faced in designing a DF engine to meet all of the performance and emissions requirements require a detailed understanding of the trade-offs for each pollutant. This paper will focus on the details of NOx formation for high substitution DF engines. Experimental results have demonstrated that NOx emission trends (as a function of lambda) for DF engines differ from both traditional diesel engines and lean burn natural gas (NG) engines. For high energy substitution (>70%) conditions, NOx emissions are a function of the premixed gas lambda (λng) and contain a local minimum, with NOx increasing as lambda is either leaned or richened beyond the local minimum which occurs from approximately λng = 1.7 – 1.85. It is hypothesized that at richer conditions (λng < 1.7), NOx formed in the burning of gaseous fuel results in increased total NOx emissions. At leaner conditions (λng > 1.85), the NOx formed in the diesel post flame regions, as a result of increased oxygen availability, results in increased total NOx emissions. Between these two regions there are competing effects which result in relatively constant NOx.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(12):122803-122803-12. doi:10.1115/1.4040517.

Homogeneous charge compression ignition (HCCI) has been considered as an ideal combustion mode for compression ignition (CI) engines due to its superb thermal efficiency and low emissions of nitrogen oxides (NOx) and particulate matter. However, a challenge that limits practical applications of HCCI is the lack of control over the combustion rate. Fuel stratification and partially premixed combustion (PPC) have considerably improved the control over the heat release profile with modulations of the ratio between premixed fuel and directly injected fuel, as well as injection timing for ignition initiation. It leverages the advantages of both conventional direct injection compression ignition and HCCI. In this study, neat n-butanol is employed to generate the fuel stratification and PPC in a single cylinder CI engine. A fuel such as n-butanol can provide additional benefits of even lower emissions and can potentially lead to a reduced carbon footprint and improved energy security if produced appropriately from biomass sources. Intake port fuel injection (PFI) of neat n-butanol is used for the delivery of the premixed fuel, while the direct injection (DI) of neat n-butanol is applied to generate the fuel stratification. Effects of PFI-DI fuel ratio, DI timing, and intake pressure on the combustion are studied in detail. Different conditions are identified at which clean and efficient combustion can be achieved at a baseline load of 6 bar IMEP. An extended load of 14 bar IMEP is demonstrated using stratified combustion with combustion phasing control.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(12):122804-122804-7. doi:10.1115/1.4040746.

An increase in lubricating oil consumption in a gasoline engine causes an increase in particulate matters in exhaust gases, poisoning the catalyst after treatment devices, abnormal combustion in a turbo-charged gasoline engine, and so on. Recent trend of low friction of a piston and piston ring tends to increase in lubricating oil consumption. Therefore, reducing oil consumption is required strongly. In this study, the effect of the position of oil drain holes on oil pressure under the oil ring and lubricating oil consumption was investigated. The oil pressure under the oil ring is measured using fiber optic pressure sensors and pressure generation mechanisms were investigated. Lubricating oil consumption was also measured using sulfur tracer method and the effects of oil drain holes hence the oil pressure were evaluated. Four types of arrangement of oil drain holes were tested. The oil pressure variations under the oil ring in the circumferential direction was measured. An increase in oil pressure was found during down-stroke of the piston. The lowest oil pressure was found for the piston with four oil drain holes. Two holes nearby the front/rear end of the piston skirt showed relatively lower pressure. The measured results of oil consumption showed good agreement to measured oil pressure under the oil ring. It was found that oil pressure under the oil ring affected oil consumption, and oil drain holes set near the front/rear end of the piston skirt were effective for reducing oil consumption.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(12):122805-122805-11. doi:10.1115/1.4040515.

Particulate matter (PM) emissions from gasoline direct injection (GDI) engines are a concern due to the health effects associated with ultrafine PM. This experimental study investigated sources of PM emissions measurement variability observed in previous tests and also examined the effect of ethanol content in gasoline on PM emissions. Some engine operating parameters (fuel and oil temperature, positive crankcase ventilation filtration) and test conditions (dilution air conditions) were studied and controlled but could not account for the level of measurement variability observed. Fourier transform infrared spectrometry (FTIR) measurements of gas phase hydrocarbon emissions provided evidence that changes in fuel composition were responsible for the variability. Exhaust emissions of toluene and ethanol were correlated positively with PM emissions, while emissions of isobutylene correlated negatively. Exhaust emissions of toluene and isobutylene were interpreted as markers of gasoline aromatic content and gasoline volatility, respectively. Tests conducted with gasoline containing added toluene (10% v/v) supported this hypothesis and led to the overall conclusion that the PM emissions variability observed can be attributed to changes in the composition of the pump gasoline being used. Tests conducted with gasoline containing added ethanol (10% and 30% v/v) found that increasing ethanol fuel content increased PM emissions at the steady-state operating condition utilized.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Eng. Gas Turbines Power. 2018;140(12):124501-124501-4. doi:10.1115/1.4039820.

This study presents additional important findings to the results of the research paper; “Optimization of the efficiency of stall control using air injection for centrifugal compressors” published in the Journal of Engineering for Gas Turbines and Power in 2015 (Halawa, T., Gadala, M. S., Alqaradawi, M., and Badr, O., 2015, “Optimization of the Efficiency of Stall Control Using Air Injection for Centrifugal Compressors,” ASME J. Eng. Gas Turbines Power, 137(7), p. 072604). The aim of this study is to make a fine determination of the injection angle, which provides the best stable condition when the compressor operates close to stall condition. A relatively narrower range of injection angles with smaller intervals was selected comparing to the results of the referred published paper, which clarified that the best injection angle is 30 deg. External air was injected close to the diffuser entrance at the shroud surface. Injection was applied with mass flow rate equals 1.5% of the design compressor inlet mass flow rate with injection angles ranged from 16 deg to 34 deg measured from the tangential direction at the vaneless region. It was found that both of injection angles of 28 deg and 30 deg achieved the best results in terms of compressor stabilization but each one of them has a specific advantage comparing to the other one. Using injection angle of 28 deg provided the lowest kinetic energy losses while the best orientation of the fluid through diffuser resulted when using an injection angle of 30 deg.

Commentary by Dr. Valentin Fuster

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