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### Research Papers

J. Eng. Gas Turbines Power. 2019;141(8):081001-081001-11. doi:10.1115/1.4042609.

In an internal combustion engine, the centrifugal compressor is placed upstream of the inlet manifold and therefore, it is exposed an unsteady flow regime caused by the inlet valves of the cylinder arrangement. This valve motion sets a pulsating state at the compressor exit, having greater influence when the operation is near the surge margin of the compressor. This paper presents the experimental results of the evaluation of the surge dynamics on a compressor with induced downstream pulsating flow. Different pulsation levels are achieved by the variation of three different parameters on the induced pulse: pulse frequency, amplitude, and system storage volume (plenum). Each pulse parameter was evaluated independently in order to assess its effect on the compressor stability limit. The main effect on the surge margin of the compressor was found to be due to the presence of a storage volume in the system for all cases (steady/pulsating condition) and at all frequencies. It was found that the magnitude of the pulse frequency determines the hysteresis behavior of the system that leads to a phase difference between the convected terms and the acoustic dominated terms, and therefore this affects the onset of flow instability, surge, in the compression system under study.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2019;141(8):081002-081002-11. doi:10.1115/1.4042650.

Tests are reported for a smooth seal with radial clearances 127 μm, 254 μm, 381 μm (1×, 2×, and 3×); length 45.72 mm, diameter 101.6 mm. An insert induced upstream preswirl. Swirl brakes (SBs), comprising 36 square cuts with axial depth 5.08 mm, radial height 6.35 mm, and circumferential width 6.35 mm each. Static and rotordynamic data were produced at ω = 2, 4, 6, 8 krpm, ΔP = 2.07, 4.14, 6.21, 8.27 bar, and eccentricity ratios ε0 = e0/Cr = 0.00, 0.27, 0.53, and 0.80. ISO VG 46 oil at a range of 46–49 °C was used, netting laminar flow (total Re ≤ 650). Dynamic measurements included components of the following vectors: (a) stator–rotor relative displacements, (b) acceleration, and (c) applied dynamic force in a stationary coordinate system. SBs were effective at the 3× clearance only. With the 3× seal, the cross-coupled stiffness coefficients have the same sign (not destabilizing). However, the seal has a negative direct stiffness K that could potentially “suck” the rotor into contact with the stator wall, along with dropping the pump rotor's natural frequency, further reducing its dynamic stability. Measurements were compared to predictions from a code by Zirkelback and San Andrés. Most predictions agree well with test data. Notable exceptions are the direct and cross-coupled stiffness coefficients for the 3× clearance. Predictions showed positive direct stiffness and opposite signs for the cross-coupled stiffness coefficients.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2019;141(8):081003-081003-11. doi:10.1115/1.4042422.

This paper presents a numerical investigation on the sealing effectiveness and unsteady flow field of a 1.5-stage turbine with the front and aft wheel-space cavities. The sealing effectiveness and flow structure are studied by solving three-dimensional unsteady Reynolds-averaged Navier–Stokes (URANS) equations and shear stress transfer (SST) turbulence model. The numerical pressure and swirl ratio distributions in cavities with two computational models are compared with experimental data to determine the position of stationary/rotating domain interface. The time-averaged mainstream pressure distribution and sealing effectiveness of the rim seal at the front and aft cavities are studied by the steady and unsteady calculations. The unsteady results agree well with experimental data by comparison of the steady calculations. The effects of coolant flow rates on the sealing effectiveness and the flow field of the rim seal at the front and aft cavities are investigated. The obtained results show that the sealing effectiveness of the rim seal at the aft cavity is much larger than that of the rim seal at the front cavity at the same coolant flow rate. The less mainstream pressure fluctuation near the aft rim seal clearance and the clockwise vortex due to the pumping effect in the aft rim seal leads to this result. The mainstream pressure fluctuation downstream of the blade and the sealing effectiveness of the rim seal at the aft cavity under five operating conditions are computed. It shows that the square root of the mainstream pressure fluctuation amplitude downstream of the blade is proportional to the mainstream flow rate. The increase of the mainstream flow results in gradual decrease of the sealing effectiveness of the rim seal at the aft cavity.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2019;141(8):081004-081004-7. doi:10.1115/1.4042725.

In order to study the in-cylinder flow characteristics, one hundred consecutive cycles of velocity flow fields were investigated numerically by large eddy simulation, and the proper orthogonal decomposition (POD) algorithm was used to decompose the results. The computed flow fields were divided into four reconstructed parts, namely mean part, coherent part, transition part, and turbulent part. Then, the dynamic mode decomposition (DMD) algorithm was used to analyze the characteristics of the reconstructed fields. The results show that DMD method is capable of finding the dominant frequencies in every reconstructed flow part and identifying the flow structures at equilibrium state. In addition, the DMD results also reveal that the reconstructed parts are related to each other through the break-up and attenuation process of unstable flow structures, while the flow energy cascade occurs among these parts through different scale vortex generation and dissipation process.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2019;141(8):081005-081005-13. doi:10.1115/1.4042975.

An advanced numerical framework to model CO2 compressors over a wide range of subcritical conditions is presented in this paper. Thermodynamic and transport properties are obtained through a table look-up procedure with specialized features for subcritical conditions. Phase change is triggered by the difference between the local values of pressure and saturation pressure, and both vaporization and condensation can be modeled. Rigorous validation of the framework is presented for condensation in high pressure CO2 using test data in a De Laval nozzle. The comparisons between computations and test data include: condensation onset locations, Wilson line, and nozzle pressure profiles as a function of inlet pressures. The framework is applied to the Sandia compressor that has been modeled over broad range of conditions spanning the saturation dome including: near critical inlet conditions (305.4 K, and 7.843 MPa), pure liquid inlet conditions (at 295 K), pure vapor inlet conditions (at 302 K), and two-phase inlet conditions (at 290 K). Multiphase effects ranging from cavitation at the liquid line to condensation at the vapor line have been simulated. The role of real fluid effects in enhancing multiphase effects at elevated temperatures closer to the critical point has been identified. The performance of the compressor has been compared with test data; the computational fluid dynamics (CFD) results also show that the head-flow coefficient curve collapses with relatively minor scatter, similar to the test data, when the flow coefficient is defined on the impeller exit meridional velocity.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2019;141(8):081006-081006-9. doi:10.1115/1.4043138.

For the two-stroke marine diesel engine, the action of exhaust valve has a significant impact on scavenging and combustion processes and ultimately affects the engine performances and emissions. In order to reduce nitrogen oxides (NOx) emissions of a two-stroke marine diesel engine, different exhaust valve lifts (EVLs) were achieved by computational fluid dynamics simulation method in this study. The NOx reduction effect and influence mechanism of EVL on a two-stroke marine diesel engine were investigated in detail. The results showed that the in-cylinder residual exhaust gas and the internal exhaust gas recirculation (EGR) rate gradually increased with the decreasing EVL. Although the total mass of charge enclosed in the cylinder did not change much, the composition changed gradually and the maximum internal EGR rate reached 13.17% in this study. The maximum compression pressure and combustion pressure both rose first and then decreased with the decreasing EVL. While the start of combustion and the maximum combustion temperature were basically unaffected by EVL, the indicated power of the engine was also not much impacted when the EVL was changed from increasing 10 mm to decreasing 20 mm. The indicated specific fuel consumption first declined slowly and then rose rapidly as the EVL reduction exceeded 20 mm. NOx emissions decreased monotonously with the decreasing EVL. The reduction of NOx formation rate and the amount of NOx formation mass mainly occurred at the middle and late stages of combustion for the downward moving of residual exhaust gas. NOx emissions were reduced by 12.57% without compromising other engine performances at medium-reduced EVL in this study. However, in order to further reduce NOx emissions at low EVLs, other measures may be needed to make the residual exhaust gas more evenly distributed during the initial stage of combustion.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2019;141(8):081007-081007-9. doi:10.1115/1.4043218.

Engine-out NOx emissions from diesel engines continue to be a major topic of research interest. While substantial understanding has been obtained of engine-out (i.e., before any aftertreatment) NOx formation and reduction techniques, not least exhaust gas recirculation (EGR) which is now well established and fitted to production vehicles, much less data are available on cycle resolved NOx emissions. In this work, crank-angle resolved NO and NOx measurements have been taken from a high-speed light duty diesel engine at test conditions both with and without EGR. These have been combined with 1D data of exhaust flow, and this used to form a mass average of NO and NOx emissions per cycle. These results have been compared with combustion data and other emissions. The results show that there is a very strong correlation (R2 > 0.95) between the NOx emitted per cycle and the peak cylinder pressure of that cycle. In addition, the crank-angle resolved NO and NOx measurements also reveal that there is a difference in NO : NO2 ratio (where NO2 is assumed to be the difference between NO and NOx) during the exhaust period, with proportionally more NO2 being emitted during the blowdown period compared to the rest of the exhaust stroke.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2019;141(8):081008-081008-8. doi:10.1115/1.4043216.

Advanced engine configuration studies have shown large advantages for an engine with counter-rotating spools with intershaft counter-rotating roller bearings. Mounted on two counter-rotating differential-speed hollow rotors, the bearing internal kinetic behavior, dynamic behavior, and then thermal behavior change greatly, causing a severe challenge to engine designers using traditional analysis methods. A special quasi-dynamic model for counter-rotating roller bearing is proposed, considering rings deformation and windage effects, to analyze the bearing mechanical and thermal behavior in different mounting configurations. Roller sliding and bearing heat generation are calculated and compared with experimental data to verify the model capabilities. It shows that the configuration that connects the inner ring to the high-speed rotor has life cycle advantage with more uniform load distribution, smaller roller/ring clearance, and lower cage speed. This leads to less drag loss due to the rotation of the rollers and cage assembly. The decrease of the total power loss is a key element to minimize the quantity of oil required to lubricate the roller bearing.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2019;141(8):081009-081009-11. doi:10.1115/1.4043251.

This paper reports on a theory for poststall transients in contra-rotating fans, which is developed from the basic Moore–Greitzer theory. A second-order hysteresis term is assumed for the fan pressure rise, which gives the theory more capabilities in predicting the fan instabilities. The effect of the rotational speed ratio of the two counter rotating rotors on the fan performance during the occurrence of surge and rotating stall are studied (the rotational speed of the front rotor is assumed to be kept constant whereas the speed of the rear rotor is variable). One of the new capabilities of the current model is the possibility of investigating the effect of the initial slope on the fan characteristic. Results reveal that unlike the conventional fans and compressors, in the current contra-rotating fan stall cannot be initiated from the negative slope portion of the fan pressure rise characteristic curve. One of the important advantages of the developed model is that it enables investigation of the effect of the rate of throttling on the instabilities. Results show that more the rotational speed of the rear rotor, the more robust to surge (caused by throttling) the fan is.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2019;141(8):081010-081010-7. doi:10.1115/1.4043243.

A dual fuel engine concept with lean premixed methane–air charge ignited by a diesel pilot flame is highly promising for reducing NOx and soot emissions. One drawback of this combustion method, however, is the high nitric dioxide (NO2) emissions observed at certain operating points. The conditions leading to increased NO2 formation have been investigated using a batch reactor model in cantera. It has been found that the high emission levels of NO2 can be traced back to the mixing of small amounts of quenched CH4 with NO from the hot combustion zones followed by postoxidation in the presence of O2, requiring that the temperatures are within a certain range. NO2 formation in the exhaust duct of a test engine has been modeled and compared to the experimental results. The well-stirred reactor model has been used that calculates the steady-state of a uniform composition for a certain residence time. An appropriate reaction mechanism that considers the effect of NO/NO2 on methane oxidation at low temperature levels has been used. The numerical results of NO–NO2 conversion in the duct at low temperature levels show good agreement with the experimental results. The partial oxidation of CH4 can be predicted well. It can be shown that methane oxidation in the presence of NO/NO2 at low temperature levels is enhanced via the reaction steps $CH3+NO2⇌CH3O+NO$ and $CH3O2+NO⇌CH3O+NO2$. In addition, the elementary reaction $HO2+NO⇌NO2+OH$ is the important NO oxidizing step.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2019;141(8):081011-081011-7. doi:10.1115/1.4043277.

Natural gas is traditionally considered as a promising fuel in comparison with gasoline due to the potential of lower emissions and significant domestic reserves. These emissions can be further diminished by using noble gases, such as argon, instead of nitrogen as the working fluid in internal combustion engines. Furthermore, the use of argon as the working fluid can increase the thermodynamic efficiency due to its higher specific heat ratio. In comparison with premixed operation, the direct injection of natural gas enables the engine to reach higher compression ratios while avoiding knock. Using argon as the working fluid increases the in-cylinder temperature at top dead center (TDC) and enables the compression ignition (CI) of natural gas. In this numerical study, the combustion quality and ignition behavior of methane injected into a mixture of oxygen and argon have been investigated using a three-dimensional transient model of a constant volume combustion chamber (CVCC). A dynamic structure large eddy simulation (LES) model has been utilized to capture the behavior of the nonpremixed turbulent gaseous jet. A reduced mechanism consists of 22-species, and 104-reactions were coupled with the CFD solver. The simulation results show that the methane jet ignites at engine-relevant conditions when nitrogen is replaced by argon as the working fluid. Ignition delay times are compared across a variety of operating conditions to show how mixing affects jet development and flame characteristics.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2019;141(8):081012-081012-9. doi:10.1115/1.4043307.

Proper orthogonal decomposition (POD) offers an approach to quantify cycle-to-cycle variation (CCV) of the flow field inside the internal combustion engine cylinder. POD decomposes instantaneous flow fields (also called snapshots) into a series of orthonormal flow patterns (called POD modes) and the corresponding mode coefficients. The POD modes are rank-ordered by decreasing kinetic energy content, and the low-order, high-energy modes are interpreted as constituting the large-scale coherent flow structure that varies from engine cycle to engine cycle. Various POD-based analysis techniques have thus been proposed to characterize engine flow field CCV using these low-order modes. The validity of such POD-based analyses rests, as a matter of course, on the reliability of the underlying POD results (modes and coefficients). Yet a POD mode can be disproportionately skewed by a single outlier snapshot within a large data set, and an algorithm exists to define and identify such outliers. In this paper, the effects of a candidate outlier snapshot on the results of POD-based conditional averaging and quadruple POD analyses are examined for two sets of crank angle-resolved flow fields on the midtumble plane of an optical engine cylinder recorded by high-speed particle image velocimetry (PIV). The results with and without the candidate outlier are compared and contrasted. In the case of POD-based conditional averaging, the presence of the outlier scrambles the composition of snapshot subsets that define large-scale flow pattern variations, and thus substantially alters the coherent flow structures that are identified; for quadruple POD, the shape of coherent structures and the number of modes to define them are not significantly affected by the outlier.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2019;141(8):081013-081013-10. doi:10.1115/1.4043274.

Composite pistons are often used in heavy duty diesel engines due to its good reliability and durability. Owing to the alternating loads, fretting wear usually happens on the mating surfaces between piston crown and skirt. In this paper, a fretting wear finite element model is developed to analyze the mating surface wear of composite piston of heavy-duty diesel engine. The fretting wear model predicts the wear depth evolution for each working cycle based on Archard model and mesh updating technique, which is validated by previous pin and disk contact experiments. The wear evolution of the top contact surface of piston skirt is simulated according to engine operating condition, and fretting wear life is estimated by the decreasing process of crown-skirt connecting bolt preload. Effects of the shape of piston skirt top surface are also evaluated. In the end, the rationality of fretting wear model is validated by durability tests of diesel engine.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2019;141(8):081014-081014-15. doi:10.1115/1.4043408.

In this study, a variety of piezoelectric pressure transducer designs and mounting configurations were compared for measuring in-cylinder pressure on a heavy-duty single-cylinder diesel engine. A unique cylinder head design was used which allowed cylinder pressure to be measured simultaneously in two locations. In one location, various piezoelectric pressure transducers and mounting configurations were studied. In the other location, a Kistler water-cooled switching adapter with a piezoresistive pressure sensor was used. The switching adapter measured in-cylinder pressure during the low-pressure portion of the cycle. During the high-pressure portion of the cycle the sensor is protected from the high-pressure and high-temperature gases in the cylinder. Therefore, the piezoresistive sensor measured in-cylinder pressure highly accurately, without the impacts of short-term thermal drift, otherwise known as thermal shock. Additionally, the piezoresistive sensor is an absolute pressure sensor which does not require a baseline or “pegging” on every engine cycle. With this measurement setup, the amount of thermal shock and induced measurement variability was accurately assessed for the piezoelectric sensors. Data analysis techniques for quantifying the accuracy of a piezoelectric cylinder pressure measurement are also presented and discussed. It was observed that all the piezoelectric transducers investigated yielded very similar results regarding compression pressure, start of combustion, peak cylinder pressure, and the overall heat release rate shape. Differences emerged when studying the impact of the transducer mounting (e.g., recessed versus flush-mount). Recessed-mount transducers tended to yield a more accurate measurement of the cycle-to-cycle variability when compared to the baseline piezoresistive sensor. This is thought to be due to reduced levels of thermal shock, which can vary from cycle-to-cycle.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2019;141(8):081015-081015-9. doi:10.1115/1.4043446.

Free piston linear engine alternators (FPLEA) may be designed following several different baseline configurations. Common designs include a translator that carries permanent magnets, with either one piston attached to one end of the translator, or a piston at each end of the translator. The single cylinder engine requires a reversing force from a spring so that it can operate, whereas, the dual cylinder version can operate without a spring. However, inclusion of stiff springs would serve to raise the operating frequency and reduces the cycle-to-cycle variations in a dual cylinder version. A matlab/simulink numerical model with translator rod dynamics and in-cylinder thermodynamics was employed to predict the overall performance and efficiency of a FPLEA. This numerical model allowed comparison of different FPLEA configurations for a variety of design variables. First, a dual cylinder FPLEA design was considered where the spring constant was varied, changing the frequency of operation and the motion of the translator. The simulation results showed that without springs the motion was far from sinusoidal, and low in frequency and power, whereas the presence of stiff springs in the system strongly dictated nearly sinusoidal motion and high power at high frequency. Effects of other parameters such as stroke and bore were also examined. Finally, the same tests were carried out for a single cylinder FPLEA design. The simulation results showed that the dual cylinder engine produced twice the electrical power output with higher efficiency than the single cylinder engine for the same cylinder geometry.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2019;141(8):081016-081016-10. doi:10.1115/1.4043472.

To enable efficient exhaust waste energy recovery (WER), it is important to characterize the exergy available in engine exhaust flows. In a recent article (Mahabadipour et al., 2018, Appl. Energy, 216, pp. 31–44), the authors introduced a new methodology for quantifying crank angle-resolved exhaust exergy (including its thermal and mechanical components) for the two exhaust phases, viz., the “blowdown” phase and the “displacement” phase. The present work combines experimental measurements with GT-SUITE simulations to investigate the effect of exhaust back-pressure (Pb) on crank angle-resolved exhaust exergy in a single-cylinder research engine (SCRE). To this end, Pb values of 1, 1.4, and 1.8 bar are considered for conventional diesel combustion on the SCRE. Furthermore, the effect of boost pressure (Pin) between 1.2 and 2.4 bar on the thermal and mechanical components of exhaust exergy is reported at different Pb. The exergy available in the blowdown and the displacement phases of the exhaust process is also quantified. Regardless of Pin, with increasing Pb, the cumulative exergy percentage in the blowdown phase reduced uniformly. For example, at Pin = 1.5 bar and 1500 rpm engine speed, the cumulative exergy percentage in the blowdown phase decreased from 34% to 17% when Pb increased from 1 bar to 1.8 bar. The percentage of fuel exergy available as exhaust exergy was quantified. For instance, this normalized cumulative exergy in the exhaust increased from 10% to 21% when Pb increased from 1 bar to 1.8 bar at 1200 rpm. Finally, although the present work focused on exhaust exergy results for diesel combustion in the SCRE, the overall methodology can be easily adopted to study exhaust exergy flows in different engines and different combustion modes to enable efficient exhaust WER.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2019;141(8):081017-081017-9. doi:10.1115/1.4043395.

According to current worldwide trends for homologation vehicles in real driving conditions is forced to test the engines in altitude and in highly dynamic driving cycles in order to approach nowadays and next future emissions standard. Up to now, there were two main options to perform this type of tests: round-robin tests of the whole vehicle or hypobaric chambers, both with high costs and low repeatability. In this paper a new device is described, which can emulate ambient conditions at whatever altitude between sea level and 5000 m high. Even it can be used to emulate ambient conditions at sea level when test bench is placed up to 2000 m high. The main advantages of the altitude simulation equipment are as follows: dynamic emulation of all the psychrometric variables affecting the vehicles during round-robin tests; lower space usage and low-energy consumption. The altitude simulator (AS) has been validated comparing with results from a hypobaric chamber at different altitudes. Previously a research about the dispersion in the measurements of both testing devices has been done for assessing the results of the comparison experiment. Final conclusion resulted in the same operating performance and emissions of the studied engine with both types of testing equipment for altitude simulation.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2019;141(8):081018-081018-13. doi:10.1115/1.4043396.

Skip-firing (or cylinder de-activation) was assessed as a method of sampling CO2 directly in the cylinder at higher speeds than previously possible. CO2 was directly sampled from one cylinder of a 1 L three-cylinder gasoline engine to determine the residual gas fraction (RGF) using a fast response CO/CO2 analyzer. Acquisition of data for similar measurements is typically limited to engine speeds of below 1300  revolutions per minute (rpm) to allow full resolution of the sample through the analyzer that has an 8 ms finite response time. In order to sample in-cylinder CO2 at higher engine speeds, a skip-firing method is developed. By shutting off ignition intermittently during engine operation, the residual CO2 from the last firing cycle can be measured at significantly higher engine speeds. Comparison of RGF CO2 at low speeds for normal and skip-fire operation shows good correlation. This suggests that skip-firing is a suitable method for directly measuring internal exhaust gas recirculation up to at least 3000 rpm. The measurements obtained may provide a useful tool for validating internal exhaust gas recirculation models and could be used to calculate combustion air–fuel ratio from the CO and CO2 content of the burned gas. These are typically complicated parameters to predict due to the slow response time and sensitivity to hydrocarbons of wide-band oxygen sensors. A differing pattern of RGF change with increasing speed was seen between normal and skip-fire operation.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2019;141(8):081019-081019-12. doi:10.1115/1.4043429.

Large eddy simulation of n-heptane spray flames is conducted to investigate the multiple-stage ignition process under extreme (low-temperature, low oxygen, and high-temperature, high-density) conditions. At low oxygen concentrations, the first-stage ignition initiates in the fuel-rich region and then moves to stoichiometric equivalence ratio regions by decreasing the initial temperature. It is also clear that at high temperatures, high oxygen concentrations, or high densities, the reactivity of the mixture is enhanced, where high values of progress variable are observed. Analysis of key intermediate species, including acetylene (C2H2), formaldehyde (CH2O), and hydroxyl (OH) in the mixture fraction and temperature space provides valuable insights into the complex combustion process of the n-heptane spray flames under different initial conditions. The results also suggest that C2H2 appears over a wider range in the mixture fraction space at higher temperature or oxygen concentration condition, implying that it mainly forms at the fuel-rich regions. The initial oxygen concentration of the ambient gas has great influence on the formation and oxidization of C2H2, and the maximum temperature depends on the initial oxygen concentration. OH is mainly formed at the stoichiometric equivalence ratio region, which moves to high-temperature regions very quickly especially at higher oxygen concentrations. Finally, analysis of the premixed and nonpremixed combustion regimes in n-heptane spray flames is also conducted, and both premixed and nonpremixed combustion coexist in spray flames.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2019;141(8):081020-081020-12. doi:10.1115/1.4043431.

Radial inflow turbines are a relevant architecture for energy extraction from supercritical CO2 power cycles for scales less than 10 MW. To ensure stage and overall cycle efficiency, it is desirable to recover exhaust energy from the turbine stage through the inclusion of a suitable diffuser in the turbine exhaust stream. In supercritical CO2 Brayton cycles, the high turbine inlet pressure can lead to sealing challenges at small scale if the rotor is supported from the rotor rear side in the conventional manner. An alternative is a layout where the rotor exit faces the bearing system. While such a layout is attractive for the sealing system, it limits the axial space claim of the diffuser. Designs of a combined annular-radial diffuser are considered as a means to meet the aforementioned packaging challenges of this rotor layout. Diffuser performance is assessed numerically with the use of Reynolds-averaged Navier--Stokes (RANS) and unsteady Reynolds-averaged Navier--Stokes (URANS) calculations. To appropriately account for cross coupling with the stage, a single blade passage of the entire stage is modeled. Assessment of diffuser inlet conditions, and off-design performance analysis, reveals that the investigated diffuser designs are performance robust to high swirl, high inlet blockage, and highly nonuniform mass flux distribution. Diffuser component performance is dominated by the annular-radial bend. The incorporation of a constant sectional area bend is the key geometric feature in rendering the highly nonuniform turbine exit flow (dominated by tip clearance flows at the shroud) more uniform.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2019;141(8):081021-081021-11. doi:10.1115/1.4043485.

Dual-fuel (DF) engines offer great fuel flexibility combined with low emissions in gas mode. The main source of energy in this mode is provided by gaseous fuel, while the diesel fuel acts only as an ignition source. For this reason, the reliable autoignition of the pilot fuel is of utmost importance for combustion in DF engines. However, the autoignition of the pilot fuel suffers from low compression temperatures caused by Miller valve timings. These valve timings are applied to increase efficiency and reduce nitrogen oxide (NOx) emissions. Previous studies have investigated the influence of injection parameters and operating conditions on ignition and combustion in DF engines using a unique periodically chargeable combustion cell. Direct light high-speed images and pressure traces clearly revealed the effects of injection parameters and operating conditions on ignition and combustion. However, these measurement techniques are only capable of observing processes after ignition. In order to overcome this drawback, a high-speed shadowgraph technique was applied in this study to examine the processes prior to ignition. Measurements were conducted to investigate the influence of compression temperature and injection pressure on spray formation and ignition. Results showed that the autoignition of diesel pilot fuel strongly depends on the fuel concentration within the spray. The high-speed shadowgraph images revealed that in the case of very low fuel concentration within the pilot spray, only the first stage of the two-stage ignition occurs. This leads to large cycle-to-cycle variations and misfiring. However, it was found that a reduced number of injection holes counteract these effects. The comparison of a diesel injector with ten-holes and a modified injector with five-holes showed shorter ignition delays, more stable ignition and a higher number of ignited sprays on a percentage basis for the five-hole nozzle.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Aircraft Engine

J. Eng. Gas Turbines Power. 2019;141(8):081201-081201-13. doi:10.1115/1.4042395.

Imaging of aeroacoustic noise sources is routinely accomplished with geometrically fixed phased arrays of microphones. Several decades of research have gone into improvement and optimization of sensor layouts, selection of basis models, and deconvolution algorithms to produce sharper and more localized images of sound-producing regions in space. This paper explores an extension to conventional phased array measurements that uses slowly, continuously moving microphone arrays with and without coupling to rigid fixed arrays to improve image quality and better describe noise mechanisms on turbofan engine sources such as jet exhausts and turbomachinery components. Three approaches are compared in the paper: fixed receiver beamforming (FRBF), continuous-scan beamforming (CSBF), and multireference CSBF (MRCSBF). The third takes advantage of transfer function matrices formed between fixed and moving sensors to achieve effective virtual arrays with spatial density one to two orders of magnitude higher, with practical sensor budgets and scan speeds. The MRCSBF technique produces array sidelobe rejection that approaches the theoretical array pattern of a continuous two-dimensional (2D) aperture. The implications of this finding are that better source localization may be achieved with conventional delay and sum (DAS) beamforming (BF) with practical sensor budgets, and that an improved starting image of the sound source can be provided to deconvolution algorithms. These findings are demonstrated on analytical and experimental examples from a low-cost rotating phased array using point sound sources, as well as linear scanning array experiments of an impinging jets point source and a near-sonic jet nozzle exhaust.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2019;141(8):082501-082501-9. doi:10.1115/1.4042651.

As performance improvements of compressors become more difficult to obtain, the optimization of stator well structure to control the reverse leakage flow is a more important research subject. Normally, the stator well can be considered as two rotor–stator cavities linked by the labyrinth seal. The flow with high tangential velocity and high total temperature exited from the stator well interacts with the main flow, which can affect the compressor aerodynamic performance. Based on the flow mechanisms in the basic stator well, four geometries were proposed and studied. For geometry a and geometry b, seal lips were attached to the rotor and stator inside downstream rim seal while impellers were positioned in the cavities for geometry c and geometry d. Leakage flow rates, tangential velocities, and pressure distributions in the cavities were analyzed using validated method of computational fluid dynamics. In the current study, where ω = 8000 rpm, π = 1.05–1.30, the maximum reductions of leakage flow rate for geometry a and geometry b are 7.9% and 15.9%, respectively, compared to the baseline model. In addition, the rotating impellers in the downstream cavity for geometry c contribute to a more significant pressure gradient along radial direction, reducing the leakage flow as much as 46%. Although the stationary impellers in the upstream cavity for geometry d appear to have little effect upon the leakage, these impellers can be used to adjust the tangential velocity of ejected flow from the stator well to the mainstream.

Commentary by Dr. Valentin Fuster

### Research Papers: Internal Combustion Engines

J. Eng. Gas Turbines Power. 2019;141(8):082801-082801-16. doi:10.1115/1.4043444.

The turbulent boundary layer flow in internal combustion (IC) engines has a significant effect on the in-cylinder flow and the wall heat transfer. A detailed analysis of the in-cylinder near-wall flow was carried out on an optical steady flow test bench by using high-resolution particle image velocimetry (PIV) in order to characterize the in-cylinder boundary layer flow in this study. The difference between the in-cylinder boundary layer and the canonical turbulent boundary layer was analyzed. The experimental results show that small-scale vortices with a length scale of about 1–2 mm in the instantaneous flow fields appeared in the wall jet region due to the entrainment of the free jet in the outer region of the wall jet. The viscous sublayer thickness decreased from 0.5 mm to 0.3 mm as the valve lift increased from 2.32 mm to 7.975 mm and the pressure drop from 0.5 kPa to 1 kPa. The dimensionless velocity profile is in good agreement with the law of the wall in the viscous sublayer. However, no obvious logarithmic law distribution region was observed in the logarithmic layer. The distribution of the Reynolds stress and the turbulent kinetic energy is similar to that of the canonical turbulent boundary layer. But the Reynolds stress had a much larger magnitude because the turbulent velocity measured in this boundary layer included not only the turbulence generated by wall shear but also the large-scale turbulent vortices caused by the wall jet.

Commentary by Dr. Valentin Fuster