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Research Papers: Gas Turbines: Aircraft Engine

J. Eng. Gas Turbines Power. 2018;140(9):091201-091201-12. doi:10.1115/1.4039732.

As the overall pressure ratio (OPR) and turbine inlet temperature (TIT) of modern gas turbines are constantly being increased in the pursuit of increasing efficiency and specific power, the effect of bleed cooling air on the engine performance is increasingly becoming important. During the thermodynamic cycle analysis and optimization phase, the cooling bleed air requirement is either neglected or is modeled by simplified correlations, which can lead to erroneous results. In this current research, a physics-based turbine cooling prediction model, based on semi-empirical correlations for heat transfer and pressure drop, is developed and verified with turbine cooling data available in the open literature. Based on the validated model, a parametric analysis is performed to understand the variation of turbine cooling requirement with variation in TIT and OPR of future advanced engine cycles. It is found that the existing method of calculating turbine cooling air mass flow with simplified correlation underpredicts the amount of turbine cooling air for higher OPR and TIT, thus overpredicting the estimated engine efficiency.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(9):091202-091202-12. doi:10.1115/1.4038794.

We systematically determine the maximally efficient manner of using water and air in a single-cycle steady-flow combustion gas turbine power plant. In doing so, we identify the upper limit to exergy efficiency for dry and wet gas turbine engines through architectures that employ regenerative work, heat, and matter transfers using imperfect practical devices. For existing device technology, the derived optimal architectures can theoretically achieve exergy efficiency above 65% without employing a bottoming cycle. This surpasses known efficiencies for both wet and combined cycles. We also show that when optimally used, nonreactive matter transfers, like water, provide an alternative, but not superior, thermal regeneration strategy to direct heat regeneration.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(9):091203-091203-11. doi:10.1115/1.4038813.

Investigations on infrared (IR) radiation suppression of axisymmetric vectoring exhaust nozzle (AVEN) are meaningful, due to the requests for maneuverability and IR stealth capability of aircrafts. In this paper, the synthetic suppression scheme of film cooling and low-emissivity coating was adopted on the center body and divergent flaps of the nozzles at 0 deg, 10 deg, and 20 deg vectoring angles. The IR signatures of both the baseline AVEN and the nozzles with IR suppression were measured. Comparing the IR signatures of the nozzles with and without IR suppression measures, the IR suppression effectiveness of the film cooling and low-emissivity coating was obtained. The investigation results indicate that the IR signatures of AVEN decrease with the increase of vectoring angle. The film cooling enables a remarkable decrease of the IR signatures of AVEN. The synthetic suppression of film cooling and low-emissivity coating enables a further decrease of IR signatures. For the case studied in this paper, the integrated radiation intensities of the nozzles with film cooling and low-emissivity coating at 0 deg, 10 deg, and 20 deg vectoring angles are decreased by 52.3%, 57.9%, and 37.2% at 0 deg measurement angle, respectively.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(9):091204-091204-8. doi:10.1115/1.4038816.

During the second half of the 90 s, NASA performed experimental investigations on six novel thrust reverser (TR) designs; core-mounted target-type thrust reverser (CMTTTR) design is one of them. To assess the CMTTTR efficiency and performance, NASA conducted several wind tunnel tests at sea level static (SLS) conditions. The results from these experiments are used in this paper series to validate the computational fluid dynamics (CFD) results. This paper is part one of the three-part series. Parts 1 and 2 discuss the CMTTTR in stowed and deployed configurations; all analyses in the first two papers are performed at SLS conditions. Part 3 discusses the CMTTTR in the forward flight condition. The key objectives of this paper are: first, to perform the three-dimensional (3D) CFD analysis of the reverser in stowed configuration; all analyses are performed at SLS condition. The second objective is to validate the acquired CFD results against the experimental data provided by NASA (Scott, C. A., 1995, “Static Performance of Six Innovative Thrust Reverser Concepts for Subsonic Transport Applications: Summary of the NASA Langley Innovative Thrust Reverser Test Program,” NASA—Langley Research Centre, Hampton, VA, Report No. TM-2000-210300). The third objective is to verify the fan and overall engine net thrust values acquired from the aforementioned CFD analyses against those derived based on one-dimensional (1D) engine performance simulations. The fourth and final objective is to examine and discuss the overall flow physics associated with the CMTTTR under stowed configuration. To support the successful implementation of the overall investigation, full-scale 3D computer aided design (CAD) models are created, representing a fully integrated GE-90 engine, B777 wing, and pylon configuration. Overall, a good agreement is found between the CFD and test results; the difference between the two was less than 5%.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2018;140(9):091501-091501-10. doi:10.1115/1.4039803.

In this study, new methodologies are introduced to analyze combustion instability in a lab-scale swirled combustor. First, with the help of radial basis function neural network (RBFNN), the flame describing function (FDF) is effectively modeled from a limited number of experimental data. This neural-network-based FDF method is able to generate more refined FDF data in an extended range. In addition, instead of a perforated plate with round holes, a slotted plate is utilized as a stabilization device. In this approach, the acoustic impedance of a slotted plate is modeled by the Dowling approach, and the dimensions of a slotted plate are optimized by simulated annealing (SA) algorithm to get the highest average absorption coefficient in a given frequency range. The present RBFNN-based FDF approach yields the reasonably good agreements with the measurements in terms of the limit-cycle velocity perturbation ratio and resonant frequency. It is also found that a slotted plate optimized by SA algorithm is quite effective to attenuate combustion instability. Numerical results obtained in this study confirm that these new methodologies are quite reliable and widely applicable for the analysis of combustion instability encountered in practical combustion systems.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(9):091502-091502-10. doi:10.1115/1.4039812.

This paper investigates the effect of the addition of natural gas (NG) and engine load on the cylinder pressure, combustion process, brake thermal efficiency, and methane combustion efficiency of a heavy-duty NG-diesel dual fuel engine. Significantly increased peak cylinder pressure (PCP) was only observed with the addition of NG at 100% load. The addition of a relatively large amount NG at high load slightly retarded the premixed combustion, significantly increased the peak heat release rate (PHRR) of the diffusion combustion, decreased the combustion duration, and advanced combustion phasing. The accelerated combustion process and increased heat release rate (HRR) at high load were supported by the increased NOx emissions with the addition of over 3% NG (vol.). By comparison, when operated at low load, the addition of a large amount of NG decreased the PHRR of the premixed combustion and slightly increased the PHRR during the late diffusion combustion. Improved brake thermal efficiency was only observed with the addition of a relatively large amount of NG at high load. The improved thermal efficiency was due to a decrease in combustion duration and the shifting of the combustion phasing toward the optimal phasing. The overall combustion efficiency of the dual fuel operation was always lower than diesel-only operation as indicated by the excess emissions of the unburned methane and carbon monoxide from dual fuel engine. This deteriorated the potential of dual fuel engine in further improving the brake thermal efficiency although the combustion duration of dual fuel engine at high load was much shorter than diesel only operation. The addition of NG at low load should be avoided due to the low combustion efficiency of NG and the decreased thermal efficiency. Approaches capable of further improving the in-cylinder combustion efficiency of NG should enable further improvement in the brake thermal efficiency.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(9):091503-091503-12. doi:10.1115/1.4039462.

In this study, we provide detailed wall heat flux measurements and flow details for reacting flow conditions in a model combustor. Heat transfer measurements inside a gas turbine combustor provide one of the most serious challenges for gas turbine researchers. Gas turbine combustor improvements require accurate measurement and prediction of reacting flows. Flow and heat transfer measurements inside combustors under reacting flow conditions remain a challenge. The mechanisms of thermal energy transfer must be investigated by studying the flow characteristics and associated heat load. This paper experimentally investigates the effects of combustor operating conditions on the reacting flow in an optical single can combustor. The swirling flow was generated by an industrial lean premixed, axial swirl fuel nozzle. Planar particle image velocimetry (PIV) data were analyzed to understand the characteristics of the flow field. Liner surface temperatures were measured in reacting condition with an infrared camera for a single case. Experiments were conducted at Reynolds numbers ranging between 50,000 and 110,000 (with respect to the nozzle diameter, DN); equivalence ratios between 0.55 and 0.78; and pilot fuel split ratios of 0 to 6%. Characterizing the impingement location on the liner, and the turbulent kinetic energy (TKE) distribution were a fundamental part of the investigation. Self-similar characteristics were observed at different reacting conditions. Swirling exit flow from the nozzle was found to be unaffected by the operating conditions with little effect on the liner. Comparison between reacting and nonreacting flows (NR) yielded very interesting and striking differences.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(9):091504-091504-9. doi:10.1115/1.4038692.

This research investigated the combustion process of an AVL Model LEF/Volvo 5312 single cylinder engine configured to simulate the operation of a heavy-duty spark ignition (SI) natural gas (NG) engine operated on stoichiometric mixture. The factors affecting the combustion process that were examined include intake pressure, spark timing (ST), and the addition of diluents including nitrogen (N2) and carbon dioxide (CO2) to the NG to simulate low British thermal unit (BTU) gases. The mixing of diluents with NG is able to slow down the flame propagation speed, suppress the onset of knock, and allow the engine to operate on higher boost pressure for higher power output. The addition of CO2 was more effective than N2 in suppressing the onset of knock and slowing down the flame propagation speed due to its high heat capacity. Boosting intake pressure significantly increased the heat release rate (HRR) evaluated on J/°CA basis which represents the rate of mass of fuel burning. However, its impact on the normalized HRR evaluated on %/°CA basis, representing the flame propagation rate, was relatively mild. Boosting the intake pressure from 1.0 to 1.8 bar without adding diluents increased the peak HRR to 1.96 times of that observed at 1.0 bar. The increase was due to the burning of more fuel (about 1.8 times), and the 12.9% increase in the normalized HRR. The latter was due to the shortened combustion duration from 23.6 to 18.2 °CA, a 22.9% reduction. The presence of 40% CO2 or N2 in their mixture with NG increased the peak cylinder pressure (PCP) limited brake mean effective pressure (BMEP) from 17.2 to about 20.2 bar. The combustion process of a turbocharged SI NG engine can be approximated by referring to the HRR measured under a naturally aspirated condition. This makes it convenient for researchers to numerically simulate the combustion process and the onset of knock of turbocharged SI NG engines using combustion process data measured under naturally aspirated conditions as a reference.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(9):091505-091505-10. doi:10.1115/1.4039731.

Over the last years, global concerns about energy security and climate change have resulted in many efforts focusing on the potential utilization of nonpetroleum-based, i.e., bioderived, fuels. In this context, n-butanol has recently received high attention because it can be produced sustainably. A comprehensive knowledge about its combustion properties is inevitable to ensure an efficient and smart use of n-butanol if selected as a future energy carrier. In the present work, two major combustion characteristics, here laminar flame speeds applying the cone-angle method and ignition delay times applying the shock tube technique, have been studied, experimentally, and by modeling exploiting detailed chemical kinetic reaction models, at ambient and elevated pressures. The in-house reaction model was constructed applying the reaction model generation (RMG)-method. A linear transformation method recently developed, linTM, was exploited to generate a reduced reaction model needed for an efficient, comprehensive parametric study of the combustion behavior of n-butanol-hydrocarbon mixtures. All experimental data were found to agree with the model predictions of the in-house reaction model, for all temperatures, pressures, and fuel-air ratios. On the other hand, calculations using reaction models from the open literature mostly overpredict the measured ignition delay times by about a factor of two. The results are compared to those of ethanol, with ignition delay times very similar and laminar flame speeds of n-butanol slightly lower, at atmospheric pressure.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Cycle Innovations

J. Eng. Gas Turbines Power. 2018;140(9):091701-091701-9. doi:10.1115/1.4039733.

This paper describes a gas turbine combined cycle (GTCC) power plant system, which addresses the three key design challenges of postcombustion CO2 capture from the stack gas of a GTCC power plant using aqueous amine-based scrubbing method by offering the following: (i) low heat recovery steam generator (HRSG) stack gas temperature, (ii) increased HRSG stack gas CO2 content, and (iii) decreased HRSG stack gas O2 content. This is achieved by combining two bottoming cycle modifications in an inventive manner, i.e., (i) high supplementary (duct) firing in the HRSG and (ii) recirculation of the HRSG stack gas. It is shown that, compared to an existing natural gas-fired GTCC power plant with postcombustion capture, it is possible to reduce the CO2 capture penalty—power diverted away from generation—by almost 65% and the overall capital cost ($/kW) by about 35%.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(9):091702-091702-9. doi:10.1115/1.4039704.

Composite cycle engines comprising piston engines (PEs) as well as piston compressors (PCs) to achieve hecto pressure ratios represent a target area of current research surpassing gas turbine efficiency. An unclear broad range of design parameters is existing to describe the design space of piston machines for this type of engine architecture. Previously published work focuses on thermodynamic studies only partially considering limitations of the design space. To untie the problem of PE design, a dimensional analysis is carried out reducing the number of parameters and deriving two basic similarity relations. The first one is a function of the mean effective pressure as well as the operating mode and is a direct result from the thermodynamic cycle. The second one is constituted of the stroke-to-bore ratio and the ratio of effective power to piston surface. Similarity relations regarding the PC design are based on Grabow (1993, “Das erweiterte “Cordier”—Diagramm Für Fluidenergiemaschinen und Verbrennungsmotoren,” Forsch. Ingenieurwes., 59, pp. 42–50). A further correlation for PCs is based on the specific compression work and the piston speed. In Part I, data of existing PEs have been subjected to the above similarity parameters unveiling the state-of-the-art design space. This allows a first discussion of current technological constraints. Applying this result to the composite cycle engine gives the design space and a first classification as a low-speed engine. Investigating various design points in terms of number and displacement volume of cylinders confirms the engine speed classification. Part II will expand this investigation using preliminary design studies.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(9):091703-091703-10. doi:10.1115/1.4038754.

In this paper, the effect of working-fluid replacement within an organic Rankine cycle (ORC) turbine is investigated by evaluating the performance of two supersonic stators operating with different working fluids. After designing the two stators, intended for operation with R245fa and Toluene with stator exit absolute Mach numbers of 1.4 and 1.7, respectively, the performance of each stator is evaluated using ANSYS cfx. Based on the principle that the design of a given stator is dependent on the amount of flow turning, it is hypothesized that a stator's design point can be scaled to alternative working fluids by conserving the Prandtl–Meyer function and the polytropic index within the nozzle. A scaling method is developed and further computational fluid dynamics (CFD) simulations for the scaled operating points verify that the Mach number distributions within the stator, and the nondimensional velocity triangles at the stator exit, remain unchanged. This confirms that the method developed can predict stator performance following a change in the working fluid. Finally, a study investigating the effect of working-fluid replacement on the thermodynamic cycle is completed. The results show that the same turbine could be used in different systems with power outputs varying between 17 and 112 kW, suggesting the potential of matching the same turbine to multiple heat sources by tailoring the working fluid selected. This further implies that the same turbine design could be deployed in different applications, thus leading to economy-of-scale improvements.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Turbomachinery

J. Eng. Gas Turbines Power. 2018;140(9):092601-092601-13. doi:10.1115/1.4039802.

The wake vortex is an important origin of unsteadiness and losses in turbines. In this paper, the development and underlying mechanisms of the shedding vortex of a high-pressure transonic turbine vane are studied and analyzed using the delayed detached eddy simulation (DDES) and proper orthogonal decomposition (POD). The goal is to understand the unsteadiness related to the wake vortex shedding and the wake evolution and mixing. Special attention is paid to the development of the wake vortex and the mechanisms behind the length characteristics. Interactions of the wake vortex with the shock wave and pressure waves are also discussed. First, the DDES simulation results are compared with published experimental data, Reynolds Averaged Navier-Stokes, and large eddy simulation (LES) simulations. Then, the development of the vane wake vortex, especially the different length characteristics from the cylinder vortex, is discussed. The reason of stronger pressure-side vortex shedding compared to suction-side vortex shedding is revealed. Wake-shock wave interaction and wake-pressure wave interaction are also investigated. The pressure waves are found to have a stronger effect than the shock wave on the spanwise motion and the dissipation of the wake vortex. An analysis of the losses through the turbine vane passage is carried out to evaluate the contributions of thermal and viscous irreversibilities. Losses analysis also confirms the strong interaction between the wake vortex and pressure waves. After that, POD study of the wake behavior was carried out. The results indicate that the shedding vortex is dominant in the unsteady flow. The phase relation between the pressure side wake vortex (PSVP) and the suction side wake vortex (SSVP) is confirmed.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(9):092602-092602-7. doi:10.1115/1.4039835.

Supercritical carbon dioxide (sCO2) power cycles require high compressor efficiency at both the design point and over a wide operating range. Increasing the compressor efficiency and range helps maximize the power output of the cycle and allows operation over a broader range of transient and part-load operating conditions. For sCO2 cycles operating with compressor inlets near the critical point, large variations in fluid properties are possible with small changes in temperature or pressure. This leads to particular challenges for air-cooled cycles where compressor inlet temperature and associated fluid density are subject to daily and seasonal variations as well as transient events. Design and off-design operating requirements for a wide-range compressor impeller are presented where the impeller is implemented on an integrally geared compressor–expander concept for a high temperature sCO2 recompression cycle. In order to satisfy the range and efficiency requirements of the cycle, a novel compressor stage design incorporating a semi-open impeller concept with a passive recirculating casing treatment is presented that mitigates inducer stall and extends the low flow operating range. The stage design also incorporates splitter blades and a vaneless diffuser to maximize efficiency and operating range. These advanced impeller design features are enabled through the use of direct metal laser sintering (DMLS) manufacturing. The resulting design increases the range from 45% to 73% relative to a conventional closed impeller design while maintaining high design point efficiency.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(9):092603-092603-10. doi:10.1115/1.4038992.

This paper proposes a hybrid method (HMRC) comprised of a radial basis function (RBF) neural net algorithm and component-level modeling method (CMM) as a real-time simulation model for triaxial gas turbines with variable power turbine guide vanes in matlab/simulink. The sample size is decreased substantially after analyzing the relationship between high and low pressure shaft rotational speeds under dynamic working conditions, which reduces the computational burden of the simulation. The effects of the power turbine rotational speed on overall performance are also properly accounted for in the model. The RBF neural net algorithm and CMM are used to simulate the gas generator and power turbine working conditions, respectively, in the HMRC. The reliability and accuracy of both the traditional single CMM model (SCMM) and HMRC model are verified using gas turbine experiment data. The simulation models serve as a controlled object to replace the real gas turbine in a hardware-in-the-loop simulation experiment. The HMRC model shows better real-time performance than the traditional SCMM model, suggesting that it can be readily applied to hardware-in-the-loop simulation experiments.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(9):092604-092604-8. doi:10.1115/1.4039057.

An experimental study has been carried out to determine how inlet total-pressure distortion affects the performance of a micro gas turbine. An inlet simulator is designed and developed to produce and measure distortion patterns at the inlet to the gas turbine. An air jet distortion generator (AJDG) is used to produce nonuniform flow patterns and total pressure probes are installed to measure steady-state total-pressure distribution at the inlet. A set of wind tunnel tests have been performed to confirm the fidelity of distortion generator and measuring devices. Tests are carried out with the gas turbine exposed to inlet flow with 60 deg, 120 deg, and 180 deg circumferential distortion patterns with different distortion intensities. The performance of the gas turbine has been measured and compared with that of clean inlet flow case. Results indicate that the gas turbine performance can be affected significantly facing with intense inlet distortions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(9):092605-092605-9. doi:10.1115/1.4038765.

Nowadays, the operative range limit of compressors is still a key aspect of the research into turbomachinery. In particular, the study of the mass flow rate lower limit represents a significant factor in order to predict and avoid the inception of critical working conditions and instabilities such as stall and surge. The identification of these instabilities and typical precursors of these two phenomena can imply many advantages, in both stationary and aeronautic applications, such as avoiding the loss of production (in industry) and efficiency of systems and reducing the maintenance and repairing cost. Many approaches can be adopted to achieve this target, but one of the most fascinating is the vibro-acoustic analysis of the compressor response during operation. At the Engineering Department of the University of Ferrara, a test bench, dedicated to the study of the performance of an aeronautic turboshaft engine multistage compressor, has been equipped with a high frequency data acquisition system. A set of triaxle accelerometers and microphones, suitable for capturing broad-band vibration and acoustic phenomena, were installed in strategic positions along the compressor and the test rig. A great amount of vibro-acoustic data were first processed through an innovative data analysis technique, and then correlated to the thermodynamic data recorded. Subsequently, the precursor signals of surge were detected and identified demonstrating the reliability of the methodology used for studying compressor instabilities. The experimental data and results offer a valid alternative way of analyzing and detecting unstable compressor behavior characteristics by means of nonintrusive measurements.

Commentary by Dr. Valentin Fuster

Research Papers: Internal Combustion Engines

J. Eng. Gas Turbines Power. 2018;140(9):092801-092801-8. doi:10.1115/1.4039809.

A partially premixed combustion (PPC) approach was applied in a single cylinder diesel research engine in order to characterize engine power improvements. PPC is an alternative advanced combustion approach that generally results in lower engine-out soot and oxides of nitrogen (NOx) emission, with a moderate penalty in engine-out unburned hydrocarbon (UHC) and carbon monoxide (CO) emissions. In this study, PPC is accomplished with a minority fraction of jet fuel injected into the intake manifold, while the majority fraction of jet fuel is delivered directly to the combustion chamber near the start of combustion (SOC). Four compression ratios (CR) were studied. Exhaust emissions plus exhaust opacity and particulate measurements were performed during the experiments in addition to fast in-cylinder combustion metrics. It was seen that as CR increased, the soot threshold equivalence ratio decreased for conventional diesel combustion; however, this afforded an increased opportunity for higher levels of port injected fuel leading to power increases from 5% to 23% as CR increased from 14 to 21.5. PPC allowed for these power increases (defined by a threshold opacity level of 3%) due to smaller particles (and lower overall number of particles) in the exhaust that influence measured opacity less significantly than larger and more numerous conventional diesel combustion exhaust particulates. Carbon monoxide levels at the higher PPC-driven power levels were only modestly higher, although NOx was generally lower due to the overall enriched operation.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(9):092802-092802-8. doi:10.1115/1.4039762.

The catalytic generation of ammonia from a liquid urea solution is a critical process determining the performance of selective catalytic reduction (SCR) systems. Solid deposits on the catalyst surface from the decomposition of urea have to be avoided, as this leads to reduced system performance or even failure. At present, reactor design is often empirical, which poses a risk for costly iterations due to insufficient system performance. The presented research project proposed a performance prediction and modeling approach for SCR hydrolysis reactors generating ammonia from urea. Different configurations of hydrolysis reactors were investigated experimentally. Ammonia concentration measurements provided information about parameters influencing the decomposition of urea and the system performance. The evaporation of urea between injection and interaction with the catalyst was identified as the critical process driving the susceptibility to deposit formation. The spray of urea solution was characterized in terms of velocity distribution by means of particle-image velocimetry. Results were compared with theoretical predictions and calculation options for processes in the reactor were determined. Numerical simulation was used as an additional design and optimization tool of the proposed model. The modeling approach is presented by a step-by-step method, which takes into account design constraints and operating conditions for hydrolysis reactors.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(9):092803-092803-7. doi:10.1115/1.4039831.

This study discusses the motion of the articulated connecting rod of an integral-engine compressor and the effect of the kinematics on in-cylinder pressure and port timings. A piston position modeling technique based on kinematics and engine geometry is proposed in order to improve the accuracy of simulated in-cylinder compression pressures. Many integral-engine compressors operate with an articulated connecting rod. For this type of engine-driven compressor, two power pistons share a crank throw with the compressor. The hinge pins that attach the power piston connecting rods to the crank are offset, causing the piston locations for each cylinder to be out of phase with each other. This causes top dead center (TDC) to occur at different crank angles, alters the geometric compression ratio, and also changes the port timings for each cylinder. In this study, the equations of motion for the pistons of the four possible compressor/piston configurations of a Cooper-Bessemer GMW are developed. With the piston profiles, the intake and exhaust port timings were determined and compared to those of a slider-crank mechanism. The piston profile was then inputted into GT-POWER, an engine modeling software developed by Gamma Technologies, in order to obtain an accurate simulation match to the experimental in-cylinder pressure data collected from a Cooper-Bessemer GMWH-10C. Assuming the piston motion of an engine with an articulated connecting rod is similar to a slider-crank mechanism can create a difference in port timings. The hinge pin offset creates asymmetrical motion about 180°aTDC, causing the port timings to also be asymmetrical about this location. The largest differences are shown in the intake port opening of about 10 deg and a difference in exhaust port opening of about 7 deg when comparing the motion of the correct configuration to the motion of a slider-crank mechanism. It is shown that properly calculating the piston motion profiles according to the crank articulation and engine geometry provides a good method of simulating in-cylinder pressure data during the compression stroke.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(9):092804-092804-9. doi:10.1115/1.4039734.

This research numerically simulates the formation and destruction of nitrogen dioxide (NO2) in a natural gas (NG)–diesel dual fuel engine using commercial CFD software converge coupled with a reduced primary reference fuel (PRF) mechanism consisting of 45 species and 142 reactions. The model was validated by comparing the simulated cylinder pressure, heat release rate (HRR), and nitrogen oxide (NOx) emissions with experimental data. The validated model was used to simulate the formation and destruction of NO2 in a NG–diesel dual fuel engine. The formation of NO2 and its correlation with the local concentration of nitric oxide (NO), methane, and temperature were examined and discussed. It was revealed that NO2 was mainly formed in the interface region between the hot NO-containing combustion products and the relatively cool unburnt methane–air mixture. The NO2 formed at the early combustion stage is usually destructed to NO after the complete oxidation of methane and n-heptane, while NO2 formed during the postcombustion process survives through the expansion process and exits the engine. The increased NO2 emissions from NG–diesel dual fuel engines was formed during the post combustion process due to higher concentration of HO2 produced during the oxidation process of the unburned methane at low temperature. A detailed analysis of the chemical reactions occurring in the NO2 containing zone consisting of NO2, NO, O2, methane, etc., was conducted using a quasi-homogeneous constant volume (QHCV) model to identify the key reactions and species dominating NO2 formation and destruction. The HO2 produced during the postcombustion process of methane was identified as the primary species dominating the formation of NO2 during the post combustion expansion process. The simulation revealed the key reaction path for the formation of HO2 noted as CH4 → CH3 → CH2O → HCO → HO2, with conversion ratios of 98%, 74%, 90%, 98%, accordingly. The backward reaction of OH + NO2 = NO + HO2 consumed 34% of HO2 for the production of NO2.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(9):092805-092805-8. doi:10.1115/1.4039750.

The design and development of high efficiency spark-ignition engines continues to be limited by the consideration of knock. Although the topic of spark knock has been the subject of comprehensive research since the early 1900s, little has been reported on the coupling of the engine thermodynamics and knock. This work uses an engine cycle simulation together with a submodel for the knock phenomena to explore these connections. First, the autoignition characteristics as represented by a recent (2014) Arrhenius expression for the reaction time of the end gases are examined for a range of temperatures and pressures. In spite of the exponential dependence on temperature, pressure appears to dominate the ignition time for the conditions examined. Higher pressures (and higher temperatures) tend to enhance the potential for knock. Second, knock is determined as function of engine design and operating parameters. The trends are consistent with expectations, and the results provide a systematic presentation of knock occurrence. Engine parameters explored include compression ratio, engine speed, inlet pressure, start of combustion, heat transfer, and exhaust gas recirculation (EGR). Changes of cylinder pressures and temperatures of the unburned zone as engine parameters were varied are shown to be directly responsible for the changes of the knock characteristics.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(9):092807-092807-11. doi:10.1115/1.4039082.

In-cylinder air flow structure makes significant impacts on fuel spray dispersion, fuel mixture formation, and flame propagation in spark ignition direct injection (SIDI) engines. While flow vortices can be observed during the early stage of intake stroke, it is very difficult to clearly identify their transient characteristics because these vortices are of multiple length scales with very different swirl motion strength. In this study, a high-speed time-resolved two-dimensional (2D) particle image velocimetry (PIV) is applied to record the flow structure of in-cylinder flow field along a swirl plane at 30 mm below the injector tip. First, a discretized method using flow field velocity vectors is presented to identify the location, strength, and rotating direction of vortices at different crank angles. The transients of vortex formation and dissipation processes are revealed by tracing the location and motion of the vortex center during the intake and compression strokes. In addition, an analysis method known as the wind-rose diagram, which is implemented for meteorological application, has been adopted to show the velocity direction distributions of 100 consecutive cycles. Results show that there exists more than one vortex center during early intake stroke and their fluctuations between each cycle can be clearly visualized. In summary, this approach provides an effective way to identify the vortex structure and to track the motion of vortex center for both large-scale and small-scale vortices.

Commentary by Dr. Valentin Fuster

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