An Experimental and Simulation Study of Early Flame Development in a Homogeneous-charge Spark-Ignition Engine

Oil & Gas Science and Technology, Jul 2018

An integrated experimental and Large-Eddy Simulation (LES) study is presented for homogeneous premixed combustion in a spark-ignition engine. The engine is a single-cylinder two-valve optical research engine with transparent liner and piston: the Transparent Combustion Chamber (TCC) engine. This is a relatively simple, open engine configuration that can be used for LES model development and validation by other research groups. Pressure-based combustion analysis, optical diagnostics and LES have been combined to generate new physical insight into the early stages of combustion. The emphasis has been on developing strategies for making quantitative comparisons between high-speed/high-resolution optical diagnostics and LES using common metrics for both the experiments and the simulations, and focusing on the important early flame development period. Results from two different LES turbulent combustion models are presented, using the same numerical methods and computational mesh. Both models yield Cycle-to-Cycle Variations (CCV) in combustion that are higher than what is observed in the experiments. The results reveal strengths and limitations of the experimental diagnostics and the LES models, and suggest directions for future diagnostic and simulation efforts. In particular, it has been observed that flame development between the times corresponding to the laminar-to-turbulent transition and 1% mass-burned fraction are especially important in establishing the subsequent combustion event for each cycle. This suggests a range of temporal and spatial scales over which future experimental and simulation efforts should focus.

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An Experimental and Simulation Study of Early Flame Development in a Homogeneous-charge Spark-Ignition Engine

Oil & Gas Science and Technology - Rev. IFP Energies nouvelles An Experimental and Simulation Study of Early Flame Development in a Homogeneous-Charge Spark-Ignition Engine Y. Shekhawat 2 3 D.C. Haworth 2 3 A. d'Adamo 1 3 F. Berni 1 3 S. Fontanesi 1 3 P. Schiffmann 0 3 D.L. Reuss 0 3 V. Sick 0 3 0 Department of Mechanical Engineering, University of Michigan , Ann Arbor, MI - USA 1 Department of Engineering “Enzo Ferrari”, University of Modena and Reggio Emilia , Modena - Italy 2 Department of Mechanical & Nuclear Engineering, The Pennsylvania State University , University Park, PA - USA 3 Clearance @ TDC , cm - An integrated experimental and Large-Eddy Simulation (LES) study is presented for homogeneous premixed combustion in a spark-ignition engine. The engine is a single-cylinder two-valve optical research engine with transparent liner and piston: the Transparent Combustion Chamber (TCC) engine. This is a relatively simple, open engine configuration that can be used for LES model development and validation by other research groups. Pressure-based combustion analysis, optical diagnostics and LES have been combined to generate new physical insight into the early stages of combustion. The emphasis has been on developing strategies for making quantitative comparisons between high-speed/high-resolution optical diagnostics and LES using common metrics for both the experiments and the simulations, and focusing on the important early flame development period. Results from two different LES turbulent combustion models are presented, using the same numerical methods and computational mesh. Both models yield Cycle-to-Cycle Variations (CCV) in combustion that are higher than what is observed in the experiments. The results reveal strengths and limitations of the experimental diagnostics and the LES models, and suggest directions for future diagnostic and simulation efforts. In particular, it has been observed that flame development between the times corresponding to the laminar-to-turbulent transition and 1% mass-burned fraction are especially important in establishing the subsequent combustion event for each cycle. This suggests a range of temporal and spatial scales over which future experimental and simulation efforts should focus. INTRODUCTION Cycle-to-Cycle Variations (CCV) of flow and combustion in spark-ignition engines have been the subject of extensive research over the last few decades [ 1 ]. High-speed/highresolution optical diagnostics and Large-Eddy Simulation (LES) have been brought to bear to understand the root causes of CCV, and significant progress has been made in that regard. Multiple sources of variability have been identified. These include local variations in the flow, temperature and mixture composition, and in the spark discharge, as well as global variations in equivalence ratio, dilution and trapped mass [ 2–5 ]. While these contributing factors have been recognized for some time, their interactions and influences on combustion remained unclear due to a lack of multi-diagnostic data and the difficulty of performing accurate LES simulations over a sufficiently large number of cycles. Lacour and Pera [5] and Baum et al. [ 6 ] performed multi-diagnostic experiments that allow a deeper understanding of the coupled physics in engines. And recently these interactions have been investigated using multi-parameter experimental [ 6, 7 ] and simulation [ 6, 8 ] approaches. Earlier experimental and simulation studies (e.g., [ 9, 10 ]) have shown that what happens during the earliest stages of combustion can determine, to a large extent, the subsequent combustion process for that cycle. Under realistic engine operating conditions, the velocity magnitude [ 7, 8 ] and velocity gradient parameters [ 7 ] in the vicinity of the spark plug at the time of ignition have been found to correlate with the Indicated Mean Effective Pressure (IMEP) for the cycle. Motivated by the difficulties of making accurate pressurebased combustion measurements during the early flame development period, some LES studies have focused on early-flame-kernel development, toward developing an understanding of the governing factors that result in cyclic variation of flame growth. Granet et al. [ 11 ] demonstrated that initial flame convection within the spark plug gap is a probable reason for incomplete combustion cycles due to local quenching, while Goryntsev et al. [ 12, 13 ] emphasized the superposition of flow variation and mixture quality for direct-injection engines, whose combined effect on combustion CCV was elucidated by LES. For these reasons, the focus here is on the early burn: ignition through fully developed turbulent flame. A specific goal is to assess the predictive capability of two different ignition and turbulent flame propagation models, using the same CFD code and computational mesh. The approach is to compare the simulated and measured combustion using a single set of metrics that are ap (...truncated)


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Y. Shekhawat, D.C. Haworth, A. d'Adamo, F. Berni, S. Fontanesi, P. Schiffmann, D.L. Reuss, V. Sick. An Experimental and Simulation Study of Early Flame Development in a Homogeneous-charge Spark-Ignition Engine, Oil & Gas Science and Technology, pp. 32, Volume 72, Issue 5, DOI: 10.2516/ogst/2017028