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Experimental and Numerical Study of the Influence of Temperature Heterogeneities on Self-Ignition Process of Methane-Air Mixtures in a Rapid Compression Machine

The self-ignition of methane-air mixtures is analyzed in conditions relevant for Homogeneous Charge Compression Ignition (HCCI) engines from both (i) experimental, and (ii) numerical modeling points of view. On the one hand, a Rapid Compression Machine (RCM) is retained as a pertaining experimental setup to carry out the first part of the study. On the other hand, the PDF method is the modeling framework chosen to describe the coupling between detailed chemical description and turbulent fluctuations. The emphasis is on the influence of the temperature heterogeneities induced by heat transfer during the compression stroke on the subsequent ignition process. Such temperature heterogeneities have already been observed in RCMs (Mittal and Sung, 2006), and they are qualitatively assessed by schlieren imaging in the present study. The simultaneous analysis of the pressure traces and direct visualizations allows to discriminate two distinct modes of ignition exhibiting different levels of coherence of the exothermic process. It is shown that the appearance of one or the other of these two modes is directly related to the temperature distribution at the onset of ignition. Then, the information gathered through the experimental results is used to check the ability of a simplified model to reproduce both qualitatively and quantitatively the observed results. The comparison with the experimental data suggests that the proposed modeling approach provides a well suited framework for further modeling studies of HCCI combustion.