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Direct Numerical Simulation of Ignition Front Propagation in a Constant Volume With Temperature Inhomogeneities

J. H. Chen, E. R. Hawkes, R. Sankaran, S. D. Mason, and H. G. Im

For more information on the project: http://scidac.psc.edu

Understanding inhomogeneous autoignition may lead to the development of control strategies for homogeneous charge compression ignition combustion (HCCI), a concept being considered for compression ignition automotive engines. HCCI is promising because of the high diesel-like efficiencies that are possible, without the incumbent NOx and soot. This is achieved by burning overall lean such that combustion occurs primarily by volumetric autoignition of a lean premixed charge, in the absence of an external spark. Since it is kinetically driven, it is difficult to control the combustion rate. One possible control strategy is to introduce temperature inhomogeneities, tailored to produce the desired heat release rate. In practice, also, the imperfect mixing of the charge along with temperature differences between the bulk gases and the cylinder walls can also lead to nonuniformities that could contribute to a range of HCCI combustion modes different than homogeneous ignition.

A parametric study of the autoignition of a thermally stratified lean hydrogen-air mixture was performed. The parametric study focused on the influence of the amplitudes and length scales of the initial temperature fluctuations and the presence of turbulence. The primary goal was to understand the effect of these parameters on the mode of combustion and on the performance of a model that is applicable to weakly stratified ignition, the multi-zone model. The combustion mode was understood by visual comparison of the observed heat release fields, and by two diagnostic techniques that we developed [9]. The first diagnostic technique was based on tracking the speed of the advancing combustion wave, and comparison of this with a nominal deflagration speed to determine the significance of molecular transport effects within the ignition front. The second technique was to use the comparison of the mixing time-scale with the ignition delay time to determine the influence of passive scalar mixing changing the probability density of temperature in the domain. These diagnostic techniques were applied to the data and used to explain the performance of the multi-zone model. In all cases the observed performance could be adequately explained using these simple arguments.

A parametric study in the RMS temperature fluctuation revealed that this parameter has a strong influence on the observed combustion mode, and on the timing and duration of heat release. Larger temperature fluctuation amplitudes were found to increase the combustion duration and advance the ignition timing. More volumetric, homogeneous combustion was observed for lower amplitudes whereas higher amplitude cases showed evidence of combustion in fronts. Using the flame speed diagnostic, it was found that higher amplitudes led to a greater prevalence of molecular transport within the fronts (i.e. deflagrations). The performance for the multi-zone model was linked to the effects of passive scalar dissipation and the prevalence of deflagrations. Very good performance was obtained in cases without significant deflagrations and when passive scalar mixing had been mostly accounted for by making the comparison at a point in time when ignition had begun. Predictions deteriorated as the prevalence of deflagrations increased, i.e. at higher amplitudes.

J. H. Chen, E. R. Hawkes, R. Sankaran, S. D. Mason, and H. G. Im, "Direct Numerical Simulation of Ignition Front Propagation in a Constant Volume With Temperature Inhomogeneities, Part I: Fundamental Analysis and Diagnostics", submitted to Combust. Flame, (2004).

E. R. Hawkes, R. Sankaran, P. Pebay, and J. H. Chen, "Direct Numerical Simulation of Ignition Front Propagation in a Constant Volume With Temperature Inhomogeneities, Part II: Parametric Study", submitted to Combust. Flame, (2004).

R. Sankaran, H. G. Im, E. R. Hawkes, and J. H. Chen, "The Effects of Nonuniform Temperature Distribution on the Ignition of a Lean Homogeneous Hydrogen-Air Mixture," Proceedings of the Combustion Institute, 30, pp. 875-882. (2004).

     
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