Numerical Prediction of the Autoignition Delay in a Diesel-Like Environment by the Conditional Moment Closure Model

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Abstract
The autoignition delay of a turbulent methane jet in a Diesel-like environment is calculated by the conditional moment closure(CMC) model. Methane is injected into hot air in a constant volume chamber under various temperatures and pressures. Detailed chemical reaction mechanisms are implemented with turbulence-chemistry interaction treated by the first order CMC. The CMC model solves the conditional mean species mass fraction and temperature equations with the source term given in terms of the conditional mean quantities. The flow and mixing field are calculated by the transient SIMPLE algorithm with the k -ε model and the assumed beta function pdf. The CMC equations are solved by the fractional step method which sequentially treats the transport and chemical reaction terms in each time step. The predictions in quiescent homogeneous mixture are presented to evaluate the effects of turbulence in jet ignition. Three major trends observed in the experiment are well reproduced by the present CMC results. First, the autoignition delay of a methane jet has strong dependence on the air temperature. It decreases significantly as the air temperature increases. Second, the chamber pressure has only a minor effect on the autoignition delay, which tends to decrease slightly at a higher ambient pressure. Third, there is a slight decrease in the autoignition delay of methane-ethane mixture as the fraction of ethane increases. The first order CMC with detailed chemical reaction mechanisms has given reasonable predictions of both the magnitude and trend of variation of the autoignition delay, although there is room for further improvement in their accuracy.

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