Modeling of autoignition of a turbulent methane jet by the conditional moment closure model

Abstract

Autoignition of a turbulent methane jet has been studied by the first-order conditional moment closure (CMC) model with die detailed chemical reaction mechanism GRI Mech 3.0. Methane was injected into hot air in a constant volume chamber under Various initial temperatures and pressures. The flow and mixing field were calculated by the transient SIMPLE algorithm with the kappa-epsilon -g turbulence model. The CMC equations were solved by the fractional step method, which sequentially treats the transport and chemical reaction terms in each time step. The stiff ordinary differential equation solver was used for chemical reaction steps. The calculated ignition delays are in good agreement in both magnitudes and major trends of variation in the measurements. The ignition delay decreases significantly as the initial air temperature increases. The chamber pressure has only a minor effect on the ignition delay, which tends to decrease slightly at a higher ambient pressure. There is a slight decrease in the ignition delay of methane/ ethane mixture as the fraction of ethane increases. Comparison with the homogeneous CMC,;which ignores spatial variation of the conditional moments, shows that the spatial dependence should be taken into account for accurate prediction of the ignition delays. It is shown that autoignition occurs on the sides of a fuel jet, where die most reactive mixture fraction is combined with a low conditional mean scalar dissipation rate.