Direct numerical simulation of DME auto-ignition with temperature and composition stratification under HCCI engine conditions

Two-dimensional (2D) direct numerical simulation (DNS) was used to reveal the effect of temperature and composition stratifications on the ignition characteristics of dimethyl-ether (DME)/air mixture at three initial mean temperatures. The simulation was conducted within a constant volume with an isotropic turbulence condition under 40 bar. Nine 2-D cases with temperature stratification (T′), composition stratification (Ф′) and different T-Ф correlations (negative-correlated, uncorrelated, positive-correlated) were performed under the mean initial temperature of T₀ = 780 K, 840 K and 1037 K. The results show that the mean heat release rate (HRR) distribution largely depends on T₀ when temperature stratification was applied. When T₀ = 780 K, T′ can obviously reduce the maximum of mean HRR and prolong combustion duration compared with 0D case. However, the HRR profile is barely changed for T₀ = 940 K and 1037 K. Whereas, it was found that composition stratification can reduce the peak value of mean HRR and spread out HRR distribution regardless of initial mean temperature. For first ignition stage, the temperature stratification has larger effect on reducing HRR than composition stratification. Three cases with different T-Ф correlations all reduce the peak value of HRR at first ignition stage. However, for high temperature stage, UC and PC T-Ф field can largely reduce the peak value of HRR and prolong combustion duration while NC T-Ф field shows no visible effect on HRR. In addition, two different combustion modes i.e. flame propagation mode and volumetric auto-ignition mode were distinguished according to HRR contours, budget term and volume averaged temperature gradient. And the mass fraction of Y_OH+HO2 and Y_H are the more reasonable choice to track the flame front.