Direct numerical simulations of the ignition of a lean biodiesel/air mixture with temperature and composition inhomogeneities at high pressure and intermediate temperature

The effects of the stratifications of temperature, T, and equivalence ratio, ϕ, on the ignition characteristics of a lean homogeneous biodiesel/air mixture at high pressure and intermediate temperature are investigated using direct numerical simulations (DNSs). 2-D DNSs are performed at a constant volume with the variance of temperature and equivalence ratio (T′ and ϕ′) together with a 2-D isotropic velocity spectrum superimposed on the initial scalar fields. In addition, three different T-ϕ correlations are investigated: (1) baseline cases with T′ only or ϕ′ only, (2) uncorrelated T-ϕ distribution, and (3) negatively-correlated T-ϕ distribution. It is found that the overall combustion is more advanced and the mean heat release rate is more distributed over time with increasing T′ and/or ϕ′ for the baseline and uncorrelated T-ϕ cases. However, the temporal advancement and distribution of the overall combustion caused by T′ or ϕ′ only are nearly annihilated by the negatively-correlated T-ϕ fields. The chemical explosive mode and Damköhler number analyses verify that for the baseline and uncorrelated T-ϕ cases, the deflagration mode is predominant at the reaction fronts for large T′ and/or ϕ′. On the contrary, the spontaneous ignition mode prevails for cases with small T′ or ϕ′, especially for cases with negative T-ϕ correlations, and hence, simultaneous auto-ignition occurs throughout the entire domain, resulting in an excessive rate of heat release. It is also found that turbulence with large intensity, u′, and a short time scale can effectively smooth out initial thermal and compositional fluctuations such that the overall combustion is induced primarily by spontaneous ignition. Based on the present DNS results, the generalization of the effects of T′,ϕ′, and u′ on the HCCI combustion is made to clarify each effect. These results suggest that temperature and composition stratifications together with a well-designed T-ϕ correlation can alleviate an excessive rate of pressure rise and control the ignition-timing in homogeneous charge compression-ignition (HCCI) combustion.