A DNS Study of Ignition of Lean PRF/Air Mixtures under HCCI Conditions

Direct numerical simulations (DNSs) of ignition of lean primary reference fuel (PRF)/air mixtures under homogeneous charge compression ignition (HCCI) conditions were performed using a 116-species reduced mechanism. The influence of fuel composition and thermal stratification on ignition characteristics of lean PRF/air mixtures is investigated. Displacement speed and Damköhler number analyses reveal that high temperature fluctuation induces deflagration waves at the reaction fronts such that mean heat release rate increases more slowly and more spread out in time, consequently preventing excessive rate of pressure rise throughout the ignition process. The propagation characteristics of deflagration waves of different PRF/air mixtures are almost the same such that the overall combustion characteristics of different PRF/air mixtures become nearly identical and thus, the effect of fuel composition on the ignition vanishes. On the contrary, for small degree of thermal stratification, spontaneous ignition prevails during the whole ignition process, and hence, those ignition characteristics are quite similar to the corresponding 0-D homogeneous auto-ignition