Direct numerical simulations of the ignition of temporally evolving n-heptane jet under RCCI combustion-relevant conditions

Abstract

The ignition characteristics of a temporally evolving n-heptane jet under reactivity controlled compression ignition (RCCI) conditions are investigated using 2-D direct numerical simulation with a 116-species PRF reduced mechanism. For RCCI combustion, n-heptane and iso-octane are selected as two different fuels that have opposite ignition characteristics. In real engine, relatively-low reactivity fuel is delivered by port-fuel injection and relatively-high reactivity fuel is directly injected. Thus, the ignition characteristics of temporally evolving jet can be investigated with different jet velocity, U0. It is found that the first-stage ignition kernels occur within n-heptane jet near the mixing layer and develop into low temperature flame, propagating into relatively fuel-rich mixture of n-heptane jet. The high temperature ignition kernel is also formed in the nheptane jet, and then rapidly propagate into both relatively fuel-rich n-heptane jet and fuel-lean iso-octane/air mixture. Finally, the end-gas autoignition occurs. It is also found that the first- and second-stage ignitions occur quickly with increasing U0; the overall combustion is prolonged and the peak of heat release rate is reduced with increasing U0.