A DNS Study of Ignition Characteristics of a Lean PRF/Air Mixture with Mixing Layer under RCCI Conditions

Abstract

The ignition characteristics of a lean PRF/air mixture with a mixing layer under reactivity controlled compression ignition (RCCI) conditions are investigated using 2-D direct numerical simulation with a 116-species PRF reduced mechanism. To account the compression heating and expansion cooling effects by piston motion, the compression heating/cooling model is adopted. For RCCI combustion, iso-octane and n-heptane are chosen as two different fuels which have low and high reactivity, respectively. The effect of the mixing layer developed by direct injection on the ignition is investigated. It is found that the first low temperature combustion, 1st stage ignition, occurs at the mixing layer, and it becomes flame and propagate into richer mixture fraction. The high temperature ignition kernels formed at n-heptane region and rapidly expanded toward both leaner and richer mixture fraction regions. The flame continued moving into leaner mixtures and, finally, the end-gas autoignition occurs. From the results, it is found that the mixing layer affects the development of low temperature combustion and subsequently the high temperature combustion.