A DNS Study of the Effects of Shear Velocity on the Ignition of a Lean PRF/air Mixture with n-Heptane Stream under RCCI Conditions

Abstract

The effects of shear velocity on the ignition characteristics of a lean primary reference fuel (PRF)/air mixture with an n-heptane stream under reactivity controlled compression ignition (RCCI) conditions are investigated using 2-D direct numerical simulations (DNSs) with a 116-species PRF/air reduced mechanism. For RCCI combustion, iso-octane and n-heptane are chosen as two different fuels which have low and high reactivity, respectively. The effects of the n-heptane stream on the ignition of PRF/ air mixture are investigated by varying relative mixing layer velocity, U0, developed between the portinjected iso-octane/air stream and directly-injected n-heptane stream. It is found that the first-stage ignition kernels governed by the low temperature chemistry develop primarily near the mixing layer in the n-heptane stream and evolve into low temperature flames, propagating into relatively fuel-rich mixtures. High temperature flames also develop in the n-heptane stream, following the trajectories of low temperature flames and propagate toward both relatively fuel-lean and fuel-rich mixtures. The high temperature flames keep propagating into fuel-lean mixtures and finally, the end-gas auto-ignition occurs. As such, both the first- and second-stage ignitions are found to be advanced in time with increasing U0, which is opposite to the results in previous DNSs of RCCI combustion with homogeneous turbulence. From the chemical explosive mode analysis (CEMA), important variables and reactions to the low-, intermediate-, and high-temperature chemistries under RCCI condition are identified.