Ignition characteristics of a temporally evolving n-heptane jet in an iso-octane/air stream under RCCI combustion-relevant conditions

Abstract

The ignition characteristics of a temporally-evolving n-heptane jet in an iso-octane/air stream under reactivity controlled compression ignition (RCCI) combustion-relevant conditions are investigated using 2-D direct numerical simulations (DNSs) with a 116-species primary reference fuel (PRF)/air reduced mechanism. For the DNSs of RCCI combustion, iso-octane and n-heptane are chosen as two different fuels delivered by the port-fuel and direct-fuel injections, respectively. Therefore, the ignition characteristics of both fuels can be investigated by simulating the ignition of a temporally-evolving n-heptane jet with relative jet velocity, U0, within iso-octane/air charge. It is found that the first-stage ignition kernels governed by the low-temperature chemistry first develop primarily within the n-heptane jet near the mixing layer regardless of U0, and evolve into low-temperature flames, propagating into relatively fuel-rich mixtures in the n-heptane jet. High-temperature flames also develop first in the n-heptane jet, following the trajectories of low-temperature flames, and then, propagate towards both relatively fuel-lean mixtures of the iso-octane/air charge and fuel-rich mixtures of the n-heptane jet. They keep propagating into fuel-lean mixtures and finally end-gas auto-ignition occurs. It is also found that the first-stage ignition occurs more quickly with increasing U0 due to enhanced mixing between relatively cold n-heptane jet and hot iso-octane/air charge, and consequently, the second-stage ignition also advances in time with increasing U0, which are opposite to the results found in previous DNSs of RCCI combustion. Such ignition characteristics are more likely to prolong the overall combustion duration and reduce the peak of heat release rate with increasing U0. In addition, chemical explosive mode analysis (CEMA) identifies important variables and reactions for the low-, intermediate-, and high-temperature chemistries under such RCCI conditions.