Measurements and simulations of mixing and autoignition of an n-heptane plume in a turbulent flow of heated air

The autoignition of a gaseous n-heptane plume in heated turbulent air has been investigated experimentally and numerically with the conditional moment closure and a CFD code. It has been demonstrated that, consistent with previous experimental results for hydrogen and acetylene, the increased scalar dissipation rate created by faster co-flowing air delays autoignition, as revealed by a disproportionate increase of ignition length with air velocity. The predicted mean and variance of the mixture fraction, the mixture fraction PDF and the conditional scalar dissipation rate are in good agreement with experimental results obtained with acetone-tracer PLIF. The first-order, spatially averaged CMC model reproduces the experimental trends quite well, despite the neglect of conditional fluctuations and spatial dependence of the conditional averages. This is attributed to the fact that for a significant period of time before autoignition the conditional scalar dissipation rate at the most reactive mixture fraction is much smaller than the critical value above which autoignition is precluded.