Influence of heat release and turbulence on scalar dissipation rate in autoigniting n-heptane/air mixtures

Abstract

Scalar dissipation rate χ is an important parameter in stratified charge combustion because it is a measure of the extent of fuel/air mixing in the flow field. In this work, the influence of heat release on the evolution of χ in laminar and turbulent autoigniting stratified n-heptane/air mixtures is studied. Multi-step kinetics is employed to model n-heptane oxidation. The pressure and temperature conditions selected are relevant to compression-ignited engines. Ignition is initiated in fuel-rich regions. Subsequently, an ignition front is observed to propagate from the initiation location to the final flame stabilization location. The influence of heat release on scalar dissipation rate is observed to be controlled by a balance of two competing effects: expansion of burned gases which decreases χ and increased laminar diffusivity due to combustion which increases χ. Behind the propagating ignition front, decrease in χ due to expansion is greater than its increase due to increased laminar diffusivity; therefore, the net effect of heat release on χ is to decrease it. The magnitude of decrease in χ due to expansion is observed to increase with increasing compositional gradients in the flow field. Ahead of the ignition front, the increased diffusivity effect is more pronounced than the expansion effect which results in an increase in χ. Turbulence, as expected, modifies the initially specified scalar dissipation rate to generate a range of scalar dissipation rates around the initial value. Turbulent mixing creates well-mixed spots with low χ, and turbulent strain creates regions with high χ. Ignition is observed in the regions with low χ, and the expansion effect due to heat release further decreases the scalar dissipation rate at such locations.