Transient soot dynamics in turbulent nonpremixed ethylene–air counterflow flames

Abstract

The dynamics of soot formation in turbulent ethylene–air nonpremixed counterflow flames is studied using direct numerical simulation (DNS) with a semi-empirical soot model and the discrete ordinate method (DOM) as a radiation solver. Transient characteristics of soot behavior are studies by a model problem of flame interaction with turbulence inflow at various intensities. The interaction between soot and turbulence reveals that the soot volume fraction depends on the combined effects of the local conditions of flow, temperature, and fuel concentration, while the soot number density depends predominantly on the high temperature regions. Depending on the relative strength between mixing and reaction, the effects of turbulence on the soot formation lead to three distinct paths in deviating the data points away from the laminar flame conditions. It is found that turbulence has twofold effects of increasing the overall soot yield by generating additional flame volume and of reducing soot by dissipating soot pockets out of high-temperature regions. The relative importance between the two effects depends on the relative length scales of turbulence and flame, suggesting that a nonmonotonic response of soot yield to turbulence level may be expected in turbulent combustion.