Simulations of soot formation in high-pressure transient spray flames

A transported probability density function (PDF) method is exercised to generate insight into soot processes in transient high-pressure turbulent n-dodecane spray flames, under diesel-engine-relevant conditions. The emphasis is on a split-injection case, where the fuel is injected in two pulses of 0.5 ms each, separated by a dwell time of 0.5 ms during which no fuel is injected. PDF model results are compared with experimental measurements and with results from a locally well-stirred reactor (WSR) model that neglects unresolved turbulent fluctuations in composition and temperature. Computed total soot mass is highly sensitive to the modeling of unresolved turbulent fluctuations. To achieve the best agreement between model and experiment and to capture the highly intermittent nature of soot in the turbulent flame, it is necessary to accurately represent mixing and the high effective Schmidt number (low diffusivity) of soot particles. This is accomplished in the PDF framework using a mixing model that enforces locality in the gas-phase composition space, and not mixing the transported soot variables in the soot model. The results suggest that mixing is at least as important as kinetics in controlling soot formation and evolution in high-pressure turbulent flames.