Turbulent flame propagation in partially premixed combustion

Stratified spark-ignition engines that use direct fuel injection into the combustion chamber feature both small- and large-scale spatial variations in unburned mixture composition. In these configurations, the spark-ignited turbulent flame propagates into a mixture with variable equivalence ratio. Under lean-rich conditions, the reaction zone can be described as a staged combustion system with a first stage corresponding to a propagating premixed flame front followed by a second stage corresponding to multiple post-diffusion flames that burn the remaining excess fuel and excess oxidizer. Direct numerical simulations (DNS) are used in the present study to bring basic information onto the effects of partial premixing both on the turbulent flame topology and the overall mean reaction rate and to determine if and how flamelet combustion models should be modified to account for these effects. The numerical configuration corresponds to three-dimensional partially premixed flames propagating into a temporally decaying turbulent flow. The simulations use different degrees of scalar inhomogeneity around mean stoichiometric conditions. It is found that while partial premixing has a negligible contribution to the premixed flame surface wrinkling, it has a net negative impact on the overall mean premixed reaction rate. This effect is related to the reduced values of mean flamelet mass burning rate per unit flame surface area observed in partially premixed configurations compared to perfectly premixed flames.