Effect of molecular transport on PDF modeling of turbulent non-premixed flames

Comprehensive studies are performed to evaluate the effect of molecular transport on the probability density function (PDF) modeling of turbulent non-premixed flames. The widely used Sandia piloted jet flame E is chosen as the validation test case. Three configuration cases are considered: neglecting the molecular transport (Case-A), considering the molecular transport (Case-B), and considering both the molecular transport and its effect on mixing (Case-C). Extensive testing is done to assess the effect of molecular transport on the performance of the different mixing models and on the prediction of the flame extinction limit. It is found that the molecular transport has a significant effect on the prediction of the upstream mixing layer between the pilot stream and the coflow, where the turbulence level is weak or it is laminar and hence the molecular transport dominates the turbulent transport. Accounting for the molecular transport increases the models' resistance (for the IEM model and the modified Curl model) to flame extinction and yields a burning flame with a lower mixing rate that can cause global extinction without molecular transport. A modification to the mixing frequency is introduced to account for the effect of the molecular diffusion on mixing. With such a modification, the modeled flame sustainability is further enhanced and a lower mixing rate can be used, which enhances the ability of the IEM model and the modified Curl model which were previously found to yield a burning flame solution only at a highly increased rate of mixing.