An analytic probability density function for partially premixed flames with detailed chemistry

Abstract

Laminar premixed flame profiles of methane/air free flames and strained flames at different fuel/air ratios and strain rates are analyzed using detailed chemistry with Lewis numbers equal to one. It is shown that the detailed chemistry flame profiles of progress variables CO2 + CO and H2O + H2 in canonically stretched coordinates can be fitted accurately by a slight generalization of recently proposed analytical presumed flame profiles over a wide range of fuel/air ratios through adaptation of a single model parameter. Strained flame profiles can be reproduced using an additional linear coordinate transformation, emulating the compression of the preheat zone by strain as predicted by premixed flame theory. The model parameter can alternatively be determined using only the laminar flame speeds and the fully burnt temperatures from the laminar flame calculations. The stretch factor of the coordinate transformation is proportional to cp/lambda, which drops by a factor up to 4 across the laminar flame. It is shown how the non-constant cp/lambda modifies the laminar flame probability density function (pdf) and a polynomial fit to cp/lambda as a function of the progress variable allows analytical results for the laminar flame pdf and the mean value of the progress variable and of the reaction source term. An analytic pdf for partially premixed flames is proposed based on Bayes's theorem as a combination of a beta pdf for the mixture fraction and the laminar flame pdf's evaluated at the respective fuel/air ratio.