Analysis of conditional statistics of a supersonic jet flame in heated coflow via direct numerical simulation

摘要

Direct numerical simulation of a three-dimensional spatially-developing supersonic turbulent round jet flame in heated coflow has been conducted. The lifted flame is auto-ignited and stabilized with a flame base. The flame surface presents to be quite corrugated due to interactions with turbulence. The mean heat release rate conditioned on curvature of the flame surface presents to be larger for positive than negative. The supersonic jet spreads slower than subsonic case. The mean center line velocity increases slightly far field due to heat release, which is definitely different from the non-reacting flows where the velocity decays monotonously. The large fluctuations of the species mass fraction and temperature in the mixture fraction space result in large discrepancy when using the first order CMC closure where the fluctuations neglected. Correction of the reaction source terms using conditional PDF approach based on doubly condition moment closure is proved to be effective, however the accuracy of the singly conditioned chemical source terms depends to a large extent on accurate assumption for the pdf of progress variable. The Beta pdf performs better in the far field of the jet flame, while very poor in the region near the nozzle.