Investigation of Reactive Scalar Mixing in Transported PDF Simulations of Turbulent Premixed Methane-Air Bunsen Flames

Abstract

Transported probability density function (TPDF) simulations have been performed in conjunction with DNS data to investigate the mixing characteristics of reactive scalars in two turbulent lean premixed methane-air Bunsen flames with Case A being close to the corrugated flamelet regime and Case C being close to the broken reaction zones regime. The study shows that with an accurate mixing timescale of progress variable being provided, TPDF simulations using the EMST mixing model predict scalar mixing and flame characteristics reasonably well. Modeling reactive scalar mixing rate remains one key challenge. For turbulent flames close to the flamelet regime, i.e. Case A, the turbulent flame structure represented by the scatter of OH, as well as the resemblance of the flame induced dissipation rate to the actual dissipation rate, highlights the necessity to account for flame structure when modeling reactive scalar mixing in flamelet region. A posteriori tests show that the hybrid mixing timescale model, which accounts for both turbulence and flame structure effects on the scalar mixing timescale, yields better performance than the constant mechanical-to-scalar timescale model for turbulent premixed flames close to the flamelet regime. Moreover, the hybrid model shows potential for modeling differential mixing rates of intermediate species featuring their own characteristic timescales. The effects of progress variable definition and turbulence modeling on the computed flame characteristics are investigated, and the significance of turbulence modeling in RANS-TPDF simulation is illustrated.