Large eddy simulation/probability density function modeling of turbulent swirling stratified flame series

Abstract

The large eddy simulation/probability density function (LES/PDF) method is applied to a turbulent swirling stratified flame series to systematically investigate its performance for accurate prediction of effects of stratification on turbulent flames under swirling conditions. The Cambridge/Sandia turbulent stratified flame series is selected as a target flame series. An augmented reduced mechanism is used for the methane/air combustion. The chemical calculations are accelerated by using the in situ adaptive tabulation method. The differential diffusion and heat loss from the bluff-body surface are taken into account in the simulations. The effect of stratification in fuel concentration is studied by increasing the stratification progressively from the pure premixed case to moderately and highly stratified cases. The performance of the LES/PDF modeling is evaluated by comparing the numerical results with the experimental data. The computed mean and rms of velocity, temperature, equivalence ratio, and mass fractions of species are found to be in good agreement with the measurements for all three conditions. Scatter plots and conditional means of mole fractions of species and temperature are presented and found to be in overall good consistency with those obtained from the experimental measurements. The recirculation zones are found to be about five times longer than those obtained under the non-swirling conditions. A low-equivalence-ratio high-temperature region near the bluff body is not captured in the computation, which is attributed to the insufficient entrainment of downstream mixtures into the recirculation zone. The parametric studies show that the differential diffusion has a minor effect on the mean and rms quantities, while the heat loss has a considerable effect on temperature and CO profiles close to the bluff body.