An LES/PDF study of a turbulent lifted ethylene jet flame in a heated coflow

We report a large-eddy simulation (LES)/probability density function (PDF) modeling study of a turbulent lifted ethylene jet flame in a heated air coflow, using a 22-species reduced chemical mechanism. In this flame, the central ethylene jet is ignited by the heated coflow, and the lifted jet flame base stabilization reaches a statistically stationary state. In this LES/PDF study, the filtered velocity field in the LES and PDF transport equations are solved by the highly-scalable NGA/HPDF code with second-order accuracy in space and time. The LES and PDF solvers are fully coupled and the transport properties used in the LES solver are obtained from CHEMKIN, based on the resolved density, temperature, and species mass fractions obtained from the PDF solver. The interaction-by-exchange with the mean (IEM) mixing model with molecular transport is applied, which includes the effects of differential diffusion on molecular transport in the PDF modeling. The performance of LES/PDF is assessed through a posteriori comparisons with the direct numerical simulation (DNS) of the same flame (C. S. Yoo et al., Proc. Combust. Inst., 2011). The comparison shows overall good agreement of the mean mixture fraction and the mean temperature. In particular, the effects of differential diffusion on the prediction of auto-ignition and the lifted flame stabilization are quantified through the conditional mean temperature. The incorporation of differential diffusion is shown to be important to obtain an accurate prediction of auto-ignition in LES/PDF calculations of this flame.