MMC-LES simulations of turbulent piloted flames with varying levels of inlet inhomogeneity

This paper presents simulations of piloted turbulent flames where compositional inlet conditions vary from homogeneous to inhomogeneous. The combustion model is sparse-Lagrangian multiple mapping conditioning (MMC) coupled with large eddy simulation (LES). This is a first attempt using MMC-LES to model flows where multi-modes of combustion may occur. The studied burner has two concentric tubes surrounded by an annular pilot. The central tube carrying methane can slide within the outer air tube to induce compositional inhomogeneity at the burner exit plane. This leads to enhanced flame stability at some optimal inner fuel tube recess distance. For comparison purposes, model results are presented firstly for a case with compositional homogeneity at the inlet plane and secondly for the inhomogeneous case. The MMC-LES model shows a very good agreement with the experiment for the homogeneous-inlet case where the prevalence of non-premixed combustion is observed. The inhomogeneous-inlet case features a transition from a premixed flame structure close to the burner to a diffusion flame further downstream. It represents an extreme test of the chosen MMC model which incorporates the enforcement of mixing locally in an extended space comprised of the LES reference mixture fraction and physical location. The computed results show a fair agreement with the data close to the nozzle; closely approaching but not quite achieving the premixed structure. The return to a broad, distributed diffusion flame away from the nozzle is, however, captured very well.