A numerical investigation of the flame structure and blowoff characteristics of a bluff-body stabilized turbulent premixed flame

Bluff-body stabilized premixed flames have been a subject of significant technological interest in practical combustion devices. A collaborative computational-experimental effort is reported here, on simulating a series of bluff-body stabilized premixed propane flames near lean blowoff. An OpenFOAM-based large eddy simulation (LES) solver is coupled with an adaptive hybrid integration solver with sparse matrix technique, to enable efficient chemistry integration. A 31-species skeletal mechanism matching interested flame characteristics is systematically reduced from USC-Mech II, and is utilized to describe the finite-rate chemistry. A procedure to convert numerical mass fractions to, pseudo Planar Laser Induced Fluorescence (PLIF) images is established to facilitate the comparison of flame topology and statistics with corresponding experiments. The study focuses on two stably burning conditions, and subsequently on a condition where global blowoff is observed. Velocity statistics, PLIF images of OH and CH2O, flame brush thickness, turbulent flame speed, and statistics of two dimensional strain rates are compared against experimental data, achieving reasonably good agreement under stably burning conditions. Approaching blowoff, the flame surfaces recede into the recirculation zone due to the reduced flame speed at lower equivalence ratios. Frequent flame-flame interaction in the narrowed necking region near the top boundary of the recirculation zone, asymmetric helical flame structure, and accumulated CH2O in the recirculation zone are observed near blowoff. Elevated levels of unburned fuel is observed in the recirculation zone, suggesting significant entrainment of fresh mixture near global blowoff. Analysis of the enstrophy transport indicates increasing strength of enstrophy approaching blowoff, which results from diminishing baroclinic torque and dilatation effects when density ratios in the anchoring region are reduced. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.