Flame-resolved transient simulation with swirler-induced turbulence applied to lean blowoff premixed flame experiment

Abstract

This article presents a flame-resolved transient simulation of the Cavaliere et al premixed flame experiment [1] to investigate the mechanisms that lead towards lean blowoff/blowout (LBO). The computational domain includes the swirler to capture the unsteady turbulent 3D velocity field generated. The computational grid design intent is to minimize the use of the subgrid scale models in order to resolve most of the turbulence scales in both the fresh and the burnt gases as well as the flame front thickness throughout the computational domain, resulting in a structured mesh with a grid count of 236 million cells. A transient sequence corresponding to a step change of equivalence ratio Φ from 0.7 to 0.55 is modeled. The combustion chemistry mechanism consists of a single step overall reaction. The computational results are compared to experimental data and an overall good agreement is observed, except for the latest times of the LBO sequence. The validated computational results are analyzed with dynamic mode decomposition (DMD) technique, flow field visualizations, signals time-traces and space-time diagrams of a posteriori reconstructed OH fields to investigate the underlying mechanism leading to lean blowoff for this flame. In addition, an evaluation of known state of the art LBO mechanisms reported in literature is carried out. It includes the precessing vortex core, flame sheet holes, inner recirculation zone (IRZ) dynamics, and heat losses. The analysis shows that a key phenomenon leading to lean blowout for the present configuration is associated with the convective motion of cooler combustion products into the IRZ as the equivalence ratio is decreased.