Local entrainment velocity in a premixed turbulent annular jet flame

Abstract

The local entrainment velocity of the enstrophy interfaces of a methane–air turbulent premixed turbulent annular jet flame stabilized on a bluff-body burner has been investigated using a high-fidelity flame-resolved three-dimensional simulation. The enstrophy (inner and outer) and the scalar interfaces have been defined and characterized by their propagation speeds, VE and Sd, relative to the fluid flow. Mean values (⟨Sd |c⟩ and ⟨VE |c⟩) conditioned on the reaction progress variable c have been obtained. A thin layer (near the enstrophy interfaces) has been used to compute mean values (⟨VE |E⟩, ⟨Sd |E⟩, and its different contributions) conditional upon enstrophy E. At the inner interface, results indicate that ⟨Sd |c⟩ > 0 and ⟨Sd |E⟩ > 0 (entrainment of fresh reactants into the flame front and hot products), while ⟨VE |c⟩ < 0 and ⟨VE |E⟩ < 0 (entrainment of hot products into the reacting jet across the inner enstrophy interface). The outer enstrophy interface displays ⟨VE |E⟩ > 0 (ambient gases are predominantly entrained into the jet of reactants), which implies a lean mixture in its neighborhood. These preliminary results aim at understanding the physical mechanisms of flame anchoring, in terms of entrainments of either hot products or fresh reactants into the diffusive-reactive region. Local geometries of the inner and outer interfaces have also been examined, through the computation of joint probability density functions of the mean curvature km and Gauss curvature kg of the iso-enstrophy surfaces, and through ⟨VE |km, kg⟩ at the inner and outer interfaces. This information has subsequently been used to discuss the physics of how the turbulent entrainment process affects premixed flames.