Vorticity in unsteady premixed flames: Vortex pair-premixed flame interactions under imposed body forces and various degrees of heat release and laminar flame thickness

The dynamics of vortical structures are investigated when a vol tex pair pr opagates into a premixed flame under different imposed body forces in the d irection of mean flame propagation and various degrees of heat release and laminar flame thicknesses. The direct numerical simulation assumes zero Mac h number, adiabatic, simple chemistry equations, and constant diffusivities . Visual pictures of the qualitatively different behaviors of the vortical structures emerge. These range from destruction of the incoming vortex pair and flame generation of flame-attached counter-rotating (rotating in the o pposite sense to the incoming vortex pair) vortical structures to amalgamat ion of the incoming vortex pair with flame-generated, flame-attached, co-ro tating (rotating in the same sense as the incoming vortex pair) vortical st ructures. Understanding of the different qualitative behaviors is aided by examination of the vorticity transport equation in two dimensions. Baroclin ic torque is found to scale more strongly with heat release and laminar fla me thickness than dilatation. As a result, increasing values of heat releas e and decreasing values of laminar flame thickness significantly strengthen the intensity of the flame-attached vortical structures. For experimentall y realizable values of heat release and laminar flame thickness the intensi ty of the: flame-attached vortical structures can be significantly greater than the incoming wrinkle-inducing vortex pair, supporting baroclinic torqu e as a mechanism for the increase in the conditional burnt gas turbulence i ntensities observed experimentally (Cheng and Shepherd, 1987). With a posit ive mean pressure gradient from reactants to products, the pressure gradien t in the unburnt gas in the flame finger formed by the incoming vortex pair is close to that in the burnt gas. For strong adverse body forces this res ults in streamwise velocities in the unburnt gas in the flame finger greate r than in the burnt gas around the finger. This potentially creates a gradi ent transport mechanism for turbulent scalar fluxes through the Bray-Moss-L ibby (BML) model of the turbulent scaler flux <(<rho>)over bar>u <(<double prime>c " )over bar> <(<rho>)over bar>(c) over bar (1-(c) over bar)((u(iu)) over bar-(u(iu)) over bar), where (u(iu)) over bar and (u(ib)) over bar ar e conditional mean velocities in unburnt and burnt gas, respectively. (C) 2 001 by The Combustion Institute.