A numerical study of large amplitude baroclinic instabilities of flames

A steady propagation of a premixed flame subject to large amplitude peturbations generated by baroclinic instabilities was studied numerically. The square flammability limit test tubes were used to model the effects of confinement. The pure Landau-Darrieus and Rayleigh-Taylor instabilities as well as the complex case of superposed instabilities were considered with different initial perturbations leading to the formation of mushroom- and tulip-shaped flames. Three dimensional simulations of Navier-Stokes equations were performed to obtain a solution of the fluid flow problem, and a Eulerian interface tracking technique was used to follow the flame, based on the level set methodology (G equation) with expansion effects included. The effects of different factors, such as gravity, density ratio, burning velocity initial perturbations, and tube end effects were investigated. When the results of the predictions are compared with experimental data, a goad agreement is obtained in spite of the disregarded heat loss and stretch effects on flames. Due to a self-adaptive nature of the flame-flow interaction reflected in the generation of extremely complex 3-D vortices, the flow behind the flames does not contain separation regions, which maximizes the flame speed. Introducing restriction on the flow structure by imposing 2-D geometry leads to much lower flame velocities. Both calculations and experiments show that spontaneous conversion of mushroom to tulip flames occurs in open vertical tubes when the flame surface is flat. The conversion is triggered by the Landau-Darrieus instability and is very. sensitive to flow asymmetries. Slight inclination of the tubes prevents the conversion.