On the velocity overshoot in a laminar boundary layer diffusion flame

The results of a numerical study of diffusion flames over a flat plate are reported. Attention was given to confinement for the flow and the resulting pressure gradient and its effects on the structure of a laminar boundary layer diffusion flame. Two-dimensional parabolic differential equations were transformed into finite difference equations using integration over control volumes and making interpolation assumptions to link downstream values. A tri-diagonal matrix algorithm was employed to solve the finite difference equations, with grid coordinates consisting of the longitudinal distance and the dimensionless stream function. A single-step irreversible reaction, an infinite reaction rate, a temperature dependence of the specific heats, and constant Prandtl and Schmidt numbers were assumed. The study showed that a velocity overshoot in the flame region is caused by pressure gradients in the confined flow. Buoyancy effects also were significant in the overshoot, and computational techniques to account for buoyancy are presented.