Stability diagram for lift-off and blowout of a round jet laminar diffusion flame

An idealized model of a lifted flame above a round laminar jet is considered where diffusion rates of species and temperature are assumed equal but differential diffusion with respect to jet momentum is allowed. The combustion is described in terms of a global Arrhenius chemistry that is symmetric in fuel and oxidizer. Theoretical results on the propagation speeds of triple flames, and, the Landau-Squire solution for a nonreacting laminar round jet are combined to arrive at a transcendental equation for the lift-off height. For given chemistry, the stability behavior is controlled by a single Schmidt number, S, characterizing the differential diffusion between species (or temperature) and momentum and a parameter B, which is inversely proportional to the square root of the jet Reynolds number. Lift-off and blowout are characterized by a pair of critical curves in this two-dimensional parameter space the region between which corresponds to a stable lifted flame. A critical value of the Schmidt number exists above which the lift-off height increases continuously from zero on increasing the jet speed but below which the flame lifts off in a discontinuous manner through a subcritical bifurcation.