Prediction of Instabilities of Spherically Propagating Flames in Laminar Premixture

A spherically expanding flame in a quiescent premixture is a bifurcation phenomenon, in which the flame becomes unstable at a radius, greater than some critical value, while remaining stable below that critical radius. Beyond this critical radius, developing instabilities are initiated by propagating cracks to form a coherent structure covering the entire flame surface and flame accelerates. The present paper reports a schlieren photographic study of spherical flame propagation in methane–air, iso-octane–air and n-heptane–air premixtures at different initial conditions where the onset of instability and the flame acceleration are clearly perceived. Hence, elapsed time and flame radius for onset of instability are correlated with the flame parameters. Predicted critical time and flame radius for the onset of instability are in good agreement with available experimental data obtained from a large-scale unconfined explosions in methane-air premixture.