Correlations for the Onset of Instabilities of Spherical Laminar Premixed Flames

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 the 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. Critical size and corresponding elapsed time for the development of such instability are measured and these values are correlated with the appropriate flame parameter.

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.

Geometry of Spherical Flame Propagation in a Disc-Shaped Combustion Chamber

In performing synthesis or analysis of the combustion process in spark-ignition engines by digital computer, it is necessary to know the enflamed volume, the flame front area, and the area of the chamber surface in contact with the enflamed material, as functions of flame travel distance. This Note examines the case of spherical flame propagation in a disc-shaped chamber, with origin at any point on one of the flat surfaces. An ‘exact’ method of computation is outlined, and a simple approximate procedure, giving high accurary for this particular case, is described.