THE DEVELOPMENT OF INSTABILITIES IN LAMINAR EXPLOSION FLAMES

Theoretical and experimental studies are presented of the growth of flame instabilities arising from perturbations of spherically propagating laminar flames in an explosion bomb. High-speed cine schlieren photographs show flame instability to be associated with the propagation of cracks, originating from disturbances due to flame movement over the spark electrodes, along the flame surface. Such cracks are the precursors of a cellular flame structure and they propagate at a rate that corresponds qualitatively with the theoretically predicted growth rate of the amplitude of a perturbation. The theoretical critical Peclet number for the growth of instabilities is associated with the onset of crack propagation, not with the completion of cell development. The theoretical approach follows that of Bechtold and Matalon and a flame stability diagram is derived in terms of dimensionless groups, including Peclet and Markstein numbers. Further experiments on vented explosions, in which stronger pressure pulses are created, are described. Taylor instabilities, arising from the baroclinic, vorticity generation term, del p x del rho/rho(2), create macro and micro vorticity at the flame front that enhance the burn rate and create a turbulent flame. Based upon this term, a flame vorticity number is proposed for the prediction of the onset of turbulent flame propagation.