Instabilities and soot formation in spherically expanding, high pressure, rich, iso-octane-air flames

Abstract

Flame instabilities, cellular structures and soot formed in high pressure, rich, spherically expanding iso-octane-air flames have been studied experimentally using high speed Schlieren cinematography, OH fluorescence, Mie scattering and laser induced incandescence. Cellular structures with two wavelength ranges developed on the flame surface. The larger wavelength cellular structure was produced by the Landau-Darrieus hydrodynamic instability, while the short wavelength cellular structure was produced by the thermal-diffusive instability. Large negative curvature in the short wavelength cusps caused local flame quenching and fracture of the flame surface. In rich flames with equivalence ratio φ > 1.8, soot was formed in a honeycomb-like structure behind flame cracks associated with the large wavelength cellular structure induced by the hydrodynamic instability. The formation of soot precursors through low temperature pyrolysis was suggested as a suitable mechanism for the initiation of soot formation behind the large wavelength flame cracks..

Instabilities and soot formation in high-pressure, rich, iso-octane–air explosion flames1. Dynamical structure

“…This is in addition to the cellular structure formed as a result of the influence of the hydrodynamic instability [1,8]. The smaller length-scale surface cellularity develops through the influence of the thermal-diffusive instability.…”

“…It appears that the hydrodynamic instability has a short wavelength cut-off, which is significantly larger than the length scale over which the thermal-diffusive instability affects the flame surface [8]. At high pressures, this can result in a decoupling of the length-scales over which the hydrodynamic and thermal diffusive instability affect the flame surface [8,10,13].…”

“…The OH concentration and the rate of fuel oxidation in the concave regions of the flame surface (cusps/troughs) are reduced through the locally large negative flame stretch, while the OH concentration and rate of fuel oxidation in convex regions of the flame surface is raised through positive flame stretch [1]. It appears that the hydrodynamic instability has a short wavelength cut-off, which is significantly larger than the length scale over which the thermal-diffusive instability affects the flame surface [8]. At high pressures, this can result in a decoupling of the length-scales over which the hydrodynamic and thermal diffusive instability affect the flame surface [8,10,13].…”

“…Cellularity in flames with positive Markstein numbers is caused by the Darrieus-Landau hydrodynamic instability [5,8]. This is of current interest as the determination of the experimental stretch-free laminar burning velocity becomes problematic as the flames become cellular.…”

“…Lockett et al have investigated this conclusion further, resulting in the identification of soot being formed behind deep cracks in the flame front, usually linked with the larger length-scale cracks induced by the hydrodynamic instability [8,10].…”