Flame stabilization on bluff bodies at low and intermediate Reynolds numbers

The problem of attaining stable combustion in ramjet power plants and in afterburners of turbojet engines has led to rather extensive studies of the processes involved in stabilizing flames on bluff bodies. One of the results of these studies was to indicate that the flame stabilization mechanism changes significantly at low Reynolds Numbers; the velocity at which flames may be stabilized drops abruptly as the Reynolds Number is decreased, and the mixture ratio for which maximum velocity of flame holding occurs shifts markedly from the stoichiometric value. This abrupt change in the mechanism of flame stabilization is investigated through photographic studies of the flame front near the flame holder. A transition from a laminar to a turbulent surface of the flame front immediately downstream from the flame holder is shown to account for the change in flame stabilization characteristics. This transition was found to occur independently of fuel type and flame holder geometry. The behavior of the low Reynolds Number stabilization limits is attributed to a diffusion process; in particular, the shift found for the mixture ratio corresponding to the maximum blowoff velocity is explained on the basis of the difference in the diffusion rate of fuel and oxygen. Detailed experiments including blowoff results, chemical analysis of gas taken from the flame holder wake, and measurements of wake temperatures are shown to confirm the suggested diffusion mechanism. Once the transition phenomenon is appreciated, re-examination of high Reynolds Number blowoff data is found possible. The results of previous experiments are found to show that the blowoff velocity depends on the square root of the characteristic dimension if the transition Reynolds Number is exceeded and if the flame holder is a bluff body of small fineness ratio