Plume Separated Region as a Flameholder

Abstract

The luminous flames in the Apollo first stage exhaust flow are considered in terms of the requirements for stabilizing hydrocarbon flames in low pressure, high speed flowfields. Above 12-15 km alt, flame stabilization depends upon a series of recirculation flows about the base of the first stage. Above 30 km alt, the flames are stabilized by the separated body flow (plume separated) region upstream of the expanding exhaust plume flowfield. The hot, fuel rich separated region is bounded by a combustible outer shear layer which contains the luminous flames. As altitude increases, the length I of the nonluminous gap, from the point of separation to the leading edge of the luminous flame-front, increases. At these altitudes heterogeneous heat release processes, involving direct attack on carbon particles by O and O2, appear to replace homogeneous two and three-body recombination processes. As a result the chemical relaxation lengths for radical formation and heat release are always smaller than the nonluminous gap length, I. Thus the Apollo body separated region flames are in the same region of rapid chemistry as the flame stabilization studies of Zukoski and Marble. After introducing a pressure factor to account for changing heating rates, their blowoff criterion is adapted to the variable pressure flame holder in the Apollo flowfield giving: /tf/w~0.5-1.5x!0' atm-sec, where/? is the separated region pressure, and u is the exterior stream speed. The flames blow off the Apollo exhaust flow when £ attains the length of the plume separated region outer shear layer.