Calculations of rocket plume afterburning coupled to reacting base recirculation regions

Afterburning of combustible plume species with air is the most striking effect noted in observations of lowflying missiles (less than 30-km alt). Because afterburning raises the temperature of the plume, it intensifies the physical and chemical processes that cause plume signature effects. In systems with relatively cool exhausts, the onset of afterburning is delayed until the missile speed exceeds some minimum value (about Mach 2.2). The onset of afterburning in these cases has been attributed to the effect of combustion in a recirculation zone at the base of the missile. A model of equilibrium combustion in the base region was coupled to a chemical kinetic model of downstream plume mixing and afterburning in order to perform the calculations for this study. The calculations made with the combined model confirm observations of plumes which afterburn only after the base region has ignited. The calculations also indicate that the rocket motor mass flow rate has an important influence on plume afterburning. It also is calculated that, for systems with hot exhausts, the base has no important effect on afterburning in the rest of the plume.