Boundary layer combustion of HTPB/paraffin fuels for hybrid propulsion applications

Abstract

HTPB/paraffin fuels exhibit high regression rates and excellent mechanical properties due to the droplet entrainment and enhancement of the HTPB matrix, however, the special structure of the fuels also leads to more complex combustion mechanisms that may be quite different from that of paraffin and HTPB alone. The boundary layer combustion characteristics of HTPB/paraffin fuels with oxygen flow in a windowed 2D slab burner were investigated by using a high speed camera, Schlieren and thermocouples. The results demonstrated that the regression rate of HTPB/paraffin fuels mainly composed of paraffin was higher than that of HTPB due to the dominant role of droplet entrainment in mass transfer. The flame height and combustion boundary layer thickness of the fuel were inversely correlated with the oxidizer mass flux, and decreased by about 2.8 times and 2.5 times when the oxidizer mass flux increased from 12.40 kg⋅m⋅−2s−1 to 64.86 kg⋅m⋅−2s−1, respectively, however, the chamber pressure exhibited little effect. The addition of HTPB limited the development of combustion boundary layer thickness and hindered the generation of entrainment droplets due to an increase in liquid layer viscosity, but both the proportion of flame area in the combustion boundary layer and the gas temperature gradient in the fuel-rich zone were positively correlated with HTPB content attributed to the higher combustion efficiency.