Numerical investigation on the droplet entrainment from gas sheared liquid film of hydroxyl-terminated polybutadiene/paraffin fuels

Hydroxyl-terminated polybutadiene (HTPB)/paraffin fuels applied in hybrid rocket motors enjoy high regression rates and excellent mechanical properties; however, the special microscopic structure of the fuels leads to more complex droplet entrainment mechanisms. The two-dimensional heterogeneous HTPB/paraffin fuels geometry model was established by a dynamic packing algorithm. Dynamic evolution characteristics of a liquid film disturbed by the interaction between HTPB and paraffin were investigated by numerical calculation, and the controlling mechanism of droplet entrainment in a hot shear flow was explored. The results showed that the motion of high-strength HTPB fragments in the liquid film distorted the structure of interfacial wave and generated uncommon rough waves with an averaged wavelength of up to 9.05 mm, for the fuel containing 60 wt. % paraffin. Additionally, there were three pathways of droplet entrainment, i.e., rupturing of paraffin-coated HTPB filaments, splashing at the crest of paraffin wavelets, and the breakup of paraffin ligaments. However, periodic interfacial wavelets were generated for the fuel with a 30 wt. % content of paraffin due to the damping of HTPB, and most of the entrained droplets were generated when the exposed molten paraffin flowed to both ends of the fuel slab. The diameter of entrained droplets was in the range of 0.02–0.74 mm, and it was shown to be positively correlated with the paraffin content, the temperature of shear flow, and the mass flux of shear flow.