Abstract Droplet entrainment contributes to excellent regression rates of HTPB/paraffin blended fuels in hybrid rocket motors, but also leads to more complicated mass transfer mechanisms in application. The liquid film dynamic behaviors and droplet entrainment mechanisms of the fuels in a windowed two-dimensional slab facility under hot gas stream were investigated using the backlight method, image processing technique and fast Fourier transform. The results showed that the liquid film with a thickness of less than 3 mm could be formed on the fuel surface, and the gas-liquid interface experienced rolling and ripple waves before evolving into singlet disturbance-crests, bags and ligaments, the sources of entrained droplets. Moreover, interfacial waves propagated periodically at frequencies within 100 Hz, which was stimulated by gas shear stress and inhibited by liquid film viscosity damping, thus the regression rate was positively correlated with the temperature and mass flux of hot gas stream, while negatively correlated with the chamber pressure and the HTPB concentration. Additionally, the burst of the main part and the rim of bag provided polarized droplet diameters among the three droplet entrainment mechanisms involving the ligament break-up and the shearing-off of crest. Therefore, the averaged droplet diameter and its coefficient of variation even reduced to 47.98 μm and 0.48 with the disappearance of bag break-up, when the HTPB content increased to 40 wt%.

Highlights • The droplet entrainment mass transfer of HTPB/paraffin blended fuels is investigated by visualization experiments. • The dynamic evolution of waves on the liquid film of HTPB/paraffin blended fuels is discussed. • The major impact of wave propagation on the regression rate of HTPB/paraffin blended fuels is demonstrated. • The droplet entrainment mechanisms and their contributions on the droplet diameter distribution are explored.