Experimental Visualization of Hybrid Combustion: Results at Elevated Pressures

This work was undertaken in an effort to improve the understanding of the combustion processes inside classical and fast-burning hybrid rocket motors. In this paper, a combustion visualization experiment and various optics were used to explore the mass transfer phenomena, boundary-layer growth rates, and the flame location above combusting fuels at atmospheric and elevated chamber pressures. The results presented provide strong confirmation of the basic droplet formation and entrainment mechanism for fast-burning fuels at operating conditions representative of those in a typical hybrid rocket motor. Entrained filament structures were not observed to be the dominant feature at chamber pressures above the critical pressure of the fuel; instead, at high pressures, numerous intense mass ejection events emitting large numbers of droplets were observed to be a major mass transfer mechanism. Imaging diagnostics were successfully used to quantify the location of the flame and the boundary-layer edge for classical and high-regression-rate fuels at a range of operating conditions. The imaging systems are also used in conjunction with chamber pressure data to examine the flameholding instability around the leading edge of the fuel grain.