Experimental Visualizations of Entrainment Phenomena in Wax-Based Fuels for Hybrid Space Propulsion

The entrainment is a physical phenomenon that concerns the transport of bubbles, droplets and, in general, particles that are spread in a fluid stream. Studies on the entrainment are very important for hybrid rocket engines that use liquefying fuels. These materials form a thin liquid layer. If the liquid film is stressed by aerodynamic forces it becomes unstable and at the liquid-gas interface waves are formed. When the forces acting on the interface are large enough some waves detach from the liquid phase; the liquid structures, in the gas phase, brake up in many droplets. The physical properties of the fuel spray define the heat and mass transfer that occurs in hybrid motors during the combustion processes. “Physical properties” are basically the detach rate of the droplets, their size, and distribution speed. This paper describes the procedure to measure the diameters of the droplets. The entrainment is reproduced, in frozen conditions, in an experimental setup which allows the process recording thanks to a fast-digital-camera. Pure wax, wax added with 5% of Sebs and wax with 15% of Sebs are the investigated fuels. Oxygen is the gas used. The results obtained by a detailed analysis are reported.