Mixing enhancement in hybrid rocket motor using vortex generators

A hybrid rocket is a compromise between a solid and a liquid rocket motor: uses a solid fuel and a liquid oxidizer. This configuration has several well known advantages, such as less security problems, smaller costs, etc. On the other hand, main problem of this type of motors, is the poor mixing, inside the combustion chamber, between the solid propellant and the oxidizer injected. As consequence, combustion chamber usually results to be quite long in order to respect mixing time between propellant and oxidizer. Several different solutions have been proposed in the past to overcome this problem, going from complex design of the internal propellant grain to different grain composition. In a previous work the authors have studied the presence of protrusions of thermal protections in a large solid rocket motor (i.e. ARIANE V - P230) because they are the reason for the production of large coherent vortex structures that are responsible of undesirable pressure oscillations. During that study by chance, it has been observed that these structures are also responsible for increased internal mixing with extraction of fluid from the boundary layer near the propellant grain. Therefore the possibility of improving internal mixing by means of obstacles placed in the combustion chamber has been discussed. Aim of the present work is to study the effects of vortex structures in the combustion chamber on propellant- oxidizer mixing in hybrid rocket motors. The proposed way for the production of these vorticity structures is the introduction in the combustion chamber of ad-hoc obstacles and localized oxidizer injection. For this reason a parametric study, by varying: The length of the combustion chamber, the height of an obstacle place in the combustion chamber and the possibility of side injection of oxidizer, has been conducted. Results, obtained by means of CFD simulations, have shown that the values of mixing intensity are several times larger than the case of the motor without obstacle and injection. Therefore starting from these results, an improved solution with the proposed devices and a reduced length has been finally tested.©2012 by the International Astronautical Federation