Experimental studies on swirl injector with varying L/D ratio of hybrid rocket motor

Abstract

In this paper, effect of swirl injector on varying the L/D (length to diameter) ratio of the hybrid rocket motor was experimentally investigated. The use of swirl injector was observed to improve regression rate and other performance parameters, but it is expected to change with the varying L/D ratio of the hybrid rocket motor. It was further compared with the conventional showerhead injector for the same L/D ratios. Gaseous oxygen was used as an oxidizer while paraffin wax was used as the fuel. Three different L/D ratio motors, specifically 9.5, 13.5 and 17 were used for the experiments. For the regression rate study, the three major factors responsible for the increment in regression rate are longer residence time due to larger motor length, diminishing effect of swirling flow with increasing motor length and the behaviour of mass flux exponent. The investigation revealed that the 13.5 L/D ratio motor can be said to have a better regression rate performance as it gives a balanced results for all three factors. Also, considering the other performance parameters such as chamber pressure, combustion efficiency and thrust generation for the showerhead as well as swirl injector, 13.5 L/D ratio motor showed superior results compared to other two L/D ratio motors. The increment observed for 13.5 L/D ratio motor were 27 %, 11 % and 25 % for chamber pressure, combustion efficiency and thrust generated values, respectively when compared with the base case of 9.5 L/D ratio motor for the swirl injector study. These experimental findings for pressure and thrust values are in corelation with the thrust equation where the thrust value is directly proportional to the chamber pressure value and the nozzle throat area.