Operational instability of a high-rotational-speed electric pump in a hybrid rocket motor

Abstract

Electric pumps capable of operating at high rotational speeds have enormous potential for application in hybrid rocket motors because of their light weight and ability to made rapid adjustments in rotational speed. Previous studies have assessed the performance of electric pumps; however, the physical mechanisms underlying the operational instabilities of high-rotational-speed pumps remain obscure. Simulations were accordingly conducted in this study to explore the physical mechanism of experimentally observed operational instabilities. The average head deviation between the simulated and experimental results was within 2%. The results of the instability analysis at low mass flow rates indicated that the reflux at the impeller outlet intensified, resulting in increased velocity, turbulent Reynolds number, and vorticity. The flow structure at a mass flow rate of 4.2 kg/s caused significant rotor torque fluctuation (2.1 times that at 5.88 kg/s), which coupled with the electric motor rotation and gradually increased the instability of the electric pump until shutdown. The minimum mass flow rate at which the electric pump operated stably was between 4.2 and 4.47 kg/s. The results of the instability analysis also indicated that severe cavitation occurred at 7.4 kg/s that reduced the head and significantly disrupted the flow stability. Furthermore, this cavitation led to an increased torque fluctuation amplitude and more dispersed frequency distribution (compared with that at 4.2 kg/s), which affected the stability of the electric motor and ultimately led to shutdown. These findings provide theoretical guidance for the development of thrust adjustment schemes for electric-pump rocket motors, benefiting the aerospace community.