Optimum characteristic length of gas generator for liquid propellant rocket engine

Abstract

Gas generator is an essential part of gas generator cycle Liquid Propellant Rocket Engine (LPRE). While designing the Gas Generator (GG), the selection of an unoptimized characteristic length result in an increase in mass flow rate of propellants required to drive turbine. As the exhaust of the gas generator is of little importance in the thrust of the engine, therefore, the specific impulse of the engine decreases with increase in the propellant flow rate to the gas generator. In literature, it is witnessed that researchers have conducted investigation of gas generator performance in terms of variation in mixing ratio or some models have been used to predict the characteristic length; however, it lacks the experimental investigation of gas generator performance in terms of characteristic length. In this paper, an experimental investigation of the gas generator of LPRE is conducted to determine the optimum characteristic length of the gas generator with liquid oxygen and kerosene. Hot tests are conducted with two different types of injectors, namely pressure swirl injector and impinging injectors in a modular water cooled gas generator. The gas generator is tested using six different characteristic lengths (i.e., from 1.75 m up to 9.67 m) for pressure swirl injector and with three different characteristic lengths (i.e., from 1.75 m up to 7.15 m) for impinging injector. The experimental results of hot tests show that the highest characteristic velocity of gas generator is achieved at a characteristic length of 4.27 m. It is demonstrated that a higher characteristic velocity of gas generator is achieved using impinging injector as compared to liquid-liquid pressure swirl coaxial injector at mixing ratio of 0.28 at optimum characteristic length. The effects of increase in mixing ratio with decrease in total mass flow rate on the chamber pressure are also discussed.