Flow field and injector heat characteristics of hybrid rocket motor with annular-gap injector

Abstract

A throttleable annular-gap axial injector, which could automatically adjust the injection gap area, is proposed to decrease injection pressure drop variation amplitude during wide range thrust regulation process in hybrid rocket motor. With the annular-gap injector adopted, liquid oxidizer is unhomogeneously injected into combustion chamber through a narrow annular injection gap. In conventional hybrid rocket motor, the oxidizer is usually homogeneously injected through full-inlet injection method. The injection area covers the whole fuel grain port. Difference of the injection methods greatly influences the oxidizer flow characteristics. Consequently, combustion and heat transfer characteristics of the motor are significantly changed. This paper is aimed to analyze the two-phase combustion flow field and coupled injector heat transfer characteristics of a lab-scale hybrid rocket motor with annular-gap injector through two-dimensional axisymmetric steady numerical simulations. The motor adopts 98% hydrogen peroxide and polyethylene as the propellants. Numerical analysis reveals that position of the injection gap influences the regression rate distribution in the first half of solid fuel grain but has little effect on that in the second half. The regression rate is relatively high when the injection gap is close to the fuel inner surface. In addition, the flowing liquid hydrogen peroxide in the injector could cool the chamber head. Sharp turns that produces vortex in oxidizer flow channel decreases the cooling effect and increases the overheating risk. Smooth bend could improve the injector heat transfer characteristics and eliminate the risk.