Modeling Instabilities due to Coupling of Acoustic and Hydrodynamic Oscillations in Hybrid Rocket Motors

Abstract

This paper presents a mathematical model that was developed to study the coupling of various types of oscillations (thermo-acoustic and hydrodynamic) in hybrid propulsion systems. The presumption is that such oscillations feed into combustion instabilities and result in poor performance of the propulsion system and/or result in mechanical vibrations that lead to failure of the rocket motor. The physical model (which the proposed computational model approximates) is a multi-node representation of a two-dimensional, subscale, low pressure hybrid rocket propulsion system. In the proposed integrated component model, the oxygen tank, the throttle valve, and the nozzle is assumed to be onedimensional and each is represented by two-nodes. The part of the combustion chamber where the solid fuel rests - including the inlet plenum (fwd) and the exit mixing chamber (aft) - are approximated by a two-dimensional, multi-node computational domain. The flow through the combustion chamber is assumed to be compressible and turbulent. The equations for the gas domain inside the combustion chamber are solved using a second order accurate implicit (CMSIP) technique where as the equations for the turbulence model and the throttle valve are solved by a second order accurate explicit technique. The numerical simulation results indicate that the mathematical model for the gas flow in the combustion chamber predicts the expected unsteady temperature and pressure distribution, and the velocity field, successfully. Furthermore, the proposed integrated component model is successful in generating the characteristics of the hydrodynamic flow instabilities coupled with thermo-acoustic pressure oscillations.