Parametric Analysis of Combustion Instability in Axial-Injected Hybrid Rocket Motors using Computational Fluid Dynamics

摘要

A computational model of a hybrid rocket motor has been developed for the purpose of simulation of internal ballistics and transient behavior such as combustion instabilities. The numerical model consists of four sub-components: i) a quasi ID gas dynamics model using Euler equations for flow field simulation ii) a chemical model using CEA iii) an analytical heat feedback model for transfer of heat from flame to solid fuel surface iv) a ID thermal conduction model inside the solid fuel. In the unsteady time-dependent simulation, it is seen that upon the application of a temporal boundary layer delay of the wall heat flux to the changes in the regression rate, an unstable region ensues. At first an oscillating periodic increase in the regression rate and chamber pressure is observed (linear regime), which then proceeds into a non-linear limit cycle. A positive DC shift in the chamber pressure is also observed. The reason for DC shift is explained with an analogy to a simple non-linear oscillating system. The frequencies of different natural modes (including the intrinsic hybrid oscillation mode) predicted by the model are found to be in good agreement with theoretical prediction. The effect of finite time needed for the unburnt fuel to move from the regressing surface to the flame region is also additionally modelled using a time delay to the heat of combustion. This results in increased amplitude of oscillations and a higher DC shift. Parametric analyses have been carried out with different boundary layer delays. It is found that the value of DC shift, frequency shift and also rms amplitude is directly proportional to the magnitude of the boundary layer delay.