Computational Fluid Dynamics Simulation of Regression Rate in Hybrid Rockets

Computational fluid dynamics has been applied to the simulation of hybrid rockets using O-2 as the oxidizer and hydroxyl-terminated polybutadiene or high-density polyethylene as the fuel. Simulations have been carried out using predefined fuel and oxidizer mass flow rates, and calculating the fuel mass flow rate as a function of the wall heat flux. In this second case, the calculated regression rate has been compared to the average value derived from the reference experiments; no tuning coefficients have been introduced. Computational fluid dynamics results are compared with the corresponding experiments in the literature. In those tests where regression rate is calculated as a function of the wall heat flux, the fuel grain is divided into segments and its average value is plotted as a function of the average oxidizer mass flux in the combustion chamber. Computational fluid dynamics regression rate is underestimated by 30% for high-density polyethylene and 50% for hydroxyl-terminated polybutadiene if only convective heat exchange is accounted for. For hydroxyl-terminated polybutadiene at relatively low gaseous oxygen, the radiative heat transfer contribution to the total heat flux is not negligible, and this is confirmed by computational fluid dynamics: when radiative heat flux is taken into account, the error on regression rate is reduced to 15% and its value is overestimated. Computational fluid dynamics does not correctly predict regression rate modulus, but it correctly calculates the n parameter of the regression rate formulation r = aG(ox)(n).