Numerical Modeling of GOX/HTPB Hybrid Rocket Flowfields and Comparison with Experiments

Numerical simulations of the flowfield in a GOX/HTPB hybrid rocket engine are carried out with a Reynolds averaged Navier-Stokes solver including detailed gas surface interaction modeling based on surface mass and energy balances. Fuel pyrolysis is modeled via finite-rate Arrhenius kinetics. A simplified two-step global reaction mechanism is considered for the gas-phase chemistry to model the combustion of 1,3-butadiene in oxygen. Results are compared with firing test data from a lab-scale hybrid rocket in which gaseous oxygen is fed into axisymmetric HTPB cylindrical grains through an axial conical subsonic nozzle. With the oxidizer fed by this kind of injector, which generates nonuniform conditions at the entrance of the fuel port, the fuel regression rate is shown to increase several times with respect to the case of homogeneous injection of the oxidizer through all the grain port area, in agreement with the experimental findings. The spatial distribution of the regression rate is substantially different from the one expected in a turbulent developing flow, because of the presence of a strong recirculation zone. Numerical simulations, yet not capturing the absolute regression rate values, are fairly able to predict the main ballistic features, i.e. the regression rate weaker dependence on the mass flux, the influence of port diameter and the pressure evolution over time.