Simulation of Gaseous Oxygen/Hydroxyl-Terminated Polybutadiene Hybrid Rocket Flowfields and Comparison with Experiments

Numerical simulations of the flowfield in a gaseous oxygen/hydroxyl-terminated polybutadiene 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 the firing test data obtained from a laboratory-scale hybrid rocket in which gaseous oxygen is fed into axisymmetric hydroxyl-terminated polybutadiene cylindrical grains through an axial conical subsonic nozzle. With the oxidizer fed by this 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 through a pipe because of the presence of a strong recirculation zone at the motor head end. Numerical simulations, although not capturing the absolute regression-rate values, are fairly able to predict the main ballistic features: that is, the regression rate's weaker dependence on the mass flux, the influence of port diameter, and the pressure evolution over time, as well as the general trends of different firing tests.