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

Abstract

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

Numerical Analysis of Port Diameter Effect on Hybrid Rocket Fuel Regression Rate with Axial Injection

“…13 The ensuing flowfield is completely different from the one of developing turbulent flows through pipes with a resulting regression rate which is both increased and differently distributed along the motor axis. 15 As was inferred in Ref. 14 and numerically proved in Ref.…”

“…14 and numerically proved in Ref. 15, two different regimes can be distinguished during the motor operation in case of axial injector: i) when the port diameter is sufficiently small with respect to the jet diameter at the fuel port entry, the flowfield is similar to a turbulent developing flow through a pipe and the solution resembles that of the injection through a stagnation chamber that uniformly distributes the oxidizer at the fuel port inlet; ii) when the port diameter is large with respect to the jet diameter, the recirculation zone penetrates farther downstream into the fuel port significantly broadening the combustion region, increasing the mixing and the fuel regression rate.…”

“…Moreover, ballistic calculations based on classical boundary-layer theory can fail to reproduce the motor behavior due to a lack of modeling of the oxidizer injection effects. 14,15 More specifically, experimental results highlighted the importance of the oxidizer entrance conditions, 13 which, as mentioned above, are often assumed to determine second order effects in classical hybrid systems. If the oxidizer is fed into the combustion port by an axial nozzle, the motor behavior is absolutely influenced by the recirculation zone induced by the oxidizer axial injection.…”

“…Considering, as a reference, a lab-scale hybrid motor tested at the University of Naples "Federico II" 13, 14, 16 equipped with an axial subsonic nozzle to feed gaseous oxygen into the port of Hydroxyl-Terminated PolyButadiene (HTPB) grains, former studies have shown the dependence of the flowfield on mass flux for different tests. 15 Goal of the present study is to exploit experimental data from Ref. 16 to extend the analysis of injection effects to that of port diameter.…”

Convective and Radiative Contributions to Wall Heat Transfer in Liquid Rocket Engine Thrust Chambers

“…25 This solver has been validated against experimental data in different operating conditions. 3,[26][27][28][29] If attention is restricted to the wall heat transfer far away from the injector plate, details of the injection geometry are relatively unimportant, and an axisymmetric solution is adequate to evaluate heat transfer models, at a limited computational cost. Accordingly, the numerical simulation can be started somewhere downstream of the injector plate, possibly once recirculation (of rather uncertain predictability) is over.…”