Evaluation of reduced-order kinetic models for HTPB-oxygen combustion using LES

Abstract

This investigation analyses three finite-rate chemistry mechanisms of 2, 4 and 6 steps to simulate the combustion process in hybrid propellant rockets of HTPB-based (solid fuel) and gaseous oxygen. Computational Fluid Dynamics is used to compare the predictions with the available experimental data of a small channel-like burner intended for reactive flow characterisation, hence to assess the pros and cons of accepting extra complexity into the chemistry model in practical engineering computations. To better isolate the effect of modelling reacting flow with different chemistry models, the frame of Large Eddy Simulation (LES) of FLUENT has been used to resolve the time-spatial evolution of the gas mixture of pyrolysed fuel (modelled as polybutadiene, which is the major species of the thermal decomposition) and a stream of oxygen injected at the head-end of the combustor. The flow exhibits a mixing layer which evolves over and downstream the solid fuel block as a result of the interaction with the vortical structures. The LES methodology is affordable because of the small Reynolds number of the conducted experiments, which corresponds to the range Re∼2000 to 8000. A set of simulations have been accomplished for each chemical mechanism with oxygen inflow velocities of 2 to 10 m/s. Being the focus to assess the behaviour of the finite-rate mechanisms, fuel regression rate has been prescribed accordingly to available empirical data. Radiation effects are not taken into account, justified by the small scale of the combustion region.The results show that dissociation reactions of the 6-reaction mechanism play a significant impact on the combustion temperature, which approaches the experimental evidence, hence it is in agreement with the theoretical flame temperature predicted by detailed chemistry solvers. On the contrary, flame temperature computed with the 2 and 4-step chemistry models is over-predicted upon unrealistic limits, above 3150 K, which is attributed to the lack of fundamental physics linked to the non-inclusion of endothermal reactions in these models. Although an improvement is clearly provided by the 6-reaction chemistry, a drawback is however observed: a less robust behaviour of the solver occurs. Therefore, a decision concerning to weight the inclusion of more accurate (more steps) finite-rate kinetics against the numerical complexities (operational and geometrical) of engineering simulations in hybrid propulsion systems, is of concern in the final approach.