Comparison of ethylene combustion mechanisms for the simulation of a supersonic combustion experiment

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Hydrocarbon fuels have certain advantages for high speed flight over liquid hydrogen, but are difficult to simulate accurately due to the expense and complexity of the available reaction mechanisms. This paper selects 5 reduced-order chemical reaction schemes for hydrocarbon combustion and applies them to the numerical simulation of an ethylene fuelled supersonic combustion experiment. The experiment consists of an axisymmetric diffuser-cavity-combustor model flowpath, tested at the University of Queensland's T4 reflected shock tunnel at an equivalent Mach 8 flight condition and dynamic pressure of 100 kPa. Data obtained from pressure sensors on the walls is compared to a set of 3D Computational Fluid Dynamics simulations of the flowpath, which employ Reynolds-Averaged Navier-Stokes turbulence modelling and quasi-laminar chemical reactions. Simulations without fuel injection are first performed to assess the underlying validity of the numerical modelling, and the subsequent wall pressure predictions match the results of a no-fuel experiment to within the ≈ 18 kPa margins of uncertainty. In the fuel-on cases however, two shockwaves are observed to have discrepancies in their positions of up to 60 mm, and some of the pressure predictions are outside of the ≈ 25 kPa mean uncertainty of the fuel-on measurements. Of the five tested mechanisms, three correctly predicted ignition of the fuel and two did not, the latter providing a poor comparison to experimental data. The three successful mechanisms produced very similar results to each other, which broadly replicate the pressure rise seen in the experiments but not the precise location of the combustion induced shockwaves. The results imply that any of these three schemes could be used, with caution, to roughly predict ignition behaviour and combustion dynamics as long as conditions are similar enough to those considered in this paper. Future work should be directed at relaxing this caveat by additional testing at different conditions and applying the same mechanisms to other experimental geometries.