Catalytic wall effects for hypersonic nozzle flow in thermochemical non-equilibrium

This paper investigates the effects of wall catalytic activity in hypersonic nozzle flows. Non-equilibrium Navier- Stokes-Fourier equations were here applied within a density-based algorithm in the OpenFOAM framework. The numerical model was validated using different test conditions conducted at NASA EAST facility and General Electric Research Laboratory. After confirming the reliability of the numerical model, a numerical investigation on the wall catalytic effects on the flow parameters using the Scirocco Plasma Wind Tunnel was performed. Four nozzles with different expansion ratios were extensively tested. It was verified that higher expansion ratio nozzles experience a greater wall catalytic effect on the flow properties. Particularly, doubling the expansion ratio causes the difference between a non-catalytic and a fully catalytic wall to increase in 21% for Mach number and 25% for trans-rotational temperature and static pressure. Furthermore, the results of this investigation allow us to define a new criterion by which to consider if the wall catalytic effects are or not important in the nozzle flow fields. It was found that catalytic effects in the core flow are negligible when simulating nozzles with expansion ratio below 186 and Mach number below 8.6. As the expansion ratio increases, the thermal boundary layer thickness also increases and the atoms recombination that occur in that region have more influence on the core flow. In these cases, it is then necessary to take into account the wall catalytic activity to achieve a better estimation of the flow properties at the nozzle exit and in the test cell.