Diffusion effects on the determination of surface catalysis in Inductively Coupled Plasma facility

•Chemistry inside the boundary layer influence the recombination coefficient determination.•The recombination coefficient is measured in the VKI Plasmatron facility on a flat plate.•The recombination coefficient is reduced with pressure.•The recombination coefficient is reduced along the plate.•The flow properties are relevant on the determination of surface kinetics.

Atomic recombination is an important process to consider when computing the heat flux transferred to the wall of a re-entry vehicle. Two chemical processes are influencing the species diffusion in the boundary layer surrounding a re-usable Thermal Protection System: gas phase reactions and catalytic recombination at the surface. The coupling between them is not normally taken into account when determining the catalytic recombination coefficient (γ) in plasma facilities. This work aims to provide evidence of such coupling based on both a theoretical analysis and an experimental campaign in the VKI-Plasmatron facility. Recombination coefficient measurements at off-stagnation point configuration on a linear copper calorimeter are provided. An evolution from a high-catalytic to a low-catalytic condition due to the boundary layer growth along the probe is observed. This result is consistent with a parametric analysis carried out using the in-house non-equilibrium boundary layer solver, which shows how the experimentally determined catalysis could be influenced by the amount of gas-phase recombination inside the boundary layer.