Ab initio air chemistry on the HEG cylinder: recombination vs catalycity

What is it about?

We ran NASA’s FUN3D on the classic High Enthalpy Shock Tunnel Göttingen (HEG) cylinder, using a custom 38-reaction, five-species air model built from ab initio dissociation and exchange rates plus Dunn–Kang recombination. Legacy Park and Dunn–Kang models underpredict surface heat flux at the high-enthalpy HEG I condition (~22 MJ/kg) even though they do well at the milder HEG III condition. A two-stage global sensitivity analysis (screening, then Sobol on Arrhenius parameters, with surrogate models) identifies which reactions actually drive stagnation-point heating, and a calibration step brings the prediction in line with the measured heat flux.

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Why is it important?

The HEG I heating shortfall has long been blamed on extra surface catalycity. This study suggests gas-phase recombination, not just the wall, may be a large part of the story under high-enthalpy, high-density conditions. If that holds, first-principles rates plus targeted calibration beat simply turning up catalycity, and they give a clearer map of which kinetic parameters still need work.

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