An introduction to the role of chemical models in the enthalpy rebuilding procedure of Inductively Coupled Plasma facilities

Plasma wind tunnels are essential in recreating the same aerothermodynamic conditions experienced during an atmospheric entry. For instance, they are central to correctly designing thermal protection systems of spacecraft. Testing conditions (e.g., free-stream enthalpy) must be accurately known to replicate a flight scenario but unfortunately, they cannot be directly measured and must rely on rebuilding procedures. These methodologies couple numerical models and experimental data increasing the total uncertainty of the envisaged quantities due to inaccuracies in the selected models and errors in the measurement chain.

The VKI’s Plasmatron rebuilding is based on heat flux measurement and two CFD codes that compute the numerical counterpart. One simulates the plasma jet in the Plasmatron chamber under the LTE assumption, the other the boundary layer region around the testing sample under chemical non-equilibrium conditions. The rebuilding process iterates on a thermodynamic variable (e.g., the temperature) until numerical and experimental quantities match inferring the quantity of interest. This work focuses on the role played by the chemical models used to describe the chemical non-equilibrium in the boundary layer. Chemical models, such as Park’s or Gupta’s, provide the coefficients that model the formation/deprecation of the species which significantly impacts the heat flux computation. Such coefficients are affected by large inaccuracies being calibrated more than 20 years ago and validated with experimental data available at that time. A forward uncertainty propagation technique and a sensitivity analysis will provide insight about the role of the chemical. Results will be applied to one test case of the Plasmatron facility.