Heat flux characterization of an under-expanded/supersonic plasma jet over a catalytic probe
Creators: name:  M. CAPRIATI 
name:   A. TURCHI 
name:   P. CONGEDO 
name:   T. MAGIN 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: FLIGHT PHYSICS
PLASMA FLOWS
ENERGY DEPOSITION

description: The von Karman Institute Plasmatron facility is used for the characterization of materials exposed to a high-enthalpy plasma environment. It is particularly helpful to predict the behaviour of Thermal Protection Materials (TPMs) for aerospace. In atmospheric entry applications, TPMs protect the vehicle from the extremely demanding aerodynamic conditions to which it is exposed. However, this normally happens at the expenses of the TPM that undergoes thermochemical decomposition and experiences mass loss and shape change. When the local heat transfer simulation constraints are met, the in-flight highly reactive boundary layer developing past the shock can be duplicated on ground by a subsonic plasma flow in the stagnation region. However, differences remain in the heat flux distributions between the hypersonic and subsonic flowfields. In particular, when a typical hemispherical ablative TPM sample is exposed to the subsonic plasma flow, the recession it experiences is almost uniform in space and the shape is preserved. To explore the effects of the shape change of the ablative samples, an experimental campaign has been recently conducted at the VKI using a supersonic plasma. The supersonic condition was achieved by I) mounting a sonic nozzle at the end of the plasma torch, that expands the gas to choked conditions, and II) adequately lowering down the chamber pressure to generate an under-expanded jet and accelerate the flow to the desired regime. The value of the pressure at the nozzle inlet and in the chamber is experimentally measured and characterized by the uncertainty of the instruments. The temperature at the nozzle inlet is not directly measurable and its values needs to be assumed. Moreover, the probe surface is catalytic, promoting the recombination of atomic nitrogen and oxygen present in the gas. Phenomenological approaches allow to model this catalytic behaviour based on a macroscopic probability that a given surface reaction takes place. Also these probabilities need to be assumed, and, thus, they suffer of uncertainty. In this work, we propose to simulate this experimental campaign using the US3D flow solver and to perform an uncertainty quantification analysis to propagate the above-mentioned sources of uncertainties to characterize the variability of the quantities of interest, i.e., the heat flux and the pressure of the probe. Additionally, an assessment of the numerical error is performed and included in the analysis. It is essential to estimate the numerical precision of the prediction for a given computational budget when the prohibitive computational cost associated to uncertainty quantification analysis prevents the systematic use of the grid-converged mesh. The catalytic behaviour of the surface was modeled using the gas-surface interaction module available in the MUTATION++ library. The numerical uncertainty on the quantities of interest is compared with the experimental values, assessed through calorimeter and Pitot measurements. Furthermore, the structure of the flow is analysed: the numerical prediction of the jet angle at the exit of the nozzle and of the shock stand-off distance are compared to the ones experimentally observed.