Optical emission spectroscopy and numerical characterization of thermochemical non-equilibrium in supersonic arc-jet plasma flows

Accurately replicating atmospheric entry conditions in ground-based facilities is critical for the development of thermal protection systems (TPSs). This study utilizes optical emission spectroscopy (OES) as a versatile non-intrusive diagnostic technique to assess the chemical composition and thermodynamic state of the plasma generated in such facilities. In this study, emission spectra of the plasma flow produced by the SPES (Small planetary entry simulator) facility at the University of Naples “Federico II” were acquired and analyzed for two gas mixtures: an air-like composition (80% N₂ and 20% O₂) and pure nitrogen. Analysis of two regions (330–440 nm and 600–875 nm) identified the spectral signatures of N, N₂, N₂⁺, CN, and O. Further analysis of the emission spectra pointed to the thermodynamic non-equilibrium of the plasma. To support interpretation of the experimental results, computational fluid dynamics (CFD) simulations were performed using a five-species, single-temperature model and an 11-species (including ions), two-temperature model. Synthetic spectra generated from CFD data using the SPECAIR code were compared to the measured spectra. The two-temperature model demonstrated better agreement, confirming the non-equilibrium nature of the flow. This work highlights the diagnostic value of OES coupled with numerical tools for characterizing supersonic plasma flows.