Thermodynamic state investigation of the hypersonic air plasma flow produced by the arc-jet facility SCIROCCO

The experimental investigation of the thermodynamic state characterizing the hypersonic air plasma flow generated by the SCIROCCO arc-jet facility has been carried out by means of high resolution optical emission spectroscopy (OES). The line-of-sight emission of the free-stream plasma jet was recorder 17 cm away from the nozzle outlet (about 0.1 nozzle diameter). The experimental nitrogen oxide (NO  ) vibrational bands, collected in the 228–335 nm UV range, were compared through a simple fitting procedure to those obtained by the spectral simulation software Specair 2.2, in order to estimate rotational (TrotTrot) and vibrational (TvibTvib) temperatures. The plasma can be assumed at roto-translational equilibrium, hence measured TrotTrot was considered a reliable estimation of the plasma static flow temperature. TvibTvib, instead, estimated using the relative intensity among vibrational bands, resulted significantly higher than TrotTrot, indicating a departure from local thermodynamic equilibrium (LTE) condition. A simple error estimation methodology was also applied.