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 ( T rot ) and vibrational ( T vib ) temperatures. The plasma can be assumed at roto-translational equilibrium, hence measured T rot was considered a reliable estimation of the plasma static flow temperature. T vib , instead, estimated using the relative intensity among vibrational bands, resulted significantly higher than T rot , indicating a departure from local thermodynamic equilibrium (LTE) condition. A simple error estimation methodology was also applied.