NO excitation and thermal non-equilibrium within a flat plate boundary layer in an air plasma

Abstract

Optical emission spectroscopy experiments are carried out by recording the radiation from the γ transitions of nitrogen monoxide in an air inductively coupled plasma in interaction with a water-cooled metallic flat plate at moderate pressure. The calibrated results allow to derive the vibrational and rotational temperatures of the NO(A ² Σ ⁺) excited state as well as its densities in the free jet and within the boundary layer by comparison with calculated spectra. Those results are compared with previous ones concerning temperatures and densities of the ground states of the majority species (N₂, O₂ and NO) that were obtained by laser techniques. As for the NO(X ² Π) ground state, vibration and rotation of the excited state are found out of equilibrium. The NO(A ² Σ ⁺) excited state is found to be populated by an energy transfer from the metastable N₂($A^{3}\varSigma ^{+}_{u}$). The steady state of the plasma allows using this property to derive N₂($A^{3}\varSigma ^{+}_{u}$) densities and N₂ electronic excitation temperatures. Close to the wall, a production of excited NO by a catalytic process is also considered involving N₂($A^{3}\varSigma ^{+}_{u}$) as source of adsorbed atoms. The present results confirm that the kinetic temperature cannot be compared to the rotational temperature derived from optical emission spectroscopy in such plasma conditions.