State-to-state modeling of a recombining nitrogen plasma experiment

Abstract

An atmospheric pressure nitrogen/argon plasma flows at high velocity through a water-cooled test-section in which it is forced to recombine within 250μs. At the test-section inlet, the plasma is in Local Thermodynamic Equilibrium (LTE) at about 7200K. Because recombination rates are finite, the plasma reaches a state of chemical nonequilibrium at the exit of the test-section, at a temperature of about 4715K. At the test-section exit, the radiation emitted by the plasma, measured by emission spectroscopy, shows significant departures from equilibrium in the populations of the excited electronic states of nitrogen (N2 B3Πg, N2 C3Πu) and of the nitrogen ion N2+B2Σu+. This experiment is thus proposed as a test-case to validate collisional-radiative (CR) models. A vibrationally specific CR model is then used to predict the emission of these states. The rate coefficients of the model are calculated with the Weighted Rate Coefficient method based on elementary cross-section data. These rates depend explicitly on the kinetic temperatures of electrons (Te) and heavy species (Tg). The predictions of the CR model are in good agreement with the measured vibrational population distribution in the N2 B state. A method is then proposed to determine ground state nitrogen atom densities based on the measured vibrational population distribution of the N2 B state.