Validation cases for recombining nitrogen and air plasmas

Abstract

We report on Raman scattering and optical emission spectroscopy (OES) measurements in recombining atmospheric pressure plasmas of air and nitrogen. An inductively coupled plasma torch is used to create an equilibrium plasma, which is then forced to rapidly recombine by flowing through a water-cooled tube. For all conditions, temperature measurements are performed using OES and Raman scattering at the exit of tubes of varying lengths. The density of atomic nitrogen is also determined. Evidence of strong chemical nonequilibrium is found in a number of cases. For these cases, we observe that the rotational temperatures measured with OES differ from those measured with Raman scattering, and that the atomic nitrogen density is elevated with respect to equilibrium. A power balance analysis confirms that a large fraction of gas enthalpy is stored in the non-recombined nitrogen atoms. For cases where the plasma remains in equilibrium, we perform numerical simulations using the Eilmer3 computational fluid dynamics (CFD) code. Eilmer3 does not predict the observed drop in gas temperature measured using Raman scattering and OES. Prior efforts by the CFD community have also failed to correctly predict this temperature drop. The results presented in this paper are therefore intended as validation test cases for CFD simulations.

Experimental study of recombining nitrogen plasmas: I. Vibronic population distributions and nonequilibrium molecular radiation

“…The results for N2(B) are similar to the results of Ref. [13] and therefore will not be presented here. For N2(C) and N2 + (B), we present the radial distribution of vibrational levels densities for each axial position in Appendix A.…”

“…ground state. Finally, CFD codes failed to predict any of the measured temperatures under any of the experimental conditionseven those where the plasma remained in equilibrium [13]. This same discrepancy between measurements and CFD codes was previously observed by other authors [14] [15].…”

“…In this case, 1.2 g/s of nitrogen is premixed with 1.5 g/s of argon. In previous work [13], we measured the emission spectrum between 500 and 800 nm and we observed a departure from chemical equilibrium at 10 and 15 cm leading to an overpopulation of the vibrational levels of N2(B). Here we extend these measurements from 250 to 900 nm in order to (a) validate the previous measurements, (b) access vibrational level v = 1 of N2(B) at 870 nm, and (c) measure the transitions of N2(C) → N2(B) and N2 + (B) → N2 + (X) between 280 and 450 nm.…”

“…In this paper, we analyze the radiation of the plasma over the range 250-900 nm to obtain the absolute densities of vibrational states of N 2 (B 3 Π g ), N 2 (C 3 Π u ) and N 2 + (B 2 Σ u + ) . This was previously done only for the N2 first positive system [13] up to 800 nm. We study two experimental conditions corresponding to two total mass flow rates.…”

Measurements and modeling of CO 4th positive (A–X) radiation

“…The procedure for these emission-based temperature measurements is documented in Refs. [24,34]. The analysis relies upon the assumption of thermochemical equilibrium and rotational symmetry of the plasma jet.…”