Measurements of Metastable N2(A3Σu+,v) Molecules in Nonequilibrium Supersonic Flows

Abstract

Nonintrusive laser diagnostics are used for the measurements of metastable nitrogen molecules in the lowest excited electronic state, ${\mathrm{N}}_2({\mathrm{A}}^3{\mathrm{\Sigma }}_u^{+})$ , in a nonequilibrium flow, blowdown supersonic wind tunnel. The tunnel is operated with nitrogen at plenum pressures of $P_0=200–250$ Torr, with the flow expanding through a two-dimensional contoured nozzle to the Mach number of $M=3.6–4.6$ . The steady-state run time of the tunnel is approximately 10 s. The flow is excited by a repetitive nanosecond pulse discharge operated in the plenum or in the nozzle throat. Time-resolved absolute ${\mathrm{N}}_2({\mathrm{A}}^3{\mathrm{\Sigma }}_u^{+},v=0)$ populations in the plenum are measured by single-pass, continuous wave tunable diode laser absorption spectroscopy (TDLAS). In the supersonic test section, absolute ${\mathrm{N}}_2({\mathrm{A}}^3{\mathrm{\Sigma }}_u^{+},v=0–2)$ populations are measured by cavity ring down spectroscopy (CRDS), using a tunable pulsed laser system operated at 10 Hz. During each run, 50 single-shot ring down traces are acquired, demonstrating good shot-to-shot reproducibility. The results demonstrate that the cavity ring down time is not affected by the supersonic flow. ${\mathrm{N}}_2({\mathrm{A}}^3{\mathrm{\Sigma }}_u^{+},v=0–2)$ populations and the flow temperature are inferred from the single-shot CRDS data. At the conditions when the flow is excited by the discharge in the nozzle throat, ${\mathrm{N}}_2({\mathrm{A}}^3{\mathrm{\Sigma }}_u^{+},v=0)$ population in the supersonic test section is measured by both CRDS and TDLAS diagnostics. The two diagnostic techniques are complementary and can be used for characterization of nonequilibrium reacting flows over a wide range of pressures, including short-run-time high-enthalpy flow facilities.