Development of a Mach 5 Nonequilibrium-Flow Wind Tunnel

Abstract

A small-scale Mach 5 blow down wind tunnel, with ample access for optical diagnostics and ability to generate steady-state nonequilibrium flows, has been designed and operated. The wind tunnel uses transverse repetitively pulsed nanosecond discharge, fully overlapped with a transverse DC discharge and operated at high plenum pressures (P0=0.5-1.0 atm) to load internal energy modes of nitrogen and oxygen molecules. The discharge remains stable at energy loadings of up to ~0.1 eV/molecule in nitrogen (discharge power up to 2.5 kW). The wind tunnel generates nonequilibrium nitrogen and air flows with steady-state run time of 5-10 seconds, translational / rotational temperature of T0~300-400 K, and estimated upper bound nitrogen vibrational temperature of Tv0~2,000 K. Internal energy mode disequilibrium in the flow is controlled by injecting nitric oxide, hydrogen, or water vapor into the subsonic flow between the discharge section and the nozzle throat. The effect of energy mode disequilibrium is studied in a flow over a cylinder model placed in the Mach 5 test section. The flow field in the supersonic test section is well predicted by a 3-D compressible Navier-Stokes flow code, indicating good flow quality. The supersonic flow field over the model is visualized by schlieren imaging and NO PLIF imaging, using a burst mode laser operated in the vicinity of 226 nm, at a pulse repetition rate of 10-20 kHz. The laser was operated in the injection-seeded mode, generating narrow linewidth (~0.1 cm -1 ) output for single-line NO excitation in the flow. Nitric oxide was either injected into the flow in the plenum or generated in a repetitively pulsed nanosecond discharge in dry air. Both single-pulse PLIF images and images integrated over 10-50 laser pulses have been obtained. Two single-line NO PLIF images on a NO(X,v˝=0→A,v´=0) transition are used for measurements of 2-D temperature distributions in nitrogen flows in the supersonic test section. Another single-line NO PLIF image on a NO(X,v˝=1→A,v´=1) transition is used to estimate NO vibrational temperature behind the bow shock, TV(NO)=550 ± 100 K. The NO vibrational temperature increases when the energy loading in the discharge is increased. Kinetic modeling calculations indicate that low NO vibrational temperature is due to fairly low vibrational energy loading per nitrogen molecule in the discharge. Schlieren images of a supersonic flow over the cylinder model demonstrate that the shock stand-off distance is reduced by approximately 5% when the discharge in the wind tunnel is in operation and water vapor or hydrogen are injected into the flow between the discharge section and the nozzle throat. This effect is attributed to additional heating of the flow in the plenum during relaxation of vibrationally excited nitrogen in the presence of water vapor or hydrogen.