Rotational Temperature Measurement in Hypersonic Shock Tunnel using Tunable Diode Laser Absorption Spectroscopy

Abstract

Atmospheric entry of a spacecraft is considered as one of the critical phases in any planetary mission. Although it lasts only for a few minutes, rapid heating and deceleration can cause severe problems to the spacecraft. Hence, the understanding of the re-entry flow field around the vehicle and design of an optimized thermal protection system (TPS) to it is important, which depends on the reliable data on aerodynamic heating. Shock tunnels are proven ground test facilities for generating a hypersonic flow to investigate the high-temperature effects around the test model. As per the shock tunnel design, the shock compressed reservoir gas at the end of shock tube will expand through a nozzle to simulate the specific kinetic energy of a flow and this may be non-equilibrium in both chemically and thermally at high enthalpy flow conditions. So, the flow generated by shock tunnels differ fundamentally from the atmosphere air flow in a real flight. A better understanding of a freestream temperature generated by a shock tunnel is strongly needed for aerothermodynamic analysis. In this work, hypersonic shock tunnel HST-2 and free piston driven shock tunnel (FPST) HST-3 in the laboratory for hypersonics and shockwave research (LHSR) were used to generate a hypersonic freestream. The rotational temperature of the freestream was measured using tunable diode laser absorption spectroscopy (TDLAS) technique by probing water vapor (𝐻2𝑂) absorption lines near 1392 nm using a vertical cavity surface emitting laser (VCSEL) scanned at 25000 Hz. Experiments were performed at different locations from the nozzle exit and different enthalpy conditions in a flow Mach number, M≈8 in HST-2 and M≈10 in HST-3. The effect of vibrational non-equilibrium over rotational/translational temperature through V-R,T relaxation process of 𝑁2 and 𝑂2 in a gas flow has been observed at low enthalpy conditions. The effect of thermal and pressure collisions in the hypersonic flow have been analyzed from the measured rotational temperature and full width at half maximum (FWHM) of the absorption line. The establishment of thermal equilibrium between the rotational and translational temperature in the flow has been analyzed and validated the assumption of rotational/translational temperature equilibrium in the hypersonic flow.