Tunable diode-laser absorption measurements of temperature, velocity, and H2O in hypervelocity flows

Abstract

Tunable diode-laser absorption measurements of temperature, velocity, and H2O partial pressures have been recorded in hypervelocity air flows using a dual-wavelength tunable diode-laser system developed at Stanford and installed at the Calspan University of Buffalo Research Center's (CUBRC) Large Energy National Shock Tunnel (LENS Tunnel) in Buffalo, New York. The measurements were recorded using a hardened probe, which contained critical optical components and photodetectors, that was installed directly into the flowfield near the nozzle exit to minimize complications due to boundary layers and facility vibration. The wavelengths of the distributed feedback diode lasers were independently current-tuned at an 8-kHz rate across H2O transitions (v,+v3 band) near 1400 nm and 1396 nm to yield high-resolution absorption lineshape measurements every 125 |0,s. Values of rotational temperature, determined from the ratio of measured absorbances, were in excellent agreement with translational temperatures, determined from the measured (Doppler-broadened) lineshapes, over the measured range (400-900 K). H2O partial pressure was determined from the measured absorbance of one transition and the gas temperature. Gas velocity was determined from Doppler-shifted absorption recorded by directing the 1400-nm beam through the flowfield at a 54° angle relative to the bulk gas velocity. The measured gas temperatures and velocities were consistent with calculated steady-state values (650 K and 4500 m/s, respectively). The results obtained demonstrate the applicability of diode-laser absorption diagnostics for direct multi-parameter gas measurements in hypervelocity flowfields for improved characterization of high-enthalpy facilities.