A tunable diode laser-based H2O absorption diagnostic has been developed to characterize transient phenomena within the Stanford Expansion Tube Flow Facility. Key parameters provided by the diagnostic include the temperature, H2O mole-fraction and velocity time-histories of the supersonic test gas, as well as the arrival time of the test gas contact surfaces. The presence of water within the test gas affords well-established rovibrational transitions near 1.4 µm that are probed using scanned wavelength-modulation absorption spectroscopy with second harmonic detection (scanned-WMS-2f). The scannedWMS-2f technique offers the sensitivity and noise resistance necessary for temperature and mole-fraction measurements within the harsh conditions of the expansion tube test section. A rigorous scanned-WMS-2f theory has been developed to account for non-ideal laser characteristics prompted by short test times and high flow velocities and is employed for the first time in an absorption diagnostic. Furthermore, by implementing a crossed beam laser configuration, second harmonic Doppler shift velocimetry is performed in concert with the temperature and mole-fraction measurements to provide flow velocity data. The measurements carried out with this sensor provide a quantitative characterization of the test gas conditions produced in the expansion tube and thus provide an essential foundation for supersonic combustion experimentation within the facility.
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