Wavelength-modulation spectroscopy near 2.5 μm for H2O and temperature in high-pressure and -temperature gases
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The design and validation of a tunable diode laser (TDL) sensor for temperature and H2O in high-pressure and -temperature gases are presented. High-fidelity measurements are enabled through the use of: (1) strong H2O fundamental-band absorption near 2.5 μm, (2) calibration-free first-harmonic-normalized wavelength-modulation spectroscopy with second-harmonic detection (WMS-2f/1f), (3) an experimentally derived and validated spectroscopic database, and (4) a new approach to selecting the optimal wavelength and modulation depth of each laser. This sensor uses two TDLs near 2,474 and 2,482 nm that were fiber coupled in free space and frequency multiplexed to enable measurements along a single line-of-sight. The lasers were modulated at 35 and 45.5 kHz, respectively, to achieve a sensor bandwidth of 4.5 kHz. This sensor was validated in a shock tube at temperatures and pressures ranging from 1,000 to 2,700 K and 8 to 50 bar. There the sensor resolved transients and recovered the known steady-state temperature and H2O mole fraction with a precision of 3.2 and 2.6 % RMS, respectively.
KeywordsShock Tube Modulation Depth Tunable Diode Laser Beam Steering Wavelength Pair
This work was sponsored by Innovative Scientific Solutions Incorporated (ISSI) with Dr. John Hoke as technical monitor and by the Air Force Office of Scientific Research (AFOSR) and the National Center for Hypersonic Combined Cycle Propulsion, grant FA 9550-09-1-0611, with technical monitors Dr. Chiping Li (AFOSR) and Dr. Richard Gaffney (NASA).
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