A scanned-wavelength-modulation absorption-spectroscopy sensor for temperature and H2O in low-pressure flames

The design and demonstration of a tunable diode laser sensor for in situ temperature and water (H₂O) measurements in laboratory flames is presented. A newly-designed probe consisting of two single-crystal sapphire rods is used to guide 2.9 μm laser light across the core of a flame while avoiding absorption by water outside of the flame. The sensor probes two H₂O transitions near 2.9 μm using a scanned-wavelength-modulation absorption-spectroscopy spectral-fitting technique, which enables measurements without calibration or knowledge of the mixture collisional-broadening coefficient. To demonstrate the sensor, temperature and water mole fraction measurements were acquired in stoichiometric, burner-stabilized flames of methane/oxygen/argon at pressures of 25 Torr and 60 Torr. Typical total uncertainties in the temperature and water mole fraction measurements were ±50 K and ±0.016, respectively. This sensor is simple, robust and accurate and enables improved chemical kinetics studies of low-pressure flames. Improvements to reduce temperature and H₂O precision to ±20 K and ±0.008, respectively, are discussed.