In situ combustion measurements of H₂O and temperature near 2.5 νm using tunable diode laser absorption

In situ combustion measurements of water vapor concentration and gas temperature were carried out with a new tunable diode laser sensor near 2.5 νm. Recent availability of room-temperature semiconductor diode lasers operating at longer wavelengths provides access to fundamental vibrational bands (ν₁ and ν₃) of H₂O. These bands have stronger absorption line strength compared to the overtone (2ν₁, 2ν₃) and combination (ν₁ + ν₃) vibrational bands in the near-infrared region probed previously with telecommunication diode lasers. The absorption transitions of H₂O vapor in the 2.5-3.0 νm region are systematically analyzed via spectral simulation, and optimal spectral line pairs are selected for combustion measurements in the temperature range of 1000-2500 K. Fundamental spectroscopic parameters (line strength, line position and line-broadening coefficients) of the selected transitions are determined via laboratory measurements in a heated cell. Absorption measurements of H₂O concentration and temperature are then made in a laboratory flat-flame burner to illustrate the potential of this sensor for sensitive and accurate measurements in combustion gases with short optical path lengths.