In situ combustion measurements of H2O and temperature near 2.5 μm 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 mu m. Recent availability of room-temperature semiconductor diode lasers operating at longer wavelengths provides access to fundamental vibrational bands (nu(1) and nu(3)) of H2O. These bands have stronger absorption line strength compared to the overtone (2 nu(1), 2 nu(3)) and combination (nu(1) + nu(3)) 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 mu m 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.