Development of a sensor for temperature and water concentration in combustiongases using a single tunable diode laser

Abstract

The water vapour spectrum in the 1–2 µmnear-infrared region is systematically analysed to find the bestabsorption transitions for sensitive measurement of H2Oconcentration and temperature in combustion environments using a single tunable diodelaser with typical distributed feedback single-mode scanning range (1 cm−1).The use of a single laser, even with relatively narrow tuning range, canoffer distinct advantages over wavelength-multiplexing techniques. Thestrategy and spectroscopic criteria for selecting optimum wavelengthregions and absorption line combinations are discussed. It should bestressed that no single figure of merit can be derived to simplify theselection process, and the optimum line pair should be chosen case bycase. Our investigation reveals that the 1.8 µmspectral region is especially promising, and we have identified 10of the best water line pairs in this spectral region for temperaturemeasurements in flames. Based on these findings, a pair of H2Otransitions near 1.8 µmwas targeted for the design and development of an initialsingle-laser sensor for simultaneously measuring H2Oconcentration and temperature in atmospheric-pressure flames. As part of thesensor development effort, fundamental spectroscopic parameters includingthe line strength, line-centre frequency and lower state energies of theprobed transitions were measured experimentally to improve the currentdatabases. We conclude with demonstration results in a steady and a forcedatmospheric-pressure laboratory combustor.