TUNABLE DIODE LASER ABSORPTION PROBE TECHNIQUES FOR SPECIES MEASUREMENTS IN COMBUSTION GASES

Two techniques to measure mole fractions of infrared-active chemical species in combustion gases with both temporal and spatial resolution have been developed, assessed and demonstrated. The techniques involve the use of tunable infrared diode laser absorption spectroscopy combined with two types of intrusive miniature optical probes. The tunable diode laser is well-suited for temporally resolved measurements because the output is continuous in time and the wavelength can be tuned to line-center of a selected absorption transition. The spectroscopic method is suitable for the measurement of any infrared-active species; in this study carbon monoxide was measured.;The miniature probes were developed to give spatial resolution to line-of-sight absorption measurements. One type of probe, called a light-guide probe, directs the laser light into a physically-limited absorption path. The other probe is a fast sampling probe with a miniature in-line absorption cell. The performance of the two probes has been assessed both experimentally and theoretically. With the light-guide probe a detection limit of less than 1000 ppm of carbon monoxide was possible, with temporal resolution of less than 0.2 ms over a 5 mm absorption path. The small sampling probe had a minimum detection limit of 100 ppm, with temporal resolution of approximately 0.5 ms over a sample region extending from 0.5 to 2 cm.;The use of the probes was demonstrated in a steady cold flow and a premixed methane/air flat flame, and in turbulent cold and flame jets of carbon monoxide. The results of the steady measurements were all within 10% of expected values. The data from both turbulent jets were analyzed to compare performance of the probes and to elucidate the structure of the flows. Quantities obtained from the carbon monoxide measurements include mean values of mole fraction, fluctuation intensities, probability density functions, power spectral densities and integral scales from autocorrelations. The turbulent behavior of the jets in this study was consistent with behavior reported previously from measurements of temperature, velocity and concentrations of ions or seed materials.