Abstract

Laser based fluorescence or ion detection methods in reactive media offer advantages such as high selectivity, nonintrusiveness and rapid time response. However, calibrating these techniques for quantitative concentration measurements is often difficult. Direct absorption methods yield quantitative measurements, but poorer sensitivity has limited their use. High frequency wavelength modulation techniques improve the minimum detectable absorbance to 10^-6 or better while retaining the advantages of direct absorption detection. We have applied high frequency wavelength modulation with GaAlAs diode lasers for measurements of temperature and O₂ concentration in laboratory flames. For the concentration measurement, the RQ(23, 24) line in the O₂ A-band is used, while the RQ(23, 24) and RQ(33, 34) line pair are utilized in temperature measurements. A single-mode optical fiber cable is used to transport the laser beam to the flame region. Atmospheric pressure premixed methane/oxygen flames at various equivalence ratios are studied in these experiments. Radiation corrected thermocouple measurements are compared with spectroscopically measured temperatures at different locations and in varying flame conditions.

© 1991 Optical Society of America