Optical measurements of soot in premixed flames

Two laser diagnostic techniques were used to measure soot volume fractions, number densities and soot particle radii in premixed propane/oxygen flat flames. The two techniques were two wavelength extinction, using 514.5 to 632.8 nm and 457.9 to 632.8 nm wavelength combinations, and extinction/scattering using 514.5 nm light. The flames were fuel rich and had cold gas velocities varying from 3.4 to 5.5 cm/s. Measurements were made at various heights above the sintered bronze, water colored flat flame burner with the equivalence ratio and cold gas velocity fixed. Also, measurements were made at a fixed height above the burner and fixed cold gas velocity while varying the equivalence ratio. Both laser techniques are based on the same underlying assumptions of particle size distribution and soot optical properties. Full Mie theory was used to determine the extinction coefficients and the scattering efficiencies. Temperature measurements in the flame were made using infrared radiometry and fine wire thermocouples. Good agreement between the two techniques in terms of soot particle radii, number density and volume fraction was found for intensity ratios between 0.1 and 0.8.