Optical Measurements of Soot Formation in Premixed Flames

Abstract

Abstract The growth of soot particles in rich premixed flames was measured using the diffusion broadening particle-sizing technique. This dynamic-light-scattering technique reduces the dependence of size measurements on particle complex index of refraction which is a problem with most light-scattering size measurements. Size measurements were made in fuel-rich premixed methaneoxygen flat flames with fuel-oxidizer ratios ranging from 2.2to 2.9 times sloichiometric values,and in propane-oxygen flames at an equivalence ratio of 2.6. Particles as small as 23nm in diameter were observed 5 mm above the burner surface (within about a millimeter of the flame front), growing t0 60-120nm (depending on the equivalence ratio) 14mm above Ihe burner. Measurements of the intensity of scattered light were used in conjunction with the size measurements to determine the soot number density in the methane-oxygen flames, which was found to be decreasing with height above the burner at positions 5 mm above the burner and higher. The soot volume fraction in the methane-oxygen flame, determined from the size and number-density measurements, continually increased in this region, although apparently starting to level off 12 mm above the burner surface. The temperature history of the soot particles was determined using optical pyrometry. Near the flame front, the measured particle temperature was slightly higher than the adiabatic flame temperature. decreasing below this level further downstream.