A study of soot formation in premixed propane/oxygen flames by in-situ optical techniques and sampling probes

Abstract

Formation of soot in premixed laminar methane and propane-oxygen flames has been studied as a function of reaction time, fuel equivalence ratio and temperature. Probe and laser-light scattering and absorption techniques were used in a complementary fashion. Measurements included soot volume fraction, individual spherical particle and agglomerate mean diameter and number density. Comparisons of probe measurements with results computed from the optical technique indicate that the soot refractive indices proposed by Dalzell (m=1.57−0.56i) and Janzen (m=2−1i) provide accurate results. Two regimes of soot particle growth have been identified. For soot volume fraction smaller than about 10−7, the particle growth is by coalescent coagulation, as originally proposed by Ulrich and Graham. At larger soot volume fraction, chain forming collisions occur, simultaneously with some surface growth. In both regimes, the particle number density is described by the free molecular collision theory, the final number of agglomerates being always close to 1010/cm3. The soot volume fraction and the particle size increase with increasing fuel equivalence ratio and decreasing temperature. The particle number density is only slightly affected by both parameters.