Observations of Soot in the Combustion of Methanol/Toluene Spray Flames

The in uence of fuel chemical composition on  ame structure and sooting trends in a swirling spray  ame was investigated semiquantitatively using a light-scattering/dissymmetry ratio technique. Binary mixtures of toluene and methanol were examined under Ž xed aerodynamic and burner inlet  ow conditions to show the effect of liquid composition. The sprays were generated using an air-assist atomizer. Methanol volume fractions Á of 0 (pure toluene), 0.25, 0.50, 0.75, 0.85, 0.90, 0.95, 0.99, and 1.0 (pure methanol) were examined. The results revealed little effect of methanol concentration on soot formation for 0:0 <– Á <– 0:75. However, for Á >0.75, the scattered light intensity decreased by almost four orders of magnitude, indicating a dramatic reduction in soot production above this concentration. Measurements of the dissymmetry ratio, complemented with Lorenz–Mie calculations of the scattering characteristics for a polydispersion of particles, indicated that no deŽ nite trends for the soot mean size were found with varying liquid composition within the resolution of the data. Direct light photography showed a hybrid  ame structure for Á >0.75, which was blue and nonsooting near the atomizer exit, and yellow and luminous downstream, indicating preferential vaporization of methanol and a delay time for soot formation. The boundary between the yellow and blue zones of the  ame moved toward the atomizer exit as the toluene concentration increased.