Abstract
The formation of soot has been investigated in flat C2H4-air and C6H6-air flames, with special reference to low flame temperatures. Measurements of the threshold for soot formation show that as well as the “high temperature threshold” (slightly temperature dependent) there exists a low temperature threshold. For a given C/O ratio the mass of soot formed passes through a maximum between low and high temperature threshold at a temperature near 1600 to 1650 K. These soot yield curves are similar to those obtained in shock wave pyrolysis of benzene etc. by Graham, Frenklach et al. With increasing pressure the soot thresholds widen slightly. Their minima shift towards lower C/O ratio and the maxima of the soot yield curves rise. For the pressure dependence of the soot volume fraction finally formed, a P2 dependence has been found at constant flame temperature for flame temperatures above 1650 K and C/O ratios from 0.65 to 0.75. For benzene-air flames the pressure dependence is almost the same as for C2H4-air flames. The dependence of soot yield on the C/O ratio at constant flame temperature is a little weaker than the data obtained for constant flow velocity. Measurements of light absorption and light scattering along the flame axis show that coagulation and mass growth of the soot particles in the low temperature flames proceed in a similar way to that in flames at temperatures above 1600 K.
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