The effect of flame structure on soot particle inception is studied by varying the mixture fraction at stoichiometry Z(st) and, consequently, flame location. Z(st) is varied by reassigning the nitrogen from the oxidizer to the fuel such that the flame temperature is not changed. Strain rates in the counterflow flame and flame heights in the coflow flame are measured in ethylene flames to identify the sooting limits based on the appearance of luminous soot. Numerical experiments with a counterflow diffusion-flame code employing C2 kinetics are also performed to understand the effects of Z(st) on flame structure and to interpret the experimental results. The results show that as Z(st) is increased and the flame shifts towards the fuel, soot inception is suppressed. In the counterflow flame, no luminous soot is detected at a strain rate greater than 60 s(-1) for Z(st) greater than or equal to 0.16, as compared to 175 s(-1) for the ethylene/air flame, Z(st) = 0.064. The laminar ethylene coflow flame is soot free at Z(st) greater than or equal to 0.72, regardless of flame height. For small Z(st), where changes in Z(st) are primarily due to changes in fuel concentration, the effect on soot inception is primarily fuel dilution. For large Z(st), where the oxygen concentration is appreciably increased, the subsequent shift in the OH profile towards the fuel side of the flame can have a dramatic influence on inception. The shift in OH essentially narrows the soot zone suggesting that it may be possible to obtain soot-free conditions for many fuels if the structure of the flame can be adjusted to the extent that significant OH exists on the fuel side of the flame at a temperature of ca. 1300 K. The experimental and numerical results demonstrate that this requirement can be satisfied for ethylene flames.
