Soot size distributions in rich premixed ethylene flames

Electrical mobility measurements of the soot particles generated in rich premixed ethylene/air flat flames reveal a characteristic lognormal size distribution that is distinct from the self preserving distribution expected from coagulation dominated aerosol dynamics. The distribution changes to a bimodal form as the equivalence ratio and the height above the burner increase. The soot particles are sampled using a three-stage ejector pump, with an overall dilution of the soot mole fraction by 3000, that quenches the flame chemistry and dilutes the sample for analysis by a nano-differential mobility analyzer. The measured soot volume fraction is in good agreement with optical extinction data, quantitatively reproducing the increases previously noted with respect to increasing equivalence ratio and height above the burner. The trends for mean particle diameter are also reproduced, but the mobility diameters are roughly threefold smaller than their light scattering counterparts. Particle number density is found to increase with height, whereas the optical data exhibit the opposite trend. Residence chamber experiments show that with sufficient time the quenched soot sample evolves from the lognormal shape found in the flame to the expected self preserving distribution.