Soot particles in premixed C2H4-air-flames at high pressures (P=30-70 bar)

In premixed C2H4-air-flames, optical measurements of the growth of soot par ticles were performed in the pressure region of 30 to 70 bar. From the lase r-scattering and molar absorptivity measurements, particle number densities , N, particle diameter, d, and soot volume fraction, f(V), were obtained at different heights above the burner for several mixture compositions and un burnt gas velocities corresponding to different flame temperatures. In addi tion soot samples were taken for electron microscopic measurements. The par ticle diameters and their size-distributions were determined and the soot p article structures were studied. The electron microscopic measurements show ed, that for the pressures applied, the primary particle diameters can be d escribed by a log-normal size distribution with a mean standard deviation o f sigma(g)approximate to 0.35 nm. In the high resolution electron micrograp hs, crystallite layers could be observed. These crystallite layers become m ore pronounced for long reaction times, high flame temperatures and high pr essures. In the pressure region of 30 to 70 bar the particle number density towards the end of the growth process, N-infinity, increases with increasi ng C/O ratio and with pressure. At 70 bar N-infinity can reach 10(13) cm(-3 ) in strongly sooting flames. The final particle diameter, d(infinity), for otherwise fixed conditions decreases with pressure, so that at 70 bar smal l diameters of d(infinity)less than or equal to 50 nm result. Therefore the mean final soot surface, A(infinity), becomes very large, with values up t o 250 cm(-1) observed (C/O greater than or equal to 0.70, T similar to 1850 K).