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

Abstract

In premixed C₂H₄–air-flames, optical measurements of the growth of soot particles were performed in the pressure region of 30 to 70 bar. From the laser-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 unburnt gas velocities corresponding to different flame temperatures. In addition soot samples were taken for electron microscopic measurements. The particle diameters and their size-distributions were determined and the soot particle structures were studied. The electron microscopic measurements showed, that for the pressures applied, the primary particle diameters can be described by a log–normal size distribution with a mean standard deviation of σ_g≈0.35 nm. In the high resolution electron micrographs, crystallite layers could be observed. These crystallite layers become more pronounced for long reaction times, high flame temperatures and high pressures. In the pressure region of 30 to 70 bar the particle number density towards the end of the growth process, N_∞, increases with increasing C/O ratio and with pressure. At 70 bar N_∞ can reach 10¹³ cm^-3 in strongly sooting flames. The final particle diameter, d_∞, for otherwise fixed conditions decreases with pressure, so that at 70 bar small diameters of d_∞⩽50 nm result. Therefore the mean final soot surface, A_∞, becomes very large, with values up to 250 cm^-1 observed (C/O0.70, T∽1850 K).