SOOT GROWTH IN ATMOSPHERIC C2H4 AIR/O-2-FLAMES, INFLUENCE OF THE FUELCARBON DENSITY/

The influence of the fuel carbon density on the growth of soot particl es at p = 1 bar was measured in 3 different hydrocarbon flames, in C2H 4/air, C2H4/(40% N-2 + 60% O-2) and C2H4/O-2-flames. The particle beha viour was measured by the laser scattering- (lambda = 488 nm) and the extinction-method (lambda = 633 nm) in a wide range of flame condition s, up to the threshold of flame stabilities. For carbon densities abov e the threshold of soot formation of rho(c,surplus) greater than or eq ual to 5 . 10(17) C-atoms/cm(3), which are in general higher than in t he previously measured hydrocarbon/air-flames, and lower flame tempera tures a rapid soot growth takes place which results in large optical s oot particle diameters, d similar to 80 nm, and in high soot volume fr actions, f(v) 10(-6). In these flames soot particles stop growing earl y in the flames. Particles also cease to coagulate early in the flame so that a high final particle number density in the later phase of the combustion process prevails. High final values of N-infinity up to 10 (10) cm(-3) are reached. The results show that the rate constants for soot mass growth, k(f), for rho(c,surplus) greater than or equal to 5 . 10(17) C-atoms/cm(3) depend on temperature and also on the carbon de nsity of the fuel, respectively on the concentrations of the growth sp ecies. For higher carbon densities of the fuel the coagulation of the particles cannot be described by the simple Smoluchowski equation gene rally used. The obtained coagulation rate constants show a similar ten dency with temperature as those obtained in the high pressure flames. With increasing temperatures, T greater than or equal to 1700 K, the c oagulation rate constants decrease to low values and become lower than the theoretical coagulation rate constants.