Soot formation in combustion processes

Abstract

Sooting tendencies of fuels are analyzed with respect to the type of experimental configuration used—per-mixed flames, normal and inverse co-annular, Wolfhard-Parker and counter-flowing diffusion flames, or shock tubes. The important effect of temperature in each type of experiment is examined. The relative tendency of various fuels to soot when consumed in pre-mixed and diffusion flames has been determined by the qualitative measurements of critical sooting equivalence ratios and smoke heights at various temperatures and quantitatively confirmed by measurements of chemical species, soot number density and volume fraction. Fuel structure has a significant effect on the sooting tendency of diffusion flames, but little influence in premixed flames. Irrespective of the fuel in diffusion flames soot inception occurs around 1400 K and is dependent somewhat on H atom diffusion. Particle burnout ceases at about 1300 K and is responsible for the smoke height. Incipient particle formation determines the volume fraction of soot and is controlled by the rate of formation of the first aromatic species containing one to two rings. The particle growth process is found to be the same regardless of flame type. A generalized chemical mechanism of soot formation irrespective of flame type or process is presented. The various routes in this mechanism support the relative sooting tendency of fuels found in both flame and shock tube experiments. The effect of oxygen added to various fuel in diffusion controlled experiments is analyzed.