Optical studies of soot particle growth in hydrocarbon oxygen flames

Abstract

Number densities, particle sizes and flame temperatures were measured in flat, premixed, laminar hydrocarbon oxygen flames at low pressure. Methods used were light scattering, extinction and sodium-D-line reversal. Soot particles in sooting hydrocarbon-oxygen flames are comparable to particles of a coagulating aerosol. This is indicated by testing the long time particle dynamics for particle number densities in the measured flames and by solving numerically the particle balance using a model based on free molecular coagulation which comprises surface growth and particle inception. The computations yield particle size distributions tending to a constant relative standard deviation after short reaction times though particle inception and surface growth should counteract the broadening of the size distributions. Changes of soot loading appear as changes of the soot mass increase by surface reactions because about 90% of the soot is built by surface reactions. Surface growth of soot particles does not follow simple pyrolytic mechanism involving thecomponents of a hydrocarbon pool. This can be concluded from the concentration profiles of those components in the soot forming region of the flames and the surface growth rates. Particle number densities and particle sizes are governed by particle inception over a small interval at incipient soot formation meaning that once solid particles are formed the intermediates of the hydrocarbon pool preferably react with the surfaces of the soot particles than with themselves to form larger intermediates or new particles.