The effect of oxygen and carbon dioxide concentration on soot formation in nonpremixed flames using time resolved LII technique

Abstract

The influence of oxygen concentration and CO2 as diluent in oxidizer side on soot characteristics was studied by Laser Induced Incandescence, Time Resolved LII and Transmission Electron Microscopy photography in non-premixed coflowing flames. Through the comparison of TEM photographs and the decay rate of LII signal, suitable two delay times of TIRE-LII method and signal sensitivity (Delta STIRE-LII/Delta d(p)) were determined. The effects of 02 and CO2 as diluent in oxidizer side on soot formation are investigated with these calibrated techniques. The O-2+CO2, N-2, and [Ar+CO2] mixture in co-flow were used to isolate CO2 effects systematically. The number concentration of primary particle and soot volume fraction abruptly decrease by the addition of CO2 to the co-flow. This suppression is resulted from the short residence time in inception region because of the late nucleation and the decrease of surface growth distance by the low flame temperature due to the higher thermal capacity and the chemical change of CO2 including thermal dissociation. As the oxygen concentration increases, the number concentration of soot particles at the inception region increases and thus this increase of nucleation enhances the growth of soot particle.