Study On Soot Formation Characteristics Of Hydrocarbon Fuel Diffusion Flames In Oxy-Fuel Atmosphere

Soot is the product of the incomplete combustion of fossil fuels,and the study of soot formation characteristics is the basis for its formation and emission control.Stoichiometric mixture fraction Z_st has been shown to be an important parameter affecting the structure and soot formation of diffusion flames,and is adjusted by relocating inert gas from the oxidant stream to the fuel stream.The detailed mechanism of Z_st affecting soot formation in oxy-fuel atmosphere remains unclear,and it is of great significance to study the effect of Z_st on soot formation characteristics of diffusion flames in oxy-fuel atmosphere for both the in-depth understanding of soot formation and the industrial application of oxy-fuel combustion.This dissertation measured the soot volume fraction and temperature based on spontaneous radiation,the effect of Z_st on soot formation characteristics of hydrocarbon fuel diffusion flame in oxy-fuel atmosphere and its detailed mechanism were investigated,and the relationship between soot formation and chemiluminescence was analyzed.A reconstruction algorithm for solving the soot volume fraction and temperature distribution of flames by spontaneous radiation measurement was constructed.Thermal radiation calibration and noise removal were carried out based on the principle of visible thermal radiation imaging.Tikhonov regularization-Generalized singular value decomposition reconstruction algorithm was applied to the spontaneous radiation measurement of hydrocarbon fuel flames,the optimal regularization parameter was determined by using the L-curve method,and the self-absorption effect of soot was considered by iteration.The uncertainties of soot volume fraction and temperature results reconstructed by this algorithm are 7.5%and 6.1%,respectively.And this algorithm is applicable to normal diffusion flames,partially premixed flames,and inverse diffusion flames with high accuracy and robustness.Based on the carbon-to-oxygen ratio（C/O）space,the effect of Z_st on soot inception,formation,and oxidant characteristics of normal diffusion flames in oxy-fuel atmosphere were analyzed.The critical temperatures of soot inception on the flame centerline and sheet are 1322 K and 1632 K,respectively.Because the comparatively low temperature on the centerline can guarantee the collision and cluster formation of polycyclic aromatic hydrocarbons,while the high temperature is required for chemical bond formation on the sheet.The increase of Z_st compresses C/O space,lowers the concentration of polycyclic aromatic hydrocarbons,and improves oxidation ability to inhibit soot inception.It also reduces soot volume fraction by suppressing soot nucleation and surface growth while promoting its oxidation.CO₂ introduced by oxy-fuel atmosphere enhances the oxidant of flames by promoting the formation of OH through the reaction CO+OH=CO₂+H.Experimental C/O measurements were carried out based on laser-induced breakdown spectroscopy,and the effects of CO₂ addition and Z_st in oxy-fuel atmosphere on soot formation characteristics of inverse diffusion flames were analyzed.CO₂ addition inhibits soot formation by reducing flame temperature and residence time,resulting in a significantly lower soot formation rate and soot loading of CO₂ addition than N₂ addition.The atomic spectral line intensity ratio after plasma temperature correction shows a good linear relationship with the atomic ratio.The critical C/O required for soot inception in inverse diffusion flame is 0.5-0.6.The increase of Z_st inhibits soot inception by reducing the C/O near the flame sheet,and it also shortens residence time and compresses C/O space to make inverse diffusion flame unfavorable for soot formation.The detailed mechanism of the competitive relationship between soot formation and chemiluminescence in hydrocarbon fuel diffusion flames was revealed.Through spontaneous radiation measurement and construction of the KM2-Radical mechanism coupled with OH^*/CH^*/C₂^*/CO₂^*species,the competitive relationship between soot formation and chemiluminescence was confirmed,specifically by competing for fuel pyrolysis products and proceeding in opposite reaction paths.Chemiluminescence mainly exists in the region where C/O is 0.45-0.50.The results indicate that CO₂addition enhances chemiluminescence and makes it appear blue.With the increase of Z_stin oxy-fuel atmosphere,chemiluminescence in counterflow diffusion flames and near the fuel nozzle of normal diffusion flames is suppressed.