Comparison of Soot Processes Inside Turbulent Acetylene Flames under Atmospheric-Pressure Conditions

Two soot-containing turbulent non-premixed flames burning gaseous acetylene in atmospheric-pressure air were investigated by conducting non-intrusive optical experiments at various flame locations. The differences in burner exit Reynolds numbers of these flames were large enough to examine the influence of flow dynamics on soot formation and evolution processes in heavily-sooting flames. By accounting for the fractal nature of aggregated primary particles (spherules), the proper interpretation of the laser scattering and extinction measurements yielded all the soot parameters of principal interest. With the separation of spherule and aggregate sizes, the axial zones of the prevailing turbulent soot mechanisms were accurately identified. With the high propensity of acetylene fuel to soot, relatively fast particle nucleation process led to high concentrations immediately above the burner exit. Soot growth process apparently dominated until the middle flame regions until which soot volume fractions and spherule diameters continuously increased. Above these axial locations, soot burnout became important as evident from the reductions in the soot spherule diameters and volume fractions. The maximum mean soot volume fractions in both acetylene flames were 8-10 ppm with these peak values being reached approximately in the middle flame portions, where the flame temperatures were also the highest. The mean diameters of soot spherules in the flames were within the narrow range of 10-45 nm. The mean aggregate sizes were 40-160 nm, depending on the flame location. When the Reynolds number was more than doubled from one flame to the other, the maximum values of soot volume fraction, and mean spherule and aggregate sizes did not change significantly. The findings reported here are relevant not only to the understanding of soot aerosol dynamics but also to the development and evaluation of predictive computational models in turbulent flames. The present measurements also provide a stringent test for particulate diagnostics in their ability to quantify the actual soot particle properties, especially in optically-thick turbulent flames that are typical of diesel engines