Abstract
Hydrogen was added to acetylene- air laminar inverse diffusion flames (IDF) to investigate the effect of hydrogen enhanced combustion on early soot formation process. A co-annular burner was used for the studies. In this study, acetylene flow rate was kept constant to maintain a fixed concentration of carbon moles in the reaction zone while the hydrogen flow rate was varied. Thermophoretic sampling of soot on transmission electron microscope (TEM) grids was employed to obtain soot samples at different flame heights. Off-line TEM analysis of the soot samples was carried out for measurement of soot volume fraction and soot structure parameters e.g., primary particle size, soot aggregates size, fractal dimensions etc.
Addition of hydrogen resulted in a significant reduction in primary soot particle size, soot volume fraction and aggregate size. The results of IDF are compared with those of normal-diffusion flames (NDF) under similar conditions where matured soot particles and aggregates are formed as these spend a longer residence time in the reaction zone. The present study in IDF reveals beneficial effect of hydrogen addition even at the nucleation and early surface growth stage in soot formation process when large reductions in primary particle size and soot volume fractions are observed. The NDF studies have shown reduction in soot volume fraction and particle size at higher flame heights attributed to enhanced soot oxidation caused by OH radicals as a result of hydrogen addition. The fractal dimensions of soot in both the flames are similar in the range 1.6 to 1.8 showing the chainlike structure of soot aggregates and are not affected by addition of hydrogen to hydrocarbon fuel.
Keywords: hydrogen, inverse diffusion flame, primary particle, soot aggregates, soot volume fraction,