Soot in flame-wall interactions: Views from nanostructure and reactivity

Abstract

The jet impinging flame is a typical flame-wall interaction configuration. This paper presented an experimental study of laboratory jet impinging ethylene diffusion flames with the special emphasis on characteristics of soot forming on the impinging plate. The soot distribution, nanostructure and reactivity which corresponded with different flame structures were investigated comparatively based on parametric variations of nozzle to plate distance (H), Reynolds number (Re) and collecting time (T). The results showed that impinging flame structures could be classified as cool central core, envelope, and disc flames based on different nozzle to plate distances and Reynolds numbers. A series of concentric soot rings for jet impinging flames were formed on the plate surface. Inner ring soot was the film-like material with the amorphous structure, and outer ring soot consisted of suborbicular primary particles with the classic core-shell structure. For soot at Re = 600 and T = 5 min, the augment of the nozzle to plate distance reduced the content of film-like soot at the inner ring because the extension of the jet free region could promote the oxidation of soot. The crystallization degree of outer ring soot was non-monotonic with the increase of the nozzle to plate distance. It might attribute to the reduction of air entrainment of the flame layer in the wall jet region. Inner ring soot at H = 10 mm had the most disordered carbon atoms with the shortest fringe length and the highest fringe tortuosity. It also presented the highest reactivity. Because of the high degree of crystallization with the longest fringe length and the smallest tortuosity, the reactivity of soot sampled at H = 30 mm was the lowest. For soot sampled with Reynolds numbers of 300, 600 and 900 at H = 10 mm and T = 5 min, inner ring soot at Re = 600 had the most amorphous structure related with the shortest fringe length and the largest tortuosity. The crystallization degrees of soot from outer rings increased with Reynolds numbers. The increase of the impinging flame layer might promote the air entrainment which could accelerate the soot oxidation. In addition, the oxidation reactivity of soot sampled at different Reynolds numbers were in a sequence of Re = 600 inner ring > Re = 300 inner ring > Re = 900 inner ring > Re = 300 outer ring > Re = 600 outer ring > Re = 900 outer ring, which was in consistent with variations of the fringe length and tortuosity. The reactivity of soot from both inner and outer rings decreased with the increase of collecting times at H = 10 mm and Re = 600, which might result from the increase of the plate temperature. The results also confirmed the relationship between the soot structure and oxidation reactivity that soot with the higher degree of crystallization was more difficult to oxidize.