Soot morphology emitted from vaporized canola methyl ester (CME)- And diesel-with varying levels of oxygen enhancement in Co-flow diffusion flames

In this study we report the effect on soot morphology that occurs when oxygen is varied in the oxidizer stream of canola methyl ester (CME), a fuel with atomic bonded oxygen, co-flow diffusion flame. Oxygen concentrations studied were 35%, 50%, and 80%; morphologies formed at these concentrations were compared to those formed in air (21%O2) as well as to a diesel flame using comparable %O2 contents. Thermophoretically extracted soot samples from within the flame, in their condensed phase, were used to create the soot profiles via a transmission electron microscopy (TEM) analysis. Spatial soot evolution profiles based on low-resolution TEM identified regions of individual discrete or "young soot" particulates and "mature soot." The presence of irregular structures, "liquid-like" or "globules," is evident in lower and middle regions of the CME/21% and CME/35%O2 flames. As the %O2 is further increased to 50% and 80% the irregular structures are no longer present. The soot primary particle diameter (dp) of the CME/21%, /35%, /50%, and /80%O2 flames decreased by approximately 27, 33, 35, and 28 percent, respectively, compared to their Diesel counterparts. In the CME flame, at a lower %O2 in the oxygen-air mixture-35%O2-there is not a significant variation of the dp compared to its air counterpart. However, introducing intermediate %O2 results in soot profiles with lower dp values. When using 80%O2 in the oxidizer stream, the peak value of dp (in both CME and diesel) is reduced by about 50% compared to their air counterparts. High-resolution TEM profiles reveal that, regardless of oxygen enrichment levels, both "young" and "mature" soot primarily consist of graphene layers, with their orientation and packing varying by oxygen concentration. The application of a special TEM grid-H2 Finder-was required when studying the 35%, 50%, and 80%O2 flames, as the introduction of oxygen resulted in confined flames: this grid allowed for a more accurate correspondence between the grid central axis and flame position.