Soot formation processes

Flame turbulence should not affect soot formation processes under premixed combustion conditions, and the near correspondence of the results from Bunsen flames [52] and stirred reactors [55] tends to support this contention. 

Flame turbulence should not affect soot formation processes under premixed combustion conditions, and the near correspondence of the results from Bunsen flames [52] and stirred reactors [55] tends to support this contention. However, the effect of turbulence on sooting diffusion flames can be very complex and unclear in most experiments unless the effect of the intensity (and scale) of turbulence on the flame structure, the temperature-time history of the pyrolyzing fuel, the rate of incipient particle formation and particle growth, and, in the case of some fuels, the transport of oxygen to the fuel stream are known. 

As discussed in the context of the conceptual spray combustion model of Dec [12], illustrated in Fig. 13.3, soot tends to form in fuel-rich regions at relatively low temperatures. The exact soot formation process is a rather complex topic more details can be found in the texts of Refs. [4, 23,44]. In the following, a relatively simple production-oxidation soot model, used in the prediction of particle emissions in diesel engines, is outlined. 

At present there is no small-scale test for predicting whether or how fast a fire will spread on a wall made of flammable or semiflammable (fire-retardant) material. The principal elements of the problem include pyrolysis of solids char-layer buildup buoyant, convective, tmbulent-boundary-layer heat transfer soot formation in the flame radiative emission from the sooty flame and the transient natme of the process (char buildup, fuel burnout, preheating of areas not yet ignited). Efforts are needed to develop computer models for these effects and to develop appropriate small-scale tests. 

Although the aromatic polycyclic hydrocarbons are usually associated with tar and soot formation, in a firing or roasting process, compounds such as fluoranthrene and pyrene have also been found in green... 

This chapter seeks not only to provide better understanding of the oxidation processes of nitrogen and sulfur and the processes leading to particulate (soot) formation, but also to consider appropriate combustion chemistry techniques for regulating the emissions related to these compounds. The combustion— or, more precisely, the oxidation—of CO and aromatic compounds has been discussed in earlier chapters. This information and that to be developed will be used to examine the emission of other combustion-generated compounds thought to have detrimental effects on the environment and on human health. How emissions affect the atmosphere is treated first. 

How this smoke effect varies with inert addition is best explained by considering the results of many early investigators who reported that incipient soot formation occurred in a very narrow temperature range. The various results are shown in Table 8.6. Since, as stated earlier, the incipient particle formation mechanisms for various fuels follow quite similar routes, it seems appropriate to conclude that a high activation energy process or processes control the incipient particle formation. The best concept and evidence to date is that given by Dobbins [77], It is likely that the slight variation of temperatures shown in Table 8.6 is attributable to the different experimental procedures... 

A detailed description of the chemical processes involved in soot formation was given in the previous chapter. Oxidation of soot particles is an important subject area because in most practical processes that form soot (e.g., diesel... 

The soot formation and its control was studied in an annular diffusion flame using laser diagnostics and hot wire anemometry [17, 18]. Air and fuel were independently acoustically forced. The forcing altered the mean and turbulent flow field and introduced coherent vortices into the flow. This allowed complete control of fuel injection into the incipient vortex shedding process. The experiments showed that soot formation in the flame was controlled by changing the timing of fuel injection relative to air vortex roll-up. When fuel was injected into a fully developed vortex, islands of unmixed fuel inside the air-vortex core led to... 

Measurements showed that the amount of soot produced by the flame was affected by the mixing process between the air jet vortices, the fuel jets, and naturally entrained external air. PIV and smoke flow visualization showed that the air vortices induced strong external air entrainment into the main jet flow very close to the exit plane when the phase angle between the fuel jets and air jet were at the value for minimized soot production. When the wrong phase angle was used, i.e., that which leads to soot formation, the air vorticity coherence was reduced, the vortices appeared to develop further downstream, and the air entrainment at the flame base was significantly reduced. 

Soot formation is a complicated process involving nucleation, surface growth, particle coagulation, and oxidation [20]. These processes pose a great challenge... 

Glassman, I. 1988. Soot formation in combustion processes. 22nd Symposium (International) on Combustion Proceedings. Pittsburgh, PA The Combustion Institute. 295-311. 

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