Luminosity, soot formation

In Fig. 14, atmospheric pressure premixed, flat flames of C2H3CI are shown under both fuel-rich and fuel-lean conditions. Under fuel-rich conditions, CHC flames exhibit similar luminosity as hydrocarbon flames. However, under excess air conditions, the flames of chlorinated hydrocarbons exhibit white luminosity, likely because of the radiative recombination of chlorine atoms (Fig. 14A). Flames of highly chlorinated hydrocarbons also exhibit soot formation even at stoichiometric conditions (Fig. 14B) owing to the suppression of oxidation reactions. 

The appearance of all of these flames is similar. Three zones appear well-defined a nonluminous zone immediately above the burner, a blue-green luminous zone associated with the primary reaction, and an extended yellow-orange luminous region associated with the carbon particles. The yellow luminosity is primarily due to incandescent carbon particles. Soot is responsible for the luminous properties of torches, candles, and kerosene lamps. Soot formation is of considerable industrial importance in both a positive and a negative sense. Carbon black is a useful commodity... 

The last point is worth considering in more detail. Most hydrocarbon diffusion flames are luminous, and this luminosity is due to carbon particulates that radiate strongly at the high combustion gas temperatures. As discussed in Chapter 6, most flames appear yellow when there is particulate formation. The solid-phase particulate cloud has a very high emissivity compared to a pure gaseous system thus, soot-laden flames appreciably increase the radiant heat transfer. In fact, some systems can approach black-body conditions. Thus, when the rate of heat transfer from the combustion gases to some surface, such as a melt, is important—as is the case in certain industrial furnaces—it is beneficial to operate the system in a particular diffusion flame mode to ensure formation of carbon particles. Such particles can later be burned off with additional air to meet emission standards. But some flames are not as luminous as others. Under certain conditions the very small particles that form are oxidized in the flame front and do not create a particulate cloud. 

For premixed fuel-air systems, results are reported in various terms that can be related to a critical equivalence ratio at which the onset of some yellow flame luminosity is observed. Premixed combustion studies have been performed primarily with Bunsen-type flames [52, 53], flat flames [54], and stirred reactors [55, 56], The earliest work [57, 58] on diffusion flames dealt mainly with axisymmetric coflow (coannular) systems in which the smoke height or the volumetric or mass flow rate of the fuel at this height was used as the correlating parameter. The smoke height is considered to be a measure of the fuel s particulate formation and growth rates but is controlled by the soot particle bumup. The specific references to this early work and that mentioned in subsequent paragraphs can be found in Ref. [50],... 

If a fuel-rich portion of an air/fuel mixture is exposed to heat, as from a hotter part of the flame, the unburned fuel molecules polymerize or suffer thermal cracking, resulting in formation of some heavy, solid molecules. These soot particles glow when hot, providing luminosity, which boosts the flame s total radiating ability. 

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