Flame soot emission from

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. 

Soot emission from hydrocarbon flames is an important snbject since it plays an important role in relation to both heat transfer by radiation and air pollution (Sha-had and Mohanuned, 2000). The use of CNG in internal combnstion (1C) engines permits operation with decreased advances and decreases NO without increasing soot formation. 

Koylii, U.O. and Feath, G.M. Carbon monoxide and soot emissions from liquid-fueled buoyant turbulent diffusion flames. Combustion and Flame, 1991. 87, 61-76. 

Emissions of soot on the other hand represent a smaller fraction of the overall emission, but are probably of greater concern from the standpoint of visibility and health effects. It has been suggested that soot emissions from fuel oil flames result from processes occurring in the vicinity of individual droplets (droplet soot) before macroscale mixing of vaporized material, and from reactions in the bulk gas stream (bulk soot) remote from individual droplets. Droplet soot appears to dominate under local fuel lean conditions (1, 2), while bulk soot formation occurs in fuel rich zones. Factors which are known to affect soot formation from liquid fuel flames include local stoichiometry, droplet size, gas-droplet relative velocity and fuel properties (primarily C H ratio). 

Finally, charge transfer reactions have been advanced also to explain the eflFect of metalhc additives on soot emission from flames. The ability of metallic additives either to diminish or to enhance the soot formation in flames has been well known for many years (3). Their role is usually linked either to modification of the production rate of hydrocarbon ions (4) or to alteration of the concentration of OH radicals (3). Although there is still some disagreement about experimental results as well as their interpretation, it now appears (29) that the soot-promoting effect is linked to ions resulting from metallic additives through reactions such as ... 

Flare generally appears as a very large turbulent diffusion flame. Radiative emission from such flames could significantly affect the surrounding environment. Radiation from a flame occurs from two sources (i) infrared emissions of CO2 and H2O, (ii) visible-infrared emissions of soot particles [65]. In order to characterize the overall radiative emission in a global sense, the flame is treated as a point source with radiation emission as a fraction, f, of the total heat release. In TDFCF, the fraction f depends on the type of fuel and aerodynamics of the flame [66]. The radiant heat flux K incident on a unit area of a surface located at a distance D from the point source is estimated as. 

Summation of Separate Contributions to Gas or Flame Emissivity Flame emissivity g -t-, due to joint emission from gas and soot has already been treated. If massive-particle emissivity ., such as from fly ash, coal char, or carbonaceous cenospheres from heavy fuel oil, are present, it is recommended that the total emissivity be approximated by... 

Flame emission was monitored using fiber optic probes and either a photodiode detector for yellow emission from hot soot or a photo multiplier tube for violet emission from CH radicals (430 nm bandpass filter). 

Dirty Flames. At this point one could well ask so what happens in real combustors which are turbulent, soot and particle laden and are highly luminous By the end of this morning s session you should be convinced that CARS can be applied to these systems. I don t want to steal all of Alan Eckbreth s slides so I will show only two more. Figure 13 shows the BOXCARS spectrum of N- with a computer fit to a temperature of 2000°K in a laminar sooting propane diffusion flame (12). Figure 14 shows the vertical temperature profile for this same flame system. It should be pointed out that care must be taken under these conditions to account for the laser interaction with carbon in the flame which can generate laser induced Swan Band emission from C2-... 

Near rich limits of hydrocarbon flames, soot is sometimes produced in the flame. The carbonaceous particles—or any other solid particles— easily can be the most powerful radiators of energy from the flame. The function k(t) is difficult to compute for soot radiation for use in equation (21) because it depends on the histories of number densities and of size distributions of the particles produced for example, an approximate formula for Ip for spherical particles of radius with number density surface emissivity 6, and surface temperature is Ip = Tl nrle ns) [50]. These parameters depend on the chemical kinetics of soot production—a complicated subject. Currently it is uncertain whether any of the tabulated flammability limits are due mainly to radiant loss (since convective and diffusive phenomena will be seen below to represent more attractive alternatives), but if any of them are, then the rich limits of sooting hydrocarbon flames almost certainly can be attributed to radiant loss from soot. 

Although various restrictions have been placed on carbon particulate emissions from different types of power plants, these particles can play a beneficial, as well as a detrimental, role in the overall plant process. The detrimental effects are well known. The presence of particulates in gas turbines can severely affect the lifetime of the blades soot particulates in diesel engines absorb carcinogenic materials, thereby posing a health hazard. It has even been postulated that, after a nuclear blast, the subsequent fires would create enormous amounts of soot whose dispersal into the atmosphere would absorb enough of the sun s radiation to create a nuclear winter on Earth. Nevertheless, particulates can be useful. In many industrial furnaces, for example, the presence of carbon particulates increases the radiative power of the flame, and thus can increase appreciably the heat transfer rates. 

Combustion processes can create pollutant emissions other than carbon monoxide and oxides of nifrogen. Unbumed hydrocarbons (UHC) is a term describing any fuel or partially oxidized hydrocarbon species that exit the stack of a furnace. The cause for these emissions is typically due to incomplete combustion of the fuel from poor mixing or low furnace temperature. A low temperature environment can be created by operating the furnace at a reduced firing rate or turndown. Particulate matter (commonly called soot) is often produced from fuel rich regions in diffusion flames. Soot becomes smoke if the rate of formation of soot exceeds the rate of oxidation of soot. Oxides of sulfur are formed when sulfur is present in the fuel. 

Low Pressure Flame Apparatus. All experiments were performed on flat, premixed low pressure, 2.7 kPa (20 Torr) C2H2/O2 flames stabilized on a water cooled burner. This 8.6 cm diam burner was constructed of approximately 900, 0.12 cm i.d. stainless steel tubes microbrazed into two stainless steel perforated plates to form a water jacket around the tubes. Gases were metered using calibrated critical flow orifices. The burner was installed in a low pressure vessel pumped by a 140 L s" (300 CFM) mechanical vacuum pump. Unburned gas velocities (298 K and 2.7 kPa) in all cases were 50 cm s . Equivalence ratios from = 1.5 to 4.0 were studied with most emphasis on a sooting ( ) = 3.0 flame. Visible soot emission became apparent at a soot threshold of <() = 2.4 to... 

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