Radiation from soot

The interaction between turbulence, chemistry, and radiation from soot in a two-dimensional, planar jet diffusion flame was investigated using LES. A djmamic... 

In technical furnaces the radiation from soot, coal and ash particles has to be considered as well as the gas radiation. Then the scattering of radiation by the suspended particles becomes important, alongside absorption and emission. P. Biermann and D. Vortmeyer [5.67], as well as H.-G. Brummel and E. Kakaras [5.68] have developed models for this. A summary can be found in [5.69] and in [5.37], p. 652-673. The calculations of heat transport in furnaces has been dealt with by W. Richter and K. Corner [5.70] as well as H.C. Hottel and A.F. Sarohm [5.48],... 

A preliminary version of the SOFM method was also applied to a commercial oil-fired burner with a nominal capacity of 60 kW. The flames in this case were yellow and highly radiating, with characteristics (physical and visual) very different from those of the blue flames described. In particular, the nature and origin of emission spectra are expected to be deeply different dominated by blackbody radiation from soot particles, with a much smaller contribution due to chemiluminescence of excited radicals. An exercise similar to that reported in Section 15.4.4 was performed to estimate NO concentration from flame images. The NO emissions varied in the range of 53 to 94 ppm for the flames analyzed the estimation error was within 5 ppm in practically all cases, very similar in relative terms to the results shown in Figure 15.7. 

Yagi, S., and lino, H. "Radiation from Soot Particles in Luminous Elames." 8th Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, PA, 288-93, 1960. 

Direct particulate radiation from soot particles within the flame to surfaces of the charged loads and walls that they can see ... 

The radiation from a flame is due to radiation from burning soot particles of microscopic andsubmicroscopic dimensions, from suspended larger particles of coal, coke, or ash, and from the water vapor and carbon dioxide in the hot gaseous combustion products. The contribution of radiation emitted by the combustion process itself, so-called chemiluminescence, is relatively neghgible. Common to these problems is the effect of the shape of the emitting volume on the radiative fliix this is considered first. 

Radiation from flames and combustion products involve complex processes, and its determination depends on knowing the temporal and spatial distributions of temperature, soot size distribution and concentration, and emitting and absorbing gas species concentrations. While, in principle, it is possible to compute radiative heat transfer if... 

The inclusion of radiative heat transfer effects can be accommodated by the stagnant layer model. However, this can only be done if a priori we can prescribe or calculate these effects. The complications of radiative heat transfer in flames is illustrated in Figure 9.12. This illustration is only schematic and does not represent the spectral and continuum effects fully. A more complete overview on radiative heat transfer in flame can be found in Tien, Lee and Stretton [12]. In Figure 9.12, the heat fluxes are presented as incident (to a sensor at T,, ) and absorbed (at TV) at the surface. Any attempt to discriminate further for the radiant heating would prove tedious and pedantic. It should be clear from heat transfer principles that we have effects of surface and gas phase radiative emittance, reflectance, absorptance and transmittance. These are complicated by the spectral character of the radiation, the soot and combustion product temperature and concentration distributions, and the decomposition of the surface. Reasonable approximations that serve to simplify are ... 

C 6 m, 1000 °C 15 m, 1100°C and 30 m, 1200 °C [14], The explanation is provided by Koseki [15] (Figure 10.12), showing how Xr decreases for large-diameter fires as eddies of black soot can obscure the flame. The eddy size or soot path length increases as the fire diameter increases, causing the transmittance of the external eddies to decrease and block radiation from leaving the flame. From Table 10.2,... 

In many process fires, heat transfer by radiation is the dominant form of heat transfer. The heat radiated from a flame is emitted by gases, in particular the products of combustion and by soot. Aflame in which the radiation is emitted solely from the gaseous products of combustion is termed nonluminous and a flame in which there is soot is termed luminous (i.e., yellow or visible). 

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. 

Considering the analogical inference, we can consider here the black body, which is perfectly black at normal temperature. (Soot has almost the same nature with the black body, and has an absorption ratio of 95% ) The intensity of the radiation from the black body increases only with the temperature, and it changes the colour from black to red, orange, white as the temperature increases. The colour is determined by the temperature only, and is shown by the M curve on the chromaticity diagram(Fig.35) ... 

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. 

Steam-assist flares use high pressure steam to entrain surrounding air and inject it into the core of the flare gas stream. The rapid mixing of the steam and air with the flare gas helps reduce soot formation that tends to lower the flame radiant fraction. Figure 30.14 shows a steam-assisted flare operating under identical flare gas flow conditions with and without steam-assist. Notice without steam-assist, the flame is more luminous and contains more soot this results in higher radiant fractions. The fraction of heat radiated from a flame can also be greatly increased by the presence of liquid droplets in the gas. Droplets within a hot flame can easily be converted to soot [21]. 

There are two origins of radiation from products of combustion to solids (1) radiation from clear flame and from gases and (2) radiation from the micron-sized soot particles in luminous flame. 

Fig. 2.18 Spectographs of radiation from ciear and luminous flames. Nonlumlnous flames top graph) are blue luminous flames lower graph) are yellow and emit soot particle radiation. Both luminous and nonlumlnous flames and Invisible poo gases emit triatomic gas radiation. Courtesy of Ceramic Industry journal, Feb. 1994, and Air Products Chemicals, Inc. reference 13).

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