Flames and Particle Clouds

Luminous Flames Luminosity conventionally refers to soot radiation. At atmospheric pressure, soot is formed in locally fuel-rich portions of flames in amounts that usually correspond to less than 1 percent of the carbon in the fuel. Because soot particles are small relative to the wavelength of the radiation of interest in flames (primary particle diameters of soot are of the order of 20 nm compared to wavelengths of interest of 500 to 8000 nm), the incident radiation permeates the particles, and the absorption is proportional to the volume of the particles. In the limit of rjX 1, the Rayleigh limit, the monochromatic emissivity e is given by 

The total emissivity of soot es can be obtained by substituting from Eq. (5-142) for 8 in Eq. (5-138a) to yield 

Clouds of Large Black Particles The emissivity zM of a cloud of black particles with a large perimeter-to-wavelength ratio is 

As an example, consider a heavy fuel oil (CH15, specific gravity, 0.95) atomized to a mean surface particle diameter of dv burned with 

20 percent excess air to produce coke-residue particles having the original drop diameter and suspended in combustion products at 1204°C (2200°F). The flame emissivity due to the particles along a path of L m, with dp measured in micrometers, is 

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According to Schack(55), a single particle of soot transmits approximately 95 per cent of the incident radiation and a cloud must contain a very large number of particles before an appreciable emission can occur. If the concentration of particles is K, then the product of K and the thickness of the layer L is equivalent to the product PgLe in the radiation of gases. For a known or measured emissivity of the flame ey, the heat transfer rate per unit time to a wall is given by. 

J.D. Buckmaster and P. Clavin. An acoustic instability theory for particle-cloud flames. Proceedings of the Combustion Institute, 24 29-36,1992. 

It is with the understanding of the above that one can give some insight to what establishes the pyrophoricity of small metal particles. The term pyrophoricity should pertain to the instantaneous combustibility of fine metal particles that have no oxide coat. This coating prevention is achieved by keeping the particles formed and stored in an inert atmosphere such as argon. Nitrogen is not used because nitrides can be formed. When exposed to air, the fine metal particle cloud instantaneously bursts into a flame. Thus it has been proposed... 

Vei y small solid fuel particles such as sawdust, agricultural grains, or coal dust can sustain flames when they are suspended in air. In fact, very serious fires have occurred in grain storage towers and coal mines because of the flammability of suspended dusts. The combustion of the individual particles follows the usual pattern of solid particle burning— devolatization and char burning. The combustion of the whole cloud of particles is similar to spray combustion and its characteristics depend on the nature of the fuel, size of the particles, and the number of particles in a given volume. 

Flame atomization and excitation can be divided into a number of stages. Firstly, the heat of the flame evaporates solvent from the droplets of sample aerosol leaving a cloud of small particles of the solid compounds originally present in the solution. These are then vaporized and molecular associations broken down releasing free atoms (atomization) some of which... 

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. 

Approach. The continuum total group combustion criterion is established by asking when will the cloud bum as a large pseudo-droplet with the flame located just outside of the cloud droplet region That is, when will the evaporating particles inside the cloud provide sufficient vapor so that fuel and oxidizer mix in stoichiometric amounts at the cloud boundary ... 

Approach. The use of continuum models is defensible only when the particles are suflBciently far apart that suitable averaged oxidizer and fuel mass fraction can be defined. This implies not only the Rp/L < < 1 but also the Rf/L < < 1, where Rf is the individual envelope flame radius, and L is the interparticle distance. Indeed, the use of a continuum approach to establish the incipient group combustion criterion may be suspect since this last inequality is not met. Further, continuum models cannot be defended when the number of particles in the cloud is small... 

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