GASES WITH SUSPENDED PARTICLES OTHER THAN SOOT

There are important applications involving radiation from gases containing suspensions of various types of particles. An example is ash particles in pulverized coal flames [Viskanta and Mengüc (1987), Im and Ahluwalia (1993)]. The particles can radiate in the regions between the gas bands, so radiation is important over a more continuous spectral region than for the radiating gas alone. In Viskanta and Mengüc it is discussed that the complex refractive index for fly ash ranges from about  = 1.43 – i0.307 to 1.5 – i0.005. When the imaginary part is small the particles are acting as a nonabsorbing dielectric. Detailed values for the complex refractive index for char and ash particles are in Im and Ahluwalia in graphical form as a function of wave number. Char and ash particles also scatter radiation. The scattering albedo of gas-char and gas-ash mixtures exceeded 0.9 in the near-visible spectral region, and was larger than 0.5 at all wave numbers. The radiative properties of fused and/or sintered slag and ash deposits were measured in Markham et al. (1992). Fused slag deposits were found to have a high spectral emittance ≥ 0.9 over a wide temperature range; the measurements included both solid and molten slag. Deposits of sintered fly ash consisted of finely packed particles. Their spectral emittance varied appreciably with wave number, and scattering was significant. A curve-fitting method to approximate Mie theory was used in Caldas and Semião (1999) to predict behavior for soot, carbon particles, and fly ash; the method reduced computation time considerably.

 

Another example is the luminosity in the exhaust plume of solid-fueled and some liquid-fueled rockets. For a solid fuel the luminosity may be caused by metal particles added to promote combustion stability. Williams and Dudley (1970) present calculations for a rocket exhaust with entrained liquid aluminum oxide particles. Edwards et al. (1987, 1990) modeled the radiation from a solid rocket motor plume with two groups of particles. The very small particles cool rapidly and are assumed to be cold and scattering. The large particles that remain hot are emitting and absorbing. The calculations in Tabanfar and Modest (1983) demonstrate the combination of radiating particles and gases where all constituents have spectrally dependent properties. Radiation by a cylindrical region at uniform temperature was analyzed in Thynell (1990) for either CO2 or H2O gas containing soot and gray particles. The absorption of the nongray gas was obtained from the exponential wide-band model. The absorption coefficient of the soot was inversely proportional to wavelength, and the scattering from the gray particles had an anisotropic component. Experiments were performed in Skocypek et al. (1987) with a heated mixture of carbon dioxide, nitrogen, and particles of BNi-2, a boron nickel alloy. The particulate scattering increased radiation in the wings of the 4.3-μm CO2 band. Experiments in Walters and Buckius (1991) show the effect of highly scattering A12O3 particles in CO2 gas. The radiative behavior of lycopodium particles during combustion in air is considered in Berlad et al. (1991).

 

The presence of particles in an otherwise weakly absorbing medium can cause the mixture to be strongly absorbing. Seeding of a gas with particles, such as finely divided carbon, can increase gas absorption and heating by incident radiation [Lanzo and Ragsdale (1964)], or the particles can shield a surface from incident radiation [Howell and Renkel (1965), Siegel (1976), Lee et al. (1984)]. These techniques have possible application in advanced energy systems and for the collection of solar energy [Abdelrahman et al. (1979)].

 

Another use for seeding is in the direct determination of flame temperatures for a nonluminous flame by the line-reversal technique. In this method, a seeding material such as a sodium or cadmium salt is introduced into an otherwise transparent flame. These materials produce a strong spectral line in the visible spectrum because of an electronic transition; cadmium gives a red line and sodium a bright yellow line. A spectrally continuous radiation source is placed so that it may be viewed through the seeded flame with a spectroscope. The intensity seen in the spectroscope at the line wavelength is [Eq. (10-15)],

 

                          (D-31)

 

If the flame is isothermal, of diameter D, and no attenuation occurs along the remainder of the path between the continuous source and the spectroscope, (D-31) becomes, as in (10-56),

 

                             (D-32)

 

where . In the wavelength region adjacent to the absorbing and emitting spectral line, the flame is essentially transparent, so the background radiation observed adjacent to the line is . By subtracting  from (D-32), the line intensity relative to the adjacent background is

 

                             (D-33)

 

If the flame is at a higher temperature than the continuous source, (D-33) shows that the line intensity in the spectroscope will be greater than the continuous background intensity. The line will appear as a bright line imposed upon a less bright continuous spectrum. Increasing the temperature of the continuous source causes the source term to override. The line then appears as a dark line on a brighter continuous spectrum. If the continuous source is a blackbody and its temperature is made equal to the flame temperature, then , and (D-33) reduces to . The line then disappears into the continuum in the spectroscope because absorption by the flame and flame emission exactly compensate. If the continuous source is a tungsten lamp, the source temperature measurement is usually made with an optical pyrometer.

 

In the derivation of (D-33) it was assumed that the flame is transparent except within the spectral line produced by the cadmium or sodium seeding. If soot is in the flame, the soot particles emit the scatter radiation in a continuous spectrum along the path of the incident beam. The line-reversal technique is of less utility in this instance, as it then depends on the soot behavior. The effect of soot is analyzed in Thomas (1968). An analysis in Dembele and Wen (2000) includes nonisothermal constituents, variable concentrations, and soot particles including scattering. Gaseous combustion products H2O, CO2, and CO are included. Spectral property variations are incorporated by using the c-k method in Sec. 9-3.3.

 

Submerged combustion, in which a flame is maintained within a porous solid matrix, is reviewed in Howell et al. (1996) for premixed gaseous fuels and in Tseng and Howell (1996) for liquid fuels. These burners combine radiative, conductive, and convective heat transfer, and combustion chemistry. Recent work is reviewed in Schoegl and Ellzey (2007) and Dixon et al. (2008).