Fluidized beds radiative

Baskakov, A. P. (1985). Heat Transfer in Fluidized Beds Radiative Heat Transfer in Fluidized Beds. In Fluidization, 2nd ed. Ed. Davidson, Clift and Harrison. London Academic Press. 

Temperature of the fluidized bed is another parameter that could influence the heat transfer coefficient. Increasing bed temperature affects not only the physical properties of the gas and solid phases, but also increases radiative heat transfer. Yoshida et al. (1974) obtained measurements up to 1100°C for bubbling beds of aluminum oxide particles with 180 pm diameter. Their results, shown in Fig. 6, indicate an increase of over 100% in the heat transfer coefficient as the bed temperature increased from 500 to 1000°C. Very similar results were reported by Ozkaynak et al. (1983) who obtained measurements for bubbling beds of sand particles (dp = 1030 pm) at temperatures up to 800°C. 

Brewster, M. O., and Tien, C. L., Radiative Heat Transfer in Packed Fluidized Beds Dependent Versus Independent Scattering, J. Heat Transfer, 104(4) 574-580 (1982)... 

Chen, J. C., Chen, K. L., Analysis of Simultaneous Radiative and Conductive Heat Transfer in Fluidized Beds, Chem. Eng. Commun., 9 255-271 (1981)... 

Chen, J. C., Cimini, R. J., and Dou, S. H., A Theoretical Model for Simultaneous Convective and Radiative Heat Transfer in Circulating Fluidized Beds, Circ. Fluid. Bed Tech. II, 255-262 (1988)... 

Han, G. Y., Experimental Study of Radiative and Particle Convective Heat Transfer in Fast Fluidized Beds, Ph.D. Dissertation, Lehigh University (1992)... 

Temperature in a fluidized bed is uniform unless particle circulation is impeded. Gas to particle heat flow is so rapid that it is a minor consideration. Heat transfer at points of contact of particles is negligible and radiative transfer also is small below 600°C. The mechanisms of heat transfer and thermal conductivity have been widely studied the results and literature are reviewed, for example, by Zabrodsky (1966) and by Grace (1982, pp. 8.65-8.83). 

Radiative heat transfer plays an important part in many fluidized bed processes operated at high temperatures, such as coal combustion and gasification. When treating a fluidized bed as a whole solid gray body, the radiative heat transfer coefficient ht between the fluidized bed at temperature 7), and a heating surface at temperature Ts is defined as... 

A unique feature of the dense-phase fluidized bed is the existence of a maximum convective heat transfer coefficient /zmax when the radiative heat transfer is negligible. This feature is distinct for fluidized beds with small particles. For beds with coarse particles, the heat transfer coefficient is relatively insensitive to the gas flow rate once the maximum value is reached. 

For radiative heat transfer, some differences between a fluidized bed and a fixed bed exist [Kovenskii, 1980]. When a bed starts to fluidize from the fixed state, the radiative heat transfer rapidly increases by 10-20 percent it remains constant as the gas velocity increases further in the bubbling bed. 

To understand the radiative heat transfer in a circulating fluidized bed, the bed can be regarded as a pseudogray body. The radiative heat transfer coefficient is [Wu et al., 1989]... 

An alternative treatment for radiative heat transfer in a circulating fluidized bed is to consider the radiation from the clusters (hcx) and from the dispersed phase (i.e., the remaining aspect of gas-solid suspension except clusters, A ), separately [Basu, 1990]... 

The bed-to-wall heat-transfer coefiicient in a fluidized bed at high temperatures is larger than at room temperature (Y19). Questions have been raised about the effect of radiative heat transfer at high temperatures (B12, Y19), and more studies are necessary on this problem. 

M. Q. Brewster, and C.-L. Tien, Radiative Transfer in Packed and Fluidized Beds Dependent versus Independent Scattering, ASMEJ. Heat Transfer, (104) 573-579,1982. 

M. Q. Brewster, Radiative Heat Transfer in Fluidized Bed Combustors, ASME paper no. 83-WA/HT-82,1983. 

G. Flamant, J. D. Lu, and B. Variot, Towards a Generalized Model for Vertical Walls to Gas-Solid Fluidized Beds Heat Transfer—II. Radiative Transfer and Temperature Effects, Chem. Eng Sci. (48/13) 2493,1993. 

For temperatures beyond 600 C radiative heat transfer plays an increasing role and must be accounted for in calculations. The reader is referred to Botterill (1986) or Kunii and Levenspiel (1990) for treatment of radiative heat transfer or for a more detailed look at heat transfer in fluidized beds. 

Brewster MO, Tien CL. Radiative heat transfer in packed fluidized beds dependent versus independent scattering. J Heat Transf 104, no. 4 574-580, 1982. 

Chen JC, Chen KL. Analysis of simultaneous radiative and conductive heat transfer in fluidized beds. Chem Eng Commun 9 255 271, 1981. 

Chen JC, Cimini RJ, Dou SH. A theoretical model for simultaneous convective and radiative heat transfer in circulating fluidized beds. In Basu P, Large JF, eds. Circulating Fluidized Bed Technology II. Oxford Pergamon Press, 1988, pp 255-262. 

Han GY. Experimental study of radiative and particle convective heat transfer in fast fluidized beds. PhD dissertation, Dept, of Chem. Eng., Lehigh University, Bethlehem, PA, 1992. 

Fang ZH, Grace JR, Lim CJ. Radiative heat transfer in circulating fluidized beds. J Heat Transfer 117 963-968, 1995. 

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