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Received for review January 21, 1980 Accepted September 10, 1980
Researchsponsoredby the ~ i v iof~chemic. i ~ ~ sciences,u.s, Department of Energy, under contract W-7405-eng-26with the Union Carbide Corporation.
Maximum Heat Transfer Coefficient between a Horizontal Tube and a Gas-Solid Fluidized Bed Nanak S. Grewal Mechanical Engineering Department, University of North Dakota, Grand Forks, North Dakota 58202
Satlsh C. Saxena' Department of Energy Engineedng, University of Illinois at Chicago Circle, Chicago, Illlnois 60680
Experiments have been conducted to measure the maxknum heat transfer coefficient, h, , between an electrically heated single horizontal tube and air-solld square fluidized beds of glass beads, dolomTe, sand, silicon carbide, and alumina particles. The effect of particle size, shape, density, and specific heat, tube size, bed depth, heat flux, and distributor design on maximum heat transfer rate has been investigated. Experimental values of hWw are compared with the values of maximum heat transfer coefficient predicted by the existlng correlations. None of these correlations is found to be adequate to reproduce the present data. Therefore a new correlation has been proposed for h, on the basis of our data and then examined to assess its reliability on the bask of available data in the literature. "I^n addition, the existing melations for bed porosity, e, and optimum mass fluidizing velocity, Gopt,have been examined on the basis of our data and a new correlation is given for the bed porosity.
Introduction Fluidized-bed combustion technolagy is being developed for the efficient utilization of high-sulfur and low-rank coals. The fluidized-bed combustion is an efficient way to generate steam because of high rates of heat transfer between immersed tubes and gas-solid fluidized bed in comparison to conventionalpulverized coal boilers. In this paper, we report data for maximum heat transfer coefficient for an electrically heated horizontal tube immersed in an air-solid fluidized bed of glass beads, dolomite, sand, silicon carbide, and alumina particles (less than 1 mm). Geldart (1973) has synthesized the fluidization characteristics of solid-gas systems of wide range of densities and sizes of solid particles. He divided the powders in four different groups. The present investigations deal with particles which fall in group B of Geldart (1973). The solid particle densities for such particles range between 1400 and 4000 kg/m3 and mean size between 40 to 500 pm. For such particles the bed expansion is small and bubble formation starts soon after the condition of minimum fluidization is passed. Bubbles rise in the bed faster than the interstitial gas and their size is independent of particle size and size distributions. The information generated here will be useful for the design of fluidized bed low-rank coal combustors where crushed coal is burnt in the presence of an inert bed of silica sand or alumina of particle size less than 1 mm (Goblirsch and Sondreal, 1977). 0196-4305/81/1120-0108$01 .OO/O
A number of excellent reviews of basic studies of fluidized bed heat transfer are available in the literature (Botterill, 1975; Gelperin and Ainshtein, 1971; Grewal, 1979; Gutfinger and Abuef, 1974; Kunii and Levenspiel, 1969; Leva, 1959; Saxena et al., 1978; Zabrodsky et al., 1976; Zabrodsky, 1966; Zenz and Othmer, 1960). The experimental and theoretical efforts have been mainly toward an understanding of the mechanisms of heat transfer to fluidized beds by unsteady-state conduction to moving solid particles a t temperatures (
