A model has been proposed by Gabor ( 1 ) for relating the rate of wall-to-bed heat transfer in a fluidized bed to the particle residence time at the heat-transfer surface. I n that model, a bed of spherical particles was approximated by a series of alternate slabs representing the solid and gas phases. The model was solved numerically by the method of Dusinberre (2).The predictions of the alternate-slab model, using a gas slab thickness of O.ld and a solid slab thickness of 2/3d, were in good agreement with data by Botterill et al.( ) ,Harakas and Beatty ( 4 ) , and Hampshire ( 5 ) on heat transfer from a wall to moving packed beds of glass and copper particles. However, existing data on heat transfer in fluidized beds are either incomplete or were not taken under applicable conditions for a satisfactory test of the model (1). Therefore, to test the applicability of the alternate-slab model to fluidized beds, measurements were made of heat-transfer coefficients as a function of particle residence time in fluidized beds. EXPERIMENTHeat transfer measurements made in two S i n . high columns operated at the same conditions were compared. One column was semicylindrical, and the other was cylindrical. The semicylindrical column was constructed by cutting a 4-in. diam. brass tube in half lengthwise. A l-in. thick glass plate coated with an electrically conducting mixture of antimony and tin oxides to reduce static charges in the fluidized bed was attached to form a flat face. A l-in. by l-in. heater surrounded by guard heaters was mounted in the back curved surface 6 in. above the sintered metal gas distributor plate at the bottom of the column. Air at the bed minimum fluidization velocity was introduced through the gas distributor. Bubbles were periodically injected from a tube mounted in the gas distributor plate adjacent to the midpoint of the base of the glass wall. A bubble that formed at the injection point was assumed to have a diameter equivalent to the dimension observed through the glass wall and was presumed to be a half-bubble whose radius dimension into the bed (perpendicular to the glass wall) was half that of the observed bubble diameter. The half bubble retained its identity as it rose along the axis of the column on the glass wall. Because of axial symmetry, the particle 'motion at the edges of the column (which was observed through the glass wall) was assumed to be identical to the particle motion within the bed at the heater surface. The effects of bubbling on both particle motion and heat transfer were studied simultaneously.The cylindrical column was constructed from a 4-in. diameter stainless steel tube. This column had a 2-in. high by 1%-in. wide glass window that was interchangeable with a heater of AlChE Journal Wol. 18, No. 1) the same dimensions. From an injection tube installed in the center of the base of the column, individual axisymmetric bubbles were injected into the bed. The cylindrical column had to be operated twice under identical conditions, once for measurement of heat-transfer r...
A study was made of the mixing of particulate solids fluidized in the voids of a packed bed af larger nonfluidized bodies. In such a system heat transfer rates are dependent on the movement of fluidized particles. Recently application has been made of inert fluidized particles in chemical reactor systems to improve the removal of the heat generated by the exothermic reaction of the larger particles. A measure of the particle movement was achieved in this study by the determination of diffusion coefficients for the rates of solids mixing in the lateral direction when metal shot is fluidized in the voids of spherical and cylindrical packing. Experimental data showed that the particle mixing was related to the void structure of the packed bed by a random walk model. A general correlation was made for fluidized-solids diffusivity in spherically packed beds.Fluidization of particulate solids with gas provides a technique for carrying on chemical reactions in a medium with high degrees of turbulence and mixing. High rates of solids mixing provide high heat transfer coefficients at the reactor wall and favor temperature uniformity of the fluidized reacting systems. Recently Argonne National Laboratory has shown interest in chemical reactors employing inert solids fluidized in the interstices of a static packed bed of larger bodies, which undergo reaction with the gas. This interest arose in connection with the development of a process for recovery of fissionable values from spent uranium dioxide reactor fuel (4, 10). Specifically, fluorine gas is reacted with massive uranium dioxide pellets in a packed bed, and inert fused alumina particles are fluidized in the voids of the packed bed to provide rapid heat removal. Such combination systems are referred to as fluidized-packed beds. The present paper describes a study of the effect of process variables on the solids-mixing behavior of such beds. It is clear from current theories (7, 17, 18) that the turbulence of the fluidized solids accounts for the heat transfer properties of fluidized-packed beds. This turbulence may be most directly measured in terms of the rate of solids mixing.The baffling effect of the packing accounts for the major differences between fluidization in open tubes and fluidization in packed beds. Thus the extremely high solids circulation rates for the unbafflled fluidized beds are not found in the fluidized-packed bed. In the latter case, the void structure of the packing determines the mixing behavior. This has been shown for gas mixing in fluidizedpacked beds ( 5 ) .There are few literature references to such combination fluidized-packed beds. Sutherland, et al.( 1 4 ) were interested in fluidized-packed beds because the baffling effect of the fixed packing permits smooth fluidization (without slugging) for large height-to-diameter ratios. Recently Ziegler (19) published data and discussed the mechanism of radial heat transfer in fluidized-packed beds; he worked at Argonne National Laboratory in cooperation with Northwestern University, Evanston,...
Experiments were conducted on heat transfer from internally heated ZnSO4-H2O pools to curved surfaces. These experiments extended existing data for nonboiling pools to higher Rayleigh numbers. The data for convective downward heat transfer from nonboiling pools to a curved surface were reasonably close to the Mayinger correlation extrapolated to higher Rayleigh numbers and lower ratios of pool depth to radius of curvature. Sideward heat transfer to a surface could be described by Nu = 0.7 Ra0.2. Insulating the upper pool surface from the atmosphere had no effect on either sideward or downward heat transfer. An investigation was also made on effects of curvature on heat transfer from boiling pools. Nusselt numbers for sideward heat transfer were proportional to a boiling Reynolds number based on superficial vapor velocity to the 0.275 power and quite close to the correlation for a pool with flat vertical walls. Downward boiling heat transfer to a curved surface was proportional to the Reynolds number to the 0.1 power.
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