A theoretical study of the radiative contribution to heat transfer between a high-temperature, large-particle gas-fluidized bed and an immersed tube Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/jq085p82k

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  • An analytical model of the radiative contribution to the total heat transfer between a large-particle (greater than 1 mm) gas-fluidized bed and an immersed horizontal tube is discussed. The model is applied to the case of a bubbling bed and the emulsion phase contribution is obtained from a detailed analysis of the first layer of particles in contact with the tube. This approximation is justified by the combined effects of large particle size and relatively short residence time at the heat transfer surface, so that cooling is limited to the first row in contact with the tube. The method involves the use of the coupled convection/transient conduction analysis of Adams [3] with modifications for the relative heat flux and addition of thermal contact resistance between rough surfaces. In the case of the emulsion phase, radiative cooling of the particles is formulated assuming that the bed, tube wall, and particle surfaces are diffuse and gray. Then the net radiation method is applied to an enclosure formed by a gray cylinder surrounding a spherical particle in order to establish the radiative heat flux to the tube wall. The bubble phase radiative heat flux is approximated by assuming the bubble boundary is isothermal and gray. Two models were considered to establish an estimate of lower and upper limits of bubble phase radiative heat transfer to the immersed tube. The first of the foregoing models assumes the gas within the bubble is optically thin and the second model takes gas absorption into account. A computer code has been developed and used to obtain heat transfer to a horizontal tube immersed in a three-dimensional, bubbling, atmospheric pressure fluidized bed. A parametric study of the effects of particle thermal and physical properties on heat transfer to the tube is presented. The numerical results for both total and radiative heat transfer are compared with experimental data reported in the literature as well as those recently obtained by Alavizadeh. The heat transfer coefficients calculated using the model were found to be within the range of experimental results obtained by others. The model is expected to be valid for mean particle diameters greater than 2.00 mm.
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