An analytical model of radiative and convective heat transfer in small and large particle gas fluidized beds Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/tm70mz79v

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  • A complete analytical model of radiative and convective heat transfer to a horizontal tube in small and large particle gas fluidized beds has been developed. In the model the bed of particles consists of an emulsion phase and a bubble phase. The emulsion phase is approximated by a series of transmitting alternate layers of gas and solid. The transmissivity of the solid slabs is obtained by treating the voids in the bed as windows through which radiation is transmitted to the immersed surface. The gas layer, the gas within the voids and the gas in the bubble are considered to be radiatively transparent. Within the emulsion phase, radiosites for all solid slab surfaces in addition to the heat transfer surface are obtained by an iterative scheme with the transmission effect included. The convective boundary condition is obtained by treating the bed as a series of contact resistances from particle to particle, starting with the initial resistance between the first layer of particles and the immersed surface. A one-dimensional unsteady conduction analysis is employed using an implicit finite difference technique to determine the temperature distribution at the surfaces and inside the solid slabs. The gas convection contribution from the emulsion phase is based upon the Adams-Welty model and justified to be an additive term. Gas convection heat transfer from the bubble phase is also included using an existing model. Radiation heat transfer through the bubble phase is obtained by considering a three-dimensional hemispherical bubble surface adjacent to a horizontal tube. The total heat transfer to the immersed surface is determined by adding the contributions of both emulsion and bubble phases, weighted by their contact fractions. Results obtained with the model show that the radiant energy transmitted through the voids increases the heat transfer by about 25% on the average over the case when the solid slabs are opaque. At low bed temperatures, the model predictions are in qualitative agreement with experimental data for small particles, and in close agreement with experimental data for large particles. At high bed temperatures, the radiative heat transfer coefficients obtained with the model are higher than the experimental results for large particles. However, the experimental local and spatial average radiative heat transfer coefficients for small particles agree closely with model predictions.
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