Numerical calculation of wall-to-bed heat transfer coefficients in Geldart B bubbling fluidized beds with immersed horizontal tubes

In dense solid-gas fluidized beds the knowledge of the wall-to-bed heat transfer is of great importance, since it often dominates reactor design and dimensioning. This heat transfer is strongly coupled to the hydrodynamic behavior of the bed material around the immersed objects. In this study the heat transfer coefficient at horizontal tubes in a Geldart B bubbling fluidized bed is determined experimentally and compared to the numerically predicted flow fields. The objective is to improve the understanding and quantify the coupling between hydrodynamics and heat transfer. The findings are applied to derive and verify a numerical-correlative approach for predicting the angle-dependent heat transfer coefficient. Experimental data are obtained from a pilot scale test-rig with different tubular heat transfer probes. Corundum is used as the solid bed material and air as the fluidization gas, entering the cylindrical geometry through a Tuyere nozzle type distributor. The angle-dependent heat transfer coefficient is measured at different superficial gas velocities and probe positions in the bed and compared to three dimensional numerical simulations. The applied CFD model of the fluidized bed treats both gas and powder as Eulerian phases. The size distribution and the shape of the particles is described by two granular phases with corresponding mean diameters and a sphericity factor. The Kinetic Theory of Granular Flow and a sphericity-adapted drag model are incorporated to consider the fluid-solid and solid-solid interactions. The hydrodynamics at the tube surface resulting from the numerical simulations (solid volume fraction, gas velocity and particle velocity fluctuation) are used for correlative calculations of the angle-dependent heat transfer coefficient between the bed material and the immersed tube. Results show that the CFD model is able to predict the magnitude and the tendency of the heat transfer around horizontal tubes correctly and therefore can be used as a valuable tool for designing heat exchanger structures in fluidized bed reactors. (C) 2017 Elsevier B.V. All rights reserved.