Experimental study on the heat transfer coefficient between a freely moving sphere and a fluidized bed of small particles

In a commercial circulating fluidized bed (CFB) boiler, coal particle combustion mostly occurs in the dense phase, where the solid particle convective heat transfer (h(pc)) is the dominant part of the overall heat transfer process. In this work, the heat transfer between freely moving spheres and the fluidized bed dense phase was investigated and parameters in the widely accepted surface resistance model were determined based on the test results. It was found that h rises initially with the increase of excess fluidizing velocity (U-ex) and becomes invariant when Ilex excesses a specific value, which demonstrates the constant surface resistance to some extent. In most conditions, when U-ex does not change, h increases as the inert particle becomes smaller. However, interestingly, a turning of h was observed with the increasing particle size at low U-ex.. h was found to decline with the increasing sphere diameter, whereas, as the sphere diameter keeps increasing, the influence of the particle size becomes more significant with respect to the sphere diameter. The higher contribution of the contact region to the surface contact resistance was verified when the particles are small enough, and the geometry of the channel between the sphere and the particle was proved to influence the contact resistance. The influences of particle size, sphere diameter and the fluidizing velocity on the surface contact resistance constant m and mean emulsion residence time theta were expressed by correlations and the correlation predictions are quite satisfactory. (C) 2014 Elsevier Ltd. All rights reserved.