DIRECT CONTACT HEAT EXCHANGE BETWEEN PARTICLES AND A HEAT TRANSFER SURFACE IN A FLUIDIZED BED

摘要

Direct-contact heat exchange between fluidizing solid particles and a heat transfer surface, which is one of the most effective heat transfer mechanisms in a fluidized bed, has been investigated. The temperature of the fluidizing particles during the period of contact with a heat transfer surface is visualized by using an Infrared (IR) imager. Based on the visualized temperature of the particles, the contact conductance between a single particle and the heat transfer surface was determined by inverse analyses. A schematic diagram of the experimental setup is shown in Fig. A-1. Visualization of particle temperature and observation of the contact status between the particles and the heat transfer surface were performed through the heat transfer surface, which is made of a MgF{sub}2 window, so as not to disturb the particle fluidization. An IR imager located external to the bed was used to visualize the temperature of the fluidizing particles heated by the heat transfer surface and a conventional VTR camera to observe the contact status. A CO{sub}2 laser was used to remotely heat the heat transfer surface so as not to obstruct visualization of the IR imager and the VTR camera. This visualization system allows measurement of the particle temperature during the contact period. Two different inverse analyses are adopted for determining the contact conductance. In one of them, the particle temperature is assumed to be uniform, and in the other, the temperature distribution within the particles is rigorously considered. The estimated contact conductance is tabulated for several particle diameters and fluidizing velocities, and their effects on the contact conductance are discussed. The contact Nusselt numbers as well as the contact conductance are determined, and are compared with Decker's prediction. The present contact Nusselt numbers, which are determined by the inverse analysis where the temperature distribution in the particles are rigorously considered, are close to Decker's prediction.