STUDY ON MICRO-SCALE STRUCTURE OF HEAT TRANSFER AUGMENTATION FOR FLUIDIZED-BED-TYPE HEAT EXCHANGER

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In this research, the micro-scale structure of heat transfer segmentation for fluidized-bed-type heat exchangers was experimentally Investigated. In order to evaluate the contribution of direct contact heat exchange between heat transfer surface and fluidized particles, which has been considered to be one of the effective mechanisms of heat transfer augmentation in the fluidized bed, contact of particles to heat transfer surface was visualized by utilizing an optical technique. The observed results clearly showed that the distribution of contact frequency of the fluidized particles is correlative with the local heat transfer augmentation. This means that the contribution of direct contact heat exchange between heat transfer surface and fluidized particles is so large as not to be disregarded.Using the observed results, heat transfer augmetitatioti due to the direct contact heat exchange was numerically estimated. The estimated results showed that the contribution of direct contact heat exchange decreases rapidly witti Increasing the particle diameter and fluid velocity because the contact frequency decreases with the increase of both of them.In order to confirm the propriety of obtained results, mass transfer experiments were also performed under the identical experimental conditions. The difference between the heat transfer augmentation and mass transfer enhancement, i.e. the contribution of direct contact heat exchange between fluidized particles and heat transfer surface, agreed well with the results estimated from the visualized contacting behavior. Therefore, it was concluded that the heat transfer augmentation of fluidized bed is dominated by the direct contact heat exchange between particles and heat transfer surface especially in the region where the flow velocity is almost identical to the minimum fluidizing velocity but this contribution decreases rapidly with increasing particle diameter and flow velocity.