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

本研究对流态化换热器换热分段的微观结构进行了实验研究。为了评价颗粒与受热面的直接接触换热的贡献，利用光学技术对颗粒与受热面的接触进行了可视化研究，该换热被认为是床层强化传热的有效机理之一。实验结果表明，流态化颗粒的接触频率分布与局部强化换热密切相关。这意味着受热面与流态化颗粒之间的直接接触换热的贡献是不容忽视的。利用观测结果，对直接接触换热引起的换热增量进行了数值估算。计算结果表明，直接接触换热的贡献随着颗粒直径和流体速度的增大而迅速减小，因为接触频率随颗粒直径和流体速度的增加而减小。为了验证所得结果的正确性，还在相同的实验条件下进行了传质实验。传热强化和传质强化之间的差异，即流态化颗粒与受热面之间直接接触换热的贡献，与可视化接触行为的估算结果吻合较好。因此，床层的强化换热主要由颗粒与受热面之间的直接接触换热决定，特别是在流速与最小流态化速度基本相同的区域，但这一贡献随颗粒直径和流速的增大而迅速减小。

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.