Abstract: The heat transfer between a dense bed and freely moving coal particles is of great significance for designing and operating a circulating fluidized bed (CFB) boiler. In this study, heat transfer coefficients were measured for graphite spheres freely moving in the dense bed of a lab-scale fluidized bed facility. It is shown that the heat transfer coefficient first increases as the fluidizing velocity rises initially from the minimum fluidizing velocity. After the fluidizing velocity reaching a critical point, the heat transfer coefficient remains invariant as the fluidizing velocity increases, which indicates that the burning coal particles remain in the emulsion phase in a CFB boiler. Under most conditions, the heat transfer coefficient decreases with the increase of bed material size, but at relatively low fluidizing velocities, after reaching a minimum value, the heat transfer coefficient increases with the bed material size. With fixed fluidizing velocity and bed material size, as the graphite diameter increases, the heat transfer coefficient declines and the dropping rate decreases. The heat transfer coefficient is higher for the graphite sphere floating in a higher bed.

Key words: circulating fluidized bed, heat transfer, two-phase flow, contact resistance, residence time

摘要： 密相区内自由移动的煤颗粒表面传热系数是循环流化床锅炉设计和运行的重要参数。利用石墨球模拟煤颗粒，在小型流化床实验台上对由粒度较小的石英砂颗粒组成的密相区内自由移动的石墨球表面传热系数进行了测量。测量结果显示，随着流化风速的增加，石墨球表面传热系数首先升高，当流化风速达到某一临界值时，继续增大流化风速，传热系数将保持不变，从传热的角度证明了流化床内煤颗粒基本停留在乳化相内。在多数情况下，石墨球表面传热系数随床料粒度的增大而减小。而在较低流化风速的情况下，随着床料粒度的增大，石墨球表面传热系数呈先下降后升高的趋势。当流化风速和床料粒径保持不变时，石墨球表面传热系数随着石墨球直径的增大而减小，且下降的趋势随石墨球直径的增大而减弱。而随着床层高度的增加，石墨球表面传热系数将会略有 升高。

关键词: 循环流化床, 传热, 两相流, 接触热阻, 停留时间