关键词: 循环流化床, 传热, 两相流, 接触热阻, 停留时间

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