快速流化床床层与内浸表面间的传热特性实验研究

EXPERIMENTAL STUDY ON HEAT TRANSFER BETWEEN SUSPENSIONS AND IMMERSED VERTICAL SURFACE IN FAST FLUIDIZED BED

在内径186mm、高8m的快速流化床内,采用特殊设计的组合式传热探头,考察了局部传热系数沿换热表面长度方向的分布规律.结果表明,只要传热表面积足够大,设置于床层径向的任何位置,都可能诱导颗粒聚集,并在传热表面形成从上向下运动的絮状物层,从而使局部传热系数沿换热表面向下逐渐降低.实验结果预示,对具有较大尺寸的换热表面,影响传热系数的主要因素是局部颗粒密度及颗粒在表面的更新颇率.气体对传热的贡献主要是通过改变颗粒在传热表面的更新频率,影响颗粒非稳态导热过程而实现的.相比之下,气体对流传热的贡献可以忽略.

The profiles of local heat transfer coefficient along the axial direction of a probe were measured in a 8m tall, 186mm ID cold model fast flu-idized bed at different radial positions. All data were obtained by using a specially designed heat probe assembly. In the experiments elimination of axial thermal conduction and constant bulk density along the probe were ensured. The experiments show that regardless of the radial position where the probe is located, the local heat transfer coefficients decrease gradually from top to bottom of the probe. It means that the particles always aggregate at the probe surface, forming a layer and moving downwards along the probe surface if the probe is large enough. In light of this mechanism, the experiments have confirmed that the local heat transfer coefficients are mainly dependent on the local bulk density around the probe and the particle renewal frequency at the surface, which in turn vary with gas velocity. The contribution of gas convection can be neglected in comparison with that of the transient heat transfer between particles and the probe surface.