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Convective heat transfer scaling at the wall of circulating fluidized bed risers.

机译：在循环流化床立管壁处的对流传热结垢。

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In this study, we examine the convective heat transfer scaling of moderately pressurized Circulating Fluidized Beds (CFB) using a single cold laboratory facility. By recycling fluidization gas mixtures of adjustable density, the experiments simulate the hydrodynamics of a combustor burning coal under a pressure of 0.64 MPa. While matching the hydrodynamics in the upper riser using Glicksman's (1993) reduced set of dimensionless numbers, we vary thermal properties of the flow and measure the effect on the convective heat transfer coefficient at the wall.; In CFB flows, the suspension partly condenses into denser clusters separated from the wall by a thin gas film of order the mean particle diameter (Glicksman, 1997). The clusters generally dominate the convective heat exchange with the wall because of their relatively high solid volume fractions and heat capacity. Accordingly, Lints and Glicksman (1993) suggested that the rate of heat transfer to the wall scales with the mean time spent by individual clusters there.; For this reason, we focus on the hydrodynamics of the cluster motion at the wall. We present a scaling of cluster velocity based on a comparison of measurements from other investigators. A novel thermal marking technique records the residence length of clusters at the wall and leads to a suggested scaling of cluster residence length.; A unique, non-intrusive probe positioned at the wall of our facility measures simultaneously the solid concentration and instantaneous heat transfer coefficient. The heat transfer sensor has a 25 ms response that is at least twice as rapid as those described by previous investigators. The fraction of the wall covered by clusters and the cluster solid volume fraction are also extracted from solid concentration measurements. Guard heaters reduce conduction losses from the probe and assist thermal development of the flow, allowing a measurement of convective heat transfer coefficient that is more representative of large surface-area combustors. Finally, we present a scaling of the Nusselt Number as a function of relevant thermal and hydrodynamic parameters of the flow.
机译：在这项研究中，我们使用单个冷实验室设备检查了中压循环流化床（CFB）的对流换热尺度。通过再循环密度可调的流化气体混合物，实验模拟了燃烧器在0.64 MPa压力下燃烧煤的流体动力学。在使用Glicksman（1993）减少的无量纲数匹配上部立管中的流体力学的同时，我们改变了流体的热特性并测量了对壁对流传热系数的影响。在CFB流动中，悬浮液部分凝结成更稠密的团簇，这些团簇通过平均粒径约为一定数量的薄气膜与壁隔开（Glicksman，1997）。由于簇的相对较高的固体体积分数和热容量，簇通常主导与壁的对流热交换。因此，Lints和Glicksman（1993）提出，热量传递到壁上的比例与单个簇在此花费的平均时间成比例。由于这个原因，我们专注于墙体簇运动的流体动力学。基于其他研究者的测量结果，我们提出了簇速度的标度。一种新颖的热标记技术记录了簇在壁的停留长度，并导致建议的簇停留长度的缩放。独特的，非侵入式的探头位于我们设施的墙壁上，可同时测量固体浓度和瞬时传热系数。传热传感器的25 ms响应速度至少是以前研究人员描述的响应速度的两倍。团簇覆盖的壁的分数和团簇固体体积分数也从固体浓度测量中提取。保护加热器减少了探头的传导损耗，并有助于流体的热发展，从而可以测量对流传热系数，该系数更能代表大面积的燃烧室。最后，我们根据流体的相关热力学和流体力学参数提出了Nusselt数的换算。

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