Heat Transport Mechanism around Tubes Immersed in Floating Particles

近年来，为实现具有优良热特性的沸腾床换热器进行了大量的研究工作。虽然已经提出了多种理论，但目前实际的设计仍依赖于经验关联式。对静态床层高度很浅的单列水平管内的传热机理进行了实验研究。为了改善流态化低密度颗粒(聚苯乙烯空心球)换热器的性能，针对传热机理的现象学理解，研制了安装在管内的快速响应电容传感器，用于研究颗粒在管内的运动情况。在相同条件下，利用微型热流传感器测量了局部瞬时换热系数。根据电容传感器和热流传感器的数据，提出了一种新的传热机理模型。将该模型与许多其他模型，特别是Mickley和Fairbank提出的填充更新模型进行了比较，结果表明：(1)电容轨迹随时间的变化与管壁附近局部固体密度的变化直接相关。(2)电容信号可分为三种主要类型：稠密乳化相、稀乳化相和夹带颗粒的气相。(3)热流传感器信号与电容式传感器信号相对应。(4)在稠密乳状液的作用下，换热系数大大提高。(5)热传递机理可以用更新模型来解释，计算结果与实验结果相吻合。

Intensive studies designed to realized to fluidized bed heat exchanger with excellent heat characteristics have been cconducted in recent years. While several theories for the heat transfer process have been proposed, actual design currently still depends on empirical correlation.The mechanism of heat transport from a single row horizontal tubes submerged in a floating fluidized bed, of which the static bed height was very shallow, was investigated experimentally. In this paper, in order to improve the performance of a heat exchanger on fluidized, low density particles (polystyrene hollow sphere) were employed.Aiming at phenomenological understanding of the heat transport mechanism, the fast response capacitance sensor mounted flush within the tube was developed to investigate the movement of particles near the surface of tubes. Local instantaneous heat transfer coefficient was also measured using a miniature heat flow sensor for the same conditions.A new model of heat transport mechanism was suggested from the data of capacitance sensor and heat flow sensor. Comparison of this model with many of the other models, in particular the packed renewal model originally developed by Mickley and Fairbanks, was tried.The results obtained were; (1) The capacitance trace versus time can be directly related to the variations in local solids density near the tube surface. (2) Capacitance signals can be divided into three major types as follows; the dense emulsion phase, the lean emulsion phase and the gaseous medium with some entrained particles. (3) Signals of heat flow sensor correspond to those of the capacitance sensor. (4) Heat transfer coefficient rises extremely by the effect of the dense emulsion. (5) Heat transport mechanism can be explained by a renewal model of results were accord with experimental results.