光管内CaCO_3结垢对管内传热影响的阶段及机理

The Effect Stages and Mechanism of Calcium Carbonate Fouling on Heat Transfer in Smooth Tubes

本实验对不锈钢管内CaCO_3污垢的沉积过程进行了分段研究。测得传热系数后,获得了污垢热阻,并用电镜扫描(SEM)获得了污垢分布随时间的变化.实验结果表明:在不锈钢管内CaCO_3污垢沉积的初始阶段,传热系数增大而非减小,出现负污垢热阻,说明CaCO_3污垢初始阶段的沉积有利于传热,经过一段时间的沉积后才会出现正污垢热阻。电镜扫描(SEM)结果表明,污垢沉积的初始阶段,污垢结晶以星状点分布于管内壁,随着沉积时间的增长星状点变得密实,这种不完全覆盖使得流体在流经污垢时产生扰流,增强了传热效果。随着时间推移,结晶污垢的覆盖面积逐渐增大,污垢热阻逐渐增加,传热系数减小。

The heat transfer coefficient,fouling resistance and the distribution of CaCO_3 fouling on the surface of stainless steel tube in different stages are experimentally studied.The experimental results of CaCO_3 fouling in the stainless steel tube show that in the initial stage of fouling,the heat transfer coefficient increases rather than decreases,the heat transfer resistance caused by the fouling is negative.This shows that the CaCO_3 fouling promotes heat transfer at the beginning of fouling.After a period of time developing of fouling,heat transfer resistance caused by the fouling becomes positive.Prom the pictures obtained by scanning electron microscope(SEM),it is found that the fouling distributes on the tube wall as star points in the initial stage of fouling,as the fouling time increases,the distribution of fouling becomes denser,but the fouling does not cover the tube surface completely.When flow flows over the fouling,disturbs in flow are produced,then heat transfer is enhanced.When fouling increases further,the area covered by crystallization fouling increases,the heat transfer resistance fouling increases,and heat transfer decreases.