垂直管内弹状气泡上升中壁面传递的实验研究

垂直管内弹状流壁面传递是诸多工业应用中需研究的重要问题之一。今用极限扩散电流技术,对弹状气泡上升时瞬时壁面剪应力和传质系数进行了测定,结果显示:当基于表观气速的Froude数FrG < 0.74时,壁面剪应力随弹状气泡和液塞的到来呈现方向相反的交替变化,壁面传质系数亦相应变化;而当FrG > 0.74时,剪应力方向一直向下,说明液膜向下流动,且弹状气泡和液塞的到来对壁面传质系数的影响很小。这说明下落液膜射流穿透了液塞段,控制了整个壁面传递过程。研究还对下落液膜区、尾迹区及液塞段的不同传递特征及机理进行了分析, 并结合气泡塔熔融结晶器中弹状气泡上升时的传热,对结晶操作条件的合理选择进行了讨论。

Experimental Study on Wall-liquid Transfer Characteristics for Gas Slugs Rising in a Vertical Tube

The instantaneous wall shear stress and wall-liquid mass transfer coefficient for gas slugs rising through the liquid column in a vertical tube were measured by using limiting diffusion current technique. Two different types of the transfer behaviors were identified. For gas Froude number FrG<0.74, the wall-liquid mass transfer coefficient and the wall shear stress changed alternatively with the coming or leaving of Taylor bubbles. For FrG >0.74, however, the wall-liquid mass transfer was influenced weakly by the arrival of Taylor bubble or liquid slug. Moreover, the liquid film flowed downward and passed through the whole trailing liquid slug, and the wall-liquid transfer was mainly controlled by the liquid film falling and jetting in this case. The different transfer mechanisms and behaviors in the falling liquid film zone, the wake zone and the developed liquid slug region were analyzed. Considering the mass and heat transfer for Taylor bubbles rising through the melts in bubble column crystallization, the selection of the suitable bubble column operation conditions for crystallization was also suggested.