FLOW PATTERNS IN LIQUID SLUGS DURING BUBBLE-TRAIN FLOW INSIDE CAPILLARIES

Bubble-train flow consists of strings of bubbles separated by liquid s lugs flowing cocurrently inside capillaries of circular or square cros s section. Bubble-train flows have interesting properties and have bee n used to enhance heat and mass transfer rates inside capillaries. The quantitative analysis of heat and mass transfer processes related to this flow requires a detailed knowledge of flow patterns inside the li quid slugs. High-speed video imaging was used to characterize liquid f low patterns. Velocity distributions inside liquid slugs were determin ed by particle imaging velocimetry (PIV). Video images recorded in a r eference frame moving with the bubbles indicate the presence of recirc ulating patterns with a high degree of mixing. Depending on the capill ary number of the flow, counter rotating vortices or a complete bypass flow inside the liquid slug were observed. The position and size of t hese vortices and the magnitude of the liquid velocity can be accurate ly predicted by theory. Mixing inside the liquid slug can be character ized by the average recirculating time of a particle of fluid trapped in the liquid vortex. (C) 1997 Elsevier Science Ltd.