Gas-liquid two-phase flow regimes in rectangular channels with mini/micro gaps

An adiabatic concurrent vertical two-phase flow of air and water in vertica l rectangular channels (12 x 260 mm) with narrow gaps of 0.3, 0.6-1.0 mm wa s investigated experimentally. Flow regimes were observed by using a CCD ca mera and were identified by examining the video images. The flow regimes fo r gaps of 1.0 and 0.6 mm were found to he similar to those in the existing literature which can be classified into bubbly flow, slug flow, churn-turbu lent flow and annular flow. With the decrease of the channel gap, the trans ition from one flow regime to another occurs at smaller gas flow rates. How ever, flow regimes for micro-gaps of 0.3 mm or less are quite different fro m the previous studies: bubbly flow was never observed even at very low gas flow rates. Due to the increased influence of the surface tension force an d the frictional shear stress in channels with a micro-gap, the liquid drop lets adhered on: the wall surface and were pushed by the gas phase. Flow re gimes in these micro-gaps can be classified into cap-bubbly flow, slug-drop let flow, churn flow and annular-droplet flow. A previous model has, been e xtended to predict the flow regime transitions from bubbly flow to slug flo w, slug flow to churn flow using the bubble rising velocity and the increas ed frictional coefficient for rectangular channels. A new criterion has bee n developed to predict the transition of the annular flow. Comparisons of o ur prediction results with experimental data are discussed for gaps larger than 0.6 mm. With micro-gaps of 0.3 mm or smaller, a new theory needs to be developed. (C) 1999 Elsevier Science Ltd. All rights reserved.