Annular two-phase flow in tubes

The paper reviews work carried out at the Atomic Energy Research Establishment, Harwell, England. Annular flow occurs in gas-liquid systems when part or all of the liquid phase flows on the tube wall in the form of a film. The first part of the paper describes investigations of two-phase flow regimes in both air-water and steam-water flows. The techniques used included high speed cine and X-ray photography. It is shown that the flow patterns can be represented in terms of superficial momentum fluxes and the dominant role of the annular flow pattern is confirmed. In annular flow, the liquid can flow partly as entrained droplets in the gas core. The existence of this phenomenon is the principal obstacle in the prediction of the characteristics of annular flow. The source of liquid droplets in the gas core is entrainment from interfacial waves and this emphasises the importance of the structure of the film-core interface. The extensive work done at Harwell on interfacial structure in annular flow is reviewed. Experiments have been carried out on both vertical upflow and vertical downflow. The most important type of interfacial wave is the disturbance wave whose amplitude is typically about five times the mean film thickness. The droplets from the gas core are continuously redeposited on the film, and eventually, an equilibrium between entrainment and redeposition is established. The various experiments carried out at Harwell to elucidate this key mass transfer process are reviewed. Finally, the implications from the work to the design of plant involving annular flow are discussed, and the failure of current design methods examined