A study of flow patterns for gas/liquid flow in small diameter tubes

Time resolved void fraction data and flow pattern information have been obtained for two-phase air/water flows in a small diameter (5 mm) vertical pipe using conductance probes. The time averaged void fractions are seen to agree with values measured using a completely different approach. Analysis of high speed videos reveals that the probability density function (PDF) technique is inadequate for accurately delineating the transition between slug and churn flows but performs better for the churn to annular flow transition. Instead a novel approach has been developed for transitions between flow patterns using the velocity of structures. This gives good agreement with the present experiments. Additionally, a modification to the bubble-to-slug flow transition of Taitel et al. (1980) gives improved predictions in narrow passages. A flow pattern specific method for void fraction prediction has been applied and its predictions are in good agreement with the measured mean void fraction. The slug flow model gives better predictions of void fraction in churn flow that the annular flow model. The velocities of disturbance waves on the wall film in annular flow are well predicted using the model of Pearce (1979). However, pipe diameter dependence of one of the constants is required.