Flow patterns in small diameter vertical non-circular channels

Experiments have been performed for upwards vertical two-phase air–water flow through one circular and four different non-circular channels (with rectangular, rhombic and equilateral triangular cross-section) with an equivalent (hydraulic) diameter of dh≈6 mm. Three of the four basic flow patterns were observed depending on the liquid and gas superficial velocity. The transition from dispersed bubble to slug flow, from dispersed bubble to churn flow and from slug to churn flow is being discussed. Based on existing models, theoretical approaches for the prediction of flow pattern transitions are qualified to achieve more satisfactory results for capillary tubes/channels. New distribution parameters for the rhombic and equilateral triangular channel are determined. For the transition from slug to churn flow, the flooding model is improved to predict the transition for non-circular channels as well as for circular tubes. A value for the unknown constant C for the flooding model providing good results is suggested. The transition from dispersed bubble to slug flow is modeled in consideration of the superficial liquid and gas velocity and in consideration of the hydraulic diameter. Good agreement between theory and experiment is obtained.