Flow patterns of two-phase flow in vertical U-type return bends

Two-phase flow patterns across vertically arranged u-bend tube set to clarify the characteristics of flow patterns were investigated. A total of five flow patterns were identified such as bubbly flow, plug flow, slug flow, annular flow, and gurgle flow. The observed flow patterns are strongly related to the influence flow direction, size of the inner diameter of the tube, and the bending ratio of the tube. Gurgle flow was found when the fluid flows through the 8-mm tubes on the downstream section. Only slug and annular flow was observed in the return bend, for a smaller diameter tube of 4-mm. All five flow patterns were identified in the flow pattern map for a larger diameter of tube. The major effect on the flow pattern map of the vertical return bend was attributed to the counterbalance among flow inertia, viscous force, buoyancy force, gravity force, flow entry direction, and geometrical parameters such as bend ratio and tube diameter.