Bubble dynamics in two-phase flows in an upward 90-degree elbow pipe*

 For a flow through a bent pipe, it is known that a centrifugal force and the boundary layer on the wall induce secondary vortical structures, which affects the interfacial structure of the flow. Since the transfer characteristics of two-phase flows are greatly influenced by this interfacial structure, the effect of pipe orientation on the two-phase flow should be studied in detail. We investigate bubble dynamics in gas-liquid bubbly flows in an upward 90-degree bent pipe, with different Reynolds numbers (Re = 0, 400 and 6000) of background flow and mean void fraction of 0-3%. The cross-section of the pipe is a square (20 mm x 20 mm), and we use high-speed shadowgraphy to track the three-dimensional path (velocity and size distributions) of the bubbles. When the flow is laminar, bubbles separate at about 15 degrees from the bent entrance, while more bubbles reside at center of the pipe as mean void fraction increases. For turbulent flow (Re = 6000), on the other hand, most of the bubbles separate from the inner wall at about half way of the bent and more bubbles migrate to outer wall as the mean void fraction increases. More detailed bubble dynamics and its mechanism will be discussed.