An experimental apparatus is setup to analyze a co-current air-water two phase flow in a 3 mm inner diameter tube with horizontal and vertical orientations. Air is axially injected through a nozzle of 260 μm of inner diameter. Air and water flow rates are accurately controlled at the inlet, covering a range of apparent velocities JL = 0.00118 to 0.0786 m/s, JG = 0.002 to 3.538 m/s for the horizontal tube and JL = 0.00078 to 0.0589 m/s, JG = 0.003 to 3.538 m/s for the upward flow. A fast camera with 250 fps is utilized to visualize the flow patterns. The experiments showed that the flow structures are very sensitive to inlet conditions. Within the covered range of velocities, several flow patterns were observed, namely bubbly flow, bubbly-slug transition flow, slug flow, slug-annular transition flow, annular flow, wavy annular flow and annular flow with dry zones. In the bubbly flow regime, a particular bubbly helical flow is observed before the dispersed bubbly flow.

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