A two-phase air–water flow experiment was carried out to obtain flow pattern, void fraction, and pressure
drop data from a vertical micro-channel, which was a fused silica tube with a diameter of 0.53 mm
and a length of 320 mm. The adiabatic two-phase flow behaviors during vertical upward direction were
experimentally investigated, and consequently, a buoyancy effect can be detected in this work. A flow
visualization study was performed, leading to observation of slug flow, throat–annular flow, churn flow,
annular flow, and annular–rivulet flow. The shape of the interfacial surface in the slug flow pattern was
deformed during vertical upward flow, which was different from the bullet-shaped gas slug observed in
the horizontal channel. The flow visualization results also indicate that the flow pattern map for vertical
upward flow is not completely compatible with that for horizontal flow. Image analysis was performed to
determine the void fraction, which increases linearly with increasing volumetric quality. The vertical
upward flow gave a lower void fraction than the horizontal flow. The frictional pressure drop can be
increased when the churn flow is formed in the channel. In addition, vertical upward flow can result
in higher pressure drop when compared with the horizontal channel.