In the present work, the phenomenon of two-phase flow reversal in crank-type vertical tubes was examined through a series of experiments. The inner diameter of the test tubes was 0.0107 m and made of pyrex glass for flow visualization. The overall height of the tubes was fixed to 2m, while the horizontal parts varied from 0 ~ 0.81m. Besides, two different radii of curvature at the elbows were tested, i.e., 0mm (sharp-turn elbow) and 15.4mm (round elbow). Water and air were used as the test fluids and the experiments were performed under the atmospheric condition. The flow rate ranges of the liquid and the gas were $0.83×10^{-3}$ ∼ $7.5×10^{-3}$ kg/s and $0.33×10^{-3}$ ~ $0.25×10^{-3}$ kg/s, respectively. The mechanism of the flow reversal was discussed by visualizing the flow inside the crank-type vertical tubes. Experimental results showed that the minimum gas flow rate to prevent the flow reversal in crank-type vertical tubes appears to be about 18% higher than that in simple vertical tubes (i.e., with zero horizontal length) when the length of the horizontal part was increased up to 0.31 m. However, the flow reversal condition did not change prominently with further increase in the horizontal length. The gas flow rates at the flow reversal with the sharp-turn elbows appear to be approximately 8% higher than that with the round elbows.
