Flow characteristics in vertical circular pipes with the square top end under flooding conditions

Abstract

Previously, we evaluated flow characteristics in a vertical pipe of diameter D = 20 mm under flooding conditions. Now, we have measured the void fraction α, pressure gradient dP/dz and countercurrent flow limitation (CCFL) characteristics in a vertical pipe of D = 40 mm with the square (i.e. sharp-edged without curvature) top end, and air and water to evaluate the film thickness δ, the wall friction factor fw and the interfacial friction factor fi. The comparison of flow characteristics between pipes of D = 20 and 40 mm showed that the CCFL characteristics and the transition from the smooth film (SF) to the rough film (RF) due to flooding near the bottom end were different. On the other hand, δ and fw for SF were expressed by the same correlation for D = 20 and 40 mm. We discussed the CCFL characteristics and fw in detail for SF; these items are important for assessment of α and fi from the dP/dz data in the absence of α data.