The influences of wave height on the interfacial friction in annular gas–liquid flow under normal and microgravity conditions

Abstract

In this paper, results for the interfacial friction factor and relative interfacial roughness on the gas–liquid interface are reported for an air–water annular flow in a small inner diameter tube (9.525 mm i.d.). The film structure was obtained through processing the time trace signal of film thickness that was measured using conductance probe technique. The interfacial friction factor and the relative interfacial roughness were altered through changing the gas mass flow rate. Changing gravity level was another way to alter the friction factor and roughness. It was found that wave height, hence the relative interfacial roughness (defined by the wave height measured from the substrate surface) decreased with increasing the gas Reynolds number. The roughness in microgravity is less than half of that in normal gravity, while the friction factor was about 10% smaller in microgravity than that in normal gravity. It was reasoned that the friction factor in annular two-phase flow decreased less significantly with the decrease of the relative interfacial roughness than that in single-phase flow, which could be explained by the flat wave shape in annular flow. The values of the interfacial shear stress at microgravity were also compared to those calculated at normal gravity. Based on the results, some similarities and differences between the single-phase flow and the gas core in annular flow were highlighted and discussed.