Effect of surface tension and gas-liquid density ratio on the wave height and interfacial shear stress in annular flows

Zhang, H.*; Umehara, Yutaro*; Horiguchi, Naoki   ; Yoshida, Hiroyuki  ; Mori, Shoji*

In the past decades, the liquid film characteristics of annular flow, such as film thickness and wave height, have been studied extensively. However, most of experimental data and analyses available in previous studies are limited to the air-water annular flows under near-atmospheric conditions. Thus, the variation in surface tension and gas-liquid density ratio is also limited, and their effects on the characteristics of annular flow are not well-understood. The objective of this study is to clarify the effect of surface tension and gas-liquid density ratio on the wave height and interfacial shear stress in annular flows. We conducted experiments for gas-liquid annular flows and measured the time-varying liquid film thickness. Using water and ethanol as working liquids, whereas nitrogen and HFC134a gas as working gases, we varied the surface tension range from 30.7 to 67.4 mN/m and density ratio range from 27 to 434. The accuracy of sensors for the measurement using the conductance probe method is 5%. By post-processing for the measurement data, liquid film thicknesses (base, average, and maximum film thickness) and wave height were estimated. As the experimental results, the data of both the maximum film thickness and the height of disturbance wave converged on a single curve when they plotted for the interfacial shear stress. The reason was considered that the shape of the disturbance wave is decided by the balance between the surface tension and the interfacial shear stress. With additional analysis based on a potential method, a direct relationship between the height of disturbance wave and interfacial shear stress was clarified. These findings enhance the understanding of annular flow dynamics and contribute to improving the two-phase flow modelling.