Instability of Interfacial Waves on Countercurrent Two-Phase Flow in a Vertical Tube

In this paper, the authors present liquid film characteristics during transition from countercurrent annular two-phase flow to flooding in a vertical tube. Time variations of liquid film thickness at four different heights in the tube are measured. Their power spectra are used to examine interfacial wave characteristics. The instability of a liquid-gas interface in countercurrent annular two-phase flow is studied using linear and nonlinear methods. In the linear analysis, the Rayleigh equation is applied. In the nonlinear analysis, the long-wave analysis considering the base harmonic and harmonics is performed. The critical wave number obtained from experiments is compared with the stability maps. In the inviscid analysis, most of the critical wave numbers for each flow regime are located in the unstable region obtained from the theoretical instability map; therefore, this analysis cannot explain the transition to flooding. The wave number obtained from the present experiment decreases with increasing gas flow rate. In the nonlinear analysis, the critical wave number increases, and ultimately, the critical wave number from the experiment is found in the unstable region immediately before flooding. This analysis is found to be consistent with the experimental results.