Instability of interfacial waves on countercurrent two-phase flow in a vertical tube

In this paper, the authors present liquid film characteristics during trans ition 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 counter current annular two-phase flow is studied using linear and nonlinear method s. In the linear analysis, the Rayleigh equation is applied. In the nonline ar analysis, the long-wave analysis considering the base harmonic and harmo nics is performed. The critical wave number obtained from experiments is co mpared with the stability maps. In the inviscid analysis, most of the criti cal wave numbers for each flow regime are located in the unstable region ob tained from the theoretical instability map; therefore, this analysis canno t explain the transition to flooding. The wave number obtained from the pre sent experiment decreases with increasing gas flow rate. In the nonlinear a nalysis, the critical wave number increases, and ultimately, the critical w ave number from the experiment is found in the unstable region immediately before flooding. This analysis is found to be consistent with the experimen tal results.