A Study on Instability of Falling Liquid Film in a Countercurrent Annular-Mist Two-Phase Flow

Abstract

Stability of liquid film in a counter-current two-phase flow of falling liquid film and upward mist flow, which is generated by spraying the liquid into gas flow, in a vertical pipe have been investigated both experimentally and theoretically. The liquid film flow rate, pressure drop, and interfacial wave velocity were measured for vapor-water and air-water systems. It is shown that, with increasing the spray flow rate under the condition of constant gas-phase velocity, the falling liquid film becomes unstable due to the impact of droplets flowing upward. With the onset of instability, flow reversal of liquid film, entrainment of droplets, and rapid increase of pressure drop occur. This instability occurs even in a stable range of conventional flooding correlation for a counter-current two-phase flow. Based on comparison between experimental results for a vapor-water system at a pressure lower then the atmospheric pressure, and an air-water system at the atmospheric pressure, the droplets' impingement effect on the stability of liquid film is correlated with the ratio of the momentum of the colliding droplets on the liquid film to the interfacial shear stress. A theoretical analysis on the stability of liquid film impinged by droplets is made. Nonlinear stability of a finite-amplitude interfacial wave is treated in the analysis. The theory predicts the existence of a stable finite-amplitude interfacial wave. It also predicts that the flow becomes unstable with increasing the gas velocity and the momentum of impinging droplets. The stability limit predicted by the analysis agrees well with the experimental results.