Flooding behaviour in countercurrent gas-liquid flow in vertical tubes with turbulence promoters

Abstract

Counterflow of gas and liquid in tubes is of great interest in many industrial processes. The liquid flowing under gravity forms a thin film coating the inner tube wall. The gas driven by a pressure difference flows up in the core of the tube. Interfacial shear stress is acting on the film surface. The phenomenon of flooding is a complex two-phase flow phenomenon. It is a major limiting factor in design and operation of reflux condensers.Inserts in shape of matrix elements are often used as turbulence promotors for in-tube condensation of mixtures. These inserts are disturbing and thinning the liquid film and the concentration boundary layer. The heat and mass transfer is enhanced significantly. These inserts also have an impact on the flooding behaviour.A test apparatus was built up to study the flooding behaviour of a countercurrent adiabatic air-water two-phase flow in a vertical glass tube. The tube length was 3000mm. Two different inner tube diameters of 15mm and 22mm were used. The superficial velocities of air and water were varied to obtain the flooding curves for the empty tubes with 15mm and 22mm inner diameter as well as for the tubes with the low and high density turbulence promoters. Counterflow is possible below the flooding curve. The flooding curve of the 15mm tube is below that of the 22mm tube. That means flooding occurs at lower superficial gas velocities. The possible region for countercurrent flow is diminished when using turbulence promoters and especially with high density. The increasing water column height in the tube and resulting climbing velocity of the liquid are presented. Correlations from literature are used for comparison.