Interfacial profiles of wavy film flow in vertical pipe at low Reynolds numbers – an experimental study

Abstract

A liquid film falling vertically along a wall results in the formation of waves at the liquid-gas interface. The principal forces in these flows are gravity, viscosity, and surface tension, which are characterized by the Reynolds number, Re and the Kapitza number, Ka. Experimental characterization of the film flows for high viscosity liquids (Ka in the order of 1) at low Re » in the order of 10 was performed for flow along the inner circumference of a vertical pipe. The three-dimensional liquid-gas interfacial profile is quantitatively reconstructed using the laser induced fluorescence technique. The waves observed had higher surface steepness at the front when compared with the back of the wave crest. The film thickness at the back of the wave crest is higher than at the front, resembling the streak-like waves observed in the literature for low-viscosity liquids (Ka in the order of 1000) at comparable Re. When the lateral surfaces of the waves are in contact, merging in the transverse direction is observed. The experimentally determined mean film thickness was approximately 15 % lower than the Nusselt’s flat film thickness, while the experimental values of average wave velocity was approximately 4 times higher than the Nusselt’s velocity for the corresponding film thickness. On the time-averaged film thickness field, a transverse variation in the film thickness, called as ridges and valleys, were observed. These transverse variations are found to be unsteady in their behaviour. Probability density distributions of the film thickness measurements showed two distinctive time-dependant patterns for the distribution of film thickness values in the ridges.