Experimental study on interfacial and wall friction factors under counter-current flow limitation in vertical pipes with sharp-edged lower ends

Experiments on air-water counter-current annular flows in circular vertical pipes of 20 and 40 mm in diameter were carried out to obtain databases of the interfacial and wall friction factors. The pipes were made of transparent acrylic resin to observe counter-current flow limitation (CCFL) in the pipe. The vertical pipe was connected to an upper and a lower tank. The liquid and gas phases were supplied to the upper and lower tanks, respectively. The liquid volume flow rate, the pressure gradient and the liquid volume fraction were measured to evaluate the friction factors. The flows in the pipe were observed using a high-speed video camera and the flow pattern under CCFL was identified using the time-strip visualization technique. The main conclusions obtained under the present experimental conditions are as follows: (1) the flows under CCFL condition can be classified into four regimes, i.e. the smooth film, the transition from smooth film to rough film regimes, the rough film with large amplitude disturbance waves forming at the lower pipe end, and the rough film with simultaneous onsets of disturbance waves inside the pipe, (2) the slope of the CCFL diagram depends on the flow regimes, (3) the flow structure strongly affects the friction factors, and therefore, available correlations of friction factors not accounting for their dependences on the flow regimes cannot give good evaluations, (4) the wall friction factor can be correlated in terms only of the liquid Reynolds number, (5) the interfacial friction factor in the rough film regimes can be well correlated in terms of the gas Wallis parameter (Froude number) since the interface structure strongly depends on the Wallis parameter, and (6) the CCFL model derived using the present empirical correlations of friction factors agrees well with the CCFL characteristics data.