Flow behaviour of drop and jet modes of a laminar falling film on horizontal tubes

Highlights

• Flow behaviour of a falling film of horizontal tubular evaporators and absorbers.

• 3-D CFD model using VOF method was built to capture gas-liquid interface.

• Effect of contact angle and specific flow rate on hydrodynamics of film and droplet.

• Falling film flow pattern is sensitive to the initial wetting condition.

• The final steady wetted region depends on the receding contact angle.

Abstract

The flow behaviour of a falling film on horizontal tubes has significant impacts on the efficiency of heat and mass transfer for the horizontal tubular evaporators and absorbers. Thus it is crucial to investigate the effect of contact angle and specific flow rate on the hydrodynamics of films and droplets in the drop and jet modes, which are the most important modes for evaporators and absorbers. A three-dimensional CFD model, using the volume of fluid (VOF) method, has been built to capture the gas-liquid interface. By employing mesh refinement and independence tests, the film spreading and droplet formation, liquid bridge breakup, droplet detachment, impact and fluctuation waviness of the simulations at the completely wetting condition are found to be in good agreement with experimental data and theory. In the drop mode, the behaviours of the droplets alternately falling from neighboring sites, including unstable slender liquid bridge and complex saddle waves were observed. This indicates that the wave propagation speed at first is higher than the impaction speed. In the in-line jet mode, the flow pattern is almost stationary but it is perturbed weakly by the impaction wave. The circumferential ring film has its maximum thickness located at the middle of the two-column in-line jet due to the wave interaction. In addition, the wetting area, spreading speed and instability time of the falling film are reduced with an increase in the contact angle, while the first two parameters increase at higher specific flow rates. In particular, an overshoot occurred at the larger contact angle. Finally, the results showed that the falling film flow pattern is sensitive to the initial wetting condition. Further, it reveals that the final steady wetted region depends on the receding contact angle, thus the introduction of the dynamic contact angle is the best approach for accurately determining the behaviour of falling films.