(234n) Numerical Simulation on Flow Behaviour of Twin-Liquid Film over a Vertical Plate with an Open Window

Liquid film, as a basic contact way between gas and
liquid, is commonly encountered in chemical engineering process. Based on the
features of film formation, it can typically be divided into two types:
wall-bounded film¹, which is supported by a solid wall,
and free film, which flows without contact with any solid
frame and has two apparent free interfaces. Different from
traditional liquid
film investigation, coexistence and interaction
of wall-bounded film and confined free film² were found on some chemical
engineering equipment, such as falling-film heat exchangers with bundles of
horizontal tubes, rotating disc reactors with large windows on the wall and
random or structured packings with holes.

VOF method was used to simulate flow behaviour of such
a novel film (twin-liquid film) over a vertical plate with an open window. According
to its development of flow structure in stream wise, three regions are divided:
Region 1, at inlet of window, velocity profile rearrangement within falling
liquid film in its transition from wall-bounded film to free film flow are
shown; Region 2, at middle of window, liquid film falls freely under gravity
and stationary varicose waves
occur; Region 3, at outlet of window, liquid film impinges on the lower wall
and flows back on the plate. The existence of confined free film apparently
changes film thickness and velocity distribution of wall-bounded film around
while the latter increases stability and waviness of confined free film in the
window. Compared with traditional
wall-bounded film, twin-liquid film owns a larger mean velocity, thinner film
thickness, stronger fluctuations and vorticity on the free surfaces, which are
all considered to improve mass transfer efficiency. Considering geometry
effects of window: with increasing window depth, amplitude of waves in the
window increases first and then fluctuation of free surface becomes dynamic and
irregular; with increasing window height, typical flow characteristics of
wall-bounded film, wavy twin-liquid film and smooth free film are seen one
after the other; with increasing window width, waves on confined free film
become three dimensional and wave intensity decreases. Considering effects of
operation condition: for varicose waves in the window, the wave length
decreases with increasing Re and Ca, while the wave amplitude increases
with increasing Re and decreasing We. Complement of
experimental observations show a good agreement with corresponding
three-dimensional simulations and offer more three-dimensional flow
characteristics of twin-liquid film. The conception of twin-liquid film will
expand traditional liquid film investigation field and offers an effective way
to improve heat and mass transfer in chemical engineering process.

Figure
1 Re=150, Ca=0.0175, D=0.28 H, Case A: (a) Liquid fraction (0-1);

(b)
Pressure (-300-200 Pa); (c) X-velocity (0-0.9 m/s); (d) and (e) streamline
(0-0.9 m/s)

Figure
2 Comparison of twin-liquid film (left) and wall-bounded film (right ) at Re=150 and Ca=0.0175. (a) Liquid fraction (0-1); (b) Y-velocity (-0.3-0.3
m/s); (c) X-velocity (0-0.9 m/s) (d) Pressure (-300-200 Pa); (e) Vorticity
(0-7000/s); (f) Swirling strength (0-3000/s)

Figure
3 Comparison between 3D simulations and experimental results (water, Re=150)

References

1.           Batchelor
GK, Moffatt HK, Worster M. Perspectives in
fluid dynamics: a collective introduction to current research: Cambridge
University Press; 2002.

2.           Hu
J, Yang X, Dai G. Numerical investigation on hydrodynamics of vertically
confined free film. The Canadian Journal
of Chemical Engineering. 2016;94(2):340-348.