Numerical study on the effect of surface wettability on the falling film flow characteristics of the spiral wound tube heat exchanger

Falling film flow is critical in the shell-side heat transfer performance of spiral-wound heat exchangers, and the wettability of the tube surface has a significant effect on the falling film flow characteristics. Therefore, a three-dimensional numerical model with two rows of tubes was established to study the influence of surface wettability on the liquid film distribution outside the tubes of a spiral wound heat exchanger. The transient spreading characteristics of the liquid film under different wettabilities were studied, the thickness of the liquid film along the axial and circumferential directions of the tube surface was quantitatively measured, and the effects of the tube spacing and Reynolds number on the flow characteristics of the falling film were analyzed. The results indicate that for the upper half-tube, increasing the contact angle to a certain extent is beneficial for improving the uniform distribution of the liquid film, whereas for the lower half-tube, decreasing the contact angle can suppress the fluctuation of the liquid film. The influence of the contact angle on the axial spreading of the liquid film was greater than that in the circumferential direction. When the contact angle does not exceed 30 degrees, increasing Re can improve the wetting ratio, otherwise, increasing Re number has a limited effect on im-proving the wetting ratio. A smaller tube spacing can increase the thickness of the liquid film outside the tube. When the tube spacing changed from 4 mm to 8 mm, the average liquid film thicknesses of the circular, elliptical and egg-shaped tube decreased by 4%, 30.7% and 59.6% respectively.(c) 2022 Institution of Chemical Engineers. Published by Elsevier Ltd. All rights reserved.