Numerical study of falling film flow and heat transfer on the horizontal tube with a porous coating layer

Porous surfaces often exhibit exceptional performance in liquid spreading and heat transfer, making them promising candidates for enhancing falling film heat transfer on solid surfaces. This paper delves into the flow and heat transfer characteristics occurring within falling films on horizontal tubes coated with a porous layer. To achieve this, a three-dimensional numerical model that couples the Volume of Fluid (VOF) method with porous media considerations was employed. The investigation comprehensively covers film spreading behavior, flow patterns, film velocity, film thickness, temperature distribution, and heat transfer coefficients, for both plain and porous tubes. Furthermore, we delve into the effects of key parameters, such as porosity, layer thickness, and material properties, on the overall heat transfer process. The results demonstrate the accuracy of our model in reproducing film thickness and heat transfer coefficients within falling films on horizontal porous tubes. When comparing plain tubes to porous ones, some key differences emerge. Specifically, the liquid film on porous tubes spreads more slowly, leading to increased film thickness and fewer occurrences of dry patches. Additionally, porous tubes result in lower film velocity and a more uniform temperature field compared to plain tubes. Interestingly, the findings reveal that smaller porosities yield thicker films and higher heat transfer coefficients, particularly when the porosity is below 0.4. Moreover, thicker porous layers result in thinner films, higher wetting ratios, and increased heat transfer coefficients. Among the materials we tested, aluminum and copper porous layers exhibit the highest heat transfer performance, while steel demonstrates the lowest.