How surface modifications enhance vertical falling film evaporation

Journal article, 2024

We use multiphase direct numerical simulations to identify, analyse and quantify components of wall-normal heat flux distributions in evaporative vertical falling films with surface modifications at industrially relevant conditions. Previous experiments showed a potential increase of the heat transfer rate through the film by up to 100 % using various types of modifications. We show that the modifications induce significant advective heat transport and hypothesise that four synergistic mixing mechanisms are behind the heat transfer rate improvement. Additionally, we examine how the important surface topology parameters, pitch p (distance between modifications), height h and the liquid Prandtl number Pr_l, influence the mode of heat transport and the Nusselt number Nu. We show that p/h ≈ 10 maximises Nu and that the optimal pitch is related to the recirculation zone length L_r behind the modification. We find that Lr/h ≈ 3.5 and that Nu ∝ Pr_l^0.42 in the investigated parameter ranges. We also show that all our cases on both smooth
and modified surfaces have Pe_l >> 1 and collapse well on a line Nu ∝ (Pe_l/Re)^0.35. This relation suggests that Nu is governed by the balance of film mixing, thermal resistance and diffusivity, and that the ratio Pe l/Re can be used to estimate Nu. Our methodology and findings extend the knowledge concerning the mechanisms behind the heat transfer improvement due to surface modifications and facilitate guidelines for designing more efficient modified surfaces in industrial evaporators.