Because of the benefits provided by the thin liquid film, droplet falling film evaporation has become a focal point for enhancing its heat transfer. A computational fluid dynamics model using VOF and dynamic contact angle methods was developed to study droplet falling film evaporation on micro-nano structured horizontal tubes, focusing on the relationship between droplet structure and transient film thickness, and comparing it to column and sheet film modes. The results showed that the transient film thickness could be divided into impact and metastable stages. The formation of a "liquid ring" caused intense fluctuations of the liquid film thickness in the impact stage. For the circumferential angle (θ) being grater than 135°, increasing the tube spacing (S) helped reduce the average liquid film thickness ( δ m ) in the metastable state. However, for θ > 135°, the enhanced droplet pulsation intensity with increasing S caused an increase in δ m . It should be noted that Nusselt's correlation could not be used to predict the δ m in droplet falling film evaporation, as both the droplet pulsation phenomenon and the effect of the droplet impact on the δ m were ignored.
Jia et al. (2026) studied this question.