Liquid supersaturation in evaporating vertical falling films — A direct numerical simulation study

Industrial fluids in heat exchanger applications frequently experience supersaturation, which can lead to crystallization and fouling on heat transfer surfaces, reducing system efficiency and performance. In this study, we investigate the roles of inverse salt solubility and interfacial evaporation, hypothesized to be the key mechanisms driving liquid supersaturation in evaporating vertical falling films. To identify where supersaturation first emerges and to elucidate the mechanisms behind its spatial distribution, a two-phase Direct Numerical Simulation (DNS) framework is developed that fully resolves hydrodynamics and heat transfer using a Volume of Fluid (VOF) approach. The gas–liquid interface is geometrically reconstructed to ensure accurate volume fraction and scalar flux calculations, with interfacial transport restricted to the liquid phase using a weighted scheme that avoids unphysical diffusion across the interface. A spatial segmentation method using periodic boundary conditions enables the simulation of an industrially relevant pipe length of $10m$ within a compact computational domain. The framework is validated against prior numerical studies, experimental data, and analytical solutions, confirming its ability to capture the coupled hydrodynamic, heat, and mass transfer processes in the film. The results show that interfacial evaporation initiates supersaturation through solute enrichment near the gas–liquid interface, while flow separation and recirculation enhance supersaturation within the film. Supersaturation dynamics are strongly influenced by the Reynolds number (closely related to the wetting rate), with higher values leading to chaotic solute redistribution. Notably, supersaturation is consistently more pronounced near the bottom region of the evaporator, indicating a higher risk of crystallization fouling in this area.