Abstract
The transient establishment of continuous liquid films is essential for the stable hydrodynamic operation of falling-film systems. However, the mechanisms governing liquid-front propagation and film establishment under carrier-gas-assisted conditions remain insufficiently understood. In this study, a three-dimensional transient volume-of-fluid (VOF) model was employed to investigate gas–liquid interaction during the startup process of a vertical falling-film system. A liquid-front arrival time (LFAT) criterion was proposed by combining liquid-area coverage and wall-attached film continuity, enabling the quantitative identification of effective continuous-film establishment during startup. The effects of liquid loading, carrier-gas velocity, and axial propagation distance on liquid-film evolution and stability were systematically investigated. The results demonstrate that carrier-gas-induced interfacial momentum transfer significantly enhances liquid-front propagation, whereas excessive gas shear may induce film thinning and instability under low liquid-loading conditions. A mechanistic correlation was further developed by coupling gravity-driven liquid transport with carrier-gas-induced interfacial momentum enhancement. The proposed correlation provides an effective approach for predicting liquid-front propagation within the investigated operating range. This study improves the understanding of carrier-gas-assisted liquid-film establishment and provides theoretical support for the design and operation of vertical falling-film systems.