Transient multiphysics coupled model for multiscale droplet condensation out of moist air

Abstract

As a key physical process, water vapor condensation has attracted significant attention because of its potential in engineering applications. The non-condensable gas in the surrounding vapor has a significant influence on condensation heat transfer. Considering as a crucial aspect, this work developed a transient multiphysics coupled solver to investigate droplet condensation in a moist air environment (considering dry air as the non-condensable gas). The current solver couples the time-dependent vapor-liquid phase-change heat transfer, mass transport of water vapor, and two-phase fluid flow. In contrast to the classical thermal resistance theory model, this solver can capture the dynamic and strong coupling characteristics during condensation comprehensively. The results demonstrate that for small-scale droplets, vapor condensation is driven by the coupled internal conduction-dominated heat transfer and external vapor diffusion. As the droplet grows and the contact angle increases, internal convection driven by the Marangoni effect becomes increasingly important. The enhanced fluid mixing inside the droplet can affect both the internal heat transfer and the external vapor diffusion. Because of the significant diffusion resistance, the droplet growth rates in a moist air environment are reduced up to 1-2 orders of magnitude compared with the case of pure steam. For large-scale droplets, the internal convection can increase the droplet growth rate up to 18.7%. Furthermore, the contact angle, the subcooling temperature, and the relative humidity have significant influences on droplet condensation in a moist air environment. This work not only promotes the mechanistic understanding of condensation heat transfer in a moist air ambient but also provides a flexible solver for vapor-liquid phase change problems.