The enhancement effects of internal convection on the heat transport of condensing droplets out of pure steam and moist air

Abstract

In this work, the effect of internal convection on the heat transport of condensing droplets is theoretically and numerically focused on considering two typical working scenes (pure steam and moist air). A three-dimensional transient multiphysics model is first constructed by elaborately coupling time-dependent multiple physics during the dynamic growth of condensing droplets. Considering variable surface wettability and industrially universal applications, heat transport characteristics of condensing droplets in these two scenes are comparatively analyzed over a wide range of the droplet radius (500–1000 μm), contact angle (60–120°), and subcooling (1–50 K). It is found that internal convection resulting from the thermocapillary effect and curved vapor/liquid interface plays a progressively prominent role as the contact angle and subcooling increase, accordingly dominating heat transport within droplets. In the steam scene, internal convection is activated neighboring the triple-phase contact line at which the temperature gradient exists solely. In comparison, in the air case, the external vapor diffusion promotes a non-uniform temperature profile over the droplet surface, and the temperature gradient is extended toward the whole surface with stronger internal convection and heat transport enhancement. In general, the quantitative analysis demonstrates that driven by strong internal convection, the total heat flow rate through the droplet can be increased by several times for both two scenes. Furthermore, using the fundamental dimensionless groups governing internal convection, we put forward an empirical correlation of the droplet Nusselt number in two condensing scenes over wide working conditions.