Heat and mass transfer boundary conditions at the surface of a heated sessile droplet

Abstract

This work numerically investigates how the boundary conditions of a heated sessile water droplet should be defined in order to include effects of both ambient and internal flow. Significance of water vapor, Marangoni convection, separate simulations of the external and internal flow, and influence of contact angle throughout drying is studied. The quasisteady simulations are carried out with Computational Fluid Dynamics and conduction, natural convection and Marangoni convection are accounted for inside the droplet. For the studied conditions, a noticeable effect of buoyancy due to evaporation is observed. Hence, the inclusion of moisture increases the maximum velocities in the external flow. Marangoni convection will, in its turn, increase the velocity within the droplet with up to three orders of magnitude. Results furthermore show that the internal and ambient flow can be simulated separately for the conditions studied, and the accuracy is improved if the internal temperature gradient is low, e.g. if Marangoni convection is present. Simultaneous simulations of the domains are however preferred at high plate temperatures if both internal and external flows are dominated by buoyancy and natural convection. The importance of a spatially resolved heat and mass transfer boundary condition is, in its turn, increased if the internal velocity is small or if there is a large variation of the transfer coefficients at the surface. Finally, the results indicate that when the internal convective heat transport is small, a rather constant evaporation rate may be obtained throughout the drying at certain conditions.

Modelling the dynamics of the flow within freezing water droplets

“…The simulations are set-up and carried out with the hybrid Finite Volume/Finite Element CFD software ANSYS CFX 15 in a similar manner as done in Ljung et al [29][30][31] for evaporating droplets. …”

Comparing Internal Flow in Freezing and Evaporating Water Droplets Using PIV

“…The evaporation of sessile droplets has been studied thoroughly both experimentally and numerically [11][12][13][14][15]. A similar setup as in the experiments performed in this study was done by Erkan and Okamoto [16] and Erkan [17], where liquid droplets were released with different impact velocities onto a dry (and later, also heated) sapphire plate.…”

Investigation on the droplet evaporation process on local heated substrates with different wettability

“…They also found that surfactant contamination has a good performance in suppressing the Marangoni effect. Anna-Lena Ljung et al [18] adopted a numerical approach in investigating the role of boundary conditions on flow pattern both inside and outside the droplet. They found that the existence of the Marangoni effect increases the velocity within the droplet with up to three orders of magnitude.…”