Inspired by the thermoregulation of mammals via perspiration, cooling strategies utilizing continuously fed evaporating droplets have long been investigated in the field, yet a comprehensive modeling capturing the detailed physics of the internal liquid flow is absent. In this study, an innovative computational model is reported, which solves the governing equations with temperature-dependent thermophysical properties in an iterative manner to handle mass and heat transfer coupling at the surface of a constant shape evaporating droplet. Using the model, evaporation from a spherical sessile droplet is simulated with and without thermocapillarity. An uncommon, nonmonotonic temperature variation on the droplet surface is captured in the absence of thermocapillarity. Although similar findings were reported in previous experiments, the temperature dip was attributed to a possible Marangoni flow. This study reveals that buoyancy-driven flow is solely responsible for the nonmonotonic temperature distribution at the surface of an evaporating steadily fed spherical water droplet.

Recent developments in fabrication techniques enabled the production of nano-andångström-scale conduits. While scientists are able to conduct experimental studies to demonstrate extreme evaporation rates from these capillaries, theoretical modeling of evaporation from a few nanometers or sub-nanometer meniscus interfaces, where adsorbed film, transition film and intrinsic region are intertwined, is absent in the literature. Using the computational setup constructed to identify the detailed profile of a nano-scale evaporating interface, we discovered the existence of lateral momentum transport within and associated net evaporation from adsorbed liquid layers, which are long believed to be at the equilibrium established between equal rates of evaporation and condensation. Contribution of evaporation from the adsorbed layer increases the effective evaporation area, reducing the excessively estimated evaporation flux values. This work takes the first step towards a comprehensive understanding of atomic/molecular scale interfacial transport at extended evaporating menisci. The modeling strategy used in this study opens an opportunity for computational experimentation of steadystate evaporation and condensation at liquid/vapor interfaces located in capillary nano-conduits.Capillary evaporation and the associated passive liquid flow are vital for numerous natural and artificial processes such as transpiration of water in plants, 1 solar steam generation, 2,3 water desalination, 4 microfluidic pumping, 5 and cooling of electronic and photonic devices. 6 Regardless of the process or the geometrical configuration, studies on evaporation focus on identification and characterization of heat transfer and flow dynamics in the vicinity of the contact line, the juncture of three phases of matter. Fig. 1a-c shows different evaporation processes schematically. Green dashed rectangles point out the liquid film distribution around the contact line, which is broadly composed of three multiscale regions as shown in Fig. 1d. Evaporation rate intensifies in evaporating thin film region due to the micro-scale liquid film thickness. The adsorbed nano-scale layer extending further is assumed to be non-evaporating due to the suppression of evaporation by strong long-range intermolecular forces. [7][8][9][10][11][12][13][14][15][16][17][18][19] While the kinetic theory of gases 20,21 is widely used to predict the theoretical maximum rate of evaporation, experiments have always calculated smaller heat fluxes than the kinetic limit. 22 However, two recent experimental studies have attracted the attention of scientific community by reporting evaporation fluxes one to two orders of magnitude higher than the prediction of kinetic theory. 23,24 These unexpectedly high flux values were attributed to the possible underestimation of evaporation area in the first study, 23 where the stretching of water meniscus over the flat surface adjacent to the channel mouth was speculated. On the other hand, the second study, 24 reported evaporation rates from a ...

h i g h l i g h t s Three performance indicators used including a newly defined heat pipe effectiveness. Optimum operation is discussed based on existence/absence of dryout in the grooves. Near optimum, dryout tendency increases with increasing groove density and heat flux. Effectiveness increases with groove density, and drops with increasing heat flux.