Experimental investigation of interfacial energy transport in an evaporating sessile droplet for evaporative cooling applications

Abstract

In this paper we experimentally examine evaporation flux distributions and modes of interfacial energy transport for continuously fed evaporating spherical sessile water droplets in a regime that is relevant for applications, particularly for evaporative cooling systems. The contribution of the thermal conduction through the vapor phase was found to be insignificant compared to the thermal conduction through the liquid phase for the conditions we investigated. The local evaporation flux distributions associated with thermal conduction were found to vary along the surface of the droplet. Thermal conduction provided a majority of the energy required for evaporation but did not account for all of the energy transport, contributing 64±3%, 77±3%, and 77±4% of the energy required for the three cases we examined. Based on the temperature profiles measured along the interface we found that thermocapillary flow was predicted to occur in our experiments, and two convection cells were consistent with the temperature distributions for higher substrate temperatures while a single convection cell was consistent with the temperature distributions for a lower substrate temperature.

An Iterative Solution Approach to Coupled Heat and Mass Transfer in a Steadily Fed Evaporating Water Droplet

“…However, the experimentally measured surface temperature distributions of Ref. [30] conform with the calculated temperature variations of the current study only in the absence of thermocapillarity, which led to a nonmonotonic temperature distribution at the interface as evident in Fig. 3.…”

“…Three different cases are simulated with and without thermocapillarity. While the ambient temperature is 30 C for all three cases, substrate temperatures are 39 C for case 1, 58 C for case 2 and 74 C for case 3 to enable a comparison with available experiments [30].…”

“…Fig. 3 Measured surface temperatures [30] and computed counterparts simulated with and without thermocapillarity 4.1 Sensitivity Analysis. A sensitivity analysis is carried out to see the effect of the droplet radius and the volumetric flow rate, two main parameters that affect the temperature field, which are prone to measurement uncertainties.…”

“…The presence of convection cells within the droplet suggests that conduction is not the sole mechanism carrying energy to the droplet surface. Substantial experimental effort has been devoted to quantify the contribution of internal convection to the energy transport at the liquid/vapor interface of water [21,30,36,[39][40][41]. While the radial temperature measurements were used to determine the conduction heat transfer to the interface, the concept of surface thermal capacity was proposed and applied to establish the energy balance at an interface with tangential velocity due to thermocapillarity [39,40].…”

“…To address this observation, the preliminary model has been improved by adding the effects of thermocapillary flow and radiative heat transfer to the surroundings in this study. The model is, then, applied to a steadily fed spherical water droplet placed on a heated substrate [30], to enable a direct comparison of predicted and experimental interface temperature variation.…”