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Droplet evaporation is a fundamental phenomenon encountered in diverse applications such as inkjet printing, DNA mapping, film coating, and electronics cooling. Modeling the evaporation process of a sessile droplet is complicated because of the coupling of several physical phenomena occurring in different phases and various magnitudes such as the buoyant convection of the liquid in millimeter size droplets and that of the surrounding air/water vapor mixture, in the order of meters. In this study, the theoretical framework presented previously for the steadily fed droplets [Int J Therm Sci, 158 (2020) 106529] is extended to resolve the evaporation of drying droplets with a pinned contact line. Based on the quasi-steady-state assumption, buoyant convection inside the droplet and diffusive-convective transport of vapor in the gas domain are modeled. As a test case, drying process of a water droplet with a 68° initial contact angle on a heated substrate is simulated and the predictions of the model are interpreted.

For the assessment of the thermal performance of heat pipes, a wide range of modeling is available in the literature, ranging from simple capillary limit analyses to comprehensive 3D models. While simplistic models may result in less accurate predictions of heat transfer and operating temperatures, comprehensive models may be computationally expensive. In this study, a universal computational framework is developed for a fast but sufficiently accurate modeling of traditional heat pipes, and an analysis tool based on this framework, named Heat Pipe Analysis Toolbox, in short H-PAT is presented. As a diagnostic tool, H-PAT can predict the fluid flow and heat transfer from a heat pipe under varying heat inputs up to the onset of dryout. During the initial estimation of phase change rates, the solutions of particular thin film phase change models are avoided by specifying an appropriate pattern for the mass flow rate of the liquid along the heat pipe rather than using finite element/volume based methods for the computational domain. With the help of a modular structure, H-PAT can simulate heat pipes with different wick structures as long as an expression for the average liquid velocity and corresponding pressure drop can be introduced. H-PAT is also capable of analyzing heat pipes with variable cross-sections, favorable/unfavorable gravity conditions and utilizes temperature dependent thermo-physical properties at evaporator, condenser and adiabatic regions together with heat input sensitive vapor pressure. In addition, H-PAT performs the computation very fast which also makes it a perfect design tool for researchers and design engineers in the field of thermal management. [ABSTRACT FROM AUTHOR]

A fundamental understanding of the evaporation/condensation phenomena is vital to many fields of science and engineering, yet there is many discrepancies in the usage of phase-change models and associated coefficients. First, a brief review of the kinetic theory of phase change is provided, and the mass accommodation coefficient (MAC, α) and its inconsistent definitions are discussed. The discussion focuses on the departure from equilibrium; represented as a macroscopic "drift" velocity. Then, a continuous flow, phase change driven molecular-dynamics setup is used to investigate steady-state condensation at a flat liquid-vapor interface of argon at various phase-change rates and temperatures to elucidate the effect of equilibrium departure. MAC is computed directly from the kinetic theory-based Hertz–Knudsen (H-K) and Schrage (exact and approximate) expressions without the need for a priori physical definitions, ad-hoc particle injection/removal, or particle counting. MAC values determined from the approximate and exact Schrage expressions ( α a p p S c h r a g e and α e x a c t S c h r a g e ) are between 0.8 and 0.9, while MAC values from the H-K expression ( α H − K ) are above unity for all cases tested. α e x a c t S c h r a g e yield value closest to the results from transition state theory [J Chem Phys, 118, 1392–1399 (2003)]. The departure from equilibrium does not affect the value of α e x a c t S c h r a g e but causes α H − K to vary drastically emphasizing the importance of a drift velocity correction. Additionally, equilibrium departure causes a nonuniform distribution in vapor properties. At the condensing interface, a local rise in vapor temperature and a drop in vapor density is observed when compared with the corresponding bulk values. When the deviation from bulk values are taken into account, all values of MAC including α e x a c t S c h r a g e show a small yet noticeable difference that is both temperature and phase-change rate dependent. [ABSTRACT FROM AUTHOR]

Droplet evaporation has been widely studied in the literature due to its key role in various applications in science and industry. The problem of droplet evaporation involves various mechanisms in both liquid and vapor phases together with the interface separating them. Modeling of this multiphase problem is not straightforward thereof studied by many researchers but in every time a few different contributing mechanisms could be highlighted. One of the pieces of this puzzle is undoubtedly the Stefan flow, which is always present during the evaporation of a liquid to an insoluble surrounding gas, yet the number of studies exploring its individual contribution to the evaporation remain very restricted. In the current study, the effect of Stefan flow is assessed by employing a recent state-of-the-art model that accounts for all pertinent physics of droplet evaporation. Results reveal that Stefan flow can be responsible for 17% of total evaporation when the droplet is placed on a high temperature substrate. Moreover, it is shown that lower performance of diffusion based models (in gas phase) can be greatly enhanced by incorporating the effect of Stefan flow into the interfacial mass flux equation. In addition, performances of existing purely diffusion and diffusion and Stefan flow based correlations in the prediction of evaporation rates are elucidated. Last but not least, under varying humidity of the surrounding gas, contribution of individual transport mechanisms in gas phase to the total evaporation rate is found to be unaffected. Based on this result, it is hypothesized that contributions of Stefan flow and natural convection have a linear dependence on the contribution of sole diffusion. The current study clearly demonstrated that Stefan flow considerably enhances the evaporation rate of droplets, especially in the case of high substrate heating. Therefore, future studies on the topic should account for the Stefan flow during the modeling of droplet evaporation. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this study, we propose and investigate the employment of elliptical micropillars as a novel evaporator wick structure to boost the performance of capillarity-driven two-phase heat spreaders through high-fidelity numerical modeling that accounts for all pertinent transport mechanisms, including thermocapillary convection. Results reveal that transforming cylindrical micropillars to elliptical ones significantly enhances the overall evaporator performance by improving the hydraulic performance associated with decreased flow resistance and boosted capillary pumping and by improving the thermal performance associated with increased contact (triple) line length. Among the scenarios tested and elliptical aspect ratios swept, it is demonstrated that the heat capacity and heat transfer coefficient can be increased by up to 350% and 52%, respectively. Overall, this study paves the way for the utilization of stretched geometries in the capillary flow direction of pillar-type evaporators by using elliptical micropillars as a demonstrator showcase. [ABSTRACT FROM AUTHOR]

In this study, an enhanced heat pipe performance for grooved heat pipes has been demonstrated through capillary boosting with the introduction of the bifurcation of grooves. Wider grooves regularly branch to narrower grooves such that the total cross-sectional liquid flow area remains approximately the same. Following the computational framework drawn by a recently developed heat pipe analysis toolbox (H-PAT), we develop a numerical model for the heat pipes with tree-like groove architecture. Then we utilize the model to design a flat-grooved heat pipe with one step groove bifurcation at the evaporator. To verify our numerical findings, two heat pipes with and without groove bifurcation are manufactured and experimented under the same conditions. Experimental results show that the numerical model can predict the thermal performance quite accurately. The results reveal that groove bifurcation can be a viable option for a better thermal performance than that of heat pipes with standard grooved heat pipes with straight grooves which leads to at least 25% higher maximum heat transport capacity. The effect of number of branching on the temperature flattening across the heat pipe is also demonstrated for different evaporator lengths. [ABSTRACT FROM AUTHOR]

Systematic investigations of self-regulation of evaporating menisci in nanochannels are conducted as a function of the surface wettability under various applied heat flux conditions. The simulation system is designed to result in steady-state response so that a stable meniscus region can be produced. Non-equilibrium molecular dynamics simulations are performed for argon fluid in platinum channels. Depending on the surface wettability and the applied heat flux the meniscus can be in the pinned regime or it can recede inside the channel. Adsorbed layer formation becomes evident for the latter case. Higher wettability enables the formation of a thicker adsorbed layer reducing the radius of curvature of the meniscus and the overall evaporation rate in the channel. Adsorbed layer reduces the thermal resistance of the evaporator, providing a higher critical heat flux. While evaporation from the adsorbed layer is negligible for macroscopic systems, it can contribute up to 80% of the total evaporating mass in nanoscale systems. The current work provides insights into the capillary-driven thin-film evaporation in ultra-small channels and the findings are meaningful for next-generation thermal management systems. [ABSTRACT FROM AUTHOR]

Estimation of condenser performance of two-phase passive heat spreaders with grooved wick structures is crucial in the prediction of the overall performance of the heat spreader. Whilst the evaporation problem in micro-grooves has been widely studied, studies focusing on the condensation on fin-groove systems have been scarce. Condensation on fin-groove systems is actually a multi-scale problem. Thickness of the film near the fin-groove corner can decrease to nanoscale dimensions, which requires the inclusion of nanoscale effects into the modeling. While a few previous studies investigated the effect of dispersion forces, the effect of structural forces has never been considered in the thin film condensation modeling on fin-groove systems. The present study utilizes a disjoining pressure model which considers both dispersion and structural forces. The results reveal that structural forces are able to dominate dispersion forces in certain configurations. Consequently, by intensifying the disjoining pressure, structural forces lead to a sudden change of the film profile (slope break) for subcooling values which are relevant to engineering applications. [ABSTRACT FROM AUTHOR]

An energy-based liquid-vapor interface detection method is presented using molecular dynamics simulations of liquid menisci confined between two parallel plates under equilibrium and evaporation/condensation conditions. This method defines the liquid-vapor interface at the location where the average kinetic energy of atoms first exceeds the average potential energy imposed by all neighboring molecules. This definition naturally adapts to the location of the menisci relative to the walls and can properly model the behavior of the liquid adsorbed layers. Unlike the density cutoff methods frequently used in the literature that suffer from density layering effects, this new method gives smooth and continuous liquid-vapor interfaces in nanoconfinements. Surface tension values calculated from the equilibrium MD simulations match the Young-Laplace equation better when using the radius of curvatures calculated from this method. Overall, this energy-based liquid-vapor interface detection method can be used in studies of nanoscale phase change processes and other relevant applications.

Recent developments in fabrication techniques have enabled the production of nano- and Ångström-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 the adsorbed film, the transition film, and the intrinsic region are intertwined, is absent in the literature. Using the computational setup constructed, we first identified the detailed profile of a nanoscale evaporating interface and then 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 toward 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 steady-state evaporation and condensation at liquid-vapor interfaces located in capillary nanoconduits.