arXiv subjects

Zafer Dursunkaya

Publications and source records attributed to Zafer Dursunkaya.

6 recordsLinked to original sources

arXiv2022

Interplay of capillary and Marangoni flows in micropillar evaporation

The evaporation from a micropillar evaporator is a problem governed by various interfacial phenomena such as the capillarity-induced liquid flow, thin-film evaporation intensifying near the contact lines, and thermocapillarity-induced Marangoni flow. However, past research has not been able to assess the effect of Marangoni flow due to the missing coupling between cell- and device-level modeling. In this work, we develop a comprehensive model for the evaporation from a micropillar evaporator by coupling the liquid flow with the energy transfer in both liquid and solid domains at both cell- and device-levels. The model is successfully validated with previous experiments. When the Marangoni number is sufficiently high, the model identifies a sharp reduction in the evaporator temperature attributed to the thermocapillary convection creating circulations beneath the liquid-vapor interface, which results in the formation of periodic reverse surface flows on the interface. This temperature drop cannot be identified when thermocapillarity is switched off in the model and the model's prediction substantially deviates from experimental measurement. Therefore, the current study reveals a hitherto unexplored role of Marangoni flow in the evaporation of water from micropillar evaporators.

physics.flu-dyn↗

arXiv2021

Interplay of transport mechanisms during the evaporation of a pinned sessile water droplet

Droplet evaporation has been intensively investigated in past decades owing to its emerging applications in diverse fields of science and technology. Yet the role transport mechanisms has been the subject of a heated debate, especially the presence of Marangoni flow in water droplets. This work aims to draw a clear picture of the switching transport mechanisms inside a drying pinned sessile water droplet in both the presence and absence of thermocapillarity by developing a comprehensive model that accounts for all pertinent physics in both phases as well as interfacial phenomena at the interface. The model reveals a hitherto unexplored mixed radial and buoyant flow by shedding light on the transition from buoyancy induced Rayleigh flow to the radial flow causing coffee ring effect. Predictions of the model excellently match previous experimental results across varying substrate temperatures only in the absence of Marangoni flow. When thermocapillarity is accounted for, strong surface flows shape the liquid velocity field during most of the droplet lifetime and the model starts to overestimate evaporation rates with increasing substrate temperature.

physics.flu-dyn↗

arXiv2020

The effect of surface morphology on the rate of phase change of micron and sub-micron sized 2-D droplets

Heat transfer via phase change is a major contributor to heat removal in numerous engineering applications. Thin films of liquid result in increased heat transfer due to a reduction of conduction resistance, in addition the pressure jump at the liquid-vapor interface also affects the rate and direction of rate of phase change. Because of these effects the morphology of the substrate surface is expected to affect the film shape, hence heat transfer, especially in thin films. In this study, the influence of surface characteristics on the rate of phase change from micron and sub-micron sized 2-D droplets-i.e. films extending to infinity-forming on a substrate are modeled. Surface film profiles are generated on both flat and non-flat surfaces, triangular or wavy in nature, and a kinetic model for quasi-equilibrium phase change is applied. Due to the presence of intermolecular forces at the contact line, which render the solution of the augmented Young-Laplace equation stiff, an implicit scheme is employed for the numerical integration. To verify the method, the predictions of a molecular dynamics (MD) simulation of a nano sized droplet present on a V-grooved surface is compared to the continuum model. The augmented Young-Laplace equation is solved numerically along with a phase change model originating from kinetic theory to calculate the shape of the two-phase interface forming the droplet and study the effect of various parameters on the rate of phase change. Results are obtained for droplets with liquid pressures higher and lower than that of vapor, resulting in opposite contribution to phase change due to the pressure jump at the interface. The results show that the heat transfer rate can be substantially altered due primarily to the combined effects of surface morphology and disjoining pressure.

physics.app-ph↗

arXiv2020

A theoretical framework for comprehensive modeling of steadily fed evaporating droplets and the validity of common assumptions

A theoretical framework is established to model the evaporation from continuously fed droplets, promising tools in the thermal management of high heat flux electronics. Using the framework, a comprehensive model is developed for a hemispherical water droplet resting on a heated flat substrate incorporating all of the relevant transport mechanisms: buoyant and thermocapillary convection inside the droplet and diffusive and convective transport of vapor in the gas domain. At the interface, mass, momentum, and thermal coupling of the phases are also made accounting for all pertinent physical aspects including several rarely considered interfacial phenomena such as Stefan flow of gas and the radiative heat transfer from interface to the surroundings. The model developed utilizes temperature dependent properties in both phases including the density and accounts for all relevant physics including Marangoni flow, which makes the model unprecedented. Moreover, utilizing this comprehensive model, a nonmonotonic interfacial temperature distribution with double temperature dips is discovered for a hemispherical droplet having internal convection due to buoyancy in the case of high substrate temperature. Proposed framework is also employed to construct several simplified models adopting common assumptions of droplet evaporation and the computational performance of these models, thereby the validity of commonly applied simplifying assumptions, are assessed. Benchmark simulations reveal that omission of gas flow, i.e. neglecting convective transport in gas phase, results in the underestimation of evaporation rates by 23-54%. When gas flow is considered but the effect of buoyancy is modeled using Boussinesq approximation instead of assigning temperature dependent density throughout the gas domain, evaporation rate can be underestimated by up to 16%.

physics.app-ph↗

arXiv2020

On the effect of structural forces on a condensing film profile near a fin-groove corner

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.

physics.app-ph↗

arXiv2018

The effect of disjoining pressure on the shape of condensing films in a fin-groove corner

Thin film condensation is commonly present in numerous natural and artificial processes. Phase-change driven passive heat spreaders such as heat pipes, which are widely used in electronics cooling, employ a continuous condensation process at the condenser region. When the wick structure of a heat pipe is composed of grooves, the top surfaces of the walls (fins) located between consecutive grooves function as the major source of condensation and the condensate flows along the fin top into the grooves. Modeling of this condensation problem is vital for the proper estimation of condensation heat transfer, which constitutes the basis for the overall performance of an heat pipe together with the evaporation process. In the current study, a solution methodology is developed to model the condensation and associated liquid flow in a fin-groove system. Conservation of mass and momentum equations, augmented Young-Laplace equation and Kucherov-Rikenglaz equation are solved simultaneously to calculate the film thickness profile. The model proposed enables the investigation of the effect of disjoining pressure on the film profile by keeping the fin-groove corner, where the film becomes thinnest, inside the solution domain. The results show that dispersion forces become effective for near isothermal systems with sharp fin-groove corners and the film profile experiences an abrupt change, a slope break, in the close proximity of the corner. The current study is the first computational confirmation of this behavior in the literature.

physics.app-ph↗