An experimental investigation was conducted to determine the thermal behavior of arrays of micro heat pipes fabricated in silicon wafers. Two types of micro heat pipe arrays were evaluated, one that utilized machined rectangular channels and the other that used an anisotropic etching process to produce triangular channels. Once fabricated, a clear pyrex cover plate was bonded to the top surface of each wafer using an ultraviolet bonding technique to form the micro heat pipe array. These micro heat pipe arrays were then evacuated and charged with a predetermined amount of methanol. Using an infrared thermal imaging unit, the temperature gradients and maximum localized temperatures were measured and an effective thermal conductivity was computed. The experimental results were compared with those obtained for a plain silicon wafer

Experimental Investigation of Micro HeatPipes Fabricated in Silicon Wafers

A lithium-ion (Li-ion) power battery and proton exchange membrane fuel cell (PEMFC) have an excellent electrical conversion performance, which makes them a hot spot in the new energy power research...

A Review of MHP Technology and Its Research Status in Cooling of Li-Ion Power Battery and PEMFC

A fundamental understanding of the evaporation/condensation phenomena is vital to many fields of science and engineering, yet there is much discrepancy in the usage of phase change models and associated coefficients. First, a brief review of kinetic theory of phase change is provided, and the mass accommodation coefficient (MAC, $\alpha$) 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 ($\alpha_{app}^{Schrage}$ and $\alpha_{exact}^{Schrage}$) are between 0.8 and 0.9, while MAC values from the H-K expression ($\alpha^{H-K}$) are above unity for all cases tested. $\alpha_{exact}^{Schrage}$ yields values 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 $\alpha_{exact}^{Schrage}$ but causes $\alpha^{H-K}$ to vary drastically emphasizing the importance of a drift velocity correction. Additionally, equilibrium departure causes a non-uniform distribution in vapor properties. At the condensing interface, a local rise in vapor temperature and a drop in vapor density are observed when compared with the corresponding bulk values.

Drifting mass accommodation coefficients: in situ measurements from a steady state molecular dynamics setup.

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 ass...

Drifting mass accommodation coefficients: in situ measurements from a steady state molecular dynamics setup

http://repository.bilkent.edu.tr/bitstream/11693/111683/1/Capillary_boosting_for_enhanced_heat_pipe_performance_through_bifurcation_of_grooves_numerical_assessment_and_experimental_validation.pdf

Capillary boosting for enhanced heat pipe performance through bifurcation of grooves: Numerical assessment and experimental validation

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

Transport mechanisms inside a drying water droplet are studied by a comprehensive model that accounts for all pertinent physics in liquid and gas phases both with and without thermocapillarity. Without thermocapillarity, a single Rayleigh cell drives convection inside the droplet at higher contact angles. With decreasing contact angle, the Rayleigh cell gets smaller and is replaced by the radial flow through a transition stage, where the rotating cell and radial flow coexist. Both thermocapillarity and buoyancy-driven drying processes end with a radial flow, which creates the well-known coffee-ring effect, but it starts at a much earlier phase of droplet lifetime for buoyancy-driven drying.

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

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

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.

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.

Osman Akdag

Papers

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

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

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

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

Modeling the evaporation of drying sessile droplets with buoyancy driven internal convection