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Improvement of Wetting Characteristics for Evaporating Meniscus Using a Binary Mixture


In order to explore an improvement of capillary pore stability and its heat transfer augmentation, a noble idea of using a binary mixture of two fluids with different surface energies is being experimentally investigated. The idea is that the preferred evaporation of one fluid of low surface energy can establish a surface tension gradient by the mixture concentration gradient and possibly compensate the well -known thermocapillary stress gradient occurring from the temperature gradient on the liquid-vapor interface. Preliminary evidence shows a noticeable delay of the instability occurrence with enhanced heat transfer rate. 


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Thermocapillary-Driven Capillary Pore Flows - Quantitative Visualization Study


In a micro-scale heat pipe transport device such as in a capillary pore, both density variation and surface tension gradients at the liquid-vapor interface prevails the detailed physics of fluid and heat transfer. For most pure liquids, the surface tension decreases with increasing temperature. Therefore, if temperature gradient exists, thermocapillary forces induce flows from a hot to a cold region. And in the bulk fluid region, buoyancy driven flows also occur due to the density gradient. To maintain the loop of heat transport phenomena, amount of liquid must be supplied to compensate the amount of evaporation on the meniscus. Otherwise, heat transport device loop will fail by dry-out of the meniscus. So understanding of the flows inside liquid region is essential. Non-intrusive PIV (Particle Image Velocimetry) technique has been used to measure the flow field inside a capillary pore. Fluorescence trace particles are being considered to eliminate the blurred image particularly near the solid surface.


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Thermocapillary-Driven Capillary Pore Flows - Numerical Study


The object of this study lies in both its contributions to engineering applications involving two-phase heat transfer in a capillary pore and its advancement to the understanding of the micro fluid behavior. Capillary Driven Heat Transfer Devices (CPHTD) have the capability of transmitting heat at a high rate over a large distance with little temperature change. Heat addition to the evaporator results in evaporation from the liquid. And the vapor flows to the condenser where the removal of heat causes the vapor to condense. The liquid is driven back to the evaporator due to capillarity. This research is conducted  to investigate the flow field and a temperature field in a capillary pore and how the meniscus changes with the surface temperature gradient and evaporation. Due to the curved meniscus and cylindrical shape, Body Fitted Coordinate (BFC) is used for grid generation. Program is made on the basis of Finite Volume Method (FVM).  

Capillary Vector01 Capillary Temperature

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Fundamental Energy Transport of Nanofluids


Nanofluids have been developed in past years and researches to examine their dramatic improvement of thermal conductivities are of growing interest. At present, however, a quantitative understanding of the fundamental mechanisms as to the energy transport in nanofluids is NOT discovered yet. Most importantly, the fundamentals of energy transport in nanofluids are necessarily understood in order to develop extremely energy-efficient nanofluids for a variety of heat transfer applications. As one of key energy transport mechanisms, the research for nanoparticle-mobility-enhanced energy transport in temperature-dependent thermal conductivities of nanofluids will lead to new knowledge about thermal conductivity enhancement. Narrowly speaking, Brownian motion between nanoparticles and base fluid molecules is a key mechanism for enhanced thermal behavior of nanofluid. Particle size, temperature of base fluid, particle concentration, and surfactants are closely related to Brownian motion between particles molecules of base fluid. Our research for nanofluids is to have those fundamental nanofluid thermal characteristics unrevealed using the relationship of Brownian motion and thermal conductivity under different related effects by using experimental heat transport and optical techniques.


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Microscale Observables for Heat and Mass Transport in Sub-Micron Scale Evaporating Thin Film


A mathematical model is developed to describe the micro/nano-scale fluid flow and heat/mass transfer phenomena in an evaporating extended meniscus, focusing on the transition film region under nonisothermal interfacial conditions. The model incorporates thermocapillary stresses at the liquid-vapor interface, a slip boundary condition on the solid wall, polarity contributions to the working fluid field, and binary mixture evaporation. The analytical results show that the adsorbed film thickness and the thin film length decrease with increasing superheat by the thermocapillary stresses, which influences detrimentally the evaporation process by degrading the wettability of the evaporating liquid film. In contrast, the slip effect and the binary mixture enhance the stability of thin film evaporation. The slip effect at the wall makes the liquid in the transition region flow with smaller flow resistance and thus the length of the transition region increases. In addition, the total evaporative heat flow rate increases due to the slip boundary condition. The mixture of pentane and decane increases the length of the thin film by counteracting the thermocapillary stress, which enhances the stability of the thin film evaporation. The polarity effect of water significantly elongates the thin film length due to the strong adhesion force of intermolecular interaction. The strong interaction force restrains the liquid from evaporation for a polar liquid compared to a non-polar liquid.


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