scholarly journals Experimental Characterization and Modeling of Capillary-Pumped Thin-Film Evaporation From Micropillar Wicks

2016 ◽  
Author(s):  
Solomon Adera ◽  
Dion Antao ◽  
Rishi Raj ◽  
Evelyn N. Wang
Keyword(s):  
Thin Film ◽  
Phase Change ◽  
Thermal Management ◽  
Thermal Transport ◽  
Nucleate Boiling ◽  
Working Fluid ◽  
Potential Candidate ◽  
Experimental Characterization ◽  
Thin Film Evaporation ◽  
Film Evaporation

Generation of concentrated heat load in confined spaces in integrated circuits and advanced microprocessors has presented a thermal management challenge for the semiconductor industry. Compared to state-of-the-art single phase cooling strategies, phase-change based approaches are promising for cooling the next generation microelectronic devices. In particular, thin-film evaporation from engineered surfaces has received significant attention in the last few decades as a potential candidate since it enables passive transport of the working fluid via capillarity in addition to increasing the evaporation area via extending the liquid meniscus and three-phase contact line. Thin-film evaporation, however, is coupled with nucleate boiling making experimental characterization as well as modeling of the fluidic and thermal transport a challenging task for thermal engineers. Furthermore, quantifying the relative contributions of nucleate boiling and thin-film evaporation from the experimentally reported heat fluxes has been difficult. Unlike previous studies, our work experimentally characterizes thin-film evaporation in the absence of nucleate boiling from arrays of silicon micropillars. In particular, we characterize the capillary-limit where the microstructured surface dries out due to liquid starvation when the capillary pressure that is generated due to the meniscus shape cannot overcome the viscous losses within the micropillar wick. We modeled the fluidic and thermal transport of the evaporating meniscus by solving the coupled heat and mass transfer equations. Compared to experiments, our model predicts the dryout heat flux with ±20% accuracy. The insights gained from this study provide a suitable platform to design and optimize micropillar wicks for phase-change based thermal management devices such as heat pipes and vapor chambers.

scholarly journals Capillary-Limited Evaporation From Well-Defined Microstructured Surfaces

2013 ◽  
Author(s):  
Solomon Adera ◽  
Rishi Raj ◽  
Evelyn N. Wang
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Heat Flux ◽  
Phase Change ◽  
Thermal Management ◽  
Latent Heat ◽  
Thin Film Evaporation ◽  
Film Evaporation ◽  
Microstructured Surfaces ◽  
Latent Heat Of Vaporization

Thermal management is increasingly becoming a bottleneck for a variety of high power density applications such as integrated circuits, solar cells, microprocessors, and energy conversion devices. The performance and reliability of these devices are usually limited by the rate at which heat can be removed from the device footprint, which averages well above 100 W/cm2 (locally this heat flux can exceed 1000 W/cm2). State-of-the-art air cooling strategies which utilize the sensible heat are insufficient at these large heat fluxes. As a result, novel thermal management solutions such as via thin-film evaporation that utilize the latent heat of vaporization of a fluid are needed. The high latent heat of vaporization associated with typical liquid-vapor phase change phenomena allows significant heat transfer with small temperature rise. In this work, we demonstrate a promising thermal management approach where square arrays of cylindrical micropillar arrays are used for thin-film evaporation. The microstructures control the liquid film thickness and the associated thermal resistance in addition to maintaining a continuous liquid supply via the capillary pumping mechanism. When the capillary-induced liquid supply mechanism cannot deliver sufficient liquid for phase change heat transfer, the critical heat flux is reached and dryout occurs. This capillary limitation on thin-film evaporation was experimentally investigated by fabricating well-defined silicon micropillar arrays using standard contact photolithography and deep reactive ion etching. A thin film resistive heater and thermal sensors were integrated on the back side of the test sample using e-beam evaporation and acetone lift-off. The experiments were carried out in a controlled environmental chamber maintained at the water saturation pressure of ≈3.5 kPa and ≈25 °C. We demonstrated significantly higher heat dissipation capability in excess of 100 W/cm2. These preliminary results suggest the potential of thin-film evaporation from microstructured surfaces for advanced thermal management applications.

Thermal Conductivity and Operating Temperature Effect on the Interline Region in a Micro/Miniature Heat Pipe

2008 ◽  
Author(s):  
Hani H. Sait ◽  
Steve M. Demsky ◽  
HongBin Ma
Keyword(s):  
Thin Film ◽  
Thermal Conductivity ◽  
Heat Flux ◽  
Operating Temperature ◽  
High Heat ◽  
Working Fluid ◽  
High Heat Flux ◽  
Thin Film Evaporation ◽  
Film Evaporation ◽  
Frictional Shear Stress

An analytical model describing thin film evaporation is developed that includes the effects of surface tension, frictional shear stress, wetting characteristics and disjoining pressure. The effects of thermal conductivity of working fluids and operating temperature on the evaporating thin film region are also studied. The results indicate that when the thermal conductivity of the working fluid increases, a high heat flux can be removed from the evaporating thin film region. The operating temperature affects the thin film evaporation. The higher the operating temperature, the more heat flux can be removed from the region. The information of thin film evaporation presented in the paper results in a better understanding of heat transfer mechanism occurring in micro heat pipes.

Heat Transfer Analysis of Flash Evaporation With MEPCM

2019 ◽  
Vol 11 (5) ◽  
Author(s):  
Yang Guo ◽  
Hongbin Ma ◽  
Benwei Fu ◽  
Yulong Ji ◽  
Fengmin Su ◽  
...  
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Phase Change ◽  
Thin Liquid Film ◽  
Heat Transfer Analysis ◽  
Seawater Desalination ◽  
Flash Evaporation ◽  
Thin Film Evaporation ◽  
Film Evaporation ◽  
Evaporation Heat

Several seawater desalination technologies have been developed and widely used during the last four decades. In the current investigation, a new approach to the seawater desalination process is presented, which utilizes microencapsulated phase change materials (MEPCMs) and thin film evaporation. In this process, the MEPCMs were placed into hot seawater. Then, the hot seawater and the MEPCMs containing the liquid phase change material (PCM) were ejected into a vacuum flash chamber. A thin liquid film of seawater was formed on the surface of the MEPCM, which subsequently vaporized. This evaporation significantly increased the evaporation heat transfer and enhanced the desalination efficiency. Film evaporation on MEPCM surfaces decreased their temperature by absorbing sensible heat. If their temperature was lower than the phase change temperature, the MEPCM would change phase from liquid to solid releasing the latent heat, resulting in further evaporation. The MEPCMs were then pumped back into the hot seawater, and the salt residue left on the MEPCMs could be readily dissolved. In this way, the desalination efficiency could be increased and corrosion reduced. A mathematical model was developed to determine the effects of MEPCM and thin film evaporation on desalination efficiency. An analytical solution using Lighthill's approach was obtained. Results showed that when MEPCMs with a radius of 100 µm and a water film of 50 µm were used, the evaporation rate and evaporative capacity were significantly increased.

Measurement of Thin-Film Evaporation Heat Transfer for Evaporators with Sintered Powder Wick Structure

2012 ◽  
Vol 579 ◽  
pp. 379-386
Author(s):  
Cho Han Lee ◽  
Yao Yang Tsai
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Phase Change ◽  
Stable Condition ◽  
Major Phase ◽  
Thin Film Evaporation ◽  
Film Evaporation ◽  
Evaporation Heat ◽  
Sintered Powder ◽  
Wick Structure

Two-phase heat transfer devices such as heat pipes and vapor chambers are composed of an evaporator, an adiabatic section and a condenser. For the dry-out prevention and capillary purpose, adiabatic sections and evaporators are covered by wick structures. Common wick structures are grooves, mesh, sintered powder and their combination. Combining with the wick structures, the major phase change effects on evaporators are thin-film evaporation. For the research between parameters of wick structure and evaporator performance, we developed a facility to measure the heat transfer on evaporators. To ensure the least heat losing, the path of heat flux and test condition were designed with several thermal guards. A pressure control system was established with balance mechanisms to maintain a stable condition of low pressure. Since temperature differences are very fast while the major phase change effect is thin-film evaporation, a high speed data acquisition system was used. Based on this test platform, the performance of evaporators can be determined at specific conditions.

scholarly journals Thin Film Evaporation Using Nanoporous Membranes for Enhanced Heat Transfer

2012 ◽  
Author(s):  
Rong Xiao ◽  
Shalabh C. Maroo ◽  
Evelyn N. Wang
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Thermal Management ◽  
High Heat ◽  
Heat Fluxes ◽  
Nanoporous Membranes ◽  
Transfer Coefficients ◽  
Thin Film Evaporation ◽  
Heat Transfer Coefficients ◽  
Film Evaporation

Recent advancements in integrated circuits demand the development of novel thermal management schemes that can dissipate ultra-high heat fluxes with high heat transfer coefficients. Previous study demonstrated the potential of thin film evaporation on micro/nanostructured surfaces [1–11]. Theoretical calculations indicate that heat transfer coefficients on the order of 106 W/m2K and heat fluxes of 105 W/cm2 can be achievable with water [1, 5–6]. However, in previous experimental setup, the coolant has to propagate across the surface which limits the increase in heat flux and the heat transfer coefficient, while adding complexity to the system design. This work aims to decouple the propagation of the coolant from the evaporation process through a novel experimental configuration. Thin nanoporous membranes of 13 mm diameter were used where a metal layer was deposited on the top surface to serve as a resistance heater. Liquid was supplied from the bottom of the membrane, driven through the nanopores by capillary force, and evaporated from the top surface. Heat transfer coefficient over 104 W/m2K was obtained with isopropyl alcohol (IPA) as the coolant, which is only two orders of magnitude smaller than the theoretical limit. This work offers insights into optimal experimental designs towards achieving kinetic limits of heat transfer for thin film evaporation based thermal management solutions.

Microengineered Surfaces for Thin Film Evaporation for Enhanced Heat Dissipation

2009 ◽  
Author(s):  
Rong Xiao ◽  
Evelyn N. Wang
Keyword(s):  
Thin Film ◽  
Liquid Film ◽  
Thermal Management ◽  
Heat Dissipation ◽  
Thin Liquid Film ◽  
High Heat ◽  
Heat Fluxes ◽  
Thin Film Evaporation ◽  
Film Evaporation ◽  
Micropillar Arrays

The increasing performance of integrated chips has introduced a growing demand for new thermal management technologies. While various thermal management schemes have been studied, thin film evaporation promises high heat dissipation rates (1000 W/cm2) with low thermal resistances. However, methods to form a thin liquid film including jet impingement and sprays have challenges associated with achieving the desired film thickness. In this work, we investigated novel microstructures to control the thickness of the thin film where the liquid is driven by capillarity. Micropillar arrays with diameters ranging from 2 μm to 10 μm, spacings between pillars ranging from 5 μm to 10 μm, and heights of 4.36 μm were studied. A semi-analytical model was developed to predict the propagation rate of the liquid film, which was validated with experiments. The heat transfer performance was investigated on the micropillar arrays with microfabricated heaters and temperature sensors. The behavior of the thin liquid film under varying heat fluxes was studied. This work demonstrates the potential of micro- and nanostructures to dissipate high heat fluxes via thin film evaporation.

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