The Visualization of Thin Film Evaporation on Thin Micro Sintered Copper Mesh Screen

2008 ◽  
Author(s):  
Chen Li ◽  
G. P. Peterson ◽  
Ji Li ◽  
Nikhil Koratkar
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Heat Flux ◽  
Thin Liquid Film ◽  
Heated Wall ◽  
Thin Film Evaporation ◽  
Film Evaporation ◽  
Sintered Copper ◽  
Mesh Screen ◽  
Copper Mesh

The thin film evaporation process through use of thin micro-scale sintered copper mesh screen was proven to be a very effective heat transfer mechanism with high critical heat flux (CHF). This efficient heat transfer mechanism is widely used in designing heat pipe, Capillary Pumped Loops (CPL), and drying process, however, the nucleation process and meniscus dynamics at the liquid-vapor-solid interface are not directly observed and systematically studied. Very few visual investigation in thin film evaporation has been conducted. In the existing two visual studies, the interface thermal resistance between coating and the heated wall was not seriously considered, and the heat flux was limited below 35 W/cm2. In this visualization investigation, the nucleation process and meniscus dynamics from initial condition to drying out were observed and well documented. To minimize the interface thermal resistance, the micro scale wicking was sintered to heated wall directly. High quality images were acquired through a well-designed visualization system. The majority of nucleate bubbles, whose diameters are at a magnitude of 10 μm, were found to form on the top wire surfaces instead of inside the porous media at moderate heat flux. Few large size bubbles were observed to grow inside capillary wicks, however, their presence did not seem to stop the evaporation process as reported before. The menisci receding process was visually captured for the first time. The minimum menisci radius was found to form at the smallest corners and pores. It is also illustrated the thin liquid area increases when the menisci recede and the thin liquid film evaporation is the dominant heat transfer mode at high heat flux. The present work visually confirms the heat transfer regimes of evaporation on micro porous media, which was proposed by Li and Peterson [2], and further improves the understanding to the nucleate boiling and thin liquid film evaporation on the surfaces of micro sintered copper mesh screen.

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.

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.

Characteristics of Extended Evaporating Meniscus on Nanotextured Rough Solid Substrate for Wenzel States

2011 ◽  
Author(s):  
J. J. Zhao ◽  
Y. Y. Duan ◽  
X. D. Wang ◽  
B. X. Wang
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Liquid Film ◽  
Thin Liquid Film ◽  
Disjoining Pressure ◽  
Roughness Effects ◽  
Maximum Heat ◽  
Thin Film Evaporation ◽  
Film Evaporation ◽  
Nanoscale Roughness

The surface nanostructure determines the system wettability and thus has significant effects on the thin liquid film spreading and phase change heat transfer. A model based on the augmented Young-Laplace equation and kinetic theory was developed to describe the nanoscale roughness effects on the extended evaporating meniscus in a microchannel. The roughness geometries in the model were theoretically related to the disjoining pressure and the thermal resistance across the roughness layer. The results show that the dispersion constant for the disjoining pressure increases with the nanopillar height when the solid-liquid-vapor system is in the Wenzel state. Thus, the spreading and wetting properties of the evaporating thin liquid film are enhanced due to the higher nanopillar height and larger disjoining pressure. Since the evaporating thin film length increases with the nanoscale roughness due to better surface wettability, the total liquid flow and heat transfer rate of the evaporating thin liquid films in a microchannel can be enhanced by increasing the nanopillar height. The effects of the nanopillar on the thin film evaporation are more significant for higher superheats. Hydrophilic nanotextured solid substrates can be fabricated to enhance the thin film evaporation and thus increase the maximum heat transport capability of the two-phase cooling devices.

Mechanisms of Heat and Mass Transfer for Thin-Film Evaporation With Velocity Slip and Temperature Jump

2019 ◽  
Author(s):  
Xiu Xiao ◽  
Chunji Yan ◽  
Yulong Ji
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Heat Flux ◽  
Temperature Jump ◽  
Slip Length ◽  
Velocity Slip ◽  
Thin Film Evaporation ◽  
Film Evaporation ◽  
Solid Liquid Interface ◽  
Solid Liquid

Abstract Velocity slip and temperature jump at the solid-liquid interface are important phenomena in microchannel heat transfer. A comprehensive mathematical model considering both velocity slip condition and temperature jump at the solid-liquid interface is developed to understand the mechanisms of heat and mass transfer during thin-film evaporation in this paper. The model structure is established based on the lubrication theory, Clausius-Clapeyron equation and Young-Laplace equation. To better formulate the film evaporation process, three dimensionless parameters representing the effects of slip length coefficient, temperature jump and wall superheat degree respectively, are introduced in the present model. The analytical solution provides insight of film thickness and heat transfer characteristics for the evaporating thin film. It shows that as the slip length and temperature jump coefficient decrease, the length of evaporating thin film region is shortened and the location of maximum heat flux moves closer to the initial evaporating point. The effect of slip condition on heat flux is small, but the increase of temperature jump can reduce the peak heat flux significantly. Furthermore, the analysis on the three thermal resistances which are caused by temperature jump, conduction through liquid film and evaporation on liquid-vapor interface result in a better understanding for effective heat transfer during thin-film evaporation.

Heat Transfer Analysis of Phase Change Microcapsules With Thin Film Evaporation

2016 ◽  
Author(s):  
Yang Guo ◽  
Benwei Fu ◽  
Yulong Ji ◽  
Fengmin Su ◽  
Corey Wilson ◽  
...  
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Phase Change ◽  
Thin Liquid Film ◽  
Water Film ◽  
Heat Transfer Analysis ◽  
Seawater Desalination ◽  
Thin Film Evaporation ◽  
Film Evaporation ◽  
Evaporation Heat

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

Theoretical Analysis of Thin Film Evaporation in the Wicks of Loop Heat Pipes

2021 ◽  
pp. 1-14
Author(s):  
Bingyao Lin ◽  
Nanxi Li ◽  
Shiyue Wang ◽  
Leren Tao ◽  
Guangming Xu ◽  
...  
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Pore Size ◽  
Momentum Conservation ◽  
Loop Heat Pipes ◽  
Thin Film Evaporation ◽  
Film Evaporation ◽  
Quantity Of Heat

Abstract In this paper, a thin film evaporation model that includes expressions for energy, mass and momentum conservation was established through the augmented Young-Laplace model. Based on this model, the effects of pore size and superheating on heat transfer during thin film evaporation were analyzed. The influence of the wick diameter of the loop heat pipe (LHP) on the critical heat flux of the evaporator is analyzed theoretically. The results show that pore size and superheating mainly influence evaporation through changes in the length of the transition film and intrinsic meniscus. The contribution of the transition film area is mainly reflected in the heat transfer coefficient, and the contribution of the intrinsic meniscus area is mainly apparent in the quantity of heat that is transferred. When an LHP evaporator is operating in a state of surface evaporation, a higher heat transfer coefficient can be achieved using a smaller pore size.

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.

Approach to Heat Transfer Mechanism Beneath Boiling Bubble With MEMS Sensor

2009 ◽  
Author(s):  
Tomohide Yabuki ◽  
Osamu Nakabeppu
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Heat Flux ◽  
Heated Surface ◽  
Bubble Nucleation ◽  
Temperature Data ◽  
Heated Wall ◽  
Measured Temperature ◽  
Back Side ◽  
Mems Sensor

Temperature variation beneath isolated bubble during saturated boiling of water was measured with a MEMS (Micro-Electro-Mechanical Systems) sensor having high temporal and spatial resolution. Then, local heat transfer from the heated surface was evaluated by a transient heat conduction analysis of the wall with measured temperature data as a boundary condition. The MEMS sensor on a 20 × 20 mm2 silicon substrate includes an electrolysis trigger and eight thin film thermocouples on the top side, and two thin film heaters on the back side. The thin film thermocouple was calibrated with a thermal scan method using two alloy samples with different melting point. The condition of the sensor was smoothly controlled with the heater. The bubble is initiated with electrolysis at a gap of the trigger electrode, where slight hydrogen gasses are supplied as bubble nuclei. Then, local and fast temperature variations in wide region are measured with the thermocouples with cutoff frequency of 100 kHz arranged in a line at 40 – 2000 μm far from the trigger gap. Measured temperature data presents formation of microlayer and expansion of dryout area in bubble growth process and rewetting in bubble departure process. The numerical analysis showed that average heat flux beneath the bubble indicated the maximum value of 19 W/cm2 during the microlayer evaporation, and then after hitting a bottom slightly lower than a heat flux at the bubble nucleation, recovers to the nucleation level. The contribution of the heat transfer from the heated wall was evaluated to approximately one-fourth of latent heat in the bubble at departure.

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