Effects of sintered powder structural thickness on thin-film evaporation heat transfer at low superheat levels

2013 ◽  
Vol 113 (13) ◽  
pp. 134903
Author(s):  
Cho Han Lee ◽  
Yao Yang Tsai
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Thin Film Evaporation ◽  
Film Evaporation ◽  
Evaporation Heat ◽  
Sintered Powder ◽  
Evaporation Heat Transfer ◽  
Low Superheat

Investigation of Thin Film Evaporation Limit in Single Screen Mesh Layer

2002 ◽  
Author(s):  
Y. X. Wang ◽  
G. P. Peterson
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Heat Pipes ◽  
Liquid Meniscus ◽  
Thin Film Evaporation ◽  
Film Evaporation ◽  
Evaporation Heat ◽  
Heat Transfer Limit ◽  
Evaporation Heat Transfer ◽  
Screen Mesh

Thin film evaporation heat transfer plays an extremely important role in capillary microstructures of the type used extensively in micro heat pipes, loop heat pipes and high-flux film heat spreaders. Because the formation of the liquid meniscus in the pore cell has a significant effect on the evaporation process occurring at the interface of the liquid meniscus, it is necessary to investigate the mechanisms and limitations of the phase-change phenomena occurring in the thin layer. In the current study, an analytical model, which combines the heat conduction in the wick layer with bubble formation mechanisms in the capillary structure, has been developed to determine the evaporation heat transfer limit. Temperature distribution, superheat, and heat flux distribution in the liquid meniscus area are investigated for a single layer of metal screen mesh. The wire diameter, the space between the wires and the contact conditions between the solid wall and mesh layer is shown to have a significant effect on the evaporation limit and capillary force. Results indicated that evaporation takes place mainly in the thin film region, and the heat transfer coefficient is much higher in this area than in the intrinsic region. The evaporation limit is restrained by the formation of the liquid meniscus, and the higher the capillary pressure, the lower the evaporation heat transfer limit.

Evaporation Heat Transfer in Sintered Porous Media

2003 ◽  
Vol 125 (4) ◽  
pp. 644-652 ◽  
Author(s):  
M. A. Hanlon ◽  
H. B. Ma
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Porous Media ◽  
Particle Radius ◽  
Thin Film Evaporation ◽  
Film Evaporation ◽  
Evaporation Heat ◽  
Structure Thickness ◽  
Wick Structure ◽  
Evaporation Heat Transfer

A two-dimensional model is presented to predict the overall heat transfer capability for a sintered wick structure. The model considers the absence of bulk fluid at the top surface of the wick, heat conduction resistance through the wick, capillary limitation, and the onset of nucleate boiling. The numerical results show that thin film evaporation occurring only at the top surface of a wick plays an important role in the enhancement of evaporating heat transfer and depends on the thin film evaporation, the particle size, the porosity, and the wick structure thickness. By decreasing the average particle radius, the evaporation heat transfer coefficient can be enhanced. Additionally, there exists an optimum characteristic thickness for maximum heat removal. The maximum superheat allowable for thin film evaporation at the top surface of a wick is presented to be a function of the particle radius, wick porosity, wick structure thickness, and effective thermal conductivity. In order to verify the theoretical analysis, an experimental system was established, and a comparison with the theoretical prediction conducted. Results of the investigation will assist in optimizing the heat transfer performance of sintered porous media in heat pipes and better understanding of thin film evaporation.

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.

Theoretical Analysis of Evaporation Heat Transfer in the Thin-Film Region of Nanofluids

2019 ◽  
Author(s):  
Nannan Zhao ◽  
Huan Lin ◽  
Fengmin Su ◽  
Benwei Fu ◽  
Hongbin Ma ◽  
...  
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Surface Tension ◽  
Transfer Process ◽  
Heat Transfer Process ◽  
Interface Temperature ◽  
Thin Film Evaporation ◽  
Film Evaporation ◽  
Evaporation Heat ◽  
Evaporation Heat Transfer

Abstract A significantly higher heat transfer coefficient can be achieved through thin-film evaporation. Nanofluids also have significant enhancements in heat transfer. In the current investigation, based on the principle of conservation of momentum and the Young-Laplace equation, considering the effects of bulk flow and nanofluids concentration variation, a mathematical model of evaporative heat transfer of nanofluids is established. The different performances of different concentrations of nanofluids in the thin film evaporation heat transfer process are discussed. The results show that with the change of nanofluids concentration, the surface tension, dynamic viscosity, thermal conductivity and density will be changed, and surface tension plays an important role in the thin film evaporation heat transfer process. That will lead to a significant effect on the thin-film profile, interface temperature, heat flux in the thin-film region of the nanofluids.

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.

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.

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.

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