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.

Heat Transport Characteristics in a Miniature Flat Heat Pipe With Wire Core Wicks

2008 ◽  
Vol 130 (5) ◽  
Author(s):  
A. J. Jiao ◽  
H. B. Ma ◽  
J. K. Critser
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Heat Pipe ◽  
Heat Transport ◽  
Curved Surface ◽  
Thin Film Evaporation ◽  
Film Evaporation ◽  
Wick Structure ◽  
Evaporation Region ◽  
Flat Heat Pipe

A mathematical model predicting the heat transport capability in a miniature flat heat pipe (FHP) with a wired wick structure was developed to analytically determine its maximum heat transport rate including the capillary limit. The effects of gravity on the profile of the thin-film-evaporation region and the distribution of the heat flux along a curved surface were investigated. The heat transfer characteristics of the thin-film evaporation on the curved surface were also analyzed and compared with that on a flat surface. Combining the analysis on the thin-film-condensation heat transfer in the condenser, the model can be used to predict the total temperature drop between the evaporator and condenser in the FHP. In order to verify the model, an experimental investigation was conducted. The theoretical results predicted by the model agree well with the experimental data for the heat transfer process occurring in the FHP with the wired wick structure. Results of the investigation will assist in the optimum design of the curved-surface wicks to enlarge the thin-film-evaporation region and a better understanding of heat transfer mechanisms in heat pipes.

Mechanisms of Thin-Film Evaporation Considering Momentum and Energy Conservation

2013 ◽  
Author(s):  
Chunji Yan ◽  
Xinxiang Pan ◽  
Xiaowei Lu
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Fluid Flow ◽  
Mathematic Model ◽  
Momentum Conservation ◽  
Momentum Equation ◽  
Maximum Heat ◽  
Thin Film Evaporation ◽  
Film Evaporation ◽  
Phase Change Heat Transfer

A mathematic model, which can be used to predict the evaporation and fluid flow in thin film region, is developed based on momentum and energy conservations and the augmented Young-Laplace equation in this paper. In the model the variations of the enthalpy and kinetics energy of the thin-film along the evaporating region are considered. By theoretical analysis, we have obtained the governing equation for thin film profile. The fluid flow and phase-change heat transfer in an evaporating extended meniscus are numerically studied. The differences between the model considering momentum conservation only and including both momentum and energy conservations are compared. It is found that the maximum heat flux of the thin-film evaporation by using two mathematical models obtained has no change, but when considering the momentum and energy conservations the total heat transfer rate unit width along the thin-film evaporation region is greater than that of only including momentum equation.

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.

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.

scholarly journals Experimental Study on the Heat Transfer Performance of Pump-Assisted Capillary Phase-Change Loop

2021 ◽  
Vol 11 (22) ◽  
pp. 10954
Author(s):  
Xiaoping Yang ◽  
Gaoxiang Wang ◽  
Cancan Zhang ◽  
Jie Liu ◽  
Jinjia Wei
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Vapor Pressure ◽  
Pressure Difference ◽  
Heat Dissipation ◽  
Vapor Pressure Difference ◽  
Thin Film Evaporation ◽  
Film Evaporation ◽  
Start Up ◽  
Liquid Vapor

To overcome the two-phase flow instability of traditional boiling heat dissipation technologies, a porous wick was used for liquid-vapor isolation, achieving efficient and stable boiling heat dissipation. A pump-assisted capillary phase-change loop with methanol as the working medium was established to study the effect of liquid-vapor pressure difference and heating power on its start-up and steady-state characteristics. The results indicated that the evaporator undergoes four heat transfer modes, including flooded, partially flooded, thin-film evaporation, and overheating. The thin-film evaporation mode was the most efficient with the shortest start-up period. In addition, heat transfer modes were determined by the liquid-vapor pressure difference and power. The heat transfer coefficient significantly improved and the thermal resistance was reduced by increasing liquid-vapor pressure as long as it did not exceed 8 kPa. However, when the liquid-vapor pressure exceeded 8 kPa, its influence on the heat transfer coefficient weakened. In addition, a two-dimensional heat transfer mode distribution diagram concerning both liquid-vapor pressure difference and power was drawn after a large number of experiments. During an engineering application, the liquid-vapor pressure difference can be controlled to maintain efficient thin-film evaporation in order to achieve the optimum heat dissipation effect.

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