scholarly journals Role of nanoscale roughness in the heat transfer characteristics of thin film evaporation

2020 ◽  
Vol 150 ◽  
pp. 119306 ◽  
Author(s):  
Han Hu ◽  
Justin A. Weibel ◽  
Suresh V. Garimella
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Heat Transfer Characteristics ◽  
Thin Film Evaporation ◽  
Film Evaporation ◽  
Transfer Characteristics ◽  
Nanoscale Roughness

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.

Experimental Investigation on Heat Transfer Characteristics of Thin Film Evaporation

2014 ◽  
Vol 960-961 ◽  
pp. 427-432
Author(s):  
Jing Ming Dong ◽  
Xin Xiang Pan ◽  
Zhi Tao Han ◽  
Zhi Jian Liu
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Experimental Investigation ◽  
Heat Transfer Performance ◽  
High Efficiency ◽  
Engineering Application ◽  
Heat Transfer Characteristics ◽  
Thin Film Evaporation ◽  
Film Evaporation ◽  
Film Evaporator

Thin film evaporation is a kind of high efficiency heat transfer. A thin film evaporator with rectangular micro groove has been designed, constructed and tested. The heat flux, superheat and thermal resistance of the thin film evaporator were investigated under different evaporating temperatures. Compared with the regular evaporator at the same condition, the thin film evaporator with rectangular micro groove has a better heat transfer performance and good temperature uniformity. The experimental investigation presented in this paper can be used as a reference to promote the engineering application and development of the thin film evaporator.

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 Transfer Characteristics of Baffled Channel Flow

2011 ◽  
Author(s):  
Changwoo Kang ◽  
Kyung-Soo Yang
Keyword(s):  
Heat Transfer ◽  
Channel Flow ◽  
Large Scale ◽  
Heat Removal ◽  
Heat Transfer Characteristics ◽  
Transfer Characteristics ◽  
Flow Heat ◽  
Two Parameters ◽  
Time Periodic

Heat transfer characteristics of baffled channel flow, where thin baffles are mounted on both channel walls periodically in the direction of the main flow, have been numerically investigated in a laminar range. In baffled channel flow, heat transfer characteristics are significantly affected by large-scale vortices generated due to flow separation at the tips of the baffles. In this investigation, a parametric study has been carried out to identify the optimal configuration of the baffles to achieve the most efficient heat removal from the channel walls. Two key parameters are considered, namely baffle interval (L) and Reynolds number (Re). We elucidate the role of the primary instability, a Hopf bifurcation from steady to a time-periodic flow, in the convective heat transfer in baffled channel flow. We also propose a contour diagram (“map”) of averaged Nusselt number on the channel walls as a function of the two parameters. The results shed light on understanding and controlling heat transfer mechanism in a finned heat exchanger, being quite beneficial to its design.

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.

Characteristics of Micro-Liquid-Layer Behavior in a Mini-Channel Vaporizer for Water and Ethanol

2006 ◽  
Author(s):  
Yoshio Utaka ◽  
Yutaka Tasaki ◽  
Kousuke Ichii
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Fuel Cell ◽  
Pool Boiling ◽  
Fuel Cell Vehicle ◽  
Bulk Liquid ◽  
Growth Speed ◽  
Heat Transfer Characteristics ◽  
Simple Method ◽  
Transfer Characteristics

With growing demands for a cleaner global environment, there is a need for automotive powertrains that can provide higher efficiency and contribute to lower C02 emissions. From this perspective, fuel cell vehicles are expected to gain a share of the automotive market in the 21st century. The reformer type fuel cell vehicle requires a heat exchanger with high efficiency and a low heat capacity to meet powertrain requirements for quick response and compactness. The use of a mini-channel vaporizer for the reformer is one possible way of meeting these requirements. In a mini-channel, the bulk liquid, bubbles and thin film that form between the heating surface and the bubbles affect the heat transfer characteristics in complex ways, just as in the case of pool boiling. However, as reported in the literature, the characteristics of evaporation in a mini-channel are completely different from those of pool boiling, because of the important role played by the superheated thin film that forms in the process of bubble expansion. For example, although deterioration of heat transfer characteristics has been reported for an extremely small gap, the mechanism involved has not yet been analyzed. In order to research and develop a mini-channel vaporizer efficiently, it is necessary to elucidate the mechanism of these phenomena, and clarify measures for improving heat transfer characteristics. Then a simple method may be devised, based on the related findings, to predict the heat transfer characteristics of a mini-channel vaporizer with sufficient accuracy at the development stage. In this study, by using ethanol instead of water as used in past research, the thickness of the thin film that plays an important role in the heat transfer characteristics was measured by application of the laser extinction method for channel gap sizes of 0.3 and 0.15 mm. The process of bubble growth was simultaneously recorded with a high-speed camera. These results were then compared with those obtained for water. Similar trends as those obtained for water were observed for ethanol. That is, observations show that the thin film thickness is essentially influenced by the growth speed of the interface between the bubble and the bulk liquid and that it approaches a certain value above a certain speed. At the same time, the thickness of the thin film decreases with a decrease in gap size.

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.

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