Mechanistic Prediction of Nucleate Boiling Heat Transfer–Achievable or a Hopeless Task?

2005 ◽  
Vol 128 (1) ◽  
pp. 1-12 ◽  
Author(s):  
Vijay K. Dhir
Keyword(s):  
Heat Transfer ◽  
Active Sites ◽  
Bubble Dynamics ◽  
Functional Dependence ◽  
Bubble Diameter ◽  
Nucleate Boiling ◽  
Complex Nature ◽  
The Past ◽  
Nucleation Sites ◽  
Nucleate Boiling Heat Transfer

Over the last half of the twentieth century, a number of purely empirical and mechanism-based correlations have been developed for pool nucleate boiling. Empirical correlations differ from each other substantially with respect to the functional dependence of heat flux on fluid and surface properties, including gravity. The mechanism-based correlations require knowledge of the number density of active sites, bubble diameter at departure, and bubble-release frequency. However, because of the complex nature of the subprocesses involved, it has not been possible to develop comprehensive models or correlations for these parameters. This, in turn, has led to the pessimistic view that mechanistic prediction of nucleate boiling is a hopeless task. However, there is an alternative to the past approaches—complete numerical simulation of the boiling process. Value of this approach for bubble dynamics and associated heat transfer is shown through excellent agreement of predictions with data obtained on microfabricated surfaces on which active nucleation sites can be controlled.

Experimental Study of Nucleate Boiling Heat Transfer Using Enhanced Space-Confined Structures

2012 ◽  
Vol 134 (6) ◽  
Author(s):  
Xuehu Ma ◽  
Chunjian Yu ◽  
Zhong Lan ◽  
Donghui Wang ◽  
Tao Bai
Keyword(s):  
Heat Transfer ◽  
Experimental Study ◽  
High Speed ◽  
Bubble Diameter ◽  
Nucleate Boiling ◽  
Large Area ◽  
Nucleation Sites ◽  
Nucleate Boiling Heat Transfer ◽  
Narrow Space ◽  
High Superheat

For narrow space boiling, it is difficult to release bubbles from the narrow space, especially on a large-area surface. To solve this problem, a new structure is designed in the present paper. An experimental study of pool boiling on the novel copper enhanced structure, with the separate ordinary confined spaces and the open channels between them, was conducted with water and ethanol. High-speed visualizations are performed to elucidate the bubble flow. The results show that the boiling performance of both water and ethanol can be enhanced effectively. The visualizations indicated that most active nucleation sites emerged in the confined channels and rarely appeared at the bare surfaces not covered by enhanced structures even at high superheat. The bubble diameter, the bubble departure frequency, and the numbers of nucleation sites are obtained using statistical methods. The results suggest that the magnitudes of bubble diameter of water are almost the same on the smooth and enhanced surfaces. The amount of nucleation sites on the enhanced surfaces is remarkably increased, indicating its key role in the boiling enhancement of water. The bubble departure frequency is increased on one of the enhanced surfaces while not increased on another, showing that it is also a significant factor for heat transfer enhancement under certain conditions. While for ethanol, all the three parameters are increased on the enhanced surfaces.

Bubble Dynamics and Boiling Heat Transfer in Microsystems

2008 ◽  
Author(s):  
Jinliang Xu ◽  
Wei Zhang ◽  
Yuxiu Li ◽  
Yunhua Gan ◽  
Qionghui Tang ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Boiling Heat Transfer ◽  
Heat Transfer Performance ◽  
Bubble Dynamics ◽  
Multiscale Model ◽  
Nucleate Boiling ◽  
Nucleation Sites ◽  
Micro Bubble ◽  
Millisecond Time Scale ◽  
Liquid Superheat

Understanding of micro boiling systems requires multiscale modeling, linking nanoscale fluid-surface interactions and micrometer channels. We present a multiscale model, successfully used for analysis of boundary conditions in microchannels. Slip lengths are found to be mainly dependent on the surface-fluid interactions, weakly on the channel sizes from nanometer to micrometer. The multiscale model is expected to be extended for bubble dynamics in microsystems, considering wall surface roughness, nano bubbles tripped in the cavities of the surface, etc. The second part gives a review on the micro bubble dynamics of pool boiling under pulse heating conditions. The third part reviews boiling heat transfer in silicon microchannels. Bubbles are being nucleated in the channel corners. Flow patterns are repeated in millisecond time-scale. Explosive boiling was found to be triggered by the higher liquid superheat, pushing liquid plugs out of microchannels. Depending on boiling numbers, three distinct heat transfer regions are identified. Heat transfer displays the nucleate boiling behavior at medium boiling numbers, and the convective heat transfer one at higher boiling numbers. The available heat transfer correlations over-predict the heat transfer performance in silicon microchannels, due to lack of nucleation sites in smooth silicon microchannels.

Experimental Study on Fundamental Phenomena of Boiling Using Heat Transfer Surfaces With Well-Defined Cavities Created by MEMS (Effect of Spacing Between Cavities)

2008 ◽  
Vol 130 (8) ◽  
Author(s):  
Takato Sato ◽  
Yasuo Koizumi ◽  
Hiroyasu Ohtake
Keyword(s):  
Heat Transfer ◽  
Microelectromechanical Systems ◽  
Nucleate Boiling ◽  
Radiation Thermometer ◽  
Heat Transfer Surface ◽  
Bubble Coalescence ◽  
Back Side ◽  
Nucleation Sites ◽  
Nucleate Boiling Heat Transfer ◽  
Cylindrical Holes

Pool nucleate boiling heat transfer experiments were performed for water using heat transfer surfaces having unified cavities. Cylindrical holes of 10μm in diameter and 40μm in depth were formed on a mirror-finished silicon wafer of 0.525mm in thickness using Microelectromechanical systems technology. The test heat transfer surface was heated by a semiconductor laser beam. Experiments were conducted in the range of up to 4.54×104W∕m2. The temperature of the back side of the heat transfer surface was measured by a radiation thermometer. When the spacing between cavities was S∕Lc<0.8, the horizontal and declining coalescence of bubbles on the neighboring cavities were dominant. Strong thermal and bubble coalescence interactions between nucleation sites were observed in this situation. This vigorous bubble coalescence created strong convection. The heat carried by this convection accounted for a large part of the heat transfer. As the cavity interval became wide, S∕Lc≥1.2, the horizontal and the declining coalescence of the bubbles ceased. The coalescence was limited to the vertical or no coalescence. The thermal and bubble coalescence interactions between the nucleation sites became quite low, to the extent of being negligible. The bubbles themselves were key in carrying heat away from the heat transfer surface.

An Overview: Analysis of Heat and Mass Transfer in Fractal Media by Fractal Geometry and Technique

2010 ◽  
Author(s):  
Boming Yu
Keyword(s):  
Heat Transfer ◽  
Mass Transfer ◽  
Porous Media ◽  
Heat And Mass Transfer ◽  
Oil Recovery ◽  
Fractal Geometry ◽  
Boiling Heat Transfer ◽  
Nucleate Boiling ◽  
Fractal Media ◽  
Nucleate Boiling Heat Transfer

In the past three decades, fractal geometry and technique have received considerable attention due to its wide applications in sciences and technologies such as in physics, mathematics, geophysics, oil recovery, material science and engineering, flow and heat and mass transfer in porous media etc. The fractal geometry and technique may become particularly powerful when they are applied to deal with random and disordered media such as porous media, nanofluids, nucleate boiling heat transfer. In this paper, a summary of recent advances is presented in the areas of heat and mass transfer in fractal media by fractal geometry technique. The present overview includes a brief summary of the fractal geometry technique applied in the areas of heat and mass transfer; thermal conductivities of porous media and nanofluids; nucleate boiling heat transfer. A few comments are made with respect to the theoretical studies that should be made in the future.

Visualization of Boiling Heat Transfer on Micro Porous Coated Surface in Confined Space

2013 ◽  
Author(s):  
Chien-Yuh Yang ◽  
Chien-Fu Liu
Keyword(s):  
Heat Transfer ◽  
Heat Exchangers ◽  
Boiling Heat Transfer ◽  
Heat Transfer Performance ◽  
Heating Surface ◽  
Nucleate Boiling ◽  
Confined Space ◽  
Two Phase ◽  
Nucleate Boiling Heat Transfer ◽  
Coated Surface

Numerous researches have been developed for pool boiling on microporous coated surface in the past decade. The nucleate boiling heat transfer was found to be increased by up to 4.5 times than that on uncoated surface. Recently, the two-phase micro heat exchangers have been considered for high flux electronic devices cooling. The enhancement techniques for improving the nucleate boiling heat transfer performance in the micro heat exchangers have gotten more importance. Previous studies of microporous coatings, however, have been restricted to boiling in unconfined space. No studies have been made on the feasibility of using microporous coatings for enhancing boiling in confined spaces. This study provides an experimental observation of the vapor generation and leaving processes on microporous coatings surface in a 1-mm confined space. It would be helpful for understanding the mechanism of boiling heat transfer and improving the design of two-phase micro heat exchangers. Aluminum particles of average diameter 20 μm were mixed with a binder and a carrier to develop a 150 μm thickness boiling enhancement paint on a 3.0 cm by 3.0 cm copper heating surface. The heating surface was covered by a thin glass plate with a 1 mm spacer to form a 1 mm vertical narrow space for the test section. The boiling phenomenon was recorded by a high speed camera. In addition to the three boiling regimes observed by Bonjour and Lallemand [1], i.e., isolated deformed bubbles, coalesced bubbles and partial dryout at low, moderate and high heat fluxes respectively in unconfined space, a suction and blowing process was observed at the highest heat flux condition. Owing to the space confinement, liquid was sucked and vapor was expelled periodically during the bubble generation process. This mechanism significantly enhanced the boiling heat transfer performance in confined space.

Pool Boiling Heat Transfer with Nanotube Arrays Surface on Titanium

2012 ◽  
Vol 550-553 ◽  
pp. 2913-2916 ◽  
Author(s):  
Jin Liang Tao ◽  
Xin Liang Wang ◽  
Pei Hua Shi ◽  
Xiao Ping Shi
Keyword(s):  
Heat Transfer ◽  
Boiling Heat Transfer ◽  
Heat Transfer Performance ◽  
Pool Boiling ◽  
Nucleate Boiling ◽  
Test Fluid ◽  
Porous Coating ◽  
Pool Boiling Heat Transfer ◽  
Nucleate Boiling Heat Transfer ◽  
Boiling Heat Transfer Coefficient

In this paper, a new porous coating was formed directly on the surface of titanium metal via anodic oxidation. And by the SEM, the morphology of the coating, which is composed of well-ordered perpendicular nanotubes, was characterized. Moreover, taking deionized water as the test fluid, a visualization study of the coating on its pool boiling heat transfer performance was made. The results demonstrated that compared with the smooth surface, the nucleate boiling heat transfer coefficient can increase 3 times while the nucleate boiling super heat was reduced 30%.

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