Visualization of Pool Boiling From Plain and Enhanced Structures

2005 ◽  
Author(s):  
Camil-Daniel Ghiu ◽  
Yogendra K. Joshi
Keyword(s):  
Atmospheric Pressure ◽  
Pool Boiling ◽  
Single Layer ◽  
Main Tool ◽  
Ccd Camera ◽  
Nucleate Boiling ◽  
Heat Fluxes ◽  
Working Fluid ◽  
Geometric Complexity ◽  
Boiling Process

A visualization study of pool boiling at atmospheric pressure from plain and enhanced structures was conducted with PF 5060 as working fluid. The single layer enhanced structures were fabricated in copper and were 1 mm thick. The parameters investigated in the present study are heat flux, width of microchannels and overall structure width. A monochrome CCD camera with attached magnifying lens served as the main tool for observation of the boiling process from the structures. The nucleate boiling regime for a plain surface is usually divided into two sub-regimes: the isolated bubbles regime and the coalesced bubbles regime. For enhanced structures, the increase in geometric complexity leads to different flow regimes that may establish under different heat fluxes. This study evaluates these regimes using movies and still photographs. A comparison with the plain case is made and the differences highlighted.

Effects of Varying Geometrical Parameters on Boiling From Microfabricated Enhanced Structures

2003 ◽  
Vol 125 (1) ◽  
pp. 103-109 ◽  
Author(s):  
C. Ramaswamy ◽  
Y. Joshi ◽  
W. Nakayama ◽  
W. B. Johnson
Keyword(s):  
Heat Dissipation ◽  
Layer Structure ◽  
Single Layer ◽  
Nucleate Boiling ◽  
Heat Fluxes ◽  
Working Fluid ◽  
Geometrical Parameters ◽  
Small Range ◽  
Two Phase ◽  
Wall Superheat

The current study involves two-phase cooling from enhanced structures whose dimensions have been changed systematically using microfabrication techniques. The aim is to optimize the dimensions to maximize the heat transfer. The enhanced structure used in this study consists of a stacked network of interconnecting channels making it highly porous. The effect of varying the pore size, pitch and height on the boiling performance was studied, with fluorocarbon FC-72 as the working fluid. While most of the previous studies on the mechanism of enhanced nucleate boiling have focused on a small range of wall superheats (0–4 K), the present study covers a wider range (as high as 30 K). A larger pore and smaller pitch resulted in higher heat dissipation at all heat fluxes. The effect of stacking multiple layers showed a proportional increase in heat dissipation (with additional layers) in a certain range of wall superheat values only. In the wall superheat range 8–13 K, no appreciable difference was observed between a single layer structure and a three layer structure. A fin effect combined with change in the boiling phenomenon within the sub-surface layers is proposed to explain this effect.

Experimental and Numerical Study of the Effects of Asymmetric Micro Ratchets on Pool Boiling Performance

2013 ◽  
Author(s):  
Lance Brumfield ◽  
Sunggook Park
Keyword(s):  
High Performance ◽  
Numerical Study ◽  
Heated Surface ◽  
Pool Boiling ◽  
Nucleate Boiling ◽  
Industrial Applications ◽  
Heat Fluxes ◽  
Working Fluid ◽  
High Heat Flux ◽  
Central Processing

Nucleate boiling is an attractive method for achieving high heat flux at low superheat temperatures. It is frequently used for industrial applications such as heat exchangers and is being considered to cool advanced central processing units (CPU) which produce heat fluxes on the order of 1 MW/m2 and are becoming increasingly less efficient to cool via forced conduction of air. The issues with implementing nucleate boiling as a cooling mechanism lies in the difficulty of quantifying the complex and numerous mechanisms which control the process. A comprehensive nucleate boiling model has yet to be formulated and will be required in order to safely and reliably cool high performance electronics. Spatially periodic systems with localized asymmetric surface structures (ratchets) can induce directed transport of matter (liquid/particles) in the absence of net force. It was hypothesized that ratchets may enhance pool boiling heat transfer by aiding in the removal of vapor which forms on the heated surface. Therefore, experiments on pool boiling using asymmetric micro ratchets of various geometries, with FC-72 as the working fluid, were investigated. Additionally, various numerical pool boiling simulations were performed using FLUENT to better understand the underlying physical principles behind pool boiling.

Effect of Heat Flux on Bubble Coalescence Phenomena and Sound Signatures During Pool Boiling

2020 ◽  
Author(s):  
Akshat Negi ◽  
Aniket M. Rishi ◽  
Satish G. Kandlikar
Keyword(s):  
Heat Flux ◽  
Pool Boiling ◽  
Nucleate Boiling ◽  
Bubble Nucleation ◽  
Frequency Region ◽  
Heat Fluxes ◽  
Working Fluid ◽  
Bubble Collapse ◽  
Maximum Heat ◽  
Acoustic Signatures

Abstract Boiling heat transfer is extensively used in various industrial applications to efficiently dissipate a large amount of heat by maintaining lower surface temperatures. The maximum heat flux dissipated during boiling is limited by the critical heat flux (CHF) and limited visualization of the boiling surface limits the identification of the impending CHF condition to rely on temperature monitoring alone. The study presented here focuses on developing a method for analyzing and identifying acoustic signatures throughout the nucleate boiling regimes that are indicative of the boiling state of the heater surface. The bubble nucleation and coalescence along with bubble collapse at the liquid-vapor interface leads to variation in acoustic emission patterns during boiling. These sound waves are studied and acoustic signatures that are representative of the impending CHF are identified over plain and enhanced copper substrates with water as the working fluid. During pool boiling study, it was observed that sound was dominant in two frequency regions (400–500 Hz dominant throughout nucleate boiling and 100–200 Hz dominant at heat fluxes > 100 W/cm2). However, just before CHF, a sudden drop in amplitude was observed in the high frequency region (400–500 Hz), while the amplitude in low frequency region (100–200 Hz) continued to rise. It was concluded that this acoustic study can be used as a tool to predict the approaching CHF condition.

Bubble Dynamics on Nanostructured Cu Surfaces

2008 ◽  
Author(s):  
Chen Li ◽  
Nikhil Koratkar ◽  
G. P. Peterson
Keyword(s):  
High Speed ◽  
Bubble Dynamics ◽  
Bubble Diameter ◽  
Ccd Camera ◽  
Nucleate Boiling ◽  
Nucleation Site ◽  
Working Fluid ◽  
Visualization System ◽  
Boiling Process ◽  
Order Of Magnitude

Nucleate boiling performance was enhanced up to an order of magnitude through direct deposition of Cu nanorods on a planar Cu surface. The methodology that enables order of magnitude improvement in boiling performance without fabricating complicated surface structures or changing the working fluid will have broad impact on metal-liquid type two-phase heat exchangers. In this study, discussion was focused on bubble dynamics on the nanostrucured Cu surfaces. We observed striking differences in bubble dynamics through nucleation boiling process for the nanostructured surface including smaller bubble diameters, higher release frequencies and nucleation site density, and large fluctuations in bubble diameter prior to release. These differences during the boiling process are responsible for the enhanced heat transfer. High quality images were captured through a well-designed visualization system, which comprises of a high-speed charge-coupled device (CCD) camera, microscope and data acquisition system. This visualization study aims to quantitatively study the bubble dynamics on the nanostructured Cu surfaces.

Pool Boiling of n-Pentane, CFC-113, and Water Under Reduced Gravity: Parabolic Flight Experiments With a Transparent Heater

1995 ◽  
Vol 117 (2) ◽  
pp. 408-417 ◽  
Author(s):  
T. Oka ◽  
Y. Abe ◽  
Y. H. Mori ◽  
A. Nagashima
Keyword(s):  
Heat Transfer ◽  
Parabolic Flight ◽  
Pool Boiling ◽  
Glass Plate ◽  
Nucleate Boiling ◽  
Heat Fluxes ◽  
Rear Side ◽  
Heater Surface ◽  
Reduced Gravity ◽  
Liquid Behavior

A series of pool boiling experiments have been conducted under reduced gravity condition (the order of 10−2 times the terrestrial gravity) available in an aircraft taking parabolic flight. A transparent resistant heater, a transparent indium oxide film plated on a glass plate, was employed so that the vapor/liquid behavior interacting with the heater surface could be observed from the rear side of the heater simultaneously with the side view of vapor bubbles above the heater surface. The experiments were performed for three different fluids—n-pentane, CFC-113, and water—under subcooled conditions. The critical heat fluxes for both n-pentane and CFC-113 under the reduced gravity were lowered to about 40 percent of the corresponding terrestrial values. Although the heat transfer characteristics in a low heat flux nucleate boiling regime for both n-pentane and CFC-113 showed no more than a slight change with the reduction in gravity, a significant heat transfer deterioration was noted with water in the reduced gravity boiling. The observation from the rear side of the heater suggested that this particular difference in the gravity dependency of heat transfer was ascribed to a considerable difference, between the organic fluids and water, in the behavior of attachment to the heater surface of the bubbles grown up, while the behavior of attachment must depend on the surface tension of each fluid and the wettability of the heater surface with the fluid.

scholarly journals Lattice Boltzmann Simulation of Nucleate Pool Boiling in Saturated Liquid

2011 ◽  
Vol 9 (5) ◽  
pp. 1347-1361 ◽  
Author(s):  
Yoshito Tanaka ◽  
Masato Yoshino ◽  
Tetsuo Hirata
Keyword(s):  
Lattice Boltzmann ◽  
Solid Wall ◽  
Bubble Diameter ◽  
Pool Boiling ◽  
Nucleate Boiling ◽  
Bubble Nucleation ◽  
Immiscible Fluids ◽  
Nucleate Pool Boiling ◽  
Two Phase ◽  
Boiling Process

AbstractA thermal lattice Boltzmann method (LBM) for two-phase fluid flows in nucleate pool boiling process is proposed. In the present method, a new function for heat transfer is introduced to the isothermal LBM for two-phase immiscible fluids with large density differences. The calculated temperature is substituted into the pressure tensor, which is used for the calculation of an order parameter representing two phases so that bubbles can be formed by nucleate boiling. By using this method, two-dimensional simulations of nucleate pool boiling by a heat source on a solid wall are carried out with the boundary condition for a constant heat flux. The flow characteristics and temperature distribution in the nucleate pool boiling process are obtained. It is seen that a bubble nucleation is formed at first and then the bubble grows and leaves the wall, finally going up with deformation by the buoyant effect. In addition, the effects of the gravity and the surface wettability on the bubble diameter at departure are numerically investigated. The calculated results are in qualitative agreement with other theoretical predictions with available experimental data.

Nucleate Boiling From Micro-Machined Surfaces

2003 ◽  
Author(s):  
Adrian M. Holland ◽  
Colin P. Garner
Keyword(s):  
Heat Transfer ◽  
High Speed ◽  
Imaging System ◽  
Laser System ◽  
Ccd Camera ◽  
Nucleate Boiling ◽  
Heat Fluxes ◽  
Frame Rate ◽  
High Speed Imaging ◽  
Machined Surfaces

This paper discusses the production and use of laser-machined surfaces that provide enhanced nucleate boiling and heat transfer characteristics. The surface features of heated plates are known to have a significant effect on nucleate boiling heat transfer and bubble growth dynamics. Nucleate boiling starts from discrete bubbles that form on surface imperfections, such as cavities or scratches. The gas or vapours trapped in these imperfections serve as nuclei for the bubbles. After inception, the bubbles grow to a certain size and depart from the surface. In this work, special heated surfaces were manufactured by laser machining cavities into polished aluminium plates. This was accomplished with a Nd:YAG laser system, which allowed drilling of cavities of a known diameter. The size range of cavities was 20 to 250 micrometers. The resulting nucleate pool boiling was analysed using a novel high-speed imaging system comprising an infrared laser and high resolution CCD camera. This system was operated up to a 2 kHz frame rate and digital image processing allowed bubbles to be analysed statistically in terms of departure diameter, departure frequency, growth rate, shape and velocity. Data was obtained for heat fluxes up to 60 kW.m−2. Bubble measurements were obtained working with water at atmospheric pressure. The surface cavity diameters were selected to control the temperature at which vapour bubbles started to grow on the surface. The selected size and spacing of the cavities was also explored to provide optimal heat transfer.

Flow Boiling on Heterogeneous Wetting Surface in a Vertical Narrow Microchannel

2020 ◽  
Author(s):  
Yuhao Lin ◽  
Junye Li ◽  
Kan Zhou ◽  
Wei Li ◽  
Kuang Sheng ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Flow Instability ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
Flow Patterns ◽  
Bubble Nucleation ◽  
Working Fluid ◽  
Hydrophilic Surface ◽  
Structured Surfaces ◽  
Boiling Process

Abstract The micro structured surfaces have significant impact on the flow patterns and heat transfer mechanisms during the flow boiling process. The hydrophobic surface promotes bubble nucleation while the hydrophilic surface supplies liquid to a heating surface, thus there is a trade-off between a hydrophobic and a hydrophilic surface. To examine the effect of heterogeneous wetting surface on flow boiling process, an experimental investigation of flow boiling in a rectangular vertical narrow microchannel with the heterogeneous wetting surface was conducted with deionized water as the working fluid. The heat transfer characteristics of flow boiling in the microchannel was studied and the flow pattern was photographed with a high-speed camera. The onset of flow boiling and heat transfer coefficient were discussed with the variation of heatfluxes and mass fluxes, the trends of which were analyzed along with the flow patterns. During the boiling process, the dominated heat transfer mechanism was nucleate boiling, with numerous nucleate sites between the hydrophilic/hydrophobic stripes and on the hydrophobic ones. In the meantime, after the merged bubbles were constrained by the channel walls, it would be difficult for them to expand towards upstream since they were restricted by the contact line between hydrophilic/hydrophobic stripes, thereby reduce the flow instability and achieve remarkable heat transfer performance.

Pool Boiling Using Thin Enhanced Structures Under Top-Confined Conditions

2006 ◽  
Vol 128 (12) ◽  
pp. 1302-1311 ◽  
Author(s):  
Camil-Daniel Ghiu ◽  
Yogendra K. Joshi
Keyword(s):  
Heat Flux ◽  
High Speed ◽  
Pool Boiling ◽  
Single Layer ◽  
Temperature Limit ◽  
Heat Fluxes ◽  
The Other ◽  
Vapor Flow ◽  
Maximum Heat ◽  
Maximum Heat Flux

An experimental study of pool boiling using enhanced structures under top-confined conditions was conducted with a dielectric fluorocarbon liquid (PF 5060). The single layer enhanced structures studied were fabricated in copper and quartz, had an overall size of 10×10mm2, and were 1mm thick. The parameters investigated in this study were the heat flux (0.8-34W∕cm2) and the top space S(0-13mm). High-speed visualizations were performed to elucidate the liquid/vapor flow in the space above the structure. The enhancement observed for plain surfaces in the low heat fluxes regime is not present for the present enhanced structure. On the other hand, the maximum heat flux for a prescribed 85°C surface temperature limit increased with the increase of the top spacing, similar to the plain surfaces case. Two characteristic regimes of pool boiling have been identified and described: isolated flattened bubbles regime and coalesced bubbles regime.

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