Directional Notches as Microstructures to Promote Nucleation and Heat Transfer in Pool Boiling PDF Download
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Author: Callum McLaughlin
Publisher:
ISBN:
Category : Ebullition
Languages : en
Pages : 44
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Book Description
"Heat generation in electronic hardware has become a major limiting factor in achieving maximum efficiency in modern computer parts. Classically forced flow convection systems are used to remove this heat at high rates but can be costly to implement and can take up space that may be needed for other critical components. In response to this, systems that use fewer parts scale in compact spaces are needed. In these situations, pool boiling as a heat transfer mechanism can excel. Pool boiling removes heat through the evaporation of fluid. On a flat surface pool boiling is chaotic and this random nature may hinder its ability to remove heat as effectively. The surface geometry of a pool boiling system can be altered to direct the flow of generated vapor bubbles to allow for increased heat flow and higher heat transfer performance. By creating paths for the vapor to follow we can induce currents in the flow of cool fluid to the heater surface, creating a faster cycle of vapor production therefore cooling the heated surface at a faster rate. The purpose of this study is to investigate angled chip notches as an alternative to already existing high heat transfer surfaces in pool boiling. These alternative chips may prove cheaper or easier to produce the alternative which may incorporate fine, hard to produce features or post process coatings like sintering and the addition of hydrophobic materials. This study will examine the effect these specifically designed notches have on the interaction between the directed vapor and the liquid pathways they create. By creating notches in the surface of the chip, vapor bubble is given sites to nucleate and form vapor pathways. The angle walls on the one side of the notch will act as a wedge when water being driven toward the notch pushes the nucleating bubble up and out of the notch. Combined with pairing nucleating notches up with another oppositely facing one the vapor bubble is departing earlier then it would have had it not been assisted by these surface elements. With just these paired notches placed in row, an HTC improvement of 158% was recorded, compared to a plain copper surface. With the inclusion of microchannels this improvement was brought up to 161%"--Abstract.
Author: Callum McLaughlin
Publisher:
ISBN:
Category : Ebullition
Languages : en
Pages : 44
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Book Description
"Heat generation in electronic hardware has become a major limiting factor in achieving maximum efficiency in modern computer parts. Classically forced flow convection systems are used to remove this heat at high rates but can be costly to implement and can take up space that may be needed for other critical components. In response to this, systems that use fewer parts scale in compact spaces are needed. In these situations, pool boiling as a heat transfer mechanism can excel. Pool boiling removes heat through the evaporation of fluid. On a flat surface pool boiling is chaotic and this random nature may hinder its ability to remove heat as effectively. The surface geometry of a pool boiling system can be altered to direct the flow of generated vapor bubbles to allow for increased heat flow and higher heat transfer performance. By creating paths for the vapor to follow we can induce currents in the flow of cool fluid to the heater surface, creating a faster cycle of vapor production therefore cooling the heated surface at a faster rate. The purpose of this study is to investigate angled chip notches as an alternative to already existing high heat transfer surfaces in pool boiling. These alternative chips may prove cheaper or easier to produce the alternative which may incorporate fine, hard to produce features or post process coatings like sintering and the addition of hydrophobic materials. This study will examine the effect these specifically designed notches have on the interaction between the directed vapor and the liquid pathways they create. By creating notches in the surface of the chip, vapor bubble is given sites to nucleate and form vapor pathways. The angle walls on the one side of the notch will act as a wedge when water being driven toward the notch pushes the nucleating bubble up and out of the notch. Combined with pairing nucleating notches up with another oppositely facing one the vapor bubble is departing earlier then it would have had it not been assisted by these surface elements. With just these paired notches placed in row, an HTC improvement of 158% was recorded, compared to a plain copper surface. With the inclusion of microchannels this improvement was brought up to 161%"--Abstract.
Author: Chunghsiung Wang
Publisher:
ISBN:
Category : Nucleate boiling
Languages : en
Pages : 256
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Book Description
Author: Ming-Chang Lu
Publisher:
ISBN:
Category :
Languages : en
Pages : 218
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Book Description
This dissertation presents a study exploring the limits of phase-change heat transfer with the aim of enhancing critical heat flux (CHF) in pool boiling and enhancing thermal conductance in heat pipes. The state-of-the-art values of the CHF in pool boiling and the thermal conductance in heat pipes are about two orders of magnitudes smaller than the limits predicted by kinetic theory. Consequently, there seems to be plenty of room for improvement. Pool boiling refers to boiling at a surface immersed in an extensive motionless pool of liquid. Its process includes heterogeneous nucleation, growth, mergence and detachment of vapor bubbles on a heating surface. It is generally agreed that the high heat transfer coefficient of boiling could be explained by the concept of single-phase forced convection, i.e., the motion of bubbles agitating surrounding liquid is similar to the process in single-phase forced convection. The occurrence of CHF results from a formation of a vapor film on the heater surface, which reduces the thermal conductance drastically and causes a huge temperature rise on the surface. Over the past few decades, researchers were struggling to identify the exact mechanism causing CHF. General observations are that both surface properties and pool hydrodynamics could affect the values of CHF. Nanowire array-coated surfaces having a large capillary force are employed to enhance the CHF. It has been shown that CHFs on the nanowire array-coated surface could be doubled compared to the values on a plain surface. The obtained CHF of 224 ± 6.60 W/cm̂2 on the nanowire-array coated surface is one of the highest values reported in the boiling heat transfer. To further enhance CHF, the mechanisms that govern CHF have been systematically explored. Experimental results show that the CHF on the nanowire array-coated surface are not limited by the capillary force. Instead, the CHF are dependent on the heater size. Corresponding experiments on plain surfaces with various heater sizes also exhibits similar heater-size dependence. The CHFs on nanowire array-coated surfaces and plain surfaces are consistent with the predictions of the hydrodynamic theory while a higher CHF is obtained on the nanowire array-coated surface as compared to the plain Si surface. This suggests that the CHFs are a result of the pool hydrodynamics while surface properties modify the corresponding hydrodynamic limits. A heat pipe is a device that transports thermal energy in a very small temperature difference and thereby producing a very large thermal conductance. It relies on evaporation of liquid at the heated end of the pipe, flow of vapor between the heated and cooled end, condensation at the other end, and capillary-driven liquid flow through a porous wick between the condenser and the evaporation. The large latent heat involved in evaporation and condensation leads to very large heat flows for a small temperature drop along the heat pipe. Despite the large thermal conductance, their operation is limited by such factors as capillary limit, boiling limit, sonic limit and entrainment limit, etc. Among these operational limits, capillary and boiling limits are most frequently encountered. The capillary limit determines the maximum flow rate provided by the capillary force of the wick structure whereas boiling limit is referred to a condition that liquid supply is blocked by vapor bubbles in the wick. Consequently, the wick structure is the key component in a heat pipe, which determines the maximum capillary force and the dominant thermal resistance. In a heat pipe using evaporation as the dominant heat transfer mechanism, a thin liquid film (̃ a few microns) extended from the solid structure in the wick causes the dominant thermal resistance. Therefore, if one reduces the pore size of a porous media, the thermal conductance could be enhanced by increasing the surface area of the thin liquid film. On the other hand, the classical thermodynamics depicts that the superheat required for evaporation is inversely proportional to the equilibrium radius of the meniscus. Consequently, enhancing thermal conductance via increasing the thin film area is contradictory to the effect of evaporation suppression for small pores. A hierarchical wick structure with multiple length scales that enhances dry-out heat flux and thermal conductance simultaneously in heat pipes was demonstrated. This hierarchical wick structure is composed of a large microchannel array to reduce flow resistance and small pin-fin arrays to provide a large capillary force. The enhancement of thermal conductance is achieved via a large number of pin-fins for increasing the total thin film area. A thermal conductance defined by the slope of the curve of ̃16.28 ± 1.33 W/cm̂2-K and a dry-out heat flux of 228.85 ± 10.73 W/cm̂2 were achieved by this design. Further, vapor transport resistance is minimized within the aligned-multi-scale wick structure. As a result, this wick does not pose a boiling limit. Artificial cavities were created in the wick structure to take the advantage of the high heat transfer coefficient of boiling heat transfer. The wick with artificial cavities successfully triggers boiling at a lower wall temperature resulting in a conductance of 9.02 ± 0.04 W/cm̂2-K compared to an evaporation mode of 3.54 ± 0.01 W/cm̂2-K. For a given heat flux, the wick with cavities effectively reduce wall temperature compared to a wick without cavities. Our experimental results display an enhancement of thermal conductance by using boiling heat transfer. This opens up a new direction for further enhancing thermal conductance in heat pipes by circumventing the limit in the evaporative heat transfer regime, in which further increase in surface area will eventually result in evaporation suppression in small pores.
Author: Michele David
Publisher:
ISBN:
Category : Bubbles
Languages : en
Pages : 76
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Book Description
Boiling heat transfer is studied for its ability to dissipate high fluxes and achieve heat transfer coefficients two orders of magnitude greater than single-phase heat transfer systems. Heater surface enhancement with increased surface area, varied geometry, wettability contrast and micro/nano-structures can further enhance boiling heat transfer performance through bubble nucleation augmentation. Bubble nucleation control, growth and departure dynamics is important in understanding boiling phenomena and enhancing nucleate boiling heat transfer performance. Bi-functional surfaces for enhanced boiling heat transfer were fabricated and studied through investigation of bubble dynamics and pool boiling experiments. For the fabrication of the surface, hydrophobic polymer dot arrays are first printed on a substrate, followed by hydrophilic ZnO nanostructure deposition via microreactor-assisted nanomaterial deposition (MAND) processing. Wettability contrast between the hydrophobic polymer dot arrays and aqueous ZnO solution allows for the fabrication of surfaces with distinct wettability regions. Bi-functional surfaces with various configurations were fabricated and their bubble dynamics were examined at elevated heat flux, revealing various nucleate boiling phenomena. In particular, aligned and patterned bubbles with a tunable departure frequency and diameter were demonstrated in a boiling experiment for the first time. A pool boiling experimental facility has been designed and built to investigate nucleate pool boiling in water at atmospheric pressure. Resulting boiling curves of enhanced surfaces showed up to 3X enhancement in heat transfer coefficients at the same surface superheat using bi-functional surfaces, compared to a bare stainless steel surface. The surfaces show promising results for energy savings in two-phase change applications.
Author: Bradley Bon
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
Conclusively, surface microstructure and topography can greatly influence nucleate boiling heat transfer. The various physical attributes employed with the structured surfaces further revealed the profound influence of surface topography on enhancing boiling heat transfer. On the atomic scale, it is seen that even differences in crystal structure can also produce noticeable variations in the boiling heat transfer rate.
Author: Michael F. Ashby
Publisher: Elsevier
ISBN: 1483297217
Category : Technology & Engineering
Languages : en
Pages : 380
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Book Description
Provides a thorough explanation of the basic properties of materials; of how these can be controlled by processing; of how materials are formed, joined and finished; and of the chain of reasoning that leads to a successful choice of material for a particular application. The materials covered are grouped into four classes: metals, ceramics, polymers and composites. Each class is studied in turn, identifying the families of materials in the class, the microstructural features, the processes or treatments used to obtain a particular structure and their design applications. The text is supplemented by practical case studies and example problems with answers, and a valuable programmed learning course on phase diagrams.
Author: Roy Elliott
Publisher: Butterworth-Heinemann
ISBN: 148319230X
Category : Technology & Engineering
Languages : en
Pages : 253
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Book Description
Cast Iron Technology presents a critical review of the nature of cast irons. It discusses the types of cast iron and the general purpose of cast irons. It also presents the history of the iron founding industry. Some of the topics covered in the book are the description of liquid metal state; preparation of liquid metal; process of melting; description of cupola melting and electric melting methods; control of composition of liquid metal during preparation; description of primary cast iron solidification structures; and thermal analysis of metals to determine its quality. Solidification science and the fundamentals of heat treatment are also discussed. An in-depth analysis of the hot quenching techniques is provided. The graphitization potential of liquid iron is well presented. A chapter is devoted to microstructural features of cast iron. The book can provide useful information to iron smiths, welders, students, and researchers.
Author: Andrea Di Schino
Publisher: MDPI
ISBN: 3039286501
Category : Technology & Engineering
Languages : en
Pages : 260
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Book Description
Stainless steels represent a quite interesting material family, both from a scientific and commercial point of view, following to their excellent combination in terms of strength and ductility together with corrosion resistance. Thanks to such properties, stainless steels have been indispensable for the technological progress during the last century and their annual consumption increased faster than other materials. They find application in all these fields requiring good corrosion resistance together with ability to be worked into complex geometries. Despite to their diffusion as a consolidated materials, many research fields are active regarding the possibility to increase stainless steels mechanical properties and corrosion resistance by grain refinement or by alloying by interstitial elements. At the same time innovations are coming from the manufacturing process of such a family of materials, also including the possibility to manufacture them starting from metals powder for 3D printing. The Special Issue scope embraces interdisciplinary work covering physical metallurgy and processes, reporting about experimental and theoretical progress concerning microstructural evolution during processing, microstructure-properties relations, applications including automotive, energy and structural.
Author: Christoph Leyens
Publisher: John Wiley & Sons
ISBN: 3527605207
Category : Technology & Engineering
Languages : de
Pages : 532
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Book Description
This handbook is an excellent reference for materials scientists and engineers needing to gain more knowledge about these engineering materials. Following introductory chapters on the fundamental materials properties of titanium, readers will find comprehensive descriptions of the development, processing and properties of modern titanium alloys. There then follows detailed discussion of the applications of titanium and its alloys in aerospace, medicine, energy and automotive technology.
Author: Michael C. Gao
Publisher: Springer
ISBN: 3319270133
Category : Technology & Engineering
Languages : en
Pages : 524
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Book Description
This book provides a systematic and comprehensive description of high-entropy alloys (HEAs). The authors summarize key properties of HEAs from the perspective of both fundamental understanding and applications, which are supported by in-depth analyses. The book also contains computational modeling in tackling HEAs, which help elucidate the formation mechanisms and properties of HEAs from various length and time scales.
