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Author: Ajay Gurung Publisher: ISBN: Category : Languages : en Pages : 91
Book Description
The present research is an experimental study of the enhancement of pool boiling and evaporative heat transfer using high temperature thermally conductive microporous coatings. Two major types of coatings were investigated: one that is based on copper powders on copper substrate and the other on aluminum powders on aluminum substrate. Both coatings were easy to fabricate with low costs compared to conventional sintering and plasma spraying techniques, yet have high bonding strength and some of them can operate at temperatures up to 670 °C. Multiple coating options were fabricated and tested in pool boiling of water in order to optimize the coating. These coating options consisted of variations of coating composition ratio, coating thickness and powder sizes. Average powder sizes ranged from 5 micron to 110 micron, and coating thicknesses from 75 micron to 340 micron, applied on flat 1x1cm2 test heaters. The heaters were tested in the horizontal, upward-facing orientation in saturated conditions at atmospheric pressure and under increasing heat flux. Pool boiling results revealed an optimum composition, powder size and thickness for each coating types. The maximum enhancement in boiling heat transfer coefficient obtained from copper microporous coatings was up to 8.7 times relative to a plain copper test surface and nearly doubled the critical heat flux while aluminum microporous coatings enhanced boiling heat transfer coefficient by 3.5 times compared to plain aluminum surface without any further enhancement in CHF. This enhancement was ascribed to the numerous microcavities of optimum shape and size formed within the porous matrix of the coating. The detail microstructures of the coatings from the top surface as well as cross-sections are also presented through optical microscope and SEM images. The optimized aluminum coatings were also explored on fluids other than water such as acetone and HFE-7100 for their boiling heat transfer enhancement. Furthermore, the same coatings were applied on evaporative spray and jetimpingement tests using water to broaden the application of aluminum microporous coatings in evaporative cooling technology.
Author: Ajay Gurung Publisher: ISBN: Category : Languages : en Pages : 91
Book Description
The present research is an experimental study of the enhancement of pool boiling and evaporative heat transfer using high temperature thermally conductive microporous coatings. Two major types of coatings were investigated: one that is based on copper powders on copper substrate and the other on aluminum powders on aluminum substrate. Both coatings were easy to fabricate with low costs compared to conventional sintering and plasma spraying techniques, yet have high bonding strength and some of them can operate at temperatures up to 670 °C. Multiple coating options were fabricated and tested in pool boiling of water in order to optimize the coating. These coating options consisted of variations of coating composition ratio, coating thickness and powder sizes. Average powder sizes ranged from 5 micron to 110 micron, and coating thicknesses from 75 micron to 340 micron, applied on flat 1x1cm2 test heaters. The heaters were tested in the horizontal, upward-facing orientation in saturated conditions at atmospheric pressure and under increasing heat flux. Pool boiling results revealed an optimum composition, powder size and thickness for each coating types. The maximum enhancement in boiling heat transfer coefficient obtained from copper microporous coatings was up to 8.7 times relative to a plain copper test surface and nearly doubled the critical heat flux while aluminum microporous coatings enhanced boiling heat transfer coefficient by 3.5 times compared to plain aluminum surface without any further enhancement in CHF. This enhancement was ascribed to the numerous microcavities of optimum shape and size formed within the porous matrix of the coating. The detail microstructures of the coatings from the top surface as well as cross-sections are also presented through optical microscope and SEM images. The optimized aluminum coatings were also explored on fluids other than water such as acetone and HFE-7100 for their boiling heat transfer enhancement. Furthermore, the same coatings were applied on evaporative spray and jetimpingement tests using water to broaden the application of aluminum microporous coatings in evaporative cooling technology.
Author: Joo Han Kim Publisher: ISBN: 9780542979903 Category : Mechanical engineering Languages : en Pages :
Book Description
The present research is an experimental study of the enhancement of boiling heat transfer using microporous coating techniques. The current research is divided into four major phases. During the first phase, the effects of different metal particle sizes in the coating compound for thermally non-conductive microporous coating on pool boiling performance of refrigerants and water are investigated. The test surfaces were solid copper blocks with 1-cm2 base at atmospheric pressure in saturated FC-72, R-123, and water. Results showed that the surface treatment by non-conductive microporous coating significantly enhanced both nucleate boiling and critical heat flux of FC-72 and R-123. However, the enhancement of boiling performance for water was merely shown. In the second phase, thermally conductive microporous coatings to enhance boiling performance of water were developed. The first phase motivated efforts to fabricate microporous coatings with conducting binder options. The second phase was stemmed from an effort to combine the advantages of both a mixture batch type (inexpensive & easy process) and sintering/machining method (low thermal resistance of conduction). Two categories of surface treatment processes were considered in the current research. The first can be achieved by a chemical process, Multi-Staged Electroplating (MSE), which uses electricity in a chemical bath to deposit a microporous structure on the surface. The second is a soldering process, Multi-Temperature Soldering Process (MTSP), which binds the metal particles to generate optimum microporous cavities. Scanning Electron Microscope (SEM) and optical microscope images were obtained for thermally conductive microporous coated surfaces. During the third phase, the pool boiling performance of developed MSE and MTSP from second phase was confirmed for water. Results showed that the MSE and MTSP augmented the boiling performance not only for refrigerants but also for water significantly compared to non-conductive microporous coatings. Further investigation for possible future industrial applications of microporous coatings, such as indirect cooling for electronic chips, nanofluids for high power generation industries, and freezing problem of water, were conducted in the final phase.
Author: Ming-Chang Lu Publisher: ISBN: Category : Languages : en Pages : 218
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: Aniket M. Rishi Publisher: ISBN: Category : Coatings Languages : en Pages : 174
Book Description
"Rapid growth and advancements in high-power electronic devices, IC chips, electric vehicles, and lithium-ion batteries have compelled the development of efficient and novel thermal management solutions. Currently used air and liquid cooling systems are unable to remove the heat efficiently due to significant pressure drops, temperature differences, and limited heat-carrying capacities. In contrast, phase-change cooling techniques can remove the larger amount of heat with higher efficiency while maintaining safer operational temperature ranges. Pool boiling heat transfer is a type of phase-change cooling technique in which vapor bubbles generated on the boiling surface carry away the heat. This pool boiling performance is limited by the maximum heat dissipation capacity, quantified by the Critical Heat Flux (CHF), and efficiency of the boiling surface, quantified by the Heat Transfer Coefficient (HTC). This work emphasizes on improving both CHF and HTC by developing highly surface functional and tunable microporous coatings using sintering and electrodeposition techniques. Initially, graphene nanoplatelets/copper (GNP/Cu)-based composite coatings were developed using a multi-step electrodeposition technique. And 2% GNP/Cu coating rendered the highest reported CHF of 286 W/cm2 and HTC of 204 kW/m2-°C with increased bond strength. To further enhance the cohesive and adhesive bond strength of the electrodeposited coatings, a novel multi-step electrodeposition technique was developed and tested on copper-based coatings. This technique dramatically improved the overall functionality, pool boiling performance, and durability of the coatings. Later, a sintering technique was used to develop the coatings using GNP and copper particles. Uniform spreading of GNP over the coatings was obtained via ball milling technique. This technique yielded a CHF of 239 W/cm2 and the HTC of 285 kW/m2-°C (~91% and ~438% higher than a plain copper surface, respectively). A novel approach of salt-templated sintering was developed in the final part to attain a better control on porosity and wicking properties of the sintered coatings. This generated interconnected porous networks with a higher nucleating activity, and attained record-breaking CHF of 289 W/cm2 and the HTC of 1,314 kW/m2-°C."--Abstract.
Author: Publisher: Springer ISBN: 9783319266947 Category : Science Languages : en Pages : 0
Book Description
This Handbook provides researchers, faculty, design engineers in industrial R&D, and practicing engineers in the field concise treatments of advanced and more-recently established topics in thermal science and engineering, with an important emphasis on micro- and nanosystems, not covered in earlier references on applied thermal science, heat transfer or relevant aspects of mechanical/chemical engineering. Major sections address new developments in heat transfer, transport phenomena, single- and multiphase flows with energy transfer, thermal-bioengineering, thermal radiation, combined mode heat transfer, coupled heat and mass transfer, and energy systems. Energy transport at the macro-scale and micro/nano-scales is also included. The internationally recognized team of authors adopt a consistent and systematic approach and writing style, including ample cross reference among topics, offering readers a user-friendly knowledgebase greater than the sum of its parts, perfect for frequent consultation. The Handbook of Thermal Science and Engineering is ideal for academic and professional readers in the traditional and emerging areas of mechanical engineering, chemical engineering, aerospace engineering, bioengineering, electronics fabrication, energy, and manufacturing concerned with the influence thermal phenomena.
Author: S. M. Sohel Murshed Publisher: BoD – Books on Demand ISBN: 9535124056 Category : Computers Languages : en Pages : 184
Book Description
Featuring contributions from the renowned researchers and academicians in the field, this book covers key conventional and emerging cooling techniques and coolants for electronics cooling. It includes following thematic topics: - Cooling approaches and coolants - Boiling and phase change-based technologies - Heat pipes-based cooling - Microchannels cooling systems - Heat loop cooling technology - Nanofluids as coolants - Theoretical development for the junction temperature of package chips. This book is intended to be a reference source and guide to researchers, engineers, postgraduate students, and academicians in the fields of thermal management and cooling technologies as well as for people in the electronics and semiconductors industries.
Author: Ajay Gurung
Author: Ajay Gurung
Author: Joo Han Kim
Author: Ming-Chang Lu
Author: N. Zuber
Author: Mehmet Arik
Author: Aniket M. Rishi
Author: Chinmay Patil
Author: 
Author: S. M. Sohel Murshed
Author: Smreeti Dahariya