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Author: Samuel Steven Cruz
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Book Description
Condensation is a phenomenon that is ubiquitous in nature and used to effectively transfer heat in many important industrial applications including thermal management of electronics, steam power generation, and natural gas processing. Industry relies mainly on filmwise condensation, where the condensate forms an insulating, thick liquid film on the condensation surface, posing a large thermal resistance. For almost a century, much work has explored developing surfaces that can promote the nucleation, growth, coalescence and effective shedding of mobile condensing droplets from surfaces, called dropwise condensation, which is known to enhance the heat transfer up to an order of magnitude. However, the requirement for ultra-thin coatings has hampered the wide adoption of this form of condensation as thin hydrophobic coatings degrade over time in various industrial applications. In this thesis, we model, fabricate, optimize, experimentally demonstrate a proof-of-concept for a novel condensation approach which we term capillary-driven condensation. The method consists of a hierarchical structure consisting of a hydrophobic porous membrane attached on top of a wicking structure that is firmly bonded to the condenser surface. The wicking structure and the membrane can be separately tailored to maximize the fluid flow in the wick and its effective thermal conductivity, as well as the maximum capillary pressure that the membrane can sustain to push fluid through a viscous pressure drop in the porous wick to an exit port. The geometry can be optimized to reduce the thermal resistance of the structure, as well as maximize the amount of condensate that can be removed passively by capillarity. To demonstrate the viability of this condensation method, we fabricated the proposed structure with highly-defined geometry utilizing silicon microfabrication technique s. The result is a surface which is able to constrain a thin film of condensate within a high thermal conductivity wicking structure while the top condensation surface appears dry despite sustaining condensation rates above those of filmwise condensation. The thickness of this layer and geometry of the membrane can be rationally designed to maximize the heat transfer coefficient even beyond dropwise condensation. Heat transfer measurements indicate a potential range of enhancement of ~~ 40% to ~~ 400% which is to be confirmed by more sensitive experiments that reduce error. The results from this thesis show a proof-of-concept and support the promise of capillary-driven condensation surfaces for various heat transfer applications.
Author: Samuel Steven Cruz
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Book Description
Condensation is a phenomenon that is ubiquitous in nature and used to effectively transfer heat in many important industrial applications including thermal management of electronics, steam power generation, and natural gas processing. Industry relies mainly on filmwise condensation, where the condensate forms an insulating, thick liquid film on the condensation surface, posing a large thermal resistance. For almost a century, much work has explored developing surfaces that can promote the nucleation, growth, coalescence and effective shedding of mobile condensing droplets from surfaces, called dropwise condensation, which is known to enhance the heat transfer up to an order of magnitude. However, the requirement for ultra-thin coatings has hampered the wide adoption of this form of condensation as thin hydrophobic coatings degrade over time in various industrial applications. In this thesis, we model, fabricate, optimize, experimentally demonstrate a proof-of-concept for a novel condensation approach which we term capillary-driven condensation. The method consists of a hierarchical structure consisting of a hydrophobic porous membrane attached on top of a wicking structure that is firmly bonded to the condenser surface. The wicking structure and the membrane can be separately tailored to maximize the fluid flow in the wick and its effective thermal conductivity, as well as the maximum capillary pressure that the membrane can sustain to push fluid through a viscous pressure drop in the porous wick to an exit port. The geometry can be optimized to reduce the thermal resistance of the structure, as well as maximize the amount of condensate that can be removed passively by capillarity. To demonstrate the viability of this condensation method, we fabricated the proposed structure with highly-defined geometry utilizing silicon microfabrication technique s. The result is a surface which is able to constrain a thin film of condensate within a high thermal conductivity wicking structure while the top condensation surface appears dry despite sustaining condensation rates above those of filmwise condensation. The thickness of this layer and geometry of the membrane can be rationally designed to maximize the heat transfer coefficient even beyond dropwise condensation. Heat transfer measurements indicate a potential range of enhancement of ~~ 40% to ~~ 400% which is to be confirmed by more sensitive experiments that reduce error. The results from this thesis show a proof-of-concept and support the promise of capillary-driven condensation surfaces for various heat transfer applications.
Author: Yajing Zhao (Mechanical engineer)
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Steam power plants, which contribute to over 50% of energy production globally, rely on condensers to control system-level energy efficiency. Due to the high surface energy of common heat exchanger materials, the vapor condenses by forming a continuous liquid film with low thermal conductivity (filmwise condensation), hindering heat transfer from the vapor side to the condenser surface. Hydrophobic surfaces achieved by either chemical methods (e.g., coating treatment) or physical methods (e.g. structures design) have shown great promise in enhancing condensation heat transfer by promoting dropwise condensation. However, the short lifetime and high fabrication cost of most of these hydrophobic surfaces remain a challenge for long-term and large-scale industrial applications. A promising solution to enhancing condensation heat transfer in a robust and scalable manner is to control the thickness and thermal conductivity of the condensate film, which we term thin-film condensation. This can be achieved by sandwiching a thin layer of porous metal wick between a hydrophobic membrane and the condenser surface to confine the condensed liquid, forming a thin liquid-metal composite film that significantly improves the effective thermal conductivity of the condensate-filled porous media. In this work, we designed, fabricated, tested, and demonstrated thin-film condensation heat transfer using commercially available materials and scalable approaches. First, we proved the concept using biphilic, microchannel-assisted hierarchical copper surfaces made of commercially available copper foams and copper meshes. Condensation heat transfer on the hierarchical copper surfaces was characterized to be up to 2x as compared to the conventional filmwise condensation, even with flooding on the surface due to the defects on the mesh and the coating. Then, we investigated electrospinning as a potential approach to customize hydrophobic membranes for the thin-film condenser surfaces. The key benefit of the hydrophobic membrane in the surface design is to generate capillary pressure through micro/nanoscale pores, which acts as the driving force for the condensate flow in the metal wick. We conducted a parametric study on the effects of several key fabrication parameters on the pore size of the electrospun membrane, with the help of the fractional factorial design. Solution feeding rate was found to be the most impactful parameter on the membrane pore size and should be considered the most during membrane optimization. A heat and mass transfer model was developed to predict the heat transfer performance of the thin-film condenser surfaces made of electrospun membranes and porous copper wicks. Upon careful design of the surface structures, an over 5x heat transfer enhancement is expected on these thin-film condensers, which is comparable to the state-of-the-art dropwise condensation. Finally, a techno-economic analysis was conducted on the thin-film condensers. The result shows that the additional material for the condenser tube modification costs less than 10% of the condenser cost. However, with the expected 5x steam-side condensation heat transfer performance, thin-film condensers will be able to increase power plants' output by 2-6%, which is equivalent to over $10B of the value proposition for steam power plants across the globe.
Author: V. G. Rifert
Publisher: WIT Press (UK)
ISBN:
Category : Science
Languages : en
Pages : 402
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Book Description
In processes with condensation of steam-to-gas compositions or refrigerative agents an intensification of condensation is often required because thermal resistance on the condensation side can be greater than the thermal resistance of the heat transfer wall on the cooling side. In this work, a pair of researchers from the National Technical U. of Ukraine and the State Academy of Refrigeration (Ukraine) provide information about the enhancement of condensation, including research results from the former USSR they feel deserves wider dissemination as well as discussion of different theoretical models of the condensation process. The US office of WIT Press is Computational Mechanics. Annotation : 2004 Book News, Inc., Portland, OR (booknews.com).
Author: Ramana Saketh Vanga
Publisher:
ISBN:
Category : Condensation
Languages : en
Pages : 100
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Book Description
Renewable energy systems operated by a thermal energy resource such as geothermal power plants and solar thermal power systems are demanding improvement in their condensation performance. While their energy resources are naturally obtained at almost no cost, heat rejecting components are relatively expensive to maintain and operate. In this research, a heterogeneous condensing surface is proposed to enhance the condensation heat transfer coefficient in vapor-to-liquid heat exchangers. Parallel stripes with hydrophobic feature and ones without it alternate on its surface. The effect of surface wettability variation that is generated by the heterogeneous surface on the dropwise condensation heat transfer of saturated steam on the flat plate copper surface is experimentally investigated. A vertical flat plate condenser is constructed to evaluate the performance of the heterogeneous condensing surface in comparison with a plain copper sample and a homogeneous hydrophobic-treated copper sample. Experimental results show that condensation heat transfer of steam on the homogeneous hydrophobic-treated sample is superior to that of the plain copper surface despite the fact that both the surfaces stably promote dropwise condensation. At the subcooling temperature of 3°C, the difference in the heat transfer coefficients between the plain copper sample and the hydrophobic-treated copper sample is almost twofold. The heat transfer coefficients for the heterogeneous surface at smaller subcooling temperatures, when its stripes situate horizontally, are as high as the heat transfer coefficients for the homogeneous hydrophobic-treated surface. The enhancement for the horizontal heterogeneous sample over the plain copper sample is approximately 100%. The heat transfer coefficient for the heterogeneous sample with its stripes being vertical at 4°C subcooling is 25% greater than that of the plain copper sample. Higher heat transfer coefficients are observed at smaller subcooling temperatures for all the samples. The results and observations of this project suggest that the heterogeneous surface has the potential to enhance the heat transfer coefficients.
Author: Karl Stephan
Publisher: Springer Science & Business Media
ISBN: 3642524575
Category : Technology & Engineering
Languages : en
Pages : 342
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Book Description
I welcome the opportunity to have my book translated, because of the great emphasis on two-phase flow and heat transfer in the English-speaking world, as related to research, university education, and industrial practice. The 1988 Springer-Verlag edition of "Warmeiibergang beim Kondensieren und beim Sieden" has been enlarged to include additional material on falling film evaporation (Chapter 12) and pressure drop in two-phase flow (Chapter 13). Minor errors in the original text have also been corrected. I would like to express my sincere appreciation to Professor Green, Asso ciate Professor of German at Rensselaer, for his excellent translation and co operation. My thanks go also to Professor Bergles for his close attention to technical and linguistic details. He carefully read the typescript and made many comments and suggestions that helped to improve the manuscript. I hope that the English edition will meet with' a favorable reception and contribute to better understanding and to progress in the field of heat transfer in condensation and boiling. February 1992 K. Stephan Preface to the German-Language Edition This book is a continuation of the series "Heat and Mass Transfer" edited by U. Grigull, in which three volumes have already been published. Its aim is to acquaint students and practicing engineers with heat transfer during condensa tion and boiling, and is intended primarily for students and engineers in mechanical, chemical, electrical, and industrial processing engineering.
Author: Aaron Stone
Publisher:
ISBN:
Category : Condensers (Steam)
Languages : en
Pages : 194
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Author: Ch Marvillet
Publisher: Elsevier Science Limited
ISBN: 9782906077782
Category : Technology & Engineering
Languages : en
Pages : 219
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Book Description
Hardbound. This book deals with condensation, one of the most complex heat and mass transfer processes, involving mass, momentum, energy and chemical species transport in the vapour and liquid phases. Condensers are of many types including large power plant condensers, shell-and-tube process condensers, in-tube condensers, and plate heat exchangers. The size of condensers varies by a factor of around a million from the condenser of a motor vehicle air conditioning unit to that used in steam power plant.
Author:
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Category :
Languages : en
Pages :
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Abstract : Extensive research has been carried out over the course of the last few decades to induce dropwise condensation as it offers 5 - 7 times better heat transfer performance compared to filmwise condensation process. A number of methods such as low surface energy hydrophobic coatings, surface modification of hydrophobic surfaces to fabricate micro, nano and hierarchical structures, and the recent incorporation of jumping droplet phenomenon have provided effective means to further enhance the condensation heat transfer. However, existing methods to enhance condensation heat transfer rate fail in the case of low surface tension, highly wetting liquids such as hydrocarbons, cryogens, and fluorinated dielectrics and refrigerants used in various industrial applications. Due to their extremely wetting behavior, such fluids almost always condense in a filmwise mode and the removal of the condensate other than by gravity has been a challenge. Here, we fabricate a novel capillary surface to decouple the removal of the condensate vapor from the condensing surface. The new surface consists of alternating capillary bridge and plain sections. The liquid condensing in the plain channels and the outer surfaces of the capillary bridge is wicked into the wick bridge, effectively decoupling the condensation surface and the condensate removal paths. We have determined that the condensation performance of the fabricated surfaces is enhanced by a factor of 3 compared to a plain surface, and further enhanced by a factor of 4.5, compared to a plain surface, by bonding an additional cover mesh layer and decreasing the channels widths of the condensation surface. This proves that the concept of employing a capillary bridge greatly enhances the rate of condensation for low surface tension liquids such as dielectric fluids. Hence, the knowledge gained from this thesis will serve as basic guideline for designing new simple, cost effective, and scalable surface technologies with enhanced condensation heat transfer for widely used low surface tension liquids.
Author:
Publisher:
ISBN:
Category : Mechanics, Applied
Languages : en
Pages : 400
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Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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In an effort to explore the possibility of building compact naval steam condensers, an experimental apparatus was designed and constructed to study enhanced condensation heat transfer of steam on horizontal tubes. Special care was taken to ensure a leak-tight apparatus so that the non-condensable gas content of the steam can be kept to a few parts-per-million. The boiler and steam piping is made of glass and stainless steel with rubber gaskets. Copper is used for the condensing tubes. The completed system has been tested satisfactorily at full power. (Author).
