CAPILLARY-ASSISTED ENHANCED CONDENSATION HEAT TRANSFER FOR LOW SURFACE TENSION LIQUIDS PDF Download
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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.
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Category :
Languages : en
Pages :
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Book Description
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: Daniel John Preston
Publisher:
ISBN:
Category :
Languages : en
Pages : 125
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Vapor condensation is routinely used as an effective means of transferring heat or separating fluids for applications ranging from personal electronic device thermal management to natural gas processing and electric power generation. Filmwise condensation, where the condensed fluid forms a liquid film, is prevalent in typical industrial-scale systems. Conversely, dropwise condensation, where the condensate forms discrete liquid droplets, results in an improvement in heat transfer performance of up to an order of magnitude compared to filmwise condensation. We explored rare earth oxides (REOs) as a potential coating to induce dropwise condensation of water; specifically, we experimentally demonstrated that the mechanism for REO hydrophobicity results from adsorption of contaminants from the atmosphere. We also used graphene, which is hydrophobic in nature, as a coating to achieve robust dropwise water condensation. With a graphene coating, we demonstrated a 4x improvement in water condensation heat transfer compared to filmwise condensation with robustness superior to state-of-the-art hydrophobic monolayer coatings. Meanwhile, low surface tension condensates pose a unique challenge since they often form a film, even on hydrophobic coatings. Lubricant infused surfaces (LIS) represent a potential solution, where a lubricant immiscible with the low surface tension condensate is infused into a rough structure on the condenser surface to repel the condensate. We developed a detailed surface-energy-based model to provide design guidelines for any arbitrary LIS system. We then characterized heat transfer coefficients during condensation of low surface tension fluids on LIS in a controlled environmental chamber for the first time, where a 5x improvement was demonstrated compared to filmwise condensation. The improved condensation heat transfer coefficients realized by LIS for low surface tension fluids and by REOs and graphene for water present opportunities for significant energy savings in device thermal management, heating and cooling, and power generation.
Author: Adam Taylor Paxson
Publisher:
ISBN:
Category :
Languages : en
Pages : 164
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Book Description
This thesis investigates the use of three classes advanced materials for promoting dropwise condensation: 1. robust hydrophobic functionalizations 2. superhydrophobic textures 3. lubricant-imbibed textures We first define the functional requirements of a hydrophobic functionalization for promoting dropwise condensation and use these guidelines to investigate two subclasses of materials: rare-earth ceramics and fluoropolymer films deposited via initiated chemical vapor deposition (iCVD). We show how both materials exhibit robust dropwise behavior, and further subject an iCVD film to an accelerated endurance trial to show how it sustains dropwise condensation throughout a 3-month equivalent trial. Next we combine hydrophobic functionalization with rough texture to obtain superhydrophobic surfaces and identify a self-similar depinning mechanism governing adhesion on surfaces with multiple roughness length scales. We introduce the metric of pinned fraction to show how these surfaces must be designed to minimize adhesion. We then show how dropwise condensation on superhydrophobic surfaces and the ensuing "jumping" behavior consists of not only binary coalescences, but multiple-drop coalescences with tangential departure that result in increased departing mass flux. However, we find that although this mode of condensation is readily achievable when condensing working fluids with high surface tension, such as water, even re-entrant structures that are known to support millimetric droplets of low-surface tension liquids in a superhydrophobic state are not sufficient to promote the dropwise mode of condensation for working fluids with low surface tension. Finally, we extend the applicability of textured surfaces by imbibing solid textures with a lubricant stabilized by capillary wicking. We show how these surfaces, when both solid texture and lubricant are properly designed, can promote dropwise condensation and reduce departing diameter of not only steam, but also of low-surface tension working fluids. In summary, we find that all three classes of surfaces provide significant increases in vapor-side heat transfer coefficient. However, when considering the overall heat transfer coefficient of a surface condenser, we find that most of the benefits of dropwise condensation can be realized by hydrophobic functionalization.
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: Yajing Zhao (S.M.)
Publisher:
ISBN:
Category :
Languages : en
Pages : 62
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Book Description
Condensation is a ubiquitous process often observed in nature and our daily lives. The large amount of latent heat released during the condensation process has been harnessed in many industrial processes such as power generation, building heating and cooling, desalination, dew harvesting, thermal management, and refrigeration. Condensation has two modes: dropwise mode and filmwise mode. Although it has been known for decades that dropwise condensation outperforms filmwise condensation in heat transfer owing to the droplet shedding effects which can efficiently reduce thermal resistance, filmwise condensation still dominates industrial applications currently due to the high costs, low robustness and technical challenges of manufacturing dropwise coatings. During water condensation, dropwise mode can be readily promoted with thin hydrophobic coatings. Superhydrophobic surfaces made out of hydrophobic coatings on micro-or-nano-engineered surfaces have shown further heat transfer enhancement in dropwise condensation of water; however, the applications of these micro- or nanoscale structured surface designs have been restricted by the high manufacturing expenses and short range of subcooling limit. Recent studies have shown that the combination of millimeter sized geometric features and plain hydrophobic coatings can effectively manipulate droplet distribution of water condensate, which provides opportunities to locally facilitate dropwise condensation at relatively low manufacturing expenses as compared to those delicate micro- and nano-structured hydrophobic surfaces. Low surface tension fluids such as hydrocarbons pose a unique challenge to achieving dropwise condensation, because common hydrophobic coatings are not capable of repelling low surface tension fluids. Recent development in lubricant infused surfaces (LIS) offers promising solutions to achieving dropwise condensation of low surface tension fluids by replacing the solid-condensate interface in conventional hydrophobic coatings with a smooth lubricant-condensate interface. However, only a few experimental studies have applied LIS to promoting dropwise condensation of low surface tension fluids (y as low as 15 mN/m). In this work, we investigated dropwise condensation of both water (y ~ 72 mN/m) and a low surface tension fluid, namely butane (y - 13 mN/m) on structured surfaces. For water condensation, we studied the effects of millimeter sized geometric structures on dropwise condensation heat transfer under two different environments: pure vapor and an air-vapor mixture. Our experimental results show that, although convex structures enable faster droplet growth in an air-vapor mixture, the same structures impose the opposite effect during pure vapor condensation, hindering droplet growth. We developed a numerical model for each case to predict the heat flux distribution along the structured surface, and the model shows good agreement with experimental results. This work demonstrates that the effects of geometric features on dropwise condensation are not invariable but rather dependent on the scenario of resistances to heat and mass transfer in the system. For butane condensation, based on a design guideline we recently developed for lubricant infused surfaces, we successfully designed an energy-favorable combination of lubricant and structured solid substrate, which was further demonstrated to promote dropwise condensation of butane. The fundamental understanding of dropwise condensation of water and low surface tension fluids on structured surfaces developed in this study provides useful guidelines for condensation applications including power generation, desalination, dew harvesting, and thermal management.
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: Sudarshan Sarathy
Publisher:
ISBN:
Category :
Languages : en
Pages : 64
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Book Description
Condensation of low surface tension fluids is important in liquefied natural gas processing and refrigeration systems. Current state of the art low surface energy low hysteresis coatings are not able to achieve dropwise condensation below 10 mN/m. Surface acoustic waves are proposed as an active method to shed thin condensate films to reduce their thermal resistance and improve heat transfer coefficients. Interdigitated electrode patterns were fabricated on piezoelectric LiNbO3 wafers and SAW waves were generated with RF voltages in the 12.5 - 100 MHz regime. These were tested in the in-house condensation rig with Ethanol, Pentane, Hexane and Perfluorohexane. Heat transfer coefficients showed more than 2X improvement over standard filmwise condensation. Further, the effect of refrigerant side heat transfer enhancement in a condenser operating in a vapor compression refrigeration cycle is studied as a potential application.
Author: P. J. Marto
Publisher:
ISBN:
Category : Condensation
Languages : en
Pages : 128
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Author: Samuel Steven Cruz
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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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: Sameer Khandekar
Publisher: Springer
ISBN: 9781461484462
Category : Science
Languages : en
Pages : 0
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Dropwise Condensation on Textured Surfaces presents a holistic framework for understanding dropwise condensation through mathematical modeling and meaningful experiments. The book presents a review of the subject required to build up models as well as to design experiments. Emphasis is placed on the effect of physical and chemical texturing and their effect on the bulk transport phenomena. Application of the model to metal vapor condensation is of special interest. The unique behavior of liquid metals, with their low Prandtl number and high surface tension, is also discussed. The model predicts instantaneous drop size distribution for a given level of substrate subcooling and derives local as well as spatio-temporally averaged heat transfer rates and wall shear stress.
