[PDF] The Evaporation Of Drops From Super Heated Nano Engineered Surfaces eBook

Author : Fiona Rachel Hughes
Publisher :
Page : 49 pages
File Size : 48,34 MB
Release : 2009
Category :
ISBN :

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In pool boiling and spray cooling the Leidenfrost point marks the transition from nucleate boiling, in which the evaporating liquid is in contact with the surface, and film boiling, in which a layer of vapor separates the fluid from the surface. For a single evaporating drop, the Leidenfrost point occurs when the capillary and gravitational forces are surpassed by the upward pressure of the escaping vapor. This thesis develops an analytical model to predict the Leidenfrost point for a microstructured surface. The microstructure consists of a regular array of square posts geometrically defined by aspect ratio and spacing ratio. The vapor pressure is modeled using the momentum equation for flow in a porous medium. Varying the geometric parameters indicated that aspect ratio and spacing ratio must be optimized to achieve the maximum Leidenfrost temperature. For a water drop evaporating from a silicon surface, the maximum Leidenfrost temperature is predicted to occur with an aspect ratio of 1.3 and a spacing ratio of 1.5. [mu]L water drops were evaporated from a smooth surface made of silicon and porous surfaces made of aluminum oxide. The microstructure of the surfaces was different from that modeled, but increased wettability and higher Leidenfrost temperatures were observed as porosity increased. Recommendations for further research in this area are made.

Author : Sameer Khandekar
Publisher : Springer Nature
Page : 462 pages
File Size : 21,23 MB
Release : 2020-09-11
Category : Science
ISBN : 3030484610

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This book is an expanded form of the monograph, Dropwise Condensation on Inclined Textured Surfaces, Springer, 2013, published earlier by the authors, wherein a mathematical model for dropwise condensation of pure vapor over inclined textured surfaces was presented, followed by simulations and comparison with experiments. The model factored in several details of the overall quasi-cyclic process but approximated those at the scale of individual drops. In the last five years, drop level dynamics over hydrophobic surfaces have been extensively studied. These results can now be incorporated in the dropwise condensation model. Dropwise condensation is an efficient route to heat transfer and is often encountered in major power generation applications. Drops are also formed during condensation in distillation devices that work with diverse fluids ranging from water to liquid metals. Design of such equipment requires careful understanding of the condensation cycle, starting from the birth of nuclei, followed by molecular clusters, direct growth of droplets, their coalescence, all the way to instability and fall-off of condensed drops. The model described here considers these individual steps of the condensation cycle. Additional discussions include drop shape determination under static conditions, a fundamental study of drop spreading in sessile and pendant configurations, and the details of the drop coalescence phenomena. These are subsequently incorporated in the condensation model and their consequences are examined. As the mathematical model is spread over multiple scales of length and time, a parallelization approach to simulation is presented. Special topics include three-phase contact line modeling, surface preparation techniques, fundamentals of evaporation and evaporation rates of a single liquid drop, and measurement of heat transfer coefficient during large-scale condensation of water vapor. We hope that this significantly expanded text meets the expectations of design engineers, analysts, and researchers working in areas related to phase-change phenomena and heat transfer.

Author : Solomon E. Adera
Publisher :
Page : 68 pages
File Size : 30,29 MB
Release : 2012
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ISBN :

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A droplet deposited on a rough, lyophilic surface satisfying the imbibition condition, results in spontaneous spreading and hence complete wetting. However, in this thesis, we demonstrate that this wetting behavior can be altered by superheating the substrate such that droplets can reside in a non-wetting state due to evaporation. Photolithography and deep reactive ion etching were used to fabricate well-defined silicon micropillar arrays with a square pattern with varying pillar diameter, height, and center-to-center spacing. Water droplets placed on these microstructured surfaces at room temperature demonstrated superhydrophilic behavior with liquid filling the voids between pillars resulting in very low contact angle, and hence complete wetting. However, when the microstructured surface was superheated above a critical superheat, the superhydrophilicity was lost and non-wetting droplets were formed on the top surface of the micropillar array structure. The superheat required to deposit a non-wetting droplet (> 75°C) was found to be significantly higher than that required to sustain an already deposited non-wetting droplet (

Author : Hyuk-Min Kwon
Publisher :
Page : 53 pages
File Size : 48,98 MB
Release : 2013
Category :
ISBN :

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Considering that contacts between liquid and solid are ubiquitous in almost all energy processes, including steam turbines, oil pumping, condensers and boilers, the efficiency of energy transportation can be maximized such that the liquid-solid interaction is optimized. Texture based super-hydrophobicity, also known as the Lotus effect, has been one of the most extensively studied topics in the last decade. Many of the recent studies have focused on how textures induce more water repellency, and how these textures can be manufactured with different methods and materials. However, few studies have shown how these surfaces benefit the real energy processes in which the interaction between liquid droplets and solid surfaces is vigorous and influences the energy transfer performances. This work focuses on altering the hydrodynamics of droplets with nano-engineered surfaces such that it enables a variety of energy transport processes to achieve better efficiency. Firstly, the wetting transition on textured super-hydrophobic surfaces is explored. The careful investigation of Cassie-Baxter to Wenzel transition of a pendant drop during the deposition explains that the rapid deceleration-induced water hammer pressure causes the transition. This new transition mechanism for large droplets enables a new wetting transition phase diagram with a previously known Laplace mechanism that explains the small drop transition. Another class of non-wetting droplet, the Leidenfrost drop, is studied with textured super-wetting surfaces. The liquid drop loses its contact to the solid by its own vapor, created by a large superheat from the solid. The Leidenfrost effect is undesirable in cooling applications as the vapor layer acts as a barrier for heat transfer. Here, it has been studied that how textured super-hydrophilic surfaces induce droplets to wet at higher superheat via capillary wicking compare to smooth surfaces. A physical model based on scaling is developed to predict the Leidenfrost drop on single length scale textures, and validated by the experiments. Additionally, the physical mechanism suggests that hierarchical textures have a higher Leidenfrost temperature compared to single-length-scale textures, confirmed experimentally. Lastly, the recently discovered rare-earth oxide ceramics are studied, which ensures the benefits of water repellency under harsh conditions such as high temperature and abrasive wear. Texturing of the rare-earth oxide ceramic is explored by the laser ablation technique. Unique micro- and nano-scale hierarchical textures are created, enhancing the water repellency, resulting in the super-hydrophobic rare-earth ceramic.

Author : David Brutin
Publisher : Academic Press
Page : 464 pages
File Size : 10,68 MB
Release : 2015-05-11
Category : Science
ISBN : 0128008083

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Droplet Wetting and Evaporation provides engineers, students, and researchers with the first comprehensive guide to the theory and applications of droplet wetting and evaporation. Beginning with a relevant theoretical background, the book moves on to consider specific aspects, including heat transfer, flow instabilities, and the drying of complex fluid droplets. Each chapter covers the principles of the subject, addressing corresponding practical issues and problems. The text is ideal for a broad range of domains, from aerospace and materials, to biomedical applications, comprehensively relaying the challenges and approaches from the different communities leading the way in droplet research and development. Provides a broad, cross-subject coverage of theory and application that is ideal for engineers, students and researchers who need to follow all major developments in this interdisciplinary field Includes comprehensive discussions of heat transfer, flow instabilities, and the drying of complex fluid droplets Begins with an accessible summary of fundamental theory before moving on to specific areas such as heat transfer, flow instabilities, and the drying of complex fluid droplets

Author : Martin Rein
Publisher : Springer Science & Business Media
Page : 328 pages
File Size : 32,64 MB
Release : 2002-10-30
Category : Computers
ISBN : 9783211836927

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This book presents a comprehensive overview of fluid mechanical, thermal and physico-chemical aspects of drop-surface interactions. Basic physical mechanisms pertaining to free-surface flow phenomena characteristic of drop impact on solid and liquid surfaces are explained emphasizing the importance of scaling. Moreover, physico-chemical fundamentals relating to a forced spreading of complex solutions, analytical tools for calculating compressibility effects, and heat transfer and phase change phenomena occurring during solidification and evaporation processes, respectively, are introduced in detail. Finally, numerical approaches particularly suited for modeling drop-surface interactions are consisely surveyed with a particular emphasis on boundary integral methods and Navier-Stokes algorithms (volume of fluid, level set and front tracking algorithms). The book is closed by contributions to a workshop on Drop-Surface Interactions held at the International Centre of Mechanical Sciences.