Download or read book Investigation of Heat Transfer Efficiency of Dropwise Condensation with Surface Modification by Coating and Structuring written by and published by . This book was released on 2015 with total page 151 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Investigation of Heat Transfer Efficiency of Dropwise Condensation with Surface Modification by Coating and Structuring Mathematical Model and Experimental Validation written by and published by . This book was released on 2015 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Drop Dynamics and Dropwise Condensation on Textured Surfaces written by Sameer Khandekar and published by Springer Nature. This book was released on 2020-09-11 with total page 462 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.

Download or read book Investigation of Heat Transfer Effiency of Dropwise Condensation with Surface Modification by Coating and Structuring Mathematical Model and Experimental Validation written by Chien-Hung Lu and published by . This book was released on 2015 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Dropwise Condensation on Inclined Textured Surfaces written by Sameer Khandekar and published by Springer Science & Business Media. This book was released on 2013-09-06 with total page 155 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.

Download or read book Parametric Studies on Dropwise Condensation Heat Transfer written by Kuok Kong Cheng and published by . This book was released on 2014 with total page 236 pages. Available in PDF, EPUB and Kindle. Book excerpt: Wettability of a solid surface with liquids could be tuned based on surface morphologies. This is significant in condensation heat transfer. Condensation is a critical heat transfer mechanism in industrial processes; Dropwise (DWC) and Filmwise (FWC) condensation processes are important and are focus of this study. There are few studies on surface morphology effect on wetting behavior of a solid in relation to condensation heat transfer. We conducted these studies to understand the phenomenon. Condensation heat transfer is affected by condensate droplets properties on the surface. Condensate drop sizes and mobility relationships are important to condensation heat transfer. Droplet sizes and mobility affect population density and condensation heat transfer coefficients directly. The goal of this study is to acquire the fundamental understanding of condensation heat transfer with relation to condensate droplet behavior. In order to achieve this goal, a liquid droplet wetting dynamic model was initially developed for predicting single drop behavior on different surfaces. Heat fluxes were then estimated by combining liquid droplet behavior models and heat transfer model. Condensation experiments were conducted to verify effectiveness of the model. Experimental results coupled with condensation models revealed relationships between wetting behaviors and condensation heat transfer. Contact angle hysteresis (CAH) remains low at both high and low droplet contact angle, i.e. whether the surface is hydrophilic or hydrophobic. CAH increases with degree of wetting, but coating thermal conductance also increases. Details of drop (DWC) and film-wise (FWC) condensation are presented.

Download or read book Dropwise Condensation of Water and Low Surface Tension Fluids on Structured Surfaces written by Yajing Zhao (S.M.) and published by . This book was released on 2018 with total page 62 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.

Download or read book Transient Droplet Growth During Dropwise Condensation written by Charles Joslin Hurst and published by . This book was released on 1966 with total page 158 pages. Available in PDF, EPUB and Kindle. Book excerpt: The study investigated the possibility of determining high heat-transfer rates during dropwise condensation by analyzing single condensate droplets growing on a convectively-cooled surface. A mathematical model included both the droplet and a portion of the condensing surface. Experiment utilized condensate droplets, growing on a thin copper surface, to verify the analysis. Experimental results agreed closely with predicted results--indicating high heat-transfer rates at droplet perimeters. Both experiment and analysis showed that no single vapor-to-condensing-surface temperature difference may be expected during dropwise condensation. Limitations on the calculation procedure pertaining to the region around the droplet perimeter restricted the accuracy of heat-transfer rate predictions; a separate study of droplet perimeter is suggested. (Author).

Download or read book Exploring the Limits of Dropwise Condensation on Nano structured Surfaces written by Hector Mendoza and published by . This book was released on 2013 with total page 294 pages. Available in PDF, EPUB and Kindle. Book excerpt: Within the types of condensation that can form on a surface, dropwise condensation has been previously shown to produce condensation heat transfer coefficients up to an order of magnitude greater than film condensation. Among dropwise condensation investigations, it has also been shown that smaller droplets result in higher heat transfer coefficients. An area that is currently under investigation within condensation advancements is creating superhydrophobic surfaces that can sustain smaller droplets during condensation. However, as droplet diameters are reduced to sizes comparable to the flow's mean free path, various mechanisms are expected to affect transport as the flow transitions from a continuum to free molecular flow: non-continuum transport effects, curvature effects on surface tension and on saturation conditions, and interactions with nearby droplets. In this dissertation, we investigate the limits of heat transfer performance on surfaces that strive to sustain dropwise condensation for smaller droplets. We explore and compare the limitations of dropwise condensation as mean droplet sizes are reduced to micro and nanoscales using three different models: one that uses an approximation for micro and nanoscale transport on an array of droplets, one that uses the DSMC method to simulate transport on a single droplet, and a third model that uses the DSMC method to simulate transport on an array of droplets. We found the three different models to show similar trends; dropwise condensation heat transfer coefficients increased as droplet sizes were reduced, but only up to a certain point where non-continuum transport and curvature effects became significant. For pure steam condensing on a cold wall at standard atmospheric condition with 3 degrees Celsius of subcooling, drop- wise condensation heat transfer coefficients were found to peak when droplets approached diameters near 200 nm. The effects of varying contact angle, thermal accommodation, pres- sure, amount of subcooling, spacing between droplets, and introduction of noncondensible particles into the system are also explored and discussed in detail.

Download or read book Investigation of Hydrophobic Coatings in Air Cooled Condensers for Heat Transfer Applications written by Kuok Kong Cheng and published by . This book was released on 2012 with total page 112 pages. Available in PDF, EPUB and Kindle. Book excerpt: In recent years, industries are replacing traditional fluoride based refrigerants for different alternative such as hydrocarbons and water based refrigerants in HVAC applications due to environmental concerns. Air cooled condensers are widely used in HVAC applications and there are ongoing researches to improve the thermodynamic efficiencies for these devices and environmental friendly at the same time. For internal condensations, there are several ways to enhance heat transfer performances, such as coil inserts and surface micro structures. Many of the researches focus on condenser surface modifications mainly on internal machined microstructure patterns. It was found that such techniques improve heat transfer performances up to 400 % compared with smooth surfaces due to enhance mixing and drainage. However, such techniques do not work with all geometries such as micro-channels and other complicated shapes. In the field of external condensation studies, dropwise condensations (DWC) are found to enhance heat transfer coefficients about 300 % to 400 % compared to filmwise condensation (FWC) in most researches. In order to promote DWC, hydrophobic coatings are applied to the condensing surface in most cases. Coatings techniques are solution based and could be easily applied to different geometries. Techniques for promoting DWC for external condensations can be applied to internal condensation as well. The objective of this project is to investigate heat transfer enhancements of DWC promoters has on internal condensation via model based on external DWC and FWC heat transfer models; experiments with hydrophobic coatings inside (coatings provided by NEI Corporation) as well as finned non-coated finned copper and aluminum tubes were conducted with water, n -hexane and n -heptane were carried out to investigate the effects of such promoters; experimental results were compared with establish correlations and models developed in this project. Models developed in this project suggest that DWC promoters could potentially enhance heat flux and heat transfer coefficients compared with FWC. It was found that hydrophobic coatings enhance heat flux and internal heat transfer coefficients by 20 % and 50 % for water and such coatings have negligible effects on n -hexane and n -heptane. Also, most established correlations over predict heat transfer coefficients at low mass flow in water, n -hexane and n -heptane

Download or read book Heat transfer Studies of Dropwise Condensation and Thin film Evaporation written by C. Saturnino and published by . This book was released on 1963 with total page 19 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book An Experimental Study of Dropwise Condensation on Horizontal Condenser Tubes written by John Talbot Manvel (Jr) and published by . This book was released on 1979 with total page 134 pages. Available in PDF, EPUB and Kindle. Book excerpt: Three types of drop promoting coatings were applied to the outside of 15.9 mm (5/8 in) outside-diameter condenser tubes to determine their effect on heat transfer performance. The coatings included a new fluorolepoxy, a commercial series of fluorocarbon coatings, and sputtered TFE. Coating thickness varied from 0.02 to 12.7 microns. Steam at about 21 KPa (3 psia) was condensed on the outside surface of each coated tube. Each test tube was cooled on the inside by water at velocities of 0.80 to 7.60 m/sec (3 to 25 ft/sec). The overall heat transfer coefficient was determined directly from experimental data. The inside and outside heat transfer coefficients were determined by using the Wilson Plot technique. Of the commercial fluorocarbon coatings, the 'Nedox' coating on a copper-nickel tube enhanced the outside heat transfer coefficient by 53% and improved the corrected overall heat transfer coefficient by 27%. Of the sputtered TFE coated tubes, the 0.08-micron thick coating on a copper-nickel tube enhanced the outside heat transfer coefficient by 45% and improved the corrected overall heat transfer coefficient by 21%.

Download or read book EXPERIMENTAL INVESTIGATION AND MODELING OF DROPWISE CONDENSATION ON A HORIZONTAL GOLD COATED TUBE written by and published by . This book was released on 2004 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The phenomenon dropwise condensation on a horizontal gold coated tube is investigated by both analytical and experimental methods in this study. A computer program is prepared to calculate the dropwise condensation heat transfer rate on the horizontal gold coated tube. An experimental setup was also manufactured to measure the dropwise condensation heat transfer rate. The effects of flow rate, temperature of cooling water and also steam to wall temperature difference have been analytically investigated by using Mathcad computer program. Experiments were carried out at different inlet temperatures of cooling water. Effects of cooling water at different flow rates are also experimentally investigated. Results of the experiments are compared to those of the literature and the analytical results.

Download or read book Experimental Investigation Into Condensation on Heterogeneous Surface written by Ramana Saketh Vanga and published by . This book was released on 2014 with total page 100 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.

Download or read book A Study on Heat Transfer in Dropwise Condensation written by Sanjay Adhikari and published by . This book was released on 2020 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Dropwise condensation (DWC) is a high intensity heat transfer phenomenon, with heat fluxes up to an order of magnitude higher than in filmwise condensation. Many industrial applications like desalination, thermal management, power generation and fog water harvesting may benefit from employing DWC. Considerable work has been done in this field and yet there are some challenges which remain open for research and further investigation. High resolution numerical studies that capture the complex hydrodynamics of larger droplets as they coalesce and move on the surface, effectively refreshing it for renucleation, have still not been realized as the drop size distribution spans six orders of magnitude. A new Volume of Fluid (VOF) based multi-scale approach, which resolves the grid scale larger droplets O(50 [mu]m) and models the heat transfer for the subgrid scale smaller droplets, is proposed in this dissertation. To supplement this approach, heat transfer models for individual droplets (Chapter 1) and droplet interaction (Chapter 2) are presented. Modeling work described in chapter 2 also solves many challenges associated with resolving grid scale droplets in VOF simulations. The proposed multi-scale approach also requires a transient heat transfer closure model for the subgrid scale droplets. Experimental studies have so far focused only on the steady state behavior of DWC. The startup period is still not well understood. Transient phenomena in DWC startup may significantly affect the performance of proposed applications like vapor chambers subjected to pulsed heat fluxes, or refrigeration cycles operating intermittently. This dissertation provides first characterization of fast transients during startup (chapter 4). A unique experimental facility and instrumentation are developed for measurements. Inverse numerical methods are employed to extract data (q" and HTC) from these measurements. Three distinct phases during DWC startup have been identified, which further highlight the importance of characterizing this period. The experimental data can be coupled with the numerical work described in this dissertation to formulate a robust, widely applicable simulation framework.

Download or read book Study of the Effects of Surface Morphology and Droplet Growth Dynamics on Condensation Heat Transfer written by Chun-Wei Yao and published by . This book was released on 2015 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Condensation heat transfer has recently received a lot of renewed attention due to the development and use of surfaces with micro- and nano-scale features. Most of the new surfaces tend to promote dropwise condensation, which leads to higher rates of heat transfer when compared with filmwise condensation. In the current study, the effects of surface morphology and surface chemistry on the condensation mechanisms have been investigated using engineered surfaces. Firstly, hybrid surfaces consisting of an array of micropillars with hydrophobic and hydrophilic sites have been designed to exhibit a distinct Cassie-Baxter wetting behavior at different temperatures. Characterization experiments have revealed that hybrid surfaces depict a unique wetting behavior. Furthermore, more types of engineered surfaces were fabricated including nanoparticle-based hydrophobic surface, polytetrafluoroethylene (PTFE) surface, and self-assembled monolayers (SAMs) surface. Experiments have been conducted to determine the heat transfer performance of all engineered surfaces under a constant humidity level, surface-to-ambient temperature difference, and laminar flow conditions. Experimental results reveal that droplet sliding can have an important effect on heat transfer performance. Also, empirical heat transfer correlations have been postulated and fitted using experimental data using condensing and air temperature difference and Reynolds number as independent variables. Results indicate that the postulated correlations are in excellent agreement with experimental data. In addition, surface temperature data obtained using an advanced IR imaging system have been analyzed to determine the effects of the surface features on droplet growth dynamics. The non-invasive IR measurement technique has been helpful in understanding the droplet growth dynamics such as droplet coalescence. Results to date show that the static contact angles and sliding angles have marked effects on droplet growth and coalescence on the surfaces in the early stages of condensation. Furthermore, results also reveal that droplet sliding angles can have an important effect on droplet sliding motion and condensed droplet dynamics play an important role during the overall condensation process. In summary, the effect of surface morphology and droplet growth dynamics on heat transfer during condensation were investigated and elucidated. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/152590

Download or read book Theoretical and Experimental Investigation of Condensation on Amphiphilic Nanostructured Surfaces written by David Milton Anderson and published by . This book was released on 2013 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Condensation of water vapor is an everyday phenomenon which plays an important role in power generation schemes, desalination applications and high-heat flux cooling of power electronic devices. Continuous dropwise condensation is a desirable mode of condensation in which small, highly-spherical droplets regularly form and shed off the surface before a thick liquid is formed, thereby minimizing the thermal resistance to heat transfer across the condensate layer. While difficult to induce and sustain, dropwise condensation has been shown to achieve heat and mass transfer coefficients over an order of magnitude higher than its filmwise counterpart. Superhydrophobic surfaces have been extensively studied to promote dropwise condensation with mixed results; often surfaces that are superhydrophobic to deposited droplets formed in the gas phase above the surface do not retain this behavior with condensed droplets nucleated and grown on the surface. Recently, nanostructured superhydrophobic surfaces have been developed that are robust to vapor condensation; however, these surfaces still are not ideal for condensation heat transfer due to the high thermal resistance of the vapor layer trapped underneath the droplets and the reduced footprint of direct contact between the highly-spherical droplets and the underlying substrate. This work has two main objectives. First, a comprehensive free energy based thermodynamic model is developed to better understand why traditional superhydrophobic surfaces often lose their properties when exposed to condensed droplets. The model is first validated using data from the existing literature and then extended to analyze the suitability of amphiphilic (e.g. part hydrophobic and part hydrophilic) nanostructured surfaces for condensation applications. Secondly, one of the promising amphiphilic surfaces identified by the thermodynamic model is fabricated and tested to observe condensation dynamic behavior. Two complementary visualization techniques, environmental scanning electron microscopy (ESEM) and optical (light) microscopy, are used to probe the condensation behavior and compare the performance to that of a traditional superhydrophobic surface. Observations from the condensation experiments are used to propose a new mechanism of coalescence that governs the temporal droplet size distribution on the amphiphilic nanostructured surface and continually generates fresh sites for the droplet nucleation and growth cycle that is most efficient at heat transfer.