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Author: David Milton Anderson Publisher: ISBN: Category : Amphiphiles Languages : en Pages :
Book Description
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.
Author: David Milton Anderson Publisher: ISBN: Category : Amphiphiles Languages : en Pages :
Book Description
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.
Author: Nicholas Gang Dou Publisher: ISBN: Category : Languages : en Pages : 33
Book Description
Condensation is an important process in many power generation and water desalination technologies. Superhydrophobic nanostructured surfaces have unique condensation properties that may enhance heat transfer through a mechanism driven by surface tension. The increased droplet removal rate and reduced size of departing droplets facilitates improvements over traditional filmwise and dropwise condensation. This work investigates copper oxide nanostructures as a potential substrate for superhydrophobic condensation. We first explored and refined a procedure for creating and functionalizing oxide nanostructures on copper surfaces. The condensation behavior of these samples was characterized using an optical microscope and an environmental scanning electron microscope by quantifying the distribution of nucleation sites and droplet growth behavior. Using the knowledge gained from these studies, an experimental test bed was designed and fabricated to measure macro-scale condensation heat transfer for flat and tubular sample geometries. Preliminary results validate previous theoretical and experimental studies of filinwise condensation and demonstrate heat transfer enhancement with dropwise condensation. The copper oxide nanostructures exhibited jumping, which shows the effectiveness of the synthesis procedure.
Author: Nenad Miljkovic Publisher: ISBN: Category : Languages : en Pages : 185
Book Description
Micro/nanostructures have long been recognized to have potential for heat transfer enhancement in phase-change processes by achieving extreme wetting properties, which is of great importance in a wide range of applications including thermal management, building environment control, water harvesting, desalination, and industrial power generation. This thesis focuses on the fundamental understanding of water vapor condensation on superhydrophobic surfaces, as well as the demonstration of such surfaces for enhanced condensation heat transfer performance. We first studied droplet-surface interactions during condensation on superhydrophobic surfaces to understand the emergent droplet wetting morphology. We demonstrated the importance of considering local energy barriers to understand the condensed droplet morphologies and showed nucleation-mediated droplet-droplet interactions can overcome these barriers to develop wetting states not predicted by global thermodynamic analysis. To minimize these droplet-droplet interactions and ensure the formation of favorable morphologies for enhanced condensation heat transfer, we show that the structure length scale needs to be minimized while ensuring the local energy barriers satisfy the morphology dependent criteria. This mechanistic understanding offers insight into the role of surface-structure length scale and provides a quantitative basis for designing surfaces optimized for condensation in engineered systems. Using our understanding of emergent droplet wetting morphology, we experimentally and numerically investigated the morphology dependent individual droplet growth rates. By taking advantage of well-controlled functionalized silicon nanopillars, the growth and shedding behavior of both suspended and partially wetting droplets on the same surface during condensation was observed. Environmental scanning electron microscopy was used to demonstrate that initial droplet growth rates of partially wetting droplets were 6 times larger than that of suspended droplets. A droplet growth model was developed to explain the experimental results and showed that partially wetting droplets had 4-6 times higher heat transfer rates than that of suspended droplets. Based on these findings, the overall performance enhancement created by surface nanostructuring was examined in comparison to a flat hydrophobic surface. These nanostructured surfaces had 56% heat flux enhancement for partially wetting droplet morphologies, and 71% heat flux degradation for suspended morphologies in comparison to flat hydrophobic surfaces. This study provides fundamental insights into the previously unidentified role of droplet wetting morphology on growth rate, as well as the need to design nanostructured surfaces with tailored droplet morphologies to achieve enhanced heat and mass transfer during dropwise condensation. To create a unified model for condensation capable of predicting the surface heat transfer for a variety of surface length scales, geometries, and condensation conditions, we incorporated the emergent droplet wetting morphology, individual droplet heat transfer, and size distribution. The model results showed a specific range of characteristic length scales (0.5 - 2 ptm) allowing for the formation of coalescence-induced jumping droplets with a 190% overall surface heat flux enhancement over conventional flat dropwise condensing surfaces. This work provided a unified model for dropwise condensation on micro/nanostructured superhydrophobic surfaces and offered guidelines for the selection of ideal structured surfaces to maximize heat transfer. Using the insights gained from the developed model and optimization, a scalable synthesis technique was developed to produce functionalized oxide nanostructures on copper surfaces capable of sustaining superhydrophobic condensation. Nanostructured copper oxide (CuO) films were formed via chemical oxidation in an alkaline solution resulting in dense arrays of sharp CuO nanostructures with characteristic heights and widths of -1 pm and -300 nm, respectively. Condensation on these surfaces was characterized using optical microscopy and environmental scanning electron microscopy to quantify the distribution of nucleation sites and elucidate the growth behavior of individual droplets with characteristic radii of -1 to 10 pm at supersaturations
Author: Ramana Saketh Vanga Publisher: ISBN: Category : Condensation Languages : en Pages : 100
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: Rebecca Lynn Winter Publisher: ISBN: Category : Amphiphiles Languages : en Pages : 118
Book Description
Condensation plays a fundamental role in our lives, from the way our climate functions to our ability to generate and use energy. The large amount of energy typically involved in phase change processes means that small improvements in efficiency could translate to impactful energy savings. Condensation can be significantly enhanced by changing the roughness and material of a surface to reduce thermal resistance. Notably, the use of hydrophobic and superhydrophobic materials enhance droplet shedding, which has been found to greatly improve performance. However, neither the most state-of-the-art, nor the simplest and most thoroughly studied of these hydrophobic designs are broadly implemented in real-world systems. These surfaces are often fragile, costly to manufacture, suffer from short usable lives, and can exhibit problematic failure modes. Here, amphiphilic surfaces, characterized by features with both hydrophilic and hydrophobic materials, demonstrate a new approach to the manipulation of wetting behavior and heat transfer.This work focuses on the experimental and analytical study of amphiphilic surfaces for use in condensation. First, an analytical foundation for the wetting behavior and heat transfer on amphiphilic surfaces is developed. The formation and stability of efficient thin filmwise condensation on microamphiphilic and hierarchical amphiphilic surfaces are presented. Next, this work is expanded to use in thermally conductive filmwise condensation on macroscale finned amphiphilic surfaces. A novel dewetting phenomenon is identified and characterized, which promotes cyclical dropwise condensation without the need for an ultrathin hydrophobic coating. Finally, the macroamphiphilic work is extended to use on cylindrical tubes in a controlled condensing environment. Both a stable conductive filmwise mode and a dynamic cyclical drainage mode are identified and characterized on tubes. Both are found to exhibit efficient condensing performance using completely distinct fundamental behaviors, and the cylindrical geometry is determined to offer additional stability to both condensing modes. The use of macroscale features with simple geometries, small dependence on coating thickness, and robustness of wetting dynamics make the work presented here particularly relevant to optimization for real-world application.
Author: Phuong Pham Publisher: BoD – Books on Demand ISBN: 1789851998 Category : Technology & Engineering Languages : en Pages : 295
Book Description
Surface sciences elucidate the physical and chemical aspects of the surfaces and interfaces of materials. Of great interest in this field are nanomaterials, which have recently experienced breakthroughs in synthesis and application. As such, this book presents some recent representative achievements in the field of surface science, including synthesis techniques, surface modifications, nanoparticle-based smart coatings, wettability of different surfaces, physics/chemistry characterizations, and growth kinetics of thin films. In addition, the book illustrates some of the important applications related to silicon, CVD graphene, graphene oxide, transition metal dichalcogenides, carbon nanotubes, carbon nanoparticles, transparent conducting oxide, and metal oxides.
Author: David Milton Anderson
Author: David Milton Anderson
Author: Merid Aboye
Author: Nataliia Fedorova
Author: Nicholas Gang Dou
Author: Nenad Miljkovic
Author: Ramana Saketh Vanga
Author: 
Author: Rebecca Lynn Winter
Author: Hadi Abbas Hadi
Author: Phuong Pham