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Author: Yen-Po Lin Publisher: ISBN: Category : Languages : en Pages :
Book Description
Spray cooling has proven to be efficient in managing thermal load in high power applications. Reliability of electronic products relies on the thermal management and understanding of heat transfer mechanisms including those related to spray cooling. However, to date, several of the key heat transfer mechanisms are still not well understood. An alternative approach for improving the heat transfer performance is to change the film dynamics through surface modification. The main goal of this study is to understand the effects of nano-scale features on flat heater surfaces subjected to spray cooling and to determine the major factors in droplet impingement cooling to estimate their effects in the spray cooling system. Single droplet stream and simultaneous triple droplet stream with two different stream spacings (500 [mu]m and 2000 [mu]m), experiments have been performed to understand the droplet-surface interactions relevant to spray cooling systems. Experiments have been conducted on nano-structured surfaces as well as on flat (smooth) surfaces. It is observed that nano-structured surfaces result in lower minimum wall temperatures, better heat transfer performance, and more uniform temperature distribution. A new variable, effective thermal diameter (de), was defined based on the radial temperature profiles inside the impact zone to quantify the effects of the nano-structured surface in droplet cooling. Results indicate that larger effective cooling area can be achieved using nano-structured surface in the single droplet stream experiments. In triple stream experiments, nano-structured surface also showed an enhanced heat transfer. In single stream experiments, larger outer ring structures (i.e. larger outer diameters) in the impact crater were observed on the nano-structured surfaces which can be used to explain enhanced heat transfer performance. Smaller stream spacing in triple stream experiments reveal that the outer ring structure is disrupted resulting in lower heat transfer. Lower static contact angle on the nano-structured surface has been observed, which implies that changes in surface properties result in enhanced film dynamics and better heat transfer behavior. The results and conclusions of this study should be useful for understanding the physics of spray cooling and in the design of better spray cooling systems.
Author: Yen-Po Lin Publisher: ISBN: Category : Languages : en Pages :
Book Description
Spray cooling has proven to be efficient in managing thermal load in high power applications. Reliability of electronic products relies on the thermal management and understanding of heat transfer mechanisms including those related to spray cooling. However, to date, several of the key heat transfer mechanisms are still not well understood. An alternative approach for improving the heat transfer performance is to change the film dynamics through surface modification. The main goal of this study is to understand the effects of nano-scale features on flat heater surfaces subjected to spray cooling and to determine the major factors in droplet impingement cooling to estimate their effects in the spray cooling system. Single droplet stream and simultaneous triple droplet stream with two different stream spacings (500 [mu]m and 2000 [mu]m), experiments have been performed to understand the droplet-surface interactions relevant to spray cooling systems. Experiments have been conducted on nano-structured surfaces as well as on flat (smooth) surfaces. It is observed that nano-structured surfaces result in lower minimum wall temperatures, better heat transfer performance, and more uniform temperature distribution. A new variable, effective thermal diameter (de), was defined based on the radial temperature profiles inside the impact zone to quantify the effects of the nano-structured surface in droplet cooling. Results indicate that larger effective cooling area can be achieved using nano-structured surface in the single droplet stream experiments. In triple stream experiments, nano-structured surface also showed an enhanced heat transfer. In single stream experiments, larger outer ring structures (i.e. larger outer diameters) in the impact crater were observed on the nano-structured surfaces which can be used to explain enhanced heat transfer performance. Smaller stream spacing in triple stream experiments reveal that the outer ring structure is disrupted resulting in lower heat transfer. Lower static contact angle on the nano-structured surface has been observed, which implies that changes in surface properties result in enhanced film dynamics and better heat transfer behavior. The results and conclusions of this study should be useful for understanding the physics of spray cooling and in the design of better spray cooling systems.
Author: Jian Shen Publisher: ISBN: Category : Nanofluids Languages : en Pages : 238
Book Description
This study investigates the hydrodynamic characteristics of droplet impinging on heated surfaces using High Speed High Resolution (HSHR) imaging and evaluates the effect of surface temperature and using water and a nanofluid on a polished and a nano-structured surface. Three types of surfaces are used in the study: polished silicon, nano-structured porous silicon and gold coated polished silicon. Seven different surface temperatures including single phase (non-boiling) and two phase (boiling) conditions are studied. Droplet impact velocity, transient spreading diameter and dynamic contact angle are determined using image processing. Five stages are observed during impingement: initial impact, boiling (if the surface temperature is high enough), near constant wetting diameter evaporation, fast receding contact line evaporation and final dry-out. Results of water and a water based single-wall-carbon-nano-tube (SWCNT) nanofluid impinging on a polished silicon surface are compared to determine effects of nano-particles on impinging dynamics. The data show that the nanofluid exhibits larger spreading velocities, larger spreading diameters and an increase in early stage dynamic contact angle. The results of water impinging on both polished silicon and nano-structured silicon disks are compared to determine effects of the nano-structured surface on impingement dynamics. It is found that the nano-structured surface enhances the heat transfer for evaporative cooling at lower surface temperatures which is indicated by a shorter evaporation time. Ultimately, using nanofluid and nano-structured surface can reduce the total evaporation time up to 37% and 20%, respectively. Experimental data are compared with models that predict dynamic contact angle and non-dimensional maximum spreading diameter. Results show that the molecular-kinetic theory's dynamic contact angle model (M-K model) agrees well with current experimental data at low velocities range corresponding to later times during impingement, but over-predict at high velocities range corresponding to early times of initial impact. Predictions of maximum spreading diameter based on surface energy analysis are compared with current experimental data. Results indicate that all the four models over-predict unless empirical coefficients are adjusted to fit the test conditions.
Author: Christof Graber Publisher: ISBN: Category : Evaporative cooling Languages : en Pages : 316
Book Description
Droplet and spray impingement cooling are typically used in high heat flux thermal management. In this thesis, droplet impingement and evaporation heat transfer characteristics are determined from measured spatially- and temporally- varying fluid and surface temperatures. Unique to this study is the documentation of the effects of using a nanofluid and a nano-structured surface for dropwise cooling by comparison of heat transfer characteristics with that of water droplet impingement on a polished surface. Temperatures were determined using radiation intensities recorded using an Infrared (IR) camera. The impingement surface is either comprised of IR transparent silicon, which permits near-surface fluid temperature measurements, or an IR opaque gold coated surface, which permits surface temperature measurements. A range of surface heat fluxes, resulting in both single-phase and boiling conditions are studied. Three different impingement surfaces have been tested, including polished silicon, nano-structured porous silicon, and gold coated polished silicon. The nanofluid is a water-based carbon nanotube suspension. Five major droplet impingement and evaporation stages have been identified: initial impact, boiling (if the surface temperature was sufficiently high), approximately constant diameter evaporation, stepwise fast receding contact line evaporation, and simultaneously decaying diameter and contact angle final dryout period. The surface temperature spatial distribution shows the lowest temperature values within the contact area bulk region and increasing temperature values toward the contact line region and beyond. The basic temperature trends and evaporation behavior are similar for the polished and nano-structured surface while the nanofluid exhibits some distinction. Evaporation times are reduced up to 20% and 37% using the nanostructured surface and nanofluid, respectively. Considering the evaporation time reduction as a measure of droplet cooling performance, the nano-enhanced surface and nanofluid may improve heat transfer in droplet impingement and spray cooling applications.
Author: Jorge Padilla Publisher: ISBN: Category : Languages : en Pages : 167
Book Description
This dissertation summarizes results of an experimental exploration of heat transfer during vaporization of a water droplet deposited on a nanostructured surface at a temperature approaching and exceeding the Leidenfrost point for the surface and at lower surface temperatures 10-40 degrees C above the saturated temperature of the water droplet at approximately 101 kPa. The results of these experiments were compared to those performed on bare smooth copper and aluminum surfaces in this and other studies. The nanostructured surfaces were composed of a vast array of zinc oxide (ZnO) nanocrystals grown by hydrothermal synthesis on a smooth copper substrate having an average surface roughness of approximately 0.06 micrometer. Various nanostructured surface array geometries were produced on the copper substrate by performing the hydrothermal synthesis for 4, 10 and 24 hours. The individual nanostructures were randomly-oriented and, depending on hydrothermal synthesis time, had a mean diameter of about 500-700 nm, a mean length of 1.7-3.3 micrometers, and porosities of approximately 0.04-0.58. Surface wetting was characterized by macroscopic measurements of contact angle based on the droplet profile and calculations based on measurements of liquid film spread area. Scanning electron microscope imaging was used to document the nanoscale features of the surface before and after the experiments. The nanostructured surfaces grown by hydrothermal synthesis for 4 and 24 hours exhibited contact angles of approximately 10, whereas the surfaces grown for 10 hours were superhydrophilic, exhibiting contact angles typically less than 3 degrees. In single droplet deposition experiments at 101 kPa, a high-speed video camera was used to document the droplet-surface interaction. Distilled and degassed water droplets ranging in size from 2.5-4.0 mm were deposited onto the surface from heights ranging from approximately 0.2-8.1 cm, such that Weber numbers spanned a range of approximately 0-99. Heat transfer coefficients were determined from thermal measurements in the test apparatus. All experiments were conducted inside an ISO Class 5 clean room enclosure. It was observed that when a liquid water droplet impinged upon the ZnO nanostructured at surface temperatures less than 140 degrees C, the nominally spherical droplet spread into a thin film over the surface. The film thickness depended on many parameters but in general it measured approximately 100-400 micrometers. As a result, it was found that the droplet evaporated by film evaporation without initiating nucleate boiling. At wall superheat levels of 10-20 degrees C, it was found in some cases that the heat transfer coefficients were nearly 4 times greater than for those of nucleate boiling at the same superheat level. For these conditions, no bubble nucleation was observed visually, and, nevertheless, extremely high heat transfer coefficients resulting from rapid evaporation of the thin liquid film formed by the spreading droplet were observed. At high wall superheat levels, the vaporization process exhibited Leidenfrost droplet vaporization. The extreme wetting of the nanostructured surfaces resulted in high Leidenfrost transition temperatures in the range of 310-376 degrees C, among the highest in the literature, exceeding those exhibited by bare metal surfaces by 100 degrees C or more. The Leidenfrost transition was detected from a recording of the acoustic signal generated from each experiment during the deposition and subsequent evaporation process. It was defined as the first point for which there is no disturbance to the acoustical signal in the form of a sizzling sound beyond the initial violent popping generated during the droplet deposition. The results document a trend of increasing Leidenfrost temperature with decreasing contact angle, which is consistent with earlier studies. The results of this study are compared with earlier work in this area and the implications for applications are discussed.
Author: Mohammad Mehdi Rashidi Publisher: Elsevier ISBN: 0443136262 Category : Technology & Engineering Languages : en Pages : 427
Book Description
Nanofluids are a new class of heat transfer fluids engineered by dispersing and stably suspending nanoparticles in traditional heat transfer fluids. Recently they have obtained global attention from the scientific community owing to their unique properties and significant applications in different engineering fields. Nanofluids: Preparation, Applications and Simulation Methods provides a comprehensive review of recent advances in this important research field. Different approaches for preparing some remarkable families of nanofluids such as aluminum oxide-based nanofluids, CuO/Cu-based nanofluids, carbon nanotubes/graphene-based nanofluids, ZnO-based nanofluids, Fe3O4-based nanofluids, and SiO2-based nanofluids are discussed in detail as well as their current and potential applications. Different approaches for numerical, semi-analytical and analytical simulations are also discussed including molecular dynamics, the Lattice Boltzmann method, and spectral methods, as well as advanced analytical techniques such as the Differential Transform Method, the Homotopy Analysis Method, and Optimal Homotopy Analysis. The book will be a valuable reference resource for academic and industrial researchers, materials scientists and engineers, nanotechnologists, and chemists working in the development of nanomaterials and nanofluids for heat transfer in energy and engineering applications. Covers the synthesis of nanostructures, preparation of nanofluids, different applications and proposed models for fluid mechanics and heat transfer Presents recent advances on preparation methods, including green chemistry-based methods for preparation of nanomaterials and nanofluids Includes novel model-based approaches such as molecular dynamics and Lattice Boltzmann methods Delves into applications in renewable energy technologies and thermal management Contains a Semi-analytical approach for solving Time-Fractional Navier-Stokes Equation
Author: Pierre-Gilles de Gennes Publisher: Springer Science & Business Media ISBN: 0387216561 Category : Science Languages : en Pages : 298
Book Description
The study of capillarity is in the midst of a veritable explosion. What is offered here is not a comprehensive review of the latest research but rather a compendium of principles designed for the undergraduate student and for readers interested in the physics underlying these phenomena.
Author: S. M. Sohel Murshed Publisher: BoD – Books on Demand ISBN: 1789848385 Category : Science Languages : en Pages : 154
Book Description
Since conventional cooling techniques are increasing falling short of meeting the ever-growing cooling demands of high heat generating devices, thermal systems, and processes, advanced and innovative cooling technologies are of immense importance to deal with such high thermal management. Hence, this book covers a number of key topics related to advanced cooling approaches, their performance, and applications, including: Evaporative air cooling; Spray impingement cooling; Heat pump-based cooling; Modular cooling for photovoltaic plant; Nucleate pool boiling of refrigerants; Transient flashing spray cooling and application; Compressor cooling systems for industry. The book is aimed at a wide variety of people from graduate students and researchers to manufacturers who are involved or interested in the areas of thermal management systems, cooling technologies, and their applications.
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: Yen-Po Lin
Author: Yen-Po Lin
Author: Jian Shen
Author: Christof Graber
Author: Jorge Padilla
Author: Mohammad Mehdi Rashidi
Author: Pierre-Gilles de Gennes
Author: Jeffrey D. Naber
Author: Patricia Barbara Weisensee
Author: S. M. Sohel Murshed
Author: Nenad Miljkovic