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Author: Daniel Frank Hanks
Publisher:
ISBN:
Category :
Languages : en
Pages : 123
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Book Description
Heat dissipation is a critical limitation in a range of electronic devices including microprocessors, solar cells, laser diodes and power amplifiers. The most demanding devices require dissipation of heat fluxes in excess of 1 kW/cm2 with heat transfer coefficients more than 30 W/cm 2K. Advanced thermal management solutions using phase change heat transfer are the most promising approach to address these challenges, yet current solutions are limited due to the combination of heat flux, thermal resistance, size and flow stability. This thesis reports the design, fabrication and experimental characterization for an evaporation device with a nanoporous membrane for high heat flux dissipation. Evaporation in the thin film regime is achieved using nanopores with reduced liquid film thicknesses while liquid pumping is enhanced using the capillary pressure of the 120 nm pores. The membrane is mechanically supported by ridges that form liquid supply channels and also serve as a heat conduction path to the evaporating meniscus at the surface of the membrane. The combination of high capillarity pores with high permeability channels facilitates theoretical critical heat fluxes over 2 kW/cm2 and heat transfer coefficients over 100 W/cm2K. Proof-of-concept devices were fabricated using a two-wafer stack consisting of a bonded silicon-on-insulator (SOI) wafer to a silicon wafer. Pores with diameters 110 - 130 nm were defined with interference lithography and etched in the SOI. Liquid supply microchannels were etched on a silicon wafer and the two wafers were fusion bonded together to form a monolithic evaporator. Once bonded, the membrane was released by etching through the backside of the SOI. Finally, platinum heaters and Resistive Temperature Detectors (RTDs) were deposited by e-beam evaporation and liftoff to heat the sample and measure the device temperature during experiments, respectively. Samples were experimentally characterized in a custom environmental chamber for comparison to the model using R245fa, methanol, pentane, water and isopropyl alcohol as working fluids. A comparison of the results with different working fluids demonstrates that transport at the liquid-vapor interface is the dominant thermal resistance in the system, suggesting a figure of merit: ... The highest heat flux recorded was with pentane at ... and the highest heat transfer coefficient recorded was with ... not including the substrate resistance. However, the samples were observed to clog with soluble, nonvolatile contaminants which limited operation to several minutes. The clogging behavior was captured in a mass diffusion model and a new configuration was suggested which is resistant to clogging. Evaporation from nanopores represents a new paradigm in phase change cooling with a figure of merit that favors high volatility, low surface tension fluids rather than water. The models and experimental results validate the functionality and understanding of the proposed approach and provide recommendations for enhancements in performance and understanding as well as strategies for resistance to clogging. This work demonstrates that nanoporous membranes have the potential for ultra-high heat flux dissipation to address next generation thermal management needs.
Author: Daniel Frank Hanks
Publisher:
ISBN:
Category :
Languages : en
Pages : 123
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Book Description
Heat dissipation is a critical limitation in a range of electronic devices including microprocessors, solar cells, laser diodes and power amplifiers. The most demanding devices require dissipation of heat fluxes in excess of 1 kW/cm2 with heat transfer coefficients more than 30 W/cm 2K. Advanced thermal management solutions using phase change heat transfer are the most promising approach to address these challenges, yet current solutions are limited due to the combination of heat flux, thermal resistance, size and flow stability. This thesis reports the design, fabrication and experimental characterization for an evaporation device with a nanoporous membrane for high heat flux dissipation. Evaporation in the thin film regime is achieved using nanopores with reduced liquid film thicknesses while liquid pumping is enhanced using the capillary pressure of the 120 nm pores. The membrane is mechanically supported by ridges that form liquid supply channels and also serve as a heat conduction path to the evaporating meniscus at the surface of the membrane. The combination of high capillarity pores with high permeability channels facilitates theoretical critical heat fluxes over 2 kW/cm2 and heat transfer coefficients over 100 W/cm2K. Proof-of-concept devices were fabricated using a two-wafer stack consisting of a bonded silicon-on-insulator (SOI) wafer to a silicon wafer. Pores with diameters 110 - 130 nm were defined with interference lithography and etched in the SOI. Liquid supply microchannels were etched on a silicon wafer and the two wafers were fusion bonded together to form a monolithic evaporator. Once bonded, the membrane was released by etching through the backside of the SOI. Finally, platinum heaters and Resistive Temperature Detectors (RTDs) were deposited by e-beam evaporation and liftoff to heat the sample and measure the device temperature during experiments, respectively. Samples were experimentally characterized in a custom environmental chamber for comparison to the model using R245fa, methanol, pentane, water and isopropyl alcohol as working fluids. A comparison of the results with different working fluids demonstrates that transport at the liquid-vapor interface is the dominant thermal resistance in the system, suggesting a figure of merit: ... The highest heat flux recorded was with pentane at ... and the highest heat transfer coefficient recorded was with ... not including the substrate resistance. However, the samples were observed to clog with soluble, nonvolatile contaminants which limited operation to several minutes. The clogging behavior was captured in a mass diffusion model and a new configuration was suggested which is resistant to clogging. Evaporation from nanopores represents a new paradigm in phase change cooling with a figure of merit that favors high volatility, low surface tension fluids rather than water. The models and experimental results validate the functionality and understanding of the proposed approach and provide recommendations for enhancements in performance and understanding as well as strategies for resistance to clogging. This work demonstrates that nanoporous membranes have the potential for ultra-high heat flux dissipation to address next generation thermal management needs.
Author: Kyle L. Wilke
Publisher:
ISBN:
Category :
Languages : en
Pages : 64
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Book Description
Cooling demands of advanced electronics are increasing rapidly, often exceeding capabilities of conventional thermal management techniques. Thin film evaporation has emerged as one of the most promising thermal management solutions. High heat transfer rates can be achieved in thin films of liquids due to a small conduction resistance through the film to the evaporating interface. In this thesis, we investigated evaporation from nanoporous membranes. The capillary wicking of the nanopores supplies liquid to the evaporating interface, passively maintaining the thin film. Different evaporation regimes were predicted through modeling and were demonstrated experimentally. Good agreement was shown between the predicted and observed transitions between regimes. Improved heat transfer performance was demonstrated in the pore level evaporation regime over other regimes, with heat transfer rates up to one order of magnitude larger for a given superheat in comparison to the flooding regime. An improved experimental setup for investigating thin film evaporation from nanopores was developed, where a biphilic membrane, i.e., a membrane with two wetting behaviors, was used for enhanced experimental control to allow characterization of the importance of different design parameters. This improved setup was then used to demonstrate the dependence of thin film evaporation on the location of the meniscus within the nanopores. This dependence on meniscus location within the pore was also shown to increase with increasing superheat. We observed a 46% reduction in heat transfer rates at a superheat of 15 °C for an L* of 14.67 compared to an L* of 2, where L* is the ratio of the depth of the meniscus within the pore to the pore radius. This work provides practical insights for the design of devices based on nanoporous evaporation. Heat transfer regimes can be predicted based on fluid supply conditions, evaporative heat flux, and membrane geometry. Furthermore, the biphilic membrane serves as a valuable experimental platform for testing the role of membrane geometry on heat transfer performance in the pore level evaporation regime. Future work will focus on demonstrating the importance of different parameters and using experimental results to either validate existing models for evaporation from nanopores or develop more suitable ones.
Author: Zhengmao Lu
Publisher:
ISBN:
Category :
Languages : en
Pages : 53
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Book Description
Heat dissipation is a limiting factor in the performance of integrated circuits, power electronics and laser diodes. State-of-the-art solutions typically use air-cooled heat sinks, which have limited performance owing to the use of air. One of the promising approaches to address these thermal management needs is liquid vapor phase-change. In this thesis, we present a study into the design and modeling of a cooling device based on thin film evaporation from a nanoporous membrane supported on microchannels. The concept utilizes the capillary pressure generated by the small pores to drive the liquid flow and largely reduces the viscous loss due to the thinness of the membrane. The interfacial transport has been re-investigated where we use the moment method to solve the Boltzmann Transport Equation. The pore-level transport has been modeled coupling liquid transport, vapor transport and the interfacial balance. The interfacial transport inside the pore also serves as a boundary condition for the device-level model. The heat transfer and pressure drop performance have been modeled and design guidelines are provided for the membrane-based cooling system. The optimized cooling device is able to dissipate 1 kW/cm2 heat flux with a temperature rise less than 30 K from the vapor side. Future work will focus on more fundamental understanding of the mass and energy accommodation at the liquid vapor interface.
Author: Jay D. Sircar
Publisher:
ISBN:
Category :
Languages : en
Pages : 63
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Book Description
We investigated the experimental performance of a nanoporous membrane for ultra-high heat flux dissipation from high performance integrated circuits. The biporous evaporation device utilizes thermally-connected, mechanically-supported, high capillarity membranes that maximize thin film evaporation and high permeability liquid supply channels that allow for lower viscous pressure losses. The 600 nm thick membrane was fabricated on a silicon on insulator (SOI) wafer, fusion-bonded to a separate wafer with larger liquid channels. Spreading effects and overall device performance arising from non-uniform heating and evaporation of methanol was captured experimentally. Heat fluxes up to 412 W/cm2, over an area of O.4x 5 mm, and with a temperature rise of 24.1 K from the heated substrate to ambient vapor, were obtained. These results are in good agreement with a high-fidelity, coupled fluid convection and solid conduction compact model, which was necessitated by computational feasibility, which incorporates non-equilibrium and sub-continuum effects at the liquid-vapor interface. This work provides a proof-of-concept demonstration of our biporous evaporation device. Simulations from the validated model, at optimized operating conditions and with improved working fluids, predict heat dissipation in excess of 1 kW/cm2 with a device temperature rise below 30 K, for this scalable cooling approach.
Author: Zhengmao Lu
Publisher:
ISBN:
Category :
Languages : en
Pages : 87
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Book Description
Evaporation, a commonly found phenomenon in nature, is widely used in thermal management, water purification, and steam generation as it takes advantage of the enthalpy of vaporization. Despite being extensively studied for decades, the fundamental understanding of evaporation, which is necessary for making full use of evaporation, remains limited up to date. It is in general difficult to experimentally characterize the interfacial heat and mass transfer during evaporation. In this thesis, we designed and microfabricated an ultrathin nanoporous membrane as an experimental platform to overcome some critical challenges including: (1) realizing accurate and yet non-invasive interface temperature measurement; (2) decoupling the interfacial transport resistance from the thermofluidic resistance in the liquid phase and the diffusion resistance in the vapor phase; and (3) mitigating the blockage risk of the liquid-vapor interface due to nonevaporative contaminants. Our nano device consisted of an ultrathin free-standing membrane (~200 nm thick) containing an array of nanopores (pore diameter ~100 nm). A gold layer deposited on the membrane served as an electric heater to induce evaporation as well as a resistive temperature detector to closely monitor the interface temperature. This configuration minimizes the thermofluidic resistance in the liquid and mitigates the contamination risk. We characterized evaporation from this nano device in air as well as pure vapor. We demonstrated interfacial heat fluxes of ~~500 W/cm2 for evaporation in air, where we elucidated that the Maxwell- Stefan equation governed the overall transport instead of Fick's law, especially in the high flux regime. In vapor, we achieved kinetically limited evaporation with an interfacial heat transfer coefficient up to 54 kW/cm2 K. We utilized the kinetic theory with the Boltzmann transport equation to model the evaporative transport. With both experiments and modeling, we demonstrated that the kinetic limit of evaporation is determined by the pressure ratio between the vapor in the far field and that generated by the interface. The improved fundamental understanding of evaporation that we gained indicates the significant promise of utilizing an ultrathin nanoporous design to achieve high heat fluxes for evaporation in thermal management, desalination, steam generation, and beyond.
Author: Rong Xiao (Ph. D.)
Publisher:
ISBN:
Category :
Languages : en
Pages : 76
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Book Description
Micro/nanostructures have been extensively studied to amplify the intrinsic wettability of materials to create superhydrophilic or superhydrophobic surfaces. Such extreme wetting properties can influence the heat transfer performance during phase-change which is of great importance in a wide range of applications including thermal management, building environment, water harvesting and power production. In particular, superhydrophilic surfaces have been of interest to achieve thin film evaporation with high heat fluxes. Meanwhile, superhydrophobic surfaces with dropwise condensation promises higher heat transfer coefficients than typical filmwise condensation. My thesis work aims at improving fundamental understanding as well as demonstrating practical enhancements in these two areas. A key challenge to realizing thin film evaporation is the ability to achieve efficient fluid transport using superhydrophilic surfaces. Accordingly, we developed a semi-analytical model based on the balance between capillary pressure and viscous resistance to predict the propagation rates in micropillar arrays with high aspect ratios. Our experimental results showed good agreement with the model, and design guidelines for optimal propagation rates were proposed. For micropillar arrays with low aspect ratio and large spacing between pillars, however, we identified that the microscopic sweeping of the liquid front becomes important. We studied this phenomenon, explained the effect of such microscale dynamics on the overall propagation behavior, and proposed a strategy to account for these dynamics. While these propagation studies provide a means to deliver liquid to high heat flux regions, we investigated a different configuration using nanoporous membrane that decouples capillarity from the viscous resistance to demonstrate the potential heat dissipation capability. With nanoporous membranes with average pore diameters of 150 nm and thicknesses of 50 [mu]m, we achieved interfacial heat fluxes as high as 96 W/cm2 via evaporation with isopropyl alcohol. The effect of membrane thickness was studied to offer designs that promise dissipation of 1000 W/cm 2 . Meanwhile, we developed new metrology to measure transient heat transfer coefficients with a temporal resolution of 0.2 seconds during the evaporation process. Such a technique offers insight into the relationship between liquid morphology and heat transfer behavior. Finally, for enhanced condensation, we demonstrated immersion condensation using a composite surface fabricated by infusing hydrophobic oil into micro/nanostructures with a heterogeneous coating. With this approach, three key attributes to maximize heat transfer coefficient, low departure radii, low contact angle, and high nucleation density, were achieved simultaneously. We specifically elucidated the mechanism for the increase in nucleation density and attribute it to the combined effect of reduced water-oil interfacial energy and local high surface energy sites. As a result, we demonstrated approximately 100% enhancement in heat transfer coefficient over state-of-the-art superhydrophobic surfaces with the presence of non-condensable gases. This thesis presents improved fundamental understanding of wetting, evaporation, and condensation processes on micro/nanostructures as well as practical implementation of these structures for enhanced heat transfer. The insights gained demonstrate the potential of new nanostructure engineering approaches to improve the performance of various thermal management and energy production applications.
Author: Young Jin Kim
Publisher:
ISBN:
Category :
Languages : en
Pages : 105
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Book Description
Nanostructured materials exhibit useful properties that are not found in the same materials in bulk form. (1) Dramatically increase surface and area and roughness of nanostructured materials is advantageous for evaporative cooling, which is one of the main subjects in this dissertating, due to the strong capillary effect and high density of gas-liquid-solid triple junctions. (2) In magnetic materials, when the size of the magnetic material is decreased to submicron or nano size, magnetic coercivity increases since magnetic domains are confined by the size of the material. (3) Also, unique optical properties of materials in nano size can be utilized for thermal managing and energy saving application. In Chapter 2 of this dissertation, I described the demonstration of using nanoporous membranes in evaporative cooling. Nanoporous membranes have been proposed and theoretically shown as a promising candidate for high heat flux evaporative heat transfer. However, the experimentally demonstrated heat flux has so far been significantly lower than the theoretical prediction, which has cast doubt on the feasibility of achieving a high heat flux from nanoporous membranes. Here we carried out evaporative heat transfer experiments using isopropyl alcohol (IPA) through anodized aluminum oxide (AAO) membranes. For membranes with a 200nm average pore size on a 0.5cm2 size area, we demonstrated a high evaporative heat flux of 210W/cm2 based on the overall AAO surface area, or ~400W/cm2 if only the pore area is considered. This heat flux is close to the theoretical value of 572 W/cm^2 (based on the pore area) for IPA evaporation through nanoporous membranes. Using time synchronized high-speed images, it was verified that evaporation was the main heat transfer mode in the high heat flux regime. The demonstration of high heat flux evaporation through nanoporous membranes, close to the theoretical limit and on a relatively large area (0.5cm2), is significant for the future development of high heat flux thermal management technology for electronic devices. In chapter 3, I presented a noble technique to achieve exchange coupling of hard phase magnetic materials and soft phase magnetic materials. Exchange coupled spring magnets have been suggested as a possible replacement of rare earth contained strong magnets. We have chosen LPT-MnBi as hard phase and FeCo as soft phase magnetic material, as many early conducted theoretical modeling suggests. The optimal size of hard phase magnet for the exchange coupling (approximately twice of single magnetic domain size, ~2[mu]m for MnBi) was achieved by conventional ball milling process, and the shell layer of FeCo was deposited by a noble process called sonic agitation assisted physical vapor deposition (SAA-PVD). TEM image and EDX mapping shows uniform coating of FeCo outer shell layer on the MnBi core. The thickness of the shell layer was in the range of 10~35nm which is slightly less than twice of single domain size of FeCo. Magnetic remanence of ball milled MnBi particles was increased from Ms = 36 emu/g to 51 emu/g after SAPVD process while the coercivity was slightly decreased from 1.1T to 1.0T. (BH)max of the particles after the SAA-PVD process was about 2.5MGOe. Smooth demagnetization curve that resembles that of single magnetic materials and high increase of magnetic remanence suggest that exchange coupling was achieved. In chapter 4, a new route to synthesize thermochromic VO2 particles and properties of the film using the particles was presented. A temperature responding fully reversible metal to insulator phase transition (MIT) accompanied by a change of optical properties only found in Vanadium dioxide monoclinic phase (VO2 (M)) has potential for huge energy saving application by controlling the amount of infrared (IR) light enter into buildings. More synthetic routes are still worthy to be explored because of difficulty in mass production of VO2 (M). In this work, we demonstrated a combination of thermal decomposition method subsequent ball milling process to produce pure VO2 (M) particles. The size of the synthesized particles was between 20 and 200nm. IR modulating smart film was fabricated by blade casting mixture of synthesized VO2 (M) particles and PVP on PET film. The thickness of the film was about 300nm and particles were uniformly dispersed in the film. Despite the irregular shape of the particles and the fact that few portion of the synthesized particles exceeding suggested optimal size range, the transmittance of little less than 40% and the IR modulation of about 20% which values are practically useful was achieved.
Author:
Publisher: Elsevier
ISBN: 0443295395
Category : Science
Languages : en
Pages : 314
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Book Description
Advances in Heat Transfer, Volume 58 presents the latest in a serial that highlights new advances in the field, with this updated volume presenting interesting chapters written by an international board of authors. Sample chapters in this new release include Nanoscale Thin Film Evaporation and Ice thermal energy storage modeling: A review. - Provides the authority and expertise of leading contributors from an international board of authors - Presents the latest release in Advances in Heat Transfer serials
Author: Vallampati Ramachandra Prasad
Publisher: BoD – Books on Demand
ISBN: 1839627115
Category : Science
Languages : en
Pages : 134
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Book Description
Written by authoritative experts in the field, this book discusses fluid flow and transport phenomena in porous media. Portions of the book are devoted to interpretations of experimental results in this area and directions for future research. It is a useful reference for applied mathematicians and engineers, especially those working in the area of porous media.
Author: Christof Graber
Publisher:
ISBN:
Category : Evaporative cooling
Languages : en
Pages : 316
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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.
