Download or read book Enhancement of Heat Transfer with Pool and Spray Impingement Boiling on Microporous and Nanowire Surface Coatings written by and published by . This book was released on 2010 with total page 10 pages. Available in PDF, EPUB and Kindle. Book excerpt: The DOE National Renewable Energy Laboratory (NREL) is leading a national effort to develop next-generation cooling technologies for hybrid vehicle electronics. The goal is to reduce the size, weight, and cost of power electronic modules that convert direct current from batteries to alternating current for the motor, and vice versa. Aggressive thermal management techniques help to increase power density and reduce weight and volume, while keeping chip temperatures within acceptable limits. The viability of aggressive cooling schemes such as spray and jet impingement in conjunction with enhanced surfaces is being explored. Here, we present results from a series of experiments with pool and spray boiling on enhanced surfaces, such as a microporous layer of copper and copper nanowires, using HFE-7100 as the working fluid. Spray impingement on the microporous coated surface showed an enhancement of 100%-300% in the heat transfer coefficient at a given wall superheat with respect to spray impingement on a plain surface under similar operating conditions. Critical heat flux also increased by 7%-20%, depending on flow rates.

Download or read book Enhancement of Pool Boiling and Evaporative Heat Transfer Using High Temperature Thermally Conductive Microporous Coatings written by Ajay Gurung and published by . This book was released on 2015 with total page 91 pages. Available in PDF, EPUB and Kindle. Book excerpt: The present research is an experimental study of the enhancement of pool boiling and evaporative heat transfer using high temperature thermally conductive microporous coatings. Two major types of coatings were investigated: one that is based on copper powders on copper substrate and the other on aluminum powders on aluminum substrate. Both coatings were easy to fabricate with low costs compared to conventional sintering and plasma spraying techniques, yet have high bonding strength and some of them can operate at temperatures up to 670 °C. Multiple coating options were fabricated and tested in pool boiling of water in order to optimize the coating. These coating options consisted of variations of coating composition ratio, coating thickness and powder sizes. Average powder sizes ranged from 5 micron to 110 micron, and coating thicknesses from 75 micron to 340 micron, applied on flat 1x1cm2 test heaters. The heaters were tested in the horizontal, upward-facing orientation in saturated conditions at atmospheric pressure and under increasing heat flux. Pool boiling results revealed an optimum composition, powder size and thickness for each coating types. The maximum enhancement in boiling heat transfer coefficient obtained from copper microporous coatings was up to 8.7 times relative to a plain copper test surface and nearly doubled the critical heat flux while aluminum microporous coatings enhanced boiling heat transfer coefficient by 3.5 times compared to plain aluminum surface without any further enhancement in CHF. This enhancement was ascribed to the numerous microcavities of optimum shape and size formed within the porous matrix of the coating. The detail microstructures of the coatings from the top surface as well as cross-sections are also presented through optical microscope and SEM images. The optimized aluminum coatings were also explored on fluids other than water such as acetone and HFE-7100 for their boiling heat transfer enhancement. Furthermore, the same coatings were applied on evaporative spray and jetimpingement tests using water to broaden the application of aluminum microporous coatings in evaporative cooling technology.

Download or read book Enhancement of Pool Boiling Heat Transfer Using Thermally conductive Microporous Coating Techniques written by Joo Han Kim and published by . This book was released on 2006 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The present research is an experimental study of the enhancement of boiling heat transfer using microporous coating techniques. The current research is divided into four major phases. During the first phase, the effects of different metal particle sizes in the coating compound for thermally non-conductive microporous coating on pool boiling performance of refrigerants and water are investigated. The test surfaces were solid copper blocks with 1-cm2 base at atmospheric pressure in saturated FC-72, R-123, and water. Results showed that the surface treatment by non-conductive microporous coating significantly enhanced both nucleate boiling and critical heat flux of FC-72 and R-123. However, the enhancement of boiling performance for water was merely shown. In the second phase, thermally conductive microporous coatings to enhance boiling performance of water were developed. The first phase motivated efforts to fabricate microporous coatings with conducting binder options. The second phase was stemmed from an effort to combine the advantages of both a mixture batch type (inexpensive & easy process) and sintering/machining method (low thermal resistance of conduction). Two categories of surface treatment processes were considered in the current research. The first can be achieved by a chemical process, Multi-Staged Electroplating (MSE), which uses electricity in a chemical bath to deposit a microporous structure on the surface. The second is a soldering process, Multi-Temperature Soldering Process (MTSP), which binds the metal particles to generate optimum microporous cavities. Scanning Electron Microscope (SEM) and optical microscope images were obtained for thermally conductive microporous coated surfaces. During the third phase, the pool boiling performance of developed MSE and MTSP from second phase was confirmed for water. Results showed that the MSE and MTSP augmented the boiling performance not only for refrigerants but also for water significantly compared to non-conductive microporous coatings. Further investigation for possible future industrial applications of microporous coatings, such as indirect cooling for electronic chips, nanofluids for high power generation industries, and freezing problem of water, were conducted in the final phase.

Download or read book Enhancement of Pool Boiling Heat Transfer Using a Combination of Open Microchannels and Microporous Surfaces written by Chinmay Patil and published by . This book was released on 2014 with total page 196 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Exploring the Limits of Boiling and Evaporative Heat Transfer Using Micro Nano Structures written by Ming-Chang Lu and published by . This book was released on 2010 with total page 218 pages. Available in PDF, EPUB and Kindle. Book excerpt: This dissertation presents a study exploring the limits of phase-change heat transfer with the aim of enhancing critical heat flux (CHF) in pool boiling and enhancing thermal conductance in heat pipes. The state-of-the-art values of the CHF in pool boiling and the thermal conductance in heat pipes are about two orders of magnitudes smaller than the limits predicted by kinetic theory. Consequently, there seems to be plenty of room for improvement. Pool boiling refers to boiling at a surface immersed in an extensive motionless pool of liquid. Its process includes heterogeneous nucleation, growth, mergence and detachment of vapor bubbles on a heating surface. It is generally agreed that the high heat transfer coefficient of boiling could be explained by the concept of single-phase forced convection, i.e., the motion of bubbles agitating surrounding liquid is similar to the process in single-phase forced convection. The occurrence of CHF results from a formation of a vapor film on the heater surface, which reduces the thermal conductance drastically and causes a huge temperature rise on the surface. Over the past few decades, researchers were struggling to identify the exact mechanism causing CHF. General observations are that both surface properties and pool hydrodynamics could affect the values of CHF. Nanowire array-coated surfaces having a large capillary force are employed to enhance the CHF. It has been shown that CHFs on the nanowire array-coated surface could be doubled compared to the values on a plain surface. The obtained CHF of 224 ± 6.60 W/cm̂2 on the nanowire-array coated surface is one of the highest values reported in the boiling heat transfer. To further enhance CHF, the mechanisms that govern CHF have been systematically explored. Experimental results show that the CHF on the nanowire array-coated surface are not limited by the capillary force. Instead, the CHF are dependent on the heater size. Corresponding experiments on plain surfaces with various heater sizes also exhibits similar heater-size dependence. The CHFs on nanowire array-coated surfaces and plain surfaces are consistent with the predictions of the hydrodynamic theory while a higher CHF is obtained on the nanowire array-coated surface as compared to the plain Si surface. This suggests that the CHFs are a result of the pool hydrodynamics while surface properties modify the corresponding hydrodynamic limits. A heat pipe is a device that transports thermal energy in a very small temperature difference and thereby producing a very large thermal conductance. It relies on evaporation of liquid at the heated end of the pipe, flow of vapor between the heated and cooled end, condensation at the other end, and capillary-driven liquid flow through a porous wick between the condenser and the evaporation. The large latent heat involved in evaporation and condensation leads to very large heat flows for a small temperature drop along the heat pipe. Despite the large thermal conductance, their operation is limited by such factors as capillary limit, boiling limit, sonic limit and entrainment limit, etc. Among these operational limits, capillary and boiling limits are most frequently encountered. The capillary limit determines the maximum flow rate provided by the capillary force of the wick structure whereas boiling limit is referred to a condition that liquid supply is blocked by vapor bubbles in the wick. Consequently, the wick structure is the key component in a heat pipe, which determines the maximum capillary force and the dominant thermal resistance. In a heat pipe using evaporation as the dominant heat transfer mechanism, a thin liquid film (̃ a few microns) extended from the solid structure in the wick causes the dominant thermal resistance. Therefore, if one reduces the pore size of a porous media, the thermal conductance could be enhanced by increasing the surface area of the thin liquid film. On the other hand, the classical thermodynamics depicts that the superheat required for evaporation is inversely proportional to the equilibrium radius of the meniscus. Consequently, enhancing thermal conductance via increasing the thin film area is contradictory to the effect of evaporation suppression for small pores. A hierarchical wick structure with multiple length scales that enhances dry-out heat flux and thermal conductance simultaneously in heat pipes was demonstrated. This hierarchical wick structure is composed of a large microchannel array to reduce flow resistance and small pin-fin arrays to provide a large capillary force. The enhancement of thermal conductance is achieved via a large number of pin-fins for increasing the total thin film area. A thermal conductance defined by the slope of the curve of ̃16.28 ± 1.33 W/cm̂2-K and a dry-out heat flux of 228.85 ± 10.73 W/cm̂2 were achieved by this design. Further, vapor transport resistance is minimized within the aligned-multi-scale wick structure. As a result, this wick does not pose a boiling limit. Artificial cavities were created in the wick structure to take the advantage of the high heat transfer coefficient of boiling heat transfer. The wick with artificial cavities successfully triggers boiling at a lower wall temperature resulting in a conductance of 9.02 ± 0.04 W/cm̂2-K compared to an evaporation mode of 3.54 ± 0.01 W/cm̂2-K. For a given heat flux, the wick with cavities effectively reduce wall temperature compared to a wick without cavities. Our experimental results display an enhancement of thermal conductance by using boiling heat transfer. This opens up a new direction for further enhancing thermal conductance in heat pipes by circumventing the limit in the evaporative heat transfer regime, in which further increase in surface area will eventually result in evaporation suppression in small pores.

Download or read book Effects of Surface Parameters on Boiling Heat Transfer Phenomena written by Bao Hoai Truong and published by . This book was released on 2011 with total page 156 pages. Available in PDF, EPUB and Kindle. Book excerpt: Nanofluids, engineered colloidal dispersions of nanoparticles in fluid, have been shown to enhance pool and flow boiling CHF. The CHF enhancement was due to nanoparticle deposited on the heater surface, which was verified in pool boiling. However, no such work has been done for flow boiling. Using a cylindrical tube pre-coated with Alumina nanoparticles coated via boiling induced deposition, CHF of water was found to enhance up to 40% compared to that of the bare tube. This confirms that nanoparticles on the surface is responsible for CHF enhancement for flow boiling. However, existing theories failed to predict the CHF enhancement and the exact surface parameters attributed to the enhancement cannot be determined. Surface modifications to enhance critical heat flux (CHF) and Leidenfrost point (LFP) have been shown successful in previous studies. However, the enhancement mechanisms are not well understood, partly due to many surface parameters being altered at the same time, as in the case for nanofluids. Therefore, the remaining objective of this work is to evaluate separate surface effect on different boiling heat transfer phenomena. In the second part of this study, surface roughness, wettability and nanoporosity were altered one by one and respective effect on quenching LFP with water droplet was determined. Increase in surface roughness and wettability enhanced LFP; however, nanoporosity was most effective in raising LFP, almost up to 100°C. The combination of the micro posts and nanoporous coating layer proved optimal. The nanoporous layer destabilizes the vapor film via heterogeneous bubble nucleation, and the micro posts provides intermittent liquid-surface contacts; both mechanisms increase LFP. In the last part, separate effect of nanoporosity and surface roughness on pool boiling CHF of a well-wetting fluid, FC-72, was investigated. Nanoporosity or surface roughness alone had no effect on pool boiling CHF of FC-72. Data obtained in the literature mostly for microporous coatings showed CHF enhancement for well wetting fluids, and existing CHF models are unable to predict the enhancement.

Download or read book Pool Boiling Heat Transfer written by Dong Soo Jung and published by . This book was released on 1984 with total page 246 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Two Phase Heat Transfer Enhancement written by Sujoy Kumar Saha and published by . This book was released on 2020 with total page 121 pages. Available in PDF, EPUB and Kindle. Book excerpt: This Brief concerns heat transfer and pressure drop in heat transfer enhancement for boiling and condensation. The authors divide their topic into six areas: abrasive treatment and coatings, combined structured and porous surfaces, basic principles of boiling mechanism, vapor space condensation, convective vaporization, and forced condensation inside tubes. Within this framework, the book examines range of specific phenomena including abrasive treatment, open grooves, 3D cavities, etched surfaces, electroplating, pierced 3D cover sheets, attached wire and screen promoters, non-wetting coatings, oxide and ceramic coatings, porous surfaces, structured surfaces (integral roughness), combined structured and porous surfaces, composite surfaces, single-tube pool boiling tests, theoretical fundamentals like liquid superheat, effect of cavity shape and contact angle on superheat, entrapment of vapor in cavities, nucleation at a surface cavity, effect of dissolved gases, bubble departure diameter, bubble dynamics, boiling hysteresis and orientation effects, basic principles of boiling mechanism, visualization and mechanism of boiling in subsurface tunnels, and Chien and Webb parametric boiling studies.

Download or read book Pool Boiling Studies on Nanotextured Surfaces Under Highly Subcooled Conditions written by Vijaykumar Sathyamurthi and published by . This book was released on 2010 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Subcooled pool boiling on nanotextured surfaces is explored in this study. The experiments are performed in an enclosed viewing chamber. Two silicon wafers are coated with Multiwalled Carbon Nanotubes (MWCNT), 9 microns (Type-A) and 25 microns (Type-B) in height. A third bare silicon wafer is used for control experiments. The test fluid is PF-5060, a fluoroinert with a boiling point of 56°C (Manufacturer: 3M Co.). The apparatus is of the constant heat flux type. Pool boiling experiments in nucleate and film boiling regimes are reported in this study. Experiments are carried out under low subcooling (5 °C and 10 °C) and high subcooling conditions (20°C to ~ 38°C). At approximately 38°C, a non-departing bubble configuration is obtained on a bare silicon wafer. Increase in subcooling is found to enhance the critical heat flux (CHF) and the CHF is found to shift towards higher wall superheats. Presence of MWCNT on the test surface led to an enhancement in heat flux. Potential factors responsible for boiling heat transfer enhancement on heater surfaces coated with MWCNT are identified as follows: a. Enhanced surface area or nano - fin effect b. Higher thermal conductivity of MWCNT than the substrate c. Disruption of vapor-liquid vapor interface in film boiling, and of the "microlayer" region in nucleate boiling d. Enhanced transient heat transfer caused by local quasi-periodic transient liquid-solid contacts due to presence of the "hair like" protrusion of the MWCNT e. Enhancement in the size of cold spots f. Pinning of contact line, leading to enhanced surface area underneath the bubble leading to enhanced heat transfer Presence of MWCNT is found to enhance the phase change heat transfer by approximately 400% in nucleate boiling for conditions of low subcooling. The heat transfer enhancement is found to be independent of the height of MWCNT in nucleate boiling regime in the low subcooling cases. About 75%-120% enhancement in heat transfer is observed for surfaces coated with MWCNT under conditions of high subcooling in the nucleate boiling regime. Surfaces coated with Type-B MWCNT show a 75% enhancement in heat transfer in the film boiling regime under conditions of low subcooling.

Download or read book High pressure Pool boiling Heat Transfer Enhancement and Mechanism on Engineered Surfaces written by Smreeti Dahariya and published by . This book was released on 2020 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Boiling has received considerable attention in the technology advancement of electronics cooling for high-performance computing applications. Two-phase cooling has an advantage over a single-phase cooling in the high heat removal rate with a small thermal gradient due to the latent heat of vaporization. Many surface modifications have been done in the past including surface roughness, mixed wettability and, porous wick copper play a crucial role in the liquid-vapor phase change heat transfer. However, the mechanisms of high-pressure pool-boiling heat transfer enhancement due to surface modifications has not been well studied or understood. The properties of water, such as the latent heat of vaporization, surface tension, the difference in specific volume of liquid and vapor, decrease at high-pressure. High-pressure pool-boiling heat transfer enhancement is studied fundamentally on various engineered surfaces. The boiling tests are performed at a maximum pressure of 90 psig (620.5 kPa) and then compared to results at 0 psig (0 kPa). The results indicate that the pressure influences the boiling performance through changes in bubble dynamics. The bubble departure diameter, bubble departure frequency, and the active nucleation sites change with pressure. The pool-boiling heat transfer enhancement of a Teflon© coated surface is also experimentally tested, using water as the working fluid. The boiling results are compared with a plain surface at two different pressures, 30 and 45 psig. The maximum heat transfer enhancement is found at the low heat fluxes. At high heat fluxes, a negligible effect is observed in HTC. The primary reasons for the HTC enhancement at low heat fluxes are active nucleation sites at low wall superheat and bubble departure size. The Teflon© coated surface promotes nucleation because of the lower surface energy requirement. The boiling results are also obtained for wick surfaces. The wick surfaces are fabricated using a sintering process. The boiling results are compared with a plain surface. The reasons for enhancements in the pool-boiling performance are primarily due to increased bubble generation, higher bubble release frequency, reduced thermal-hydraulic length modulation, and enhanced thermal conductivity due to the sintered wick layer. The analysis suggests that the Rayleigh-critical wavelength decreases by 4.67 % of varying pressure, which may cause the bubble pinning between the gaps of sintered particles and avoids the bubble coalescence. Changes in the pitch distance indicate that a liquid-vapor phase separation happens at the solid/liquid interface, which impacts the heat-transfer performance significantly. Similarly, the role of the high-pressure over the wicking layer is further analyzed and studied. It is found that the critical flow length, [lambda]u reduces by three times with 200 [mu]m particles. The results suggest that the porous wick layer provides a capillary-assist to liquid flow effect, and delays the surface dry out. The surface modification and the pressure amplify the boiling heat transfer performance. All these reasons may contribute to the CHF, and HTC enhancement in the wicking layer at high-pressure.

Download or read book Pool Boiling Enhancement Through Improved Liquid Supply Pathways Over Open Microchannels written by Arvind Jaikumar and published by . This book was released on 2014 with total page 158 pages. Available in PDF, EPUB and Kindle. Book excerpt: "Boiling is an efficacious mode of heat transfer and is utilized in various energy conversions, heat exchange systems and in cooling of high energy density electronic components. Fundamental pool boiling mechanisms suggest that liquid rewetting on a heated surface is a key factor in delaying critical heat flux (CHF) for enhancing pool boiling performance. In this study, pool boiling enhancement is achieved by providing improved liquid supply pathways to nucleation sites in open microchannels. A two part study is conducted to enhance pool boiling performance of open microchannels. Micromachined and porous surfaces are identified as enhancement techniques in Part-I and Part-II respectively. The results obtained in part-I showed significant improvement in the pool boiling performance when tested with water and FC-87. In part-II of the study, porous coatings are deposited on the boiling surface of an open parallel microchannel fin tops, channel bottoms and both, and individually investigated for their pool boiling performance. The best performing surface was with porous coatings throughout the geometry and had a CHF of 313 W/cm2 at a wall superheat of 7.5 °C. High speed images for the three surfaces show that bubble nucleation occurred at the location of porous deposits. Furthermore, additional nucleation sites are identified as the main contributing factor in the best performing surface which had an enhancement of 150% in CHF when compared to a plain surface. Efficient liquid recirculation provided by open microchannels also contributed to improved microconvection in the channels."--Abstract.

Download or read book Effects of Nano and Micro Surface Treatments on Boiling Heat Transfer written by Hai Trieu Phan and published by LAP Lambert Academic Publishing. This book was released on 2011-04 with total page 176 pages. Available in PDF, EPUB and Kindle. Book excerpt: This work investigates the flow boiling heat transfer in microchannels with the aim of developing compact cooling systems which can be adapted to miniaturized power components. Nano and micro-surface treatments were used as innovative techniques to improve the heat transfer performance as well as to delay the intermittent dryout. Initially, pool-boiling experiments were performed to highlight the impact of nanocoatings on nucleate-boiling mechanisms. It was observed that the surface wettability modified by nanoparticle deposition had significant effects on the boiling processes. Afterwards, a second experimental campaign was conducted to investigate the flow boiling in a microchannel with nanocoated and microstructured samples. These studies highlighted the impacts of surface wettability and of micro-patterning on two-phase flow patterns, pressure drop and heat transfer coefficient. In particular, significant enhancements in heat transfer coefficient and in intermittent dryout were obtained with micro structured samples

Download or read book Pool Boiling Heat Transfer from Porous coated Surfaces in FC 72 the Effects of Subcooling and Non boiling Immersion Time written by Kuiyan Xu and published by . This book was released on 2005 with total page 188 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Nucleate Pool Boiling of Surface modified Nano porous Alumina written by Bong June Zhang and published by . This book was released on 2012 with total page 294 pages. Available in PDF, EPUB and Kindle. Book excerpt: In nucleate pool boiling, heat transfer augment is an important issue. Among various techniques for enhancement, porous surfaces with millimeter to micron-sized diameter have been studied in the past few decades. In this study, the nano porous surface (NPS) of alumina was created to address technical issues in submicron regime pool boiling. Especially, influence of surface modifications (structural and chemical treatments) of the NPS over heat transfer enhancement was the subject of the reported research. There are a few important aspects of the research reported in this dissertation: to develop the optimal NPS to increase heat transfer in nucleate pool boiling, to suggest the mechanistic model to explain heat transfer augment in submicron regime, and to assess the feasibility of the NPS for nucleate pool boiling enhancement. First of all, an optimal NPS with surface modifications for heat transfer in nucleate pool boiling was developed. The surface-modified NPS provides enlarged heating surface area, active nucleation site enhancement, and improvement of vapor-liquid menisci through reentrant-shaped cavities. Secondly, the mechanistic model suggested in this study explained that the combination of convection-driven and phase change-induced heat transfers and liquid thin film evaporation (LTFE) achieves heat transfer augment. Especially, the model supported the hypothesis that the NPS is appropriate to dissipate heat through the LTFE at lower heat flux regime. Lastly, this study shows that heat transfer enhancement phenomena of the NPS that was evaluated and quantified by using various physical and electrochemical methods. For instance, most contributing factor of the CHF enhancement was assumed the liquid spreading and the absorption-induced capillary pressure. They were assessed by using Apparent Water Contact Angle (AWCA) and Electrochemical Impedance Spectroscopy (EIS) measurements of which trends were well matched with CHF experimental data.

Download or read book Multiscale Mechanistic Approach to Enhance Pool Boiling Performance for High Heat Flux Applications written by Arvind Jaikumar and published by . This book was released on 2017 with total page 218 pages. Available in PDF, EPUB and Kindle. Book excerpt: "The advent of cloud computing and the complex packaging architecture of next generation electronic devices drives methods for advanced thermal management solutions. Convection based single-phase cooling systems are inefficient due to their large pressure drops, fluid temperature differences and costs, and are incapable of meeting the cooling requirements in the high power density components and systems. Alternatively, phase-change cooling techniques are attractive due to their ability to remove large amounts of heat while maintaining uniform fluid temperatures. Pool boiling heat transfer mechanism centers on the nucleation, growth and departure of a bubble from the heat transfer surface in a stagnant pool of liquid. The pool boiling performance is quantified by the Critical Heat Flux (CHF) and Heat Transfer Coefficients (HTC) which dictate the operating ranges and efficiency of the heat transfer process. In this work, three novel geometries are introduced to modify the nucleation characteristics, liquid pathways and contact line motion on the prime heater surface for a simultaneous increase in CHF and HTC. First, sintered microchannels and nucleating region with feeder channels (NRFC) were developed through the mechanistic concept of separate liquid-vapor pathways and enhanced macroconvection heat transfer. A maximum CHF of 420 W/cm2 at a wall superheat of 1.7 °C with a HTC of 2900 MW/m2°C was achieved with the sintered-channels configuration, while the NRFC reached a CHF of 394 W/cm2 with a HTC of 713 kW/m2°C. Second, the scale effect of liquid wettability, roughness and microlayer evaporation was exploited to facilitate capillary wicking in graphene through interlaced porous copper particles. A CHF of 220 W/cm2 with a HTC of 155 kW/m2°C was achieved using an electrodeposition coating technique. Third, the chemical heterogeneity on nanoscale coatings was shown to increase the contribution from transient conduction mechanisms. A maximum CHF of 226 W/cm2 with a HTC of 107 kW/m2°C was achieved. The enhancement techniques developed here provide a mechanistic tool at the microscale and nanoscale to increase the boiling CHF and HTC."--Abstract.

Download or read book Enhancement of Pool Boiling Heat Transfer in Confined Space written by Chia-Hsiang Hsu and published by . This book was released on 2015 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Pool boiling is an effective method used in many technical applications for a long time. Its highly efficient heat transfer performance results from not only the convection effect but also the phase change process in pool boiling. Pool boiling enhancement has been studied in the past decade. However, the mechanisms of pool boiling has not yet been fully understood because of the many parameters that affect its behavior including the latent heat of vaporization, nucleation density, bubble and fluid motion, interaction at the interface, and the physical properties of surface. Among the current studies, bubble departure rate is viewed as one of the dominant factors that affect heat transfer. This research considers the effect of bubble confinement on pool boiling. In the study, confinement was achieved by placing a flat plate over heated surface. The flat plate has a hole in the middle, and there is a gap between the flat plate and the heater. The diameters of hole are 2 mm, 3 mm, and 4 mm; the gap distances are 2.3 mm, 3.6 mm, and 5 mm. The heater consists of an indium-tin-oxide layer deposited on a silicon wafer. An IR camera and high speed cameras are used to acquire the surface temperature distribution and bubble image. By controlling the plate hole size and the gap distance, the effect of confinement on heat transfer performance can be evaluated. Moreover, heat transfer performance of pool boiling with three-2mm-holes plate was investigated and compared with that of single-2mm-hole plate with the smallest gap size. At the lower heat flux values, heat transfer enhancement in confined space was experimentally observed. Surface temperature can be reduced by 4 °C at most. Results indicate that higher bubble departure rate and coalescence effect might be the dominant factor for improving heat transfer performance in a confined space caused by induced shear flow. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/152843

Download or read book New Surfaces for the Control of Boiling Heat Transfer written by Nicole Lea Aitcheson and published by . This book was released on 2003 with total page 210 pages. Available in PDF, EPUB and Kindle. Book excerpt: