Download or read book Roles Played by Heater Size Contact Angle Surrounding Vessel Size and Surface Structure During Pool Boiling on Horizontal Surfaces written by Zhenyu She and published by . This book was released on 2021 with total page 186 pages. Available in PDF, EPUB and Kindle. Book excerpt: Nucleate boiling is ubiquitous from daily lives to engineering applications, and the increasing demand for efficient heat transfer in the industry requires further understanding of it, especially on small scales where the knowledge on conventional scales may no longer apply. This study investigated the effects of heater size, contact angle, surrounding vessel size, and surface structure on nucleate boiling heat transfer occurring on horizontal flat surfaces where the heater size is comparable to the two-dimensional ''most dangerous'' Taylor wavelength, aiming to comprehend the parametric effects on nucleate boiling heat flux and critical heat flux (CHF) and revisit the hydrodynamic theory near critical condition. Saturated water at one-atmosphere pressure was boiled on horizontal flat copper discs of diameters 1.0, 1.5, and 2.0 cm, respectively. The contact angle was varied from about 10 to 80 deg by controlling thermal oxidation of the discs, while the surrounding vessel size was changed by placing glass tubes of different inner diameters around the discs. The surface structure in the form of microgrooves was fabricated by sanding the disc top surface. Boiling heat transfer data were obtained up to CHF. Boiling curves and CHFs measured under different experimental configurations were compared in terms of each parameter. Rohsenow's correlation was employed to assess the parametric effects on nucleate boiling heat flux quantitatively. Vapor removal patterns were photographed in nucleate boiling regime and near CHF. Vapor jet diameter and the dominant wavelength at water-steam interface were measured from the photographs for the well wetted discs and used to predict the corresponding CHF based on the hydrodynamic theory. For well wetted surfaces, the boiling curve was insensitive to the heater size, but the CHF increased when the heater size was reduced from 2.0 to 1.0 cm. Improving the wettability delayed the onset of nucleate boiling and shifted the boiling curve to the right while enhancing the CHF substantially. Enlarging the liquid-holding vessel hardly affected the boiling curve at low heat fluxes but improved the CHF slightly. The structured surface featured higher nucleate boiling heat flux resulting from more active nucleation sites but showed no advantage in CHF over the plain surface of similar size and contact angle. In this study, the highest measured CHFs for plain and structured surfaces are close and both about 2.1 times Zuber's CHF prediction for infinite horizontal flat plates. They were obtained on well wetted 1.0-cm-diameter discs surrounded by large vessels.

Download or read book A Study of Heat Transfer Accompanying Pool Boiling from a Horizontal Flat Surface written by Warren W. Bowden and published by . This book was released on 1959 with total page 196 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Hydrodynamic Aspects of Boiling Heat Transfer written by N. Zuber and published by . This book was released on 1959 with total page 216 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book The Effect of Wettability on Boiling Heat Transfer Performance with Water on Copper Surfaces written by Adam Ryon Girard and published by . This book was released on 2016 with total page 468 pages. Available in PDF, EPUB and Kindle. Book excerpt: The current study aims to investigate the effect of wettability on the Boiling Heat Transfer Coefficient (BHTC) and Critical Heat Flux (CHF) in both pool and flow boiling of water. In this study, hot alkali solutions were utilized to promote cupric and cuprous oxide growth on copper surfaces, with thicknesses on the order of a couple of micrometers. This growth exhibited micro and nano-scale structures, resulting in differing levels of wettability and roughness while maintaining the effusivity of the bare copper surface. Teflon® was utilized to modify the surface energy of these surfaces, creating a range of wettability with apparent contact angles spanning from ~0° to ~160°. Pool boiling tests were conducted with these surfaces using saturated water at 1 atm on 1×1 cm2 square blocks, heated via resistance heaters. Flow boiling tests were conducted using both 1×1 cm2 square and 1×3 cm2 rectangular heaters, for slightly subcooled (1.5–3.5°) conditions over the velocity range of 0.3 to 0.9 m/s. This study showed that for both Pool and Flow boiling, the BHTC has an inverse relationship to wettability; the BHTC decreases as wettability increases. Furthermore, it was shown that this dependency between BHTC and wettability is more significant than the relationship between BHTC and surface roughness. The CHF was found to increase with increased wettability, roughness and mass flux. Based on the results of this study and published data, it is postulated that there exists a true hydrodynamic CHF limit for pool boiling with water on flat surfaces independent of heater material, near 2,000 kW/m2, representing an 80% increase in the limit suggested by Zuber. This study recommends this topic be explored with more focused research, and also suggests future boiling surface characterizations be performed in terms of both surface roughness as well as apparent contact angle to aid in the creation of a robust boiling dataset.

Download or read book Interacting Effects of Gravity and Size Upon the Peak and Minimum Pool Boiling Heat Fluxes written by John H. Lienhard and published by . This book was released on 1970 with total page 98 pages. Available in PDF, EPUB and Kindle. Book excerpt: This report summarizes a two-year study of the peak and minimum pool boiling heat fluxes, q[superscript max] and q[superscript min], under a wide variety of conditions. The study was motivated by the fact that gravity, g, and size exert inter-related effects upon the extreme heat fluxes. Covered in this report are the mathematical correlation of effects of gravity, size, boiled liquid, pressure, and configuration on peak and minimum boiling heat fluxes.

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 Bubble Behavior in Subcooled Flow Boiling on Surfaces of Variable Wettability written by Emily W. Tow and published by . This book was released on 2012 with total page 63 pages. Available in PDF, EPUB and Kindle. Book excerpt: Flow boiling is important in energy conversion and thermal management due to its potential for very high heat fluxes. By improving understanding of the conditions leading to bubble departure, surfaces can be designed that increase heat transfer coefficients in flow boiling. Bubbles were visualized during subcooled nucleate flow boiling of water on a surface of variable wettability. Images obtained from the videos were analyzed to find parameters influencing bubble size at departure. A model was developed relating the dimensions of the bubble at departure to its upstream and downstream contact angles based on a rigid-body force balance between momentum and surface tension and assuming a skewed truncated spherical bubble shape. Both experimental and theoretical results predict that bubble width and height decrease with increasing flow speed and that the width increases with the equilibrium contact angle. The model also predicts that the width and height increase with the amount of contact angle hysteresis and that the height increases with equilibrium contact angle, though neither of these trends were clearly demonstrated by the data. Several directions for future research are proposed, including modifications to the model to account for deviations of the bubbles from the assumed geometry and research into the parameters controlling contact angle hysteresis of bubbles in a flow. Additionally, observations support that surfaces with periodically-varying contact angle may prevent film formation and increase the heat transfer coefficients in both film and pool boiling.

Download or read book The Effects of Contact Angle and Gravity on the Critical Heat Flux from Horizontal Cylindrical Heaters in Saturated Pool Boiling written by Hesham Abdulrahman and published by . This book was released on 1996 with total page 124 pages. Available in PDF, EPUB and Kindle. Book excerpt:

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 Transport Phenomena in Multiphase Systems written by Amir Faghri and published by Academic Press. This book was released on 2006 with total page 1072 pages. Available in PDF, EPUB and Kindle. Book excerpt: Engineering students in a wide variety of engineering disciplines from mechanical and chemical to biomedical and materials engineering must master the principles of transport phenomena as an essential tool in analyzing and designing any system or systems wherein momentum, heat and mass are transferred. This textbook was developed to address that need, with a clear presentation of the fundamentals, ample problem sets to reinforce that knowledge, and tangible examples of how this knowledge is put to use in engineering design. Professional engineers, too, will find this book invaluable as reference for everything from heat exchanger design to chemical processing system design and more. * Develops an understanding of the thermal and physical behavior of multiphase systems with phase change, including microscale and porosity, for practical applications in heat transfer, bioengineering, materials science, nuclear engineering, environmental engineering, process engineering, biotechnology and nanotechnology * Brings all three forms of phase change, i.e., liquid vapor, solid liquid and solid vapor, into one volume and describes them from one perspective in the context of fundamental treatment * Presents the generalized integral and differential transport phenomena equations for multi-component multiphase systems in local instance as well as averaging formulations. The molecular approach is also discussed with the connection between microscopic and molecular approaches * Presents basic principles of analyzing transport phenomena in multiphase systems with emphasis on melting, solidification, sublimation, vapor deposition, condensation, evaporation, boiling and two-phase flow heat transfer at the micro and macro levels * Solid/liquid/vapor interfacial phenomena, including the concepts of surface tension, wetting phenomena, disjoining pressure, contact angle, thin films and capillary phenomena, including interfacial balances for mass, species, momentum, and energy for multi-component and multiphase interfaces are discussed * Ample examples and end-of-chapter problems, with Solutions Manual and PowerPoint presentation available to the instructors

Download or read book High pressure Pool Boiling and Physical Insight of Engineered Surfaces written by Nanxi Li and published by . This book was released on 2017 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Boiling is a very effective way of heat transfer due to the latent heat of vaporization. Large amount of heat can be removed as bubbles form and leave the heated surface. Boiling heat transfer has lots of applications both in our daily lives and in the industry. The performance of boiling can be described with two important parameters, i.e. the heat transfer coefficient (HTC) and the critical heat flux (CHF). Enhancing the performance of boiling will greatly increase the efficiency of thermal systems, decrease the size of heat exchangers, and improve the safety of thermal facilities. Boiling heat transfer is an extremely complex process. After over a century of research, the mechanism for the HTC and CHF enhancement is still elusive. Previous research has demonstrated that fluid properties, system pressures, surface properties, and heater properties etc. have huge impact on the performance of boiling. Numerous methods, both active and passive, have been developed to enhance boiling heat transfer. In this work, the effect of pressure was investigated on a plain copper substrate from atmospheric pressure to 45 psig. Boiling heat transfer performance enhancement was then investigated on Teflon© coated copper surfaces, and graphene oxide coated copper surfaces under various system pressures. It was found that both HTC and CHF increases with the system pressure on all three types of surfaces. Enhancement of HTC on the Teflon© coated copper surface is contributed by the decrease in wettability. It is also hypothesized that the enhancement in both HTC and CHF on the graphene oxide coated surface is due to pinning from micro and nanostructures in the graphene oxide coating or non-homogeneous wettability. Condensation and freezing experiments were conducted on engineered surfaces in order to further characterize the pinning effect of non-homogeneous wettability and micro/nano structure of the surface.

Download or read book Evaluation of External Surface Modification Techniques to Enhance Pool Boiling of Dielectric Fluids written by Farhan Mody and published by . This book was released on 2019 with total page 75 pages. Available in PDF, EPUB and Kindle. Book excerpt: "The miniaturization trend of transistors and increase in packing density of electronic devices has resulted in high heat flux generation, which has created a need for efficient heat removal systems. The present research is an experimental study of pool boiling using plain copper chip and microchannel chip with boiling surface of 34.5mm x 32mm. Three dielectric fluids, Perfluoro-2-methylpentane (PP1), perfluoro-methyl-cyclopentane (PP1C), and fluorocarbon (FC-87) were used in a closed loop pool boiling system to determine their performance at atmospheric pressure. The pool boiling results have been compared with literature for a boiling surface of 10 mm x 10 mm to study the effect of heater size. To improve the performance of the pool boiling system, we desire high critical heat flux and low surface temperatures. In the current study, we introduced two external structures fitted on the test surfaces for regulating the flow of vapor through specific structures and generating independent liquid-vapor pathways without any deposition and/or chemical surface modifications of the test surface. Firstly, an array of hollow conical structures (HCS) called volcano manifold are printed using additive manufacturing technique. A critical heat flux (CHF) of 28.1 W/cm2, 38.3 W/cm2 and 32.5 W/cm2 was achieved for volcano manifold with plain copper chip using PP1, PP1C and FC87 respectively giving 19%, 33% and 6.5% enhancement in CHF respectively as compared to a plain chip without volcano manifold. Secondly, dual taper manifold having taper angle of 15° is printed using a stereolithography (SLA) additive manufacturing technique. Plain chip with dual taper manifold gave the CHF of 25.6 W/cm2, 31.7 W/cm2 and 32.3 W/cm2 for PP1, PP1C and FC-87, respectively. These results indicate a deterioration in CHF caused by vapor constriction. In addition, the heater size effect was studied by comparing the pool boiling performance of a plain copper boiling surface of 34.5 mm x 32 mm (Large heater) with 10 mm x 10 mm (Small heater) from published literature for all three refrigerants. It was noted that 31%, 66% and 104% increment in maximum heat transfer coefficient was obtained for PP1, PP1C and FC-87 respectively with larger heater over smaller heater at CHF. The geometrical parameters of the enhancement structures were based on published results for water. The results show that the external surface modification techniques require further geometrical parameter optimization as the current designs based on water performance caused vapor constriction effects that caused performance deterioration for dielectric fluids."--Abstract.

Download or read book Experimental and Analytical Study of the Effects of Wettability on Nucleation Site Density During Pool Boiling written by Chunghsiung Wang and published by . This book was released on 1992 with total page 256 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Separate Effects of Surface Roughness Wettability and Porosity on Boiling Heat Transfer and Critical Heat Flux and Optimization of Boiling Surfaces written by Harrison Fagan O'Hanley and published by . This book was released on 2012 with total page 161 pages. Available in PDF, EPUB and Kindle. Book excerpt: The separate effects of surface wettability, porosity, and roughness on critical heat flux (CHF) and heat transfer coefficient (HTC) were examined using carefully-engineered surfaces. All test surfaces were prepared on nanosmooth indium tin oxide - sapphire heaters and tested in a pool boiling facility in MIT's Reactor Thermal Hydraulics Laboratory. Roughness was controlled through fabrication of micro-posts of diameter 20[mu]m and height 15[mu]m; intrinsic wettability was controlled through deposition of thin compact coatings made of hydrophilic SiO2 (typically, 20nm thick) and hydrophobic fluorosilane (monolayer thickness); porosity and pore size were controlled through deposition of layer-by-layer coatings made of SiO2 nanoparticles. The ranges explored were: 0 - 15[mu] for roughness (Rz), 0 - 135 degrees for intrinsic wettability, and 0 - 50% and 50nm for porosity and pore size, respectively. During testing, the active heaters were imaged with an infrared camera to map the surface temperature profile and locate distinct nucleation sites. It was determined that wettability can play a large role on a porous surface, but has a limited effect on a smooth non-porous surface. Porosity had very pronounced effects on CHF. When coupled with hydrophilicity, a porous structure enhanced CHF by approximately 50% - 60%. However, when combined with a hydrophobic surface, porosity resulted in a reduction of CHF by 97% with respect to the reference surface. Surface roughness did not have an appreciable effect, regardless of the other surface parameters present. Hydrophilic porous surfaces realized a slight HTC enhancement, while the HTC of hydrophobic porous surfaces was greatly reduced. Roughness had little effect on HTC. A second investigation used spot patterning aimed at creating a surface with optimal characteristics for both CHF and HTC. Hydrophobic spots (meant to be preferential nucleation sites) were patterned on a porous hydrophilic surface. The spots indeed were activated as nucleation sites, as recognized via the IR signal. However, CHF and HTC were not enhanced by the spots. In some instances, CHF was actually decreased by the spots, when compared to a homogenous porous hydrophilic surface.

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 The Mechanism of Heat Transfer in Nucleate Pool Boiling written by Chi-Yeh Han and published by . This book was released on 1962 with total page 76 pages. Available in PDF, EPUB and Kindle. Book excerpt: A criterion is developed for bubble initiation from a gas filled cavity on a surface in contact with a superheated layer of liquid. It is found that the temperature of bubble initiation on a given surface is a function of the temperature conditions in the liquid surrounding the cavity as well as the surface properties themselves and the delay time between bubbles is a function of the bulk liquid temperature and the wall superheat and is not constant for a given surface. With a thermal layer thickness and a critical wall superheat relation for the cavity, a bubble growth rate is obtained. Bubble departure is considered. The Jakob and Fritz relation works as long as the true (non-equilibrium) bubble contact angle is used. The effect on the departure size of the virtual mass in the surrounding liquid is negligible at one gravity. The initiation, growth and departure criterions are each experimentally checked and used to compute the heat transfer near the knee of the boiling curve using only an experimental determination of the number of bubbles as a function of wall superheat and other known quantities. (Author).

Download or read book The Role of Surface Microstructure and Topography in Pool Boiling Heat Transfer written by Bradley Bon and published by . This book was released on 2011 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Conclusively, surface microstructure and topography can greatly influence nucleate boiling heat transfer. The various physical attributes employed with the structured surfaces further revealed the profound influence of surface topography on enhancing boiling heat transfer. On the atomic scale, it is seen that even differences in crystal structure can also produce noticeable variations in the boiling heat transfer rate.