Author: Indranil M. Joshi
Publisher:
ISBN:
Category : Ebullition
Languages : en
Pages : 71
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Book Description
"The miniaturization trend in electronics has spurred the development of efficient thermal management solutions. Single phase techniques are reliable but are limited by large fluid temperature differences and pressure drop. Two phase cooling has very little pressure drop with large heat absorption capacity. Boiling stands out as one of the most effective methods of heat dissipation which utilizes phase change. However, the design of two-phase systems is limited by the critical heat flux condition where a vapor layer prevents the liquid from contacting the heater surface. The current research study is directed towards increasing the CHF and maintaining low wall superheats to design efficient heat removal systems. In this study, different surface modification techniques are studied with an aim to identify various mechanisms that affect the heat transfer. Different surface enhancements in the form of Circumferential rectangular microchannels(CRM) and fin are used over cylindrical surface. Cylindrical tube with outer diameter of 15 mm was used for testing with water as working fluid. Tubular surface with fin attached performed the best yielding the CHF of 115 W/cm2 at wall superheat of 18oC which translated to an enhancement of 76%. The best performance of 110 W/cm2 at 9 oC without reaching CHF was obtained amongst CRM. Different mechanisms were identified by analyzing the results from pool boiling experiments. Area enhancement and contact line substantially affected the heat transfer performance in CRM. Area enhancement increased performance by providing additional area for heat transfer. Contact line region has higher heat flux. Single bubble growing over multiple grooves has increased contact line density which increases heat transfer performance. Increment in CHF was obtained by employing any one of these surface enhancements. High speed imaging enabled to analyze the behavior of bubble after nucleation on the fin surface thus deciphering the flow modulation over the cylindrical surface. Presence of bubble diverter at the bottom surface ensured higher evaporative momentum force towards the cylindrical surface. This displaced nucleating bubble at the bottom away from the fin, enabling liquid to rewet the surface. This allowed the formation of separate liquid vapor pathways which resulted in increased performance."--Abstract.
Author: Jeet S. Mehta
Publisher:
ISBN:
Category : Heat-transfer media
Languages : en
Pages : 222
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Book Description
"Pool boiling is a stable and efficient method for transferring large quantities of heat. It is employed in a wide range of applications, including steam generation in boilers, petrochemical, pharmaceutical, cryogenic and many other industrial processes. The objective of this work was to investigate the augmentation in the boiling heat transfer rates with an array of open microchannels over a cylindrical tube. In order to develop high performance surfaces, rectangular and V-groove cross-sectional geometry microchannels were fabricated and tested over tubular test sections. These microchannels were manufactured in two configurations: circumferentially around the test section and axially along the length. The effects of the microchannel geometric parameters on pool boiling performance were studied under horizontal and vertical orientations. Twenty uniquely modified microchannel surfaces were designed, fabricated and tested. The best performance was obtained with a circumferential rectangular microchannel test section in the horizontal orientation. A maximum heat transfer coefficient of 129 kW/m2*K was achieved at a heat flux of 1095 kW/m2, while maintaining a wall superheat of 8.5 K. The overall enhancement factors obtained at the maximum heat flux condition, ranged between 1.9 and 3.4 in the horizontal orientation, and 2.1 and 3.1 in the vertical orientation. The critical heat flux for almost all the designed test surfaces was increased by a factor of at least 1.6 over a plain tube. Area normalized results indicated that factors other than area enhancement are responsible for augmenting the heat transfer performance. High-speed videography of bubbles nucleating, growing and departing from the heated surface was performed. The bubble behavior over these open microchannels was analyzed to understand the fundamental mechanism during pool boiling. The bubble interactions in and over the open microchannels, and the liquid rewetting phenomenon greatly influence the heat transfer performance for these surface."--Abstract.
Author: Scott Gayton Liter
Publisher:
ISBN:
Category :
Languages : en
Pages : 246
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Book Description
Author: Richard W. Murphy
Publisher:
ISBN:
Category :
Languages : en
Pages : 108
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Book Description
The study was oriented toward the fabrication and testing of improved repeatable, high-porosity turbulators for augmenting heat transfer in cylindrical tubes. It was found that the spiral brush insert was superior in repeatability, uniformity, and ease of emplacement to the other types (fiber-filled and knitted wire) considered. Experiments were performed to evaluate the effectiveness of spiral brush inserts in augmenting forced convection heat transfer at high fluxes with water as the fluid under both single-phase and subcooled boiling conditions. Based on equal Reynolds number, use of the spiral brush turbulators resulted in an eightfold increase in heat transfer coefficient over values achievable with comparable empty tubes. Judged on the same criterion, the burnout (critical) heat flux was raised by a factor of three using the spiral brush turbulators. On an equal-pumping-power basis, single-phase heat transfer performance was approximately equal for the two configurations. The results indicate that the spiral brush turbulator method of augmentation is highly advantageous in situations where high heat fluxes must be transferred while, at the same time, restrictions are imposed on available amounts of heat transfer area and coolant. (Author).
Author: Smreeti Dahariya
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
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.
Author:
Publisher: Springer
ISBN: 9783319266947
Category : Science
Languages : en
Pages : 0
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Book Description
This Handbook provides researchers, faculty, design engineers in industrial R&D, and practicing engineers in the field concise treatments of advanced and more-recently established topics in thermal science and engineering, with an important emphasis on micro- and nanosystems, not covered in earlier references on applied thermal science, heat transfer or relevant aspects of mechanical/chemical engineering. Major sections address new developments in heat transfer, transport phenomena, single- and multiphase flows with energy transfer, thermal-bioengineering, thermal radiation, combined mode heat transfer, coupled heat and mass transfer, and energy systems. Energy transport at the macro-scale and micro/nano-scales is also included. The internationally recognized team of authors adopt a consistent and systematic approach and writing style, including ample cross reference among topics, offering readers a user-friendly knowledgebase greater than the sum of its parts, perfect for frequent consultation. The Handbook of Thermal Science and Engineering is ideal for academic and professional readers in the traditional and emerging areas of mechanical engineering, chemical engineering, aerospace engineering, bioengineering, electronics fabrication, energy, and manufacturing concerned with the influence thermal phenomena.
Author: R.T. Lahey Jr.
Publisher: Elsevier
ISBN: 1483291383
Category : Technology & Engineering
Languages : en
Pages : 629
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Book Description
This volume covers the modern developments in boiling heat transfer and two-phase flow, and is intended to provide industrial, government and academic researchers with state-of-the-art research findings in the area of multiphase flow and heat transfer technology. Special attention is given to technology transfer, indicating how recent significant results may be used for practical applications. The chapters give detailed technical material that will be useful to engineers and scientists who work in the field of multiphase flow and heat transfer. The authors of all chapters are members of the CMR at Rensselaer, a research centre specializing in the state-of-the-art in multiphase science.
Author: Sujoy Kumar Saha
Publisher:
ISBN: 9783030207564
Category : Fluid mechanics
Languages : en
Pages : 121
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Book Description
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.
Author: John C. Chen
Publisher: CRC Press
ISBN: 1000716031
Category : Science
Languages : en
Pages : 402
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Book Description
This book comprises selected papers from the First International Conference on Convective Flow Boiling. The purpose of the conference is to examine state-of-science and recent developments in technology of flow boiling, i.e., boiling systems which are affected by convective flows.
Author: John R. Thome
Publisher: CRC Press
ISBN: 9780891167457
Category : Technology & Engineering
Languages : en
Pages : 220
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Book Description
This is a comprehensive survey of boiling heat transfer augmentation, one of the most dynamic areas in the field. The text covers fundamental aspects of boiling augmentation and provides an in-depth treatment of enhanced boiling surface applications in industry.
