Дисертації з теми "Heat transfer enhancement, Homogenization"
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1
Webber, Helen. "Compact heat exchanger heat transfer coefficient enhancement." Thesis, University of Bristol, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.540881.
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2
Ozerinc, Sezer. "Heat Transfer Enhancement With Nanofluids." Master's thesis, METU, 2010. http://etd.lib.metu.edu.tr/upload/12611862/index.pdf.
Повний текст джерелаАнотація:
A nanofluid is the suspension of nanoparticles in a base fluid. Nanofluids are promising for heat transfer enhancement due to their high thermal conductivity. Presently, discrepancy exists in nanofluid thermal conductivity data in the literature, and enhancement mechanisms have not been fully understood yet. In the first part of this study, a literature review of nanofluid thermal conductivity is performed. Experimental studies are discussed through the effects of some parameters such as particle volume fraction, particle size, and temperature on conductivity. Enhancement mechanisms of conductivity are summarized, theoretical models are explained, model predictions are compared with experimental data, and discrepancies are indicated.
Nanofluid forced convection research is important for practical application of nanofluids. Recent experiments showed that nanofluid heat transfer enhancement exceeds the associated thermal conductivity enhancement, which might be explained by thermal dispersion, which occurs due to random motion of nanoparticles. In the second part of the study, to examine the validity of a thermal dispersion model, hydrodynamically developed, thermally developing laminar Al2O3/water nanofluid flow inside a circular tube under constant wall temperature and heat flux boundary conditions is analyzed by using finite difference method with Alternating Direction Implicit Scheme. Numerical results are compared with experimental and numerical data in the literature and good agreement is observed especially with experimental data, which indicates the validity of the thermal dispersion model for explaining nanofluid heat transfer. Additionally, a theoretical analysis is performed, which shows that usage of classical correlations for heat transfer analysis of nanofluids is not valid.
3
Reddy, M. A. "Single phase heat transfer enhancement." Thesis, University of Manchester, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.616903.
Повний текст джерелаАнотація:
This thesis presents investigations into the enhancement of heat transfer inside tubes using HiTRAN® tube inserts manufactured by Cal Gavin Ltd. The applicability of heat transfer enhancement in vertical thermo syphon reboilers was investigated using a computer simulation. In vacuum operation, reboilers can have a significant subcooled length (as high as 60 % of the tube length). Heat transfer coefficients in this region are lower than in the two-phase region. Using tube inserts, an increase is made in the heat transfer rate occurring in the sub-cooled region and, a corresponding increase in the length of the tube subjected to two-phase heat transfer and improvement of heat transfer performance results. Geometric variables of the tube insert were investigated experimentally, to study their influence on heat transfer and pressure drop performance. Loop density, loop wire diameter, core wire diameter, loop material and uniformity of loop density were investigated. Two experimental facilities were designed, commissioned and constructed to measure the heat transfer and pressure drop performance of these tube inserts. The new rig at UMIST is located in a flameproof location and was constructed with the intention of investigating a wide range of other processes in the future. Two tube inserts were tested over a Reynolds number range of 200 to 200000 using water as the working fluid. Adiabatic, cooling and heating tests were performed using an inside tube diameter of28.25 mm. At the Cal Gavin Ltd. facility, the rig was redesigned to extend the operating range of data collection. It was enhanced by the provision of automatic data collection, improved accuracy of temperature measurement and new equipment to allow cooling experiments. Tube inserts were tested between a Reynolds number of 100 to 2000 using a viscous oil as the working fluid. Again adiabatic, cooling and heating tests were performed. An inside tube diameter of 21.18 mm was used in the maj ority of the tests, but some preliminary results using a tube diameter of 28.45 mm are also reported. Using the results of the experimental work, pressure drop performance was correlated using an approach similar to that used for packed beds. It was found that 90 % of the data were correlated between ± 15 % of the prediction using specific insert dimensions and inside tube diameter. Further investigations into the prediction of heat transfer coefficients were made. However no general correlation could be developed from a fundamental basis, to predict heat transfer across the full range of Reynolds numbers investigated in this study. A recommendation is made for a suitable correlation. The influence of the insert geometry was associated with the fundamental pressure drop and heat transfer performance of the tube insert, leading to recommendations for the optimisation of tube insert design.
4
Wang, Yufei. "Heat exchanger network retrofit through heat transfer enhancement." Thesis, University of Manchester, 2012. https://www.research.manchester.ac.uk/portal/en/theses/heat-exchanger-network-retrofit-through-heat-transfer-enhancement(c504dc06-f261-4968-8c58-4f4de153c694).html.
Повний текст джерелаАнотація:
Heat exchanger network retrofit plays an important role in energy saving in process industry. Many design methods for the retrofit of heat exchanger networks have been proposed during the last three decades. Conventional retrofit methods rely heavily on topology modifications which often results in a long retrofit duration and high initial costs. Moreover, the addition of extra surface area to the heat exchanger can prove difficult due to topology, safety and downtime constraints. These problems can be avoided through the use of heat transfer enhancement in heat exchanger network retrofit. This thesis develops a heuristic methodology and an optimization methodology to consider heat transfer enhancement in heat exchanger network retrofit. The heuristic methodology is to identify the most appropriate heat exchangers requiring heat transfer enhancements in the heat exchanger network. From analysis in the heuristic roles, some great physical insights are presented. The optimisation method is based on simulated annealing. It has been developed to find the appropriate heat exchangers to be enhanced and to calculate the level of enhancement required. The new methodology allows several possible retrofit strategies using different retrofit methods be determined. Comparison of these retrofit strategies demonstrates that retrofit modification duration and pay-back time are reduced significantly when only heat transfer enhancement is utilised. Heat transfer enhancement may increase pressure drop in a heat exchanger. The fouling performance in a heat exchanger will also be affected when heat transfer enhancement is used. Therefore, the implications of pressure drop and fouling are assessed in the proposed methodology predicated on heat transfer enhancement. Methods to reduce pressure drop and mitigate fouling are developed to promote the application of heat transfer enhancement in heat exchanger network retrofit. In optimization methodology considering fouling, the dynamic nature of fouling is simulated by using temperature intervals. It can predict fouling performance when heat transfer enhancement is considered in the network. Some models for both heat exchanger and heat transfer enhancement are used to predict the pressure drop performance in heat exchanger network retrofit. Reducing pressure by modifying heat exchanger structure is proposed in this thesis. From case study, the pressure drop increased by heat transfer enhancement can be eliminated by modifying heat exchanger structure.
5
Lagos, Arcangel. "Heat transfer enhancement in DX evaporators." Thesis, London South Bank University, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.311210.
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6
Staats, Wayne Lawrence. "Active heat transfer enhancement in integrated fan heat sinks." Thesis, Massachusetts Institute of Technology, 2012. http://hdl.handle.net/1721.1/78179.
Повний текст джерелаАнотація:
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2012.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 205-211).
Modern computer processors require significant cooling to achieve their full performance. The "efficiency" of heat sinks is also becoming more important: cooling of electronics consumes 1% of worldwide electricity use by some estimates. Unfortunately, current cooling technologies often focus on improving heat transfer at the expense of efficiency. The present work focuses on a unique, compact, and efficient air cooled heat sink which addresses these shortcomings. While conventional air cooled heat sinks typically use a separate fan to force air flow over heated fins, the new design incorporates centrifugal fans directly into the body of a loop heat pipe with multiple planar condensers. These "integrated fans" rotate between the planar condensers, in close proximity to the hot surfaces, establishing a radially outward flow of cooling air. The proximity of the rotating impellers to the condenser surfaces results in a marked enhancement in the convective heat transfer coefficient without a large increase in input power. To develop an understanding of the heat transfer in integrated fan heat sinks, a series of experiments was performed to simultaneously characterize the fan performance and average heat transfer coefficients. These characterizations were performed for 15 different impeller profiles with various impeller-to-gap thickness ratios. The local heat transfer coefficient was also measured using a new heated-thin-film infrared thermography technique capable of applying various thermal boundary conditions. The heat transfer was found to be a function of the flow and rotational Reynolds numbers, and the results suggest that turbulent flow structures introduced by the fans govern the transport of thermal energy in the air. The insensitivity of the heat transfer to the impeller profile decouples the fan design from the convection enhancement problem, greatly simplifying the heat sink design process. Based on the experimental results, heat transfer and fan performance correlations were developed (most notably, a two-parameter correlation that predicts the dimensionless heat transfer coefficients across 98% of the experimental work to within 20% relative RMS error). Finally, models were developed to describe the scaling of the heat transfer and mechanical power consumption in multi-fan heat sinks. These models were assessed against experimental results from two prototypes, and suggest that future integrated fan heat sink designs can achieve a 4x reduction in thermal resistance and 3x increase in coefficient of performance compared to current state-of-the-art air cooled heat sinks.
by Wayne L. Staats, Jr.
Ph.D.
7
Dellorusso, Paul Robert. "Electrohydrodynamic heat transfer enhancement for a latent heat storage heat exchanger." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk2/tape17/PQDD_0027/MQ31562.pdf.
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Abed, Waleed Mohammed. "Heat transfer enhancement in micro-scale geometries." Thesis, University of Liverpool, 2016. http://livrepository.liverpool.ac.uk/3004993/.
Повний текст джерелаАнотація:
Micro-geometries or 'microfluidics' are commonly utilised in a widespread variety of applications such as, bioengineering devices, microelectronic devices, electronics cooling, chemical micro-reactors and mini or micro-heat exchangers. In the microscale systems (with 'small' dimensions typically less than 1 millimeter), however, fluid mixing has been understood as one of the most fundamental and difficult-to-achieve issues because the flow of Newtonian fluids becomes increasingly controlled by viscous forces rather than inertia (as molecular diffusion is dominant at these small scales). As a consequence, the enhancement of convective heat transfer is problematic under these conditions (steady and laminar flow regime). In this thesis, two different regimes of instabilities, namely 'purely-inertial' and 'purely-elastic', have been adopted to enhance the convective heat transfer in the micro-scale geometries. Purely-inertial instability refers here to the secondary flow that arise in curved channels, also known as Dean flows, due to the centrifugal forces and also in crossed channels (cross-slot), symmetry-breaking bifurcations, which results in an axially-oriented spiral vortex along the outlet channels. While, purely-elastic instability is created in the flow of non-Newtonian viscoelastic fluids through curved channels due to the non-linear interaction between elastic stresses generated within the flowing viscoelastic solutions and the streamline curvature or through cross-slot device as a consequence of the planar extensional flow field (strong elongational flow) at the stagnation point. Fluid flow and convective heat transfer characteristics have been investigated experimentally and supporting numerical calculations for Newtonian flow within two different micro-geometries: a square cross-section serpentine microchannel and a square cross-section crossslot micro-device. A group of Newtonian fluids, aqueous glycerine solutions and aqueous sucrose solutions, was utilised to carry out the experiments for purely-inertial flows whilst high-viscosity polymeric viscoelastic fluids, shear-thinning and approximately constant-viscosity Boger solutions, were used for the experiments to investigate purely-elastic instabilities.
9
Shi, Haifeng. "Surfactant Drag Reduction and Heat Transfer Enhancement." The Ohio State University, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=osu1343664380.
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10
Stuart, Dale. "Heat Transfer Enhancement using Iron Oxide Nanoparticles." VCU Scholars Compass, 2012. http://scholarscompass.vcu.edu/etd/425.
Повний текст джерелаАнотація:
Two different iron oxide nanofluids were tested for heat transfer properties in industrial cooling systems. The nanofluids either had 30 nm particles with a wide size distribution to include particles greater than 1 micrometer or 15 nm particles with greater than 95% of the particles less than 33 nm. Calorimetry and thermal circuit modeling indicate that the 15 nm particle ferrofluid enhanced heat capacity. The smaller particle ferrofluid also demonstrated up to a 39% improvement in heat transfer, while the larger particle ferrofluid degraded the heat transfer performance. Particles from the larger particle ferrofluid were noted as settling out of a circulating system and therefore not participating in the bulk fluid properties. Application of 0.32% 15nm particles in an open cooling system improved cooling tower efficiency by 7.7% at a flow rate of 11.4 liter per minute and improved cooling tower efficiency by 3.3% at a flow rate of 22.7 liter per minute, while applying 0.53% 15 nm particles also improved cooling tower efficiency but was less effective than the lower concentration.
11
Hu, Shih-Yung. "Heat transfer enhancement in thermoelectric power generation." [Ames, Iowa : Iowa State University], 2009.
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12
Mori, Hiromi. "Enhancement of heat transfer for ground source heat pump systems." Thesis, University of Nottingham, 2010. http://eprints.nottingham.ac.uk/11483/.
Повний текст джерелаАнотація:
Uptake of geothermal heat pump (GSHP) systems has been slow in some parts of the world due to the unpredictable operational performance, large installation space requirement and high installation costs. Therefore, design modification was searched in order to improve the feasibility. With regard to relatively small impact of the construction costs, efficient thermal energy collection was targeted in horizontal ground-loop systems with shallow underground construction. The research started with a sensitivity analysis of the underground heat collection system using computational fluid dynamics (CFD). The results indicated essential design parameters to enhance the performance. Strategies to improve one of the parameters, thermal conductivity of soil surrounding the heat exchanger, were investigated through lab experiments. Subsequently, further design optimisation with the CFD intended to select the most competitive modified design against the existing design. It was discovered that an indication to achieve economic and practical modifications for efficient heat collection was to increase the moisture content of sub-soil up to the optimum moisture content (OMC). Annual operation analysis with the CFD disclosed that additional costs for even simple design modifications could easily worsen the payback period. Consequently, solutions to improve the performance of the GSHP within reasonable payback period were proposed.
13
Cooper, Paul. "Electrically enhanced heat transfer in the shell/tube heat exchanger." Thesis, Imperial College London, 1986. http://hdl.handle.net/10044/1/37978.
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14
Bisetto, Alberto. "Enhancement of single- and two- phase heat transfer: inside heat generators." Doctoral thesis, Università degli studi di Padova, 2015. http://hdl.handle.net/11577/3423946.
Повний текст джерелаАнотація:
Heat generators are actually the most used system for producing domestic heat and hot water. These systems are most commonly fire tube heat generators, which consist in a shell-and-tube heat exchanger with the flue gases produced by a stationary combustion process flowing inside the tubes, and the secondary fluid, i.e. water, located in the shell.
The first Chapter of this Thesis presents an experimental and theoretical analysis of the working conditions of a three pass fire tube heat generator, while operating both in stationary and transient regimes. In the literature, at the best of the author’s knowledge, very limited research is published on such systems.
Experimental tests were performed varying heat generator working conditions as well as inserting or not turbulence generator inserts inside the tube composing the last flue gases pass.
A dynamic MatLab/Simulink model for the fire tube heat generator is presented. This has been validated using the new experimental dataset. The model is characterized by a subsystem structure which makes it easily adaptable to different geometries, and it can be used to predict the behavior of heat generators working in stationary and transient conditions.
Heat generators performances can be increased by enhancing heat transfer between the flue gases and the water inserting turbulence generators in the tube where the combustion products flow. In particular, inserting turbulators inside the last pass of fire tube heat generators leads to an increase of the system efficiency, due to reduced flue gases exit temperature which implies reduced thermal losses at the chimney. Thus, single phase heat transfer enhancement by means of turbulence generator inserts is an important field of research for the heat transfer industry.
However, it must be kept in mind that the convective heat transfer coefficient enhancement due to the turbulators presence is associated with an augmentation of the frictional losses in the system, thus both factors have to be taken into account when evaluating inserts performances.
In the second part of the present Thesis, after a review of the most common solutions presented in the literature, performance enhancement of turbulators geometries actually used by the heat generators manufacturers is evaluated by means of CFD simulations. Effects of several geometrical parameters, such as turbulator position inside the tube and pipe diameter, are analyzed with the simulations. Equations for predicting the inserts working conditions are presented. Finally, a modified geometry is presented with the aim of proposing a solution with enhanced thermal and frictional characteristics.
By recovering the latent heat in the exhaust products coming out from the heat generator it is possible to achieve extremely high system efficiencies. Thus, two-phase heat transfer is a field of interest in the heat generators industry.
During pure steam condensation the mayor resistance to the thermal transport is associated to the condensate layer which forms at the wall. Reducing, or eventually removing, the liquid film thickness at the wall will thus lead to an enhancement of the condensation heat transfer coefficient, increasing the heat transfer per unit area. This means the possibility to get higher performances of the system maintaining the same geometry, or to reduce the heat transfer area (thus the system cost) maintaining the same net effect.
The last Section of the present Thesis is focused on the enhancement of the condensation heat transfer coefficient by means of nano-engineered surfaces. Effect of surface wetting properties on the condensation process and performance is analyzed by studying the behavior of conventional, superhydrophilic, hydrophobic and superhydrophobic surfaces during pure steam flow condensation at different mass velocities. The aim of the research is to remove (dropwise condensation mode) or eventually reduce (filmwise condensation mode) the condensate layer over the wall during the two-phase process, by acting on surface superficial characteristics, as well as to evaluate the effect of surface roughness on the filmwise condensation heat transfer coefficient.I generatori di calore sono attualmente i sistemi più utilizzati nelle applicazioni di riscaldamento domestico. Questi sono di norma generatori di calore a tubi di fumo, i quali consistono in uno scambiatore a fascio tubiero in cui i fumi prodotti da un processo di combustione stazionaria fluiscono all’interno dei tubi, mentre il fluido secondario, comunemente acqua, si trova nel mantello. Nel primo Capitolo di questa Tesi viene presentata un’analisi sperimentale e teorica del funzionamento di un generatore di calore a tre giri di fumo, operante sia in condizioni stazionarie che in condizioni dinamiche. In letteratura è estremamente difficile trovare dati teorici e sperimentali riguardanti questi sistemi. Le prove sperimentali sono state svolte variando le condizioni di lavoro del generatore di calore e lavorando sia con che senza generatori di turbulenza all’interno dei tubi che compongono l’ultimo passaggio del sistema. Un modello dinamico, sviluppato in ambiente MatLab/Simulink, del generatore di calore a tre giri di fumo è quindi presentato in questo elaborato. Il modello è caratterizzato da una struttura a sotto-sistemi che lo rende facilmente adattabile a geometrie diverse, e può essere utilizzato per predirre il comportamento dei generatori di calore durante funzionamento in regime stazionario o dinamico. Le prestazioni dei generatori di calore possono essere incrementate aumentando lo scambio di calore tra i gas combusti e l’acqua attraverso l’inserimento di generatori di turbulenza nei tubi dove fluiscono i gas. In particolare, l’inserimento di turbulatori all’interno dell’ultimo passaggio dei fumi nei generatori di calore comporta un aumento dell’efficienza globale del sistema, per via della minore temperatura di uscita dei gas che si riflette in minori perdite al camino. Per questo motivo, l’aumento dello scambio termico monofase attraverso l’inserimento di generatori di turbulenza è un importante ambito di ricerca per l’industria dei generatori di calore. Ad ogni modo, è importante considerare che ad un aumento del coefficiente di scambio termico è associato un incremento delle perdite di carico per attrito nel sistema, ed entrambi gli elementi devono essere considerati nel valutare le prestazione dei turbulatori. Nella seconda parte di questo elaborato, dopo aver presentato una review delle soluzioni più comuni in letteratura, sono analizzate attraverso simulazioni CFD le prestazioni delle geometrie attualmente utilizzate nei generatori di calore. Gli effetti di diversi parametri geometrici, come la posizione del turbulatore all’interno del tubo e il diametro dello stesso, sono stati analizzati. Inoltre, attraverso le simulazioni si sono ricavate delle equazioni predittive del comportamento degli inserti. In ultimo è proposta una modifica alle geometrie attuali al fine di proporre una soluzione più performante. L’efficienza dei generatori di calore può essere incrementata attraverso la condensazione del vapore presente nei gas combusti allo scarico. Per questo, lo studio dello scambio termico bifase è di interesse per l’industria dei generatori di calore. Durante il processo di condensazione del vapore la maggior parte della resistenza termica è localizzata nel condensato che si forma a contatto con la superficie fredda. Riducendo, o eventualmente annullando, lo spessore del film di liquido alla parete si possono quindi ottenere coefficienti di scambio termico bifase estremamente elevati, incrementando quindi il flusso termico specifico scambiato. Questo permetterebbe di incrementare le performance del sistema a parità di geometria, o di ridurre l’area di scambio (e quindi i costi del generatore di calore) a parità di effetto utile. Per questo, nell’ultimo Paragrafo di questa Tesi si analizza l’incremento del coefficiente di scambio termico durante condensazione di vapore su superfici nano-ingegnerizzate. L’effetto delle proprietà di bagnabilità delle superfici sulla modalità e sulle prestazioni del processo di condensazione è studiato analizzando il comportamento di superfici convenzionali, superidrofiliche, idrofobiche e superidrofobiche durante condensazione di vapore puro fluente a diverse velocità. Lo scopo della ricerca è di rimuovere (condensazione a gocce) o ridurre (condensazione a film con scivolamento del condensato) il film di liquido che si forma durante il processo bifase, agendo sulle proprietà della superficie, e di valutare l’effetto della rugosità superficiale sul coefficiente di scambio durante condensazione a film.
15
Bansal, Aditya. "Alumina Nanofluid for Spray Cooling Heat Transfer Enhancement." Scholar Commons, 2007. http://scholarcommons.usf.edu/etd/3759.
Повний текст джерелаАнотація:
Nanofluids have been demonstrated to be promising for heat transfer enhancement in forced convection and boiling applications. The addition of carbon, copper, and other high-thermal-conductivity material nanoparticles to water, oil, ethylene glycol, and other fluids has been determined to increase the thermal conductivities of these fluids. The increased effective thermal conductivities of these fluids enhance their abilities to dissipate heat in such applications. The use of nanofluids for spray cooling is an extension of the application of nanofluids for enhancement of heat dissipation.
In this investigation, experiments were performed to determine the level of heat transfer enhancement with the addition of alumina nanoparticles to the fluid. Using mass percentages of up to 0.5% alumina nanoparticles suspended in water, heat fluxes and surface temperatures were measured and compare. Compressed nitrogen was used to provide constant spray nozzle pressures to produce full-cone sprays in an open loop spray cooling system. The range of heat fluxes measured were for single-phase and phase-change spray cooling regimes.
16
Al, Husseny Adel Ahmed Niameh Mehdy. "Heat transfer enhancement using rotating metallic porous media." Thesis, University of Manchester, 2016. https://www.research.manchester.ac.uk/portal/en/theses/heat-transfer-enhancement-using-rotating-metallic-porous-media(1a50759b-fd87-4836-b7ab-5fa0ab6d6c5d).html.
Повний текст джерелаАнотація:
The present research is dedicated to numerically optimise the heat transfer enhancement achieved by means of utilising rotating metallic porous media. Three systems that play a crucial role in power generation systems are examined. The first is the cooling passages used to dissipate the heat generated within the rotor conductors in electrical generators, the second is the double pipe-heat exchanger, and the last is the rotary thermal regenerator. In all of the systems under consideration, the main objective is to improve the overall performance achieved and to determine the optimum values of the parameters affecting it. Concerning the first system examined, an enhancement way is proposed through filling such rotating cooling passages either fully or partially with open-cell metal foams. A compound heat transfer enhancement is utilised in the second system through applying an axial rotation to a double-pipe heat exchanger occupied with open-cell metal foams structures. The enhancement process carried out using the third system is examined through proposing an effective and less-costly way to simulate and design rotary regenerators based on a porous medium approach. The developing three-dimensional convective fluid flow across the porous media utilised in all the systems investigated is considered laminar and incompressible. Both thermal dispersion and rotational effects, including centrifugal buoyancy and Coriolis force, are taken into account in the first and second systems, while they are neglected in the latter. The momentum equations are mathematically formulated using the generalised model with considering that fluid and solid phases are in a local thermal non-equilibrium. The governing equations are discretised based on the finite volumes method and then solved iteratively using the SIMPLE algorithm, where an in-house FORTRAN code has been developed to investigate the first system, while the other two systems have been simulated using the STAR-CCM+ CFD commercial code. Inspecting the worth of using metal foams to enhance the heat dissipated by the rotating channels reveals the potential of this proposal to outperform the turbulent flow in rotating clear channels, which confirms that this enhancement way is practically justified and efficient. Moreover, the overall system performance achieved using the compound enhancement employed in the second system is incomparable, i.e. OSP=O(102). The porous medium approach, on the other hand, has been found to be sufficient to accurately simulate the convective flow across the third system. Hence, it can be utilised as an effective and more economical alternative to design such sort of heat exchangers rather than the traditional experimental or numerical ways. The resulted data reveal that heat transfer can be improved considerably by manipulating the design factors including the operating conditions as well as the geometrical and thermal characteristics of the porous medium used. However, care must be taken to avoid unnecessary expenses resulting from potential augmentation in pressure drop.
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Martinez, Christian David. "Heat transfer enhancement of spray cooling with nanofluids." [Tampa, Fla] : University of South Florida, 2009. http://purl.fcla.edu/usf/dc/et/SFE0003237.
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Haji, Aghaee Khiabani Reza. "Heat transfer in nano/micro multi-component and complex fluids with applications to heat transfer enhancement." Diss., Georgia Institute of Technology, 2010. http://hdl.handle.net/1853/41154.
Повний текст джерелаАнотація:
Thermal properties of complex suspension flows are investigated using numerical computations. The objective is to develop an efficient and accurate computational method to investigate heat transport in suspension flows. The method presented here is based on solving the lattice Boltzmann equation for the fluid phase, as it is coupled to the Newtonian dynamics equations to model the movement of particles and the energy equation to find the thermal properties. This is a direct numerical simulation that models the free movement of the solid particles suspended in the flow and its effect on the temperature distribution. Parallel implementations are done using MPI (message passing interface) method. Convective heat transfer in internal suspension flow (low solid volume fraction, φ<10%), heat transfer in hot pressing of fiber suspensions and thermal performance of particle filled thermal interface materials (high solid volume fraction, φ>40%) are investigated. The effects of flow disturbance due to movement of suspended particles, thermo-physical properties of suspensions and the particle micro structures are discussed.
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Ko, Kang-Hoon. "Heat transfer enhancement in a channel with porous baffles." Texas A&M University, 2004. http://hdl.handle.net/1969.1/1519.
Повний текст джерелаАнотація:
An experimental and numerical investigation of heat transfer enhancement in a
three dimensional channel using wall mounted porous baffles was conducted. The
module average heat transfer coefficients were measured in a uniformly heated
rectangular channel with staggered positioned porous baffles. A numerical procedure
was implemented, in conjunction with a commercially available Navier-Stokes solver, to
model the turbulent flow in porous media. The Brinkman-Forchheimer-Extended Darcy
model was used for modeling fluid flow through the porous baffles. Conventional, oneequation,
and two-equation models were used for heat transfer modeling. The accuracy
and characteristics of each model were investigated and discussed. The results were
compared with experimental data.
Baffles were mounted alternatively on the top and bottom walls. Heat transfer
coefficients and pressure loss for periodically fully developed flow and heat transfer
were obtained for different pore densities (10, 20, and 40 pores per inch (PPI)) with two
different baffle heights ( / h h B D = 1/3 and 2/3), and two baffle thicknesses ( / t h B D = 1/3
and 1/12). The Reynolds number (Re) was varied from 20,000 to 50,000. To compare
the effect of foam metal baffles, the data for conventional solid-type baffles was
obtained for ( / t h B D =1/3). The maximum uncertainties associated with the module
Nusselt number and friction factor were 5.8% and 4.3%, respectively. The experimental
procedure was validated by comparing the data for the straight channel without baffles
( / h h B D = 0) with those in the literature.
The use of porous baffles resulted in heat transfer enhancement as high as 300%
compared to heat transfer in straight channels without baffles. However, the heat transfer
enhancement per unit increase in pumping power was less than one for the range of
parameters studied in this work. Correlation equations were developed for the heat
transfer enhancement ratio and the heat transfer enhancement per unit increase in
pumping power in terms of Reynolds number.
The conventional theoretical model, the dispersion conductivity model, and the
modified two-phase model using the local thermal non-equilibrium theory were
considered. The results from each model were compared against the experimental data,
and compared to each other to investigate the efficiency of each model. Also, the
characteristics of each model were discussed.
20
Anantawaraskul, Siripon. "Heat transfer enhancement under a turbulent impinging slot jet." Thesis, McGill University, 2000. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=33321.
Повний текст джерелаАнотація:
Heat transfer characteristics under a single turbulent confined slot jet were determined experimentally. New enhancement techniques for the impingement heat transfer rate are proposed and tested experimentally. The results from each enhancement technique are compared with those for a smooth slot nozzle configuration with the same apparatus.The impingement heat transfer rate was observed to increase due to internal finning of the slot nozzles. Both rectangular and triangular fins were tested. The fins acted as roughness elements. Experimental results with the "rough" nozzle show that the stagnation and average heat transfer rates can be enhanced by up to 15% and 10%, respectively. However, an increase in pressure drop across the nozzles is also noted.
Use of inclined confinement surfaces of 10° and 20° angles accelerate the exit flow provides average impingement heat transfer rates comparable with those for parallel wall confinement. Experimental results show no significant change in the heat transfer distribution for the inclination angle of 10°, while the average heat transfer coefficient is in fact decreased slightly for the inclination angle of 20° at high jet Reynolds numbers.
It was found that insertion of a single turbulence generator in the jet flow provides superior impingement heat transfer without any increase in the system pressure drop. Two types of turbulence generators (square rod and thin plate) were investigated. Both turbulence generators provide the same level of average heat transfer enhancement (up to 15%).
21
Egger, Robert A. "Enhancement of boiling heat transfer in di-electric fluids." Thesis, Monterey, California. Naval Postgraduate School, 1991. http://hdl.handle.net/10945/28168.
Повний текст джерела
22
Tetlow, David. "Heat transfer enhancement in integrated phase change drywall system." Thesis, Nottingham Trent University, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.446610.
Повний текст джерела
23
Maxson, Andrew. "Heat Transfer Enhancement in Turbulent Drag Reducing Surfactant Solutions." The Ohio State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=osu1500419520976994.
Повний текст джерела
24
Wang, Xiaolin. "A numerical study of vorticity-enhanced heat transfer." Diss., Georgia Institute of Technology, 2014. http://hdl.handle.net/1853/54017.
Повний текст джерелаАнотація:
In this work, we have numerically studied the effect of the vorticity on the enhancement of heat transfer in a channel flow. In the first part of the work, we focus on the investigation of a channel flow with a vortex street as the incoming flow. We propose a model to simulate the fluid dynamics. We find that the flow exhibits different properties depending on the value of four dimensionless parameters. In particularly, we can classify the flows into two types, active and passive vibration, based on the sign of the incoming vortices. In the second part of the work, we discuss the heat transfer process due to the flows just described and investigate how the vorticity in the flow improves the efficiency of the heat transfer. The temperature shows different characteristics corresponding to the active and passive vibration cases. In active vibration cases, the vortex blob improves the heat transfer by disrupting the thermal boundary layer and preventing the decay of the wall temperature gradient throughout the channel, and by enhancing the forced convection to cool down the wall temperature. The heat transxfer performance is directly related to the strength of the vortex blobs and the background flow. In passive vibration cases, the corresponding heat transfer process is complicated and varies dramatically as the flow changes its properties. We also studied the effect of thermal parameters on heat transfer performance. Finally, we propose a more realistic optimization problem which is to minimize the maximum temperature of the solids with a given input energy. We find that the best heat transfer performance is obtained in the active vibration case with zero background flow.
25
Rutledge, Jeffrey. "Direct simulation of enhancement of turbulent heat transfer by micro-riblets /." Thesis, Connect to this title online; UW restricted, 1989. http://hdl.handle.net/1773/9839.
Повний текст джерела
26
McCleave, Robert W. (Robert William). "Impinging jet heat transfer with turbulence enhancement at the nozzle." Thesis, McGill University, 1993. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=68045.
Повний текст джерелаАнотація:
The effect of turbulence enhancement at the nozzle exit on fluid flow and heat transfer characteristics was investigated for confined jets from sharp-edged nozzles.Average turbulence intensity of the jet flow was characterised by integrating the local turbulence intensity values over the width of the nozzle and at several axial positions from the nozzle exit to the near approach to the impingement surface. Average impingement heat transfer was obtained by integrating the local Nusselt number over an area of the impingement surface relevant to the process engineering application of impingement drying of paper.
Of the several simple methods of turbulence generation examined, the most effective was the simple expedient of placing a bar with a diameter 1/8 that of the nozzle width along the centreline of the slot nozzle. For a heat transfer averaging area equivalent to a nozzle area of 5% of the impingement surface and a nozzle to impingement surface spacing of 1.0 to 1.5 times the nozzle width, this simple method increased average heat transfer rates over those of the plain nozzle by 14%, with only a 7% increase in nozzle operating pressure. The results are presented as enhancement in average heat transfer as a graphical function of mean turbulence intensity, and as an empirical correlation between mean Nusselt number, mean intensity of turbulence and Reynolds number.
27
Hejazian, Majid. "Magnetofluidics for Enhancement of Heat and Mass Transfer in Microscale." Thesis, Griffith University, 2017. http://hdl.handle.net/10072/366857.
Повний текст джерелаАнотація:
Magnetofluidics is the science and technology that combine magnetism and fluid dynamics to modify transport phenomena for a variety of applications. Magnetofluidics often works with conventional microfluidics to take advantage of the small size, the low cost and the low consumption of sample for chemical and biological studies. Magnetofluidics has been used for actuation and manipulation of fluid flow and suspended particles or cells in microfluidic devices.
The use of bio-compatible ferrofluids as a paramagnetic carrier fluid in the field of microfluidics has attracted great interest recently. Ferrofluid is a colloidal liquid made of ferromagnetic or ferrimagnetic nanoparticles suspended in a carrier fluid. In the presence of a magnetic field, a ferrofluid becomes strongly magnetized. Thus, a small amount of samples containing ferrofluid could be manipulated for applications such as mixing, pumping, sorting of particles and cells, enhancement of heat and mass transfer phenomena and chemical reactions. On the other hand, magnetic force can be induced wirelessly and is suitable for biological studies as it sustains cell viability. Therefore, the combination of magnetofluidics and microfluidics has proven to be a low cost, efficient and versatile technology for a number of applications.Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Natural Sciences
Science, Environment, Engineering and Technology
Full Text
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Chaer, Issa Namr. "Refrigeration performance with electrically enhanced heat transfer at the evaporator." Thesis, London South Bank University, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.313001.
Повний текст джерела
29
Stromberger, Jöerg H. "An experimental investigation of electrohydrodynamic (EHD) enhancement of boiling heat transfer." Thesis, Georgia Institute of Technology, 1997. http://hdl.handle.net/1853/17050.
Повний текст джерела
30
Chen, Lingling [Verfasser]. "Heat Transfer Enhancement in Impingement Systems with Surface Enlargements / Lingling Chen." München : Verlag Dr. Hut, 2017. http://d-nb.info/1137023996/34.
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31
Al-Dadah, Raya Khalil. "Electrohydrodynamic boiling heat transfer enhancement at the evaporator of refrigeration plants." Thesis, London South Bank University, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.386293.
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32
Greiner, M. (Miles). "Experimental investigation of resonance and heat transfer enhancement in grooved channels." Thesis, Massachusetts Institute of Technology, 1986. http://hdl.handle.net/1721.1/15094.
Повний текст джерелаАнотація:
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1986.MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING
Bibliography: leaves 85-89.
by Miles Greiner.
Ph.D.
33
Rahman, Aevelina. "Characterization of Heat Transfer Enhancement for an Oscillating Flat Plate-Fin." Thesis, Virginia Tech, 2020. http://hdl.handle.net/10919/98919.
Повний текст джерелаАнотація:
Heat transfer augmentation is of paramount importance in energy transfer and storage systems and the idea of using the inherent vibrations in a system to enhance heat transfer needs to be thoroughly researched upon. The current study numerically investigates an infinitesimally thin plate-fin undergoing forced oscillations over a range of amplitudes and frequencies in the presence of an approach flow. Reduced frequencies of 0.25 ≤ k ≤16 and plunge amplitudes of 0.03125 ≤ h ≤ 8 are investigated at Re=100 and Pr = 0.71. It is shown that the combined effect of frequency and amplitude on heat transfer enhancement can be accounted for as a single parameter “plunge velocity” (0.25 ≤ kh ≤ 4) instead of the individual frequency and amplitude values. For kh > 0.5 a significant increase in Nusselt number ( is observed compared to a stationary plate. With increasing kh or more vigorous oscillations, the increase in becomes more prominent and similar trends and comparable magnitudes were observed for a constant value. Unlike the hydrodynamic counterpart of the study, both Leading Edge Vortices (LEVs) and Trailing Edge Vortices (TEVs) are found to act positively to induce enhanced heat transfer on the plate. Finally, the dependence of heat transfer augmentation on the frequency and amplitude of vibration is quantified with a simple parameterization for a plate-fin in a fluid medium.M.S.
Heat transfer enhancement is of paramount importance in energy transfer and storage systems. The idea of using the inherent mechanical vibrations in a heat producing system to enhance transfer of unwanted heat from that system needs to be thoroughly researched upon. To investigate this idea, we numerically study an infinitesimally thin plate-fin undergoing forced oscillations over a range of amplitudes and frequencies in the presence of an incoming air flow. It is shown that the combined effect of frequency and amplitude on heat transfer enhancement can be accounted for as a single parameter called “plunge velocity” instead of the individual frequency and amplitude values. For a significant plunge velocity, a significant increase in Nusselt number ( is observed compared to a stationary plate representing an increase in the extent of heat transferred. With more vigorous oscillations, the increase in becomes more prominent and similar trends and comparable magnitudes were observed for a constant value. Finally, the dependence of heat transfer augmentation on the frequency and amplitude of vibration is quantified with a simple parameterization for a plate-fin in a fluid medium.
34
Coursey, Johnathan Stuart. "Enhancement of spray cooling heat transfer using extended surfaces and nanofluids." College Park, Md.: University of Maryland, 2007. http://hdl.handle.net/1903/7635.
Повний текст джерелаАнотація:
Thesis (Ph. D.) -- University of Maryland, College Park, 2007.Thesis research directed by: Dept. of Mechanical Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.
35
Herrmann, Priesnitz Benjamín. "Heat transfer enhancement strategies in a swirl flow channel heat sink based on hydrodynamic receptivity." Tesis, Universidad de Chile, 2018. http://repositorio.uchile.cl/handle/2250/153339.
Повний текст джерелаАнотація:
Doctor en Ciencias de la Ingeniería, Mención FluidodinámicaEl disipador de calor de canal con flujo espiral ha demostrado ser una alternativa prometedora para el manejo térmico de aplicaciones de alto flujo de calor, como electrónica y fotovoltaica concentrada. Temperaturas indeseadas son perjudiciales para el despempeño, la seguridad y la vida útil de estos dispositivos, por lo cual la investigación de tecnologías de enfriamiento de alto flujo de calor es una de las áreas de transferencia de calor con mayor actividad en la actualidad. En este trabajo se identifican estrategias eficientes para el aumento de la transferencia de calor en el disipador de calor de canal con flujo espiral estudiando la respuesta de las perturbaciones de temperatura frente a un forzamiento de momentum. Se presentan simulaciones numéricas de los campos de velocidad y temperatura estacionarios en el dispositivo para investigar el efecto de los parámetros de diseño en el desempeño termohidráulico. La rotación del fluido induce una componente de flujo cruzado, y se encuentra que esto aumenta considerablemente la transferencia de calor convectiva debido a movimiento del fluido hacia la superficie de intercambio térmico. En este estudio, se usa el marco de la teoría de estabilidad no modal para estudiar la estabilidad y receptividad del flujo estacionario en el canal de flujo espiral. Para este propósito, se formula un problema de perturbaciones lineales con un forzamiento armónico usando las aproximaciones de flujo local y paralelo. Al contrario del flujo de Poiseuille plano, se encuentra que el crecimiento transiente de las perturbaciones es pequeño, y por lo tanto, no juega un rol en el mecanismo de transición. La transición se le atribuye a la inestabilidad de flujo cruzado que ocurre por el cambio en la forma del perfil de velocidad debido a los efectos rotacionales. Se lleva a cabo un experimento de visualización de flujo y se encuentra una concordancia cualitativa entre los patrones de difusión observados y el número de Reynolds crítico predicho. La mayor amplificación en la respuesta de temperatura frente al forzamiento de momentum es presentada por vórtices y trazas longitudinales, seguidas por ondas viajeras radiales, y luego por ondas viajeras longitudinales. Se lleva a cabo un experimento para medir el desempeño del disipador de calor usando un flujo pulsante con frecuencias de forzamiento dentro del rango sugerido por el análisis de receptividad. Para obtener la misma temperatura de pared que en el caso sin pulsaciones, se observa una reducción de la potencia de bombeo de hasta un 26.6%, y usando la misma potencia de bombeo se obtiene un aumento del Nusselt de hasta un 10.3%. Este enfoque para identificar estrategias para el aumento de la transferencia de calor basado en física se puede extender a otras técnicas, por ejemplo, para seleccionar la longitud de onda en una superficie ondulada, la periodicidad de elementos de rugosidad, o la frequencia de vibraciones acústicas.
36
Srinivasan, Shreyas. "Experimental Investigation of Dimples as a Heat Transfer Enhancement Feature in Narrow Diverging and Converging Channels." Thesis, Virginia Tech, 2013. http://hdl.handle.net/10919/51422.
Повний текст джерелаАнотація:
Detailed heat transfer coefficient distributions have been obtained for narrow converging and diverging channels with and without enhancement features. The enhancement feature considered for this study is dimples (inline and staggered) on the main heat transfer surfaces. All the measurements are presented at Reynolds numbers of 3500, 8900, 18000, and 7000, 14000, 28000 for converging and diverging channels respectively. Pressure drop measurements for the overall channel are also presented to evaluate the heat transfer enhancement geometry with respect to pumping power requirements. The test models were studied for wall heat transfer coefficient measurements using the transient liquid crystal technique. The modeled wall inner surfaces were sprayed with thermochromic liquid crystals, and a transient test was used to obtain the local heat transfer coefficients from the measured color change. Analysis of results shows that dimples, in general, have very good enhancement capabilities and staggered dimpled surfaces provide considerably higher heat transfer coefficients and a reasonable pressure drop compared to inline dimpled configuration. Additionally, this study was extended to understand the effect of strategic placement of dimples (staggered) at various locations along the channel to understand regions that contribute significantly to the overall enhancement.
Master of Science
37
Down, Edward M. "Enhancement of plate heat exchanger performance using electric fields." Thesis, City University London, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.339994.
Повний текст джерела
38
Chan, Jui-Hua, and 詹瑞華. "Heat Transfer Enhancement of Sintered Miniature Heat Pipe." Thesis, 2001. http://ndltd.ncl.edu.tw/handle/31183717189972705560.
Повний текст джерелаАнотація:
碩士國立臺灣大學
機械工程學研究所
89
This study reports the heat transfer enhancement in miniature heat pipe. First, the theoretical analysis has been investigated. We found the porosity is the most important wick structure parameter in miniature heat pipe. And the porosity has been chosen to discuss mainly in this study. In the experiment, we sintered copper powder with the pore former to enhance the porosity and attain the heat transfer enhancement. The Na2CO3 has been chosen to be the pore former in this study.The sintering results are: the porosity is increasing and then decreasing with increasing the mixture ratio of pore former. The best mixture ratio can be found at the maximum porosity. The experimental results are: the heat transfer has the promotional effect with increasing porosity. The reason can be explained as follow: the channel for liquid flowing back to evaporator increases by increasing pores in wick. Therefore, the working fluid circulates more easily and then increase heat transfer rate. Except the porosity discussion, this study also discusses the angle effect in miniature heat pipe. It was found that with increasing the included angle between miniature heat pipe and horizontal line, the maximum heat transfer rate would decrease by adverse gravity. But the effect of the operational angle in sintered miniature heat pipe is less significant.
39
Chang, Cheng-Yan, and 張丞硯. "Heat Transfer Enhancement of a Spiral Heat Exchanger." Thesis, 2016. http://ndltd.ncl.edu.tw/handle/90056852319219196201.
Повний текст джерелаАнотація:
碩士國立中興大學
機械工程學系所
104
The heat transfer enhancement due to flow disturbance rods in a spiral heat exchanger constructed by four Archimedes’ spirals originated from the same center is experimentally investigated. The four spirals form two channels. The widen one with spacing of 10 mm is the cold fluid channel (air), while the narrower one with spacing of 1 mm is the hot fluid channel (water). Isothermal wall condition is assumed and the outer most surface of the heat exchanger is considered to be insulated. Seven threaded rods ( ) are vertically installed at different locations in the air channel. At various air volumetric flow rates ( 0.214、0.446、0.716、0.89、1.055、1.385 m3/min ) and with/without the flow disturbance rods, the air-side fluid friction factors (f) and average Nusselt numbers (Nu) for the heat exchanger (including inlet and spiral channel) and the spiral channel alone are measured, respectively. Using the data, a set of correlation equations are established. The result shows that, for 4000 ≤ Re ≤ 24000, the Nu enhancement index (r1) for the heat exchanger is in the range of 1.38 to 1.503 ; for the spiral channel, it is in the ranger of 1.541 to 1.796. This verifies that the flow disturbance rods are quite effective to enhance the heat transfer in the heat exchanger. The mechanical energy consumption indexes (r2) for the heat exchanger and the spiral channel are in the range of 0.357 to 0.4 and 0.164 to 0.23, respectively. In the spiral channel, 60% of the cross-sectional area is blocked by the flow disturbance rods. This causes the r2 values to be small.
40
Mendes, Luís Pedro Martins e. "Heat transfer enhancement in viscoelastic fluids." Master's thesis, 2017. https://hdl.handle.net/10216/103019.
Повний текст джерела
41
Mendes, Luís Pedro Martins e. "Heat transfer enhancement in viscoelastic fluids." Dissertação, 2017. https://hdl.handle.net/10216/103019.
Повний текст джерела
42
Leu, Shiann-Woei, and 呂憲偉. "Heat Transfer Enhancement by Myltilobe Vortex Generators." Thesis, 1997. http://ndltd.ncl.edu.tw/handle/55917457988715735524.
Повний текст джерелаАнотація:
碩士國立交通大學
機械工程研究所
85
The flow and heat transfer in a tube with multilobe vortex generators inserted are studied using numerical method. The governing equations are transferred to curvilinear coordinates to fit the irregular geometry. Discretization is made by using the finite-volume integral method with non-staggered grid arrangement. The pressure-correction equation is solved over 19 neighboring points. The system of algebra equations are solved by the SIMPLE-type algorithm. Categories of the thesis have three departments: 1. Analysis a tube flow by putting multilobe vortex generator in it. The Reynolds number is mainly fixed at 2000 and the Prandtl number is 5. 2: inside the wall of the above-mentioned tube, we added different length of fins, to analysis the heat enhancement 3: Analysis of the turbulent flow of the smooth tube. When fluid flows through the vortex generator, secondary velocities are induced and axial vortices are formed after a short distance downstream of the exist of the lobe. The axial vortices leads to enhancement of heat transfer as well as friction and pressure loss.. Heat transfer can be improved by increasing the axial slope of the lobe through increasing the lobe penetration, reducing the lobe length, and making a concave lobe shape. Five or six lobes for the vortex generator are sufficient to y ield good performance. The heat transfer can be improved by increasing the length of fins. A three-dimensional flow calculation procedure has been successfully incorporated to study the turbulent flow for Reynolds number is 2×105. It's heat transfer, circulation and friction coefficient is much larger than laminar flow.
43
Li, Po-Yen, and 李柏諺. "Condensation Heat Transfer Enhancement by Porous Microchannel." Thesis, 2012. http://ndltd.ncl.edu.tw/handle/28395483904510377861.
Повний текст джерелаАнотація:
碩士國立臺灣大學
機械工程學研究所
100
The microchannel evaporator with two phase heat transfer is considered to be one of the most potential cooling techniques because of its high heat flux, good temperature uniformity and the lesser requirement for coolant flow rate. In recent years, the heat dissipation rate of the high tech products has increased day by day. The traditional single-phase heat exchanger could not efficiently cool down in a limited area, so the microchannel condenser with two phase heat transfer is regarded as a high potential cooling component in the future. The central purpose of the present research is to enhance the condensation heat transfer by utilizing the two pore size distributions of a biporous surface structure. This surface is sintered from the mixture of dendritic copper powders and the pore former, Na2CO3, which formed the different size pores in the microchannel. By changing the volumetric ratio of pore former, it was able to alter the porosity and the numbers of larger pores, further increasing the heat transfer coefficient. During condensation, vapor could go through the larger pores. The smaller pores could absorb the liquid and help to reduce the liquid film thickness. It decreased the heat resistant and increased the heat transfer coefficient. First, a plane surface microchannel system was built as a compared base. The test section of the 30 channels where the width and the depth is 500μm and 155μm, respectively. The test section was made by oxygen-free copper. Water steam is using as working fluid. In the experiment of the plane surface microchannel, the heat transfer coefficient and the pressure drop werw positively related to the increasing mass flux. When increasing the mass flux, the velocity of working fluid becomes faster due to the increasement of wall shear stress. Therefore, it caused the thickness of the liquid film much thinner, decreased the heat resistance and also increased the heat transfer coefficient. Compared with the heat transfer correlation of the conventional channel, the result showed the MAE is quite large. That means there is much room to make progress on the heat transfer correlation of the microchannel. With regard to the pressure drop, comparing with the correlation of microchannel in recent years, it considerably correlated with the results. That shows the result is reliable. For experiment of the biporous surface microchannel, the parameter with copper powder is 61~70 μm diameter and volumetric ratio of Na2CO3 is 30%. Comparing with the plane surface microchannel, the results showed that the heat transfer coefficient is enhanced to 72.9% on average when increased the pressure drop to 28.6% on average. The main reasons of enhancing the heat transfer are high water absorbing capacity and good ability for reducing liquid thickness.
44
Liu, Bing-Han, and 劉秉翰. "Heat Transfer Enhancement in Porous Microchannel Evaporator." Thesis, 2011. http://ndltd.ncl.edu.tw/handle/25368331614143280375.
Повний текст джерелаАнотація:
博士國立臺灣大學
機械工程學研究所
99
The microchannels evaporator, which possesses the advantages of high heat transfer coefficient, good temperature uniformity, and small requirement for coolant flow rates, is considered as a potential cooling technology. In recent years, the raising of heat dissipation rate in electrical products becomes an important issue. The heat-transfer enhanced microchannels are suitable for the applications. The porous structure with a large number of nuclear sites as well as the re-entrance cavities is expected to enhance the heat transfer performance in a microstructure. In the present study, porous microchannel evaporators are designed and manufactured. The effects of powder size, thickness of structure, and pore size distribution upon the heat transfer performance are investigated. The comparisons of heat transfer characteristics, pressure drop, pressure instability, and heat transfer enhanced effects between the plane and the porous microchannel evaporator are made. The flow boiling experiments were conducted with a plane and a porous microchannel evaporator using R-134a as coolant. Both microchannels had 62 channels (225μm in width; and 660μm in depth) on copper substrates with one square inch in area. For the plane microchannel evaporators, the results showed that the nucleation boiling and the force convection boiling mechanisms both appeared in microchannels. When the quality in the microchannels was smaller than 0.4, the heat transfer coefficient mainly increased with increasing heat flux and did not vary with the mass flow rate or the quality. This region (quality was under 0.4) was dominated by the nucleation boiling mechanism. On the other hand, when the quality was larger than 0.4, the heat transfer coefficient increased with a increasing mass flux. This region (quality was over 0.4) was dominated by the force convection boiling. The experiment results were substituted into the correlations in which the surface tension force was taken into consideration. The predictions showed a good agreement with experimental data. The critical heat flux (CHF) increased with increasing flow rates. A CHF correlation that incorporates the surface tension force showed an excellent accuracy for the experimental data. Pressure drop were raised by increasing flow rates and heat fluxes. The separation model incorporating surface tension force had a good agreement. The pressure drop oscillation suggested that the presence of instability inside the plane microchannels as well as the maximum amplitude of oscillation were found near the onset of nucleation (ONB). For the porous microchannels evaporator, the experimental results depicted that the heat transfer coefficient reached a peak value at low quality and decreased with a increasing quality. However, the heat transfer coefficient did not vary with the mass flow rate. This was apparently different from the plane microchannels. The heat-transfer behavior dominated by the mass fluxes belongs to the force convection boiling mechanism. In contrast of the plane microchannel evaporator, the heat transfer coefficient in the porous microchannels evaporator had an enhancement of 5 times in average. The CHF in porous microchannel evaporator increased with increasing mass fluxes and did not enhanced significantly. Furthermore, the trend of pressure drop in porous microchannel was similar in the plane microchannels. The pressure drop was higher than plane microchannels; however, the maximum pressure drop was not over 50%. The amplitude of average pressure drop oscillation near the high heat flux as well as ONB was 1/6 and 1/2 smaller than in the plane microchannels. This result presented that the porous microchannels evaporators provided a stable boiling behavior when the nucleation began. The porous microchannel evaporators were sintered under the following parameters: the powder diameter dp ranged from 1~100μm, thickness of porous structure δ ranged from 150~375μm, and δ/dp ranged from 2~20, respectively. The investigation on the effect of particle size dp as well as thickness δ indicated that the ratio of the thickness to the particle size δ/dp had a significance in the heat transfer performance. This ratio must be properly chosen in order to reach a better heat transfer performance. The better ratio of δ/dp was between 8~12 in our work. Moreever, the pore size distribution dominated the heat transfer behavior. Smaller pore size with a higher heat transfer capacity. The bi-porous structure was better than the mono-porous structure in about 2 times. To conclude the present study, the porous microchannel evaporator is highly potential for the industrial applications.
45
Chen, Hou-Ren, and 陳厚任. "Heat Transfer Enhancement of Heat Sink by Inverse Design Method." Thesis, 2017. http://ndltd.ncl.edu.tw/handle/y8w6pg.
Повний текст джерелаАнотація:
碩士國立臺北科技大學
車輛工程系所
105
In this study, perforated plate-fin heat sinks are designed to enhance heat transfer performance and reduce flow power numerically and experimentally. Moreover, the heat transfer enhancement of a pin-fin heat sink with vortex generators is also investigated numerically and experimentally. The vortex generators will be used to modify the flow fields and enhance the heat transfer of the heat sink. First of all, the flow and the temperature fields of the perforated plate-fin heat sinks in a cross flow wind tunnel are analyzed numerically. The optimized radius, distance and position of the perforated holes are obtained for the perforated heat sinks with the expected thermal resistance and flow power by the inverse design method. In addition, the numerical model is established and the flow and temperature fields of the pin-fin heat sink with the vortex generators is be solved by computational fluid dynamics software. The optimized height and attack angle of vortex generator, distance between vortex generators, and distance between vortex generator and heat sink are obtained for the expected thermal resistance and flow power by the inverse design method. Then, the thermal resistance and flow power of the optimized perforated heat sinks are measured by infrared thermography in the cross flow wind tunnel. Besides, the thermal resistance and flow power of the optimized vortex generator and heat sink system are also measured in a cross flow wind tunnel. Finally, the experimental results are compared with the numerical data to ensure their accuracy.
46
Chu, Mao-Long, and 朱茂榕. "Heat Transfer Enhancement of Loop Heat Pipe with Self-Rewetting fluid." Thesis, 2014. http://ndltd.ncl.edu.tw/handle/62422980327924299494.
Повний текст джерелаАнотація:
碩士國立臺灣大學
機械工程學研究所
102
The objective of this study is the application of self-rewetting fluid as the working fluid on loop heat pipe (LHP), with sintered copper as the chosen capillary structure material; this study also investigates the effect of using different contents and concentrations of self-rewetting fluid on heat transfer performances of LHP as well as compares the results with those from using water as working fluid. Previous studies have shown that using self-rewetting fluid as working fluid can enhance the heat transfer mechanisms of pool boiling, traditional heat pipes, and wickless heat pipes. Compared to using pure substance as working fluid, where the surface tension decreases linearly with increasing temperature, self-rewetting fluid’s surface tension has a non-linear relationship with temperature changes; therefore, at a certain temperature, the self-rewetting fluid’s surface tension increases with increasing temperature, resulting in the Marangoni effect, and the condensed liquid can be transported to the heating surface, delaying the occurrence of dry out and thus increasing the critical heat load. Concerning the effect of varying the concentration of butanol and petanol aqueous solutions on heat transfer performance of LHP, butanol concentrations ranging from 2% to 8% is investigated, and pentanol concentrations ranging from 1% to 3% is investigated. Experimental results show that 6% butanol aqueous solution results in the best heat transfer performance of LHP; compared with that of water, the critical heat load is increased by 100% and the total thermal resistance is decreased on average by 30%. Concerning the effect of changing the components of self-rewetting working fluid, the fluids considered are butanol, pentanol, hexanol, with the concentration of each being the maximum solubility concentration in water under standard conditions. Experimental results show that, compared with those from using water as working fluid, using self-rewetting fluid can allow the total thermal resistance of LHP system to decrease, increasing the critical heat load. Concerning the heat transfer performance of different self-rewetting fluids, under operating temperature of 90°C or lower, hexanol aqueous solution achieves the largest heat load of 200W and lowest total thermal resistance of 0.33°C/W; at operating temperatures higher than 90°C, hexanol aqueous solution has already reach the critical heat load, causing the system to be unstable, but butanol aqueous solution achieves the best results, with maximum critical heat load of 500W and minimum total thermal resistance of 0.26°C/W. Therefore, after analysis of the heat transfer performance of various self-rewetting fluids, butanol water solution has the largest operating temperature range, highest critical heat load, and lowest total thermal resistance, indicating that butanol aqueous solution is the most effective in enhancing the heat transfer performance of LHP.
47
Juan, Chun-Chia, and 阮俊嘉. "Heat Transfer Enhancement of a Loop Heat Pipe with Bidispersed Wicks." Thesis, 2010. http://ndltd.ncl.edu.tw/handle/89053937052331807590.
Повний текст джерелаАнотація:
碩士臺灣大學
機械工程學研究所
98
The purpose of this article is to develop high-performance bidispersed wicks utilized in a LHP’s evaporator and to improve heat transfer crisis in formerly monoporous wicks. The influence of heat transfer performance about difference pore-size parameters will be discussed. The study was conducted by sintering the nickel powder clusters, which mixed with binder into a bidispersed wick. A two-level factorial plan of statistical design was introduced involving two variables: the average cluster size (53、82μm), and the cluster size distribution range (14、42μm). Moreover, the statistical model was built to determine the optimized parameter combination of the bidispersed wick. Finally, the heat transport capability of the LHP between monoporous wicks and bidispersed wicks has been investigated. The experimental results indicated that average cluster size is the major effect (92.8%) on LHP’s performance about pore-size parameters, and the effect of cluster size distribution range was not significant (2.9%) within the parametric range. The better pore-size parameter tended to smaller average cluster size and wider cluster size distribution range. The best pore size parameter of the bidispersed wick was 20μm~62μm, which obtained by follow-up experiments. Experimental results also showed that at the sink temperature of 10℃ and the allowable evaporator wall temperature of 85℃ the maximum heat transfer capacity of monoporous wick achieved 400W and the minimum total thermal resistance was 0.16℃/W. Comparing to the monoporous wick, the corresponding values of the best bidispersed wick was 575W for heat transfer capacity and 0.13℃/W for total thermal resistance. In addition to the heat transfer ability of porous wicks, the best Bidispersed wick showed higher heat transfer coefficient of 23.3 kW/m2℃ at 400W than monoporous wick of 10.3 kW/m2℃ by 2.3 times. In conclusion, bidispersed wicks are very attractive for high heat flux applications.
48
Chang, Pai-Yu, and 張百禹. "Laminar Heat Transfer Enhancement in Horizontal Rectangular Channels." Thesis, 1999. http://ndltd.ncl.edu.tw/handle/29757907578462068315.
Повний текст джерелаАнотація:
博士國立中央大學
機械工程研究所
88
The dissertation primarily focuses on the passive techniques of heat transfer enhancement, derived from special configuration of the surface geometry. This includes three methods of the passive techniques: additives for liquids or additives for gases, rough surfaces and polymer solutions. First, additives for liquids or additives for gases; Present a numerical study of stagnant conductivity effect on the thermal developed non-Darcian mixed convection in horizontal packed-sphere channels. The effect of stagnant conductivity is characterized by the conductivity ratio of solid sphere-to-fluid (ks /kf). The results show that the buoyancy effect affects significantly the secondary flow structure and enhanced heat transfer when ks /kf and Peclet number are all low. But under a fixed Rayleigh number, the effect of buoyancy will be suppressed when the value of ks /kf or Peclet number increases. Based on the criterion of 5% deviation of Nusselt number from that for forced convection, the critical Rayleigh number for the onset of buoyancy effect is shown to increase more than 1 order of magnitude when ks /kf increases from 1.3 to 26.8 or 38.8 at low Peclet number. Second, rough surfaces; In microchannels, the effect of the surface roughness has been simulated by a numerical study to investigate the laminar flow and heat transfer behaviors on the forced convection by different parameter''s change. A result show that both parameters include larger surface relative roughness, smaller roughness geometry slip coefficient which have all more momentum transfer on near the wall induce heat transfer enhancement. Another result as shown heat transfer''s effect are also dependent on thermal boundary conditions and microchannel''s aspect- ratio, when microchannel''s aspect-ratio , constant axial wall heat flux with uniform peripheral wall heat flux which had larger local Nusselt number Nul value than constant wall temperature peripherally as well as axially. Larger microchannel''s aspect-ratio had larger local Nusselt number Nul value than aspect-ratio , which appeared on the constant wall temperature peripherally as well as axially. But, for constant axial wall heat flux with uniform peripheral wall heat flux, local Nusselt number Nul value is not regularly increasing to increase microchannel''s aspect-ratio. Last, polymer solutions; A numerical study has been performed on the heat transfer mechanism of Newtonian and non-Newtonian fluids in 2:1 horizontal rectangular ducts. The effects of temperature which dependent on fluid''s viscosity, shear thinning, and buoyancy-induced secondary flow are all considered. Experimental data for Newtonian fluid, water, and non-Newtonian fluid, 1000 weight parts per million (wppm) aqueous Separan AP-273 solution (0.1%), were chosen for the comparison with the numerical results. For water, the present numerical results are all in good agreement with the experimental data. The heat transfer enhancement is caused by the buoyancy-induced secondary flow. For Separan AP-273 solution (0.1%), the present numerical results agree with the experimental data in the region near the entrance, but the present modeling underestimates the value of Nul in the fully-developed region. In the region near the entrance, the heat transfer enhancement is caused mainly by the axial velocity distortion, which is mainly due to the temperature dependence of viscosity. The effect of buoyancy-induced secondary flow which is much weaker in the case for Separan AP-273 solution (0.1%) rather than that for water. It is mainly caused by the relatively high viscosity of fluid around the central zone of rectangular duct.
49
Tseng, Ching-Chi, and 曾慶祺. "Enhancement of Heat Transfer in Circular Tube Flow." Thesis, 1994. http://ndltd.ncl.edu.tw/handle/09104198612537846705.
Повний текст джерела
50
Chen, Kuei-Yen, and 陳奎延. "Wettability Effect on Microchannel Condenser Heat Transfer Enhancement." Thesis, 2012. http://ndltd.ncl.edu.tw/handle/18284612827192116300.
Повний текст джерелаАнотація:
碩士國立臺灣大學
機械工程學研究所
100
In recent years, the microchannel evaporator with two phase heat transfer, due to its highly heat flux and little requirement for coolant flow rate, is considered as one of potential cooling techniques. When the traditional single phase heat exchanger cannot efficiently cool in a limited area, collocating the microchannel condenser with two phase heat transfer is regarded as a potential cooling component. Because of the highly developed technology, the heat dissipation rate raise in many products. Therefore the microchannel condenser with enhanced heat transfer is more applicale. The hydrophobic surface has wide contact angle, and worse wettbility. In the process of condensation it will form dropwise condensation. The heat transfer coefficient is increased dramatically in the large scale condenser within hydrophobic surface. We assume that it will show the same result in the microchannel condenser. Thus, this research design and manufacture the hydrophobic and hydrophilic microchannel condenser, compare to the heat transfer coefficient and pressure drop with uncoated microchannels condenser. The test section has 30 channels 500μm in width and 155μm in depth using water as working fluid. Using layer-by-layer (LbL) assembly method manufacture the hydrophobic and hydrophilic structure the same geometric dimensions microchannel condenser. In the experiment of the uncoated microchannel, the heat transfer coefficient and pressure drop is positive correlative with the increasing mass flux. Because increasing the mass flux, the velocity becomes faster, along with increasing the wall shear stress. This will make the thickness of the liquid film become much thinner, and reduce the heat resistant, further increase the heat transfer coefficient. Compared with the heat transfer correlation of the conventional channel, the result shows the MAE is still large. Currently, there is still much room to make progress on the heat transfer correlation of the microchannels. With regard to the pressure drop, compare with the correlation of micrchannel developed recently, it correlated with our result and shows that our result is reliable. In coated surface, we use layer-by-layer (LbL) assembly method to change the contact angle between water and copper, to manufacture the hydrophobic and hydrophilic structure. The contact angle of the hydrophilic structure change from 87°to 43°. Also the contact angle of the hydrophobic structure rise up to 135°. Compared with the heat transfer coefficient of the uncoated surface, the hydrophobic microchannels can increase roughly 100% on average, with remarkable difference. The droplet cannot adhere on the hydrophobic surface to cause the dropwise condensation. The mechanism is different from the filmwise condensation. Therefore, the heat transfer coefficient is much higher. When the mass flux is small, the heat transfer coefficient doesn’t increase remarkably. Because the velocity is slower, the liquid film is easy to form. Therefore, the mechanism that dropwise condensation could increase the heat transfer coefficient cannot be observed within the small mass flux in this experiment. As increasing the mass flux, the velocity is so fast that is not easy to form the liquid film. This will make the heat transfer coefficient increase remarkably. For the pressure drop, the contact angle of the hydrophilic structure is smaller, and it can extend liquid, making the flow easier. In this experiment the pressure drop decrease about 40% on average. It shows that the hydrophilic structure can greatly improve the pressure drop for the microchannel condenser.