Heat pipes are passive heat transfer devices in which a working fluid is sealed inside a metal enclosure. Properly designed wick structures on the inner surface of the heat pipe are critical as the wick aids in the return of the condensed liquid from the cold end back to the hot end where the vaporization-condensation cycle begins again. Additive manufacturing techniques allow for manufacturing complex parts that are typically not feasible with conventional manufacturing methods. Thus, additive manufacturing opens the possibility to develop high performance heat pipes with complex shapes. In this study, an additive manufacturing technique called Binder Jetting is used to fabricate a fully operational compact (78 mm x 48 mm x 8 mm) flat plate heat pipe. Rectangular grooves with converging cross section along the length act as the wicking structure. A converging cross section was designed to enhance the capillary force and to demonstrate the capability of additive manufacturing to manufacture complex shapes. This work describes the challenges associated with the development of heat pipes using additive manufacturing such as de-powdering and sintering. Multiple de-powdering holes and internal support pillars to improve the structural strength of the heat pipe were provided in order to overcome the manufacturing constraints. The heat pipe was experimentally characterized for thermal performance with acetone as the working fluid for two different power inputs. The heat pipe operated successfully with a 25% increase in effective thermal conductivity when compared to solid copper. / Master of Science / The number of transistors in electronic packages has been on an increasing trend in recent decades. Simultaneously there has been a push to package electronics into smaller regions. This increase in transistor density has resulted in thermal management changes of increased heat flux and localization of hotspots. Heat pipes are being used to overcome these challenges. Heat pipes are passive heat transfer devices in which a working fluid is sealed inside a metal enclosure. The fluid is vaporized at one end and condensed at the other end in order to efficiently move heat through the pipe by taking advantage of the latent heats of vaporization and condensation of the fluid. Properly designed wick structures on the inner surface of the heat pipe are used to move the condensed fluid from the cold end back to the hot end, and the wick is a critical component in a heat pipe. Additive manufacturing techniques offer the opportunity to manufacture complex parts that are typically not feasible with conventional manufacturing methods. Thus, additive manufacturing opens the possibility to develop high performance heat pipes with complex shapes as well as the ability to integrate heat exchangers with the heat source. In this study, an additive manufacturing technique called Binder Jetting is used to fabricate a fully operational compact (78 mm x 48 mm x 8 mm) flat plate heat pipe. Rectangular grooves with converging cross section along the length act as the wicking structure. This work describes the challenges associated with the development of heat pipes using additive manufacturing such as depowdering and sintering. The heat pipe was experimentally characterized for thermal performance with acetone as the working fluid for two different power inputs. The heat pipe was found to operate successfully with a 25% increase in effective thermal conductivity when compared with solid copper.

Yes / This article presents the homotopy perturbation method (HPM) employed to investigate the effects of inclination on the thermal behavior of a porous fin heat sink. The study aims to review the thermal characterization of heat sink with the inclined porous fin of rectangular geometry. The study establishes that heat sink of an inclined porous fin shows a higher thermal performance compared to a heat sink of equal dimension with a vertical porous fin. In addition, the study also shows that the performance of inclined or tilted fin increases with decrease in length–thickness aspect ratio. The study further reveals that increase in the internal heat generation variable decreases the fin temperature gradient, which invariably decreases the heat transfer of the fin. The obtained results using HPM highlights the accuracy of the present method for the analysis of nonlinear heat transfer problems, as it agrees well with the established results of Runge–Kutta. / Supported in part by the Tertiary Education Trust Fund of Federal Government of Nigeria, and the European Union’s Horizon 2020 research and innovation programme under grant agreement H2020-MSCA-ITN-2016SECRET-722424.

Doubling transistor count for every two years in a computer chip, transmitter and receiver (T/R) module of a phased-array antenna that demands higher power with smaller dimensions are all results of miniaturization in electronics packaging. These technologies nowadays depend on improvement of reliable high performance heat sink to perform in narrower volumes. Employing microchannels or open cell metal foam heat sinks are two recently developing promising methods of cooling high heat fluxes. Although recent studies especially on microchannels can give a rough estimate on performances of these two methods, since using metal foams as heat sinks is still needed further studies, a direct experimental comparison of heat exchanger performances of these two techniques is still needed especially for thermal design engineers to decide the method of cooling. For this study, microchannels with channel widths of 300 &micro / m, 420 &micro / m, 500 &micro / m and 900 &micro / m were produced. Also, 92% porous 10, 20 and 40 ppi 6101-T6 open cell aluminum metal foams with compression factors 1,2, and 3 that have the same finned volume of microchannels with exactly same dimensions were used to manufacture heat sinks with method of vacuum brazing. They all have tested under same conditions with volumetric flow rate ranging from 0,167 l/min to 1,33 l/min and 60 W of heat power. Channel height was 4 mm for all heat sinks and distilled water used as cooling fluid. After experiments, pressure drops and thermal resistances were compared with tabulated and graphical forms. Also, the use of metal foam and microchannel heat sinks were highlighted with their advantages and disadvantages for future projects.

A presente tese envolve um estudo experimental da ebulição convectiva no interior de um dissipador de calor baseado em multi-microcanais. Resultados experimentais para perda de pressão e coeficiente de transferência de calor foram levantados para os hidrocarbonetos R600a (isobutano), R290 (propano) e R1270 (propileno), fluidos com reduzido GWP (Global Warming Potential) e ODP (Ozone Depletion Potential) nulo. O desempenho termo-hidráulico destes fluidos foi avaliado em um dissipador de calor de cobre, contendo cinquenta canais paralelos com seção transversal retangular de 123x494 µm2 , 15 mm de comprimento e área de base de 15x15 mm2. Os experimentos foram realizados para fluxos de calor de até 400 kW/m2, velocidade mássica variando entre 165 e 823 kg/m2s, graus de sub-resfriamento do líquido na entrada da seção de testes de 5, 10 e 15°C e temperaturas de saturação de 21 e 25°C. Os dados experimentais foram amplamente analisados e discutidos, focando o efeito do fluido refrigerante. Oscilações dos sinais de temperatura e pressão foram analisadas parametricamente visando caracterizar efeitos de instabilidades térmicas. Adicionalmente, realizou-se análise comparativa de desempenho dos refrigerantes baseada na 2ª Lei da Termodinâmica. Os dados para hidrocarbonetos foram comparados com resultados de trabalhos prévios para o refrigerante R134a levantados na mesma seção de testes e utilizando a mesma bancada experimental. A partir destes dados, conclui-se que os hidrocarbonetos proporcionam coeficientes de transferência de calor superiores ao R134a. Em geral, o coeficiente de transferência de calor apresenta a seguinte ordem decrescente: R290, R1270, R600a e R134a. No entanto, o R290 necessitou superaquecimentos da parede superiores ao R1270 para iniciar o processo de ebulição. O refrigerante R1270 proporcionou perdas de pressão totais inferiores aos demais fluidos segundo a seguinte ordem decrescente: R600a, R134a, R290 e R1270. O refrigerante R1270 apresentou frequências de oscilação inferiores na temperatura da câmara de saída. Baseado na análise de desempenho da 2ª Lei da Termodinâmica, conclui-se que, as irreversibilidades devido ao processo de transferência de calor foram predominantes quando comparadas àquelas devido à perda de pressão. Através desta análise também constatou-se o melhor desempenho para o refrigerante R290. / The present thesis concerns an experimental study on flow boiling inside a microchannel array. Experimental results for two-phase pressure drop and heat transfer coefficient were acquired for the hydrocarbons R600a (isobutane), R290 (propane) and R1270 (propylene). These fluids present low Global Warming Potential (GWP) and null Ozone Depletion Potential (ODP). The cooling performance of these hydrocarbons were evaluated for a copper heat sink containing fifty parallel microchannels. The microchannels are rectangular with cross section of 123x494 µm2, 15 mm length and a footprint area of 15x15 mm2. The experimental evaluation was performed in a test facility located at the Laboratory of Thermal and Fluid Engineering of School of Engineering of São Carlos, University of Sao Paulo. The experiments were performed for heat fluxes up to 400 kW/m2, mass velocities from 165 to 823 kg/m2s, degrees of liquid subcooling at the test section inlet of 5, 10 and 15°C and saturation temperatures of 21 and 25°C. The experimental data were carefully analyzed and discussed focusing on the effects of the fluid on the heat sink thermal hydraulic performance. Fluctuations in the temperature and pressure were analyzed parametrically in order to evaluate thermal instability effects. Additionally, an exergy analysis was performed to evaluate the refrigerant efficiency during convective evaporation. Subsequently, the parametric effects and performance of hydrocarbons were compared with previous results for refrigerant R134a obtained in the same test facility and under the same experimental conditions. The refrigerant R290 provided heat transfer coefficients higher than R600a and R1270. However, R290 needed a degree of wall superheating for the onset of nucleate boiling higher than R1270. Based on the exergy analysis it was concluded that, the irreversibility associated to the heat transfer process are predominant compared with the irreversibility due to the pressure drop. According to the Second Law analyses it was also concluded R290 as the fluid providing the best performance.

Heat stress is a well documented hazard across industries. The combination of environmental conditions, work demands, and clothing contribute to heat strain. Left unchecked, heat strain causes changes in an individual's physiological state that can lead to serious and fatal conditions with little warning. Although engineering and administrative controls are the first choice to abate this hazard, they frequently are not feasible. In these cases, personal cooling is often employed. There are three main types of personal cooling: liquid, air, and passive. Each has its own advantages and disadvantages. This study focuses on continuous cooling using a portable liquid cooling system (LCS). The LCS used a vest with tubes circulating water from an ice heat sink. The experiment consisted of five males each completing seven tests in random order. The subjects wore work clothes as the control then in conjunction with a firefighter, vapor barrier, and bomb suits. Each suit was tested with and without the benefit of the LCS. All of the tests took place at 35oC dry bulb and 50% relative humidity while attempting to walk 90 minutes on a treadmill at a 300 W metabolic rate. The study found continuous use of the LCS significantly reduced heat storage (S) and the rate of rise of heart rate (rrHR), core temperature (rrTre), and mean skin temperature (rrTsk) for the firefighter and vapor barrier suits as compared to no-cooling. Although the LCS didn't significantly affect the rate of rise for physiological responses with the bomb suit, it did however, significantly increase the endurance time. Interestingly, the study also found when wearing either the vapor barrier or firefighter suits in conjunction with the LCS that the rrHR and rrTre were not significantly different from only wearing work clothes.

Low grade heat sources have a large amount of thermal energy content. Due to low temperature, the conventional power generation technologies result in lower efficiency and hence cannot be used. In order to efficiently generate power, alternate methods need to be used. In this study, a supercritical organic Rankine cycle was used for heat source temperatures varying from 125°C to 200°C. Organic refrigerants with zero ozone depletion potential and their mixtures were selected as working fluid for this study while the cooling water temperature was changed from 10-25°C. Operating pressure of the cycle has been optimized for each fluid at every heat source temperature to obtain the highest thermal efficiency. Energy and exergy efficiencies of the thermodynamic cycle have been obtained as a function of heat source temperature. Efficiency of a thermodynamic cycle depends significantly on the sink temperature. At areas where water cooling is not available and ambient air temperature is high, efficient power generation from low grade heat sources may be a challenge. Use of passive cooling systems coupled with the condenser was studied, so that lower sink temperatures could be obtained. Underground tunnels, buried at a depth of few meters, were used as earth-air-heat-exchanger (EAHE) through which hot ambient air was passed. It was observed that the air temperature could be lowered by 5-10°C in the EAHE. Vertical pipes were used to lower the temperature of water by 5°C by passing it underground. Nocturnal cooling of stored water has been studied that can be used to cool the working fluid in the thermodynamic cycle. It was observed that the water temperature can be lowered by 10-20°C during the night when it is allowed to cool. The amount of water lost was calculated and was found to be approximately 0.1% over 10 days. The different passive cooling systems were studied separately and their effects on the efficiency of the thermodynamic cycle were investigated. They were then combined into a novel condenser design that uses passive cooling technology to cool the working fluid that was selected in the first part of the study. It was observed that the efficiency of the cycle improved by 2-2.5% when passive cooling system was used.

<p>Abstract</p><p>This thesis is studying the heatsinks new designs for copper heatsinks which utilizes modelling and simulation by CFD, construction of prototypes and experimental works. Challenges and complications in manufacturing of copper heatsinks are expressed and finding the solutions to these hindrances involve in this work. Numerical efforts supported by fluent are made to promote investigation and approaching the goal in which serves the new opportunities for wider application of copper material in heat sinks.</p><p>However the thermal conductivity of copper is about double as aluminium but still aluminium heatsinks are commonly used for heat dissipation in computers.</p><p>Comparing of heat performance of two analogous heatsink of different materials, aluminium and copper, is conducted by numerical analysis in the CFD environment.</p><p>In addition to larger surface area and airflow velocity another solution for enhancement of heat dissipation is suggested.</p><p>Manufacturing solutions of copper heatsinks are proposed which will facilitate fabrication of more high performance copper heatsinks than the current heavy and expensive models.</p><p>Our first copper heat sink model is designed exclusively based on the technical observations and analyses of numerical simulation of two identical copper and aluminium heatsinks by CFD and as well as manufacturability concerns.</p><p>This heat sink is fabricated mechanically and is tested by a number of heat sources and high sensitive devices such as adhesive K type thermocouple, data acquisition 34970A in associated with HP Bench Link program.</p><p>An extent experimental work on aluminium heatsinks, integrated with forced convection, is performed in order to measure their thermal capacities.</p><p>Comparison of the heat performance of a typical aluminium heatsink, which was the best among the all aluminium heat sinks and proposed copper heatsink under identical experimental conditions, is performed.</p><p>Also in some numerical efforts, optimizing and predicting of the thermal characterization of the proposed heatsink with inclined free fins is developed. The model is scaled up in the fluent environment to predict its application in the cooling of larger heat generated electronic devices.</p><p>Impingement air-cooling mode of force-convection is adopted for heat dissipation from high power electronic devices in associated with the proposed inclined fin model.</p><p>Components of airflow velocity in the hollow spaces of the heatsink are discussed. Pressure drop and other thermal variables are analyzed analytical and by CFD code.</p><p>Another mechanical manufactured copper heat sink is investigated. A new design of the base and fins is optimized.</p><p>A three-dimensional finite volume method is developed to determine the performance of the proposed heatsink.</p><p>Thermal and hydraulic characterization of the heat sink under air-forced convection cooling condition is studied. The flow behavior around the fins and some other parts of the heat sink is analyzed by utilizing CFD code.</p><p>The hydraulic parameters including velocity profiles, distribution of static pressure, dynamic pressure, boundary layer and fluid temperature between the fins and in the passageway at the middle of the heat sink are analyzed and presented schematically.</p><p>Furthermore the thermal characteristic of the proposed heatsink is studied by contouring the three dimensional temperature distributions through the fins and temperature of the heat source by CFD code.</p>

Abstract This thesis is studying the heatsinks new designs for copper heatsinks which utilizes modelling and simulation by CFD, construction of prototypes and experimental works. Challenges and complications in manufacturing of copper heatsinks are expressed and finding the solutions to these hindrances involve in this work. Numerical efforts supported by fluent are made to promote investigation and approaching the goal in which serves the new opportunities for wider application of copper material in heat sinks. However the thermal conductivity of copper is about double as aluminium but still aluminium heatsinks are commonly used for heat dissipation in computers. Comparing of heat performance of two analogous heatsink of different materials, aluminium and copper, is conducted by numerical analysis in the CFD environment. In addition to larger surface area and airflow velocity another solution for enhancement of heat dissipation is suggested. Manufacturing solutions of copper heatsinks are proposed which will facilitate fabrication of more high performance copper heatsinks than the current heavy and expensive models. Our first copper heat sink model is designed exclusively based on the technical observations and analyses of numerical simulation of two identical copper and aluminium heatsinks by CFD and as well as manufacturability concerns. This heat sink is fabricated mechanically and is tested by a number of heat sources and high sensitive devices such as adhesive K type thermocouple, data acquisition 34970A in associated with HP Bench Link program. An extent experimental work on aluminium heatsinks, integrated with forced convection, is performed in order to measure their thermal capacities. Comparison of the heat performance of a typical aluminium heatsink, which was the best among the all aluminium heat sinks and proposed copper heatsink under identical experimental conditions, is performed. Also in some numerical efforts, optimizing and predicting of the thermal characterization of the proposed heatsink with inclined free fins is developed. The model is scaled up in the fluent environment to predict its application in the cooling of larger heat generated electronic devices. Impingement air-cooling mode of force-convection is adopted for heat dissipation from high power electronic devices in associated with the proposed inclined fin model. Components of airflow velocity in the hollow spaces of the heatsink are discussed. Pressure drop and other thermal variables are analyzed analytical and by CFD code. Another mechanical manufactured copper heat sink is investigated. A new design of the base and fins is optimized. A three-dimensional finite volume method is developed to determine the performance of the proposed heatsink. Thermal and hydraulic characterization of the heat sink under air-forced convection cooling condition is studied. The flow behavior around the fins and some other parts of the heat sink is analyzed by utilizing CFD code. The hydraulic parameters including velocity profiles, distribution of static pressure, dynamic pressure, boundary layer and fluid temperature between the fins and in the passageway at the middle of the heat sink are analyzed and presented schematically. Furthermore the thermal characteristic of the proposed heatsink is studied by contouring the three dimensional temperature distributions through the fins and temperature of the heat source by CFD code.

A novel hybrid cooling scheme is proposed to remove non-uniform heat flux in real time from the microprocessor. It consists of a liquid cooled microchannel heat sink to remove the lower background heat flux and superlattice coolers to dissipate the high heat flux present at the hotspots. Superlattice coolers (SLC) are solid-state devices, which work on thermoelectric effect, and provide localized cooling for hotspots. SLCs offer some unique advantage over conventional cooling solutions. They are CMOS compatible and can be easily fabricated in any shape or size. They are more reliable as they don't contain any moving parts. They can remove high heat flux from localized regions and provide faster time response. Experimental devices are fabricated to characterize the steady-state, as well as transient performance, of the hybrid cooling scheme. Performance of the hybrid cooling scheme has been examined under various operating conditions. Effects of various geometric parameters have also been thoroughly studied. Heat flux in excess of 300 W/cm² has been successfully dissipated from localized hotspots. Maximum cooling at the hotspot is observed to be more than 6 K. Parasitic heat transfer to the superlattice cooler drastically affects its performance. Thermal resistance between ground electrode and heat sink, as well as thermal resistance between ground electrode and superlattice cooler, affect the parasitic heat transfer from to the superlattice cooler. Two different test devices are fabricated specifically to examine the effect of both thermal resistances. An electro-thermal model is developed to study the thermal coupling between two superlattice coolers. Thermal coupling significantly affects the performance of an array of superlattice coolers. Several operating parameters (activation current, location of ground electrode, choice of working fluid) affect thermal coupling between superlattice coolers, which has been computationally as well as experimentally studied. Transient response of the superlattice cooler has also been examined through experiments and computational modeling. Response time of the superlattice cooler has been reported to be less than 35 µs.

In this study, an entropy generation minimization procedure is employed to optimize the overall performance (thermal and hydrodynamic) of isolated fin geometries and pin-fin heat sinks. This allows the combined effects of thermal resistance and pressure drop to be assessed simultaneously as the heat sink interacts with the surrounding flow field. New general expressions for the entropy generation rate are developed using mass, energy, and entropy balances over an appropriate control volume. The formulation for the dimensionless entropy generation rate is obtained in terms of fin geometry, longitudinal and transverse pitches, pin-fin aspect ratio, thermal conductivity, arrangement of pin-fins, Reynolds and Prandtl numbers. It is shown that the entropy generation rate depends on two main performance parameters, i. e. , thermal resistance and the pressure drop, which in turn depend on the average heat transfer and friction coefficients. These coefficients can be taken from fluid flow and heat transfer models. An extensive literature survey reveals that no comprehensive analytical model for any one of them exists that can be used for a wide range of Reynolds number, Prandtl number, longitudinal and transverse pitches, and thermal conductivity. This study is one of the first attempts to develop analytical models for the fluid flow and heat transfer from single pins (circular and elliptical) with and without blockage as well as pin-fin arrays (in-line and staggered). These models can be used for the entire laminar flow range, longitudinal and transverse pitches, any material (from plastic composites to copper), and any fluid having Prandtl numbers (&ge;0. 71). In developing these models, it is assumed that the flow is steady, laminar, and fully developed. Furthermore, the heat sink is fully shrouded and the thermophysical properties are taken to be temperature independent. Using an energy balance over the same control volume, the average heat transfer coefficient for the heat sink is also developed, which is a function of the heat sink material, fluid properties, fin geometry, pin-fin arrangement, and longitudinal and transverse pitches. The hydrodynamic and thermal analyses of both in-line and staggered pin-fin heat sinks are performed using parametric variation of each design variable including pin diameter, pin height, approach velocity, number of pin-fins, and thermal conductivity of the material. The present analytical results for single pins (circular and elliptical) and pin-fin-arrays are in good agreement with the existing experimental/numerical data obtained by other investigators. It is shown that the present models of heat transfer and pressure drop can be applied for a wide range of Reynolds and Prandtl numbers, longitudinal and transverse pitches, aspect ratios, and thermal conductivity. Furthermore, selected numerical simulations for a single circular cylinder and in-line pin-fin heat sink are also carried out to validate the present analytical models. Results of present numerical simulations are also found to be in good agreement.