Thermal energy storage (TES) in a concentrating solar power (CSP) plant allows for continuous operation even during times when solar radiation is not available, thus providing a reliable output to the grid. Energy can be stored either as sensible heat or latent heat, of which latent heat storage is advantageous due to its high volumetric energy density and the high Rankine cycle efficiency owing to the isothermal operation of latent thermal energy storage (LTES) system. Storing heat in the form of latent heat of fusion of a phase change material (PCM), in addition to sensible heat, significantly increases the energy density, thus potentially reducing the storage size and cost. However, a major technical barrier to the use of latent thermal energy of PCM is the high thermal resistance to energy transfer due to the intrinsically low thermal conductivity of PCMs, which is a particularly acute constraint during the energy discharge. Secondly, for integration of TES in CSP plants, it is imperative that the cyclic exergetic efficiency be high, among other requirements, to ensure that the energy extracted from the system is at the maximum possible temperature to achieve higher cycle conversion efficiency in the power block. The first objective is addressed through computational modeling and simulation to quantify the effectiveness of two different approaches to reduce the thermal resistance of PCM in a LTES, viz. (a) developing innovative, inexpensive and passive heat transfer devices that efficiently transfer large amount of energy between the PCM and heat transfer fluid (HTF) and (b) increase the heat transfer area of interaction between the HTF and PCM by incorporating the PCM mixture in small capsules using suitable encapsulation techniques.   The second portion of the research focuses on numerical modeling of large scale latent thermal storage systems integrated to a CSP plant with the aforementioned enhancement techniques and cascaded with more than one PCM to maximize the exergetic efficiency. Based on systematic parametric analysis on the various performance metrics of the two types of LTES, feasible operating regimes and design parameters are identified to meet the U.S. Department of Energy SunShot Initiative requirements including storage cost < $15/kWht and exergetic efficiency > 95%, for a minimum storage capacity of 14 h, in order to reduce subsidy-free levelized cost of electricity (LCE) of CSP plants from 21¢/kWh (2010 baseline) to 6¢/kWh, to be on par with the LCE associated with fossil fuel plants. / Ph. D.

No description available.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: A reactor cavity cooling system (RCCS) is used in the PBMR to protect the concrete citadel surrounding the reactor from direct nuclear radiation impingement and heat. The speci ed maximum operating temperature of the concrete structure is 65 ±C for normal operating conditions and 125 ±C for emergency shut-down conditions. A conceptual design of an entirely passive RCCS suitable for the PBMR was done by using closed loop thermosyphon heat pipes (CLTHPs) to remove heat from a radiation heat shield over a horizontal distance to an annular cooling dam placed around the PBMR. The radiation shield is placed in the air space between the Reactor Pressure Vessel (RPV) and the concrete citadel, 180 mm from the concrete citadel. A theoretical heat transfer model of the RCCS was created. The theoretical model was used to develop a computer program to simulate the transient RCCS response during normal reactor operation, when the RCCS must remove the excess generated heat from the reactor cavity and during emergency shut-down conditions, when the RCCS must remove the decay heat from the reactor cavity. The main purpose of the theoretical model is to predict the surface temperature of the concrete citadel for di erent heat generation modes in the reactor core and ambient conditions. The theoretical model assumes a 1D geometry of the RCCS. Heat transfer by both radiation and convection from the RPV to the radiation heat shield (HS) is calculated. The heat shield is modelled as a n. The n e ciency was determined with the experimental work. Conduction through the n is considered in the horizontal direction only. The concrete structure surface is heated by radiation from the outer surface of the heat shield as well as by convection heat transfer from the air between the heat shield and the concrete structure surface. The modelling of the natural convection closed loop thermosyphon heat pipes in the RCCS is done by using the Boussinesq approximation and the homogeneous ow model. An experiment was built to verify the theoretical model. The experiment is a full scale model of the PBMR in the horizontal, or main heat transfer, direction, but is only a 2 m high section. The experiments showed that the convection heat transfer between the RPV and the HS cannot be modelled with simple natural convection theory. A Nusselt number correlation developed especially for natural convection in enclosed rectangles found in literature was used to model the convection heat transfer. The Nusselt number was approximately 3 times higher than that which classic convection theory suggested. An optimisation procedure was developed where 121 di erent combinations of n sizes and heat pipe sizes could be used to construct a RCCS once a cooling dam size was chosen. The purpose of the optimisation was to nd the RCCS with the lowest total mass. A cooling dam with a diameter of 50 m was chosen. The optimal RCCS radiation heat shield that operates with the working uid only in single phase has 243 closed loop thermosyphon heat pipes constructed from 62.72 mm ID pipes and 25 mm wide atbar ns. The total mass of the single phase RCCS is 225 tons. The maximum concrete structure temperature is 62.5 ±C under normal operating conditions, 65.8 ±C during a PLOFC emergency shut-down condition and 80.9 ±C during a DLOFC emergency shut-down condition. In the case where one CLTHP fails and the adjacent two must compensate for the loss of cooling capacity, the maximum concrete structure temperature for a DLOFC emergency shut-down will be 87.4 ±C. This is 37.6 ±C below the speci ed maximum temperature of 125 ±C. The RCCS design is further improved when boiling of the working uid is induced in the CLTHP. The optimal RCCS radiation heat shield that operates with the working uid in a liquid-vapour mixture, or two phase ow, has 338 closed loop thermosyphon heat pipes constructed from 38.1 mm ID pipes and 20 mm wide atbar ns. The total mass of the two phase RCCS is 198 tons, 27 tons less than the single phase RCCS. The maximum concrete structure temperature is 60 ±C under normal operating conditions, 2.5 ±C below that of the single phase RCCS. During a PLOFC emergency shut-down condition, the maximum concrete structure temperature is 62.3 ±C, 3.5 ±C below that of the single phase RCCS and still below the normal operating temperature of the single phase RCCS. By inducing two phase ow in the CLTHP, the maximum temperature of the working uid is xed equal to the saturation temperature of the working uid at the vacuum pressure. This property of water is used to limit the concrete structure temperature. This e ect is seen in the transient response of the RCCS where the concrete structure temperature increases until boiling of the working uid starts and then the concrete structure temperature becomes constant irrespective of the heat load on the RCCS. An increased heat load increases the quality of the working uid liquid-vapour mixture. Working uid qualities approaching unity causes numerical instabilities in the theoretical model. The theoretical model cannot capture the heat transfer to a control volume with a density lower than approximately 20 kg/m3. This limits the extent to which the two phase RCCS can be optimised. Recommendations are made relating to future work on how to improve the theoretical model in particular the convection modelling in the reactor cavities as well as the two phase ow of the working uid. Further recommendations are made on how to improve the basic design of the heat shield as well as the cooling section of the CLTHPs. / AFRIKAANSE OPSOMMING: 'n Reaktor lug spasie verkoelingstelsel (RLSVS) word in die PBMR gebruik om die beton wat die reaktor omring te beskerm teen direkte stralingskade en hitte. Die gespesi seerde maksimum temperatuur van die beton is 65 ±C onder normale bedryfstoestande en 125 ±C gedurende die noodtoestand afskakeling van die reaktor. 'n Konseptuele ontwerp van 'n geheel en al passiewe RLSVS geskik vir die PBMR is gedoen deur gebruik te maak van geslote lus termo-sifon (GLTSe) om hitte van die stralingskerm te verwyder oor a horisontale afstand na 'n ringvormige verkoelingsdam wat rondom die reaktor geposisioneer is. Die stralingskerm word in die lug spasie tussen die reaktor drukvat (RDV) en die beton geplaas, 180 mm vanaf die beton. 'n Teoretiese hitteoordrag model van die RLSVS was geskep. Die teoretiese model was gebruik vir die ontwikkeling van 'n rekenaar program wat die transiënte gedrag van die RLSVS sal simuleer gedurende normale bedryfstoestande, waar die oorskot gegenereerde hitte verwyder moet word vanuit die reaktor lug spasie, asook gedurende noodtoestand afskakeling van die reaktor, waar die afnemingshitte verwyder moet word. Die primêre doel van die teoretiese model is om the oppervlak temperatuur van die beton te voorspel onder verskillende bedryfstoestande asook verskillende omgewingstoestande. Die teoretiese model aanvaar 'n 1D geometrie van die RLSVS. Hitte oordrag d.m.v. straling asook konveksie vanaf die RDV na die stralingskerm word bereken. The stralingskerm word gemodelleer as 'n vin. Die vin doeltre endheid was bepaal met die eksperimente wat gedoen was. Hitte geleiding in die vin was slegs bereken in die horisontale rigting. Die beton word verhit deur straling vanaf die agterkant van die stralingskerm asook deur konveksie vanaf die lug tussen die stralingskerm en die beton. The modellering van die natuurlike konveksie GLTS hitte pype word gedoen deur om gebruik te maak van die Boussinesq benadering en die homogene vloei model. 'n Eksperiment was vervaardig om the teoretiese model te veri eer. Die eksperiment is 'n volskaal model van die PBMR in die horisontale, of hoof hitteoordrag, rigting, maar is net 'n 2 m hoë snit. Die eksperimente het gewys dat die konveksie hitte oordrag tussen die RDV en die stralingskerm nie met gewone konveksie teorie gemodelleer kan word nie. 'n Nusselt getal uitdrukking wat spesi ek ontwikkel is vir natuurlike konveksie in geslote, reghoekige luggapings wat in die literatuur gevind was, was gebruik om die konveksie hitteoordrag te modelleer. Die Nusselt getal was ongeveer 3 maal groter as wat klassieke konveksie teorie voorspel het. 'n Optimeringsprosedure was ontwikkel waar 121 verskillende kombinasies van vin breedtes en pyp groottes wat gebruik kan word om 'n RLSVS te vervaardig nadat 'n toepaslike verkoelingsdam diameter gekies is. Die doel van die optimering was om die RLSVS te ontwerp wat die laagste totale massa het. 'n Verkoelingsdam diameter van 50 m was gekies. Die optimale RLSVS stralingskerm, waarvan die vloeier slegs in die vloeistof fase bly, bestaan uit 243 GLTSe wat van 62.72 mm binne diameter pype vervaardig is met 25 mm breë vinne. The totale massa van die enkel fase RLSVS is 225 ton. Die maksimum beton temperatuur is 62.5 ±C vir normale bedryfstoestande, 65.8 ±C vir 'n PLOFC noodtoestand afskakeling en is 80.9 ±C vir 'n DLOFC noodtoestand afskakeling. In die geval waar een GLTS faal gedurende 'n DLOFC noodtoestand afskakeling en die twee naasgeleë GLTSe moet kompenseer vir die vermindering in verkoelings kapasiteit, is die maksimum beton temperatuur 87.4 ±C. Dit is 37.6 ±C laer as die gespesi seerde maksimum temperatuur van 125 ±C. Die RLSVS ontwerp kan verder verbeter word wanneer die vloeier in die GLTSe kook. Die optimale RLSVS stralingskerm met die vloeier wat kook, of in twee fase vloei is, bestaan uit 338 GLTSe wat van 38.1 mm binne diameter pype vervaardig is met 20 mm breë vinne. The totale massa van die twee fase vloei RLSVS is 198 ton, 27 ton ligter as die enkel fase RLSVS. Die maksimum beton temperatuur is 60 ±C vir normale bedryfstoestande, 2.5 ±C laer as die enkel fase RLSVS. Gedurende 'n PLOFC noodtoestand afskakeling is die maksimum beton temperatuur 62.3 ±C, 3.5 ±C laer as die enkel fase RLSVS en nogtans onder die maksimum beton temperatuur van die enkel fase RLSVS vir normale bedryfstoestande. Deur om koking te veroorsaak in die GLTS word die maksimum temperatuur van die vloeier vasgepen gelyk aan die versadigings temperatuur van die vloeier by die vakuüm druk. Hierdie einskap van water word gebruik om 'n limiet te sit op die maksimum temperatuur van die beton. Hierdie e ek kan gesien word in die transiënte gedrag van die RLSVS waar die beton temperatuur styg tot en met koking plaasvind en dan konstant raak ongeag van die hitte belasting op die RLSVS. 'n Toename in die hitte belasting veroorsaak net 'n toename in die kwaliteit van die vloeistof-gas mengsel. Mengsel kwaliteite van 1 nader veroorsaak numeriese onstabiliteite in die teoretiese model. The teoretiese model kan nie die hitteoordrag beskryf na 'n kontrole volume wat 'n digtheid het laer as ongeveer 20 kg/m3. Hierdie plaas 'n limiet op die optimering van die twee fase RLSVS. Aanbevelings was gemaak met betrekking tot toekomstige werk aangaande die verbetering van die teoretiese model met spesi eke klem op die modellering van konveksie in die reaktor asook die modellering van twee fase vloei. Verdere aanbevelings was gemaak aangaande die verbetering van die stralingskerm ontwerp asook die ontwerp van die verkoeling van die GLTSe.

The cold chain industry has a need for a standalone, electricity independent cooling unit that is used for both storage of warehouse product and on deliveries [1]. Mixed temperature fresh and frozen food deliveries are problematic without the distributor having specialized duel compartment refrigerated trucks [2]. These trucks permanently reduce the available capacity for payload delivery [2]. It would be valuable to the cold chain industry to have a passive, independent, storage unit that can be moved using a forklift and placed anywhere within a reefer or warehouse [1]. This versatile unit is a simple mechanical system, but presents a complicated thermal problem. One of the design challenges is to thermally isolate the load from the environment and to maintain thermal conditions for a specified length of time. A proposed storage system uses heat pipes to connect the cargo compartment to a heat sink containing solid CO2. Heat pipes are a simple, passive, and quiet way to transfer heat. Heat pipe design and theory is an active area of research with numerous papers in the literature; however, there is less reported about the actual process of manufacturing. This thesis investigates a new potential application of heat pipes, with a focus on the manufacturing process and experimental performance. A total of four heat pipes and two thermosyphons are created using acetone and methanol as the working fluids, and copper and aluminum as the heat pipe housing. Performance is compared to an insulated copper tube with the same outer dimensions, where the primary performance metric is steady-state thermal resistance. In addition, transient performance is quantified as well as the temperature distribution along the outer in the evaporator, adiabatic and condenser regions. Results show that the prototypes made out of copper reached steady-state faster than the aluminum pipes, while also having a smaller temperature differential between the evaporator and condenser. Methanol and acetone have similar performance over the temperature ranges of 198 K to 358 K. The best performing prototype is a copper thermosyphon containing methanol which achieves an effective thermal resistance of 2.0 K/W with an applied load of 40.7 W, when the condenser is cooled with dry ice in acetone. When cooled with ice water the copper thermosyphon achieves an effective thermal resistance of 0.5 K/W with a load of 40.7 W. / Graduate / 0548 / jstrain@uvic.ca

This manuscript has been realized in the frame of SELENE experiment research activities. SELENE is the ac-ronym of Self-rewetting fluids for ENErgy management and consists of a space project aiming to investigate heat and mass transfer phenomena in mono-groove configuration with self-rewetting fluids (SRFs). Self-rewetting fluids are mixture showing an anomalous trend of surface tension with temperature, an inversion of the surface tension slope after certain temperature. As consequence, when the minimum in surface ten-sion is crossed, surface tension gradient at the meniscus interface pulls the liquid towards the warmest region, preventing hot spots. This mechanism is completely spontaneous and has an interesting potential when applied to heat transfer applications as heat pipes (HPs). In HPs heat is removed by the liquid at the warmest region (the evaporator) and transported at the coldest zone (the condenser) by phase change; here, heat is removed by the pipe and dissipated outside through a radiator. To operate correctly, liquid is supplied to the evaporator by capillarity and the liquid vapour is allowed to flow back to condenser from a dedicated pipe region where liquid is not allowed. Vapour condensation releases at the condenser the heat to be dissipated. When SRFs are replacing working fluid in HP applications and temperatures are higher than the characteristic minimum in surface tension, capillary force is assisted by inverse Marangoni flow at the vapour-liquid interface.Since heat pipe performances are related to liquid supplied at the evaporator, in order to compare SRFs and not SRFs working fluids, it is needed to split the contribution of Marangoni and capillary force in the liquid flow. Marangoni effect is related to surface tension gradient that, in a mixture as SRF, is dependent on temperature and local composition at the liquid interface. For all these reasons, SELENE is designed to be the link between scientific research on HPs and heat transfer applications using SRFs. SELENE consists of a mono-groove with trapezoidal section that can be considered as a “clump” of an Inner Grooved Heat Pipe (IGHP) and, in order to split capillary and Marangoni contribution, it is integrated dedicated tools providing the required data in terms of concentration and liquid meniscus shape. Experimental data are used to build a simplified thermo-soluto-fluido dynamic model describing the thermo-mechanic mechanisms between the liquid bulk and the vapour flow. In the manuscript here presented it has been carried on a technology development of the required diag-nostics for the SELENE space project. The diagnostics have been designed to work in microgravity condi-tions even if they are tested on ground. As concentration diagnostic, in the text are proposed several tech-niques and more interest is spent on the adaptation of I-VED (In vivo Embolic Detection) technology meas-uring fluid AC impedance to retrieve composition information; the technology is not yet mature to be inte-grated in SELENE but it presents interesting features to be investigated in microgravity conditions. As me-niscus reconstruction technique it is proposed a new and innovative technology developed in the frame of the presented thesis and it consists of a non-intrusive optical technique aiming to retrieve liquid meniscus shape (and so curvature) from a single visualization window mounted at the top of the SELENE breadboard.An analytical approach aiming to retrieve a simplified mathematical model of the transfer mechanisms is also provided in the text. The analytical analysis clearly shows the relations between the experimental measured data and the velocity profiles in the liquid and vapour regions. In addition, since in SELENE exper-iment the heat conduction across the groove itself is not negligible, in the text it is provided a semi-empirical thermal model based on the Multi Lumped Model (MLM) theory and able to retrieve local heat exchanged information along the pipe length. The model is used to compare experiments with different working fluids at different operational regimes. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished

A aplicação de nanofluidos em tubos de calor em geral apresenta resultados experimentais satisfatórios em estudos buscando obter uma redução na resistência térmica do tubo de calor. No entanto, os estudos computacionais existentes associando tubos de calor e nanofluidos apresentam resultados conflitantes e carecem de uma discussão mais aprofundada a respeito da validade dos modelos utilizados para a representação computacional do comportamento de um nanofluido em tubo de calor, especialmente utilizando materiais e fluidos não convencionais como nanotubos de carbono ou etilenoglicol. Assim, o presente estudo busca avaliar a exatidão e a precisão obtida em uma série de simulações computacionais utilizando diferentes equações disponíveis na literatura para a modelagem de um nanofluido em um tubo de calor por meio da comparação com dados experimentais da literatura. Esta modelagem utiliza o método dos volumes finitos e permite determinar o efeito da variação dos modelos de propriedades e da concentração volumétrica de um nanofluido nos campos de temperaturas e nas resistências térmicas resultantes das simulações. Os resultados obtidos apresentam concordância com o comportamento esperado do ponto de vista qualitativo, mas falham em representar quantitativamente o comportamento da seção do evaporador dos tubos de calor estudados, apresentando variações máximas entre 1,5% e 23,9% em relação às temperaturas medidas experimentalmente. Isso pode ser justificado pelo fato de que a modelagem do fenômeno de ebulição de um nanofluido é mais complexa do que a modelagem utilizada atualmente em simulações computacionais. Essa consideração possui suporte na literatura e cria possibilidades para pesquisas futuras. / Application of nanofluids in heat pipes usually presents satisfactory experimental results in studies seeking to reduce the thermal resistance of the heat pipe. However, the existing computational studies connecting heat pipes and nanofluids present conflicting results and lack a deeper discussion regarding the validity of the models currently used for the computational representation of the behavior of a nanofluid in a heat pipe, especially using unusual materials and fluids, like carbon nanotubes or ethylene glycol. Thus, the present study seek to analyze the accuracy and the precision obtained in a set of computational simulations using pre-established equations for the modeling of a nanofluid in a heat pipe by using a direct comparison with existing experimental data. This modeling uses the finite volume method and permits to determine the effect of the variation of the properties models and the volume fraction of a nanofluid in the resulting temperature fields and the thermal resistances of the simulations. The obtained results show agreement with the expected behavior qualitatively, but fail to represent the phenomenon quantitatively, presenting maximum variations between 1,5% and 23,9% comparing to the experimentally measured average temperatures. This is justified by the hypothesis that the ebullition phenomenon modeling is more complex than the modeling currently used for computational simulations. This hypothesis is supported by the literature and creates possibilities for future researches.

The intention of this study is to investigate the main source of unacceptable humidity levels at the U.S. Coast Guard Housing located in Bayamon, Puerto Rico. The aim of this research is to use a systematic approach to resolve the humidity and mold issues by testing the least expensive solutions first. This study involves the recording of indoor air quality conditions for six months as an analysis tool to investigate current air conditions and to document how physical changes to the air conditioning units will affect the resulting air conditions. This research will investigate and implement different approaches geared to solving the high humidity issues. Some of the most relevant changes that will be tested are the installation of heat pipe technology, the addition of fresh air to existing air conditioning units to create positive pressure, and the review of the space load design of currently installed air conditioning units to determine if the units were over-designed. In addition, this study will verify the relationship between energy-saving thermostats and high humidity, determine any connection between roof leaks and high humidity indoors, and determine the estimated cost to the Coast Guard to implement the recommended changes.

The Facility Manager of a laboratory environment continuously walks a fine line between safe and economical operation of that facility. The primary responsibility of the laboratory is to provide a safe environment for personnel while optimizing the space for experiment. Energy efficiency is not a necessary goal. Laboratories typically require HVAC systems utilizing 100% outside air to protect the occupants. Facilities demanding the basic design requirement of 100% outside air can result in annual energy costs 4 to 5 times greater than that of the typical office building requiring 20 CFM per person. With energy costs typically representing a substantial part of an organizations operating budget is it prudent for facility managers to seek opportunities to reduce these costs. The intent of this research is to show that participation of a knowledgeable Facility Manager, during the initial design phase of a laboratory facility, can result in a finished product capable of easily incorporating a variety of energy efficiency technologies. The scope of this research is limited to smaller chemical laboratories supported with less than 20,000 CFM of comfort air. When the Facility Manager actively participates in the design process for laboratory environments there is potential for increased HVAC energy efficiency. A substantial portion of this research has been conducted from the authors daily experience and responsibility for a small chemical laboratory. Additional data was collected using personal interviews among industry experts and fellow colleagues working in the Atlanta metropolitan area with significant laboratory experience. This research focused on the mechanical systems supporting laboratories as they represent the largest percentage in first costs, energy consumption, and offer the greatest opportunity for energy reduction. The results of this research are intended to provide guidance to Facility Managers to incorporate cost effective energy recovery systems in either new construction or at a future date. The results of this research project the impact of energy consumption in a small chemical laboratory from the hypothetical installation of a customized energy recovery system.

Capillary pumped loop (CPL) and loop heat pipe (LHP), are two-phase heat transport devices which rely on surface tension induced by a fine pore wick to drive a working fluid in a loop. These are based on a working principle similar to that of heat pipes -closed evaporation and condensation cycle being maintained by capillary pumping. CPLs and LHPs are gaining importance as a part of the thermal control system of modern high power spacecraft, electronic thermal management, cryogenics, etc. A mathematical model to simulate the thermo-hydraulic performance of CPLs and LHPs is developed to aid in the design of such a spacecraft thermal control system. In this study a unified mathematical model to estimate thermal and hydraulic performance of a CPL and an LHP -with a two-phase or a hard-filled reservoir is presented. The steady state model is based on conservation of energy and mass in the system. Heat exchanges between the loop and the surroundings and pressure drops in the loop are calculated. The constant conductance regime in a CPL or an LHP occurs when the reservoir is hard-filled. It also occurs in an LHP if the condenser is fully utilised. The heat leak across the wick becomes significant in a hard-filled LHP since the core is no longer saturated and hence the mass flow rate must be calculated using an energy balance on the outer surface of the wick. Theoretical studies indicate that the core of a hard-filled CPL and LHP is always sub-cooled. Hard-filled LHPs (with a bayonet) cannot be operated under all conditions. If the heat exchange between the compensation chamber (of an LHP with bayonet) and the ambient is small then such an LHP will not deprime if the hard-filling occurs before the condenser opens. Deprime due to hard-filling is not expected if it occurs after the condenser opens. A laboratory model is built to demonstrate the operation of these two devices and to correlate the theoretical predictions with the experimental observations. The CPL/LHP laboratory model is fabricated and designed so that different evaporator and reservoir designs can be integrated into the test-rig and tested with different working fluids. Experiments are conducted on a three-port CPL with a tubular axially grooved (TAG) evaporator. This CPL is operated with three different fluids -namely -Ammonia, Acetone and R134a. The CPL is operated for heat loads in the range of 75W to 400W with sink temperatures of -10◦C and 0◦C. The influence of reservoir temperature (35◦C and 43◦C) is also studied. The TAG evaporator is modified to operate in an LHP mode with R134a as the working fluid with heat loads in the range of 75W to 150W. This LHP does not exhibit typical “√” shaped operating characteristic due to large liquid inventory in the compensation chamber (CC). The R134a based LHP results suggest that large liquid inventory (in the CC) and absence of secondary wick significantly influence the thermal coupling between the core and the compensation chamber. Experiments are also conducted with a flat plate (FP) evaporator, in LHP operating mode, with Ammonia as the working fluid. This LHP can transport heat loads from 25W to 300W with a sink temperature at -15◦C. The experimental results indicate that ammonia is the best working fluid (moderate temperature regime) among all the working fluids tested, and that evaporation heat transfer coefficients in sintered Ni-wick are better. The correlation of the predicted temperatures on the transport lines and the saturation temperature (in LHPs) with the observations is good. Some of the salient conclusions from these experiments are that mass of charge can significantly influence the operating characteristics of a TAG LHP, even though the fluid in the CC is in two-phase condition. Theoretical predictions can be significantly affected when thermal and hydraulic development lengths in the condenser are comparable with the length of the sub-cooling section.

Capes / Neste trabalho, o processo de eletroerosão a frio (wire electrical discharge machining ou wire EDM) foi utilizado como método de fabricação alternativo para a confecção de microranhuras axiais em tubos de calor. Com isso, material foi retirado ao invés de ser adicionado ao invólucro do tubo de calor para a concepção da estrutura capilar, contribuindo para a redução de massa no dispositivo passivo de transferência de calor por mudança de fase. Uma modelagem baseada no projeto térmico e nos limites operacionais (limites capilar, de arrasto, viscoso, sônico e de ebulição) foi proposta para os tubos de calor com microranhuras axiais de geometria semicircular como estrutura capilar considerando diferentes definições disponíveis na literatura. Estes modelos, implementados no software EEStm (Engineering Equation Solver)tm, foram utilizados como ferramenta para o projeto dos tubos de calor ranhurados propostos. Os tubos de calor foram produzidos a partir de um tubo reto de cobre com um diâmetro externo de 9,45 mm, um diâmetro interno de 6,20 mm e um cumprimento total de 200 mm. O fluido de trabalho utilizado foi água deionizada e os tubos e calor foram carregados com uma razão de preenchimento de 60% do volume evaporador. O condensador foi resfriado por convecção forçada de ar, a seção adiabática foi isolada por uma fita de fibra de vidro e o evaporador foi aquecido utilizando um resistor elétrico em fita de liga de níquel-cromo e isolado do ambiente externo por um isolamento térmico aeronáutico. Os tubos de calor foram testados experimentalmente para as inclinações de operações iguais a 0º, 45º, 90º, 225º e 270º com relação ao plano horizontal, sob cargas térmicas compreendidas entre 5 W e 50 W. Os resultados experimentais do desempenho térmico dos tubos de calor mostraram que as microranhuras axiais fabricadas pelo processo de eletroerosão a frio como estrutura capilar funcionaram com sucesso em todos os casos estudados. Além disso, na maioria dos casos estudados, o tubo de calor com micriranhuras apresentou melhor desempenho térmico quando comparado com um tubo de calor contendo tela metálica como estrutura capilar. / In this master's dissertation an alternative fabrication method (wire electrical discharge machining, or wire EDM) was used to manufacture axial microgrooves in heat pipes. With this, material has been removed rather than being added to the heat pipe shell for the design of the capillary structure, contributing to mass reduction in the passive heat transfer device by phase change. A model based on thermal design and operational limits (capillary, trailing, viscous, sonic and boiling limits) was proposed for the heat pipes with axial microstrips of semicircular geometry as capillary structure considering different definitions available in the literature. These models, implemented in the EEStm (Engineering Equation Solver) software, were used as a tool for the design of the proposed grooved heat pipes. The heat pipes were produced from a straight copper tube with an outside diameter of 9.45 mm, an inner diameter of 6.20 mm and a total compliance of 200 mm. The working fluid used was deionized water and the tubes and heat were charged with a fill ratio of 60% of the evaporator volume. The condenser was cooled by forced convection of air, the adiabatic section was insulated by a fiberglass tape and the evaporator was heated using an electric resistor in nickel-chromium alloy tape and isolated from the external environment by an aeronautical thermal insulation. The heat pipes were experimentally tested for slopes of operations equal to 0º, 45º, 90º, 225º and 270º with respect to the horizontal plane, under thermal loads between 5 W and 50 W. The experimental results showed that the axial grooves manufactured by the Wire-EDM process worked satisfactorily in all analyzed cases. In most of the cases, the heat pipe with grooves showed a better performance when compared with the heat pipe with metallic mesh.