No description available.

No description available.

The first Molten Salt Reactor (MSR) was designed and tested at Oak Ridge National Laboratory (ORNL) in the 1960's, but recent technological advancements now allow for new components, such as heat exchangers, to be created for the next generation of MSR's and molten salt-cooled reactors. The primary (fuel salt-to-secondary salt) heat exchanger (PHX) design is shown here to make dramatic improvements over traditional shell-and-tube heat exchangers when changed to a compact heat exchanger design. While this paper focuses on the application of compact heat exchangers on a Molten Salt Reactor, many of the analyses and results are similarly applicable to other fluid-to-fluid heat xchangers. The heat exchanger design in this study seeks to find a middle-ground between shell- and-tube designs and new ultra-efficient, ultra-compact designs. Complex channel geometries and microscale dimensions in modern compact heat exchangers do not allow routine maintenance to be performed by standard procedures, so extended surfaces will be omitted and hydraulic diameters will be kept in the minichannel regime (minimum channel dimension between 200 μm and 3 mm) to allow for high-frequency eddy current inspection methods to be developed. High aspect ratio rectangular channel cross-sections are used. Various plant layouts of smaller heat exchanger banks in a "modular" design are introduced. FLUENT was used within ANSYS Workbench to find optimized heat transfer and hydrodynamic performance. With similar boundary conditions to ORNL's Molten Salt Breeder Reactor's shell-and-tube design, the compact heat exchanger interest in this thesis will lessen volume requirements, lower fuel salt volume, and decrease material usage. / Master of Science

There are several different types of compact heat exchangers used in applications where small size and weight are required. One particular type of compact heat exchanger, the louvered fin heat exchanger, has been used heavily in the automotive and air conditioning industries. Over the last several decades, the majority of the work towards improving louvered fin exchanger efficiency has focused on designing more efficient fins by optimizing fin parameters like louver angle, fin pitch, louver pitch, and louver length. At this point in time, many improvements to standard louver geometry have been made, so other surfaces and methods of enhancing exchanger performance need to be studied if any significant future efficiency gains are to be expected. This thesis presents a detailed experimental study that has two major foci relative to the performance of the louvered fin compact heat exchanger. The first is to determine the effect of louver length on pressure drop and tube wall heat transfer, which is the primary heat transfer surface in the heat exchanger. The second is to augment tube wall heat transfer with the use of delta winglets placed on the fins near the tube wall. These studies were completed on a 20X scale model of a louvered fin exchanger with a fin pitch to louver pitch ratio of 0.76 and a louver angle of 27°, over a Reynolds number range based on louver pitch of 230 < ReLp < 1016. The three louver lengths evaluated were 100%, 82%, and 70% of the fin height and delta winglet experiments were performed for louver length to fin pitch ratios of 100% and 70%. Heat transfer results for the louver length tests show that decreasing louver length leads to increases in tube wall heat transfer of 0% to 50% depending on Reynolds number. Also, delta winglets placed on the fins near the tube wall have been shown to produce average tube wall heat transfer augmentations of up to 52%. / Master of Science

Compact heat exchangers are widely used by the automotive industry in systems that cool engine components. Louvered fin heat exchangers are used over their continuous fin counterparts because of the significant advantages they provide in heat transfer efficiency, while only causing small increases in overall pressure losses. With the recent emphasis that has been placed on reducing fuel consumption, decreasing the size of the compact heat exchanger has become an important concern. With reduction in size comes not only weight savings, but also a decrease in frontal area in a vehicle that must be dedicated to the heat exchanger, allowing for more aerodynamic vehicle designs. Air-side resistance on the tube wall and louvered fin surfaces comprises over 85% of total resistance to heat transfer in louvered fin heat exchangers. The tube wall surface is considered the primary surface for heat transfer, where the temperature between the working fluid and convecting air is at a maximum. Recent studies have shown that implementing delta winglets on louvered fins along the tube wall is an effective method of augmenting tube wall heat transfer. In this thesis, the effect of delta winglets is investigated in both two- and three-dimensional louvered fin arrays. For both geometries, winglets are simulated in a manufacturable configuration, where piercings in the louvered fins that would result from the winglet manufacturing process are modeled. Using the two-dimensional geometry to model tube wall heat transfer was shown not to accurately predict heat transfer coefficients. In a two-dimensional geometry, winglets were found not to be an effective means for augmenting tube wall heat transfer and caused only 8% augmentation. Using the three-dimensional geometry, winglets with simulated piercings were observed to cause up to 24% tube wall heat transfer augmentation, with a corresponding increase in pressure losses of only 10%. / Master of Science

This PhD work was motivated by the CEA R&D program to provide solid technological basis for the use of Brayton power conversion system in Sodium-cooled Fast nuclear Reactors (SFRs). Multi-channel compact heat exchangers are necessary for the present application because of the low heat transfer capacity of the gas foreseen. In ASTRID project, a minimum size of Na channels section is required to avoid the plugging risk. However, this induces very low pressure losses in the bundle. Considering an additional inlet flow condition, a real risk of bad flow distribution remains. As a result, the thermal performance and thermal loading of the heat exchanger degrades due to it. The main goal of this work was to overcome the flow maldistribution problem by means of an innovative design of sodium distribution system (PATENT FR1657543), the development of a numerical strategy and the construction of an experimental database to validate all theoretical studies. The innovative sodium distribution system consists on an inlet header which tries to guide the evolution of the impinging jet flow while a system of bifurcating pre-distribution channels increases pressure drops in the bundle. Lateral communications between pre-distribution channels are introduced to further homogenize the flow. Two experimental facilities have been conceived to study the flow behavior in bifurcating channels and in the inlet header, respectively. At the same time, their effect on the flow distribution between channels is evaluated. The acquired PIV aerodynamic database allows to validate the numerical models and to prove the design basis for the proposed distribution system. Once having validated the CFD turbulence models and the strategy to study the flow maldistribution in the SGHE module, a decisive and trustworthy optimization of each component of the sodium distribution system has been performed. Finally, an optimal configuration has been proposed for the actual phase of ASTRID project.

No description available.

Cette thèse de doctorat a pour objet l'étude d'échangeurs géothermiques hélicoïdaux implantés dans le proche sous-sol. Ces objets font partie de la famille des échangeurs géothermiques compacts, et sont une alternative aux sondes verticales et aux nappes horizontales plus couramment utilisées pour chauffer et rafraîchir des bâtiments (géothermie sèche, très basse enthalpie). L'étude à la fois théorique et expérimentale de ces échangeurs mène à différents modèles, numériques comme analytiques, permettant de prévoir le comportement en température du sous-sol comme du fluide caloporteur. Des résultats de simulation intégrant le couplage aux bâtiments sont présentés, et des outils logiciels ont été créés pour aider au dimensionnement d'installations. De surcroît, un "test de réponse thermique" adapté à ces échangeurs a été développé, afin de permettre d'estimer les propriétés thermiques moyennes du sous-sol et la résistance thermique liée à l'installation de l'échangeur. Enfin, des conseils pratiques sont donnés, dont le but est d'obtenir les températures d'utilisation les mieux adaptées aux pompes à chaleur, et en conséquence d'améliorer les coefficients de performance du système. / This thesis consists in a study of geothermal helical heat exchangers buried in the close underground. These objects belong to the compact exchangers kind and may be used instead of borehole heat exchanger or geothermal horizontal exchangers to cope with the needs in heating and cooling buildings with a heat pump. A theoretical and experimental study of these exchangers leads to numerical and analytical models which can be used to estimate the temperatures in the underground and in the exchanger over years. Results of simulations including a building are given, and softwares were developed to help with the sizing of installations. Moreover, a specific "thermal response test" for helical heat exchangers was developed, in order to estimate the mean thermal properties of the underground and the thermal resistance between the heat transfer fluid and the underground. At least, practical advices are given, which goal is to get more valuable temperatures at the heat pump and as a consequence better coefficients of performance of the geothermal system.

Domestic solar hot water units, if properly designed, are capable of providing all hot water needs in an environmentally friendly and cost-effective way. Despite 50 years of development, commercial technology has not yet achieved substantial market penetration compared to mainstream electric and gas options. Therefore, alternate designs are warranted if they can offer similar or greater performance for a comparable cost to conventional units. This study proved that such alternatives are possible by designing and testing two novel solar hot water systems (SHWS). The first system used compound parabolic collector (CPC) panels to concentrate solar energy and produce steam. The steam moved from a rooftop downward into a heat exchange pipe within a ground level water tank, heating the water, condensing and falling into a receptacle. The operation was entirely passive, since the condensate was pulled up due to the partial vacuum that occurred after system cooling. Efficiencies of up to 40% were obtained. The second system used an air heater panel. Air was circulated in open and closed loop configuration (air recycling) by means of a fan/blower motor and was forced across a compact heat exchanger coupled to a water tank. This produced a natural thermosiphon flow heating the water. Air recycling mode provided higher system efficiencies: 34% vs. 27%. The concurrent development of an analytical model that reasonably predicted heat transfer dynamics of these systems allowed 1) performance optimisation for specific input/starting operating conditions and 2) virtual design improvements. The merit of this model lay in its acceptable accuracy in spite of its simplicity. By optimising for operating conditions and parameter design, both systems are capable of providing over 30 MJ of useful domestic hot water on clear days, which equates roughly to an increase of 35°C in a 200 L water tank. This will satisfy, on average, daily hot water requirements for a 4-person household, particularly in low-latitude regions (eg. Queensland). Preliminary costing for these systems puts them on par with conventional units, with the passive, remotely coupled, low maintenance, CPC SHWS comparable to higher end models. The air heater SHWS, by contrast, was much more economical and easier to build and handle, but at the trade-off cost of 1) the need for an active system, 2) increased maintenance and running costs and 3) the requirement for a temperature control mechanism that would protect the panel body by dumping hot air trapped inside if stagnation were to occur.

Ce travail de thèse s’inscrit dans le cadre du programme R&D du CEA en support au système de conversion d’énergie à gaz du prototype industriel de Réacteur à Neutrons Rapides refroidi au Sodium (RNR-Na). Cette technologie représente une alternative aux cycles Rankine conventionnels à eau/vapeur, ayant pour avantage principal l’élimination du scenario accidentel de réaction sodium-eau. Cependant, la faible capacité de transfert de chaleur du gaz nécessite une technologie d’échangeurs compacts à plaques avec un nombre élevé de canaux à alimenter. Coté sodium, une section minimale de passage est nécessaire pour éviter le risque de bouchage par impureté. Cela induit de très faibles pertes de pression dans le faisceau qui, couplées à une condition de vitesse élevée à l’entrée, génèrent un risque réel de mauvaise distribution du débit. Les performances d’échange thermique et la tenue mécanique du composant sont alors dégradées. L’objectif principal de ce travail de thèse a été de résoudre ce problème de mauvaise distribution, en s’appuyant sur une conception innovante (BREVET FR16 57543), sur une stratégie de calcul numérique et l’établissement d’une base de données expérimentale pour la validation des travaux théoriques. Le nouveau système de distribution sodium se compose d’un collecteur d'entrée dont le design permet de guider la trajectoire du jet et d’un système de bifurcation de canaux qui augmente les pertes de pression dans le faisceau. De plus, des communications latérales entre les canaux sodium aident à homogénéiser davantage le flux. Deux installations expérimentales ont été conçues pour caractériser l'écoulement dans les canaux de bifurcation et dans le collecteur d'entrée. La conception des maquettes a permis de quantifier leur effet sur la distribution du flux entre les canaux. La base de données aérodynamiques PIV acquises a permis de valider les modèles numériques et de prouver l’efficacité du système de distribution proposé. Après avoir validé les modèles de turbulence CFD et la stratégie d'étude de la distribution dans le module SGHE, une optimisation de chaque composant du système de distribution de sodium a été réalisée. Le travail de cette thèse s’achève par la description de la conception optimale retenue pour la phase actuelle du projet ASTRID. / This PhD work was motivated by the CEA R&D program to provide solid technological basis for the use of Brayton power conversion system in Sodium-cooled Fast nuclear Reactors (SFRs). Multi-channel compact heat exchangers are necessary for the present application because of the low heat transfer capacity of the gas foreseen. In ASTRID project, a minimum size of Na channels section is required to avoid the plugging risk. However, this induces very low pressure losses in the bundle. Considering an additional inlet flow condition, a real risk of bad flow distribution remains. As a result, the thermal performance and thermal loading of the heat exchanger degrades due to it. The main goal of this work was to overcome the flow maldistribution problem by means of an innovative design of sodium distribution system (PATENT FR1657543), the development of a numerical strategy and the construction of an experimental database to validate all theoretical studies. The innovative sodium distribution system consists on an inlet header which tries to guide the evolution of the impinging jet flow while a system of bifurcating pre-distribution channels increases pressure drops in the bundle. Lateral communications between pre-distribution channels are introduced to further homogenize the flow. Two experimental facilities have been conceived to study the flow behavior in bifurcating channels and in the inlet header, respectively. At the same time, their effect on the flow distribution between channels is evaluated. The acquired PIV aerodynamic database allows to validate the numerical models and to prove the design basis for the proposed distribution system. Once having validated the CFD turbulence models and the strategy to study the flow maldistribution in the SGHE module, a decisive and trustworthy optimization of each component of the sodium distribution system has been performed. Finally, an optimal configuration has been proposed for the actual phase of ASTRID project.