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[en] The investigation into a full-scale 27 m high, by 6 m wide, thermosyphon loop. The simulation model is based on a one-dimensional axially-symmetrical control volume approach, where the loop is divided into a series of discreet control volumes. The three conservation equations, namely, mass, momentum and energy, were applied to these control volumes and solved with an explicit numerical method. The flow is assumed to be quasi-static, implying that the mass-flow rate changes over time. However, at any instant in time the mass-flow rate is constant around the loop. The boussinesq approximation was invoked, and a reasonable correlation between the experimental and theoretical results was obtained. Experimental results are presented and the flow regimes of the working fluid inside the loop identified. The results indicate that a series of such thermosyphon loops can be used as a cavity cooling system and that the one-dimensional theoretical model can predict the internal temperature and mass-flow rate of the thermosyphon loop.
[en] This paper presents the development of a one-dimensional mathematical model for simulate the processes of mass and energy transfer during cooling of the molten nuclear fuel relocated to the vessel bottom during a severe accident. The cooling effect is produced by the thermosyphon phenomenon, in which removal of heat from nuclear decay is carried out by the water that possibly penetrates between the bottom wall of the vessel and the molten material. The water heats up and changes phase producing steam. This steam generates a pressure strong enough to lift the molten material into a cavity. Through this cavity, water vapor flows to keep the molten material partially cooled. The mathematical model includes a model of the molten material and the remaining water. The growth of the crust of the molten material and its temperature profile, with heat generation by decay, are described by the approach Stefan problem in one dimension. The results show that for powers less than 4.15 k W m-3 is possible to solidify and cool the molten material in less than 2.5 x 106 s, that is in about 29 days. (author)
[en] The world market for solar water heaters (SWH) is in great expansion. In fact, SWHs have become a challenging alternative system to gas and electric heating devices, because of their low CO2 emissions and their explicit involvement in the reduction of the building's electric bills. The 'Institut de recherche en energie solaire et energies nouvelles' (IRESEN-Morocco) is actually providing the necessary funding support to promote research and development field of SWHs in Morocco. Hence, in February 2016, SOL'R SHEMSY project has been defined in order to design and commercialize the first SWH integrating Evacuated Tube Collectors (ETC) with heat pipes, under the constraints of being accessible to the Moroccan public users. For the time being, several technologies of solar collectors with different brands are commercialized in Morocco such as flat plat collectors (FPC) and Evacuated Tube Collectors (ETC), besides to thermosyphon and forced circulation SWHs to convert solar energy to thermal energy, in order to produce hot water with the temperature required in several applications (individual, collective, industrial and the tertiary applications). However, a serious challenge of adjustment, integration and technology transfer should be considered before importing SWHs with ETC into the Moroccan market from supplier countries such as China, Spain, Germany and Turkey described as the leading manufacturer of SWH in the world. Indeed, the imported SWHs in Morocco presented thermo-mechanical failure modes after a short period of use such as the limestone deposited inside the storage tanks, corrosion and cracking of their inner shell... Taking into account the challenge of adjustment, integration and technology transfer of SWHs in Morocco is necessary, because the conditions of use differ from one country to another in terms of climatic conditions, which can be summarized in solar radiations, ambient temperature, wind velocity and the percentage of humidity in the air. In addition, the consumer's random consumption profile usually affects the thermal efficiency of the collector field and the overall solar fraction of SWHs. The main aim of this thesis is to carry out numerical simulations and optimizations of SWHs taking into account the technico-economic context of Morocco and its intrinsic weather conditions. Two simulation approaches were used to achieve this purpose. The first approach was based on unsteady Computational Fluid Dynamics (CFD) numerical simulation in order to enhance the efficiency of the heat pipes integrated inside ETCs and to increase the thermal stratification indicators of horizontal thermal storage tanks with submerged heat pipes. Finally, improve the storage tank's structure using thermo-mechanical coupling simulations. The second approach is based on dynamic simulations, which have been performed to assess and optimize the energy performance of a collective hot water process intended to produce domestic hot water (DHW) under the climatic conditions of Fez city located in Morocco. Indeed, several parameters have been considered, in particular the effect of the storage tank volume, the brands of collectors and their technology from different manufacturers, the connection between the solar panel and the overall surface of the ETC and FPC which constitute the collector field etc. (author)
[fr]Le marche mondial des chauffe-eau solaires (CES) est en essor. En effet, les CES sont devenus des systemes competitifs aux appareils de chauffage par gaz naturel et energie electrique, en raison de leurs faibles emissions de CO2 et de leur participation explicite a la reduction des factures d'electricite du consommateur marocain. Actuellement, l'Institut de Recherche en Energie Solaire et Energies Nouvelles (IRESEN-Maroc) pourvoit le soutien financier necessaire pour promouvoir la recherche et le developpement dans le domaine des CES au Maroc. Ainsi, en fevrier 2016, le projet SOL'R SHEMSY a ete defini pour concevoir et commercialiser le premier CES integrant la technologie des capteurs a tubes sous vide (ETC) avec des caloducs, en rendant son acquisition accessible au sociale marocain. Il parait qu'a l'heure actuelle, plusieurs technologies de capteurs solaires de marques differentes sont commercialisees au Maroc tels que les capteurs plan (FPC) et les capteurs a tube sous vide (ETC), outre le thermosiphon et les CES a circulation forcee pour convertir l'energie solaire en une energie thermique, afin de produire de l'eau chaude avec la temperature requise dans plusieurs applications individuelles, collectives, industrielles et tertiaires. Cependant, une serieuse problematique d'ajustement, d'integration et de transfert technologique devrait etre envisagee avant d'importer les CES a capteur ETC munis de caloduc sur le marche marocain aupres de pays fournisseurs tels que la Chine, l'Espagne, l'Allemagne et la Turquie consideres comme des leaders internationaux dans le domaine des CES. Car en effet, les CES importes au Maroc ont presente des modes de defaillances thermomecaniques apres une courte periode d'utilisation, comme le depot de calcaire a l'interieur des ballons de stockage, la corrosion et la fissuration de leur coque interne...Prendre en consideration la problematique d'ajustement, de l'integration et du transfert technologique des CES au marche marocain est necessaire, etant donne que les conditions d'utilisation different d'un pays a un autre en termes de conditions climatiques, qui peuvent etre resumees par l'amplitude de l'irradiation solaire, la temperature ambiante, la vitesse du vent et le pourcentage de l'humidite dans l'air. Par ailleurs, le profil de consommation aleatoire du consommateur affecte a son tour l'efficacite thermique du champ de capteurs et la fraction solaire globale des CES. L'objectif principal de cette these est de mener des simulations numeriques et des optimisations qui portent sur les CES, en prenant en compte le contexte technico-economique du Maroc et ses conditions climatiques intrinseques. Deux approches de simulation ont ete utilisees pour atteindre cet objectif. La premiere approche s'est basee sur des simulations numeriques CFD (Computational Fluid Dynamics) instationnaires pour ameliorer l'efficacite energetique des caloducs integres dans les ETC et d'augmenter les indicateurs de stratification thermique des ballons horizontaux dans lesquels les caloducs sont immerges. Enfin, d'ameliorer la structure du reservoir du CES en utilisant des simulations qui permettent de modeliser un couplage thermomecanique. La deuxieme approche est basee sur des simulations dynamiques, qui ont ete menees pour evaluer et optimiser la performance energetique d'un systeme collectif destine a produire de l'eau chaude sanitaire (ECS) sous les conditions climatiques de la ville de Fes-Maroc. En effet, plusieurs parametres ont ete consideres, en particulier l'effet du volume du reservoir de stockage, les marques des capteurs, leurs rendements et leurs technologies issues de differents fabricants, et finalement l'effet de la connexion serie/parallele et mixte entre les panneaux solaires (ETC et FPC) qui constituent le champ capteur, etc. (auteur)
[en] This paper presents the derivation of a closed form solution for density driven natural circulation flows (i.e. – thermosiphon) in a thermal system typical of power plants. The assumptions and methodology to properly define the mathematical system are described and an example provided. The solution is shown for a common power plant system. (author)
[en] Heat pipes are passive heat transfer devices, of long lives. Material and testing reactors (MTRs) have residual heat after shutdown. Usually MTRs have also spent fuel storage tanks to compromise heat that need to be removed. Gravity assisted two-phase closed heat-pipe loop (GTPHL) covered by removal of decay heat (or heat after shutdown) with evaporator and condenser lengths each 100 m helical coil shape with outer diameter 15 cm and 3 mm thickness as a passive cooling system for a nuclear spent fuel storage pool. This study proposes a completely passive cooling system using thermosyphon loop for cooling and dissipation of the residual heat of wet spent fuel storage by running as main or alternative cooling system. The design focuses on heat removal from the spent fuel storage tank of a research reactor. The model considers natural convection by air for the condenser part of the heat-pipe loop to confine the residual heat. A numerical simulation, using special design of GTPHLs, was used to investigate the thermal performance of the GTPHL. The effects of heat loads were analyzed. Demineralized water was used as the GTPHL working fluid. The atmospheric air was circulated around the condenser as a cooling system. The thermal performance of the GTPHL is evaluated at heat input ranging from 25 to 15 degree kW with filling ratio of the working fluid of 100%. The results show that a good thermal performance is obtained at high evaporator heat load obtained from nuclear spent fuel storage tank.
[en] Kilopower is a power generation system based on nuclear processes for Space, Moon and Mars applications. The system is designed to operate from 1 to 10 kWe and is expected to occupy the gap between fission power systems and thermoelectric systems. The heat generated in the reactor core will be transported by alkali metal heat pipe to the Stirling converters for electric power generation. The Stirling converters need a radiator system to dump the excess of heat. This radiator system is also based on heat pipes. This work aims to analyze the thermo-hydraulic characteristics of the heat extraction system of the micro-reactor for the Stirling converters. This analysis will be made in Ansys Fluent. As boundary conditions, the operation temperature was fixed in 1023K with a thermal power of 1250W in the evaporator, and the external temperature of the condenser in 973K. The analysis results were very close to the experimental results released by the Advanced Cooling Technologies team (ACT). (author)
[en] In a two-step approach the applicability of 10 m long two-phase closed thermosyphons (TPCT) is investigated for a passive heat removal system for spent fuel pools. The basic operational behavior of TPCT is measured for predefined thermal conditions at various pipe diameters (20, 32 and 45 mm) and pipe filling ratios in a laboratory setup. The influence on the thermal operation and the heat flux in dependency on the inner pipe diameter is measured and presented. First, the experiments are performed with direct electric heating and then with indirect water-heating. In the second step, the demonstration facility ATHOS (Atmospheric THermosyphon cooling System) with water tank heating and ambient air cooling is built, in order to investigate in a small-scale model experiment the heat transfer performance of TPCTs towards application-oriented thermal conditions of a spent fuel pool (SFP). First results of the ATHOS experiments are presented, demonstrating the applicability of a TPCT bundle using the ambient air as ultimate heat sink.
[en] Two-phase thermosyphons, which use sodium at 960 K as a refrigerant for heat transportation in small nuclear reactors, are promising for manned exploration to Mars. This is because the concentric-tube type thermosyphon may not have a flooding limit, and so the heat transfer performance per unit volume is comparatively large. Moreover, since its external form is a single tube, the reactor core can be made smaller. Experimental investigations of the energy transport properties when using water, R113, ethanol, and nitrogen for the refrigerant, as well as formulation of prediction relations, are progressing for establishing the design of the concentric-tube two-phase thermosyphon. However, the model needs to be improved and fluidization phenomena must be clarified in order to establish a model for predicting the maximum heat transfer rate of the thermosyphon which uses sodium at 960 K as a refrigerant. We have proposed a model based on bubble pump theory in order to take into consideration the rise of liquid level of the heating section. We conducted an experimental study on the flow inside a thermosyphon made of transparent material, and evaluated the maximum heat transfer rate using a low boiling point refrigerant, HFE-7100. As a result, even when the heat transfer rate was close to the maximum, it was shown that the flow in the outer tube of the adiabatic section of the concentric tube is two-phase flow. However, the experimental value of the maximum heat transfer rate was found to be about 30% smaller than the calculated value. We therefore investigated the cause of this difference by making pressure measurements and flow observations, and found that bubble entrainment in the inner tube of the concentric tube of the adiabatic section greatly influences the pressure distribution. It is thought that taking mixing into consideration would help improve the model. (author)
[en] Nanofluids have been introduced as an alternative to conventional fluids to improve energy efficiency in heat transfer systems. However, their stability problems before and after operation cycles can produce inconsistent results in different heat transfer technologies that use them. This review summarizes different experimental results obtained using nanofluids in heat pipes, particularly in two-phase closed thermosyphons, and it focuses on the role of preparation and stability issues of nanofluids before and after their use in these devices. Additionally, the effects of nanofluids on heat pipes’ thermal performance were compiled and compared from available experimental studies in the literature. Nanoparticles’ deposition on the evaporator surface and wick or groove structures were the most common mechanism to explain the reported increase or decrease in the thermal performance of heat pipes. This review also identifies the research problems that need to be solved in order to use nanofluids that outperform conventional fluids in heat pipes.
[en] Stable nanofluids based on DG-100 grade carbon black and carbon nanotubes have been prepared, and their influence on the maximum heat-transfer capacity and thermal resistance of closed-loop two-phase thermosyphons (TPTs) intended for electronics cooling have been studied. A more than twofold increase in the critical heat flux of these TPTs as compared to those filled with water has been obtained along with a sharp decrease in their thermal resistance. It is suggested that this effect is not only due to the high thermal conductivity of the proposed nanofluids, but is also related to the formation of a specific porous structure hindering the appearance of a vapor film and enhancing the boiling process.