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[en] Solar energy is one of the promising resources of renewable energy. It is of particular interest due to the energy shortage and environment pollution problems. Water heating by solar energy for domestic use is one of the most successful and feasible applications of solar energy. The thermosyphon SDHWS and the loop type thermosyphon systems are widely used for domestic hot water system. The loop type thermosyphon is a circulation device for transferring the heat produced at the evaporator area to the condenser area in the loop by a working fluid. The system has the advantage of high heat transfer rate. A phase change of the working fluid occurs at the evaporator section and the vapor is transported to the condenser by the density gradient. The loop type thermosyphon collector can be made of smaller area and has higher efficiency than the present thermosyphon SDHWS. In this study, the operating characteristics of various working fluids being used have been identified. The working fluids employed in the study were ethanol, water and a binary mixture of ethanol and water. The volume of working fluid used in this study were 30%, 40%, 50%, 60% and 70% of evaporator volume. An increased heat was applied with the increased volume of working fluid. It is observed that, in the thermosyphon with low volume of working fluid, such as 30% or 40%, the fluid was dried out. The average efficiency of the loop type thermosyphon was 46% with high solar irradiation and 43% with low irradiation. The flow pattern and mechanism of the heat transfer were identified through this study. Flow patterns of the binary mixture working fluid were also investigated, and the patterns were recorded in the camera. The system parameters were calculated using the thermal performance data. Modelling of the system was carried out using PSTAR method and TRNSYS program
[en] Experimental studies were carried out for the new thermosyphon type of double tube evaporator. thermosyphons are utilized for transporting large quantities of heat energy. thermosyphons have been widely used in energy and different industrial applications due to their simple construction , small thermal resistance, broad operating limits and low fabrication costs. thermal analysis of thermosyphons performance is experimentally presented in the steady state operation of the closed two-phase thermosyphon of double tube evaporator. the experimental study was performed by inserting an inner pipe into the evaporator of thermosyphon. this study aims to examine the thermosyphons performance against the traditional thermosyphon and analysis of the characteristics of double tube evaporator
[en] The issue concerning the dynamic response of closed two-phase thermosyphon with inner tube inserted inside evaporator creating natural circulation system is getting on increasing degree of attention due to its relevance during startup. Thermosyphons transient operation for startup from ambient condition to steady state is considered a stringent necessity for vital applications such as electronic, solar, geothermal and even nuclear reactors safety systems. This typically returns to the need to keep the temperature within certain limits before reaching critical conditions. Also, a greater understanding of the thermosyphon and its transient behavior is needed.Transient thermal-hydraulic analysis of the closed two-phase water/copper thermosyphon with inner tube is theoretically and experimentally presented. The main objective of the current study is to develop a theoretical model that can predict the dynamic behavior of the double-tube evaporator by tracing various transient parameters during operation from start up to steady state condition.The model numerically describes thermosyphon of double tube evaporator (double tube thermosyphon DTT) by seven coupled sub-models. These sub-models are: heater, wall of thermosyphon, fluid in the riser hot channel inside evaporator, vapor core in adiabatic and condenser sections, liquid in the down-comer of thermosyphon, the condensate film in the condenser, and finally the cooling water of the condenser. The mathematical model is derived by applying the energy balance for each section and sub-section in addition to continuity, and momentum equations for the liquid and vapor in different sections of double tube thermosyphon (DTT). The model simulates the thermosyphon by several differential equations which are solved to predict the various parameters. A computer program is designed to solve these differential equations by an explicit finite difference method. To validate the predicted theoretical model results, an experimental investigation has been conducted. An experimental setup is constructed from: thermosyphon main tube of 42 mm inner diameter, 4 mm thickness and 1950 mm long. The evaporator and condenser section lengths are 1100 mm and 400 mm respectively, while the adiabatic section is 450 mm long. An inner tube made of poly propylene is installed inside the evaporator. The inner tube has outer diameter of 34 mm with thickness 3.5 mm and 1100 mm length. This structure forms a hot channel with 4 mm annular gap width, where the natural flowing liquid circulated between inner and outer tubes. The heat source is simulated by 4 electrical coils while the heat sink is directly performed by a cooling process at condenser section, using cooling water of 0.0456 kg /s.
[en] Thermosyphons transient response for startup from ambient temperature to steady state until shutdown conditions, is considered a stringent necessity for applications such as electronic, solar, geothermal and even nuclear reactors safety systems. This typically returns to the need to keep the temperature within certain limits before reaching critical conditions. A simple network model is derived for describing the transient response of closed two-phase thermosyphon (CTPT) at startup and shutdown states. In addition, for predicting the effect of operational characteristics of water/copper closed two-phase thermosyphon such as thermal load, filling ratio, evaporator length, and thermosyphon tube diameter. The thermosyphons operation was considered a thermal network of various components with different thermal resistances and dynamic responses. The network model consists of six sub-models. These models are pure conduction in walls of evaporator, adiabatic and condenser, and convection in evaporator pool, evaporator film, and condenser film. So, an energy balance for each sub-model was done to estimate temperatures, heat transfer coefficients, thermal resistances, time constant, and other thermal characteristics that describe the required transient response of the closed two-phase thermosyphon. Governing equations of the transient thermosyphon behavior can be simplified into a set of first-order linear ordinary differential equations. The Runge-Kutta method can be used to obtain transient thermosyphon temperatures from these equations.
[en] Energy conservation is becoming increasingly important as the cost of fuel continuously rises. The heat pipe and the closed two-phase thermosyphon are particularly effective tools in the heat transfer process.A theoretical and experimental investigation was conducted to study the double-tube two-phase closed-thermosyphon (DTTH) behavior in transient regimes. Experiments were performed to investigate the effects of changing the heating and cooling rate as well as the evaporator length on the double tube thermosyphon in actual integrated operation (start-up, steady-state and shut-down). he necessity for a dynamic model of DTTH for some applications of discontinuous operation imposed the need to the current applied investigation. Therefore, the main objective of the current study is to develop a theoretical model that can predict the dynamic behavior of the double-tube evaporator by tracing various transient parameters during operation from start up to steady state until shut down condition. A model describing both thermal and phase flows of the closed two-phase double tube thermosyphon (DTTH) has been simulated. The theoretical model provides a general description of the behavior of our practical setup based on experimental observations which show a simple exponential behavior. It is based on a two thermal body description (evaporator wall and working fluid) there is good agreement between experiments data and numerical prediction.A computer simulation program based on the method was developed to estimate temperature and the other performance of double tube thermosyphon as well as the time needed to reach steady state condition. The governing equations of the simple 1-D model were solved by Engineering Equation Solver program (EES) using finite difference Euler method. A computer program is designed to solve these differential equations by an explicit finite difference method. The results from this model were found to be in general agreement with the experimental data.To validate the predicted theoretical model results, an experimental investigation has been conducted. An experimental setup is constructed from: thermosyphon main tube made of copper with 32 mm inner diameter, 1.5 mm thickness and 1000 mm long. The evaporator and condenser section lengths are 600 mm and 250 mm respectively, while the adiabatic section is 150 mm long. Inner tubes made of Pyrex are installed inside the evaporator. The inner tubes have thickness of 1.8 mm and 600, 450, 300 and 150 mm lengths. This structure forms a hot channel, where the natural flowing liquid circulated between inner and outer tubes. The heat source is simulated by 4 electrical coils while the heat sink is directly performed by a cooling process at condenser section, using cooling water at ambient temperature and constant pressure at 0.05 MPa.Variation of the transported thermal energy of thermosyphon is experimentally applied by constant-heat flux electric heat source to cover ranges of 900-1650 W. Direct measurements of the axial local wall and vapor temperature distribution are performed along the entire length of DTTH. The liquid in down-comer and the vapor in adiabatic and condenser sections are measured using a special long probe developed for the current study. In addition to eight tiny temperature probes were inserted in the annulus hot channel of the evaporator to determine the single liquid and two-phase fluid local temperatures. The relevant thermocouples are connected to a data logger device to monitor the transient temperatures during experimental investigations.From the previous studies it was found that the main objectives of this investigation was to study the effects of changing the heating and cooling rate as well as the evaporator length on the double tube thermosyphon in actual integrated operation as follows:The stability of the double-tube thermosyphon which included:1-Average wall and vapor temperature.2-The output power.Also, the characteristics transient study includes the analysis of:1-Boiling and non-boiling heat transfer coefficients of the eva porator.2-Temperature profile along the double-tube thermosyphon.In addition to the performance study for comparison purposes mainly involves:1-Transient response of the double-tube thermosyphon.2-Average evaporator heat transfer coefficient.3-Equivalent overall heat transfer coefficient.4-Effective thermal conductivity of thermosyphon.
[en] The present work analyses thermosyphons and their non dimensional numbers. The mathematical model considers constant pressure, single-phase incompressible flow. It simulates both open and closed thermosyphons, and deals with heat sources like PWR cores of electrical heaters and cold sinks like heat exchangers or reservoirs. A computer code named STRATS was developed based on this model. (author)
[en] heat pipes and thermosyphons have their great advantages of transporting large amount of heat energy through small temperature difference, in addition, to their special outstanding characteristics. the gravity -assisted wickless heat pipe or thermosyphon has many enhanced characteristics over the capillary forced heat pipes. however, thermosyphons involve some technical problems in unsteady and steady operation . therefore, development of thermosyphons performance did not cease in last decades.of these improvements is the double tube thermosyphon, that substantially avoided both critical heat flux and occurrence of the dryout . for this invention, several researches have been performed, but still there is not a clear understanding for design selection of the most sufficient hot channel (riser) width dimension and hot fluid behavior in the evaporator section. therefore, the current study is an attempt for the examination of the influence of changing the annulus spacing dimensions on the characteristics of hot fluid channel and as a whole on thermosyphons performance . this research is considered the first and unique experimental parametric study which uses a setup with the ability of changing the gap width
[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] The method of cooling based on the thermosyphon principle is of great interest because of its simplicity, its passivity and its low cost. It is adopted to cool down to 4,5 K the superconducting magnet of the CMS particles detector of the Large Hadron Collider (LHC) experiment under construction at CERN, Geneva. This work studies heat and mass transfer characteristics of two phase He I in a natural circulation loop. The experimental set-up consists of a thermosyphon single branch loop mainly composed of a phase separator, a downward tube, and a test section. The experiments were conducted with varying several parameters such as the diameter of the test section (10 mm or 14 mm) and the applied heat flux up to the appearance of the boiling crisis. These experiments have permitted to determine the laws of evolution of the various parameters characterizing the flow (circulation mass flow rate, vapour mass flow rate, vapour quality, friction coefficient, two phase heat transfer coefficient and the critical heat flux) as a function of the applied heat flux. On the base of the obtained results, we discuss the validity of the various existing models in the literature. We show that the homogeneous model is the best model to predict the hydrodynamical properties of this type of flow in the vapour quality range 0≤x≤30%. Moreover, we propose two models for the prediction of the two phase heat transfer coefficient and the density of the critical heat flux. The first one considers that the effects of the forced convection and nucleate boiling act simultaneously and contribute to heat transfer. The second one correlates the measured critical heat flux density with the ratio altitude to diameter. (author)