Dissertations / Theses on the topic 'Ejector refrigeration'

Refrigeration systems, air-conditioning units and heat pumps have been recognized as indispensable machines in human life, and are used for e.g. food storage, provision of thermal comfort. These machines are dominated by the vapor compression refrigeration system and consume a large percentage of world-wide electricity output. Moreover, CO2 emissions related to the heating and cooling processes contribute significantly to the total amount of CO2 emission from energy use. The ejector refrigeration system (ERS) has been considered as a quite interesting system that can be driven by sustainable and renewable thermal energy, like solar energy, and low-grade waste heat, consequently, reducing the electricity use. The system has some other remarkable merits, such as being simple and reliable, having low initial and running cost with long lifetime, and providing the possibility of using environmentally-friendly refrigerants, which make it very attractive. The ERS has received extensive attention theoretically and experimentally. This thesis describes in-depth investigations of vapor ejectors in the ERS to discover more details. An ejector model is proposed to determine the system performance and obtain the required area ratio of the ejector by introducing three ejector efficiencies. Based on this ejector model, the characteristics of the vapor ejector and the ERS are investigated from different perspectives. The working fluid significantly influences the ejector behavior and system performance as well as the ejector design. No perfect working fluid that satisfies all the criteria of the ERS can be found. The performance of nine refrigerants has been parametrically compared in the ERS. Based on the slope of the vapor saturation curve in a T-s diagram, the working fluids can be divided into three categories: wet, dry and isentropic. A wet fluid has a negative slope of the vapor saturation curve in the T-s diagram. An isentropic expansion process from a saturated vapor state will make the state after the expansion to fall inside the liquid-vapor area of the T-s diagram which will result in droplet formation. Generally, an isentropic expansion for a dry fluid will not occur inside the liquid-vapor area, and consequently no droplets will form. An isentropic fluid has a vertical slope of the vapor saturation curve in the T-s diagram and an isentropic expansion process will hence follow the vapor saturation curve in the T-s diagram, ideally without any droplet formation. However, when the saturation condition is close to the critical point, it is possible that the isentropic expansion process of a dry fluid and an isentropic fluid occurs inside the liquid-vapor area of the T-s diagram, resulting in formation of droplets. In order to avoid droplet formation during the expansion, a minimum required superheat of the primary flow has been introduced before the nozzle inlet. Results show that the dry fluids have generally better performance than the wet fluids and the isentropic fluid. Hence the thesis mostly focuses on the features of vapor ejectors and the ERS using dry fluids. Exergy analysis has been proven to be very useful to identify the location, magnitude, and sources of exergy destruction and exergy loss, and to determine the possibilities of system performance improvement. This method is applied to the ejector and the ERS. The ejector parameters are closely interacting. The operating condition and the ejector area ratio have a great impact on the ejector overall efficiency and system COP. The ejector efficiencies are sensitive to the operating conditions, and they significantly influence the system performance. A so-called advanced exergy analysis is adopted to quantify the interactions among the ERS components and to evaluate the realistic potential of improvement. The results indicate that, at the studied operating condition, the ejector should have the highest priority to be improved, followed by the condenser, and then the generator. Thermoeconomics, which combines the thermodynamic analysis and economic principles, is applied to reveal new terms of interest of the ERS. The economic costs of the brine side fluids (fluids that supply heat to the generator and evaporator and remove heat from the condenser) play very essential roles in the thermoeconomic optimization of the ERS. Depending on different economic conditions, the system improvement from a thermodynamic point of view could be quite different from the thermoeconomic optimization. The ERS is economically sound when using free heat sources and heat sink. An ejector test bench has been built to test the entrainment ratio of different ejectors. Although the experiments do not achieve the desired results, they could still be discussed. The insignificant effect of the superheat of the secondary flow found in the theoretical study is validated. The assumption of neglecting the velocities at the ejector inlets and outlet are confirmed. The quantification of the ejector efficiencies shows that they largely depend on the operating conditions and the ejector dimensions.

QC 20141102

This thesis describes a theoretical and experimental investigation of the ejector re-compression lithium bromide absorption refrigeration cycle. In this novel cycle, a steam ejector is used to enhance the concentration process by compressing the vapour to a state that it can be used to re-heat the solution from where it was evolved. Since this cycle recovers the heat otherwise wasted in a conventional absorption cycle, the energy performance of the cycle is improved. The theoretical study shows that the improvement of the efficiency is proportional to the performance of the steam ejector. A COP of 1.013 was achieved from the experiment in this investigation. The novel cycle does not only improve the energy efficiency but also avoids the corrosion that will happen when high temperature heat sources are used to drive a lithium bromide absorption refrigerator. The steam ejector in the novel cycle acts as an efficient temperature converter in acceptance of different temperature heat sources, which reduces the energy loss when the temperature difference between the solution and the heat source is big. Therefore, the solution temperature can be set to a low level while the heat source temperature is high. This is significant to avoid the corrosion of lithium bromide solution at high temperature. Furthermore, the construction of the machine based on the novel cycle is simpler than that based on the conventional double-effect cycle. This refrigerator will be more reliable and have a lower initial capital cost. The cycle was investigated comprehensively in this thesis. In the theoretical study, a mathematical model for this novel cycle was established. The theoretical study reveals the operation characteristics and the factors that affect the energy efficiency of the cycle as well as how to design a refrigerator based on the novel cycle. In the experimental study, a concept-approved refrigerator was manufactured and tested. The part-load performance of the novel cycle was investigated from the experiment. The theoretical results had a good agreement with the experimental ones. NB. This ethesis has been created by scanning the typescript original and contains some inaccuracies. In case of difficulty, please refer to the original text.

Apresenta-se neste trabalho uma análise termodinâmica comparativa entre o desempenho de um ciclo de refrigeração que usa um ejetor como um pré-expansor do fluido refrigerante e um ciclo padrão de refrigeração por compressão de vapor. Na primeira etapa do trabalho é desenvolvido um modelo matemático em regime é desenvolvido um modelo matemático em regime permanente baseado na primeira Lei da Termodinâmica para cada um dos ciclos estudados. O modelo é capaz de prever o funcionamento de cada um dos componentes do ciclo, assim como do sistema geral. Ao modelo do ejetor é dada uma especial atenção. Para este componente do sistema apresenta-se uma simulação de desempenho, isto é, a partir de condições termodinâmicas previamente estabelecidas, seu modelo matemático é resolvido separadamente através de um processo iterativo pelo método sequencial de solução. Na segunda etapa do trabalho é apresentada para cada um dos equipamentos de cada ciclo uma análise, também em regime permanente, baseada na Segunda Lei da termodinâmica, ou análise Exergética. Os resultados encontrados demonstram que teoricamente, o uso de um ejetor nas condições propostas no trabalho, melhora de maneira significativa o desempenho de um ciclo padrão de refrigeração por compressão de vapor.
The present work is about a comparative thermodynamics performance analysis between a refrigeration cycle that uses an ejector as a refrigerant expander and a standard vapor compression cycle. In the first step, a steady state mathematical model based on the first law of thermodynamics is developed for each one of previous cycles. A special attention is given to the ejectors model. A performance simulation is shown to this component, from previously thermodynamics conditions set up your mathematical mode is separately solved through a iterative process by a sequential solution method. In the second step a second Law of thermodynamics, or exergy analysis, in the steady state, is presented for each one of the equipament of the cycles. The obtained results showed that theoretically, the use of an ejector at the proposed conditions of this work, improves in a significant way the performace of standard vapor- compression cycle.

A CFD model to simulate two-phase flow in refrigerant ejectors is described. This work is part of an effort to develop the ejector expansion refrigeration cycle, a device which increases performance of a standard vapor compression cycle by replacing the throttling valve with a work-producing ejector. Experimental results have confirmed the performance benefit of the ejector cycle, but significant improvement can be obtained by optimally designing the ejector. The poorly understood two-phase, non-equilibrium flow occuring in the ejector complicates this task. The CFD code is based on a parabolic two-fluid model. The applicable two-phase flow conservation equations are presented. Also described are the interfacial interaction terms, important in modelling non-equilibrium effects. Other features of the code, such as a mixing length turbulence model and wall function approximation, are discussed. Discretization of the equations by the control volume method and organization of the computer program is described. Code results are shown and compared to experimental data. It is shown that experimental pressure rise through the mixing section matches well against code results. Variable parameters in the code, such as droplet diameter and turbulence constants, are shown to have a large influence on the results. Results are shown in which an unexpected problem, separation in the mixing section, occurs. Also described is the distribution of liquid across the mixing section, which matches qualitative experimental observations. From these results, conclusions regarding ejector design and two-phase CFD modelling are drawn.
Ph. D.

The purpose of this thesis is to experimentally determine the feasibility of an ejector based, waste heat recovery driven refrigeration system applied to the data center environment in order to reduce operational cooling costs. A comprehensive literature review is detailed to determine the current state of the ejector refrigeration research and assess the initial direction of this thesis. A simplified model was created to perform preliminary performance estimations and system sizing before constructing an experimental system apparatus to evaluate the model predictions. The pressures and temperatures used in the model and instituted in the experimental system are based on the maximum temperatures typically observed in computing servers (50-75°C). Precision controlled heaters are used to simulate the computer server heat, and R245fa is used as the working fluid. Performance results ranged from 0.06 to 0.13.

The ejector expansion refrigeration cycle is a modified vapor compression cycle in which a two phase ejector is used to recover a portion of the work otherwise lost in the expansion valve. The ejector improves cycle performance by increasing compressor inlet pressure and by lowering the quality of the fluid entering the evaporator. Theoretically, a cooling COP improvement of approximately 21 % is achievable for a typical refrigerating cycle and an ideal ejector. If the ejector performed as well as typical single-phase ejectors, an improvement of 12% could be achieved. Previous tests have demonstrated a smaller 3.7% improvement; the difference is in the poor performance of the two-phase ejector. The purpose of this research is to understand the operating characteristics of the two phase ejector and to improve design. A two-phase ejector test rig has been constructed and tested. Preliminary data show performance superior to previously tested two-phase ejectors, but still inferior to single phase ejectors. Ejector performance corresponds to refrigeration cycle COP improvements ranging from 3.9010 to 7.6%. This performance was obtained with an ejector designed from single-phase ejector and wet steam ejector design methods. The poor performance indicates the design methods must be improved for two-phase ejectors. This research has begun the development of design methods for the two-phase ejectors and this research has developed models to describe the fluid dynamics and thermodynamics of the ejector.
Ph. D.

In an attempt to reduce the dependence on fossil fuels, a variety of research initiatives has focused on increasing the efficiency of conventional energy systems. One such approach is to use waste heat recovery to reclaim energy that is typically lost in the form of dissipative heat. An example of such reclamation is the use of waste heat recovery systems that take low-temperature heat and deliver cooling in space-conditioning applications. In this work, an ejector-based chiller driven by waste heat will be studied from the system to component to sub-component levels, with a specific focus on the ejector. The ejector is a passive device used to compress refrigerants in waste heat driven heat pumps without the use of high grade electricity or wear-prone complex moving parts. With such ejectors, the electrical input for the overall system can be reduced or eliminated entirely under certain conditions, and package sizes can be significantly reduced, allowing for a cooling system that can operate in off-grid, mobile, or remote applications. The performance of this system, measured typically as a coefficient of performance, is primarily dependent on the performance of the ejector pump. This work uses analytical and numerical modeling techniques combined with flow visualization to determine the exact mechanisms of ejector operation, and makes suggestions for ejector performance improvement. Specifically, forcing the presence of two-phase flow has been suggested as a potential tool for performance enhancement. This study determines the effect of two-phase flow on momentum transfer characteristics inside the ejector while operating with refrigerants R134a and R245fa. It is found that reducing the superheat at motive nozzle inlet results in a 12-13% increase in COP with a 14-16 K decrease in driving waste heat temperature. The mechanisms of this improvement are found to be a combination of two effects: the choice of operating fluid (wet vs. dry) and the effect of two-phase flow on the effectiveness of momentum transfer. It is recommended that ejector-based chillers be operated such that the motive nozzle inlet is near saturation, and environmentally friendly dry fluids such as R245fa be used to improve performance. This work provides critical methods for ejector modeling and validation through visualization, as well as guidance on measures to improve ejector design with commensurate beneficial effects on cooling system COP.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2006.
A small scale steam jet ejector experimental setup was designed and manufactured. This ejector setup is of an open loop configuration and the boiler can operate in the temperature range of Tb = 85 °C to 140 °C. The typical evaporator liquid temperatures range from Te = 5 °C t o 10 °C while the typical water cooled condenser presure ranges from Pc = 1 . 70 kPa t o 5. 63 kPa (Tc = 15 °C to 35 °C). The boiler is powered by by two 4kW electric elements, while a 3kW electric element simulates the cooling load in the evaporator. The electric elements are controlled by means of variacs. The function ...
Centre for Renewable and Sustainable Energy Studies

Abstract : This research project has as main objective to study in detail the internal flow features of single-phase supersonic ejectors for refrigeration applications, and the potential effects of injecting droplets on the performance of the device. To this end, a numerical approach is proposed which has been separated into two parts: First, a RANS modelling strategy for supersonic ejectors has been outlined combining the NIST real gas equations database [NIST, 2010] and the k − ω SST turbulence model in its low-Reynolds number formulation. The proposed approach agrees within 5% (resp. 2%) to the experimental entrainment ratio (resp. compression ratio) data of García del Valle et al. [2014], properly captures the main internal flow features and has a reasonable computational cost. This RANS model has been applied in the analysis of a supersonic R134a ejector for refrigeration purposes, showing in particular that the secondary flow is entrained by momentum transfer through the mixing shear layer, that the distance between the primary nozzle exit and the shock-waves in the constant area section varies between 9 and 16 times the primary nozzle exit diameter and that the important axial character of the flow limits mixing of both inlet flows until after the shock train. Furthermore, an exergy analysis through the device shows that the mixing and the oblique shock waves are responsible for between 50% and 70% of the generated losses, the latter might be attenuated through droplet injection in the constant area section. Moreover, it has been shown that drop-in replacement of the working fluid with HFOs R1234yf and R1234ze(E) leads to mild changes in the ejector performance but reduces the HDRC system COP (resp. cooling capacity) in average by 7.1% (resp. 23.3%). Lastly, a comparison of the model predictions with the thermodynamic model of Galanis and Sorin [2016] for an air ejector, shows that as the working fluid approaches the ideal gas behaviour, the flow can be adimensionalized in terms of the secondary inlet temperature and pressure, the motive nozzle throat and the entrainment and compression ratios. In the second part, the influence of droplets has been studied from a local perspective by extending the RANS model to include a discrete phase, which affects the main flow through exchanges of momentum and thermal energy, and from a global perspective by building a thermodynamic model, which predicts the entrainment and limiting compression ratio given a fixed geometry and operating conditions. Both approaches present very good agreement in terms of p, T and M a internal profiles. Results for a supersonic ejector with R134a as baseline working fluid and droplets injected at the constant area section show that the flow structure has perceptible changes only at the highest injection fraction considered 10%, which induces boundary layer detachment, reduces the shock intensity by 8% and diminishes the superheat at the ejector outlet by 15 ◦C. Nonetheless, ejector performance metrics are severely affected as the limiting compression ratio, Elbel efficiency and exergy performance reduce respectively by 5%, 11% and 15%, due mainly to the additional entropy generated through droplet injection and mixing with the main flow.
Ce projet de recherche a pour objectif principal d’étudier en détail les caractéristiques de l’écoulement interne dans des éjecteurs supersoniques monophasiques pour des applications en réfrigération, et les effets potentiels de l’injection de gouttelettes sur les performances de l’appareil. A cette fin, une approche numérique est proposée et a été séparée en deux parties. Tout d’abord, une stratégie de modélisation RANS pour les éjecteurs supersoniques a été décrite en combinant la base de données pour les gaz réels NIST [NIST, 2010] et le modèle de turbulence k − ω SST dans sa formulation à bas nombre de Reynolds. L’approche proposée prédit avec un accord d’environ 5% (resp. 2%) le rapport d’entraînement (resp. rapport de compression) avec les données expérimentales de García del Valle et al. [2014]. Il capte également correctement les principales caractéristiques de l’écoulement interne et a un coût de calcul raisonnable. Ce modèle RANS a été appliqué à l’analyse d’un éjecteur supersonique au R134a utilisé à des fins de réfrigération, montrant en particulier que le flux secondaire est entraîné par un transfert d’impulsion à travers la couche de cisaillement, que la position de départ des ondes de choc dans la section constante se situe dans une plage de 9 à 16 fois le diamètre de sortie de la buse primaire et que l’important caractère axial du flux limite le mélange des deux écoulements d’entrée au-delà du train d’ondes de choc. De plus, une analyse exergétique à travers le dispositif montre que le mélange et les ondes de choc obliques sont responsables de 50% et 70% des pertes générées, ces dernières pouvant être atténuées par injection de gouttelettes dans la section à zone constante. De plus, il a été démontré que le remplacement direct du fluide de travail par les HFO R1234yf et R1234ze(E) entraîne de légers changements dans la performance de l’éjecteur mais réduit en moyenne le COP du système HDRC (resp. la capacité de refroidissement) de 7.1% (resp. 23.3%). Enfin, une comparaison des prédictions du modèle avec le modèle thermodynamique de Galanis and Sorin [2016] pour un éjecteur à air montre que lorsque le fluide de travail se rapproche du comportement de gaz idéal, l’écoulement peut être normalisé en fonction de la température et de la pression à l’entrée secondaire, la gorge de la tuyère principale et les rapports d’entraînement et de compression. Dans la seconde partie, l’influence des gouttelettes a été étudiée d’un point de vue local en étendant le modèle RANS à une phase discrète qui affecte le flux principal par des échanges de quantité de mouvement et d’énergie thermique, et d’un point de vue global en construisant un modèle thermodynamique qui prédit l’entraînement et le rapport de compression limitant étant donné une géométrie fixe et les conditions de fonctionnement. Les deux approches présentent un très bon accord en termes de profils internes de p, T et Ma. Les résultats pour un éjecteur supersonique au R134a comme fluide de base, avec des gouttelettes injectées à mi-chemin dans la section de la zone constante, montrent que la structure d’écoulement dans cette région présente des changements perceptibles seulement à la fraction d’injection la plus élevée, 10%, en diminuant l’intensité du choc de 8% et la surchauffe à la sortie de l’éjecteur de 15 ◦C. Néanmoins, la performance de l’éjecteur est sévèrement affectée vu que le rapport de compression, l’efficacité d’Elbel et le performance exergétique sont réduites respectivement de 5%, 11% et 15%, principalement en raison de l’entropie supplémentaire générée par l’injection de gouttelettes et le mélange avec le flux principal.

With the increasing importance of CO2 as natural refrigerant with low Global Warming Potential(GWP) ejectors have been used in a number of recent installations to recover expansion work atthe high operating pressures of these systems. In colder climates, this is particularly seen in combinationwith heat recovery due to the high compressor discharge pressures.This work analyses the field measurement data of two ice rink refrigeration systems with integratedvapor ejectors and two supermarket refrigeration systems with integrated liquid ejectors, alllocated in northern Europe. The aim is to evaluate the interaction of the ejector with the refrigerationsystem in practical applications. A theoretical model of the ejector systems is developed andevaluated in parallel as a reference for the analysed system installations.The model of the analysed vapor ejector system shows an increasing eciency improvement potentialby the ejector for higher gas cooler outlet temperatures, while the liquid ejector systemmodel indicates higher eciency improvement potential at relatively lower gas cooler outlet temperaturesand pressures.From the vapor ejector field data evaluation, this is confirmed with additional findings of lowejector work recovery eciencies at low gas cooler outlet temperatures. Furthermore, problemsin the ejector operation are found for too low evaporation temperatures in one of the systems. Inaddition, an unstable ejector control at certain operating conditions is linked to a decreasing ejectorperformance. While the ejector is found not to provide any significant savings in one of the systemsmainly due to low evaporation temperatures, the other ice rink system is found to achievetotal energy savings of 7% from the ejector.For the liquid ejector field data evaluation, the ejectors are found to work as expected for the purposeof removing liquid from the low-pressure receiver. However, overfed evaporation conditionsare only found temporarily for most cabinets in the analysed systems, with remaining high averagesuperheat values. Low required air supply temperatures in the cabinets and the dimensioning of theexpansion valves at the evaporator inlet are identified as possible limitations for a further decreaseof the superheat and increase of the evaporation temperature.
Med den ökande betydelsen av CO2 som naturligt köldmedium med låg global uppvärmningspotential(GWP) har ejektorer använts i ett antal nya installationer för att återvinna expansionsarbetevid de höga drifttrycken i dessa system. I kallare klimat är detta särskilt vanligt i kombination medvärmeåtervinning på grund av de höga utloppstrycken i kompressorerna.I detta arbete analyseras fältmätdata från två kylsystem för isbanor med integrerade ångejektoreroch två kylsystem för livsmedelsbutiker med integrerade vätskeejektorer. Samtliga system finns inorra Europa. Syftet med studien är att utvärdera ejektorns samverkan med kylsystemet i praktiskatillämpningar. En teoretisk modell av ejektorsystemen utvecklas och utvärderas parallellt som referensför de analyserade systeminstallationerna.Modellen för det analyserade ångejektorsystemet visar att potentialen för e ektivitetsförbättringgenom ejektorn ökar vid högre utloppstemperaturer för gaskylare, medan modellen försystemet med vätskeutkastare visar att potentialen för e ektivitetsförbättring ökar vid relativt lägreutloppstemperaturer och tryck för gaskylare.Detta bekräftas i utvärderingen av fältdata från ångejektorsystemen som vid låga utloppstemperatureri gaskylaren samtidigt ger låg e ektivitet för ejektorn. Dessutom noteras problemmed ejektorns funktion vid för låga förångningstemperaturer i ett av systemen. En instabilstyrning av ejektorn vid vissa driftsförhållanden leder vidare till en minskad ejektore ektivitet.Medan ejektorn inte ger några betydande besparingar i det ena systemet, främst på grund av lågaavdunstningstemperaturer, har en total energibesparing på 7% från ejektorn hittats i den andra isbanan.När det gäller utvärderingen av fältdata för vätskeejektorer konstateras att ejektorerna fungerar somförväntat för att avlägsna vätska från vätskeavskiljaren. För de flesta kyldiskar i de analyseradesystemen syns dock bara kortvarigt flödad tillstånd i förångrarna, och i övrigt en kvarvarande höggenomsnittlig överhettning. Låg erforderlig tilluftstemperatur i kyldiskarna och dimensioneringenav expansionsventilerna vid förångarens inlopp identifieras som möjliga begränsningar för enytterligare minskning av överhettningen och en ökning av förångningstemperaturen.

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The use of waste heat of energy conversion equipment to produce a cooling effect, consists currently in a very interesting way of efficiency improvement of energy systems. The present research has as intention the theoretical and experimental study of a new intermittent refrigeration system ejector cycle characteristics, with use of waste heat. Initially, was doing a bibliographical survey about the vapor ejector refrigeration system technology. In the following stage was doing a simulation of the corresponding thermodynamic cycle, with preliminarily intention to evaluate the performance of the system for different refrigerants fluids. On the basis of the results of the simulation were selected the refrigerant fluid and developed an experimental group of benches of the refrigeration system considered, where pressure and temperature sensory had been inserted in strategical points of the refrigeration archetype and connected to a computerized data acquisition system for measure the refrigerant fluid properties in the thermodynamic cycle. The test results obtained show good agreement with the literature
A utiliza??o do calor residual de equipamentos para convers?o de energia para produzir frio, ? uma forma de melhoria da efici?ncia dos sistemas energ?ticos. A presente pesquisa destina-se ao estudo te?rico e experimental das caracter?sticas de um novo sistema de refrigera??o intermitente por jato-compress?o (ejetor) com uso de calor residual. Inicialmente, foi realizado um levantamento bibliogr?fico sobre a tecnologia de sistemas de refrigera??o por jato-compress?o. Na fase seguinte, investigou-se a concep??o do principal elemento da proposta de sistema de refrigera??o, o ejetor. A metodologia emp?rica utilizada no c?lculo do ejetor est? dispon?vel na literatura. Com o aux?lio do software EES (Engineering Equation Solver) foram realizados estudos (simula??es) para diferentes fluidos refrigerantes sint?ticos. O fluido refrigerante R141b apresentou as propriedades termodin?micas e de transporte necess?rios para o funcionamento eficaz do sistema proposto. Com base nos resultados da simula??o foi selecionado o fluido refrigerante e desenvolvida uma bancada experimental do sistema de refrigera??o proposto, onde sensores de press?o e temperatura foram inseridos em pontos estrat?gicos do prot?tipo de refrigera??o e conectados a um sistema de aquisi??o de dados computadorizado para medi??o das propriedades do fluido refrigerante no ciclo termodin?mico. Os resultados obtidos nos ensaios revelam boa concord?ncia com os encontrados na literatura

L’objectif de cette étude est d’optimiser un cycle frigorifique au CO2 pour une application dans le transport frigorifique. Les performances de l’unité doivent être supérieures à celles d’une machine avec une injection de vapeur et un compresseur bi-étagé déjà commercialisé. Suite à l’étude de plusieurs solutions, un éjecteur couplé à un échangeur interne semble la solution la plus intéressante.Un banc expérimental est créé à partir de l’unité possédant une injection de vapeur. L’éjecteur est muni d’une aiguille pour pouvoir assurer une recherche de la haute pression optimale. L’échangeur interne est équipé d’un by-pass pour modifier la puissance échangée. Un ensemble de points d’essais est testé avec deux températures d’évaporation : 0 °C et -20 °C, et trois températures de source chaude : 30 °C, 38 °C et 50 °C.L’éjecteur avec aiguille est capable de s’adapter à différentes températures extérieures en modifiant la haute pression. L’échangeur interne permet d’augmenter les performances du cycle mais présente une limite, la température au compresseur devient élevée et présente un risque de détérioration de ses performances ou de l’huile.Avec le cycle présentant un éjecteur, une amélioration du coefficient de performance est observée pour les points avec une température d’évaporation de 0 °C mais celle-ci chute drastiquement pour les températures d’évaporation de -20 °C. Une analyse exergétique du cycle confirme les faibles performances de celui-ci pour des températures d’évaporation négatives.À partir des résultats expérimentaux, des modèles numériques sont mis en place. Les échangeurs, vannes de détente sont modélisés de manière conventionnelle. Pour le compresseur, le modèle de Winandy est modifié afin de fonctionner pour un compresseur bi-étagé avec injection de vapeur. L’éjecteur est modélisé à travers un modèle unidimensionnel basé sur des transformations simplifiées décrites à travers des rendements isentropiques. Tous les modèles sont validés mais ceux des échangeurs ont un temps de calcul CPU trop important pour pouvoir être utilisés sur une modélisation dynamique
The aim of this study is to optimize a CO2 cooling cycle for transport application. The efficiency of the unit needs to be superior that of a cycle with vapor injection and a two stages compressor. The solution proposed is to install an ejector with an internal heat exchanger.A test bench is created from a pre-existing unit. Tests are made for two evaporation temperatures: 0 °C and -20 °C and three external temperatures: 30 °C, 38 °C and 50 °C. The ejector is equipped with a needle to seek the optimal high pressure. The internal exchanger is equipped with a by-pass to modify the thermal power exchanged.The ejector with needle can change the high pressure to seek the optimal conditions. The internal heat exchanger increases the efficiency of the cycle but the rising of temperature at the compressor can degrade its efficiencies or the oil. The new cycle increases the COP for evaporation temperature of 0 °C but the COP is lower than without ejector for evaporation temperature of -20 °C. An exergetic analysis shows that indeed the cycle is less efficient for low evaporation temperature.From the experimental results, numerical models are created to realize a system simulation and to test different scenarii to drive the unit. Exchangers and valves modeled with conventional tools. Wynandy’s model is adapted to be used on a two-stage compressor with vapor injection. The ejector is modeled with a one-dimensional model, based on simplified transformations described with isentropic efficiencies. All models seem to work but the CPU time is too high to use the exchanger models for dynamic simulation

Refrigerated transport systems are employed in the cold chain to supply the consumer with safe, high-quality perishable freights. Differently from static refrigeration systems, refrigerated transport equipment is required to perform reliably in a wide range of ambient temperatures and under extremely variable weather conditions (solar radiation, cloudiness, rain etc…). Refrigerated vehicles include vans, rigid trucks and semi-trailers. The refrigerating system has to ensure a precise temperature control of the internal cargo space: given a certain perishable freight requiring a certain set-point temperature, a lower temperature might damage it while a higher temperature might reduce its shelf life or result in unsafe or low quality product for the consumer. Close temperature control systems for chilled goods require continuous, modulated refrigeration combined with high rates of air circulation. This thesis contains the results of a study carried out during three years of PhD activity, conducted at the Construction Technology Institute (ITC) of the National Research Council (CNR) in Padova, where an official ATP test station is located. The ATP (Agreement on the International Carriage of Perishable Foodstuffs and on the Special Equipment to be Used for such Carriage) is an international agreement regulating the transport of perishable foodstuff. Starting from this experience, dedicated experimental activity was set up to collect data: ATP K-value measurement tests have been performed, along with step tests intended to evaluate the characteristic time constant of the insulated body of a brand new refrigerated van, designed to the carriage of chilled products (0°C-class refrigerated vehicle). The experimental data collected during the ATP test and step test of the brand new refrigerated van were used to develop and validate a 0-D, unsteady, lumped capacitance, simple numerical model of its insulated box. The numerical model presented in this thesis introduces a novel approach to model the average transient response of the insulated body of a refrigerated vehicle: the model is able to reproduce the actual performance of the structure without the need for the detailed drawing of the structure or the knowledge of the actual properties of the material used for the construction. Under the stimulus of environmental sustainability and in order to offer the market with a natural alternative to F-gases, CO2 was considered as a possible working fluid for refrigerating units serving refrigerated vehicles. A new lay-out for the cooling unit, developed for refrigerated transport application, which utilizes carbon dioxide as the working fluid, was developed. Starting from the experience developed in stationary refrigeration, but taking care of constraints specific of refrigerated transport, the cooling unit was designed to operate according to three different configurations: traditional low-pressure receiver cycle configuration, ejector cycle configuration and ejector cycle configuration utilizing an auxiliary evaporator located in the line between the ejector outlet and the low-pressure receiver. Numerical simulations of the refrigerating system operation with an internal cargo space temperature varying between -5°C and +15°C and an external ambient temperature spanning between 15 °C and 45 °C demonstrated that the operation of the refrigerating system with the ejector cycle configuration was convenient in hot climates and internal air temperature between +5 °C and -5 °C. For ambient temperature lower than 30 °C the operating range of the ejector could be extended with an auxiliary evaporator at the outlet of the ejector. Taking the coefficient of performance of the system as a thermal performance indicator, the ejector cycle configuration provided a maximum advantage by 15.9 % over the traditional cycle configuration at 42°C ambient temperature and -5°C internal cargo space temperature. On the other hand, when the auxiliary evaporator was employed in the refrigerating system, the maximum advantage of 14.2 % on the COP was identified for an ambient temperature of 27 °C and an internal cargo space temperature of 5 °C. After the numerical models of the cooling unit and the insulated body were developed, they were coupled and a typical delivery mission was defined. The selected delivery mission was characterized by a total of 12 door opening operations where doors are kept completely opened and heat enters directly from the external environment. Numerical results gave the input to insight on the correct dimensioning and sizing of a refrigerated transport cooling unit, but were also utilized to compare the thermal performance of the state of the art (i.e. cooling unit operating with R-134a as the working fluid) with the innovation proposed in this thesis. Moreover, numerical simulations were able to assess the main heat fluxes that influence the performance of the refrigerating system during operation.

Les fluides naturels employés en réfrigération et en conditionnement d’air possèdent de faibles PRG et sont de ce fait une véritable alternative aux HFC. Cependant, leur généralisation se heurte à des limites provenant de leur caractère toxique (NH3), inflammable (hydrocarbures, NH3) ou de leurs caractéristiques thermodynamiques défavorables (CO2). Leur utilisation accrue nécessite la mise en œuvre de composants spécifiques (échangeurs de chaleur intermédiaire, éjecteur) qui sans qui les performances seraient inférieures à celles obtenues avec les HFC (COPCO2 = 55 % du COPHFC-134a pour des températures de sources de 0 °C et 40 °C). L’utilisation d’un éjecteur comme organe de détente est une solution envisagée pour réduire les irréversibilités. Les éjecteurs diphasiques constituent une alternative intéressante pour les dispositifs de détente classiques utilisés depuis plusieurs décennies. Le principal avantage de l’éjecteur est de récupérer une partie de l’énergie cinétique du processus de détente de la haute pression à la basse pression pour augmenter la pression d’aspiration du compresseur. Ceci entraîne une diminution du travail consommé par ce dernier et, par suite, une augmentation du coefficient de performance du système. Néanmoins, une bonne conception d’un éjecteur diphasique nécessite une analyse détaillée en termes de simulations numériques et travaux expérimentaux. Ainsi, l’objectif de ce travail est d’apporter une contribution expérimentale à l’étude des machines frigorifiques au CO2 transcritique équipées d’éjecteur diphasique. Des efforts importants ont été investis dans la conception d’un éjecteur diphasique avec diverses géométries pour évaluer les principales caractéristiques à savoir le facteur d’entraînement et le rapport de compression. Les essais effectués ont permis de mettre en évidence l’influence des différents paramètres géométriques sur les performances de la machine (différents diamètres au col des tuyères primaires, différents diamètres de mélangeurs, longueurs de mélangeurs, distance entre le plan de sortie de la tuyère primaire et l’entrée du mélangeur, l’angle de divergent des tuyères primaires…) ainsi que les paramètres thermodynamiques (température d’évaporation, température à l’entrée de la tuyère primaire)
Natural refrigerants used in refrigeration and air conditioning have low GWP and are therefore a real alternative to HFCs. However, their generalization comes up against limits due to their toxic (NH3), flammable (hydrocarbons, NH3) or their unfavorable thermodynamic characteristics (CO2). Their increased use requires the implementation of specific components (intermediate heat exchangers, ejector) which without performance would be lower than those obtained with HFCs (COPCO2 = 55% of COPHFC-134a for temperatures source of 0 °C and 40 °C). The use of an ejector as an expansion device is a solution considered to reduce irreversibility. Two-phase ejector has been an interesting alternative for conventional expansion devices for several decades. The main advantage of the ejector is to recover some of the kinetic energy of the process of expansion from high pressure to low pressure to increase the suction pressure of the compressor. This results in a reduction of the work consumed by the latter and, consequently, an increase in the coefficient of performance of the system. Nevertheless, a good design of a two-phase ejector requires a detailed analysis in terms of numerical simulations and experimental work. Thus, the objective of this work is to make an experimental contribution to the study of transcritical CO2refrigeration machines equipped with two-phase ejector. Significant efforts have been invested in the design of a two-phase ejector with various geometries to evaluate the main characteristics namely the entrainment ratio and the compression ratio. The tests carried out made it possible to highlight the influence of the various geometrical parameters on the performances of the machine (different diameters of the throat of the primary nozzle, different mixers diameters and lengths, distance between the exit of the primary nozzle and the inlet of the mixer, the divergence angle of the primary nozzles ...) as well as the thermodynamic parameters (evaporation temperature, temperature at the inlet of the primary nozzle)

Dans le contexte des recherches de réductions de l’impact environnemental des machines frigorifiques, l’utilisation du gaz carbonique comme fluide frigorigène est aujourd’hui une réalité. Toutefois, les propriétés thermodynamiques du CO2 impliquent un cycle frigorifique transcritique à basses performances énergétiques pour une température de source chaude proche de l’ambiante. Pour étendre le champ d’application de ce fluide, il est nécessaire d’augmenter l’efficacité des machines transcritiques. L’analyse exergétique du cycle montre que les principales pertes de performances proviennent essentiellement de la détente isenthalpique et de la compression. Afin de réduire ces pertes, l’utilisation d’un éjecteur comme organe principale de détente se présente comme une solution prometteuse. Ce travail apporte une contribution à l’étude des machines frigorifiques aux CO2 transcritique équipées d’éjecteur à la fois expérimentale et numérique pour développer la compréhension des phénomènes qui se produisent à l’intérieure de l’éjecteur afin d’améliorer les outils de dimensionnement de cet organe. L’étude numérique comporte un modèle unidimensionnel de l’écoulement du dioxyde de carbone à travers l’éjecteur. Ce modèle constitue un bon outil de prédiction des points de fonctionnement de l’éjecteur et des caractéristiques globales de l’écoulement : débit, vitesse, enthalpie... Le modèle reste une approche perfectible d'un milieu complexe. Il constitue néanmoins un bon outil pour l'optimisation de la géométrie de l’éjecteur. Après le dimensionnement et la fabrication de l’éjecteur, des essais comparatifs ont été menés sur la machine frigorifique au CO2 en fonctionnement avec et sans éjecteur. L’étude expérimentale a montré que l’éjecteur améliore jusqu’à 12,5 % la puissance frigorifique produite et 17 % le coefficient de performance de la machine. Les résultats expérimentaux réalisés ont été utilisés pour valider le modèle unidimensionnel développé, un accord satisfaisant a été trouvé entre les résultats issus du modèle et ceux expérimentaux, particulièrement en terme de débits avec un écart de l’ordre de 9 %
Carbon dioxide is being advocated to reduce the environmental impact of the refrigeration systems. However, the thermodynamic properties of CO2 imply supercritical refrigerating cycle with low energy performance when the hot source temperature is near that of the environment. The expansion losses of an isenthalpic throttling process have been identified as one of the largest irreversibilities of transcritical refrigeration cycles, which contribute to the low efficiency of such cycles. In order to recover the expansion losses and increase the cycle efficiency, it has been proposed to replace the expansion valve with an ejector expansion device. This work is devoted to the numerical and experimental study of the ejector expansion devices used in a transcritical vapor compression system using carbon dioxide as the refrigerant. The numerical study includes a one-dimensional model of the CO2 two-phase ejector. The developed model is a good tool for predicting the operation conditions of the ejector and the overall characteristics of the flow (mass flow, velocity, enthalpy.. The model is a good tool to optimizing the geometry of the ejector, although it can be improved. The ejector was manufactured and incorporated into an instrumented test bench. Experimental study showed that the transcritical CO2 refrigeration system using an ejector as the expansion device outperformed a conventional expansion-valve transcritical CO2 system in COP and cooling capacity by approximately 17 % and 12,5 %, respectively. The experimental results were used to validate the one-dimensional model, a satisfactory agreement was found between the numerical and experimental results, especially in terms of mass flow with a difference of 9 %

Les études expérimentales effectuées dans diverses conditions de fonctionnement du système permettent de déterminer les conditions thermodynamiques optimales pour une géométrie d'éjecteur donnée ou de définir une conception optimale de l'éjecteur dans des conditions thermodynamiques imposées. L'efficacité mesurée au cours des essais atteint 0,7 pour des températures de sources égales à 85°C, 30°C et 15°C. Un modèle global unidimensionnel confirme les données expérimentales. Une étude comparative du système à éjecteur avec les systèmes à absorption montre que le système à éjecteur est compétitif dans les conditions d'assez haute température à l'évaporateur. Ce système compact est bien adapte a la climatisation

Work proposes the optimization of an ejector refrigeration cycle, based on one-dimensional modelling techniques and some innovative design criteria for the ejector. A specific model is proposed for mixing chamber and the method of Constant Rate of Momentum Change integrating friction losses is used for supersonic diffuser design. An experimental apparatus has been developed and realized to validate optimization results and design criteria.

碩士
國立臺灣大學
機械工程學研究所
87
The purpose of this dissertation is to improve the efficiency of a binary-cycle refrigeration system that combines a compression system with a heat driven ejector cooling system. This paper will introduce the theoretical analysis of the binary-cycle refrigeration system , reformulate the frame design of the binary hardware , estimate its performance , and draw conclusions from the real applications of the binary-cycle refrigeration system . This study advances the theory for the binary-cycle refrigeration system developed by the Jet Empirical Correlation . (According to the literature , the jet empirical correlation predicts the system entrainment ratio ω with an error range between -10﹪and +14﹪) The theoretical prediction and the experiment shows that the ejector cooling system can enhance COP by 10﹪or more (which is 2.6﹪more than 7.4﹪recorded in the literature) in comparison with the single-compressor system at the design point of the air condition system( =+5℃) and the ice-storage condition system( = - 5℃) . In addition , the experiment successfully increases the critical condensing temperature ( )of the ejector cooling system , making the integration of the ice-storage condition system possible . To put the binary-cycle refrigeration system - the ice-storage condition system into practice , this study investigates the possibility of operation in the binary-cycle ice-storage condition system in a new coil-type ice-storage tank . This new combination is tested in a real and unsteady environment to provide as a groundwork for future studies . This study has successfully applied the ejector technology to the binary-cycle refrigeration system , and makes a significant contribution to this field of studies .

碩士
國立臺北科技大學
能源與冷凍空調工程系
107
This study proposes an ejector based on the R-404A. The earliest ejector refrigeration system uses steam as the working fluid. Compared to the application of the ejector as a heat-driven refrigeration system, there is relatively little research on the application of the ejector as an expansion device. The ejector is considered to be one of the ways to solve energy shortages and improve the efficiency of refrigeration systems. R-404A refrigerant is used in the refrigeration system of the general freezer car in the tropical regions of Southeast Asia. Since R-404A refrigerant ejectors are currently less researched, this study wanted to develop a refrigeration system ejector device suitable for R-404A refrigerant to improve the efficiency of the overall refrigeration system. This study will use the simulation software ANSYS Fluent to predict the ejector entrainment ratio and pressure recovery ratio for R-404A refrigerant. This study is focused on how will the impact be when the ejector is on the refrigeration system. It is expected to find the ejector model that close to the thermodynamic modeling result with numerical simulation. The results show that under the same conditions, the entrainment ratio of the ejector will decrease as the throat diameter (Dt) of the ejector rises, and the pressure recovery ratio will increase with the throat diameter (Dt) of the ejector rises. And compare the error of entrainment ratio and thermodynamic modeling under numerical simulation under reasonable conditions of system operation. Finally, the results are deduced from the refrigeration system capability to compare the performance differences between the ejector refrigeration system and the general refrigeration system. The results obtained here can be a good guide for developing ejector expansion refrigeration technology for practical refrigeration systems.

This experim ental project aims to describe the influencing factors in the vacuum boiling o f w ater in w ater vapour refrigeration system s Testing was conducted using a 2 kW three-stage steam je t ejector system, w ith barom etric condensers, as the com pression device. Three direct-contact evaporators were used to investigate the boiling phenom ena. T hese were : a through-flow evaporator w here heal and mass transfer rates were established for boiling m echanism s at various positions within the evaporator; a vertical cylinder where small quantities o f w ater were subjected to rapid decom pression and the effects m easured, and a sim ple channel for photographic studies o f the process. Boiling in direct-contact water vapour systems is described herein The vacuum boiling proo ss was found to be controlled by a com bination o f the w ater surface tem perature and the hydrostatic pressure gradient, these being governed by the w ater vapour flow geometry between the w ater surface and ejector suction and convective heat transfer below the boiling region. The contributions o f the various boiling regim es to the total heat transfer are discussed. Heat and mass transfer coefficients and their applicability to evaporator design are presented

During the recent years the interest of industry and scientific community in refrigeration systems working with natural fluids has considerably grown because of the more and more strict regulations regarding environmentally safe refrigerants. This thesis mainly describes a theoretical and experimental investi-gation of the jet pump refrigerator, and the application of Com-putational Fluid Dynamic (CFD) to validate the performance of the system. The present work is divided into two main parts plus introduction chapter which is devoted to literature reviews and theoretical concept of refrigeration system especially heat-powered ejector refrigeration cycle. Part I is devoted to the presentation of the results obtained with the industrial prototype developed by the University of Florence. Chapter 1 propose a detailed examina-tion of the phenomena occurring in the various ejector regions with R245fa as refrigerant. The knowledge and experience of these tests sets the basic to move on the new refrigerant, R1233zd, due to the same thermodynamic properties with the previous one and low GWP. The numerical and analytical mod-elling of the ejector validate the experimental results of the new refrigerant that is explored in Chapter 2. Part II is examined the fundamental study on water vapour con-densation inside a supersonic nozzle operated through shock tunnel. Chapter 3 is dedicated to this issue for the final goal of the Thermo Group which is substituting synthetic refrigerant with steam as the best natural and environmentally friendly fluid. Experimental data of the condensation shocks inside a nozzle and shock behavior through the tunnel was validated with the thermodynamic theoretical and recorded photos.

In this study a semi-passive pulse thermal loop (PTL) was designed and experimentally validated. It provides improved heat transfer over passive systems such as the loop heat pipe in the moderate to high heat flux range and can be a sustainable alternative to active systems as it does not require an electric pump. This work details the components of the engineering prototype and characterizes their performance through the application of compressible and two-phase flow theory. A custom LabVIEW application was utilized for data acquisition and control. During operation with refrigerant R-134a the system was shown to be robust under a range of heat loads from 100 W to 800 W. Operation was achieved with driving pressure differentials ranging from 3 bar to 12 bar and pulse frequencies ranging from 0.42 Hz to 0.08 Hz. A smaller pressure differential and an increased pulse frequency results in improved heat transfer at the boilers. An evolution of the PTL is proposed that incorporates a novel, ejector-based pump-free refrigeration system. The design of the pulse refrigeration system (PRS) features valves at the outlet of two PTL-like boilers that are alternately actuated to direct pulses of refrigerant through an ejector. This is intended to entrain and raise the pressure of a secondary stream of refrigerant from the cooling loop, thereby replacing the compressor in a conventional vapor-compression cycle. The PRS is therefore characterized by transient flow through the ejector. An experimental prototype has been constructed which is able to operate as a conventional PTL when the cooling section is bypassed, although full operation of the refrigeration loop remains to be demonstrated. The design of the ejector is carried out using a one-dimensional model implemented in MATLAB that accounts for compressibility effects with NIST REFPROP vapor data sub-routines. The model enables the analysis of ejector performance in response to a transient pressure wave at the primary inlet. The high driving pressures provided by the PTL permit operation in a micro-gravity environment with minimal power consumption. Like the PTL, the proposed PRS is therefore well suited to terrestrial and aerospace applications where it could be driven by waste heat from electronics or solar thermal energy. As a novel semi-passive thermal management system, it will require complex control of the valves. Further analysis of the transient thermodynamic cycle is necessary in order to characterize and effect successful operation of the PRS.
Thesis (M.Sc.)-University of KwaZulu-Natal, Durban, 2013.

碩士
國立成功大學
工程科學系
102
In order to establish a generalizable small-capacity ejector refrigerating system, this study aims at optimizing the entrainment rate of the system by re-designing and fabricating the core ejector. The process of designing an optimized core ejector has been constructed, and the design verification test was also employed. The design processes are as follows. First, construct a simulation model by CFD. Second, use the ejector principle and choke effect to validate the model. Third, design a nozzle throat, a nozzle exit, and cross-sectional area via the one-dimension ejector theory. Furthermore, the Taguchi method (L9 orthogonal array) and two dimensional numerical simulations were employed to optimize the length and angle. Finally, verify the gas entrainment rate under different pressure and temperature settings. In the pilot run, I used two acrylic sheets to fabricate an ejector, and selected Novec 649 as the working fluid. Through the Taguchi designing process, the suction efficiency of the analog part was increased by 16.2%, and the interaction was increased by 4.7%, while the designing process can be the basis of establishing plate type ejectors. In the verification section, the data can not be effectively verified because the pressure vessels cannot maintain a stable pressure for sufficient time; therefore, the measured data is in transient form, which can only prove the suction effect.

At a first glance, the nature of this thesis may be considered somehow “sui generis”. By the author’s choice, its structure is not conceived as a report of the doctoral activities, but rather as a guide that can help the reader to appreciate the phenomena and critical points arising in the context of supersonic ejector research. The rationale for this choice has progressively appeared clear in the course of the doctoral period. During the last two decades, the literature on ejector refrigeration has become mostly self-referential. However, deep understanding of the physics governing both single- and two-phase high-speed phenomena requires a strong effort in terms of “broadening the research horizons”. Work of high relevance to supersonic ejectors can be found in the fields of aerospace propulsion, wind tunnel, steam turbines and turbomachinery cavitation research areas. Consequently, the character of this dissertation is shaped by the firm belief that major advancements may only come by joining together the knowledge from all these different disciplines. Under this perspective, the main results of this three-year doctoral research should not only be considered the achievements in ejector chiller performances or the developments of analytical and numerical tools, but also the opening-up to different research fields and the elaboration of a logical structure where all the themes are rationally presented.

碩士
國立臺灣大學
機械工程研究所
81
The objective of present study is to investigate the performance characteristics of ejectors in the jet refrigeration system experimentally. By changing the sizes of the ejectors, the effects of changes of sizes are understood, compared and applied in the future design. The comparison of pressure distribution in the ejectors between the results calculated from the literatures and the experimental data shows good agreement, and clearly demonstrates the relation- ships of the flow field, the shock positions and the back pressure in the ejectors. By analyzing the suction phenomena at different primary, secondary and mixed pressure, the mass flow rate ratios of choking and the critical pressure are determined. Therefore, the ejectors with different working fluid and sizes can be compared. The working fluid used are R114 and R113, the performance of ejectors is compared. The performance curves of R113 are compared with literatures in order to study the effects of different sizes of ejectors. The ejector size variables are defined as the throat area and position of the primary nozzle, and the lengthes of the throat region and the diffuser of the ejectors for the theoreti- cal analysis. By the effects and performance of changes of sizes, the best size of ejectors is approached. In the mean time, to upgrade the efficiency of the jet refrigeration system, the per- formance curves of the ejectors can be used to match in multi- stage and multi-effect systems.

碩士
國立臺灣大學
機械工程研究所
81
Using two-dimensional axisymmetric flow model and MacCormack TVD scheme, the flow fields of freon ejectors are obtained. The comparison of pressure distribution in the ejectors between experiment data and the results caculated by computer shows good agreement. Therefore, this analytic model is adaptable to evaluate ejector flow. The performance curves of ejectors with various geometry are constructed from the calculated results. Through these graphs the influence of main geometric parameter on ejector performance is shown, and the features of high performance ejector are summarized. Finally, further analysis is made to obtain the optimum geometry of an ejector with the best performance in order to improve the efficiency of the jet refrigeration system. In order to predict the performance of the ejectors more correctly, it's necessary to change the analytic model. Two new models are used in this paper, the first one is real gas model and the second one is viscous model. The real gas model can promit more exactly performance, but the viscous effect can't make a conculsion untill more computations are done.