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Atualmente, a análise de desempenho de sistemas de refrigeração domésticos é realizada através de ensaios experimentais normalizados. Durante esses experimentos, diversas variáveis como pressões de trabalho, temperaturas em diversos pontos do sistema, corrente elétrica e potência consumida, são monitoradas. Porém, em muitos casos são necessárias mais de 24 horas para execução de um teste experimental (e.g., teste abaixamento de temperatura). Tendo em vista o tempo despendido nestes testes, propõe-se no presente trabalho um modelo matemático semi-empírico capaz de predizer o comportamento das variáveis do sistema testado e, com isso, antecipar o final do ensaio. O modelo, desenvolvido através das leis de conservação da massa e da energia, apresenta parâmetros que são ajustados a partir de informações experimentais obtidas durante a execução do próprio teste. Após a inicialização do ensaio, a cada período de tempo prédeterminado, os dados medidos são utilizados para determinar os parâmetros empíricos do modelo. Obtidas as constantes, simula-se o comportamento das principais variáveis do sistema de refrigeração até a condição de regime permanente. Com isso, o teste experimental pode ser finalizado com antecedência. O modelo desenvolvido é capaz de prever com boa precisão, a partir de duas horas de teste, a variação da vazão mássica e da pressão de sucção (com diferenças da ordem de 10% em regime permanente quando comparadas às variáveis experimentais), da pressão de condensação (com diferença da ordem de 5%) e da temperatura da parede do condensador (diferença da ordem de 2°C). / The performance of household refrigeration systems are usually evaluated through experimental tests carried at in temperature and humidity controlled chambers. During the tests, the discharge and suction pressures, the temperature in several system positions, and the compressor power are measured. These tests are expensive and time-demanding, e.g., a single pull-down test can take more than 24 hours to be performed. Although the mathematical models have been proposed for decades as an alternative to the experiments, they are not sufficiently reliable to substitute completely the tests. Therefore, the current work proposes a semiempirical mathematical model to predict the system performance with the purpose of reducing the test time instead of replacing it. The model is based on the mass and energy conservation equations in which the constant parameters, such as conductance and capacitances, are calibrated from previous measured values of temperature and pressure. As soon as the parameters are obtained, a simulation is performed to forecast future values of temperature, pressure and compressor power and therefore, to anticipate the end of the test. Calibrations and simulations can be continuously performed as the test evolves. Preliminary results show that steadystate values of discharge and suction pressures can be predicted within error bands of 5 and 10%, respectively, after only two hours of a pull-down test being performed.

L'application de la brumisation d'eau dans l'air en amont des condenseurs à air permet de réduire la consommation d'énergie des machines frigorifiques. L'intérêt de la brumisation d'eau réside dans l'abaissement de température de l'air obtenu par évaporation du spray. L'optimisation de cette méthode permet d'obtenir une efficacité se rapprochant de celle des systèmes évaporatifs tout en supprimant le risque de développement et de transports de bactéries tel que les légionelles. Le problème est abordé ici de deux manières : la première théorique et numérique (utilisation du code MIRABELLES et développement d'un modèle d'échange thermique air-goutte/condenseur) et la seconde expérimentale (mise en place d'un pilote et d'une pompe à chaleur). A l'aide du code MIRABELLES, on a déterminé les caractéristiques d'un spray évoluant dans un milieu ambiant donné et son impact sur l'abaissement de température de l'air. Les données numériques obtenues ont guidé le choix du matériel et la stratégie de brumisation à appliquer expérimentalement. Les mesures effectuées sur le pilote montrent l'amélioration des échanges thermiques entre l'air et le condenseur grâce à la brumisation et quantifient également l'influence de l'impact des gouttes avec les ailettes du condenseur. L'application de la brumisation sur la pompe à chaleur a mis en avant les gains sur la consommation électrique et l'augmentation de la production frigorifique. La comparaison de l'ensemble des résultats expérimentaux avec les résultats numériques a permis de valider le modèle thermique simulant les échanges entre un air chargé de gouttelette d'eau et un condenseur. / The application of water mist in the air upstream of the air-cooled condensers can reduce the energy consumption of refrigeration. The advantage of water mist is the lowering of air temperature obtained by evaporation of the spray. The optimization of this method provides an efficiency approaching that of evaporative systems while eliminating the risk of development and transport of bacteria such as legionella. The problem is addressed here in two ways: the first theoretical and numerical (using the code MIRABELLES and development of a model heat exchange air-droplet/condenser) and the second experiment (establishment of a pilot and a heat pump). Using the code MIRABELLES, it was determined the characteristics of a spray operating in a given environment and its impact on the lowering of air temperature. The digital data obtained have guided the choice of materials and the experimental using condition of water mist. Measurements on the pilot show the improvement of heat exchange between air and the condenser by means of the mist and quantify the influence of the impact drops/condenser fins. The application of the mist on the heat pump has highlighted the gains in power consumption and the increase in refrigeration. The comparison of all experimental results with the numerical results has validated the thermal model simulating the interaction between a droplet-laden air and water condenser.

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 %.

Dynamic finite element modeling and analysis of a refrigeration compressor was investigated as part of a noise emission study. Natural frequencies and normal mode shapes were calculated for the major structural components of the compressor. The components were later combined to form a model of the compressor assembly which was subsequently solved for its dynamic properties. Model development included coordination with test data for verification and revision to improve model prediction accuracy. Considerable efforts were made to accurately represent the hermetic shell which presents several inherent modeling difficulties due to its geometry and other characteristics which result from a deep drawn manufacturing process. The importance of physical simplifications such as geometry representation, thickness variation, attachments, the welded seam, and residual stresses were established. In addition, theoretical limitations of the finite element method were addressed as a cause for analysis-test discrepancies. Housing models developed were found to agree within 12% of experimental natural frequencies up to 1100 Hz. Compatibility of analytical normal modes with resonant dwell experimental deflection shapes was considered. Analytical forced vibration response showed situations when the deflected shapes can be a superposition of modes rather than the pure mode shape. Analytical simulation of the test setup improved the agreement of analysis and test data. Additional components modeled include the internal compressor mechanism and its supports. Analysis showed that interactions with the internal components, particularly resonances within the suspension springs, are important for a valid representation of the compressor assembly. Resonances within the internal suspension components more than double or nearly triple the number of resonance frequencies in the compressor assembly. / Master of Science

The second law is still rarely used as a design decision tool. However, information obtained from second law analysis is valuable in the design process for thermodynamic systems. This investigation reviews the past development of second law analysis. A clear, operational vocabulary is established. Then two examples of microcomputer-based design procedures are presented. The first is a second law analysis of refrigeration cycles. As a part of this example, the correlation of physical property data by simple methods is demonstrated. The second example is a second law analysis of gas turbine systems. The salient point is that all of this can be done on a microcomputer, and consequently is readily available to any engineer. / M.S.

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

[pt] O presente trabalho trata da caracterização e simulação de compressores alternativos dos tipos automotivo, hermético e semi-hermético, motivado pela necessidade de estudo de novos refrigerantes, com menor impacto ambiental, isto é, sem potencial de destruição da camada de ozônio e baixo potencial de efeito estufa. O estudo apresenta uma metodologia para a modelagem, mediante a qual, dependendo do tipo de compressor, este é dividido nos seguintes volumes de controle: mufla de sucção, câmara de sucção, cilindro de compressão, câmara de descarga, linha de descarga, motor elétrico, carcaça do compressor, massa metálica interna e gás escoando no interior da carcaça. Equações fundamentais de conservação, de troca de calor, de queda de pressão e de propriedades termofísicas do refrigerante são aplicadas a cada volume de controle. Buscando o desenvolvimento de modelos simples, porem, ainda capazes de identificar o desempenho do compressor operando com diferentes refrigerantes, optou-se pelo desenvolvimento de modelos semi-empíricos, determinando-se coeficientes empíricos, característicos do compressor e independentes do refrigerante ou das condições de operação. Foram efetuados ensaios calorimétricos normalizados em duas instalações laboratoriais existentes para os compressores hermético e semihermético. Para o compressor hermético foi utilizado o HFC-134a como referência e foram testados os refrigerantes HFO-1234yf e HFO-1234ze(E) e a mistura HDR-17. Para o compressor semi-hermético foi utilizada uma instalação de refrigeração comercial instrumentada do tipo ar-ar, operando com a mistura R404A (referência), tendo sido testados oito novos fluidos. Para o compressor automotivo foram utilizados dados experimentais do refrigerante HFO-1234yf, existentes na literatura. A caracterização dos compressores alternativos foi bem sucedida na medida em que os parâmetros empíricos determinados a partir de diferentes refrigerantes mostraram-se com valores suficientemente próximos. Nos testes experimentais foram identificados fluidos refrigerantes com desempenho maior e baixo potencial de aquecimento global. Atingiu-se, com a modelagem, o desenvolvimento de uma ferramenta computacional capaz de predizer as condições de operação de compressores alternativos operando com novos refrigerantes, a partir do modelo baseado em parâmetros empíricos obtidos de testes experimentais com refrigerantes convencionais, de fácil obtenção. O método de gradiente reduzido generalizado (GRG) foi utilizado na solução do sistema de equações não lineares, para a caracterização dos compressores alternativos. O modelo de simulação foi desenvolvido na linguagem Fortran. As propriedades termodinâmicas dos fluidos refrigerantes foram obtidas pelo pacote computacional REFPROP (NIST Standard Reference Database 23, Version 8.0). Os valores previstos pela simulação apresentaram boa concordância com os resultados experimentais. / [en] This work shows a methodology for calculating the characteristic parameters of an open, a hermetic and a semi-hermetic reciprocating compressor. This study was motivated by the need to study new refrigerants with lower environmental impact with reduced global warming potential and zero ozone depletion potential. The compressor is divided in control volumes: suction muffler, suction chamber, compressor cylinder, discharge chamber, discharge line, electric motor, flowing gas through the compressor, compressor shell and inner metallic mass. Fundamental equations of conservation, heat exchange, pressure drop and thermophysical properties of the refrigerant are applied to each control volume. A semi-empirical model and standard calorimetric tests are used to obtain empirical parameters independents of refrigerant and operating conditions. For the hermetic compressor was used as reference the refrigerant HFC-134a and tested refrigerants HFO-1234yf, HFO-1234ze(E) and a mixture HDR-17. For the semihermetic compressor, an instrumented commercial refrigeration system operating with the mixture R404A was used as reference. In this system eight new fluids have been tested. For the automotive compressor experimental data from refrigerant HFO-1234yf obtained from literature were used. The characterization of the reciprocating compressors has been successful because the empirical parameters determined from different refrigerants proved to have sufficiently close values. A computational tool, able to predict the operating conditions of reciprocating compressors (open automotive, hermetic and semihermetic), working with new and untested refrigerants, was developed from the simulation models. The generalized reduced gradient (GRG) method was implemented in order to obtain a numerical solution for the characteristic parameters and the simulation computer program was developed in FORTRAN. Refrigerant properties were calculated using the software REFPROP version 8.0, developed by NIST, U.S.A.

Le contrôle précis des boucles de régulation existantes sur une machine frigorifique est essentiel à son bon fonctionnement. Il existe plusieurs méthodes de régulation parmi lesquelles on retrouve la plus ancienne et la plus connue : la commande PID (Proportionnelle, Intégrale et Dérivée). Malgré la simplicité apparente des relations mises en jeu, le régulateur PID est assez délicat à ajuster sur des processus présentant de fortes perturbations comme les installations frigorifiques. L’objectif du travail présenté est de mettre en œuvre une commande prédictive fonctionnelle (PFC) afin de réguler la surchauffe en sortie d’évaporateur, la pression de condensation et la puissance frigorifique sur une installation munie d’un compresseur à vitesse variable. L’utilisation d’une commande PFC nécessite de réaliser une prédiction de l’évolution à venir de la sortie du procédé. C’est un modèle interne qui fait office de modèle de connaissance. En assimilant le procédé à un système du premier ordre, la mise en œuvre de cette commande ne nécessite que trois paramètres : un gain statique, une constante de temps et un retard pur. L’originalité de la démarche développée a consisté à réaliser une modélisation physique de la machine afin de déterminer les paramètres précédents par une approche macroscopique. Une bibliothèque de modèles physiques a été élaborée permettant de simuler le comportement de différents évaporateurs, condenseurs, compresseurs ou vannes de détente. La commande développée a ensuite été implantée sur un automate industriel programmable et des expériences ont été réalisées sur deux machines différentes. La première est constituée de deux échangeurs à tubes et calandre et d’un compresseur à pistons, alors que la seconde comprend un évaporateur à plaques, une batterie à ailettes et un compresseur à vis. / An accurate control of fluids flow is essential in any refrigeration system. As the conventional Proportional-Integral-Derivative (PID) control with invariable parameters can lead to unsatisfactory performance because of the variation of refrigeration unit parameters under disturbances, the aim of the work presented here is to develop a method for using Predictive Functional Control (PFC) to regulate the evaporator superheat, the condensing pressure and the cooling capacity on a variable-speed refrigeration system. Using a predictive controller requires to predic future change at the output of the process. This prediction is based on an internal model used as known model. By assuming that the behaviour of refrigerating machine heat exchangers can be represented using a first-order model, the implementation of PFC requires only three parameters: gain, time constant and time delay. In order to determine these parameters an original method has been developed which is based on the physical modelling of the machine. Physical models for different types of evaporator, condenser, compressor or expansion valve have been established to quantify heat transfer and refrigerant flow rate in these components. The control system created has been incorporated into an industrial programmable logic controller and used for experiments on two different refrigerating machines: the first one is composed of two shell and tube heat exchangers and a reciprocating compressor, whereas the second one is composed of a plate evaporator, a finned-tube condenser and a screw compressor. The tests performed show that PFC controller succeed in maintaining a precise chilled liquid temperature.

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).