Diese Arbeit umfasst experimentelle und theoretische Untersuchungen zum Verständnis des Siedenverhaltens von Kältemittel R-134a an glatten und porösen Oberflächen und beschäftigt sich mit der Herausforderung der Optimierung geeigneter Oberflächenstrukturen zur Steigerung des Wärmeübergangskoeffizienten. Die in der Einleitung vorgestellte Anforderung an immer kompaktere Wärmeübertrager wird durch poröse Schichten, welche mittels Vakuum-Plasma-Beschichtung hergestellt werden, Rechnung getragen. Innerhalb des in der Praxis üblichen Bereichs des Siededrucks und der Wärmstromdichten werden im Vergleich zum Stand der Technik beim Sieden von R-134a am Einzelrohr und am Rohrbündel für optimierte, poröse Wärmeübertrageroberflächen deutlich verbesserte Wärmeübergangskoeffizienten nachgewiesen. Zum besseren Verständnis des gesteigerten Wärmeübergangs beim Sieden liefert diese Arbeit im Weiteren einen Einblick in die Blasenbildung an unterschiedlichen Oberflächenstrukturen mittels optischer Analyse. Detaillierte Angaben zu Grundlagen des Siedens und der Blasenbildung sowie zur verwendeten Beschichtungstechnik sind im Anhang ab Seite 185 wiedergegeben. / This work includes experiments and theoretical analyses on boiling of refrigerant R-134a on smooth and porous surfaces, and deals with the challenge of optimizing suitable surface structures to increase the heat transfer coefficient. The targets presented in the introduction with respect to increasing compact heat transfer are taken into account by different porous layers produced by means of vacuum plasma coating. Within the usual range of pressure and heat flux density the heat transfer coefficient was found to be significantly improved, compared to the state of the art for single tubes and tube bundles by means of optimized, porous heat transfer surfaces. For a better understanding of the increased heat transfer for boiling on porous surfaces, this work provides an insight into the formation of bubbles on different surfaces via optical analysis.

abstract: Recent literature indicates potential benefits in microchannel cooling if an inlet orifice is used to suppress pressure oscillations that develop under two-phase conditions. This study investigates the costs and benefits of using an adjustable microchannel inlet orifice. The focus is on orifice effect during steady-state boiling and critical heat flux (CHF) in the channels using R134a in a pumped refrigerant loop (PRL). To change orifice size, a dam controlled with a micrometer was placed in front of 31 parallel microchannels. Each channel had a hydraulic diameter of 0.235 mm and a length of 1.33 cm. For steady state two-phase conditions, mass fluxes of 300 kg m-2 s-1 and 600 kg m-2 s-1were investigated. For orifice sizes with a hydraulic diameter to unrestricted hydraulic diameter (Dh:Dh,ur) ratio less than 35 percent, oscillations were reduced and wall temperatures fell up to 1.5 °C. Critical heat flux data were obtained for 7 orifice sizes with mass fluxes from 186 kg m-2 s-1 to 847 kg m-2 s-1. For all mass fluxes and inlet conditions tested, CHF values for a Dh:Dh,ur ratio of 1.8 percent became increasingly lower (up to 37 W cm-2 less) than those obtained with larger orifices. An optimum orifice size with Dh:Dh,ur of 35 percent emerged, offering up to 5 W cm-2 increase in CHF over unrestricted conditions at the highest mass flux tested, 847 kg m-2 s-1. These improvements in cooling ability with inlet orifices in place under both steady-state and impending CHF conditions are modest, leading to the conclusion that inlet orifices are only mildly effective at improving heat transfer coefficients. Stability of the PRL used for experimentation was also studied and improved. A vapor compression cycle's (VCC) proportional, integral, and derivative controller was found to adversely affect stability within the PRL and cause premature CHF. Replacing the VCC with an ice water heat sink maintained steady pumped loop system pressures and mass flow rates. The ice water heat sink was shown to have energy cost savings over the use of a directly coupled VCC for removing heat from the PRL. / Dissertation/Thesis / Ph.D. Mechanical Engineering 2012

I Kraftringens geotermianläggning cirkulerar köldmediet R134a som är en fluorerande växthusgas med relativt hög GWP (Global Warming Potential). När den nya F-gasförordningen trädde i kraft 2015 resulterade det i att priserna ökade markant på de köldmedierna med hög GWP. Kraftringen vill därför undersöka vilka konverteringsalternativ det finns till R134a, dels för att minska på kostnaderna men också för att minimera verksamhetens klimatpåverkan.   Olika köldmedier har jämförts med avseende på hur de påverkar anläggningens prestanda. Det har också undersökts vilka köldmedier som ligger inom rätt tryck med hänsyn till vad kompressorn och anläggningens rör är konstruerade för. Undersökningen har gjorts genom teoretiska beräkningar som är grundade på uppmätta värden från när värmepumparna går på maximal belastning. Vid urvalet av köldmedier har det utgåtts från samma förångningstemperatur och kondenseringstemperatur som vid mätningen.   I undersökningen fann vi två köldmedier, R152a och R450A, som skulle fungera som konverteringsalternativ i avseende att de ligger inom rätt tryck och temperatur. Båda alternativen har lägre GWP och en högre värmefaktor än nuvarande köldmediet R134a. Dock är R152a brandfarligt och har högre säkerhetsklass än de två andra medierna. / In the geothermal plant of 'Kraftringen' the refrigerant R134a, which is a fluorescent greenhouse gas with relatively high GWP (Global Warming Potential), circulates. When the new F-Gas regulation was introduced in 2015, it resulted in a significant price increase for high-pressure GWP refrigerants. 'Kraftringen' therefore wants to investigate which conversion options there are for R134a, partly to reduce costs, but also to minimize the climate impact of the business.   Different refrigerants have been compared with regard as to how they affect the plants performance. Moreover, it has been investigated which refrigerants are in the correct pressure range with regard to what the plants compressor and pipes are designed for. The survey has been done by performing theoretical calculations based on measured values from when the heat pumps operate at maximum load. In the refrigerant selection process, the same evaporation and condensation temperature as during the measuring was used.   In the study we found two refrigerants, R152a and R450A, which would serve as conversion options in terms of them being in the correct pressure and temperature range. Both options have lower GWP and a higher heat factor than the current refrigerant R134a. However, R152a is flammable and has a higher degree of safety than the other two refrigerants.

La compréhension des écoulements multiphasiques dans les évaporateurs à mini-canaux est primordiale pour la performance des boucles de climatisation dans le secteur automobile notamment. Cette thèse s’est principalement intéressée à l’écoulement d’entrée de tels évaporateurs ainsi qu’à la répartition des phases dans les mini-canaux. Dans un premier temps, l’écoulement adiabatique diphasique en entrée d’évaporateur a été étudié. Un dispositif expérimental transparent, respectant au mieux la géométrie d’entrée de l’évaporateur, a été réalisé afin de reproduire l’écoulement diphasique d’entrée en eau-air mais en respectant les régimes d’écoulement rencontrés avec du R134a. Plusieurs techniques de caractérisation ont été mises en œuvre (visualisation, conductimétrie, tube de Pitot et prises de pression) afin de quantifier les pertes de pression, les épaisseurs de film et les vitesses du gaz dans un régime principalement annulaire. Suivant le même principe, un autre module en acier-inox a été développé pour caractériser l’écoulement directement en entrée d’évaporateur avec du réfrigérant R134a. Dans un second temps, nous avons étendu l’étude au cas d’un évaporateur compact à mini-canaux. Dans deux situations adiabatiques : monophasique (eau) et diphasique (eau-air), les pertes de pression, la répartition des phases le long de l’évaporateur et le régime d’écoulement dans les mini-canaux ont été étudiés sur un échangeur fabriqué en polycarbonate dont la géométrie s’approche au mieux de celle d’un échangeur réel. Les nombreux résultats ainsi obtenus constituent une base de données conséquente utile à la simulation numérique de ce type d'écoulements diphasiques / Understanding of multiphase flows in mini-channel evaporators is essential for the performance of air-conditioning systems, particularly in automotive sector. This thesis is mainly interested in behavior of inlet flow and phase distribution in the mini-channels. Initially, an adiabatic two-phase flow at the evaporator's inlet was studied. A transparent experimental apparatus with the same geometry as an evaporator's inlet has been designed. This test section helped us to reproduce the same flow regimes with air-water as flow regimes encountered with R134a in an evaporator. Several characterization techniques were used (visualization, conductance probes, Pitot tube and pressure taps) to determine pressure losses, liquid film thickness and gas velocity in a predominantly annular flow regime. Following the same principle, another experimental facility in stainless steel was developed to directly characterize the R134a flow at the evaporator's inlet. Finally, we have extended the study to the case of a compact evaporator in two adiabatic situations: single-phase (water) and two-phase (air-water). Pressure losses, phase distribution along the evaporator and flow regime in mini-channels were studied on an evaporator made of transparent materials (polycarbonate) with a close geometry to that of a real evaporator. Numerous results were obtained to provide a consistent database that would be useful for numerical simulation of this type of two-phase flows

Compressed air is widely used particularly in industry. In order to increase the quality of the process and lifetime of the machine, the compressed air should be dried. Therefore the air is used after compression and drying processes. The most commonly used machines that do this process are called &ldquo / Refrigerant Air Dryers&rdquo / . These air dryers are designed to cool and dehumidify the moist air. The process of decreasing temperature is carried out by a refrigerant, R134a. Unlike design conditions, dryers are working in variable loads (variable compressed air flow rates). An experimental setup is prepared for analyzing the variance on the machine and the performance under these variable loads. This thesis includes the design, preparation and the modification of the refrigeration experimental setup for refrigerant air dryers. The setup is tested under three different conditions and the results are compared.

This thesis investigates the refrigerant (R-134a) flow in a minitube experimentally. The small scale heat transfer is a relatively new research area and has been in favor since the end of 1970&rsquo / s. Refrigerant flow in mini- and microscale media is a potential enhancement factor for refrigeration technology in the future. For the forthcoming developments and progresses, experimental studies are invaluable in terms of having an insight and contributing to the establishment of infrastructure in the field in addition to leading the numerical and theoretical approaches. The studies in the literature show that low mass flow rate and constant wall temperature approach in minitubes and minichannels were not among the main areas of interest. Therefore, an experimental set-up was prepared in order to perform experiments of two-phase refrigerant flow in a 1.65 mm diameter copper minitube with the constant wall temperature approach. The design, preparation, and modifications of the experimental set-up are explained in this thesis. Two-phase flow and quality arrangements were done by pre-heating the refrigerant at saturation pressure and the constant wall temperature was achieved by a secondary cycle with water and ethylene glycol mixture as the working fluid. The heat transfer coefficient and the pressure drop for the two-phase flow with varying quality values and saturation temperatures of the refrigerant were calculated and compared with the results available in literature.

Two-phase heat transfer in mini/micro-channels is capable of meeting the high cooling demands of modern high heat flux applications. The phase change process ensures better temperature uniformity and control for local hot spots. Furthermore, these compact channels could be helpful in reducing the required charge and material inventories.Environmental concerns—mainly ozone depletion and global warming—have instigated a search for new alternatives in refrigeration industry. While new compounds are being developed to address stringent legislative demands, natural alternatives are also coming into prominence. A limited number of investigators have reported on thermal performance of such alternatives. The current study is therefore focused on saturated flow boiling heat transfer, pressure drop and dryout characteristics for three low global warming potential (GWP) refrigerants (R152a, R600a and R1234yf) in a vertical mini-channel.In this study experiments were carried out by uniformly heating a test section (stainless steel tube with 1.60 mm inside diameter and 245 mm heated length) at 27 and 32 oC saturation temperature with 50-500 kg/m2s mass velocities. The effects of various parameters of interest (like heat flux, mass flux, system pressure, vapor quality, operating media) on flow boiling heat transfer, frictional pressure drop and dryout characteristics were recorded. R134a, which has been widely used in several applications, is utilized as a reference case for comparison of thermal performance in this study.Experimental results for saturated boiling heat transfer showed strong influence of heat flux and system pressure with insignificant contributions from mass flux and vapor quality. Two phase frictional pressure drop increased with mass flux, vapor quality and with reduced operating pressure. The dryout heat flux remained unaffected with variation in saturation temperature, critical vapor quality in most cases was about 85%. The experimental results (boiling heat transfer, two-phase pressure drop and dryout heat flux) were compared with well-known macro and micro-scale correlations from the literature. / <p>QC 20141124</p>

Submitted by Automa??o e Estat?stica (sst@bczm.ufrn.br) on 2016-06-03T23:57:09Z No. of bitstreams: 1 BrunoDanielAlvesDoNascimento_DISSERT.pdf: 2697025 bytes, checksum: 8a93610daedd22a97964b68f4766a99b (MD5) / Approved for entry into archive by Arlan Eloi Leite Silva (eloihistoriador@yahoo.com.br) on 2016-06-07T20:32:39Z (GMT) No. of bitstreams: 1 BrunoDanielAlvesDoNascimento_DISSERT.pdf: 2697025 bytes, checksum: 8a93610daedd22a97964b68f4766a99b (MD5) / Made available in DSpace on 2016-06-07T20:32:39Z (GMT). No. of bitstreams: 1 BrunoDanielAlvesDoNascimento_DISSERT.pdf: 2697025 bytes, checksum: 8a93610daedd22a97964b68f4766a99b (MD5) Previous issue date: 2015-06-19 / A redu??o no consumo de energia ? o principal requisito a ser satisfeito em sistemas de refrigera??o e condicionamento de ar por compress?o mec?nica de vapor. Em sistemas automotivos essa realidade n?o ? diferente. A an?lise t?rmica nesses sistemas ? de fundamental import?ncia para um bom desempenho do ar-condicionado veicular. Este trabalho tem o objetivo de avaliar as condi??es de utiliza??o do g?s refrigerante R134a (utilizado em ve?culos automotivos), variando a velocidade no eletroventilador do evaporador, comparando a performance do mesmo com o g?s R437a, submetido as mesmas condi??es. Todos os testes foram realizados na unidade de climatiza??o automotiva ATR600, que simula as condi??es t?rmicas do sistema e do habit?culo do ve?culo. O equipamento est? instrumentado para aquisi??o dos dados de temperatura, press?es de alta e baixa e pot?ncia el?trica consumida, para determina??o do coeficiente de performance do ciclo. O sistema foi ensaiado nas rota??es de 800, 1600 e 2400 RPM com carga constante de R134a / R437a, recomendada pelo fabricante. Os resultados mostram que a melhor performance do sistema ocorre na rota??o de 800 RPM para ambos os refrigerantes. / The reduction in energy consumption is the main requirement to be satisfied in refrigeration and air conditioning by mechanical vapor compression system. In automotive system isn?t different. Thermal analyses in these systems are crucial for a better performance in automotive air conditioner. This work aims to evaluate the conditions of use of R134A refrigerant (used in vehicles) and compare with R437A (alternative refrigerant), varying the speed of the electric fan in the evaporator. All tests were performed in automotive air conditioning unit ATR600, simulating the thermal conditions of the system. The equipment is instrumented for data acquisition temperature, condensation and evaporation pressures and electrical power consumed to determine the coefficient of performance of the cycle. The system was tested under rotations of 800, 1600 and 2400 rpm with constant load of R- 134a. It occurred with the same conditions with R437A. Both recommended by the manufacturer. The results show that the best system performance occurs in the rotation of 800 RPM for both refrigerants.

Submitted by Automa??o e Estat?stica (sst@bczm.ufrn.br) on 2016-06-03T23:57:10Z No. of bitstreams: 1 LuizHenriquePinheiroDeLima_DISSERT.pdf: 2055438 bytes, checksum: a2892c7c538f0c5ba015c03438289d3f (MD5) / Approved for entry into archive by Arlan Eloi Leite Silva (eloihistoriador@yahoo.com.br) on 2016-06-07T21:59:39Z (GMT) No. of bitstreams: 1 LuizHenriquePinheiroDeLima_DISSERT.pdf: 2055438 bytes, checksum: a2892c7c538f0c5ba015c03438289d3f (MD5) / Made available in DSpace on 2016-06-07T21:59:39Z (GMT). No. of bitstreams: 1 LuizHenriquePinheiroDeLima_DISSERT.pdf: 2055438 bytes, checksum: a2892c7c538f0c5ba015c03438289d3f (MD5) Previous issue date: 2015-09-11 / Os refrigeradores de uso dom?stico s?o equipamentos que apresentam uma parcela significativa do consumo de energia el?trica nas resid?ncias brasileiras. O uso destes equipamentos com baixa efici?ncia energ?tica contribui para o aumento do consumo de energia. A efici?ncia energ?tica de um refrigerador ? fun??o da intera??o o fluido refrigerante e os componentes do ciclo termodin?mico. Altera??es na carga e/ou natureza do refrigerante podem modificar as press?es de condensa??o e/ou evapora??o. A capacidade volum?trica do compressor, a vaz?o m?ssica de refrigerante e a pot?ncia de compress?o s?o par?metros dependentes das press?es de evapora??o e condensa??o. Assim, os dispositivos de expans?o exercem um papel importante no equil?brio destas press?es, sendo fundamentais para o melhor desempenho do ciclo de refrigera??o. A presente pesquisa experimental tem como prop?sito investigar a sensibilidade dos par?metros de desempenho de um refrigerador dom?stico operando com R134a e em diferentes press?es de evapora??o. Para tanto, um refrigerador de pequeno porte foi instrumentado com sensores de temperatura, de press?o e de outras vari?veis de interesse, instalados ao longo do circuito frigor?fico, de modo a permitir o mapeamento t?rmico e a avalia??o dos par?metros de desempenho do equipamento. A varia??o da perda de carga no fluido refrigerante resultante da atua??o da v?lvula de expans?o de ajuste microm?trico modifica a temperatura de evapora??o, influenciando sensivelmente os par?metros de desempenho do ciclo termodin?mico de refrigera??o. / Household refrigerators are equipments that represent a significant portion on the eletricity consumption of Brazilian homes. The use of these devices with low energy efficiency contributes to increase the energy consumption. The energy efficiency of a refrigerator is a function of the interaction between the coolant fluid and the components of the thermodynamic cycle. Changes in load and/or nature of the coolant may modify the condensing and/or evaporation pressures. The volumetric capacity of the compressor, the mass flow of coolant and the compression power are dependent parameters of the condensation and evaporation pressures. Thus, the expansion devices exert an importante role in the balance of these pressures, being fundamental for the better performance of the refrigeration cycle. This experimental research aims to investigate the sensitivity of the performance parameters of a household refrigerator operating with R134a and at different evaporation pressures. Therefore, a small refrigerator was instrumented with temperature, pressure sensors and other variables of interest, installed along the cooling circuit, in order to allow the thermal mapping and the evaluation of the equipment performance parameters. The variation of pressure loss in the coolant fluid resulting from the operation of the expansion valve with micrometric adjustment that modifies the evaporation temperature, influencing significantly the performance parameters of the thermodynamic refrigeration cycle.

Este estudo avalia o desempenho de um sistema em cascata em operação subcrítica utilizando o par R134a/R744, como uma opção a sistemas convencionais em supermercados que normalmente utilizam R404A ou R22. O aparato experimental consiste em um compressor alternativo de velocidade variável para R744 e uma válvula de expansão eletrônica que promove a evaporação direta do CO2 dentro de uma câmara fria (2,3m x 2,6m x 2,5m) para manter a temperatura interna do ar estável. O ciclo de alta temperatura consiste em um compressor alternativo para R134a, uma válvula de expansão eletrônica, e um condensador a ar. Um trocador de calor de placas, que é ao mesmo tempo, o condensador para o R744 e o evaporador para R134a completa a configuração. Durante os testes experimentais, dois parâmetros foram manipulados: o grau de superaquecimento do R744, 515 K, e a frequência de operação do compressor R744, 45-65 Hz. Os valores da capacidade de refrigeração, para R134a/R744 variaram entre 4,30 ± 0,01 e 5,57 ± 0,02 kW, demonstrando a aplicabilidade desse sistema cascata em condições de carga térmica variável. Essa flexibilidade operacional também se estendeu aos valores estabelecidos para a temperatura do ar dentro da câmara fria, sendo o menor valor de -17,7 °C e o mais alto -0,8 °C. A fim de contribuir para a melhoria dos processos de refrigeração, principalmente no que se refere à eficiência energética, foi feito um drop-in no ciclo de alta temperatura, cuja carga de R134a foi substituída por R438A. A comparação foi feita de forma que os sistemas proporcionem capacidades de refrigeração e valores de temperatura do ar dentro da câmara fria semelhantes. Como resultado, o consumo de compressor com R438A foi maior em todos os testes. Os valores de COP para o sistema R134a/R744 variaram de 1,81 ± 0,01 a 2,09 ± 0,01, enquanto no sistema R438A/R744, de 1,41 ± 0,01 a 1,66 ± 0,01. O impacto total equivalente de aquecimento global para o sistema R438A/R744 foi maior em relação ao sistema original, devido ao maior potencial de aquecimento global do fluido R438A e ao maior consumo de potência elétrica do compressor do ciclo de alta temperatura. Logo, o refrigerante proposto no drop-in do sistema de alta temperatura de R134a por R438A provou-se uma alternativa menos eficiente, para as aplicações específicas. / This study evaluates the performance of a cascade system in subcritical operation using the pair R134a/R744, as an option to conventional systems in supermarkets, which usually uses R404A, or R22. The experimental apparatus consists of a variable speed reciprocating compressor for R744 and an electronic expansion valve that promotes direct evaporation of the CO2 inside a cold room (2,3m x 2,6m x 2,5m) to maintain the internal air temperature stable. The high-temperature cycle consists of a reciprocating compressor for R134a, an electronic expansion valve, and an air-cooled condenser. A plate heat exchanger, which is at the same time, the condenser for the R744 and evaporator to R134a completes the setup. During the experimental tests, two parameters were manipulated: The superheating degree of the R744, 5-15 K, and the R744 compressor operation frequency, 45-65 Hz. The cooling capacity values for R134a/R744 ranged between 4,30 ± 0,01 and 5,57 ± 0,02 kW, demonstrating an application to the cascade system under variable thermal load conditions. This operational flexibility also extends to the air temperature values established inside the cold room, being the lower value of -17,7 ° C and higher -0,8 ° C. In order to contribute to the improvement of the cooling processes, mainly about the energy efficiency, a drop-in has been made at the high- temperature cycle, which R134a refrigerant charge has been replaced by R438A. The comparison was done regard both systems providing similar cooling capacities and air temperature values inside the cold room. As a result, the consumption of the compressor operating with R438A was higher in all tests. The COP values for the R134a/R744 system ranged from 1,81 ± 0,01 to 2,09 ± 0,01, while with the R438A/R744 system, from 1,41 ± 0,01 to 1,66 ± 0,01. The total equivalent warming impact for the R438A/R744 system was higher compared to the original system, as a result of the higher GWP for the R438A and higher electrical power consumptions. It has been proven the proposed drop-in, at the high temperature cycle from R134a by R438A, is a less efficient choice for such specific applications. / Dissertação (Mestrado)