Dissertations / Theses on the topic 'Refrigerants'

When running a refrigeration system, the refrigerant will always leak into the atmosphere in various ways. The leakage of currently used refrigerants have a non-negligible impact on the global warming. The European Commission recently approved the Regulation No 517/2014 on fluorinated greenhouse gases (FGR). This regulation implies, among other, that refrigerants with GWP of 2500 or more will be restricted from being used in stationary refrigeration equipment being placed on European refrigeration market. In this study a number of new refrigerants are analysed and evaluated as potential replacements for R404A which is a popular commercially used refrigerant with high global warming potential. A few refrigerants from different manufacturers have been evaluated using a computer model of a basic vapour-compression system, as well as analysed from the position of refrigerant safety. It was found that the best short term replacement for R404A are two refrigerants called R448A and R449A and the best long term replacement is DR7.<br>I ett kylsystem kommer köldmediet alltid att, på ett eller annat sätt, läcka ut i atmosfären. Läckage av all världens köldmedium har en icke försumbar inverkan på den globala uppvärmningen. Den europeiska kommissionen nyligen godkänt en förordning kallad Regulation No 517/2014 on fluorinated greenhouse gases (FGR). Denna förordning innebär, bland flera saker, att kommersiella köldmedium med en global uppvärmningspotential på 2500 eller mer kommer att förbjudas för användning i stationära kylsystem. I denna studie har ett antal nya köldmedier analyserats och utvärderats i syfte att hitta ersättare till R404A vilket är ett populär kommersiellt köldmedium med en hög global uppvärmningspotential. Jämfört med R404A har nya köldmedier from olika producenter utvärderas i en databaserad modell av ett enkelt kylsystem för att utvärdera deras effektivitet. Studien fokuserar också på säkerhet kring köldmedier och tillsammans med resultaten från modellen framkom det att på kort sikt är det bästa alternativet för att ersätta R404A är två köldmedium kallat R448A R449A och på lång sikt ett köldmedium kallat DR7.

The European Parliament has shown its legislative capabilities by adopting a tough approach on the reduction of fluorinated gases (F-gases). The EU Regulation No 517/2014, will phase out F-gases with high global warming potential (GWP) within a well-defined step down time schedule. This will affect refrigerants which are commonly used in commercial cooling applications. While the time schedule implies existence of replace- ment refrigerants, the market continues to develop alternatives for the refrigerants already in deployment. Though the R404A,which is a commonly used refrigerant in station- ary cooling applications, will be prohibited of use 1 January 2020. In this paper, the two alternatives R448A from Honeywell and R449A from DuPont were compared to the R404A baseline, within a theoretical model with empirical compressor and system input data. The key points of comparison were cooling capacity, compressor discharge temperature, coefficient of performance (COP) and total equivalent warming impact (TEWI). The outcome of the model showed a decrease in cooling capacity for both alternatives, as well as an overall increase in compressor discharge temperature. For low evaporator temperatures, a decrease in COP was present and vice versa for high evaporator temperatures, both for medium and high condenser temperature. However, the TEWI for the both refrigerants, showed a decrease in carbon dioxide (CO2) equivalent emissions during the refrigerant system lifespan, running on both R448A and R449A, regardless of which European country the electricity was produced in.<br>Det Europeiska Parlamentet har visat sin lagstiftande förmåga i och med antagandet av en tuff förordning om reducering av F-gaser. EU Förordningen Nr 517/2014 kommer att fasa ut flourinerade gaser med högt GWP värde och det med en väl definierad nedtrappningsperiod. Detta kommer bland annat att påverka köldmedier som ofta förekommer i kommersiella kylapplikationer. Medan tidsschemat förutsätter redan existerande ersättare fortsätter marknaden att utveckla alternativ för de köldmedier som idag är i bruk, samtidigt som användandet av det vanligt förekommande kylmediet R404A kommer att förbjudas i och med 1 januari 2020. I den här rapporten jämförs de två alternativa kylmedierna R448A från Honeywell och R449A från DuPont i en teoretiskt modell, med empiriska data för kompressor- och systemvariabler, där kylmediet R404A används som referens. Huvudsakligen jämfördes kyleffekt, kompressorns utströmningstemperatur, COP och TEWI. Resultaten från modellen visade en minskad kyleffekt för båda de båda alternativen, såväl som en ökad utströmningstemperatur för kompressorn. Låga evaporatortemperaturer resulterade i en minskning av COP och vice versa för höga evaporatortemperaturer, både för mellan och hög kondensortemperatur. Dock konstaterades för TEWI, en minskning av de sammanlagda CO2 ekvivalenta utsläpp under kylsystemets livslängd, under drift på både R448A och R449A oavsett i vilket Europeiskt land elektriciteten producerats i.

Approved for public release; distribution is unlimited<br>The construction of an apparatus for the condensation performance testing of a horizontal bundle of four tubes with various refrigerants was completed. The apparatus was instrumented, and data reduction software was developed to provide bundle and single tube condensation data. Two tube bundles were tested, smooth copper tubes and low integral-fin copper-nickel tubes, with two refrigerants, R-114 and R-113. An enhancement ratio of about 2.0 for the overall heat transfer coefficient was demonstrated for the finned tubes over the smooth tubes. Internal contamination, possibly due to a breakdown of the refrigerant molecules when subjected to high temperatures in the boiling chamber, inhibited further meaningful data collection. Recommendations for improvement of the test apparatus are made.<br>http://archive.org/details/condensationofre00mabr<br>Lieutenant, United States Navy

Made available in DSpace on 2014-10-09T12:38:23Z (GMT). No. of bitstreams: 0<br>Made available in DSpace on 2014-10-09T14:05:53Z (GMT). No. of bitstreams: 1 05724.pdf: 6566331 bytes, checksum: 217859b9c420011d3a1761390d6692db (MD5)<br>Tese (Doutoramento)<br>IPEN/T<br>Escola Politecnica, Universidade de Sao Paulo - POLI/USP

Aqueous solutions of organic or inorganic salts are used as secondary refrigerants in indirect refrigeration systems to transport and transfer heat. Water is known for its corrosive character and secondary refrigerants based on aqueous solutions have the same tendency. The least corrosive from the aqueous solutions are glycols and alcohols. Salt solutions, such as chlorides and potassium salts, are much more corrosive. Nevertheless, it is possible to minimize corrosion risks at the beginning stage while designing system. Proper design can significantly help in improving system performance against corrosion. There are several aspects which need to be taken into account while working with secondary refrigerants: design of system, selection of secondary refrigerant, proper corrosion inhibitors, compatible materials used to build the installation and proper preparation of system to operation. While choosing proper materials it is advised to avoid the formation of a galvanic couple to reduce the risk of the most dangerous type of corrosion. Oxygen present in installation is another important factor increasing the rate of corrosion. Even small amounts of oxygen can significantly affect the system lifetime. The methods of cleaning, charging the system with refrigerant, and deaeration procedures are extremely important. The purpose of this thesis work is to present the problems of corrosion occurring in the indirect systems with secondary refrigerants. The thesis describes the mechanism of corrosion and its different types, most commonly used materials in installation, different corrosion inhibitors used to protect system. This thesis also lists the available secondary refrigerants on the market and briefly describes them. Further, it describes the important aspects related with designing, preparing and maintaining of indirect systems. This thesis is giving some clues and shows what should be done in order to reduce risks of corrosion.<br>Effsys 2 P2 project

Le contexte de ce travail est etroitement lie au probleme de la substitution des cfc, chloro fluoro carbones. L'etude de systemes pour le domaine particulier de la refrigeration concerne d'une part l'aspect experimental (technique et mesures) et l'aspect modelisation des donnees obtenues. Sur le plan experimental, de tres nombreuses donnees (pres de 89000 points experimentaux) de type pvt et d'equilibre on ete obtenues par une methode basee sur la densimetrie en tube vibrant. Les produits etudies sont des melanges (binaires et ternaires) des composes r32, r125 et r134a d'interet tres actuel pour les industries de la climatisation et la refrigeration. Les donnees fournies par ce travail sont realisees essentiellement dans le domaine de temperature de 253 a 333 k et des pressions inferieures a 20 mpa. L'incertitude sur les mesures de temperature est de 0. 02 k. L'incertitude sur la pression est de 0. 001 mpa pour les basses pressions (phase vapeur), de 0. 003 mpa pour les pressions en phase liquide. Les masses volumiques sont representees avec une incertitude de l'ordre de 0. 3 kg/m 3 sur toute la gamme. Pour les donnees a saturation, l'incertitude sur les pressions est en moyenne de 0. 004 mpa, celle sur les masses volumiques a saturation est de l'ordre de 0. 4 kg/m 3. Sur l'aspect modelisation, deux approches physiques ont ete abordees. Le premier modele fait appel a l'equation d'etat bwrs (des regles de melange sont appliquees pour chacun des coefficients du viriel). Il decrit la surface thermodynamique entiere avec une deviation standard inferieure a 2. 7%. La deuxieme voie a ete la mise en uvre d'une application originale du principe des etats correspondants qui ne contient que deux parametres ajustables. Bien que la forme de

This thesis consists of two parts; the first presents a critical review of the state of the art of research on internal forced convection boiling in microchannels and in microgravity while the second focuses on the experimental investigation of the onset of nucleate boiling. The critical review wants to answer to the necessity to define in the most complete way the status-of-the-art of this important research field and to critically investigate the successes and the weaknesses of the current scientific literature. The idea to consider the channel microsize together with the microgravity effects in a single review is due to the fact that the transition between confined and unconfined bubble flows may be defined using dimensionless numbers, such as the Eötvös number. This is linked to the tube diameter and the gravity forces and it tends to zero either when the gravity tends to zero or when the tube diameter tends to zero, but physical phenomena appear different considering separately either only the tube size or only the microgravity condition. It is pointed out that some literature results are still incongruous and that the global picture of boiling process remains still unclear. Part II contains the experimental investigation of boiling and in particular of onset of nucleate boiling in a glass minichannel using two different refrigerants R-134a and R-245fa. During the experimental activity some new interesting trends appeared, linked to the nucleation that always starts from the upper side of the heater and only later from the lower side of the heater. The onset of nucleate boiling was investigated thanks to the transparent ITO heater that allow the visualization of the boiling process. The heat transfer coefficients are evaluated as function of the vapour quality, heat flux and wall superheat and they increase increasing the mass flux values.

This thesis consists of two parts; the first presents a critical review of the state of the art of research on internal forced convection boiling in microchannels and in microgravity while the second focuses on the experimental investigation of the onset of nucleate boiling. The critical review wants to answer to the necessity to define in the most complete way the status-of-the-art of this important research field and to critically investigate the successes and the weaknesses of the current scientific literature. The idea to consider the channel microsize together with the microgravity effects in a single review is due to the fact that the transition between confined and unconfined bubble flows may be defined using dimensionless numbers, such as the Eötvös number. This is linked to the tube diameter and the gravity forces and it tends to zero either when the gravity tends to zero or when the tube diameter tends to zero, but physical phenomena appear different considering separately either only the tube size or only the microgravity condition. It is pointed out that some literature results are still incongruous and that the global picture of boiling process remains still unclear. Part II contains the experimental investigation of boiling and in particular of onset of nucleate boiling in a glass minichannel using two different refrigerants R-134a and R-245fa. During the experimental activity some new interesting trends appeared, linked to the nucleation that always starts from the upper side of the heater and only later from the lower side of the heater. The onset of nucleate boiling was investigated thanks to the transparent ITO heater that allow the visualization of the boiling process. The heat transfer coefficients are evaluated as function of the vapour quality, heat flux and wall superheat and they increase increasing the mass flux values.

The wide fluctuations that occur in the aggregate electrical demand of a generating utility are punitive with respect to total system efficiency. Demand side management techniques have been applied to reduce such fluctuations including the conversion of electrical energy to thermal energy during periods of low demand for use during peak demand periods. For thermal processes requiring energy above ambient temperature it is feasible to use sensible heat due to the existence of stable storage mediums and efficient methods of heating at the high temperatures required. However where energy is required below ambient temperatures, efficiency of cooling limits the use of sensible heat, hence latent heat storage has been adopted. Conventional cold storage systems use ice banks to store cooling energy at 0°C in order to capture the high latent heat of fusion of water. The rate of discharge for such stores is limited by thermal resistance in the store and the thermal capacity of secondary coolants (such as glycol solutions). This investigated the use of hydrophilic materials to overcome the limitations of current cold-storage technology. Such materials have the capacity to absorb and retain up to 95% by mass of water (or other aqueous solutions) regardless of how the materials is subdivided. Furthermore the thermal properties of the polymers in their hydrated state resemble those of the free hydration fluid, including any phase transitions. By supporting the hydrated materials in a non-freeing, non-aqueous fluid the resultant mixture provides a medium for cold storage that can be pumped and used at the point of load, and is not limited by the thermal resistance of an encapsulating material. Three aspects concerning the utilisation of hydrophilic materials for thermal engineering applications have been investigated; (i) the physical properties of the materials in their hydrated state, (ii) methods of fluidising material in a high density store, and (iii) the heat transfer properties of hydrophilic based slurries while undergoing phase transition. Material tests have shown that currently available hydrophilic materials have thermo- physical properties that depend principally upon the hydrating fluid, regardless of particle size, and are stable over long periods (>3years). Suitable hydration fluids can lower the temperature of the phase transition thus extending their potential as storage mediums beyond those of ice-based technologies. Novel materials, of very high water content (95%) have been produced and investigated. These appear to be very suitable for thermal storage because they increase the maximum achievable energy densities of a fluidised storage system and potentially reduce cost. A number of thermal storage devices to utilise hydrophilic based slurries have been designed and evaluated. The resultant devices has been shown to provide a means of taking hydrophilic materials to, and from, a packed bed and feeding them at a controlled rate into a fluid stream. The thermal charge/discharge rates of such a device are limited only by the choice of external heat exchange systems. An experimental apparatus has been designed to investigate the effects of phase change particles on the heat transfer properties of flowing mixtures. The results have shown that (i) at temperatures above the phase transition temperature the presence of the particles causes an increase in the measured heat transfer coefficient for concentrations above 10% by volume, (ii) there is a significant interaction of particles at the heat transfer surface, and (iii) that under high flow rate conditions, with phase change occurring, heat transfer coefficients are considerably enhanced (ie 80%) above those of the support fluid when used alone or with non-active particles. Further work is recommended to extend this study to produce an engineering prototype storage system for trial evaluation.

A presente pesquisa teve por objetivo o estudo da ebulição convectiva dos novos refrigerantes halogenados no interior de tubos horizontais de cobre. Uma busca na literatura pertinente ao assunto mostrou que a determinação do coeficiente de transferência de calor pode ser determinado por correlações que, aqui, foram classificadas como: 1- Estritamente Convectivas, 2- Superposição de Efeitos e 3- Estritamente Empíricas. Essas correlações mostraram-se inadequadas a generalizações. Uma bancada experimental foi concebida e construída, o que permitiu a obtenção de dados experimentais envolvendo uma ampla faixa de condições operacionais. Efeitos de parâmetros físicos como a velocidade mássica, fluxo de calor, temperatura de saturação e título, foram investigados. Os dados experimentais obtidos foram utilizados no desenvolvimento de uma correlação para o coeficiente de transferência de calor na ebulição convectiva que satisfizesse adequadamente esses dados obtidos para condições operacionais típicas de aplicações frigoríficas. A partir da análise efetuada foi proposto um modelo no qual foram introduzidos os adimensionais que envolvem os principais efeitos relacionados à mudança de fase. O modelo proposto apresentou resultados bastante adequados não apenas na correlação dos resultados experimentais obtidos na presente pesquisa como também em dados encontrados na literatura.<br>Present research has aimed at the study of convective boiling of recently introduced halocarbon refrigerants inside horizontal copper tubes. A comprehensive literature survey has revealed that the correlations for the convective boiling heat transfer coefficient can be divided into three main categories: (1) strictly convective; (2) superposition of effects; (3) strictly empirical. As a general rule these correlations are not fitted for generalizations. An experimental set up has been developed and constructed in order to raise data involving a relatively wide range of operational conditions. These data have been used to investigate effects of such parameters as heat flux, mass velocity, quality and evaporating temperature. In addition gathered data have been used in the development of a correlation for the heat transfer coefficient under convective boiling conditions typical of refrigeration applications. The model has been developed in terms of the main intervening non dimensional groups. The proposed equation correlated very well not only experimental data from present investigation but data obtained else where.

The aim of this work was to develop a new methodology, which can be used to design new refrigerants that are better than the currently used refrigerants. The methodology draws some parallels with the general approach of computer aided molecular design. However, the mathematical way of representing the molecular structure of an organic compound and the use of meta models during the optimization process make it different. In essence, this approach aimed to generate molecules that conform to various property requirements that are known and specified a priori. A modified way of mathematically representing the molecular structure of an organic compound having up to four carbon atoms, along with atoms of other elements such as hydrogen, oxygen, fluorine, chlorine and bromine, was developed. The normal boiling temperature, enthalpy of vaporization, vapor pressure, tropospheric lifetime and biodegradability of 295 different organic compounds, were collected from open literature and data bases or estimated. Surrogate models linking the previously mentioned quantities with the molecular structure were developed. Constraints ensuring the generation of structurally feasible molecules were formulated and used in commercially available optimization algorithms to generate molecular structures of promising new refrigerants. This study was intended to serve as a proof-of-concept of designing refrigerants using the newly developed methodology.

Ever since the computer was invented, there has been a need of data storage and the demand has strictly grown since. This has resulted in a huge amount of data centers and the trend has shown no signs of changing. The data centers are powered by electricity and in 2010 the electricity consumption for data centers stood for 1.3% of the world’s electricity usage. The most energy consuming part of a data center is the servers themselves, but the second largest energy consuming part is the cooling system which, in a normal data center, stands for two fifths of the energy usage. Besides the energy consumption, the cooling systems are in most cases a cooling machine using HCFC and HFC refrigerants. These refrigerants are all bad for the environment since HCFCs have high ODP and GWP values and HFCs have high GWP values. The purposes of this work is: A) Find a way to make the cooling systems more efficient. Previous work has shown that using free cooling from the ambient air is an effective method of reducing the yearly electricity demand. Further the systems use a two-phase thermosiphon to move heat from the servers to the ambient, which means that there is no need of pumping power. B) Find solutions using natural refrigerants that have no ODP and very low or zero GWP. C) Evaluated if there is a possibility to recover the waste heat from the data center to e.g. an office building. This work contains two systems being mathematically modeled with the software Engineering Equation Solver: a direct R744 system and an indirect system running with R290 and R744. Both systems are using a thermosiphon application, connected to a condenser, to use free cooling up to a certain set point temperature and the rest is covered with a vapor compression cycle. These systems are then matched to temperature profiles for five cities, Stockholm, Paris, Phoenix, Tokyo and Madrid, to see how many hours of the year are covered by free cooling. The systems are then evaluated considering both energy consumption and cost. To be able to compare these systems to a present cooling system, a reference system is modeled which uses R22 as refrigerant, that is the most commonly used refrigerant in the world today for the data center cooling application. The results show that a direct R744 system or an indirect system with R290/R744 with a thermosiphon application have both energy and economical savings compared to the reference system. The energy savings are up to 88% and the total annual cost savings are up to 69%. The Power Usage Effectiveness is reduced with up to 6% and up to 8% if only cooling is considered. These savings are for an optimized condenser with a 2000 m2 fin area and 6 fans with a set point temperature of 22°C. The indirect R290/R744 system is the best in all cities considering energy efficiency. Both systems are also well suited for use with heat recovery. The Seasonal Performance Factor for the heat recovery is between 8.3 and 15.2, which is a consequence of the high evaporation temperature and low supply temperature to the heating system.<br>Ända sedan datorn uppfanns har det funnits ett behov av datalagring, ett behov som ökat stadigt. Detta har resulterat i en stor mängd datacenter och det finns inget som tyder på att trenden kommer ändras. Datacenter drivs av el och under 2010 var elförbrukningen för datacenter 1.3% av världens totala elanvändning. Den mest energikrävande delen av ett datacenter är de faktiska servrarna och den näst största energikrävande delen är kylsystemet, vilket i ett normalt datacenter står för två femtedelar av energianvändningen. Förutom energiförbrukningen, är kylsystemen i de flesta fall, en kylmaskin med HCFC- och HFC-köldmedier. Dessa köldmedier är dåliga för miljön eftersom HCFC har högt ODP- och GWP-värden och HFC har höga GWP-värden. Syftet med detta arbete är: A) Hitta ett sätt att göra kylsystem effektivare. Tidigare arbeten har visat att användning av frikyla från den omgivande luften är en effektiv metod för att minska det årliga elbehovet. Det finns även system som använder en två-fas termosifon för att flytta värme från servrar till den omgivande luften, vilket innebär att det inte behövs några pumpar. B) Hitta systemlösningar med naturliga köldmedier som har noll ODP och mycket låg eller noll GWP. C) Utvärdera om det finns möjlighet att återvinna spillvärme från ett datacenter till exempelvis en kontorsbyggnad. Detta arbete innehåller två system vilka modelleras matematiskt med hjälp av programvaran Engineering Equation Solver: ett direkt R744-system och ett indirekt system som använder R290 och R744. Båda systemen använder en termosifonslinga som är ansluten till en kondensor för att kunna använda frikyla upp till en viss brytpunktstemperatur och det resterande behovet täcks av en kylmaskin. Dessa system matchas sedan mot temperaturprofiler för fem städer, Stockholm, Paris, Phoenix, Tokyo och Madrid, för att se hur många timmar av året som frikylakan användas. Systemen utvärderas sedan utifrån både energiförbrukning och kostnad. För att kunna jämföra dessa system mot ett befintligt kylsystem modelleras ett referenssystem med R22 som kylmedel, vilket är det vanligaste köldmediet i världen idag för kylning av datacenter. Resultaten visar att ett direkt R744-system eller ett indirekt system med R290/R744, båda med en termosifonslinga, är både energieffektivare och ekonomiskt fördelaktigare jämfört med referenssystemet. Energibesparingen uppgår till 88% och de totala årliga kostnadsbesparingarna uppgår till 69%. Power Usage Effectiveness värdet reduceras med upp till 6% och om enbart hänsyn tas till nedkylning, upp till 8%. Dessa besparingar är för en optimerad kondensor med en flänsyta på 2000 m2 samt 6 stycken fläktar då kondensatorn har en brytpunktstemperatur på 22° C. Det indirekta R290/R744-systemet är det bästa i alla städer vad gäller energieffektivitet. Båda systemen är också väl lämpade för användning med värmeåtervinning. Årsvärmefaktorn för värmeåtervinningen är mellan 8.3 och 15.2, vilket är en följd av den höga förångningstemperaturen och den låga framledningstemperaturen till värmesystemet.

This study aimed to facilitate evaluation of alternative refrigerants for the air conditioner market. In order to achieve this target, two objectives were set. The first being the identification of alternative refrigerant with regard to regulations and trends. The second being enabling of thermodynamic evaluation of refrigerants for single components in air conditioners via design and installation of a test rig. A literature review was conducted to investigate the regulations, standards and trends for nine regions with high market shares. Furthermore, refrigerant substitutes for R410A and R22 where identified. The literature showed strong indications that a global HFC phase down is on the horizon, but that the approach and urge for such a phase down varies between regions where three main priorities could be seen; efficiency, GWP values and safety. Comparing these priorities with simulated refrigerant characteristics such as Carnot efficiency, GWP values, volumetric capacity and the liquid density using RefProp, identified alternatives. Further the literature showed that the lower flammability limit (LFL) and the amount of charge allowed were limiting factors for flammable refrigerant. There are many new refrigerants that could have less charge for the same capacity the LFL however restricts the refrigerants like R290, R441A and R443A only to be used in small systems. A test rig was designed and constructed in order to enable testing of single components in air conditioners. To verify the test rig, tests conducted with an evaporator using two refrigerants: R410A and R32. The reliability and validity of the tests was studied by comparing the results from the air side and refrigerant side, via uncertainty calculations using the GUM method and by a thermodynamic evaluation. The results of the repetition tests showed an expanded uncertainty, with a confidence interval of 95%, of 26 W for a cooling capacity of 2190W for R410A (Q̇evap,R410A=2190±26W). For R32 the expanded uncertainty was 27W for a cooling capacity of 2795W (Q̇evap,R32=2795±27W). Also the test rig was verified to be used for analyzing detailed evaluation of evaporators such as looking at heat transfer and the differential pressure drop. Further the study presented six conclusions: When evaluating refrigerants regional priorities between efficiency, safety and GWP values, refrigerant characteristics and type of unit need to be considered. Safety standards need to be more acceptable towards flammable refrigerants in order to meet low GWP targets. The change in market trends leads to new roles for the refrigerant and compressor suppliers and put higher demand on air conditioner manufacturers to consider refrigerant characteristics in product development. The importance of experimental thermodynamic evaluation on refrigerants effect on single components is increasing. Performance of different refrigerant for single components can be evaluated for optimizing cycle performance. Leapfrogging in the developing world can lead to a faster introduction of low GWP refrigerants.

Environmental considerations have led to the phase out of chlorofluorocarbon (CFC) refrigerants from the domestic refrigeration industry. One intriguing aspect is that the chlorine in CFCs is a good lubricating agent and any deterioration of system performance may adversely influence other environmental considerations. Based on the above, the aim of this research is to address the sustainable development of domestic refrigeration systems using the replacement refrigerant HFC-134a. The work focuses on the emissions that may arise if the electrical consumption of the product deteriorates or its durability is curtailed. Tribological characteristics on compressor components influence both of these product attributes and therefore a thorough system analysis was carried out. An in-house built experimental test rig, which monitored slight variations in the electrical power drawn by a reciprocating hermetic compressor, was used under different experimental conditions. Furthermore, a detailed life cycle assessment on a domestic refrigerator was performed to help quantify the ensuing environmental burdens. In this way, a relation between tribological characteristics, power consumption and environmental impact was studied. Results have shown that the CFC substitute will increase friction and wear characteristics on the aluminium alloy connecting rod and the steel gudgeon pin. These characteristics led to an increase in the electrical energy consumption of the compressor such that the indirect global warming implications are set to rise with HFC-134a. If the sustainable development of this product is to be ascertained then a change in refrigerants alone will not suffice. New design considerations, primarily aimed at servicing and extending the life of the hermetic compressor itself, are considered. This work helps stimulate new ideas to address environmental issues influenced by traditional engineering disciplines. For this reason additional future research work, which will help determine these implications further, is outlined.

International environmental concern led to the control and phase out of traditional chlorofluorocarbon refrigerants (CFCs) under the terms of the Montreal protocol. CFCs used in domestic applications were initially replaced with hydrofluorocarbons (HFCs) such as R134a which has a zero ozone depletion potential (ODP). The use of HFCs has also come under scrutiny as they have high global warming potential (GWP) and inferior thermodynamic and lubricating properties and have been replaced by hydrocarbon (HC) refrigerants such as R600a in much of the domestic European and Asian markets. Despite this, there has been little research into the long-term environmental consequences of their application. Domestic refrigeration compressors were analysed to ascertain the tribological contact conditions for both R600a and R134a systems. A novel pressurised micro-friction test machine was developed to simulate the tribological conditions of the critical components using aluminium on steel samples. Refrigerant charges of R600a with mineral oil (MO) and poly-ol-ester (POE) lubricant and R 134a with POE were tested for their tribological performance within the test rig. Experimental tribological information is presented from the physical test procedures to establish wear mechanisms and friction coefficients within the critical components. The tribological performance is used to predict deterioration in energy consumption and system durability. Results indicate that for higher contact stresses R600a MO charges provide a lower wear regime than R600a and R134a POE charges. At lower contact stresses the R600a and R134a POE charges provide a very low wear, very low friction regime. Despite contact conditions lead to a faster deterioration in durability, hence increase in energy consumption compared to the R600a system.

This thesis addresses the general subject of implementation of refrigerants that are alternatives for ozone depleting substances. In particular it covers three topics, which are environmental assessment, system performance and safety aspects related to flammability. The first Part examines the current means of assessing the global wanning impact of systems, and proposes an improved approach. The second Part presents a new balancing technique for designing systems that use refrigerants with a temperature glide, as an advancement over the conventional technique for pure refrigerants. The third Part derives a quantitative risk assessment model to be employed for evaluating the safety of hazard of flammable refrigerants. The concept of'Total Equivalent Warming Impact'' is introduced in Part 1, which is currently used by manufacturers, installers and end-users of refrigerating systems to evaluate the contribution to climate change resulting from energy consumption and refrigerant leakage. It was found that existing methods rely on many assumptions, so a thorough approach was proposed to assess the relevance of the assumptions and providing ways of avoiding them. An alternative measure for interpretation of consequences of greenhouse gas emissions is also included. Part 2 investigated the existing approach for rating and balancing sy stem components, which were found to be incompatible when applied to zeotropic refrigerants. Differences in the performance of components using pure and zeotropic refrigerants were identified, and so a new method for component rating and an advanced system balancing technique were developed based on the properties specific to zeotropes. Within Part 3, a framework for calculating ignition frequencies and consequences of a release of hydrocarbon refrigerant is presented. A major element of these calculations is the evaluation of certain quantities of the flammable mixture, and the model developed for this purpose w as based on the results from a series of gas dispersion experiments.

In this thesis an experimental study on flow boiling of refrigerants inside horizontal tubes is presented. Vaporization inside horizontal tubes is commonly used in evaporators of machines for refrigeration and air-conditioning applications. Most of flow boiling data, available in the open literature for HFC fluids, refer to a narrow range of saturation temperatures, resulting in a limited range of reduced pressure in the refrigerant vaporization data sets. The present data are taken at higher saturation temperature as compared to common air-conditioning applications. This allows to extend the reduced pressure range in the databases. The adoption of new refrigerants, such as carbon dioxide, with different thermodynamic and thermo-physical properties, requires to extend the pressure range of application of existing heat transfer predictive methods. When evaporating at 2 °C, for instance carbon dioxide has a saturation pressure of 3673 kPa and 0.5 reduced pressure, which is far higher as compared to common halogenated refrigerants. For R410A, 2 °C saturation temperature corresponds to 0.17 reduced pressure, while 0.39 reduced pressure is associated to 30 °C saturation temperature. On the other hand, an example of high evaporating temperature application of HFC refrigerants is represented by heat pump heat dryers. In this thesis new experimental measurements of heat transfer coefficient during flow boiling of R134a and R410A at 30 °C and 40 °C saturation temperature (reduced pressure from 0.19 to 0.5) inside a horizontal microfin tube are reported and discussed for a wide range of operative conditions: mass velocity from 80 to 600 kg/(m2 s), heat flux from 14 to 83.5 kW/m2 and vapour quality from 0.1 to 0.99. As compared to plain tubes, the presence of the fins modifies the two-phase flow pattern; for instance, capillarity forces play a role in microfin tube, at low mass velocity, and there is need to assess the prediction capability of the heat transfer models at these same conditions. Moreover, the fins also affect the conditions leading to dry-out and partial dry-out. The heat transfer coefficients in microfin tubes are compared with heat transfer smooth tube data for R134a and R410A at the same operating conditions. The heat transfer models by Koyama et al. (1996), Thome et al. (1999), Cavallini et al. (2006), Chamra and Mago (2007) and Hamilton et al. (2008) are compared with the new measurements of heat transfer coefficient in microfin tube. In the case of smooth tube, heat transfer data for R134a and R410A are used to assess the accuracy of the models by Gungor and Winterton (1987), Kandlikar (1991), Liu and Winterton (1991) and Wojtan et al. (2005b). The development of diabatic two-phase flow pattern maps is a key aspect to improve the understanding of the role of heat transfer mechanisms, but a flow pattern map is not yet available for flow boiling in microfin tube. Present flow boiling data in smooth and microfin tube have been here plotted in the diabatic two-phase flow pattern map by Wojtan et al. (2005a) for plain tube: the predicted flow patterns are discussed by analyzing the experimental heat transfer results. The transition boundary between stratified and annular flow regime, provided by Wojtan et al. (2005a), is compared with the transition curve from stratified to annular flow for condensation in microfin tube; in fact, during vaporization and condensation in microfin tube annular flow pattern occurs earlier as compared to a plain tube, at the same operating conditions. The prediction of the onset of dry-out is fundamental to get a more comprehensive understanding of the validity range of flow boiling heat transfer models and then to improve the design of an evaporator; but, few predicting correlations are available in the open literature, especially for microfin tube. New measurements of dry-out inception vapour quality in microfin tube, are here presented and discussed; the measured values are then compared with the correlation by Mori et al. (2000) for horizontal microfin tube. Measurements of pressure drop in microfin tube during adiabatic two-phase flow at 30 °C and 40 °C are reported and compared with the model by Cavallini et al. (1999). Addendum An experimental study on solar energy conversion for applications in solar cooling have been also carried out during the Doctorate course. In particular a new test rig for the measurement of efficiency of solar collectors has been set up and calibrated. Moreover, a new measuring system for the measurement of solar radiation in horizontal and tilted-oriented planes has been set up. The experimental measurements of solar radiation can be used to assess models for the prediction of solar radiation components on tilted-oriented surfaces. This part of the research is presented in the appendix, by reporting two papers on this activity.<br>In questa tesi viene presentato uno studio sperimentale sulla vaporizzazione di refrigeranti all’interno di un tubo orizzontale con superficie liscia e microalettata. La vaporizzazione interno tubo trova applicazione nelle macchine per la refrigerazione ed il condizionamento dell’aria. La maggior parte dei dati di scambio termico in vaporizzazione per fluidi HFC, disponibili in letteratura, si riferiscono ad un ristretto intervallo di temperature di saturazione, e quindi sono caratterizzati da un intervallo limitato di pressione ridotta. I dati nella presente tesi sono misurati a temperature più alte rispetto alle comuni applicazioni di condizionamento dell’aria. Ciò consente di estendere l’intervallo di pressione ridotta nei database. L’adozione di nuovi refrigeranti come l’anidride carbonica, con proprietà termodinamiche e termofisiche differenti rispetto ai refrigeranti di comune impiego, richiede di estendere l’intervallo di validità dei modelli per la previsione del coefficiente di scambio termico. Infatti, quando evapora alla temperature di 2 °C, l’anidride carbonica ha una pressione di saturazione di 3673 kPa e una pressione ridotta di 0.5, che è notevolmente più alta rispetto a quella dei comuni refrigeranti. Nel caso di R410A, alla temperatura di saturazione di 2 °C corrisponde una pressione ridotta di 0.17, mentre alla temperatura di saturazione di 30 °C è associata una pressione ridotta di 0.39. Un esempio applicativo di vaporizzazione di fluidi HFC ad alta temperatura è rappresentato dalle macchine asciugatrici a pompa di calore. In questa tesi vengono presentate nuove misure del coefficiente di scambio termico durante la vaporizzazione di R134a e R410A alla temperatura di saturazione di 30°C e 40°C (pressione ridotta da 0.19 a 0.5) all’interno di un tubo orizzontale microalettato. Le misure coprono un ampio intervallo di condizioni operative: portata di massa specifica tra 80 e 600 kg/(m2 s), flusso termico specifico tra 14 e 83.5 kW/m2 e titolo del vapore tra 0.1 e 0.99. Rispetto ad un tubo liscio, la presenza delle alette modifica il regime di deflusso in vaporizzazione. Per esempio a bassa portata di massa specifica, le forze di capillarità giocano un ruolo importante e c’è necessità di valutare l’accuratezza dei modelli di previsione di scambio termico a queste condizioni. La alette influenzano inoltre le condizioni operative che possono portare al dry-out. Le misure del coefficiente di scambio termico in tubo microalettato sono state confrontate con dati di scambio termico durante la vaporizzazione di R134a e R410A in tubo liscio alle stesse condizioni operative. I modelli per la stima del coefficiente di scambio termico di Koyama et al. (1995), Thome et al. (1997), Cavallini et al. (2006), Chamra e Mago (2007) and Hamilton et al. (2008) sono stati confrontati con le misure ad alta temperatura di saturazione effettuate per il tubo microlaettato. Nel caso del tubo liscio i dati di scambio termico in vaporizzazione per R134a e R410A sono stati confrontati con i modelli di Gungor e Winterton (1987), Kandlikar (1991), Liu e Winterton (1991) e Wojtan et al. (2005b). Lo sviluppo di mappe di deflusso bifase è un aspetto decisivo per migliorare la comprensione del ruolo dei meccanismi di scambio termico; tuttavia non esiste ancora una mappa di deflusso per la vaporizzazione entro tubo microalettato. I dati di vaporizzazione di R134a e R410A in tubo liscio e microalettato sono sono stati qui diagrammati nella mappa di deflusso per vaporizzazione in tubo liscio di Wojtan et al. (2005a): il regime di deflusso stimato dalla mappa è stato discusso, analizzando il corrispondente comportamento dei coefficienti in scambio termico. La curva di transizione tra regime di deflusso stratificato ed anulare, fornita dalla mappa di Wojtan et al. (2005a), è stata confrontata con la curva di transizione tra regime stratificato ed anulare, che si ha nel caso della condensazione in tubo microalettato; sia durante la vaporizzazione che la condensazione all’interno di un tubo microalettato il regime di deflusso anulare avviene a portate più basse rispetto al caso di un tubo liscio. La previsione dell’inizio del regime di dry-out è un altro aspetto fondamentale per ottenere una maggiore comprensione dell’intervallo di applicazione dei modelli di scambio termico e migliorare così la progettazione dell’evaporatore. Tuttavia, poche correlazioni sono disponibili in letteratura per la stima del titolo di dry-out incipiente, soprattutto nel caso di tubo microalettato. Nuove misure del titolo di vapore di inizio dry-out, nel tubo micro alettato, sono qui presentate confrontate con la correlazione di Mori et al. (2000). Misure sperimentali della caduta di pressione attraverso il tubo microalettato durante deflusso bifase adiabatico alla temperatura di saturazione di 30 °C e 40 °C sono qui riportate e confrontate con il modello di previsione di Cavallini et al. (1999) per tubi con superficie intensificata. Addendum Durante il corso di dottorato è stato inoltre condotto uno studio sperimentale sulla conversione dell’energia solare per applicazioni di “solar cooling” (raffrescamento solare). In particolare è stato installato e calibrato un nuovo impianto di prova per la misura dell’efficienza dei collettori solari ed è stato messo a punto nuovo sistema di misura della radiazione solare in piani diversamente orientati. Le misure sperimentali hanno consentito di verificare l’accuratezza dei modelli per la stima delle componenti della radiazione solare su superfici con diversa orientazione. Questa parte della ricerca è presentata in appendice riportando due articoli su questa attività.

Negli ultimi anni è aumentato l’interesse verso i refrigeranti a basso potenziale di riscaldamento globale e le loro miscele come sostituti dei refrigeranti convenzionali in modo da ridurre il loro impatto ambientale. Le proprietà termofisiche dei fluidi alternativi sono necessarie per valutare le loro prestazioni nelle applicazioni HVAC&amp;R. Di conseguenza, il presente lavoro di tesi presenta un’analisi teorica e sperimentale di alcune delle loro principali proprietà termofisiche. Sono stati testati alcuni dei più conosciuti modelli per determinare differenti proprietà termodinamiche e di trasposto. È stata analizzata la capacità di diverse equazioni di stato cubiche nel descrivere la pressione di vapore e l’equilibrio liquido-vapore di refrigeranti alternativi e loro sistemi binari, trovando buoni risultati. Correlazioni scalate basate sul principio degli stati corrispondenti sono state proposte per la tensione superficiale di diversi refrigeranti alternativi e loro sistemi binari. I risultati di queste equazioni sono in accordo con quelli di correlazioni semi-empiriche specifiche per refrigeranti e loro miscele. È stata sviluppata una versione semplificata dell’equazione di Kardos specifica per la conducibilità termica di refrigeranti allo stato liquido e allo stato vapore. La versione modificata ha dato risultati accurati per la conducibilità termica dei refrigeranti studiati. Le proprietà PvTz di diversi sistemi binari a basso potenziale di riscaldamento globale sono state misurate sia nella regione liquido-vapore che in quella di vapore surriscaldato mediante un apparato isocorico. Dalle misure realizzate in bifase, è stata derivata una rappresentazione accurata dell’equilibrio liquido-vapore dei sistemi binari studiati. I dati misurati nella regione del vapore surriscaldato sono stati elaborati mediante diverse equazioni di stato. L’equilibrio solido-liquido del sistema R32 + R1234ze(E) è stato misurato con un apposito apparato sperimentale nel range di temperature da 168.2 K fino a 132.0 K. Questo sistema binario ha mostrato il punto eutettico a 132.0 K.<br>In the last years, the interest in low global warming potential refrigerants and their blends as replacements of conventional refrigerants has been increasing to reduce their environmental impact. To evaluate their potential performance in HVAC&amp;R applications, the thermophysical properties of these alternative working fluids are needed. Thus, this study presents a theoretical and experimental analysis of some of their main thermophysical properties. Some of the most well-known models to estimate different thermodynamic and transport properties have been investigated. The capability of different cubic equations of state to describe the vapor pressure and the vapor-liquid equilibrium of alternative refrigerants and their binary systems was analyzed, obtaining good results. Scaled correlations based on the corresponding states principle were presented for the surface tension of different alternative refrigerants and their binary systems. The results of these correlations agree with those of different reliable semi-empirical correlations designed for pure refrigerants and blends. An empirically simplified Kardos equation specifically oriented to both liquid and vapor thermal conductivity of refrigerants was developed. The modified equation provided very accurate results for the liquid and vapor thermal conductivity of the studied refrigerants. The PvTz properties for different low global warming potential binary systems were measured both in the two-phase and superheated vapor regions with an isochoric apparatus. From the two-phase measurements, an accurate vapor-liquid equilibrium behavior of the measured binary systems was derived. The vapor-phase data were correlated through different equations of state. The solid-liquid equilibrium of the R32 + R1234ze(E) binary pair was measured with an experimental setup in the temperature range from 168.2 K to 132.0 K. The studied binary pair showed an eutectic point estimated at 132.00 K.

Projeto de Pós-Graduação/Dissertação apresentado à Universidade Fernando Pessoa como parte dos requisitos para obtenção do grau de Mestre em Ciências Farmacêuticas<br>O presente trabalho foca o consumo da cafeína, relatando os seus efeitos a nível do apetite e sono. As consequências benéficas ou maléficas do consumo de cafeína ainda não estão verdadeiramente esclarecidas, e por isso merecem cada vez mais atenção, dada a constante preocupação pela saúde e bem-estar da sociedade em geral. Esta relação foi estudada a partir de um estudo estatístico onde foi realizado um inquérito a 100 alunos da Universidade Fernando Pessoa. A amostra populacional foi constituída por 29% homens e 71% mulheres, que possuíam idades compreendidas entre os 18 e 40 anos. Destes indivíduos, 49% consomem chá, 48% refrigerantes ou bebidas energéticas, 65% chocolate e 71% café. Quanto à qualidade de sono 68% apresenta boa qualidade e 32% má qualidade de sono. Na avaliação da sonolência diurna 60% não apresenta e 40% apresenta sonolência diurna. Quanto à alteração do apetite por parte dos consumidores de café apenas 12,7% demonstraram redução do apetite, 31% afirmaram que este se manteve e 56,3% revelaram nunca averiguar qualquer influência. Após análise estatística dos dados com o teste de Qui-Quadrado, verificou-se que as variáveis eram independentes. Embora os resultados não sejam significativamente estatísticos, pode dizer-se que a nível da avaliação da capacidade do consumo de cafeína influenciar o apetite, a maioria dos inquiridos revelou nunca averiguar qualquer alteração. Relativamente à avaliação do sono avaliou-se duas variáveis, designadamente a qualidade do sono e a sonolência diurna. Quanto à qualidade do sono os resultados demonstram que o consumo de cafeína não afeta esta variável. Quanto à avaliação da sonolência diurna, para as fontes de cafeína derivadas do café e do chocolate verificou-se uma diminuição desta. O mesmo não se verificou para as outras fontes estudadas, como o chá e os refrigerantes ou bebidas energéticas. Neste estudo, a cafeína revelou de alguma forma interferir no sono mas não no apetite. This study focuses on the consumption of caffeine, reporting their effects on appetite and sleep. The beneficial or harmful effects of caffeine consumption are not truly enlightened, and therefore deserve increased attention, given the constant concern for the human health and well-being of society. This relationship has been studied from a statistical study which was conducted a survey of 100 students at the University Fernando Pessoa. The sample population consisted of 29% men and 71% women where aged ranged between 18 and 40 years old. The study reported that 49% of the individuals consume tea, 48% refrigerants and drinks energy, 65% chocolate and 71% coffee. Relatively to the quality of sleep 68% presents good quality and 32% poor quality. In the assessment of daytime sleepiness, 60% presents and 40% has no features daytime sleepiness. Regarding the change in appetite from consumers of coffee only 12.7% showed reduced appetite, 31% said it remained 56.3% and never revealed to ascertain any influence. After statistical analysis of the data with the Chi-square test, it was found that the variables were independent. Although the results are not statistical significant, the evaluation of the capacity level of caffeine consumption influence the appetite, most of the population found no changes. The evaluation of sleep involved two variables, the study of the quality of sleep and the daytime sleepiness. Relatively to the quality of sleep, the results demonstrated that caffeine does not affect this variable. Regarding the assessment of daytime sleepiness, the sources of caffeine derived from coffee and chocolate, were the only that decreases this variable. The same results were not similar for the other sources studied, such as tea and refrigerants or energy drinks. In this study, caffeine showed otherwise interferes with the sleep but not on appetite.

Thesis (M.S.)--University of Missouri--Rolla, 2007.<br>Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed May 11, 2007) Includes bibliographical references (p. 115-119).

Thesis (PhD) - Swinburne University of Technology, Faculty of Engineering and Industrial Sciences, 2006.<br>A thesis submitted for the degree of Doctor of Philosophy, School of Engineering and Science, Swinburne University of Technology, 2006. Typescript. Includes bibliographical references (p. 123-127).

Thesis (M.S.)--Missouri University of Science and Technology, 2009.<br>Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed April 14, 2009) Includes bibliographical references (p. 201-203).

Two-phase flow, boiling, and condensation in microchannels have received considerable attention in the recent past due to the growing interest in the high heat fluxes made possible by these channels. This dissertation presents a study on the condensation of refrigerant R134a in small hydraulic diameter (100 < Dh < 160 mm) channels. A novel technique is used for the measurement of local condensation heat transfer coefficients in small quality increments, which has typically been found to be difficult due to the low heat transfer rates at the small flow rates in these microchannels. This method is used to accurately determine pressure drop and heat transfer coefficients for mass fluxes between 300 and 800 kg/m2-s and quality 0 < x < 1 at four different saturation temperatures between 30 and 60oC. The results obtained from this study capture the effect of variations in mass flux, quality, saturation temperature, hydraulic diameter, and channel aspect ratio on the observed pressure drop and heat transfer coefficients. Based on the available flow regime maps, it was assumed that either the intermittent or annular flow regimes prevail in these channels for the flow conditions under consideration. Internally consistent pressure drop and heat transfer models are proposed taking into account the effect of mass flux, quality, saturation temperature, hydraulic diameter, and channel aspect ratio. The proposed models predict 95% and 94% of the pressure drop and heat transfer data within ±25%, respectively. Both pressure drop and heat transfer coefficient increase with a decrease in hydraulic diameter, increase in channel aspect ratio and decrease in saturation temperature. A new non-dimensional parameter termed Annular Flow Factor is also introduced to quantify the predominance of intermittent or annular flow in the channels as the geometric parameters and operating conditions change. This study leads to a comprehensive understanding of condensation in microchannels for use in high-flux heat transfer applications.

After a brief introduction on the global warming effect and on the measures that have been adopted to limit it, this thesis is focused on low GWP refrigerants and, above all, on HFOs, which have been studied during two phase flow in several operating test conditions. To cover a great portion of the existing devices, four heat exchangers have been investigated: a tube in tube heat exchanger, a Brazed Plate Heat Exchanger (BPHE), a roll-bond type heat exchanger, and a Finned Heat Pipe Heat Exchanger (HPFHE). In Chapter 2 the four experimental test rigs used to collect experimental data are explained and the data reduction processes used to analyze the recorded data are shown. In Chapter 3 all the experimental data points collected are presented, critically discussed, and compared against some existing correlations. In addition, new analytical procedures are proposed to evaluate the heat transfer coefficient and the pressure drop during vaporization and condensation inside BPHEs respectively. Furthermore a new computational procedure to calculate the heat capacity and the heat transfer coefficients of a HPFHE is presented. In Chapter 4, the experimental data obtained with different fluids under the same working conditions are grouped and compared to highlight the refrigerant effect on the global performance of the heat exchangers. Finally some performance evaluation criteria to discriminate the best refrigerants on the basis of thermophysical properties and to select the ones that perform better in terms of heat transfer and pressure drops are presented.<br>Il Capitolo 1 si apre con una breve introduzione riguardante la questione, purtroppo sempre attuale, dell’impatto ambientale dovuto all’attività umana e del surriscaldamento globale, facendo rifermento alle misure adottate negli anni per limitarlo. In seguito viene posta attenzione sul tema dei fluidi refrigeranti a basso impatto ambientale – e dunque a basso indice GWP – e in special modo sulle applicazioni in cui sussiste uno scambio termico bifase. Con l’interesse e lo scopo di coprire il maggior numero di casistiche adottate nella tecnica applicativa, la tesi approfondisce quattro tipi diversi di scambiatore di calore: uno scambiatore tubo in tubo, uno scambiatore a piastre, un evaporatore di tipo roll-bond e un recuperatore a tubi di calore. Il Capitolo 2 comprende la descrizione degli impianti sperimentali e dei procedimenti di elaborazione dei dati ottenuti. Il Capitolo 3, invece, presenta tutti i dati sperimentali acquisiti nel corso di questa tesi suddivisi per tipologia di scambiatore di calore, ne riporta un commento critico e li confronta con modelli e correlazioni presenti in letteratura. Inoltre vengono proposti due nuovi modelli basati su dati sperimentali per stimare i coefficienti di scambio termico durante i processi di vaporizzazione e di condensazione di refrigeranti all’interno di scambiatori a piastre. Viene altresì illustrata una procedura di calcolo che permette di ottenere il calore scambiato da un recuperatore a tubi di calore e i coefficienti di scambio termico del fluido operativo all’interno degli stessi tubi di calore. Il Capitolo 4, infine, riporta un confronto tra dati sperimentali ottenuti nelle medesime condizioni operative con diversi fluidi refrigeranti che mette in luce l’effetto del fluido stesso sulle prestazioni dello scambiatore. Inoltre, per ogni scambiatore, vengono adottati ed implementati dei criteri di valutazione delle prestazioni dei soli refrigeranti sulla base delle proprietà termofisiche e delle prove sperimentali condotte. In tal modo è possibile racchiudere in un unico indice l’effetto combinato del refrigerante su coefficiente di scambio termico e su perdite di carico. Tale discussione può guidare nella scelta di un nuovo fluido, scelta che al giorno d’oggi, inserita all’interno di uno scenario che esige la tutela dell’ambiente e del clima, sta diventando di fondamentale importanza.

En aquesta tesi s'ha realitzat un estudi experimental i teòric de la solubilitat de nous absorbents en refrigerants naturals. El mesurament de la temperatura de solubilitat dels nous fluids de treball NH3/LiNO3, NH3/NaSCN i NH3/H2O+NaOH s'ha realitzat a que les dades de solubilitat en la literatura són escassos, antics i incomplets. Per això s'ha dissenyat i construït un nou dispositiu basat en el mètode politèrmic-visual capaç de treballar a pressions moderades-altes per mantenir l’amoníac en estat líquid. També s'ha modelitzat la solubilitat dels sistemes H2O/LiNO3 (validació a pressió atmosfèrica) amb el model LIQUAC i els sistemes NH3/LiNO3 i NH3/NaSCN amb el model Symmetric Electrolyte-NRTL. Per millorar la solubilitat de la barreja H2O/LiBr i fer-la apta en sistemes aero-refrigerats s'ha optimitzat, mitjançant mètode politèrmic-visual, la mescla ) H2O/(LiBr+LiNO3+LiI+LiCl) proposada per Koo et al. (1999) en un rang de composicions de 65% a 69 %, obtenint un ràtio molar òptim 7:1:0.5:0 i un descens d'en la temperatura de solubilitat de 8 K a 35 K respectivament. Així mateix, s'ha modelitzat amb el model Electrolyte-NRTL la pressió de vapor de la mescla de sals de liti òptima obtinguda i l'efecte que té sobre aquesta l'addició de LiCl, obtenint un lleuger descens en la pressió de vapor respecte la mescla proposada per Koo et al. (1999) i una insignificant millora amb l'addició de LiCl. Finalment, s'ha estudiat la metodologia de calorimetria de flux per mesurar el límit de solubilitat de CO2 en amines. Aquesta tècnica permet la simultània determinació de entalpies de dissolució i límit solubilitat, obtenint desviacions menors del 5 % respecte a metodologies específiques.<br>En esta tesis se ha realizado un estudio experimental y teórico de la solubilidad de nuevos absorbentes en refrigerantes naturales. La medición de la temperatura de solubilidad de los nuevos fluidos de trabajo NH3/LiNO3, NH3/NaSCN y NH3/H2O+NaOH se ha realizado debido a que los datos de solubilidad en la literatura son escasos, antiguos e incompletos. Por ello se ha diseñado y construido un nuevo dispositivo basado en el método politérmico-visual capaz de trabajar a presiones moderadas-altas para mantener el amoníaco en estado líquido. También se ha modelizado la solubilidad de los sistemas H2O/LiNO3 (validación a presión atmosférica) con el modelo LIQUAC y los sistemas NH3/LiNO3 y NH3/NaSCN con el modelo Symmetric Electrolyte-NRTL. Para mejorar la solubilidad de la mezcla H2O/LiBr y hacerla apta en sistemas aero-refrigerados se ha optimizado la mezcla H2O/(LiBr+LiNO3+LiI+LiCl) propuesta por Koo et al. (1999) en un rango de composiciones de 65 % a 69 %, obteniendo un ratio molar óptimo 7:1:0.5:0 y un descenso de en la temperatura de solubilidad de 8 K a 35 K respectivamente. Así mismo, se ha modelizado con el modelo Electrolyte-NRTL la presión de vapor de la mezcla de sales de litio óptima obtenida y el efecto que tiene sobre ésta la adición de LiCl, obteniendo un ligero descenso en la presión de vapor respecto la mezcla propuesta por Koo et al. (1999) y una insignificante mejora con la adición de LiCl. Finalmente, se ha estudiado la metodología de calorimetría de flujo para medir el límite de solubilidad de CO2 en aminas. Esta técnica permite la simultánea determinación de entalpias de disolución y límite solubilidad, obteniendo desviaciones menores del 5 % respecto a metodologías específicas.<br>This thesis has made an experimental and theoretical study of the solubility of new absorbents in natural refrigerants. The measurement of solubility temperature of the new working fluids NH3/LiNO3, NH3/NaSCN and NH3/(H2O+NaOH) is due to the fact that solubility data available in literature is scarce, old and incomplete. For this purpose it has designed and built a new experimental device based on visual-polythermal method with the capacity to work at moderate-high pressures to maintain ammonia in liquid sate. It has also been modelled the solubility of H2O/LiNO3 systems (validation at atmospheric pressure) and the model LIQUAC NH3/LiNO3 and NH3/NaSCN systems with Symmetric Electrolyte-NRTL model. To improve the solubility of the mixture H2O/LiBr and make it suitable in aero-cooled systems it has been optimized, by polythermal-visual method, the mixture H2O/(LiBr + LiNO3 + LiI + LiCl) proposed by Koo et al. (1999) in a composition range from 65% to 69%, obtaining an optimum molar ratio of 7:1:0.5:0 and a decrease in the solubility temperature from 8 K to 35 K respectively. Furthermore, it has been modelled with Electrolyte NRTL model the vapour pressure of the optimum mixture of lithium salts obtained and the effect of the LiCl addition, obtaining a slight reduction in the vapour pressure regarding the mixture proposed by Koo et al. (1999) and an insignificant improvement with the addition of LiCl. Finally, it has been studied the flow-calorimetric methodology to measure the solubility limit of CO2 in amines. This technique allows the simultaneous determination of enthalpies of solution and solubility limit, obtaining deviations lower than 5% regarding specific methodologies.

A high pressure pool boiling experiment of pressurized R134a is designed and built, utilizing thermochromatic liquid crystal techniques. Liquid crystals thermo-chromatography uses encapsulated liquid crystals that are sensitive to temperature. When exposed to hot temperatures the crystal reflect a blue/violet color, and when exposed to cooler temperatures it reflects a red/orange color. The color value or hue is proportional to its temperature. Using this technique this experiment is capable of studying the physics and thermodynamics of refrigerants under nucleate pool boiling. The main objective of this experiment was the design of the experimental setup. Various designs were tested and validated, of which all incorporated a pressure resistant chamber constructed out of aluminum and glass viewing ports. Design parameters such as the heating element thickness were verified using a transient FEA thermal model. This model, which was developed in ANSYS, verified that this design would be able to capture the thermal response of the thermochromatic liquid crystals. This analysis concluded that a negligible error of 0.02&deg;C is expected due to transient effects. Difficulties were encountered during early stages of development; most notable were imaging limitations such as low camera frame-rates and poor resolution. Since a TLC technique was used to measure the temperature of the boiling surface, a camera system fast enough to capture the thermal response was needed. At bubble frequencies of 30 nucleations per second, it was necessary for the camera to have much higher frame rates. Through the use of two synchronized cameras, the surface temperature, position, size and shape of the bubbles were recorded simultaneously. Two camera systems were designed and tested. The first system consisted of a high speed CMOS camera capable of capturing 1,000 frames per second, and an RBG CCD color camera capable of 30 Frames per second. However, this system was limited the slow frame rate and low resolution of the RBG camera. The second system used two high resolution and fast shutter speed cameras, which were able to capture fast bubble nucleations. This method required the assumption that under constant operating conditions, the path of one bubble was identical to the next. This method was tested utilizing the high speed camera, and was shown that there was less than a .04% deviation from the path any bubble to that of the next. Detailed analysis of nucleating surface temperatures using thermochromatic liquid crystal technique and temporal-temperature response under various heat flux and at 813.6kPa (118Psia) and 882.5kPa (128Psia) was performed. It is seen that temperature distribution is quite varied in each case. At high pressures the size of nucleation site decreases, giving rise to an increase in the surface temperature. Bubble growth is also analyzed through the use of high speed cameras and compared to temperature distributions. Simultaneous temperature and bubble size measurements provided a correlation between bubble growth and heat transfer. Boiling parameters such as bubble frequency, bubble size, and contact area are also analyzed. From the surface temperature plots, the local and average heat transfer coefficients were calculated as a function of time and bubble dynamics.<br>M.S.<br>Department of Mechanical, Materials and Aerospace Engineering;<br>Engineering and Computer Science<br>Mechanical Engineering

O presente estudo envolve a análise teórico-experimental da transferência de calor através do mecanismo de ebulição em um único tubo e em banco com três tubos horizontais. A análise da literatura permitiu levantar os parâmetros que podem influenciar o coeficiente de transferência de calor na ebulição em banco de tubos e algumas correlações. O aparato experimental foi adaptado para realização de ensaios envolvendo refrigerantes halogenados, 3 tubos dispostos em fileiras paralelas e 3 distintos diâmetros. Nos experimentos foram utilizados os refrigerantes R-11, R-123 (baixa pressão) e R-134a (média pressão), tubos de latão aquecidos internamente com resistências elétricas, com fluxo de calor específico variando entre 1 e 40 kW/'M POT.2'. Através dos resultados foram observados alguns comportamentos inéditos na influência do acabamento superficial e em banco de tubos. Com base no banco de dados levantado, foi desenvolvida uma correlação para o coeficiente de transferência de calor em ebulição em banco de tubos. Os resultados obtidos por esta correlação apresentaram reduzidos desvios em relação aos experimentais.<br>The research reported herein is a theoretical and experimental investigation of nucleate boiling heat transfer in an isolated tube and a row of three horizontal tubes. The literature review provided enough information to raise the intervening physical parameters and several correlations. The experimental bench has been developed and adapted to perform experiments with several refrigerants, three different tube diameters, and to accommodate a row of three parallel tubes. The experiments have been carried out with refrigerants R-11, R-123 and R-134a. Heating of the brass tubes has been provided by tubular electrical heaters inserted inside the tubes. The heat flux varied from 1 to 40 kW/'M POT.2'. Experiments have been carried out by successively heating two and three tubes. Effects of boiling in tubes underneath (lower level) have been investigated. Finally a correlation for the heat transfer coefficient in successive tubes of a tube bank has been developed. The correlation presents good accuracy with respect to data from the present investigation.

The temperature separation effect (TSE) is a unique thermal phenomenon occurring in a vortex tube (VT). This creates the possibilities of incorporating a VT in various thermal systems to improve their overall system efficiency. Any improvement will be strongly dependent on the working fluid choices, VT geometric parameters, and the system configurations and operating conditions. However, there appears that no systematic approach for selecting the possible working fluid and for evaluating the performance of a VT when operating in a system is available. Therefore, this research aims at developing a systematic approach to screen possible choices of working fluids, and a system integration procedure to achieve optimal matching of the working fluid choice, the VT geometries and the operation conditions, based on using a combined thermodynamic and CFD simulation analysis. A 2-D CFD VT model, created using Ansys Fluent, is used to assess the influence of the VT boundary conditions on the TSE, and to provide detailed information on the flow velocities, temperature and shear stress distributions inside the VT, as well as the cooling/heating effect of the VT. The shape of refrigerant’s T-s diagram is initially used for grouping various refrigerants to either cooling or heating applications of VT. The fluid state at the VT nozzle exit is set as the criterion to identify the suitable VT entry regions on the T-s diagram for individual refrigerants. The thermal-physical properties including isentropic expansion exponent, J-T (Joule-Thomson) coefficient, thermal diffusivity, kinematic viscosity and density are employed to appraise the relative heating or cooling performance of individual refrigerants. One cooling and one heating system are chosen to illustrate the development and implementation of the proposed system integration procedure. In developing the procedure, a boundary line concept is introduced, which allows suitable VT entry conditions in a system be identified for cooling applications. An iteration procedure is designed to identify the best combination of the VT inlet pressure and degree of superheat for the heating applications for individual refrigerants. A guideline for re-selecting alternative refrigerants and re-dimensioning of VT for improving heating or cooling effect is presented, based on examining their thermal-physical properties under system conditions. The results show that the pressure drop in the VT plays an important role in determining the final heating effect. Key thermal-physical properties, such as thermal diffusivity and kinematic viscosity, are shown to be able to reliably assist the evaluation of the relative cooling/heating performance of different working fluids in closed VT systems. The proposed integration procedure is developed in such a way that it could be easily adapted for evaluation of different system configurations.

We have investigated the coordination chemistry of the tripodal alcohols H₃thme, H₃tmp, Br-mpH₃, H₄peol and their analogues aepH₂and ampH₂, with a particular focus on Mn. In combination with other bridging/chelating ligands such as carboxylates or β-diketonates a plethora of polymetallic clusters whose topologies are based on shared triangular Mn₃ units have been synthesised. 25 New complexes are reported. These are: a family of seven “rod-like” structures; a family of three “wheel-like” structures; two supertedrahedra; a family of deca- and tetradecametallic “planar discs”; edge-sharing bioctahedra and a bioctahedral polymer.

Thesis (Ph. D.)--Michigan State University. Dept. of Mechanical Engineering, 2006.<br>Title from PDF t.p. (viewed on June 19, 2009) Includes bibliographic references (p. 84-89). Also issued in print.

O estudo consistiu em uma análise teórica experimental, com o desenvolvimento de uma correlação, dos parâmetros que afetam a ebulição nucleada de refrigerantes halogenados em superfícies cilíndricas lisas. Uma ampla análise da literatura permitiu levantar os distintos tópicos envolvendo este tema destacando entre eles o presente estudo. A análise da literatura envolveu, ainda, um estudo detalhado dos fundamentos da ebulição e das correlações para a previsão do coeficiente de transferência de calor. Foi projetado e construído um aparato experimental no qual foram realizados ensaios envolvendo superfícies de aço inoxidável, cobre e latão, os refrigerantes R-11, R-123, R-12, R-22 e R-134a, fluxos específicos de calor entre 0,8 e 120 kW/m&sup2, pressões reduzidas de 0,008 a 0,26 e rugosidade aritmética média variando entre 0,02 e 3,3 &#956m. A análise destes resultados possibilitou a verificação de comportamentos físicos inéditos segundo a literatura consultada. Destaca-se a análise de efeitos da condução longitudinal e da resistência térmica de contato entre a superfície de transferência de calor e os termopares. Esta análise foi incorporada ao procedimento de determinação da temperatura superficial, e permitiu estabelecer a distribuição de temperaturas ao longo do perímetro da superfície. Finalmente, com base no banco de dados levantado, foi desenvolvida uma correlação para o coeficiente de transferência de calor em ebulição nucleada plenamente desenvolvida de refrigerantes halogenados.<br>The present research has been focused in a theoretical and experimental analysis of the parameters that affect the pool boiling of halocarbon refrigerants in smooth cylindrical surfaces with the development of a correlation. A wide survey of the literature allowed to list the distinct topics related to this subject and to highlight among them the theme of this study. The literature analysis still involved a detailed study of the pool boiling fundamentals and of the correlations for the pool boiling heat transfer coefficient. An experimental set up has been developed and constructed in which were raise data involving surfaces of stainless steel, copper and brass, the refrigerants R-11, R-123, R-12, R-22 and R-134a, specific heat flux range of 0.8 to 120 kW/m&sup2, reduced pressures from 0.008 to 0.26 and arithmetic mean roughness varying between 0.02 and 3.3 &#956m. The analysis of these results revealed, according to the consulted literature, unpublished physical behaviors. In this work the effects of the longitudinal conduction and of the thermal contact resistance between the heat transfer surface and the thermocouple were analyzed carefully. This analysis was incorporated to the superficial temperature determination procedure and allowed to establish the temperature distribution along the surface perimeter. Finally, using the data base raised in this research, a simple and accurate correlation for the fully developed pool boiling heat transfer coefficient for halocarbon refrigerants applications has been developed.

The use of mixtures of refrigerants in refrigeration cycles can lead to improvements in cycle efficiency. The majority of refrigerant working fluids have been pure fluids. With pure refrigerants temperature profiles between the working fluid and the heat source and sink fluids may not be well matched. Mixtures of refrigerants boil and condense across a temperature range. This property can be used to reduce the mean temperature differences in the heat exchangers, and to improve the matching of the temperature profiles. This leads to higher coefficients of performance (COP). In this thesis, the improvement in refrigeration COP due to mixtures of refrigerants is investigated. Ratification of the Montreal protocol led to the phasing our of chlorofluorcarbon (CFC) refrigerants. Hydrofluorocarbon (HFC) refrigerants are the leading candidates to replace CFCs. Mixtures of HFC refrigerants are examined. An existing pilot plant refrigeration cycle was adapted and modified for use with HFC refrigerants. A binary mixture of difluoromethane (R32) and 1,1,1,2-tetrafluoroethane (R134a) is examined experimentally. The phasing out of CFC refrigerants means that there exists a need for methods which can predict accurately the thermodynamic properties of a proposed replacement refrigerant, from sparse amounts of data. The Cubic Chain-of-Rotators (CCOR) equation of state requires relatively little knowledge of the fluid it describes. CCOR predictions of pure and mixed HFC thermodynamic properties, were compared with published experimental data. Comparisons were also made with the more complex Carnahan-Starling-DeSantis (CSD) equation. The CCOR equation predicted saturated and superheated vapour pressure with satisfactory accuracy. Liquid density was not predicted with the same precision. Vapour density was described no worse than the CSD equation. CCOR description of binary vapour-liquid equilibrium (VLE) was superior to that of the CSD equation. Prediction of VLE data was improved by using optimal interaction constants. It was shown that if an optimal set of interaction constants was located for each experimental data point for bubble point VLE data, the interaction constants exhibited a regular dependence upon temperature and composition. The CCOR equation can be used to provide approximate preliminary thermodynamic data for a new refrigerant, for which little data exists.