La connaissance des propriétés critiques des mélanges binaires est importante pour la conception et le développement des procédés des industries chimiques, mais également, pour l'amélioration et l'extension des modèles thermodynamiques prédictifs. Cependant, des données expérimentales critiques liquide - vapeur de mélanges binaires sont relativement rare dans la littérature. C'est la raison pour laquelle, nous avons acquis un nouvel appareil permettant de mesurer les points critiques de corps purs et de mélanges. Dans une première étape, nous avons mis au point et validé le protocole expérimental, en comparant les résultats de nos mesures aux données de la littérature. Dans une deuxième étape, nous montrons l'intérêt que la mesure de points critiques de mélanges binaires peut présenter pour le développement de deux types de procédés : i) la production de biodiesel par voie supercritique et ii) l'étude de comportement de phases de nouveaux réfrigérants. Les résultats expérimentaux obtenus ont été également utilisés pour déterminer les paramètres d'interactions entre groupes du modèle PPR78 / Knowledge of mixture critical data is not only important for the design and the improvement of chemical processes but also to extend the range of applicability of predictive equations of state. However, mixture critical data comprise only a small percentage of the fluid phase equilibria data available in the open literature. This is the reason why it was decided to acquire a new synthetic-dynamic apparatus capable of determining the critical points of pure substances and multi-component mixtures. In a first step, the validation of the experimental procedure for critical point measurements was performed by comparing the experimental results obtained in this work with literature data. In a second step, the application of mixture critical data to the development of two types of process: i) biodiesel production via supercritical process and ii) phase behaviour of new refrigerants is presented. The experimental critical points measured in this work were also used for determining new group interaction parameters of the PPR78 model

O estudo presente constitui uma análise da influência do acabamento superficial no coeficiente de transferência de calor na ebulição nucleada de refrigerantes halogenados. Dados para três superfícies distintas (cobre, latão e aço inoxidável), dois fluidos refrigerantes (R123 e R134a) e pressões reduzidas entre 0,023 e 0,26 são analisados com o intuito de verificar a influência da rugosidade nestes três parâmetros. O efeito da rugosidade foi avaliado com três acabamentos distintos (massa polidora, lixa e jato de areia) cobrindo uma faixa de rugosidades médias variando desde 0,03 até 10,5 micrômetro. Uma análise de diversas publicações da literatura foi levada a cabo, tendo sido particularmente investigadas algumas correlações que consideram o efeito do acabamento superficial em sua estimativa do coeficiente de transferência de calor. As tendências destas correlações são comparadas entre si e com os dados experimentais. A análise dos resultados permitiu levantar tendências inéditas na literatura consultada. A superfície em ebulição recebeu especial atenção com a obtenção de microfotografias e o cálculo de diversos parâmetros de rugosidade. Foram, ainda, investigados efeitos de envelhecimento da superfície, caracterizado pela diminuição do coeficiente de transferência de calor ao longo do tempo de ebulição. / The present research has been focused in an analysis of the effect of surface finishing on nucleate boiling heat transfer coefficient of halocarbon refrigerants. Experimental data for three different surface material (cooper, brass and stainless steel), two refrigerants (R123 and R134a) and reduced pressures between 0.023 and 0.26 have been analyzed aiming to verify the roughness effects on these three parameters. Three different finishing processes (polishing, emery papering and shot pining) have been used to result in an average roughness range from 0.03 to 10.5 micrometer. An analysis of varied publications and some correlations, particularly those which estimate the effect of surface roughness in heat transfer coefficient, has been done. The tendencies from these correlations have been compared with themselves and with experimental data. These results have shown some effects still unpublished. The boiling surface has received an especial attention, micro-photography has been taken and various parameters have been evaluated. Ageing effects, characterized by the reduction of heat transfer coefficient, have been verified and analyzed.

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.

No description available.

O estudo presente constitui uma análise da influência do acabamento superficial no coeficiente de transferência de calor na ebulição nucleada de refrigerantes halogenados. Dados para três superfícies distintas (cobre, latão e aço inoxidável), dois fluidos refrigerantes (R123 e R134a) e pressões reduzidas entre 0,023 e 0,26 são analisados com o intuito de verificar a influência da rugosidade nestes três parâmetros. O efeito da rugosidade foi avaliado com três acabamentos distintos (massa polidora, lixa e jato de areia) cobrindo uma faixa de rugosidades médias variando desde 0,03 até 10,5 micrômetro. Uma análise de diversas publicações da literatura foi levada a cabo, tendo sido particularmente investigadas algumas correlações que consideram o efeito do acabamento superficial em sua estimativa do coeficiente de transferência de calor. As tendências destas correlações são comparadas entre si e com os dados experimentais. A análise dos resultados permitiu levantar tendências inéditas na literatura consultada. A superfície em ebulição recebeu especial atenção com a obtenção de microfotografias e o cálculo de diversos parâmetros de rugosidade. Foram, ainda, investigados efeitos de envelhecimento da superfície, caracterizado pela diminuição do coeficiente de transferência de calor ao longo do tempo de ebulição. / The present research has been focused in an analysis of the effect of surface finishing on nucleate boiling heat transfer coefficient of halocarbon refrigerants. Experimental data for three different surface material (cooper, brass and stainless steel), two refrigerants (R123 and R134a) and reduced pressures between 0.023 and 0.26 have been analyzed aiming to verify the roughness effects on these three parameters. Three different finishing processes (polishing, emery papering and shot pining) have been used to result in an average roughness range from 0.03 to 10.5 micrometer. An analysis of varied publications and some correlations, particularly those which estimate the effect of surface roughness in heat transfer coefficient, has been done. The tendencies from these correlations have been compared with themselves and with experimental data. These results have shown some effects still unpublished. The boiling surface has received an especial attention, micro-photography has been taken and various parameters have been evaluated. Ageing effects, characterized by the reduction of heat transfer coefficient, have been verified and analyzed.

[pt] Um gerador de pasta de gelo foi desenvolvido para o estudo experimental de suas características de transferência de calor. Uma das características da pasta de gelo é que pode ser bombeada como qualquer líquido. O gerador de pasta de gelo é um evaporador, do tipo trocador de calor de superfície raspada com intensificação mecânica de transferência de calor. Foi estabelecida a influência de vários parâmetros na transferência de calor no gerador de pasta de gelo: a vazão mássica, a velocidade de rotação do raspador, a temperatura da parede na interface, a temperatura de operação da solução aquosa, entre outros. Estudou-se tanto a transferência de calor com ou sem mudança de fase. Utilizaram-se soluções aquosas de etanol com diferentes concentrações. A pasta de gelo era produzida continuamente sem acumulação no evaporador. O gerador era parte de um sistema integral contendo os seguintes componentes: o sistema de medição da fração de gelo on-line, através da medição da massa específica da pasta de gelo; uma bomba helicoidal de cavidade progressiva, que permitia controlar a vazão, por meio de um variador de freqüência; um aquecedor elétrico, atuando como carga térmica, com a possibilidade de variar a capacidade desta carga térmica e, com isto, o controle da temperatura de operação do sistema; e o sistema de aquisição de dados. Foi estabelecido um modelo para determinar, experimentalmente, o número de Nusselt e, por conseguinte, o coeficiente interno de transferência de calor da pasta de gelo. Utilizou-se uma unidade condensadora convencional, com R22 como fluido refrigerante. A pasta de gelo resultante era bombeada continuamente em um circuito fechado, assegurando o estudo de um fluido homogeneamente distribuído, visto que o circuito não permitia a acumulação de pasta de gelo. Os sistemas de geração de pasta de gelo apresentam potencial para reduzir significativamente os custos de capital inicial e operação, quando comparados com tecnologias de sistemas de termoacumulação estáticos de gelo ou dinâmicos, como o ice harvesting. / [en] An ice slurry generator for was developed for the experimental study of its heat transfer characteristics. One of the main characteristic of ice slurry is that it can be pumped as any liquid. The ice slurry generator is an evaporator, with mechanical heat transfer enhancement, by surface scraping. The dependence of several parameters on heat exchanger performance was established. They included: mass flow rate, scraped rotational velocity, surface temperature, solution operating temperature. Single and two-phase flow was studied. Aqueous ethanol solutions, of different concentrations, were used. Ice slurry was produced on a continuous basis. The experimental apparatus consisted of the heat exchanger itself, an on-line ice mass fraction measurement device, a helicoidal positive displacement variable flow rate pump, an electrical heater, the data acquisition system and a R22 condensing unit. The ice slurry was pumped continuously on a closed circuit, providing a homogeneous fluid, as no accumulation of ice was possible. Ice slurry systems have the potential of significant reduction on capital and maintenance costs, when compared to traditional technologies, static or dynamic, of ice thermoaccumulation.

A presente pesquisa trata de um estudo experimental da transferência de calor e da perda de carga de fluidos refrigerantes puros e suas misturas em mudança de fase convectiva no interior de tubos lisos e aqueles dotados de ranhuras internas. Para tanto, foi desenvolvido um equipamento experimental cujo componente básico é composto por um tubo horizontal, aquecido por intermédio de uma resistência elétrica do tipo fita, aderida à superfície externa do tubo. As condições de ensaio variaram numa ampla faixa, permitindo cobrir as condições verificadas na maioria das instalações frigoríficas. Os resultados experimentais foram agrupados em duas faixas de velocidades mássicas: elevadas (G > ou = 200 kg/s.m2), onde prepondera o padrão anular de escoamento, e reduzidas (G < 200 kg/s.m2), predominando o padrão estratificado. Os principais parâmetros que afetam o coeficiente de transferência de calor, tais como, velocidade mássica, fluxo de calor, tipo de refrigerante, temperatura de evaporação e diâmetro do tubo foram analisados. O desempenho termo-hidráulico, relativo ao efeito combinado da transferência de calor e da perda de carga dos tubos ranhurados, foi sensivelmente superior quando comparados aos tubos lisos. A análise dos resultados experimentais permitiu a proposição de correlações para a perda da carga, avaliada através do multiplicador bifásico, &#966L, e para coeficiente de transferência de calor, em tubos lisos e ranhurados. As correlações propostas se mostraram adequadas para aplicações práticas, proporcionando desvios reduzidos em relação aos resultados experimentais. Destacam-se as correlações obtidas para o multiplicador bifásico para tubos microaletados e para o coeficiente de transferência de calor para vazões reduzidas em tubos lisos. Diversos registros fotográficos dos principais padrões de escoamento foram levantados, tendo sido importante na análise e entendimento da mudança de fase. / Present research deals with an experimental study of the heat transfer and pressure drop of pure and mixtures of refrigerants undergoing convective boiling inside horizontal smooth and microfin tubes. An experimental apparatus has been developed and constructed whose main component is a horizontal tube electrically heated. Experimental results have been grouped into two mass velocity ranges: the one corresponding to mass velocities lower than 200 kg/s.m2, where the stratified flow pattern is dominant, and that for mass velocities higher than 200 kg/s.m2, where typically the annular flow pattern can be found. Effects over the heat transfer coefficient of physical parameters such as mass velocity, heat flux, diameter, saturation temperature, and refrigerant have been investigated and analyzed. It has been found out that the thermo-hydraulic performance of microfin tubes is better than that of the smooth ones. Empirical correlations have been proposed for both the two-phase flow multiplier and the heat transfer coefficient for different ranges of operating conditions as well as for smooth and microfin tubes. Results from the proposed correlations can be deemed adequate for practical applications given the limited dispersion obtained with respect to their experimental counterpart. Noteworthy are the results obtained from correlations for both the two phase flow multiplier for microfin tubes and the heat transfer coefficient for the lower range of mass velocities in smooth tubes. Finally, worth mentioning is the photographic essay developed in present research involving the flow patterns that occur under convective boiling of refrigerants in horizontal tubes.

A presente dissertação trata de um estudo teórico-experimental sobre a perda de pressão em canais de diâmetro reduzido durante escoamento bifásico de refrigerantes halogenados. Trocadores de calor baseados na ebulição convectiva, em condições de micro-escala são amplamente estudados devido à intensificação da troca de calor proporcionada e a possibilidade de compactação de sistemas de resfriamento. Proporcionam também a redução do inventário de refrigerante e do material utilizado no processo de fabricação do trocador. Porém, o incremento da transferência de calor é acompanhada pelo aumento da perda de pressão, parâmetro também fundamental para o desempenho do sistema. Para o projeto satisfatório e otimizado destes dispositivos são necessários métodos de previsão de transferência de calor e perda de pressão. Entretanto, no caso de canais de diâmetro reduzido, tais ferramentas não encontram-se disponíveis e trocadores de calor baseados em escoamentos bifásicos no interior de canais de diâmetro reduzido vêm sendo desenvolvidos heuristicamente. Desta forma, inicialmente neste estudo, realizou-se uma revisão crítica da literatura envolvendo critérios de transição entre padrões de escoamento, fração de vazio superficial, perda de pressão no interior de canais com diâmetro reduzido durante escoamento bifásico e os principais métodos de estimativa da perda de pressão para macro e micro-escala. Resultados experimentais para perda de pressão levantados neste estudo em condições adiabáticas para os fluidos R245fa e R134a e tubo com 1,1 mm de diâmetro interno foram descritos e comparados aos métodos preditivos encontrados na literatura. Finalmente um novo método da previsão da perda de pressão foi proposto baseado na correlação de Müller-Steinhagen e Heck (1986), ajustando os valores do coeficiente e do expoente com base nos resultados experimentais levantados. / A theorical and experimental study on two-phase pressure drop inside micro-scale channels has been developed. Recently, the study of flow boiling in micro-scale channel have received special attention from academia and industry due to several advantages that they offer such as minimization of fluid inventory, high degree of compactness of the heat exchangers, better performance and the capacity of dissipate extremely high heat fluxes. The significant heat transfer coefficient enhancement provided by micro-scale channels comes together with a huge pressure drop penalty that impacts the efficiency of the overall cooling system. So, accurate predictive methods to evaluate the pressure drop are necessary for the appropriate design of the system and for its optimization. In the present study, firstly, a critical review on studies from literature was performed that covers criteria of transition between micro- and macro-scale flow boiling, void fraction, frictional pressure drop on micro-scale channels and the leading frictional pressure drop predictive methods. Experimental pressure drop results were acquired under adiabatic conditions for R245fa and R134a fluids and internal diameter tube of 1.1 mm. Then, the leading pressure drop predictive methods were compared against the present database. Also a new correlation based on Muller-Steinhagen e Heck (1986) method was proposed in this work by adjusting new empirical constants based on the present database together with previous results obtained by Tibiriçá et al. (2011) for a 2.3 mm ID tube.

Secondary working fluids (secondary refrigerants, heat transfer fluids, antifreezes, brines) have long been used in various indirect re-frigeration and heat pump systems. Aqueous solutions (water solu-tions) have long been used as single phase (liquid only) secondary working fluids for cooling in supermarkets, ice rinks, heat recovery systems, heat pumps and other applications. However, aqueous solutions are increasingly used also for freezers in supermarkets and other applications in low temperature refrigeration. Of importance when comparing different secondary working fluids for indirect systems are the following basic thermophysical properties: freezing point, density, viscosity, specific heat, and thermal conductivity. Reliable data of these properties are needed to choose suitable fluid and to make technical calculations of an indirect refrigeration or heat pump system. The main intention of this work has been to select thermophysical property data with good or acceptable technical accuracy of a number of aqueous solutions that can be used by the refrigeration and heat pump industry, rather than focusing on a limited number of property values or scientifically very accurate measuring techniques. A thorough literature search was in view of this made to find the most reliable property values of aqueous solutions. Detailed literature references are given for thermo-physical properties of the following aqueous solutions, without other additives: Ethylene and propylene glycol, ethyl and methyl alcohol, glycerol, ammonia, potassium carbonate, calcium, lithium, magnesium and sodium chlorides as well as potassium acetate and potassium formate. Some laboratory measurements were made of most of the fluid types when literature values were incomplete or deemed unreliable. Methods used are briefly described and results are given. Much of the work was reported on in the Engineering Licentiate Thesis: Thermophysical properties of liquid secondary refrigerants, A Critical Review on Literature References and Laboratory Measure-ments (Melinder 1998a). That material forms the basis for the charts and tables used in the IIR-publication Thermophysical properties of liquid secondary refrigerants (Melinder, 1997). The present thesis reports on an update made since 1998, including re-view work done on two additional fluids not covered in Melinder (1998a). The thesis describes how the selection of property values results in tables and charts intended for the industry. Coefficients for poly-nomial equations are generated from these property values using a Matlab program and this material is intended as a useful tool for computer treatment. Aqueous solution of ethyl alcohol is used as example to see how this process is made. This choice of fluid can also be seen as a test of this method, as the basic thermophysical properties of aqueous solutions of ethyl alcohol present more chal-lenges than the other fluids examined. A comparison is made of a few types of aqueous solutions used as secondary working fluids for two types of applications. The first example is bedrock heat pumps and the second is cooling cabinets in a supermarket. An effort is made to see how the additive con-centration affects the thermal performance. Most aqueous solutions used as single-phase secondary fluids can also be used as ice slurry, a fluid consisting of liquid and ice where small ice crystals are produced, usually with some type of ice generator. The ice crystals are then transported to the cooling object from which heat is removed when ice crystals melt. This results in less temperature change in the cooling object and makes it also possible to reduce the volume flow rate and to use smaller pipe dimensions in the system. In order to choose a secondary fluid for ice slurry use and to make correct technical calculations of the ice slurry system there is a need to examine and evaluate thermo-physical properties and other aspects of ice and of the aqueous solution used. For dimensioning purposes it is of interest to estimate ice mass fraction and enthalpy values and enthalpy-phase diagrams can serve that purpose. This thesis presents enthalpy-phase diagrams made by author that besides isotherms contain lines with ice fraction and lines connecting enthalpies at freezing point and 1, 2, etc. to 10 K below the freezing point curve. / QC 20100609

A comprehensive study of heat transfer and pressure drop of refrigerant R410A during condensation and supercritical cooling at near-critical pressures was conducted. Investigations were carried out at five nominal pressures: 0.8, 0.9, 1.0, 1.1 and 1.2xpcrit. The refrigerant was tested in commercially available horizontal smooth tubes of 6.2 and 9.4 mm I.D. Heat transfer coefficients were measured using a thermal amplification technique that measures heat duty accurately while also providing refrigerant heat transfer coefficients with low uncertainties. For condensation tests, local heat transfer coefficients and pressure drops were measured for the mass flux range 200 G 800 kg/m2-s in small quality increments over entire vapor-liquid region. For supercritical tests, local heat transfer coefficients and pressure drops were measured for the same mass flux range as in the condensation tests for temperatures ranging from 30 110oC. Condensation heat transfer coefficients and pressure drops increased with quality and mass flux. The effect of reduced pressure on heat transfer is not very significant, while this effect is more pronounced on the pressure gradient. The flow regime transition criteria of Coleman and Garimella (2003) were used to initially designate the prevailing flow regimes for a given combination of mass flux and quality. The condensation data collected in the present study were primarily in the wavy and annular flow regimes. During supercritical cooling, the sharp variations in thermophysical properties in the vicinity of the critical temperature resulted in sharp peaks in the heat transfer coefficients and sudden jumps in the pressure drop. Based on the characteristics of the specific work of thermal expansion (contraction), the data from the supercritical tests were grouped into three regimes: liquid-like, pseudo-critical transition and gas-like regimes. Flow regime-based heat transfer and pressure drop models were developed for both condensation and supercritical cooling. For condensation, the overall heat transfer model predicts 98% of the data within 15% while the overall pressure drop model predicts 87% of the data within 15%. For supercritical cooling, the heat transfer model predicted 88% of the data within 25% while the pressure gradient model predicts 84% of the data within 25%.