Em sistemas frigoríficos de compressão a vapor, o evaporador é o equipamento responsável direto pela retirada de calor de ambientes refrigerados. Por esta razão, este componente é a fonte de investigação do presente estudo, o qual teve por objetivo principal a busca por alternativas para a melhoria de desempenho térmico em sistema frigoríficos. Isto posto, o trabalho foi desenvolvido em três frentes de estudo, as quais encontram-se interligadas entre si: desenvolvimento de programa de simulação de serpentinas resfriadoras; realização de ensaios para determinação das capacidades de trocadores de calor; e revisão bibliográfica circunstanciada do coeficiente de transferência de calor do lado do ar. O programa de simulação desenvolvido, o qual tomou como base o modelo proposto por Rich, mostrou-se ser uma ótima ferramenta de trabalho, tanto no meio acadêmico como no industrial. Como conseqüência da elaboração do programa, fez-se uso de correlações já existentes para determinação dos coeficientes de transferência de calor do lado do refrigerante, Bo Pierre, e do lado do ar, McQuiston, sendo esta última escolhida a partir de um sumário de correlações levantadas quando no estudo dos coeficientes de transferência de calor descritos acima. Para que o programa fosse validado como ferramenta de trabalho, foram realizadas simulações dos ensaios das serpentinas testadas e seus resultados confrontados. Tais resultados apresentam uma adequada concordância,possibilitando validar o programa. Os ensaios foram realizados segundo recomendações da norma ASHRAE 25/1992. Dos dados obtidos nos testes, também foi proposto uma correlação para o fator j-Colburn para superfícies secas; no entanto, tratou-se apenas de especular acerca de seu comportamento com as demais correlações, as quais obtiveram boa concordância. / In refrigerating systems of compression to vapour, the evaporator is the direct responsible equipment for remove of heat of cooling environments. In this way, the component is the source of investigation of the present study, which had for main objective the search for alternatives to improvement the performance thermal in refrigeration systems. The research was developed in three studies fronts, that are connected each other: development of program of simulation of cooling coils; tests to find the performance of heat exchange; and revision bibliographical of the coefficient of heat transfer of air. The developed simulation program, based in the Rich\'s model, it showed to be a good work tool, as in the academic, as in the industrial behaviour. As consequence of the elaboration the program, it was used existents correlations to calculation the coefficients of refrigerant heat transfer, Bo Pierre, and to air, McQuiston, the last one was choose from a correlation\'s summary. Same simulations were made to validated the program of the tested serpentines and the results were confronted. Such results presented an appropriate agreement that contributed to validate the program. The tests were accomplished according to recommendations of the norm ASHRAE 25/1992. One correlation was proposed for the factor j-Colburn for dry surfaces in base of the testes results; however, the purpose was to speculated its behaviour with the other correlations, which obtained good agreement.

Dissipadores de calor baseados em microcanais são apresentados como solução para a remoção de fluxos de calor elevados em espaços restritos, pois proporcionam elevados coeficientes de transferência de calor quando comparados a canais convencionais. Tais trocadores também proporcionam elevadas razões entre a área superficial em contato com o refrigerante por unidade de volume do dissipador. Além dos microcanais, a utilização de nanofluidos também se apresenta como tecnologia com potencial de incremento do coeficiente de transferência de calor. Os nanofluidos consistem na adição de nanopartículas a um fluido base visando alterar suas propriedades de transporte termodinâmicas. Neste contexto, o objetivo do presente estudo é avaliar o coeficiente de transferência de calor para escoamentos monofásicos e ebulição convectiva de nanofluidos aquosos no interior de microcanais. Para isto, foram realizados experimentos em canais com diâmetro de 1,1 mm e comprimento de 200 mm para água deionizada, nanofluidos de alumina com diâmetros de 20-30 e 40-80 nm, nanofluidos de dióxido de silício com diâmetros de 15 e 80 nm, e nanofluidos de cobre com diâmetro de 25 nm. Estas soluções foram ensaiadas para concentrações volumétricas de nanopartículas de 0,001, 0,01 e 0,1, velocidades mássicas de 200, 400 e 600 kg/m2s e fluxos de calor de 20 a 350 kW/m2. A análise dos resultados revelou que a adição de nanopartículas a água deionizada proporciona o incremento do número de Nusselt para escoamentos monofásicos, principalmente na região inicial do tubo. Concluiu-se que os efeitos da adição de nanopartículas a um fluido base no coeficiente de transferência de calor durante a ebulição convectiva estão relacionados ao recobrimento da superfície com uma camada porosa. A deposição de nanopartículas com diâmetro inferior a 30 nm resultou na redução do coeficiente de transferência de calor e das instabilidades térmicas do escoamento em relação a água deionizada. O coeficiente de transferência de calor e as instabilidades térmicas não apresentaram variações significativas da deposição de nanopartículas com diâmetro superior a 40 nm. Por meio da análise da textura das superfícies recobertas e do critério de nucleação proposto por Kandlikar et al. (1997) concluiu-se que tal comportamento encontra-se associado aos efeitos do acabamento superficial na densidade de cavidades de nucleação ativas. / Microchannels based heat exchangers were introduced as a solution to high heat flux removal in restrict spaces due to their high heat transfer coefficients compared to heat exchangers based on conventional channels. The high ratio of surface are per volume is an additional advantage to microchannels in relation to conventional channels. Beside the microchannels technology, the nanofluids also present itself as a technique with potential to increase the heat transfer coefficient. Nanofluids consist of a solution containing nanoparticles dispersed in a base fluid with the goal to improve its thermodynamic and transport properties. In this context, the objective of the present study is to evaluate the heat transfer coefficient for single-phase flow and convective boiling of aqueous nanofluids inside microchannels. Experiments were performed for channels with internal diameter of 1.1mm and 200 mm long for DI-water, nanofluids containing alumina- (nanoparticles diameters of 20-30 and 40-80 nm), silicon dioxide (nanoparticles diameters of 15 and 80 nm), and copper (nanoparticles diameter of 25 nm). These solutions were evaluated for volumetric concentrations of 0.001, 0.01 and 0.1%, mass velocities of 200, 400 and 600 kg/m2s and heat fluxes from 20 to 350 kW/m2. The analysis of the results revealed that the addition of nanoparticles to DI-water causes an increment in the Nusselt number for single phase flows, especially at the inlet of the tube. The results for flow boiling indicated that the effects of adding nanoparticles to the base fluid are related to the deposition on the heating surface of a nanoparticles porous layer due to the boiling process. The deposition of nanoparticles smaller than 30 nm promoted a reduction of the heat transfer coefficient compared to DI-water on a clean surface, and thermal instabilities were minimized. For the deposition of nanoparticles larger than 40 nm these parameters did not presented significant variations in comparison to DI-water. A combined analysis of the surfaces finishing and the criterion of Kandlikar et al. (1997) for bubble nucleation revealed that such behaviors are correlated to the effects of the surface texture associated to the boiling process on the density of active nucleation cavities.

Orientadores: Carlos Teofilo Salinas Sedano, Kamal Abdel Radi Ismail / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-25T08:57:38Z (GMT). No. of bitstreams: 1 Silva_RaqueldaCunhaRibeiroda_D.pdf: 3855986 bytes, checksum: dc3f065365a2224c57128ddbedffab68 (MD5) Previous issue date: 2014 / Resumo: O fenômeno de formação de frost é encontrado em muitas aplicações comerciais e industriais de baixa temperatura. A camada de gelo que se forma em uma superfície fria cresce ao longo do tempo, tornando-se mais densa e por este motivo, a formação de frost em equipamentos é responsável pela queda no seu desempenho. O processo de formação de frost é um fenômeno transiente e complicado de avaliar, no qual os processos de transferência de calor e de massa ocorrem simultaneamente em um meio poroso. Neste trabalho apresentam-se medições experimentais da espessura de frost ao redor de cada um dos tubos de um arranjo triangular de cilindros verticais, realiza-se a estimação experimental de uma correlação empírica para a obtenção do número de Nusselt Global sobre o arranjo de tubos com formação de frost baseada nas medições experimentais feita numa seção de teste no interior de um túnel de vento do Laboratório de Armazenamento térmico e tubos de Calor da FEM. Para verificação é feita a estimação de uma correlação empírica para o Número de Nusselt Local em cada cilindro baseado em dados empíricos da literatura e um procedimento inverso de aproximação sucessiva utilizando a solução numérica unidimensional que simula o problema direto de formação de frost em geometria cilíndrica. O mesmo procedimento utilizando os dados experimentais deste trabalho. Encontra-se que os valores dos Números de Nusselt para as duas formas de estimação tem boa concordância. Comparações dos resultados numéricos e experimentais são realizadas e mostra-se boa concordância dos resultados das simulações numéricas para o crescimento da camada de frost utilizando ambas as correlações empíricas / Abstract: In practice, it is found the phenomenon of frost formation in a lot of commercial and industrial refrigeration applications. The layer of ice that it is formed in a cold surface grows over time, becoming more dense and bigger, and for that reason, this formation of frost in refrigeration equipment is responsible for performance decrease in this equipment. The formation of frost is a complicated and transient process, where different process of heat transfer and mass occurs simultaneously in a porous and fine layer. The convective and mass diffusive coefficients are difficult to determinate in fact of the complex physic phenomenon and the variation of the different properties that occurs over time like velocity, temperature, humidity and the geometry of this problem himself. In this work some experimental results of the frost thickness around every cylinder in a triangular arrangement of vertical cylinders are presented, an estimated experimental correlation in order to find Nusselt Global number in this triangular arrangement of cylinders is presented. This correlation is based in the experimental measurements in a wind tunnel situated in the Laboratory of Thermal storage and Fluids in FEM. In order to verify this correlation, another empiric correlation have been estimated in order to find the Nusselt number Global and Local in each cylinder. In this case, it is used data reported in the literature and an inverse method that approximates the numerical solution in one dimension that simulates this problem in cylindrical frost formation geometry. Then the same procedure has been made using the experimental results founded in this work. The different results of the two different correlations have an acceptable concordance. Different comparisons of the numerical and experimental results have been made for the frost growth always validating the results with the numerical simulations of the frost growth using both of the empirical correlations and comparing them with the experimental results that have been founded in this work and with other experimental results founded in the literature / Doutorado / Termica e Fluidos / Doutora em Engenharia Mecânica

Hydrodynamic multiplicity in the trickle flow, or low interaction, regime is a well documented phenomenon. Multiple hydrodynamic states are often presented in the form of hysteresis loops where the hydrodynamic parameter studied are shown as a function of the operating history of the bed, i.e. liquid and gas flow rates. In extreme cases the lower leg, representing an increase in liquid flow rate on a pre-wetted and drained bed, is commonly referred to as the Levec mode. The upper extreme, referred to as the Kan-liquid mode, represents a decrease in liquid flow rate after operation in the high interaction regime. The many reported studies investigating liquid-solid mass transfer in trickle beds have generally used either the dissolution or electrochemical techniques. Numerous researchers have used their data to develop correlations predicting solid-liquid mass transfer coefficients. Most of these studies do not specify the multiplicity mode of operation. Only two studies (Sims et al. (1993) and Van der Merwe, Nicol&Al-Dahhan (2008)) use both the Levec and Kan-liquid operating modes. Both of these studies suggest that solid-liquid mass transfer also exhibit multiplicity behaviour although the trends suggested or speculated differ from each other. Sims et al. (1993) found that a Kan-liquid operated bed will outperform a Levec operated bed; however in contrast to this Van der Merwe et al. (2008) speculated that a Levec operated bed is better suited for liquid limited reactions due to enhanced liquid-solid mass transfer in the Levec mode as a result of faster interstitial velocity. This study showed that solid-liquid mass transfer coefficients, measured with both the dissolution and electrochemical technique, show multiplicity behaviour. Two distinct operating regions were found, which corresponds to the Levec and Kan-liquid modes. Measurements taken using the electrochemical technique yielded solid-liquid mass transfer coefficients larger than those measured using the dissolution method. The experimental results agree with the trend found by Sims et al. (1993) but the mass transfer coefficients in this study were significantly lower. Additionally it was shown that the difference in mass transfer coefficients, in the two modes, cannot be explained by merely compensating for the differences in wetting efficiency and interstitial velocity, suggesting that the Levec mode has a larger percentage of stagnant or poorly irrigated zones. It was also shown that mass transfer coefficients measured at the top of the column is higher than those measured at the bottom, suggesting that the flow structure is changing as a function of axial length. Lastly, with regards to electrochemical measurements of liquid-solid mass transfer, it was shown that measurements using a single particle electrode compared well to that of a multiple packing electrode. / Dissertation (MEng)--University of Pretoria, 2009. / Chemical Engineering / unrestricted

This thesis deals with the calculation procedures for determining the heat transfer coefficient for steam condensers. The first section provides a basic overview for industrial steam condensers and possible procedure for selecting the appropriate type of tube condenser. The next section describes the types of condensation and equations for determining the heat transfer coefficient for various geometries of tubular condensers. In the final section there is performed practical calculation of condenser in Maple software using the above relations. The results obtained in this calculation are compared with the results obtained by professional computational software Chemcad and HTRI.

This thesis in introductory part aims to analyze the available literature on the heat transfer coefficient in labyrinth seals and rotating discs of steam turbines. The available experiment studies were processed to summarize heat transfer coefficients on the rotating parts of the turbine. Then, this thesis specifies a design calculation to determine the heat transfer coefficient in selected parts of the turbine, exercisable for specific geometric and operating parameters. The outcome of this work is simulation of rotor dilation of operating steam turbine in the program Ansys during cold start of turbine.

A great deal of work has been conducted on in-tube condensation in horizontal and vertical smooth tubes. The available literature points to mechanisms governing two-phase condensation heat transfer coefficients and pressure drops, which are directly linked to the local flow pattern for both horizontal and inclined configurations. However, the work has been limited to flow pattern observations, heat transfer, pressure drops and void fractions for both horizontal and inclined tubes at high mass fluxes. No work has been conducted on the analysis of the observed flow patterns and the effect of temperature difference between the average wall temperature and average saturation temperature for different inclination angles at mass fluxes of 100 kg/m2.s and below. The purpose of this study is to carry out a qualitative analysis of flow patterns, and show the effect of temperature difference on the heat transfer coefficient for inclination angles from +90° (upward flow) to -90° (downward flow) at mass fluxes below 100 kg/m2.s. An experimental set-up provided the measurements for the two-phase condensation of R-143a in a smooth tube with an inside diameter of 8.38 mm and a length of 1.5 m. The mass fluxes were 25 kg/m2.s to 100 kg/m2.s, the saturation temperature was 40 °C and the mean qualities were 0.1 to 0.9. A high-speed camera was used to visually analyse and determine the flow patterns for both the inlet and the outlet of the test section. Through the results, eight flow patterns were observed: stratified-wavy, stratified, wavy, wavy-churn, intermittent, churn, annular and wavy-annular. The maximum heat transfer was observed for downward flow between inclination angles of -15° and -30°. The Thome-Hajal flow pattern map correctly predicted horizontal flow patterns, but failed to predict most of the inclined flow patterns. Various flow pattern transitions were identified and proposed for all the investigated inclination angles in this study. Finally, the heat transfer coefficient was found to be dependent on quality, mass flux, temperature difference and inclination angle. / Dissertation (MSc)--University of Pretoria, 2016. / Mechanical and Aeronautical Engineering / MSc / Unrestricted

Nowadays, the filling and solidification of macro-scale injection mouldings can be predicted using commercial CAE software. For micro-injection moulding, the conventional tools do not work for all process conditions. The reasons might be the lack of high quality database used in the simulation and the improperly specified boundary conditions which do not reflect the real state in the cavity. Special aspects like surface tension or "size dependent" viscosity might also be responsible for the inaccuracy of the simulations. In this paper, those aspects related to the boundary conditions were taken into consideration, especially the thermal contact behaviour and the melt compression in the barrel which affects not only the temperature of the melt due to the compression heating, but also reduces the actual volume rate in the cavity. It can be shown that the heat transfer coefficient between the melt and the mould wall has a significant influence on the simulation results. In combination with precise material data and considering the reduction of the volume rate due to the melt compression in the barrel, the heat transfer coefficient may be quantified by means of reverse engineering. In general, it decreases when either the cavity thickness or the injection speed increases. It is believed that a pressure dependent model for the heat transfer coefficient would be more suitable to describe the thermal contact behaviour in micro injection moulding. The melt compression in the barrel affects definitely the filling behaviour and subsequently the heat transfer in the cavity as well, which is especially true for micro parts of high aspect ratio.

There exist several different numerical models for predicting bubble coalescence and breakup using computational fluid dynamics (CFD). Various combinations of these models will be employed to model a bioreactor process in a stirred reactor tank. A mass transfer coefficient, Kla, has been calculated and compared to those found experimentally by Thermo-Fisher Scientific, to validate the accuracy of currently available mathematical models for population balance equations. These include various combinations of bubble breakup and coalescence models coupled with the calculation of mass transfer coefficients.

No / The results of computational fluid dynamics simulation for convective heat transfer of turbulent flow in a cooled convergent-divergent nozzle are reported. The importance of the heat transfer coefficient is to find the most suitable metals for the nozzle wall as well as its application for producing nano-particles. ansys-icem and ansys-cfx 13.0 are used to mesh and simulate fluid flow in the nozzle, respectively. Effects of grid resolution and different turbulence models on the heat transfer coefficient are investigated. Three turbulence models of k-omega, k-epsilon and shear stress transport are applied to calculate the heat transfer coefficient. Stagnation absolute pressure and temperature are 10.3 bara and 840 K, respectively, the same as those in the experimental work. The heat transfer coefficients obtained from simulation are compared with the available experimental data in literature to find out the best suitable mesh grid and the turbulence model. Under the selected operating conditions, k-epsilon and k-omega models have shown the best agreement with the experimental data with the average error of 6.5% and 10%, respectively, while shear stress transport under predicts the values with 16% error.