[pt] Hoje em dia trocadores de calor com tubos aletados são largamente empregados em sistemas de condicionamento de ar, radiadores, etc. O presente trabalho é uma tese teórico-experimental sobre trocadores de calor com uma e duas fileiras de tubos aletados. É feita uma análise bi-dimensional da aleta do trocador e coeficientes locais de troca de calor são utilizados na obtenção da distribuição da temperatura de mistura do ar ao alongo da aleta. Este procedimento é feito para vários números de Reynolds e materiais da aleta. Através da analogia entre transferência de massa e de calor, os coeficientes locais de troca de massa obtidos por Saboya e Sparrow 2, 3 e 6 foram transformados em coeficientes locais de troca de calor. Esta analogia emprega um expoente que geralmente é desconhecido. Devido ao desconhecimento do expoente da analogia, para o caso de trocadores com duas fileiras de tubos, foi realizada uma experiência onde foram coletados dados para o cálculo do expoente. São também apresentados os coeficientes médios de troca de calor para aletas isotérmicas, na forma adimensional de números de Nusselt, para trocadores com uma fileira de tubos aletados centrados e recuados e para trocadores com duas fileiras de tubos aletados. Com a eficiência da aleta e com números de Nusselt médios de aletas isotérmicas, é possível calcular a taxa real de transferência de calor ou a área real de trocar de calor. / [en] Nowadays heat exchangers consisting of tubes and plate fins have many engineering applications such as air contioning systems, radiators, etc. The present work is a theoretical-experimental thesis about one and two row tubes and plate fins heat exchangers. By means of a two-simensional analysis performance, and using local heat trasnfer coefficients, the plate fin temperature distrubution, the air bulk temperature along the stream path and the fin efficiency can be obtained, for several Reynolds numbers and fin materials. Using heat-mass transfer analogy, the local mass transfer coefficientes measured by Saboya and Sparrow 2, 3 and 6 were transformed into local heat trasnfer coeficcients. This analogy employs an exponent usually unknown. An experiment was accomplished, and data was collected for the calculus of such exponent in the case of two row tubes heat exchangers, confugurations. Herein are also presented the average heat trasnfer coefficientes for isothermal plate fins, referring to heat exchangers with central-tube and rear-tube row and two-row tubes heat exchangers configurations. It is possible to obtain the real tax or the real area of heat trasnfer, using the average heat trasnfer coefficients for isothermal plate fins and the fin efficiency.

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M.Ing. / Heat transfer and pressure drop characteristics during in-tube condensation of nonazeotropic mixtures of R22/R142b in a smooth helically coiled copper tube with an inside diameter of 8.11 mm are investigated. The experimental results are compared with prediction from correlation. The coefficient of performance of.the heat pump built and used for experiments has been studied. The mass flux of the refrigerant was varied during the course of the experiments. At similar mass flow rate of fluids, the average heat transfer coefficients for mixtures were lower than those for pure refrigerant R22 used as reference for comparison. Also, the heat transfer coefficients of all the refrigerants increased with increasing mass flux.

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When water vapour condenses at a sub-atmospheric pressure, the pressure drop may be a significant fraction of the absolute pressure. Furthermore the pressure drop in a condenser passage also reduces the absolute vapour temperature and therefore affects the heat transfer capacity of a condenser. For a tubular heat exchanger the pressure loss in the heat exchanger tubes can be minimized by the use of contoured or rounded inlet sections at the inlets of the tubes instead of using a sudden contracting inlet section or a protruding inlet section for the tubes. The pressure loss characteristics of different inlet sections to the tubes were obtained through a literature survey of the pressure loss coefficients. The pressure loss at the inlet sections were also investigated with computational fluid dynamics, using the Star-CD software system. The flow regimes for which the pressure loss was investigated were for the laminar incompressible and turbulent incompressible flow regimes. The inlet sections investigated were a sudden contraction and two rounded inlet sections with a rounding radius of 52% and 105% of the tube diameter respectively. The computational fluid dynamics results of the laminar flow simulations revealed that the pressure loss coefficients of the sudden contraction and rounded inlet sections were very similar. The pressure loss coefficient of the sudden contraction inlet sections only being 3 to 6% higher than the rounded inlet sections. This is due to the dominant effect of viscosity in the laminar flow regime. The viscosity reduces the extent of flow contraction occurring since transverse momentum is damped by the viscous dissipation. The dominant pressure loss mechanism in the laminar flow regime is hydrodynamic flow development. With hydrodynamic flow development the flow velocity profile changes from a uniform velocity profile before the inlet section into a pointed parabolic profile downstream in the tube. The turbulent flow simulation results revealed that the pressure loss coefficients of the rounded inlet sections investigated in this study were very similar. The pressure loss coefficient of the sudden contracting inlet section was higher than the rounded inlet sections’ pressure loss coefficient. The results indicated that rounded tubular inlet sections would be of limited value in the laminar flow regime; it would however be beneficial in the turbulent flow regime. / Prof.J.P. Meyer Prof. L. Pretorius

[pt] Coeficientes de perda de carga foram determinados experimentalmente no escoamento transversal a feixes de uma, duas e três fileiras de tubos de secção circular ou elíptica, com aletas contínuas, e correlacionados em função do Número de Reynolds característico do canal formado entre as aletas, na faixa de fluxo laminar, e isotérmico. A técnica utilizada consistiu no levantamento do campo de pressão do escoamento no canal para diversos valores de vazão. Os resultados são confiáveis, e apresentam um grau de incerteza satisfatório, compatível com a finalidade de aplicação a que esta pesquisa se propõe. A geometria estudada é utilizada em permutadores de calor compactos, principalmente em sistemas de condicionamento de ar. Uma das aplicações deste trabalho é o cálculo da potencia de bombeamento necessária para manter o escoamento através do feixe de tubos aletados de um permutador compacto. / [en] Head loss coefficients were determined by an experimental procedure, in the flow normal to banks with one, two and three rows of tubes of circular or elliptical cross-section, with continuous fins. The coefficients were correlated as function of the Reynolds Number of the channel space between the fins, for the laminar and isothermical flow. The pressure field was measured in the flow direction and in the vicinity of the tubes at several flow rates. The results are reliable, and show a satisfactory degree of uncertainty. The geometry studied here is used in compact heat exchangers, mainly in air conditioning systems. The data gathered in this work may be useful in the evaluation of the pumping power necessary to maintain the flow trhough the bundle of finned tubes of a compact heat exchanger.

[pt] A motivação principal do presente trabalho é analisar numericamente o desempenho de um trocador de calor de tubo duplo, sem impor condições de contorno artificiais na superfície de troca de calor e considerando a possibilidade de cada um dos escoamentos ser laminar ou turbulento. As condições de contorno utilizadas são apenas as compatibilidades de temperatura e fluxo térmico, isto é, condições de contorno naturais. Considera-se, para ambos os fluidos, escoamento incompressível desenvolvido hidrodinamicamente e se desenvolvendo termicamente. Os perfis de velocidades e de difusidades turbulentas a quantidade de movimento e do calor para ambos os escoamentos no tubo e no espaço anular são determinados por um método inverso (semi-analítico), baseado na utilização de dados globais sobre o coeficiente de atrito e o número de Nusselt. O objetivo a ser alcançado na elaboração de um método específico para o problema aqui abordado é a obtenção de formas analíticas fechadas para os perfis desejados, as quais sejam continuas em todo domínio de cálculo. Também é desejável que as equações referentes a um dado escoamento turbulento englobem o escoamento laminar como um caso particular. Os resultados obtidos podem ser classificados em duas categorias diferentes (i) resultados semi-analíticos referentes às difusividades turbulentas em ambos os escoamentos e (ii) resultados da análise numérica de trocador de calor, considerando tanto escoamentos de mesmo sentido (correntes paralelas) quanto de sentidos opostos (contra corrente). Quanto ao trocador de calor, os principais resultados são os referentes aos coeficientes convectivos locais de transferência de calor, com base no cálculo das temperaturas médias de mistura, aos valores locais do coeficiente global de transferência de calor, aos valores médios de todos os coeficientes e, finalmente, ao número de unidades de transferência e à efetividade do trocador de calor. É feita uma ampla variação dos parâmetros relevantes, tais como parâmetros geométricos, números de Reynolds e Prandtl de ambos os escoamentos e a razão das taxas de capacidade térmica. A grande quantidade de informações sobre o presente problema, que é encontrada na literatura permite, por comparação, não apenas a validação do método empregado, como também a verificação da exatidão dos resultados. As equações diferenciais do problema e as condições de contorno pertinentes são resolvidas através de uma análise numérica acoplada para os dois escoamentos. Admite-se que as difusidades térmicas turbulentas independam da condição térmica de contorno na superfície de troca de calor. O método de volumes finitos é utilizado dentro de um procedimento numérico iterativo. O aumento do trabalho computacional devido à análise acoplada dos dois domínios de cálculo é compensado através da parte analítica previamente estabelecida, evitando-se o risco de descontinuidades e eliminando-se a necessidade de interpolações durante o refinamento da malha. / [en] In the present work a coupled numerical analysis for the two flows in a double pipe heat exchanger is intended, considering counter or co-current hydrodynamically incompressible fully developed flow with constant properties, and thermally developing flow. A coupling between the two fluid currents is reached by means of the natural boundary conditions, a compatibility of temperatures and heat fluxes closed to the solid wall which separates the fluid currents. Wide ranges of Reynolds and Prandtl numbers are used, including laminar and turbulent flows and involving different kinds of fluids. Semi-analytical equations are derived for the time-averaged profiles of velocity and both momentum and thermal eddy diffusivities in the tube and concentric annulus. The most relevant features of the proposed profiles are their applicability through all flow field without discontinuities and the laminar flow considered as a particurlar case of the turbulent flow. The determination of the proposed profiles is done by using an inverse method, wich is based on some experimental data or correlations for the fiction factor and the Nusselt number. The results of the present study are classified into two different categories as follows (i) semi-analytical results for the turbulent thermal and momentum diffusivity profiles for both flows and (ii) numerical results for the double pipe heat exchanger, mainly the local values of the mean bulk temperatures, local and average values of the heat transfer coefficients and overall heat transfer coefficient. Finally, the number of transfer units and effectiveness of the double pipe heat exchanger are evaluated. Wide ranges of the characteristics parameters (Reynolds and Prantdl numbers of each stream, geometric parameters and thermal capacity rate ratio) are covered. The differential equations of the problem accompanied by suitable boundary conditions are solved nemerically, considering the thermal diffusivities as independent of the thermal boundary condition at the solid wall which separate the fluid currents. The finite difference scheme, based on the control volume approach is used, and iterative procedures are developed, in order to implement the calculations. The advantage of having a previously stablished analytical solution for some parts of the problem is to compensate the greater computational work due to the coupled calculation domains. Also, it eliminates the risk of discontinuities and the necessity of interpolation during the grid refining. Both the validation of the proposed method and the check of the accuracy of the results are possible, by comparison with the great amount of data, which is available in the bibliography.

[pt] Da segunda lei da termodinâmica, os conceitos de disponibilidade e irreversibilidade são aplicados à trocadores de calor. Equações relacionando a irreversibilidade com a efetividade de trocador de calor são desenvolvidas e aplicadas à vários casos. Relações entre o decréscimo de irreversibilidade e custos operacionais são também estabelecidos. Embora seja comumente dito que o trocador de calor em contra-correntes é menos irreversível que o trocador de calor em correntes paralelas, o presente trabalho mostra que isto não é sempre verdadeiro. Um critério muito simples, baseado na mínima perda de disponibilidade, foi estabelecido. Este critério indica, para os dois tipos de trocadores de calor, qual deles tem a irreversibilidade mínima. Foi também provado que a irreversibilidade do trocador de calor tem um ponto de máximo dado por E igual a Cmax/(Cmin mais Cmax), onde e é a efetividade do trocador de calor. Este valor de e é um parâmetro importante e não depende do tipo de trocador de calor considerado. / [en] From the second Law of Thermodynamics, the concepts of availability and irrervesibility are applied to heat exchangers. Equations relating the irreversibility to the heat-exchanger effectiveness are developed and applied to several cases. Relations between the decrease of irreversibility and operational costs are also established. Although it is commonly Said that the counterflow heat-exchanger is less irrerversible than the parallel flow heat-exchanger, the present work shows that this is not always true. A very simple criterion, based on the minimum loss of availability, was established. This criterion indicares, for the two types of heat exchangers, which one has the minimum irrerversibilty. It has been also proved that the heat-exchanger irreversibility has a point of maximum given by E iqual Cmax/ (Cmin more Cmax), where e is the heat-exchanger effectiveness. This value of e is na important parameter and it does not depend on the type of heat-exchanger under consideration.

This thesis provides an assessment of various computational strategies for modelling the turbulent flow and heat transfer around in-line tube banks. The research has direct application to the heat exchanger of an Advanced Gas-cooled Reactor (AGR). The suitability and accuracy of different Computational Fluid Dynamic (CFD) techniques were investigated first on generic square in-line tube banks where experimental data are available. The assumption of flow periodicity in all three Cartesian directions is initially investigated whereby the domain size was varied. Wall-resolved Large Eddy Simulations (LES) predict an increasing flow asymmetry with decreasing tube spacing. Two dimensional (2D) and three dimensional (3D) Unsteady Reynolds Averaged Navier-Stokes (URANS) models were simulated at the tube spacing known to be close to the flow pattern transition from symmetric to asymmetric. Marked differences were observed between the flow pattern predicted by turbulence models resolving the boundary layer and those that rely on wall functions. Ultimately, an improved understanding of the flow physics and heat transfer mechanisms encountered within in-line tube banks was gained. The assumption of flow periodicity was then removed and the effects of confining walls were investigated by reproducing experimental conditions. The correct pressure forces and heat transfer around the central tubes could only be accurately predicted when the walls in the crossflow direction were modelled. The inclusion of walls in the spanwise direction gave rise to small flow asymmetries which have been reported on similarly-spaced in-line tube banks. The latter half of the thesis focuses on the reasons for the enhanced thermal mixing and 3D secondary flow patterns observed in the in-line section of the AGR heat exchanger. A wall-resolved periodic LES was conducted at the lower Reynolds number of 11,000 along with URANS calculations of the full experimental conditions at both Reynolds numbers 11,000 and 66,000. These calculations required the use of High Performance Computing (HPC) facilities. Large 3D secondary flow structures were predicted that produced the same level of crossflow temperature drifting as that reported experimentally. Multiple upward and downward flow paths were observed which qualitatively explained why the experimental temperature profiles reported at different spanwise locations indicated multiple spirals (or secondary vortices). Quantification of the levels of thermal diffusion were investigated using both decaying temperature spikes and blanked tube platens. Thus the CFD provided recommendations about the thermal diffusivity assumptions used by the AGR heat exchanger code.

M.Ing. / Gold ore deposits are depleted near the surface of the earth and the remaining deposits are located at great depths of over 3000m. Geothermal energy will cause the ventilation air temperature to increase above 50°C, and therefore it must be cooled to an effective working temperature. The cost of providing acceptable air temperatures to the workers in the mine plays an important role in the feasibility of whether to proceed with a mining project. At present the air in South African gold mines is cooled by a number of processes including bulk air coolers and direct contact spray chambers. In all of the above mentioned processes large quantities of water need to be pumped into the mine where it is used as the cooling fluid to reduce the air temperature. One of the recent developments for cooling the ventilation air uses ice, produced on surface in large quantities and transported underground in conventional plastic piping. It is regarded as the most effective system for creating chilled cooling water. To create an even more effective process, the properties of a new coolant will be investigated. In the new process, ice is transported into the mine and then mixed with a brine solution. The presence of the salt in the brine creates a freezing point depression and an endothermic chemical reaction causing the temperature of the mixture to decrease well below zero degrees Celsius. This chilled brine solution will be used instead of water to decrease the temperature of the ventilation air. The aim of the project is to conduct an analytical and experimental evaluation by using ice and brine as the coolant. In the experimental investigation the coolant is pumped over a flat, multi-plate heat transfer surface where the air flow is maintained at a constant rate. This simplifies the mathematical modeling and gives useful relationships between the experimental results and the theoretical results.