No description available.

Determination of the heat transfer coefficients for natural and mixed convection in horizontal annuli is important for designing double pipe heat exchangers and for energy storage systems. In part one and two of this study, the 2D numerical solution of the laminar natural convection of water in six internally finned horizontal annuli has been obtained. The fins are attached to the external surface of the inner cylinder. Only the symmetrical half of the horizontal annulus with three equally spaced longitudinal divergent solid and porous fins are considered. The parameters of the problem are Rayleigh number, fin height, permeability and porosity of the porous fin, etc. The above parameters are suitably varied to ascertain their effects on fluid flow and heat transfer. The results show that traditional solid fins provide much higher heat transfer rates compared to the porous fins. Part three of this work deals with mixed convective heat transfer (laminar natural and forced convections) of water in a vented annulus. The forced flow conditions are imposed by providing an inlet at the top and an outlet at the bottom. For various parameters of the problem, the average and local Nusselt numbers along the inner cylinder are calculated for water for both aiding and opposing flows. The fourth part of this study deals with numerical modeling of natural convection of nanofluids in a horizontal cylindrical annulus. Simulations are carried out for Cu-water nanofluids. The results, in general, show that nanoparticles systematically decrease the natural convective heat transfer coefficient on the inner cylinder. Practical and useful correlations are provided for calculating average heat transfer rates from the inner cylinder in the form of average equivalent thermal conductivity and average Nusselt number for all of the four cases discussed above. These correlations are new and will be helpful in designing heat exchangers.

No description available.

Heat exchanger fouling is the deposition of material onto the heat transfer surface causing a reduction in thermal efficiency. A study using Computational Fluid Dynamics (CFD) was conducted to increase understanding of key aspects of fouling in desalination processes. Fouling is a complex phenomenon and therefore this numerical model was developed in stages. Each stage required a critical assessment of each fouling process in order to design physical models to describe the process???s intricate kinetic and thermodynamic behaviour. The completed physical models were incorporated into the simulations through employing extra transport equations, and coding additional subroutines depicting the behaviour of the aqueous phase involved in the fouling phenomena prominent in crystalline streams. The research objectives of creating a CFD model to predict fouling behaviour and assess the influence of key operating parameters were achieved. The completed model of the key crystallisation fouling processes monitors the temporal variation of the fouling resistance. The fouling rates predicted from these results revealed that the numerical model satisfactorily reproduced the phenomenon observed experimentally. Inspection of the CFD results at a local level indicated that the interface temperature was the most influential operating parameter. The research also examined the likelihood that the crystallisation and particulate fouling mechanisms coexist. It was found that the distribution of velocity increased the likelihood of the particulate phase forming within the boundary layer, thus emphasizing the importance of differentiating between behaviour within the bulk and the boundary layer. These numerical results also implied that the probability of this composite fouling was greater in turbulent flow. Finally, supersaturation was confirmed as the key parameter when precipitation occurred within the bulk/boundary layer. This investigation demonstrated the advantages of using CFD to assess heat exchanger fouling. It produced additional physical models which when incorporated into the CFD code adequately modeled key aspects of the crystallisation and particulate fouling mechanisms. These innovative modelling ideas should encourage extensive use of CFD in future fouling investigations. It is recommended that further work include detailed experimental data to assist in defining the key kinetic and thermodynamic parameters to extend the scope of the required physical models.

Proefschrift (doctoral)--Technische Universiteit Eindhoven, 1990. / Includes bibliographical references (p. 265-275).

Thesis (M. Phil.)--University of Hong Kong, 1998. / "1998"--Cover. Includes bibliographical references (leaves 176-184).

Low-temperature energy systems are processes that operate below ambient temperatures and make use of refrigeration cycles, where the main energy consumption is due to the shaft work required to drive the compressors. Very-low-temperature energy systems, also known as cryogenic processes, operate at around -150°C and below. Due to increasing demand of products from cryogenic processes and tighter environmental regulations, existing plants need to be revamped to increase their energy efficiency or adapt to new processing capacities. So, accurate models of the performance of cryogenic processes are needed in order to optimise their operation. The present work proposes a new approach for optimising the operating conditions of existing refrigeration cycles in cryogenic processes, using pure refrigerants, for different plant operating conditions. In this work, the process conditions are considered as given and not considered as variables during the optimisation. The operational optimisation is achieved by integrating models for the part-load performance of centrifugal compressors and models for the simulation of plate-fin heat exchangers (PFHEs), into a single optimisation approach. An optimisation approach similar to the one proposed in this work was not found in the open literature. The optimisation approach varies the refrigerant evaporation temperatures, flow rates and cooling duties, minimum temperature difference in PFHEs, and rotational speed of compressors. The objective function seeks to minimise shaft work demand and the constraints consider the operational limitations of centrifugal compressors (minimum and maximum flow rates) and PFHEs (no temperature crosses and meeting the target temperatures of the process streams). In order to explore the solution space that is generated by the complex interactions between the variables and find an approximation to a global optimum, a multistart optimisation algorithm is implemented. The part-load centrifugal compressor model implemented in this work uses regressed data from their performance curves together with the fan laws. The proposed simulation model of PFHEs represents these units as a ‘fictitious’ heat exchanger network of two-stream matches. The simulation model accounts for single and two-phase streams and for the temperature-dependent physical properties of pure refrigerants (e.g. viscosity, heat capacity, etc.). In addition to the simulation model, design and rating models for PFHEs with single and two-phase streams are also proposed. The examples presented in this work for the design, simulation and rating of single and two-phase streams in PFHEs show that the models proposed can find feasible designs, and can predict the outlet temperature of the process streams within ±3°C for different inlet conditions. The example presented in this work for the operational optimisation of refrigeration cycles shows that savings of around 3% in shaft work consumption (up to £0.86 million per year), for different process throughput, can be achieved using the proposed methodology.

[pt] Trocadores de calor consistindo de tubos aletados são largamente aplicados em sistemas de ar condicionado, aquecedores de ar, radiadores, etc. No presente trabalho é executada uma análise bidimensional da aleta, utilizando-se coeficientes de troca de calor locais, para a obtenção de sua distribuição de temperatura, eficiência e da distribuição da temperatura da corrente de ar ao longo da aleta, em trocadores de calor de tubos elípticos, de excentricidade 0,5 e 0,65 e com uma e duas fileiras, para diversos números de Reynolds. Através da analogia entre transferência de massa e de calor, os coeficientes locais de troca de massa obtidos por Ximenes[1] foram transformados em coeficientes de troca de calor. Novos resultados numéricos de eficiência de aleta para trocadores de calor de tubos elípticos foram comparados com aqueles para trocadores de calor de tubos circulares e a comparação revelou que o arranjo com tubos elípticos é superior ao arranjo com tubos circulares. Esta é uma conclusão positiva, porque o uso de tubos elípticos pode reduzir consideravelmente a queda de pressão, devido a sua melhor configuração aerodinâmica, o que, associado à comparação anterior, mostra que os trocadores de calor de tubos elípticos podem apresentar desempenho bastante superior aos convencionais trocadores de tubos cilíndricos. A comparação entre o uso de coeficientes médios de troca de calor para aletas isotérmicas e coeficientes locais para o cálculo da eficiência de aleta evidenciou que a utilização de coeficientes médios é uma hipótese muito boa, pois o erro relativo entre os dois resultados foi menor do que 7,52 por cento, nos casos analisados. Com a eficiência da aleta e com os números de nusselt médios de aletas isotérmicas é possível calcular a taxa real de transferência de calor dos trocadores de calor nas diversas configurações analisadas neste trabalho. / [en] Heat exchangers consisting of tubes and plate fins have many engineering applications such as air conditioning systems, air heaters, radiators, etc. In the present work it is performed a two-dimensional heat trasnfer analysis of the heat exchangers, using the local heat coefficients, in order to obtain the plate fin temperature distribuition, the fin efficiency and the air bulk temperature distribuition along the free stream. Two arrangements were tested, one-row and two-row elliptical tubes and plate fin heat exchangers of eccentricity 0.5 and 0.65, for several Reynolds numbers. The analogy between the processes of heat and mass trasnfer was invoked to transform the local mass trasnfer coefficients measured by Ximenes [1] in to local heat trasnfer coefficients. Novel numerical results of the fin efficiency elliptical tubes and plates fin hear exchangers were compared with existing data for circular tubes and plate fin heat exchangers [3] and this comparison has shown that the elliptical tubes configuration is better than circular tubes configuration, from the heat transfer point of wiew. This is a important conslusion because the use of elliptical tubes would reduce dramatically the drag force, owing to the aerodynamical configuration, which, in addition to the preceding comparison, shows that the elliptical heat exchangers have the potential to deliver a performance greater than the conventional circular tubes heat exchangers. The comparison netween the use of average heat transfer coefficients for isothermal fins and local coefficients to obtain the fin efficiency revealed that the use of average coefficients is a very good hypothesis, because the relative error between the two results was less than 7.52 por cento for all cases analyzed. With the fin efficiency and the average Nusselt numbers for isothermal fins it is possible to obtain the actual heat trasnfer rate of the heat exchangers analyzed in this work.

[pt] O problema de convecção forçada em dutos triangulares isósceles, com condições de escoamento em regime laminar e perfis de velocidade e temperatura não desenvolvidos, é analisado neste trabalho. Aplicando-se a Técnica de Sublimação de Naftaleno são determinados experimentalmente os valores médios dos coeficientes de transporte de massa em dutos triangulares com as laterais constituídas em naftaleno e a base inerte, para ângulos de 30 graus, 45 graus, 60 graus e 90 graus. São obtidos resultados para números de Reynolds compreendidos entre 100 e 1800, coma relação entre o comprimento do duto e o diâmetro hidráulico entre 2 e 40, cobrindo, desta forma, ampla faixa para o comprimento adimensional, notadamente na região próxima à entrada do canal. A partir da Analogia entre Transferência de Calor e Massa são estimados os coeficientes de transporte de calor para dutos triangulares isósceles com paredes isotérmicas e base adiabática. Esta configuração corresponde à simulação de um coletor solar para aquecimento de ar, cuja placa absorvedora é formada por cavidades triangulares. É realizada análise teórica deste coletor utilizando-se, também, superfícies, seletivas. São apresentadas curvas de performance para diversas condições de trabalho. / [en] Overall mass-transfer coefficients in the laminar entrance regions of 30, 45, 60 ans 90 degrees isósceles triangular ducts were determinated by application of the naphthalene sublimation technique. I in accordance with the analogy between heat and mass- transfer, the experimental conditions corresponded to a heat-transfer situation carachterized by simultaneously developing velocity and temperature fields in a isósceles triangular duct with isothermal lateral walls and adiabatic base. Results were obtained for Reynolds numbers in the range between 100 and 1800, and length-to-hydraulic diameter ratio from 2 to 40, in order to cover a long range of the dimensionless streamwise coordinate values, mainly in the neighborhood of the entrance region. Analytical expressions for the mass-transfer coefficients were determined using the least squares method. In the present analysis the obtained results were employed in a theoretical study of a solar energy collector consisting of a vee-corrugated selective plate. Performace curves for many working conditions were presented.

[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.