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.

D.Ing.

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.

D.Ing.

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