Parabolic trough systems are one of the most commercially and technically developed technologies for concentrated solar power. With the current research and development efforts, the cost of electricity from these systems is approaching the cost of electricity from medium-sized coal-fired power plants. Some of the cost-cutting options for parabolic trough systems include: (i) increasing the sizes of the concentrators to improve the system’s concentration ratio and to reduce the number of drives and controls and (ii) improving the system’s optical efficiency. However, the increase in the concentration ratios of these systems requires improved performance of receiver tubes to minimise the absorber tube circumferential temperature difference, receiver thermal loss and entropy generation rates in the receiver. As such, the prediction of the absorber tube’s circumferential temperature difference, receiver thermal performance and entropy generation rates in parabolic trough receivers therefore, becomes very important as concentration ratios increase. In this study, the thermal and thermodynamic performance of parabolic trough receivers at different Reynolds numbers, inlet temperatures and rim angles as concentration ratios increase are investigated. The potential for improved receiver thermal and thermodynamic performance with heat transfer enhancement using wall-detached twisted tape inserts, perforated plate inserts and perforated conical inserts is also evaluated. In this work, the heat transfer, fluid flow and thermodynamic performance of a parabolic trough receiver were analysed numerically by solving the governing equations using a general purpose computational fluid dynamics code. SolTrace, an optical modelling tool that uses Monte-Carlo ray tracing techniques was used to obtain the heat flux profiles on the receiver’s absorber tube. These heat flux profiles were then coupled to the CFD code by means of user-defined functions for the subsequent analysis of the thermal and thermodynamic performance of the receiver. With this approach, actual non-uniform heat flux profiles and actual non-uniform temperature distribution in the receiver different from constant heat flux profiles and constant temperature distribution often used in other studies were obtained. Both thermodynamic and multi-objective optimisation approaches were used to obtain optimal configurations of the proposed heat transfer enhancement techniques. For thermodynamic optimisation, the entropy generation minimisation method was used. Whereas, the multi-objective optimisation approach was implemented in ANSYS DesignXplorer to obtain Pareto solutions for maximum heat transfer and minimum fluid friction for each of the heat transfer enhancement techniques. Results showed that rim angles lower than 60o gave high absorber tube circumferential temperature differences, higher receiver thermal loss and higher entropy generation rates, especially for flow rates lower than 43 m3/h. The entropy generation rates reduced as the inlet temperature increased, increased as the rim angles reduced and as concentration ratios increased. Existence of an optimal Reynolds number at which entropy generation is a minimum for any given inlet temperature, rim angle and concentration ratio is demonstrated. In addition, for the heat transfer enhancement techniques considered, correlations for the Nusselt number and fluid friction were obtained and presented. With heat transfer enhancement, the thermal efficiency of the receiver increased in the range 5% – 10%, 3% – 8% and 1.2% – 8% with twisted tape inserts, perforated conical inserts and perforated plate inserts respectively. Results also show that with heat transfer enhancement, the absorber tube’s circumferential temperature differences reduce in the range 4% – 68%, 3.4 – 56% and up to 67% with twisted tape inserts, perforated conical inserts and perforated plate inserts respectively. Furthermore, the entropy generation rates were reduced by up to 59%, 45% and 53% with twisted tape inserts, perforated conical inserts and perforated plate inserts respectively. Moreover, using multi-objective optimisation, Pareto optimal solutions were obtained and presented for each heat transfer enhancement technique. In summary, results from this study demonstrate that for a parabolic trough system, rim angles, concentration ratios, flow rates and inlet temperatures have a strong influence on the thermal and thermodynamic performance of the parabolic trough receiver. The potential for improved receiver thermal and thermodynamic performance with heat transfer enhancement has also been demonstrated. Overall, this study provides useful knowledge for improved design and efficient operation of parabolic trough systems. / Thesis (PhD)--University of Pretoria, 2015. / tm2015 / Mechanical and Aeronautical Engineering / PhD / Unrestricted

Orientador: Leonardo Goldstein Junior / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-24T09:03:09Z (GMT). No. of bitstreams: 1 Fassani_FabioLuis_D.pdf: 4581004 bytes, checksum: 522f53b51ed91bca9f748a16cea9d03e (MD5) Previous issue date: 2014 / Resumo: A combustão de óleo combustível em equipamentos industriais é um processo que fornece energia para aquecimento, geração de vapor e produção de energia elétrica, entre outros. A fuligem que se forma durante a combustão ocasiona aumento da taxa de transferência de calor por radiação entre os gases produtos da combustão e as superfícies de troca de calor do equipamento de processo, o que ocorre devido à luminosidade da chama. O gás natural tem sido utilizado para substituir o óleo combustível, porém esse gás produz pouca fuligem e pode ocasionar diminuição das trocas térmicas nos equipamentos projetados para óleo combustível. Assim, observa-se que o conhecimento da relação entre a concentração de fuligem na chama e a transferência de calor fornece subsídios para o projeto de caldeiras e fornalhas. O problema abordado nesse trabalho consiste em determinar a relação entre a transferência de calor e a fração volumétrica de fuligem em uma câmara de combustão cilíndrica horizontal, considerando a rotação imposta ao escoamento. O foco principal do trabalho é experimental. Para conduzi-lo, foi projetada e construída uma câmara de combustão dotada de um queimador que induz rotação ao escoamento. A concentração de fuligem foi determinada a partir da variação da intensidade de feixes de luz laser ao passar pela câmara. Complementarmente ao trabalho experimental realizado, para estimar a influência da presença de fuligem, do comprimento da chama e outros parâmetros sobre as trocas térmicas, foi elaborada uma simulação numérica da transferência de calor em uma câmera de combustão cilíndrica, empregando-se o método das zonas. A principal contribuição do trabalho foi indicar que a fração volumétrica de fuligem depende tanto do excesso de ar quanto do número de rotação do escoamento e que para o maior excesso de ar considerado, quanto menor a fração volumétrica de fuligem, menor a taxa de transferência de calor. Os resultados numéricos sugerem que a adoção de um padrão de liberação de energia pela chama foi o fator preponderante para a intensificação das trocas de calor / Abstract: Fuel oil combustion in industrial equipment is a process that provides energy for heating, steam generation and production of electrical energy. Soot produced during combustion causes an increase in the radiation heat transfer rate between the combustion gases and the heat exchange surfaces of the process equipment, due to the flame luminosity. Natural gas has been used as a substitute to fuel oil, but this gas produces little soot and may cause a decrease in the thermal exchanges that take place in equipment originally designed to burn fuel oil. Thus, the knowledge of the relationship between soot concentration in the flame and heat transfer provides subsidies for the design of boilers and furnaces. The issue addressed in this work is to determine the relationship between heat transfer and the soot volume fraction in a horizontal cylindrical combustion chamber, considering the swirling intensity of the flow. The main focus of the work is experimental. To accomplish it, a combustion chamber equipped with a swirl burner to induce rotation to the flow was designed. Soot volume fraction was assessed by means of a laser beam directed towards the chamber. Additionally to the experimental work, a numerical simulation of the heat transfer in a cylindrical combustion chamber was carried out to estimate the influence of soot, flame release pattern and other parameters on the thermal exchanges. The main conclusion of the work was to indicate that the soot volume fraction depends both on the excess air and the swirl number of the flow. In the highest excess air employed, the lower the soot volume fraction, the lower the heat transfer rate. The numerical results suggest that the flame energy release pattern was the most important contributing factor to increase the heat transfer / Doutorado / Termica e Fluidos / Doutor em Engenharia Mecânica

Submitted by Fabio Sobreira Campos da Costa (fabio.sobreira@ufpe.br) on 2016-04-13T12:23:40Z No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) DISSERTAÇÃO CORRIGIDA E IMPRESSA.pdf: 8529514 bytes, checksum: 22404f929127279cf0e8547f5c3df094 (MD5) / Made available in DSpace on 2016-04-13T12:23:40Z (GMT). No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) DISSERTAÇÃO CORRIGIDA E IMPRESSA.pdf: 8529514 bytes, checksum: 22404f929127279cf0e8547f5c3df094 (MD5) Previous issue date: 2015-08-14 / CNPq / O elevado consumo de energia para climatização das edificações tem motivado a realização de muitos trabalhos de pesquisa na área de eficiência energética. Os telhados das construções, em particular, são responsáveis por uma grande parcela do consumo de energia para aquecimento ou refrigeração. Os telhados dos galpões industriais, por serem de grandes extensões, são os principais meios de absorção energética advinda da radiação solar durante os dias, em especial, os dias de verão. Este trabalho apresenta um modelo, desenvolvido em Matlab, que parte do balanço de energia para a superfície do telhado e resolução do mesmo por meio de equações consolidadas da literatura científica, considerando dados de radiação solar incidente e influências da longitude e latitude locais. Com isto, objetiva-se determinar o aquecimento sofrido por cada tipo de telhado analisado e o fluxo de calor que atravessa os mesmos, a fim de que se possa inferir qual telhado possui maior eficiência energética para cada região do país. Foram analisados 4 tipos de telhado, com e sem revestimento reflexivo e isolamento térmico. Constatou-se que tanto as aplicações do isolamento térmico quanto do revestimento reflexivo proporcionam redução do fluxo para o interior dos edifícios, ocasionando melhorias no desempenho térmico dos mesmos. / The high energy consumption for air conditioning of buildings has motivated the implementation of many research papers in the area of energy efficiency. The roofs of buildings, in particular, account for a large share of energy consumption for heating or cooling. The roofs of industrial buildings, because they are large areas, are the main means of energy absorption arising from solar radiation during the day, especially the summer days. This paper presents a model, developed in Matlab, that part of the energy balance for the roof surface and resolution through consolidated equations of scientific literature, considering data of solar radiation and influences the longitude and latitude locations. With this, the objective is to determine the heating suffered by each type of roof and analyzed the heat flow through them, so that one can infer what roof has greater energy efficiency for each region of the country. 4 roof types were analyzed with and without reflective coating and thermal insulation. It was found that both the thermal insulation applications as reflective coating provide the reduction in flow to the interior of buildings, resulting in improved thermal performance thereof.

Heat treatment processes are used to develop the desired mechanical properties for steels. Unfortunately, heat treatment, especially quenching, can cause distortion. Failure to meet geometry specifications can result in extensive rework or rejection of the parts. A series of quenching simulations, using DANTE, have been conducted on an AISI 4140 steel Navy C-ring distortion coupon and a WPI designed plate with a hole to determine the effects of quenching process parameters including part geometry, agitation during quenching, and quench start temperatures on distortion. The heat transfer coefficients (HTC) of the quenchant with selected pump speeds were measured by CHTE quench probe system, which is the key input for heat treatment simulation. The maximum HTC of the quenching oil was increased from 2350 W/m2K to 2666 W/m2K with higher pump speed. Quenching experiments were also conducted. It was found that the experimental measured gap opening of the standard Navy C-rings increased from 0.307mm without agitation to 0.536mm at a high agitation. Quench start temperature does not have a significant effect on the gap opening. The experimental results showed good agreement with simulation results. The important processing parameter identification was conducted using design of experiments (DoE) coupled with analysis of variance (ANOVA). The effect of processing parameters in decreasing order of importance were determined to be: quenchant type, part geometry, agitation speed, quenching orientation, quenchant temperature, immersion rates, and quench starts temperature. Based on the simulation and experimental results, it was found that the two most import parameters are: 1. The part geometry and size (product design) 2. The temperature dependent heat transfer coefficients between the part and the quenchant (process design) The coupling of these product and process parameters is necessary to apply the systems analysis that must be accomplished to understand the interaction between the part design and process design parameters. This coupling can be accomplished by locally applying the well-known Biot number. Bi (T) = h(T) * L / k(T) Where h(T) = film coefficient or convective heat transfer coefficient [W/m2*K]. LC = characteristic length, which is generally described as the volume of the body divided by the surface area of the body [m]. k(T) = thermal conductivity of the body [W/m*k] The concept of a local Biot number is introduced to quantify the local variations of part size, geometry and heat transfer coefficient. First, a large Bi indicates large temperature gradients within the part. Second, large local (geometry dependent) variations in Bi number will lead to large lateral temperature gradients. Therefore, variations in local Bi can lead to large temperature gradients and therefore high stress during quenching and finally distortion. This local Bi concept can be used in a systems approach to designing a part and the quenching system. This systems approach can be designated as design for heat treating.

With the increasing pressure of providing reliable potable water in a sustainable way, it is important to understand water phase change phenomena (condensation and evaporation) as the water phase change is involved in many processes such as membrane distillation and solar still which can be a feasible choice of supplementing the present potable water access. In the present thesis, we first elucidate the role of wettability of water condensation substrate by combining the droplet growth dynamics and droplet population evolution. The results show that wettability has a negligible effect on water condensation rate in an atmospheric environment. After confirming the role of substrate wettability, we provide a quantitative analysis of the effect of substrate geometry on water condensation in the atmospheric environment. The analysis can help to predict the efficiency of water condensation rate with a given substrate of a certain geometry with the aid of computational simulation tools. The results show that water condensation can be increased by 40% by rationally designing the geometry of the condensation surface. However, the condensation rate in the atmospheric environment is relatively slow due to the presence of non-condensable gas. In order to increase the condensation rate, a relatively pure vapor environment is desired, in which condensed water will be the major heat transfer barrier. Coalescence induced jumping of condensed droplets on superhydrophobic surfaces is an interesting phenomenon to help faster removal of condensed droplets. However, it is still not clear how to optimize the overall heat transfer efficiency by condensation on such surfaces. We observed an interesting phenomenon on a superhydrophobic nano-cones array, on which water preferentially condenses within larger cavities among the nanocones. Droplets growing form larger cavities have larger growth rate. This finding can possibly provide a solution to optimizing heat transfer efficiency. Finally, a nylon-carbon black composite is prepared by electrospinning to enhance water evaporation under solar radiation. The composite shows an interesting light absorption property. In a wet state, the composite can absorb around 94% of the incident sunlight. The composite also shows strong mechanical and chemical stability. Thus, the composite is considered to be a practical candidate to be applied in the solar distillation process.

Eulerian-Eulerian Computational Fluid Dynamics (CFD) techniques continue to show promise for characterizing the internal flow and near-field spray for various fuel injection systems. These regions are difficult to observe experimentally, and simulations of such regions are limited by computational expense or reliance on empiricism using other methods. The physics governing spray atomization are first introduced. Impinging jet sprays and Gasoline Direct Injection (GDI) are selected as applications, and modern computational/experimental approaches to their study are reviewed. Two in-house CFD solvers are described and subsequently applied in several case studies. Accurate prediction of the liquid distribution in a like-doublet impinging jet spray is demonstrated via validation against X-Ray data. Turbulence modeling approaches are compared for GDI simulations with dynamic mesh motion, with results validated against previously available experimental data. A new model for turbulent mixing is discussed. Code performance is thoroughly tested, with new mesh motion techniques suggested to improve scaling. Finally, a new workflow is developed for incorporating X-Ray scanned geometries into moving-needle GDI simulations, with full-duration injection events successfully simulated for both sub-cooled and flash-boiling conditions.

Heat exchangers are used in a wide variety of industrial applications. Augmentation of heat transfer can realize a reduction in heat transfer size and increase the effectiveness and efficiency of heat exchangers. Heat transfer can be enhanced with various methods where the turbulence of the fluid flow is enhanced: by adding ribs, grooves or steps to the channel wall, using helical inserts, or by adding bluff bodies in the channel flow. By using these methods, there is also an increase in pressure drop penalty and larger pumping power is required to achieve the same flow rate. Circular cylindrical bluff bodies have been found to have smaller drag coefficients than square, rectangular or triangular cylindrical bluff bodies in the channel flow. Heat transfer and pressure drop experimental tests were done for eight different circular cylindrical cross-bar arrays at 15 different Reynolds numbers, in the range of 640 to 12 500. Eight different cross-bar configurations were tested: the cylinder diameter to pitch ratios were, d/p = 0.025, d/p = 0.05, d/pi=i0.1 and d/p = 0.2, and the angle to the flow direction, was θ = 90° and θ = 45° for each of the four different diameter-to-pitch ratios. Transient CFD simulations were done using Ansys fluent for d/p = 0.05 and d/p = 0.2, for θ = 90°, at Reynolds numbers 920 and 9 700, to analyze the secondary flow structures in the wake of the cylinders, partly responsible for the heat transfer and pressure drop increase in the channel flow in comparison to the smooth channel. The k-Ω shear stress transport (SST) model was used for the simulations. A mesh dependence study was done for spatial discretization, temporal discretization and validated against the experimental setup. The pressure drop gradient was found from the test data for the hydraulically developed part of the test section to calculate the friction factors. With an increase in Reynolds number, the friction factors decreased until reaching an asymptotic value for all the cross-bar configurations. For θi=i90° the friction factors were larger than for θ = 45° for the same d/p ratio and Reynolds number. With an increase in d/p, the friction factors increased. The largest measured friction factor was f = 0.3, for configuration d/p = 0.2, θ = 90°, at Re = 640 and the smallest measured friction factor f = 0.02, for d/pi= 0.025, θ = 45°, at Re = 12 500. The friction factor ratio was then used to quantify the pressure penalty for using cylindrical cross-bars in the channel flow to enhance heat transfer. The maximum friction factor ratio, f/f0 = 16.7 occurred at Re = 9 700, for d/pi=i0.2, θ = 90° and the minimum friction factor ratio, f/f0 = 2.1, at Re = 640, for d/pi=i0.025, θ = 45°. The average Nusselt numbers were then calculated using the spatial integral average of the local Nusselt numbers. With an increase in Reynolds number, there was an increase in the average Nusselt number for all the cylindrical cross-bar configurations. For larger d/p ratios and θ = 90° cases, the average Nusselt numbers were larger than for smaller d/p ratios and θ = 45°. The largest average Nusselt number was Nuavg = 66.3, at Re = 9 700 for d/p = 0.2, θ = 90° and the smallest average Nusselt number, Nuavg = 8.7, at Re = 640 for d/p = 0.025, θ = 45°. The Nusselt number ratio could then be used to quantify the heat transfer enhancement of the cylindrical cross-bar channel to that of the smooth channel, where the largest Nusselt number ratio was, Nuavg /Nu0,avg = 3.3, for d/p = 0.2, θ = 90°, at Rei=i3 000 and the smallest Nuavg /Nu0,avg = 1.1, for d/p = 0.025, θ = 45°, at Re = 640. The CFD results concluded that the pressure drop increase and heat transfer enhancement were caused by the flow acceleration, flow separation, eddy formation, vorticity increase, and boundary layer deformation next to and behind the cylinders. The Strouhal number for the larger d/p ratios suggested that the unsteadiness in the flow is higher for the cylinder arrays with a larger diameter, increasing both the heat transfer enhancement and friction factor in comparison with the smaller diameter cylinder arrays. Finally, the thermal performance coefficients could be calculated by using the friction factor ratios and Nusselt number ratios. The thermal performance coefficient combines the effects of the heat transfer and pressure penalty increase. The thermal performance coefficients increased from Re = 640 until Rei=i3 000 after which it decreased with an increase in Reynolds number. This is because the pressure penalty starts to outweigh the heat transfer increase caused by the turbulators. The largest thermal performance coefficient was η = 1.6, for d/p = 0.025, θ = 45°, at Re = 3 000, and the lowest, η = 0.79, for d/p = 0.05, θ = 90°, at Re = 640. / Dissertation (MEng (Mechanical Engineering))--University of Pretoria, 2021. / Mechanical and Aeronautical Engineering / MEng (Mechanical Engineering) / Unrestricted

Compressors are widely used across the all technical fields and current pressure on ecology increases the demand for more effective compressor with economical operation costs. The reasons for inefficiencies must be identified during the development process of a new compressor, where simulation tools might become very useful. There are many different tools for compressor analysis and choosing the right one is mostly dependent on the level of detail that must be analyzed. Models based on energy balance seem to be appropriate when the global parameters of a compressor are demanded. These models offer quick results with reasonable degree of accuracy in terms of basic compressor characteristics. The goal of this thesis is to develop such a simulation tool for a reciprocating compressor. The tool can predict compressor behavior based on compressor dimensions and valve properties. The processes inside the cylinder and heat transfer between the components of a compressor are analyzed using energy balance equation. Simulation tools were verified and experimentally validated using two different types of compressors, therefore they might be used for any reciprocating compressor under some conditions. Mathematical solution was developed in Matlab and therefore it is possible to add new sub-models or to couple the actual model with other simulation tools. This work also contains an analysis of heat transfer models used to predict heat transfer coefficient inside the cylinder and comparison with complex numerical approach. Impact of heat transfer on the compressor efficiency was evaluated too.

The aim of this diploma thesis is to investigate the influence of different types of HVAC system outlets on convective heat transfer from a human body. The first part of the thesis consists of an overview of essentials important for understanding the issue, specifically, metabolism, thermoregulation, heat transfer mechanisms, thermal vote and fluid dynamics. The second part defines the main working hypothesis and describes the used experimental approach leading either to confirmation or disproval of the hypothesis. The chosen approach is based on a measurement with thermal mannequin “Newton” using two different configurations: constant surface temperature and constant generated heat flux. In case of the first configuration, the convection intensity indicator was the value of heat flux generated from each of surface segments of the thermal mannequin. Their surface temperature was the indicator when running the experiment using the second configuration. The value was evaluated by the thermal mannequin as well as the thermal camera Flir i7 which provided more detailed division of the surface. The final part of the thesis describes the progress of the experiment itself, represents gathered values involving analysis of contaminants and confirms or disproves the original thesis.

The thesis deals with the evaluation of indoor environment cab passenger car, with a focus on thermal comfort of passengers. The theoretical part contains the fundamentals of heat transfer, analysis of the aspects affecting human thermal comfort and its assessment methods. A brief description of the ventilation and air conditioning systems used in passenger cars is mentioned. The practical part includes numerical simulations of indoor environment, in selected driving modes and at different climatic conditions. Results of simulations lead to evaluation the status of the internal environment in terms of thermal comfort.