This work investigated a numerical approach to the search of a maximum heat transfer rate density (the overall heat transfer dissipated per unit of volume) from a two-dimensional laminar multiscale array of cylinders in cross-flow under an applied fixed pressure drop and subject to the constraint of fixed volume. It was furthermore assumed that the flow field was steady state and incompressible. The configuration had two degrees of freedom in the stationary state, that is, the spacing between the cylinders and the diameter of the smaller cylinders. The angular velocity of the cylinders was in the range 0 ≤ ϖ, ≤ 0.1. Two cylinders of different diameters were used, in the first case, the cylinders were aligned along a plane which lay on their centrelines. In the second case, the cylinder leading edge was aligned along the plane that received the incoming fluid at the same time. The diameter of the smaller cylinder was fixed at the optimal diameter obtained when the cylinders were stationary. Tests were conducted for co-rotating and counterrotating cylinders. The results were also compared with results obtained in the open literature and the trend was found to be the same. Results showed that the heat transfer from a rotating array of cylinders was enhanced in certain cases and this was observed for both directions of rotation from an array which was aligned on the centreline. For rotating cylinders with the same leading edge, there is heat transfer suppression and hence the effect of rotation on the maximum heat transfer rate density is insignificant. This research is important in further understanding of heat transfer from rotating cylinders, which can be applied to applications ranging from contact cylinder dryers in the chemical processes industry and rotating cylinder electrodes to devices used for roller hearth furnaces. / Dissertation (MEng)--University of Pretoria, 2011. / Mechanical and Aeronautical Engineering / unrestricted

Heat exchangers have a wide range of applications and engineers need accurate correlations to optimise the design of these heat exchangers. During the design process, the best compromise between high heat transfer coefficients and relatively low pressure drops is usually in the transitional flow regime. Limited research has been done on tube flow in the transitional flow regime. These studies considered either fully developed flow, or average measurements of developing flow across a tube length. No research has been done with the focus on developing flow in smooth tubes in the transitional flow regime. Therefore, the purpose of this study was to experimentally investigate the heat transfer and pressure drop characteristics of developing flow in the transitional flow regime. An experimental set-up was designed, built and validated against literature. Heat transfer and pressure drop measurements were taken at Reynolds numbers between 500 and 10 000 at three different heat fluxes (6.5, 8.0 and 9.5 kW/m2). A total of 398 mass flow rate measurements, 19 158 temperature measurements and 370 pressure drop measurements were taken. Water was used as the test fluid and the Prandtl number ranged between 3 and 7. The test section was a smooth circular tube and had an inner diameter and length of 11.52 mm and 2.03 m, respectively. An uncertainty analysis showed that the uncertainties of the Nusselt numbers and Colburn j-factors varied between 4% and 5% while the friction factor uncertainties varied between 1% and 17%. Five different flow regimes (laminar, developing laminar, transitional, low-Reynolds-number-end and turbulent) were identified in the first part of the tube during the experiments and nomenclature was developed to more clearly identify the boundaries of the different flow regimes. The developing laminar regime was unique to developing flow and decreased along the tube length. Both the start and end of transition were delayed along the tube length and the width of the transition region decreased slightly. This is in contrast with the results obtained in literature where the effect of the non-dimensional distance from the inlet on fully developed flow in the transition region was investigated. Transition was also slightly delayed with increasing heat flux, but secondary flow effects had no significant influence on the width of the transition region. The relationship between heat transfer and pressure drop was investigated and correlations were developed to predict the Nusselt number as a function of friction factor, Reynolds number and Prandtl number in the laminar, transitional, low-Reynolds-number-end and turbulent flow regimes. Overall, it can be concluded that the heat transfer characteristics of developing and fully developed flow differ significantly and more work needs to be done to fully understand the fundamentals before the heat transfer and pressure drop characteristics are fully understood. / Dissertation (MEng)--University of Pretoria, 2015. / Mechanical and Aeronautical Engineering / Unrestricted

[pt] O fato que a dissolução de pequenas quantidades de polímero de cadeias moleculares longas em fluxo turbulento através dutos reduz drasticamente o atrito, é conhecido há muitos anos. No entanto, muitos aspectos fundamentais deste fenômeno ainda não são bem compreendidos, particularmente os mecanismos de interação entre o polímero e o solvente. O presente trabalho descreve os resultados de um estudo voltado para fornecer informações detalhadas sobre as características da turbulência no escoamento de água com aditivo polimérico através de tubos. Para isto, foram realizados experimentos para medir o campo instantâneo de velocidade em uma seção longitudinal de um tubo, empregando a técnica de Velocimetria de Partículas por Imagem, PIV, em sua versão bidimensional. As grandezas estatísticas turbulentas foram obtidas a partir de uma série de 2.500 campos instantâneos de velocidade medidos para cada configuração de escoamento caracterizada pelo número de Reynolds e pela concentração de polímero em água. Foram medidos campos de velocidade média, de flutuações de velocidade dadas por valores RMS e perfis de tensão turbulenta. Especial atenção foi dedicada ao projeto da seção de testes, de modo a minimizar as distorções de imagem junto à parede do tubo. Na região de visualização, a parede do tubo foi usinada com uma espessura de 500 μm para minimizar a refração óptica da superfície curva. Esta tubulação de paredes finas, em combinação com uma caixa de acrílico preenchida com água posicionada ao redor do tubo, permitiu medições de velocidade com a técnica PIV em posições a cerca de 250 μm da parede do tubo, o que equivale a menos de 10 unidades em coordenadas de parede. A análise dos campos de deslocamento das partículas traçadoras nas imagens capturadas empregou uma combinação da técnica PIV clássica com técnicas de acompanhamento de partículas, resultando em medidas com resolução espacial substancialmente melhorada. Os testes foram conduzidos para uma faixa de números de Reynolds de 23000 a 80000, e para 20 ppm de concentração de polímero Superfloc A110 em água. As medidas da queda de pressão indicaram reduções no coeficiente de atrito da ordem de 50%. Os resultados obtidos demonstraram que a presença do aditivo polimérico produz alterações na estrutura do escoamento turbulento junto à parede, o que se reflete em mudanças na camada de amortecimento do perfil de velocidade. / [en] The fact that dissolving a minuscule amount of long-chain polymer molecules in a turbulent pipe flow reduces drastically the frictional drag, has been known for many years. However, many fundamental aspects of this phenomenon are still not well understood, particularly the mechanisms of interaction between the polymer and the solvent. The present paper describes the results of an ongoing project aimed at providing detailed information on the turbulence characteristics of turbulent pipe flow of water with polymer additive. To this end, an experimental program was conducted for measuring the instantaneous longitudinal velocity field employing a two-dimensional Particle Image Velocimetry technique – PIV. Turbulence statistics were obtained from a series of approximately 2500 instantaneous velocity fields measured for each flow configuration characterized by the pipe Reynolds number and the polymer concentration. Profiles of Time-averaged velocity, RMS velocity fluctuations and turbulent shear stresses were measured. Special attention was dedicated to the test section design so as to minimize image distortions close to the pipe wall. At the visualization region, the pipe wall was machined to a thickness of 500 μm to minimize optical refraction of the curved surface. This thin-walled pipe in combination with a plexiglass box filled the water surrounding the pipe, allowed for PIV velocity measurements at approximately 250 μm from the pipe wall, which is equivalent to less than 10 wall units. A combination of classical PIV with particle tracking techniques was employed in the analysis of the particle displacements, what substantially enhanced the spatial resolution of the measurements. Tests were conducted for a Reynolds number range from 23000 to 80000, and for 20 ppm of Superfloc A110 polymer in water. Wall pressure measurements indicated reductions in the friction factor of the order of 50%. The results obtained demonstrated that the presence of the polymer additive alters the turbulence structure at the wall, what produces a shift in the buffer layer of the velocity profile.

This master’s thesis deals with the design of heat recovery system from melting furnace waste gases. The first part is devoted to a brief description of heat exchangers, with the special importance being placed on the shell-and-tube heat exchanger. The second part contains a calculation of stoichiometric combustion, design of geometrical dimensions, calculation of pressure drops and power. At the end of the thesis there are various possibilities of utilization of the obtained waste heat and their basic economic assessment.

Target of the master´ thesis is a design of heat exchanger air-water with the air flowing inside the pipes and with staggered pipe configuration, based on the engaged and chosen values. Next target is to determine heat exchanger with cross parallel flow and cross countercurrent flow of mediums, pressure drop on both sides air and water and dimensions of intake and outgoing gate on both sides air and water.

The phase-out of ozone-depleting refrigerants, such as R-12 and R-22, according to the Montreal Protocol of 1987, has provided the incentive to increase the thermal efficiency of current heating and refrigeration systems. The purpose of this study was to increase the accuracy of the predictions of both the heat transfer and pressure drop correlations for condensing refrigerants in the Intermittent flow regime. This was done utilizing a novel method involving the temporal and spectral analysis of the light intensity of the local flow regime, as seen through a sight glass. An experimental setup was designed, built and commissioned specifically for this purpose using refrigerant R-22 and a smooth tube. It was found that the accuracy of the mean heat transfer coefficient predictions increased substantially compared to other leading correlations,particularly at low mass fluxes. In terms of the pressure drop,the predictions also increased in accuracy, and it was found that the time fraction method allows for continuous predictions over flow regime transitions when using local flow-pattern-based pressure drop models. This was previously not possible. / Dissertation (MEng (Mechanical Engineering))--University of Pretoria, 2007. / Mechanical and Aeronautical Engineering / unrestricted

<div>In this study, three ducts have been built in Herrick Labs, Purdue University, West Lafayette, representing the real HVAC systems. Different types of filters have been tested in each duct, including MERV8, MERV14 mechanical filters, and MERV13 electret filter. One of this study's main objectives is to compare in-situ filter performance with the MERV rating, which comes from laboratory testing. Eventually, we can explain the variations and have a better understanding of the in-situ filter performance. Another primary objective is to evaluate the aging process of tested filters. Typically, HVAC filters will not be replaced or maintained frequently, so they tend to stay in HVAC systems for an extended period. In this study, the evolution of filter aging performance is tested continuously for the experiment's entire duration, including temporal evolution of pressure drops, filtration efficiencies, and loaded particulate mass on the filter media.</div><div><br></div><div>This study will continue for 52 full weeks. This thesis is a part of the on-going study, including the data up to the first 18 weeks so far. As a result, the pressure drops are steadily increasing over time for most filters due to the natural filter loading process, except for the MERV13 electret filter, which shows no significant change at all. In terms of the filtration efficiencies, two efficiency categories are included: mass-based efficiencies (ePM x ) for sizeintegrated particulate mass (PM) and size-resolved efficiencies. It can be seen that there is no significant change in ePM x for MERV8 and MERV14 filters, MERV13 electret filter whereas shows a degradation in ePM x efficiencies. Size-resolve efficiencies provide additional filtration efficiencies as a function of particle sizes, therefore more comprehensive. It can be found that for MERV8 and MERV14 filters, the efficiencies are slowly increasing for particles above 300 nm, with some variations. MERV13 electret filter has an explicitly decreasing trend for efficiencies across all particle size ranges.</div><div><br></div><div>Besides the filter loading process, another component affecting the filter performance is the environmental factor, such as temperature, relative humidity (RH), and precipitation events, including rainfall and snowfall. As a result, it can be observed that when precipitation events, the pressure drops trend to decrease for that period. It is also evident that decreasing temperature tends to promote pressure drops.</div>

The current study focuses on improving and testing the CTF thermalhydraulics computer code. CTF is a thermalhydraulic code used for subchannel analysis of nuclear power reactors developed as part of the US DOE CASL program and distributed by North Carolina State University. Subchannel analyses are used to predict the local fuel temperatures and coolant conditions inside a complex nuclear fuel assembly. Such calculations are used to improve designs of nuclear fuel, improve operating margins, or perform safety analysis. An important part of the code development process is the verification and validation for its intended use. In this work validation activities are performed using the RISO experiments are modeled in CTF for adiabatic and diabatic cases in annular flow regimes and a limited set of tests in CANDU geometries. The CTF predictions significantly overpredicted the pressure drop for cases involving annular flow conditions. Depending on the application, such overprediction can result in significant errors in the computation of fuel element dryout and other figures of merit. For example, an analysis using fixed pressure boundary conditions CTF predicts much lower subchannel flows and hence fuel element temperatures may be overestimated. On the other hand, for a scenario with mass flux and inlet pressure as boundary conditions, the impact of pressure drop discrepancies on dryout predictions may be lower. Therefore, there is a particular focus in this thesis on the two-phase pressure drop models and the RISO experiment specifically, since the RISO tests involve a range of annular flow conditions which is prototypical of many CANDU accident analysis conditions. In addition to the RISO experiments, 28-element CANDU full scale rod bundle experiments are modeled in CTF for single-phase and two-phase flow conditions. Cases are modeled for crept and uncrept conditions with different bearing pad heights i.e., 1.17 mm and 1.35mm. Pressure drop predictions are compared with the experimental results where single-phase comparisons are in good agreement while an overprediction of ~25% is observed for two-phase conditions. The effect of bearing pads on the subchannel local parameters, like mass flow rate, are also studied. Furthermore, the effect of turbulent mixing rate on subchannel enthalpy distribution in the bundle and CHF in different subchannels is also analyzed. Based on the comparison to the RISO and CANDU 28 element test databases, the overprediction of pressure drop in the annular flow regime needs improvement in the current version of CTF. This overprediction of the frictional pressure drop results from either wall drag or interfacial shear stress phenomena. In this study, it is demonstrated that the issue occurs mostly as a result of interfacial friction factor modelling this work examines several alternative approaches. The results show the Ju’s and Sun’s interfacial friction factor better predicts the results among all the other six correlations implemented in CTF. The major impediment in further testing of CTF is that it lacks the capability to simulate R-134a fluids. Given there is a large database of R-134a two-phase tests, another aspect of this thesis is to extend CTF for application and validation using refrigerants. The current CTF version only supports fluid properties for water and FLiBe salts. By adding R-134a fluid properties the testing and validation range of CTF is broadened for different experiments performed using R-134a fluids. CHF experiments are modeled in CTF and results are compared with experimental data. For local conditions correlation, 2006 water LUT are used to predict CHF and DNBR. The fluid-to-fluid scaling method is applied in CTF when using CTF with R-134a fluid properties for CHF and DNBR predictions to account for the difference in fluid properties between R-134a and the CHF look-up table. / Thesis / Master of Applied Science (MASc) / COBRA-TF (CTF) is a thermalhydraulic code, based on the historical code COBRA-TF, used for subchannel analysis of nuclear power reactors. Subchannel analysis can be used to predict the local fuel temperatures and coolant conditions inside a complex nuclear fuel assembly. CTF is a transient code that simultaneously solves conservation equations for mass, momentum, and energy for the three coolant phases present, i.e. vapor, continuous liquid, and entrained liquid droplet phases. The scope of the current study includes 1) testing the code for conditions relevant to CANDU accident analysis, 2) refinement of the models that are used in two-phase interfacial friction calculations, and 3) inclusion of alternate fluid properties. The testing of CTF is performed with different experimental databases covering CANDU thermalhydraulic conditions. The refinement is done by improving the pressure drop prediction in the annular flow regime by using different interfacial friction factor correlations from earlier studies in the literature. The current CTF version includes water and liquid salt properties (FLiBe) for coolant fluids. Freon (R-134a) fluid properties have been added in CTF in order to broaden the testing range of CTF for different experimental database using R-134a as working fluid.

No description available.

Analytical, closed-form solutions governing thermoelectric behavior are derived. An analytical model utilizing a thermal circuit is presented involving heat transfer into, through, out of, and around a thermoelectric device. A nondimensionalization of the model is presented. Linear heat transfer theory is applied to the model to obtain a series of closed form equations predicting net power output for the thermoelectric device. Fluid streams flowing through shrouded heat sinks with square pin fins are considered for the thermal pathways to and from the device. Heat transfer and pressure drop are characterized in a manner conducive to an analytical model using previously published experimental results. Experimental data is presented which validates and demonstrates the usefulness of the model in predicting power output for commercially available thermoelectric generators. A specific design for a thermoelectric power harvester is suggested consisting of a pattern of thermoelectric generators. An economic model for calculating payback time is developed. An optimization process is demonstrated that allows for the payback time of such a system to be minimized through optimization of the physical design of the system. It is shown that optimization of the thermal pathways dramatically reduces payback time. Optimized design of a system is discussed in light of theoretical cases with feasible payback times.