Varying diameter ratios associated with smooth concentric tube-in-tube heat exchangers are known to have an effect on its convective heat transfer capabilities. Much literature exists for predicting the inner tube’s heat transfer coefficients, however, limited research has been conducted for the annulus and some of the existing correlations are known to have large errors. Linear and nonlinear regression models exist for determining the heat transfer coefficients, however, these are complex and time consuming methods and require much experimental data in order to obtain accurate solutions. A direct solution to obtain the heat transfer coefficients in the annulus is sought after. In this study a large dataset of experimental measurements on heat exchangers with annular diameter ratios of 0.483, 0.579, 0.593 and 0.712 was gathered. The annular diameter ratio is defined as the ratio of the outer diameter of the inner tube to the inner diameter of the outer tube. Using various methods, the data was processed to determine local and average Nusselt numbers in the turbulent flow regime. These methods included the modified Wilson plot technique, a nonlinear regression scheme, as well as the log mean temperature difference method. The inner tube Reynolds number exponent was assumed to be a constant 0.8 for both the modified Wilson plot and nonlinear regression methods. The logarithmic mean temperature difference method was used for both a mean analysis on the full length of the heat exchanger, and a local analysis on finite control volumes. Friction factors were calculated directly from measured pressure drops across the annuli. The heat exchangers were tested for both a heated and cooled annulus, and arranged in a horizontal counter-flow configuration with water as the working medium. Data was gathered for Reynolds numbers (based on the hydraulic diameter) varying from 10 000 to 28 000 for a heated annulus and 10 000 to 45 000 for a cooled annulus. Local inner wall temperatures which are generally difficult to determine, were measured with thermocouples embedded within the wall. Flow obstructions within the annuli were minimized, with only the support structures maintaining concentricity of the inner and outer tubes impeding flow. / Dissertation (MEng)--University of Pretoria, 2013. / gm2014 / Mechanical and Aeronautical Engineering / unrestricted

Master of Science / Department of Mechanical Engineering / Bruce Babin / Recent world events are motivating the United States Government to invest in the development of Directed Energy Weapons (DEW). One defense contractor developing the technology, Raytheon Missile Systems Company, is addressing the cooling requirements. To this end, Raytheon has proposed some two-phase (liquid and vapor) heat transfer devices capable of dispersing the high energy densities associated with DEW. The Kansas State University Mechanical and Nuclear Engineering Department has been contracted to characterize the performance of the devices using ammonia as the working fluid. To this end, an Ammonia Test Chamber was reconfigured to perform the experiments. The chamber is now configured to deliver liquid ammonia at saturation pressures ranging from 45 to 115 psia, a sub-cooled liquid temperature of -25oC, and mass flow rates ranging from 0.01 to 0.03 kg/s. The Ammonia Test Chamber can absorb heat loads of up to 5000 W. Measurements of the Critical Heat Flux (CHF) of the device ranged from 173 W/cm2 to 488 W/cm2. This data agrees characteristically with published correlations of CHF values, however the correlations predict lower magnitudes.

Conventional gas turbine combustors operate with a designed drop in pressure over the length of the device. This is desired in order to encourage mixing within the combustor. Compared to this, pulse pressure gain combustors are an alternative to the conventional combustor that produces an increase in static pressure between the inlet and exhaust of the device. The removal of the combustor pressure loss increases the efficiency of the combustion process by increasing the amount of work produced. Many types of pulsed pressure gain combustors exist. Of these, the valveless pulse combustor is the simplest featuring no moving parts. Whilst some research has been conducted into investigating the performance and workings of a pulse combustor, little has been conducted with the view of cooling the combustor. This has been the focus for the research contained herein. The research has focussed on establishing an understanding of the heat transfer characteristics within a pulse combustor tailpipe. This has involved experimental, analytical and computational research on a pulse combustor as well as on a cold-flow model of a pulse combustor tailpipe. This has enabled a study into the feasibility of cooling a pulse combustor to be conducted. The research has found that for conditions where the unsteady velocity amplitude within the cold-flow model of the pulse combustor tailpipe exceeds the mean velocity, an enhancement to the heat transfer coefficient is measured compared to the value expected in a similar non-oscillating flow. When there is no enhancement to the heat transfer coefficient, the cyclic variation of the unsteady heat flux follows the variation of the unsteady pressure within the device. However, at times of enhancement, the instantaneous heat flux structure shows a large deviation from the structure of the pressure field driving the oscillations. This change is shown to be caused by the reversal in the near-wall velocity and may indicate a mechanism for the enhancement in the mean heat flux. The cooling feasibility study showed that with further investigation, it may be possible to cool a pulse combustor within a gas turbine engine.

In order to meet the increasingly strict emissions targets required in modern civil aviation, lean burn combustors are being pursued as a means to reduce the environmental impact of gas-turbine engines. By adopting a lean air/fuel mixture NOx production may be reduced. The increase in proportional amount of high pressure air entering directly into the combustor reduces the amount available for cooling of the combustor liner tiles. A reduced mass of air places restrictions on the porosity of cooling arrays, requiring a departure from applications of pedestal and slotted film cooling typically used to cool double skin combustor liners. An alternative approach applied to lean burn combustors places impingement and effusion arrays on the cold and hot skins respectively for cooling of both sides of the hot liner skin. Although impingement cooling is well established as a means of promoting forced convection cooling, there are many areas on a liner tile where cooling behaviour is not well characterised. Additionally, film cooling reduces combustive efficiency and increases the production of NOx and CO, prompting interest in reducing its use in combustor cooling. The research for this thesis has focussed on investigations into current and proposed geometries to identify methods to enhance cold side cooling in lean burn applications. A fully modelled combustor liner tile has been used for investigation into the impact of structural and pressure blockages on cold side cooling performance of an impingementeffusion array using a transient liquid crystal technique to measure heat transfer performance. Research has found structural blockages can reduce heat transfer performance to ~60% of typical values, with crossflow development due to pressure blockage producing similar reductions in Nusselt values to ~70% of typical. A second investigation explored enhanced cooling geometries combining a distributed impingement feed over roughened channels of pedestals at variable height (H/D) and pitch (P/D). A newly proposed 'Shielded Impingement' concept combines full height pedestals, to protect impingement jets from developing crossflow, with quarter height pedestals for turbulence enhancement of crossflow cooling. The research has found that Shielded Impingement geometries displayed the strongest cooling performance of all tested designs due primarily to increased downstream Nusselt numbers. Pressure losses were comparable to short pedestal geometries, with little apparent effect of full height pedestals. Low pressure losses mean that application to extended channels in line with the full tile geometry is possible.

[pt] Superpondo uma estrutura adequada em forma de colméia a um coletor plano é possível absorver mais eficientemente energia solar. Na formulação de modelo, a troca de calor por radiação entre as superfícies envolvidas foi considerada em duas bandas: solar e infravermelho. As equações resultantes formam um sistema íntegro-diferencial não-linear. Utilizou-se iteração de resolução de equações lineares algébricas para a solução do sistema discretizado. As eficiências obtidas foram comparadas com as de sistema planos, e determinadas para uma faixa dos dez parâmetros adimensionais envolvidos no modelo. Por fim, um programa é colocado à disposição para atender necessidades específicas de usuários interessados. / [en] By placing na appropriate structure in the form of a honeycomb on a plane collector it is possible to absorb solar energy in a more efficient manner. In formulating the model, the radiation heat transfer between the involved surfaces was considered in two bands: solar and infra-red. The resulting equations form a non-linear integro-differential system, an iteration of the resolution of algebraic linear equations was used. The efficiencies obtained were compared with those of plane systems and determined for a range of the ten non-dimensional parameters involved in the model. At the end, a program is offered for attending to specific needs of interested users of same.

[pt] Coeficientes de queda de pressão e transferência de calor foram determinados experimentalmente para escoamento turbulento em dutos circulares e semi-circulares lisos e pinados. Pinos de seção transversal circular foram colocados na superfície interna dos dutos. As experiências foram realizadas utilizado-se vários tipos de trocadores de calor, eram ar e água. Para se atingir condições totalmente desenvolvidas, os trocadores de calor possuíam um cumprimento de entrada de 25 diâmetros . Os coeficientes médios de troca de calor foram determinados através da medida dos coeficientes globais de troca de calor dos trocadores de calor. Números de Nusselt e coeficientes de atrito são apresentados como funções do número de Reynolds do escoamento. A finalidade de tal comparação foi estudar a influência dos pinos de seção circular na queda de pressão e taxa de transferência de calor . Números de Nusselt e coeficientes de atrito são apresentados como funções do número de Reynolds do escoamento. Os resultados para dutos lisos e pinados foram comparados. A finalidade de tal comparação foi estudar a influência dos pinos da seção circular na queda de pressão e taxa de transferência de calor. O desempenho térmico global de superfícies pinadas depende principalmente do número de Nusselt e da eficiência da região. Assim, resultados para a eficiência do pino foram também apresentados. / [en] Pressure drop and heat transfer coeficients have been experimentally determined for turbulent flow in smoth and pin finned circular and semi-circular ducts. Spines of circular cross section were attached to the internal surface of the ducts. The experiments were perfomermed by utilizing several types of heat exchanges. The flowing fluids, in the heat exchangers, were air and water. In order to attain fully developed conditions, the heat exchangers had a starting longth of 25 diameters. The average heat tranfer coefficients were determined by measuring the overall heat tranfer coefficients of the heat exchangers. Nusselt numbers and friction factors are presented as fuctions of the flow Reynolds number . The purpose of such comporison was to study the influence of the spines of circular cross section on the pressure drop and heat transfer rate. The overall thermal performance of pin finned surfaces depends mainly on the Nusselt number ando n the region efficiency. Then, results for the pin fin efficienty were also presented.

[pt] Neste trabalho desenvolveu-se um programa numérico geral capaz de resolver as equações de transporte tridimensionais parabólicas típicas da modelagem de problemas da mecânica dos fluidos e transferência de calor e massa. Utilizou-se o método de diferença finitas com formulação em volumes de controle (volumes finitos) e o esquema de discretização power law. O método de discretização utilizado na direção principal do escoamento (direção parabólica) é totalmente implícito. O sistema de equações obtido é resolvido por métodos iterativos que simultaneamente incorporam a não-lineariade do problema. O aclopamento velocidade-pressão recebe tratamento especial para o plano transversal ao escoamento e para a direção parabólica. O algoritmo SIMPLEC (semi-implicit method for pressure-linked equations consistent) foi utilizado para resolver o aclopamento pressão-velocidade foi proposto no presente trabalho um método análogo ao SIMPLEC. O programa permite resolver escoamentos em coordenadas cartesianas (dutos retos com seção retangular) e coordenadas cilíndricas (dutos retos de seção circular assim como dutos curvos de seção retangular). Problemas teste tridimensionais parabólicos foram resolvidos e comparados com dados disponíveis na literatura, validando o programa e comprovando a generalidade do mesmo. / [en] In the present work, a general purpose computer program hás been developed to solve the three-dimensional transport phenomena equations governing problems of fluid flow, heat and mass transfer. The finite volume method has been used with the power-law discretization scheme. A totally implicit discretization method was used in the main flow direction (parabolic direction). The resulting system of algebraic equations is solved by iterative methods which take into account the non-linearity of the situation. The pressure – velocity coupling has received special treatment for the cross-section plane and for the parabolic direction. The SIMPLEC algorithm (Semi-Implicit Method for Pressure-Linked Equations Consistent) was used to resolve the pressure-velocity coupling in cross-section plane. In the present work, and approach similar to SIMPLEC was used to resolve the pressure-velocity coupling in the parabolic direction. The program allows one to obtain the flow field using cartesian cordinates (straight ducts with rectangular corss-section) and cylindrical coordinates (straigth ducts with circular cross-section) and cylindrical coordinates (straight ducts with circular cross-section as well as curved ducts with rectangular cross-section) Three-dimensional parabolic test problems were solved and the results were compared with available data from the literature, validating the program and ensuring the generality of it.

It is common practice to design water chiller units and heat exchangers in such a way that they do not operate within the transition region. This is mainly due to the perceived chaotic behaviour as well as the paucity of information in this region. Due to design constraints or change of operating conditions, however, exchangers are often forced to operate in this region. This is even worse for enhanced tubes as much less information within this region is available. It is also well known that the entrance has an influence on where transition occurs, adding to the woes of available information. The purpose of this study is thus to obtain heat transfer and friction factor data in the transition region of fully developed and developing flows inside smooth and enhanced tubes, using water as the working fluid, and to develop correlations from these results. The use of different inlets, tube diameters and enhanced tubes was also investigated with regards to the commencement of transition. Heat transfer and pressure drop data were obtained from six different types of tubes with diameters of 15.88 mm (5/8′′) and 19.02 mm (3/4′′). Low fin enhanced tubes with a fin height to diameter ratio of 0.4 and helix angles of 18◦ and 27◦ were investigated. Heat transfer was obtained by means of an in-tube heat exchanger with the cooling of water being used as the test fluid. Reynolds numbers ranged between 1 000 and 20 000 while Prandtl numbers were in the order of 4 to 6. Uncertainties in heat transfer coefficient and friction factors were on average below 2.5% and 10% respectively. Adiabatic friction factor results showed that the use of different inlets influenced the commencement of transition. The smoother the inlet profile the more transition was delayed, confirming previous work done. The effect of increasing tube diameters had a slight delay in transition. Enhanced tubes caused transition to occur at lower Reynolds numbers which was accounted for by the fin height and not the helix angle. Heat transfer results showed that transition occurred at approximately the same Reynolds number for all the different inlets and enhanced tubes. This was attributed to the secondary flow forces influencing the growing hydrodynamic boundary layer. These secondary flow forces also influenced the laminar heat transfer and diabatic friction factors with both these parameters being higher. Turbulent enhanced tube heat transfer results were higher than those of the smooth tube, with the tube with the greatest helix angle showing the greatest increase. Correlations were developed for all the tubes and their inlets and predicted all the data on average to within 3%. / Thesis (PhD)--University of Pretoria, 2010. / Mechanical and Aeronautical Engineering / unrestricted

Energy efficiency has become a growing concern in a world driven by a fossil fuel economy. Recuperated micro-gas-turbine systems offer the possibility of high efficiency power generation for low output power systems. To this end, increasing the performance while decreasing the cost, the weight and the volume of heat exchangers such as recuperators has become a critical research focus. Recent work done by Brayton Energy Canada (BEC) has renewed interest in Wire Mesh Heat Exchangers (WMHE) by introducing a new production method that uses cells of stacked wire mesh sheets that have a thick external shell deposited by cold spray. Fins are then machined in this external shell, creating a heat exchanger. Net shaped pin fins were successfully deposited using Cold Gas Dynamic Spraying (CGDS or simply cold spray) as an additive manufacturing technique to replace the plate fin arrays currently used. This new development is envisioned to save costs while providing higher heat transfer efficiency than traditional fin arrays. Increasing the performance of such fin arrays would yield higher heat exchanger efficiencies and increase the total efficiency of the gas turbine system. The present thesis provides a description of the research performed, as well as the results thereof, with regards to the performance of pin fin arrays produced using cold spray. A review of the relevant literature is performed to establish the motivation of this study and to describe the relevant work that has been performed by other authors in this respect. The research objectives are to evaluate the thermal and hydrodynamic performance of these fin arrays and relate those to the flow structures arising from fluid motion between these extended surfaces. Furthermore, the proposed approach and the experimental equipment that will be used are described in this work. The research objectives were successfully met, with the results obtained from this work presented in the form of peer-reviewed publications.

This research is concerned with the development of a mathematical approach for energy and mass transport in solidification modelling involving a control volume (CV) technique and finite element method (FEM) and incorporating non-physical variables in its solution. The former technique is used to determine an equivalent capacitance to describe energy transport whilst the latter technique provides temperatures over the material domain. The numerical solution of the transport equations is achieved by the introduction of two concepts, i.e. weighted transport equations and non-physical variables. The main aim is to establish equivalent transport equations that allow exact temporal integration and describe the behaviour of non-physical variables to replace the original governing transport equations. The variables defined are non-physical in the sense that they are dependent on the velocity of the moving CV. This dependence is a consequence of constructing transport equations that do not include flux integrals. The form of the transport equations facilitate the construction of a FEM formulation that is applicable to heat and mass transport problems and caters for singularities arising from phase-change, which can prove difficult to model. However, applying the non-physical enthalpy method (NEM) any singularity involved in the solidification process is precisely identified and annihilated.