Forced air cooling of electronic packages is enhanced through the use of extended surfaces or heat sinks that reduce boundary resistance allowing heat generating devices to operate at lower temperatures, thereby improving reliability. Unfortunately, the clearance zones or bypass regions surrounding the heat sink, channel some of the cooling air mass away from the heat sink, making it difficult to accurately estimate thermal performance. The design of an "optimized" heat sink requires a complete knowledge of all thermal resistances between the heat source and the ambient air, therefore, it is imperative that the boundary resistance is properly characterized, since it is typically the controlling resistance in the path. Existing models are difficult to incorporate into optimization routines because they do not provide a means of predicting flow bypass based on information at hand, such as heat sink geometry or approach velocity. <br /><br /> A procedure is presented that allows the simultaneous optimization of heat sink design parameters based on a minimization of the entropy generation associated with thermal resistance and fluid pressure drop. All relevant design parameters such as geometric parameters of a heat sink, source and bypass configurations, heat dissipation, material properties and flow conditions can be simultaneously optimized to characterize a heat sink that minimizes entropy generation and in turn results in a minimum operating temperature of an electronic component. <br /><br /> An analytical model for predicting air flow and pressure drop across the heat sink is developed by applying conservation of mass and momentum over the bypass regions and in the flow channels established between the fins of the heat sink. The model is applicable for the entire laminar flow range and any type of bypass (side, top or side and top both) or fully shrouded configurations. During the development of the model, the flow was assumed to be steady, laminar, developing flow. The model is also correlated to a simple equation within 8% confidence level for an easy implementation into the entropy generation minimization procedure. The influence of all the resistances to heat transfer associated with a heat sink are studied, and an order of magnitude analysis is carried out to include only the influential resistances in the thermal resistance model. Spreading and material resistances due to the geometry of the base plate, conduction and convection resistances associated with the fins of the heat sink and convection resistance of the wetted surfaces of the base plate are considered for the development of a thermal resistance model. The thermal resistance and pressure drop model are shown to be in good agreement with the experimental data over a wide range of flow conditions, heat sink geometries, bypass configurations and power levels, typical of many applications found in microelectronics and related fields. Data published in the open literature are also used to show the flexibility of the models to simulate a variety of applications. <br /><br /> The proposed thermal resistance and pressure drop model are successfully used in the entropy generation minimization procedure to design a heat sink with bypass for optimum dimensions and performance. A sensitivity analysis is also carried out to check the influence of bypass configurations, power levels, heat sink materials and the coverage ratio on the optimum dimensions and performance of a heat sink and it is found that any change in these parameters results in a change in the optimized heat sink dimensions and flow conditions associated with the application for optimal heat sink performance.

In this study, an entropy generation minimization procedure is employed to optimize the overall performance (thermal and hydrodynamic) of isolated fin geometries and pin-fin heat sinks. This allows the combined effects of thermal resistance and pressure drop to be assessed simultaneously as the heat sink interacts with the surrounding flow field. New general expressions for the entropy generation rate are developed using mass, energy, and entropy balances over an appropriate control volume. The formulation for the dimensionless entropy generation rate is obtained in terms of fin geometry, longitudinal and transverse pitches, pin-fin aspect ratio, thermal conductivity, arrangement of pin-fins, Reynolds and Prandtl numbers. It is shown that the entropy generation rate depends on two main performance parameters, i. e. , thermal resistance and the pressure drop, which in turn depend on the average heat transfer and friction coefficients. These coefficients can be taken from fluid flow and heat transfer models. An extensive literature survey reveals that no comprehensive analytical model for any one of them exists that can be used for a wide range of Reynolds number, Prandtl number, longitudinal and transverse pitches, and thermal conductivity. This study is one of the first attempts to develop analytical models for the fluid flow and heat transfer from single pins (circular and elliptical) with and without blockage as well as pin-fin arrays (in-line and staggered). These models can be used for the entire laminar flow range, longitudinal and transverse pitches, any material (from plastic composites to copper), and any fluid having Prandtl numbers (&ge;0. 71). In developing these models, it is assumed that the flow is steady, laminar, and fully developed. Furthermore, the heat sink is fully shrouded and the thermophysical properties are taken to be temperature independent. Using an energy balance over the same control volume, the average heat transfer coefficient for the heat sink is also developed, which is a function of the heat sink material, fluid properties, fin geometry, pin-fin arrangement, and longitudinal and transverse pitches. The hydrodynamic and thermal analyses of both in-line and staggered pin-fin heat sinks are performed using parametric variation of each design variable including pin diameter, pin height, approach velocity, number of pin-fins, and thermal conductivity of the material. The present analytical results for single pins (circular and elliptical) and pin-fin-arrays are in good agreement with the existing experimental/numerical data obtained by other investigators. It is shown that the present models of heat transfer and pressure drop can be applied for a wide range of Reynolds and Prandtl numbers, longitudinal and transverse pitches, aspect ratios, and thermal conductivity. Furthermore, selected numerical simulations for a single circular cylinder and in-line pin-fin heat sink are also carried out to validate the present analytical models. Results of present numerical simulations are also found to be in good agreement.

Thermal radiation is of significant importance in a broad range of engineering applications including high-temperature and large-scale systems. Although the governing equations of thermal radiation have been known for many years, the complexities inherent in the phenomenon, such as the multidimensionality and integro-differential nature of these equations, have made it difficult to obtain an accurate, efficient, and robust computational method. Developing the finite volume radiation method in the 1990s was a significant progress but not a panacea for computational radiation. The major drawback of this method, which is common among all methods that solve for directional intensities, is its slow convergence rate in many situations which increases the solution cost dramatically. These situations include large optical thicknesses, strongly reflecting boundaries, and any other factor that causes strong directional coupling like complex geometries. Several acceleration schemes have been developed in the heat transfer and neutron transport communities to expedite the convergence and reduce the solution cost, but none of them led to a general and reliable method. Among these available schemes, the two most promising ones, the multiplicative scheme and coupled ordinates method, suffer from failing on fine grids and being very complicated for complex scattering phase functions, respectively. In this research, a new computational method, called the QL method, has been introduced. The main idea of this method is using the phase weight concept to relate the directional and average intensities and re-arranging the Radiative Transfer Equation to find a new expression for the radiant heat flux. This results in an elliptic-type equation for the average intensity at each control volume which conserves the radiant energy in all directions in the control volume. This formulation gives the QL method a great advantage to solve for the average intensity while including the directional effects. Since the directional effects are included and the radiant energy is conserved in each control volume, this method is expected to be accurate and have a good convergence rate in all conditions. The phase weight distribution required by the QL method can be provided by a method like the finite volume method or discrete ordinates method. The QL method is applied to several 1D and 2D test cases including isotropic and anisotropic scattering, black and partially reflecting boundaries, and emitting absorbing problems; and its accuracy, convergence rate, and solution cost are studied. The method has been found to be very stable and efficient, regardless of grid size and optical thickness. This method establishes very accurate predictions on the tested coarse grids and its results approach the exact solution with grid refinement.

Shading devices have the potential to reduce peak cooling load and annual energy consumption because they can be used to control solar gain. Thus, the need to model shading devices in a glazing system analysis is important. This thesis deals with various measurement techniques and model development related to solar optics in windows with shading devices. It also considers longwave radiative properties of shading devices via model development and experimentation. The different shading devices examined were roller blinds, insect screens, pleated drapes and venetian blinds. The energy performance of windows with shading devices was modeled using a two step procedure. Solar radiation was considered in the first step by developing a multi-layer solar optical model for glazing/shading systems. This newly developed model is an extension of an existing model for systems of specular glazing layers and includes the effect of layers that create scattered, specifically diffuse, radiation in reflection and/or transmission. Spatially-averaged (effective) optical properties were used to characterise shading layers, including their beam-diffuse split. The multi-layer solar optical model estimates the system solar transmission and absorbed solar components. The absorbed solar components appear as energy source terms in the second step – the heat transfer analysis. The heat transfer analysis involves the formulation of energy balance equations and requires both effective longwave properties and convective heat transfer coefficients as input. The simultaneous solution of the energy balance equations yields the temperature as well as the convective and radiative fluxes. The effective solar optical properties of flat materials like drapery fabrics, roller blinds and insect screens were obtained by developing a new measurement technique. Special sample holders were designed and fabricated to facilitate measurements using an integrating sphere installed in a commercially available spectrophotometer. Semi-empirical models were then developed to quantify the variation of solar optical properties with respect to incidence angle. In turn, effective layer properties of venetian blinds and pleated drapes were modeled using a more fundamental net radiation scheme. The effective longwave properties of flat materials were obtained by taking measurements with an infrared reflectometer using two backing surfaces. The results enabled simple models to be developed relating emittance and longwave transmittance to openness, emittance and longwave transmittance of the structure. In turn, effective longwave properties of venetian blinds and pleated drapes were modeled using a net radiation scheme. Convective heat transfer correlations were readily available. Finally, the newly developed models were validated by measuring the solar gain through various shading devices attached to a double glazed window using the National Solar Test Facility (NSTF) solar simulator and solar calorimeter. Solar gain results were also obtained from simulation software that incorporated the models. There was good agreement between the measured and the simulated results thus strengthening confidence in the newly developed models.

Over the last decade, much research has been done to understand the role of nanoparticles in heat transfer fluids. While experimental results have shown "anomalous" thermal enhancements and non-linear behavior with respect to CNT loading percentage, little has been done to replicate this behavior from an analytical or computational standpoint. This study is aimed towards using molecular dynamics to augment our understanding of the physics at play in CNT-nanofluid systems. This research begins with a heat transfer study of individual CNTs in a vacuum environment. Temperature gradients are imposed or induced via various methods. Tersoff and AIREBO potentials are used for the carbon-carbon interactions in the CNTs. Various chirality CNTs are explored, along with several different lengths and temperatures. The simulations have shown clear dependencies upon CNT length, CNT chirality, and temperature. Subsequent studies simulate individual CNTs solvated in a simple fluidic box domain. A heat flux is applied to the domain, and various tools are employed to study the resulting heat transfer. The results from these simulations are contrasted against the earlier control simulations of the CNT-only domain. The degree by which the solvation dampens the effect of physical parameters is discussed. Effective thermal conductivity values are computed, however the piecewise nature of the temperature gradient makes Fourier's law insufficient in interpretting the heat transfer. Nevertheless, the computed effective thermal conductivities are applied to classical models and better agreement with experimental results is evident. Phonon spectra of solvated and unsolvated CNTs are compared. However, a unique method utilizing the Irving-Kirkwood relations reveals the spatially-localized heat flux mapping that fully illuminates the heat transfer pathways in the solid-fluid composite material. This method confirms why conventional models fail at predicting effective thermal conductivity. Specifically, it reveals the volume of influence that the CNT has on its surrounding fluid.

In modern gas turbines parts of combustion chamber and turbine section are under heavy heat load, for example, the rotor inlet temperature is far higher than the melting point of the rotor blade material. These high temperatures causes thermal stresses in the material, therefore it is very important to cool the components for safe operation and to achieve desired component life. But on the other hand the cooling reduces the turbine efficiency, for that reason it is vital to understand and optimize the cooling technique. In this project Thermochromic Liquid Crystals (TLCs) are used to measure distribution of heat transfer coefficient over a scaled up combustor liner section. TLCs change their color with the variation of temperature in a particular temperature range. The color-temperature change relation of a TLC is sharp and precise; therefore TLCs are used to measure surface temperature by painting the TLC over a test surface. This method is called Liquid Crystal Thermography (LCT). LCT is getting popular in industry due to its high-resolution results, repeatability and ease of use. Test model in present study consists of two plates, target plate and impingement plate. Cooling of the target plate is achieved by impingement of air coming through holes in the impingement plate. The downstream surface of the impingement plate is then cooled by cross flow and re-impingement of the coolant air. Heat transfer on the target plate is not uniform; areas under the jet which are called stagnation points have high heat transfer as compare to the areas away from the center of jet. It is almost the same situation for the impingement plate but the location of stagnation point is different. A transient technique is used to measure this non-uniform heat transfer distribution. It is assumed that the plates are semi-infinitely thick and there is no lateral heat transfer in the plates. To fulfill the assumptions a calculated time limit is followed and the test plates are made of Plexiglas which has very low thermal conductivity. The transient technique requires a step-change in the mainstream temperature of the test section. However, in practical a delayed increase in mainstream temperature is attained. This issue is dealt by applying Duhamel’s theorem on the step-change heat transfer equation. MATLAB is used to get the Hue data of the recorded video frames and calculate the time taken for each pixel to reach a predefined surface temperature. Having all temperatures and time values the heat transfer equation is iteratively solved to get the value of heat transfer coefficient of each and every pixel of the test surface. In total fifteen tests are conducted with different Reynolds number and different jet-to-target plate distances. It is concluded that for both the target and impingement plates, a high Reynolds number provides better overall heat transfer and increase in jet-to-target distance decreases the overall heat transfer.

Idag värms glasugnar upp med antingen gasol eller olja, detta projekt vill visa på möjligheten att istället använda gas från biobränsleförgasning som förbränns utan synliga flammor. Detta skulle miljömässigt ge fördelarna att biobränslen inte bidrar till växthuseffekten och ge förutsättningar för att minska kväveoxidutsläppen genom bättre teknik än dagens. För att visa att det är möjligt att både behålla produktionen och reducera kväveoxiderna med förgasningsgas konstruerades en modell av ugnen och strömningsbilden studerades i vattenmodell. För att undersöka värmeöverföringen i ugnen behöver en eller flera kalorimetrar konstrueras för att kunna användas vid varma försök. Dimensionsberäkningar gjordes som visade att detta är möjligt med vissa typer av kalorimetrar. / Today glassfurnaces are heated with LPG or oil, this project will show the possibility to use gas from biofuel gasification combusted without visible flames. This would give the environmental benefits that biofuels don´t contribute to the greenhouse effect and reduce nitrogenoxide emissions by better technique than today. To prove the possibility to retain todays production and reduce nitrogenoxide emissions a model of the furnace was constructed and the flow field was studied using water model technique. To examine the heat transfer in the furnace one or more calorimeters need to be constructed to be used in hot experiments. Dimensioning calculations were made that showed that this is possible provided certain specific designs.

The ability and efficiency of today’s gas turbine engines are highly dependent on development of cooling technologies, among which film cooling is one of the most important. Investigations have been conducted towards discovering different aspects of film cooling, utilizing both experiments and performing CFD simulations. Although, investigation by using CFD analysis is less expensive in general, the results obtained from CFD calculations should be validated by means of experimental results. In addition to validation, in cases like simulating a turbine vane, performing CFD simulations can be time consuming. Therefore, it is essential to find approaches that can reduce the computational cost while results are validated by experiments. This study has shown the potential for reduced models to be utilized for investigation of different aspects of film cooling by means of CFD at low turn-around time. This has been accomplished by first carrying out CFD simulations and experiments for an engine-like setting for a full vane. Then the computational domain is reduced in two steps where all results are compared with experiments including aerodynamic validation, heat transfer coefficient and film effectiveness. While the aerodynamic results are in close agreement with experiments, the heat transfer coefficient and film effectiveness results have also shown similarities within the expected range. Thus this study has shown that this approach can be very useful for e.g. early vane and film cooling design.

Plasma sprayed Thermal Barrier Coating systems (TBCs) are commonly used for thermal protection of components in modern gas turbine application such as power generation, marine and aero engines. The material that is most commonly used in these applications is Yttria Partially Stabilized Zirconia (YPSZ) because of this ceramic’s favourable properties, such as low thermal conductivity, phase stability to high temperature, and good erosion resistance. The coating microstructures in YPSZ coatings are highly heterogeneous, consisting of defects such as pores and cracks of different sizes which determine the coating’s final thermal and mechanical properties, and the service lives of the coatings. Determination of quantitative microstructure–property correlations is of great interest as experimental procedures are time consuming and expensive. Significant attention has been given to this field, especially in last fifteen years. The usual approach for modelling was to describe various microstructural features in some way, so as to determine their influence on the overall thermal conductivity of the coating. As the analytical models over-simplified the description of the defects, various numerical models were developed which incorporated real microstructure images.This thesis work describes two modelling approaches to further investigate the relationships between microstructure and thermal conductivity of TBCs. The first modelling approach uses a combination of a statistical model and a finite element model which could be used to evaluate and verify the relationship between microstructural defects and thermal conductivity. The second modelling approach uses the same finite element model along with a coating morphology generator, and can be used to design low thermal conductivity TBCs. A tentative verification of both the approaches has been done in this work.

A literature survey on modelling of the resistance spot welding process has been carried out and some of the more interesting models on this subject have been reviewed in this work. The underlying physics has been studied and a brief explanation of Heat transfer, electrokinetics and metallurgy in a resistance spot welding context have been presented.\nl\hsLastly a state of the art model and a simplified model, with implementation in the FEM software LS-DYNA in mind, have been presented.