Advancements in electronic performance lead to a decrease in device size and an increase in power density. Because of these changes, current cooling mechanisms for electronic devices are beginning to be ineffective. Microchannels, with their large heat transfer surface area to volume ratio, cooled with either gas or liquid coolant, have shown some potential in adequately maintaining a safe surface temperature. By modifying the walls of the microchannel with fins, the cooling performance can be improved. Using computational fluid dynamics software, microfins placed in a staggered array on the bottom surface of a rectangular minichannel are modeled in order to optimize microstructure geometry and maximize heat transfer dissipation through convection from a heated surface. Fin geometry, dimensions, spacing, height, and material are analyzed. Correlations describing the Nusselt number and the Darcy friction factor are obtained and compared to recent studies. These correlations only apply to short fins in the laminar regime. Triangular fins with larger fin height, smaller fin width, and spacing double the fin width maximizes the number of fins in each row and yields better thermal performance. Once the effects of microfins were found, an experiment with multi-walled carbon nanotubes (MWNTs) grown on the surface were tested using both water and Al2O3/H2O nanofluid as the working medium. Minichannel devices containing two different MWNT structures – one fully coated surface of MWNTs and the other with a circular staggered fin array of MWNTs - were tested and compared to a minichannel device with no MWNTs. It was observed that the sedimentation of Al2O3 nanoparticles on a channel surface with no MWNTs increases the surface roughness and the thermal performance. Finally, using the lattice Boltzmann method, a two dimensional channel with suspended particles is modeled in order to get an accurate characterization of the fluid/particle motion in nanofluid. Using the analysis based on an ideal fin, approximate results for nanofluids with increase surface roughness was obtained. Microchannels have proven to be effective cooling systems and understanding how to achieve the maximum performance is vital for the innovation of electronics. Implementation of these modified channel devices can allow for longer lasting electronic systems.

The objective of current study was to numerically investigate the flow and heat transfer characteristics in a stationary one pass rectangular channel (AR=4:1) with circular and elliptical pin fin arrays. Two types of elliptical pin fins (a SEF and an N fin whose minor axis length is equal to the diameter of the circular fin) were used. The analysis was performed with an array of six rows of staggered pin fins in the streamwise direction for Reynolds numbers (Re) of 10,000, 20,000, 30,000, 40,000 and 50,000. 3-D, steady simulations were performed using the low Reynolds number k-omega SST turbulence model in the FLUENT CFD code. The data predicted by the current numerical model showed favorable agreement with the experiments in the validation study. It was observed that SEF array produces minimum pressure loss and the highest thermal performance. It was also observed that N fin array produces minimum hot spots and the highest channel averaged Nusselt number ratio values.

The mass-market adoption of electrification in the transportation sector mandates stringent and aggressive requirements in terms of cost, power rating, efficiency, power density, and specific density of power electronics. Modular packaging of power electronics is advantageous and thus ubiquitously used by the automotive industry. A trend of shrinking die sizes and increased integration is evident and will inevitably continue. The thermal management system has become ever more significant as it is one of the main obstacles to higher power densities. The cooling system must be cost-effective, simple, efficient, reliable, and compatible with system requirements. Pin fins are a reliable and effective means of augmenting heat transfer. They rely on inducing turbulence, increasing the effective wetted surface, and accelerating fluid velocity. Unavoidably the pin fin array also produces an undesirable pressure drop that is commensurate to the pumping power required for the system. In this thesis, a tool is developed for the geometry optimization of pin fin arrays to dissipate the heat at a rate large enough to ensure junction temperatures do not exceed the maximum value possible at a minimal pressure drop. It is hoped that this tool would contribute to the multi-physics optimization and integration of power electronics for electrified vehicles. This optimization is confined to equalaterally spaced short pin fins, aspect ratios less than three. The tool employs empirical correlations since flow is too complex to solve analytically and numerical solutions or CFD-simulations are too time and computationally extensive. The tool development is done in a comprehensive manner. Starting from the first principles of a two-level voltage source inverter's operation. Next, the inevitable power losses from the operation are explained and a method for their calculations is presented. Correlations in the literature related to both pressure drop and heat transfer are reviewed afterward. Then the methodology of the construction of the tool is explicated in detail. Employing a commercial power module to benchmark results; three scenarios with different flow rates and inlet temperatures are optimized for. Simulations in ANSYS Fluent are run to verify the accuracy of correlations used in the tool. Comparing the optimized geometry of pin fins to the original benchmarking geometry it is evident that employing this tool on a per-application basis provides superior performance. / Thesis / Master of Applied Science (MASc)

Due to increasing power demands and decreasing component size, thermal management has become the bottleneck for many power electronic applications. The aerospace industry has focused on reducing weight, operating temperature, and pumping power of power converters since these will limit an aircrafts' range and load carrying capacity. This paper outlines a tool created in MATLAB to automate the cold plate design process for DC-DC converters (or similar applications). The tool incorporates a genetic algorithm to fi nd the optimal aligned or staggered pin fi n confi guration that maintains the devices below their critical junction temperature while reducing the system's overall weight and pressure drop. Utilizing this MATLAB design tool, a cold plate was designed, manufactured, and tested. The convection coefficient calculated within MATLAB (via empirical correlations) was veri fed using simplifi ed CFD simulations within 5% of each other. The same CFD setup, boundary condition types, and methodology are then applied for the full-sized prototype cold plate simulations. These simulations were then validated using the experimental results. For all cases, the percentage error between the simulated convection coefficient values (CFD) and the experimental results was less than 12%. The experiments' measured surface temperature and pressure drop errors were less than 8% of the predicted CFD results. Therefore, the MATLAB tool and its correlations/calculations could be veri fied (via CFD) and validated (experimentally) based on good agreement between the CFD and the experimental results. This three-pronged approach (analytical calculations, CFD simulations, and experimental validation) is an effective and robust method to solve heat transfer problems. Overall, with the framework outlined in this thesis, a complete cold plate design can now be completed in weeks instead of months. This streamlined approach will save companies signifi cant time and money in the design and simulation phases, making this tool a valuable addition to the current literature available. / Thesis / Master of Applied Science (MASc)

The effects of greenhouse gases have seen a significant rise in recent years due to the use of fossil fuels like gasoline and diesel. Conversion of the energy stored in these fossil fuels to mechanical work is an extremely inefficient process which results in a high amount of energy rejected in the form of waste heat. Thermoelectric materials are able to harness this waste heat energy and convert it to electrical power. Thermoelectric devices work on the principle of the Seebeck effect, which states that if two junctions of dissimilar materials are at different temperatures, an electrical potential is developed across them. Even though these devices have small efficiencies, they are still an extremely effective way of converting low grade waste heat to usable electrical power. These devices have the added advantage of having no moving parts (solid state) which contributes to a long life of the device without needing much maintenance. The performance of thermoelectric generators is dependent on a non-dimensional figure of merit, ZT. Extensive research, both past and ongoing, is focused on improving the thermoelectric generator's (TEG's) performance by improving this figure of merit, ZT, by way of controlling the material properties. This research is usually incremental and the high performance materials developed can be cost prohibitive. The focus of this study has been to improve the performance of thermoelectric generator by way of improving the heat transfer from the exhaust gases to the TEG and also the heat transfer from TEG to the coolant. Apart from the figure of merit ZT, the performance of the TEG is also a function of the temperature difference across it, By improving the heat transfer between the TEG and the working fluid, a higher temperature gradient can be achieved across it, resulting in higher heat flux and improved efficiency from the system. This area has been largely neglected as a source of improvement in past research and has immense potential to be a low cost performance enhancer in such systems. Improvements made through this avenue, also have the advantage of being applicable regardless of the material in the system. Thus these high performance heat exchangers can be coupled with high performance materials to supplement the gains made by improved figure of merits. The heat exchanger designs developed and studied in this work have taken into account several considerations, like pressure drop, varying engine speeds, location of the system along the fuel path, system stability etc. A comprehensive treatment is presented here which includes 3D conjugate heat transfer modeling with RANS based turbulence models on such a system. Various heat transfer enhancement features are implemented in the system and studied numerically as well as experimentally. The entire system is also studied experimentally in a scaled down setup which provided data for validation of numerical studies. With the help of measured and calculated data like temperature, ZT etc, predictions are also presented about key metrics of system performance. / Ph. D.

The heat transfer characteristics through arrays of pin fins were studied for the further development of internal cooling methods for turbine airfoils. Low aspect ratio pin fin arrays were tested through a range of Reynolds numbers between 5000 and 30,000 to determine the effects of pin spacing as well as aspect ratio on pin and endwall heat transfer. Experiments were also conducted to determine the independent effects of pin spacing and aspect ratio on arrays with different flow incidence angles. The pin Nusselt numbers showed almost no dependence on pin spacing or flow incidence angle. Using an infrared thermogaphy technique, spatially-resolved Nusselt numbers were measured along the endwalls of each array. The endwall results showed that streamwise spacing had a larger effect than spanwise spacing on array-averaged Nusselt numbers. Endwall heat transfer patterns showed that arrays with flow incidence angles experienced less wake interaction between pins than arrays with perpendicular flow, which caused a slight decrease in heat transfer in arrays with flow incidence angles. The effect of flow incidence angle on array-average Nusselt number was greater at tighter pin spacings. Even though the pin Nusselt number was independent of pin spacing, the ratio of pin-to-endwall Nusselt number was dependent on flow conditions as well as pin spacing. The pin aspect ratio had little effect on the array-average Nusselt number for arrays with perpendicular flow; however, the effect of flow incidence angle on array-average Nusselt number increased as aspect ratio decreased. / Master of Science

The performance of many engineering devices from power electronics to gas turbines is limited by thermal management. Pin fins are commonly used to augment heat transfer by increasing surface area and increasing turbulence. The present research is focused on but not limited to internal cooling of turbine airfoils using pin fins. Although the pin fins are not limited to a single shape, circular cross-sections are most common. The present study examines heat transfer from a single row of circular pin fins with the row oriented perpendicular to the flow. The configurations studied have spanwise spacing to pin diameter ratios of two, four, and eight. Low aspect ratio pin fins were studied whereby the channel height to pin diameter was unity. The experiments are carried out for a Reynolds number range of 5000 to 30,000. Heat transfer measurements are taken on both the pin and on the endwall covering several pin diameters upstream and downstream of the pin row. The results show that the heat transfer augmentation relative to open channel flow is highest for the smallest spanwise spacing for the lowest Reynolds number flows. The results also indicate that the pin fin heat transfer is higher than on the endwall. / Master of Science

This study examines the local and averaged endwall heat transfer effects of a staggered array of porous pin fins within a rectangular channel. The porous pin fins were made from aluminum and had a pore density of 10 pores per inch (PPI). The pressure drop through the channel was also determined for several flow rates and presented in terms of the friction factor. Local heat transfer coefficients on the endwall were measured using Thermochromic Liquid Crystal (TLC) sheets recorded with a charge-coupled device (CCD) camera. Static and total pressure measurements were taken at the entrance and exit of the test section to determine the overall pressure drop through the channel and explain the heat transfer trends through the channel. Results are presented for Reynolds numbers between 25000 and 130000 and a blockage ratio (blocked channel area divided by open channel area) of 50%. All results were compared to the corresponding results obtained using solid pins. All experiments were carried out in a 150 mm by 500 mm channel with an X/D of 1.72, a Y/D of 2.0, and a Z/D of 1.72 for the porous pins. It was found that for the range of Reynold's numbers tested in this study, the porous pin array consistently resulted in a larger friction factor, and therefore greater losses than a geometrically similar array of solid pins. The friction factors for the solid pin array were between 9.5 and 10.5, similar to the results found in the literature. For the porous pins, however, the friction factors were significantly increased as the Reynold's number increased, reaching as high as 15.3 at the highest Reynold's number tested. The heat transfer enhancement for the porous pins was found to be between 150 and 170% while the solid pins resulted in a heat transfer enhancement between 190 and 230%.

With the demand of computing power from electronic chips on a constant rise, innovative methods are needed for effective and efficient thermal management. Forced convection cooling through an array of micro pin-fins acts not only as a heat sink, but also allows for the electrical interconnection between stacked layers of integrated circuits. This work performs a multi-objective optimization of three shapes of pin-fins to maximize the efficiency of this cooling system. An inverse design approach that allows for the design of cooling configurations without prior knowledge of thermal mapping was proposed and validated. The optimization study showed that pin-fin configurations are capable of containing heat flux levels of next generation electronic chips. It was also shown that even under these high heat fluxes the structural integrity is not compromised. The inverse approach showed that configurations exist that are capable of cooling heat fluxes beyond those of next generation chips. Thin film heat spreaders made of diamond and graphene nano-platelets were also investigated and showed that further reduction in maximum temperature, increase in temperature uniformity and reduction in thermal stresses are possible.