This work presents an experimental and numerical study of phase change behaviour in a horizontal cylindrical latent heat energy storage system (LHESS). Fins with two orientations, straight fins and angled fins, are added into the PCM to enhance heat transfer. The PCM used in this study is lauric acid which has desirable thermal properties for LHESS. The experimental work concentrates on studying the heat transfer mechanism during phase change, impacts of the HTF inlet temperature and HTF flow rates. Moreover, heat transfer enhancement effectiveness of straight fins and angles fins is compared. Numerical model is simulated using COMSOL Multiphysics software package. It is observed that conduction is the dominant heat transfer mechanism during the initial stage of charging, and natural convection plays a more important role afterwards. Conduction plays a major role during solidification. Complete melting time is affected by the HTF inlet temperature and HTF flow rates.

Le but de l'étude présentée dans ce mémoire de thèse est de caractériser localement l'intensification des transferts thermiques observée le long d'une plaque chauffante lorsqu'elle est soumise à des ultrasons. Ces derniers induisent des effets hydrodynamique dans les fluides qu'ils traversent. Premièrement, la cavitation acoustique, qui permet de produire de forts effets mécaniques dans les liquides. Viens ensuite le courant acoustique qui génère une écoulement convectif sous l'action d'une dissipation visqueuse de l’énergie acoustique. Ce travaille à permis de mettre en évidence la relation existant entre ces effets hydrodynamiques et l'intensification des transferts thermiques observée. Ces résultats ont permis de définir quelles conditions expérimentales sont favorables à l'intégration d'ultrasons dans un échangeur de chaleur. / The aim of the study presented in this thesis is to characterize heat transfer enhancement by ultrasound observed along a hot plate in forced convection. These induced hydrodynamic effects in the fluids they cross. The first one is the acoustic cavitation, which can produce strong mechanical effects in liquids. The second one is the acoustic streaming which generates convective flows thanks to viscous dissipation of the acoustic energy. This work helped to highlight the relationship between ultrasound hydrodynamic effects and heat transfer enhancement. Thanks to those results, experimental conditions which allowed ultrasound integration in a heat exchangers, could be determined.

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Heat transfer is a critical part of engineering design, from the cooling of rocket engines to the thermal management of the increasingly dense packaging of electronic circuits. Even for the most fundamental modes of heat transfer, a topic of research is devoted to finding novel ways to improve it. In recent decades, investigators experimented with the idea of exposing systems to acoustic waves with the hope of enhancing thermal transfer at the surface of a body. Ultrasound has been applied with some success to systems undergoing nucleate boiling and in single-phase forced and free convection heat transfer in water. However, little research has been done into the use of sound waves to improve heat transfer in air. In this thesis the impact of acoustic waves on natural convection heat transfer from a horizontal cylinder in air is explored. An experimental apparatus was constructed to measure natural convection from a heated horizontal cylinder. Verification tests were conducted to confirm that the heat transfer could be described using traditional free convection heat transfer theory. The design and verification testing of the apparatus is presented in this work. Using the apparatus, experiments were conducted to identify if the addition of acoustic waves affected the heat transfer. For the first set of experiments, a 40 kHz standing wave was created along the length of the heated horizontal cylinder. While our expectation was that our results would mirror those found in the literature related to cooling enhancement using ultrasound in water (cited in the body of this thesis), they did not. When a 40 kHz signal was used to actuate the air surrounding the heated cylinder assembly, no measurable enhancement of heat transfer was detected. Experiments were also performed in the audible range using a loudspeaker at 200 Hz, 300 Hz, 400 Hz, 500 Hz, and 2,000 Hz. Interestingly, we found that a 200 Hz acoustic wave causes a significant, measurable impact on natural convection heat transfer in air from a horizontal cylinder. The steady-state surface temperature of the cylinder dropped by approximately 12℃ when a 200 Hz wave was applied to the system.

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<p>The present work relates to a new procedure, to perform temperature measurements with unprecedented accuracy. The new approach relies on a correction based on a two-wire probe thermocouple that enables a precise estimation of the conduction error. The difference between measured temperature by a thermocouple and total gas temperature for steady conditions can be decomposed into three main contributions: velocity error, conduction error and radiation error. Radiation error can be considered negligible for temperatures lower than 800K. The velocity error can be corrected using dedicated experimental calibrations to measure the recovery factor. However, the conduction error, remains an unresolved challenge in the aerospace and power-energy community. The proposed method includes a comprehensive correction with different options for the postprocessing. The method has been demonstrated using high-fidelity aero-structural computational simulations.</p>

The effect of using high voltage DC and AC on the heat transfer process during the melting of a Phase Change Material (PCM) in a rectangular enclosure was studied experimentally and numerically. The experiments were conducted for two configurations: (a) a horizontal rectangular enclosure in which the initial melting process is governed by heat conduction, (b) a vertical rectangular enclosure in which the initial melting process is governed by heat convection. The level of heat transfer enhancement was quantified by using a novel experimental facility for the horizontal configuration. The experimental methodology was verified first against non-EHD melting cases and then was further expanded to include the EHD effects. The experiments showed that EHD forces can be used to enhance a conduction dominated melting up to a maximum of 8.6-fold locally and that the level of enhancement is directly related to the magnitude of the applied voltage. It was found that the main mechanism of enhancement in these cases can be attributed to the electrophoretic forces and that the role of the dielectrophoretic forces is minimal under the applied voltages. In the vertical configuration, the effect of the magnitude of the applied voltage, the applied voltage wave-form, the gravitational Rayleigh number, Stefan number and the aspect ratio of the enclosure on the heat transfer enhancement were investigated experimentally. A novel shadowgraph experimental measurement system was developed and verified against the analytical correlations of natural convection in rectangular enclosures and the non-EHD melting performance was verified against the bench mark experiments of Ho (1984). The shadowgraph system was used to measure the local heat transfer coefficient across the heat source wall (the heat exchanger surface). The local heat transfer measurements along with the melting temporal profiles were used to explain and visualize the coupling between the Electrohydrodynamics (EHD) forces and the gravitational forces. It was found that the EHD forces could still enhance the melting process even for an initially convection dominated melting process. The mechanism of enhancement was found to be a bifurcation of the initial convection cell into multiple electro-convective cells between the rows of the electrodes. The shadowgraph system was used to assess the interaction between the electrical and the gravitational forces through the visualization of these cells and quantifying their size. The EHD heat transfer enhancement factor was found to increase by the increase of the applied voltage, reaching a 1.7 fold enhancement at the lower gravitational Rayleigh number tested and 1.45 fold for the highest gravitational Rayleigh and Stefan number. The effect of the polarity of the applied voltage was tested for the different cases and it was found that there was no significant difference between the positive and the negative polarities when the magnitude of the applied voltage was below 4 kV. At higher voltages- 6kV- the negative polarities showed better level of enhancement when compared to the positive applied voltage. It was again found that the main mechanism of enhancement is attributed to charge injection from the high voltage electrodes. A scaling analysis was conducted based on the previous conclusions and the dominant mechanism of enhancement to describe the problem in non-dimensional form. An electrical Rayleigh number was introduced and its magnitude was correlated to the magnitude of the injected current. The melt volume fraction was then represented against the non-dimensional parameter (n+1)(H/W)Fo.Ste.RaE^0.25 and the melt fraction temporal profiles for the different voltages collapsed well against this parameter. Finally, a numerical analysis was conducted on the role of the dielectrophoretic forces during the melting of Octadecane and when they would become of significant importance. The results of the numerical model supported the experimental findings and suggested that a minimum of 15 kV is needed in order to realize the effect of the dielectrophoretic forces. The numerical model was used to understand the interaction between the gravitational and the dielectrophoretic forces at different ranges of both gravitational Rayleigh number and electrical Rayleigh number. The model was complemented with scaling analysis to determine the governing scales of the problem and the dielectrophoretic Rayleigh number was deduced from the study. / Thesis / Doctor of Philosophy (PhD)

Heat transfer technologies based on boiling refer to applications like heat pumps, waste heat recovery systems, power plants and electronic components cooling. The widespread use of boiling as the heat transfer mode is due to high heat transfer coefficients associated with the phase change from liquid to vapor. Boiling heat transfer coefficients can be further enhanced by modifying the texture or chemical composition of the interface at which boiling occurs. The objective of this research is to fabricate textured surfaces with electrical discharge machining (EDM) and investigate the enhancement in pool boiling heat transfer, concerning machining and surface characterization parameters. It is complemented by a qualitative analysis of bubble dynamics with high-speed imaging, to provide insights into the differences in boiling performance associated with the changes in surface topography. Sink electrical discharge machined surfaces demonstrated ten times higher heat transfer coefficient compared to a polished surface during these studies. / Thesis / Master of Applied Science (MASc)

Thermoelectric generators (TEGs) are currently being explored for their potential in harvesting energy from automobile exhaust. TEGs in form of an appropriate TEG- Heat exchanger module can utilize the temperature difference between the hot exhaust gases and the automobile coolant and convert it into electrical voltage. The amount of power is anticipated to be a few hundred watts depending on the temperature gradient and the material of the TEGs. The focus of this study is increasing the hot side heat transfer for improved performance of the thermoelectric generators using two different internal fins — louvered fins and herringbone wavy fins. The multi-louvered fins basically have 'multi flat plate' behavior and will enhance the heat transfer by deflecting the air from its original path and aligning it with the plane of the louvers. Herringbone fins are used to lengthen the path of airflow allowing for greater residence time and better mixing of the flow. They also provide for greater wetted surface area achieving higher heat transfer. The flow and heat transfer behavior inside the exhaust pipe test section with internal fins is modeled using commercial numerical software. The thermal and flow behavior through both these internal fins depends to a large extent on geometric parameters and fin arrangement. Optimization of the fin design is considered to determine the configuration that provides highest heat transfer while providing least pressure drop across the pipe length. The heat transfer and pressure drop characteristics are compared to the baseline flow without any fin enhancement. / Master of Science