A novel, idealized mathematical description is presented of the cooling process of electric power cable system by an integral liquid coolant configuration. A model is first obtained for constant fluid velocities, by the use of Laplace transform and subsequent solution of an ordinary differential equation. A simple integral expression is obtained, which is numerically integrated by the Simpson's rule. The set of curves so obtained, clearly show the delay of the temperature peak vs. the peak of the power demand. A steady state form of control is proposed. Block diagrams of two implemetations are shown. These control systems are designed to maintain the temperature of the effluent constant. An optimization algorithm based on developments given by Sage is presented. This algorithm will optimize the transfer of the cooling system from the initial condition of zero velocity to the steady state operation described above. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

L’ébullition en micros canaux est une technique de refroidissement très prometteuse pour les composants en microélectronique. Ces derniers, de plus en plus miniaturisés, nécessitent souvent la dissipation de densités de flux importantes, pouvant atteindre quelques MW m-2 pour maintenir des températures acceptables. L’objet de cette étude est de mieux comprendre les instabilités des écoulements, le transfert thermique par changement de phase tout comme l’effet des rapports de forme (a) sur l’apparition de sites de nucléation à la surface de micro canaux. L’analyse des échanges convectifs locaux lors de l’ébullition a été réalisée dans le cas de micro canaux de rapport de forme allant de 0.3 à 3 et de diamètres hydrauliques allant de 50 à 150 μm. Le banc d’essai a été instrumenté de manière à pourvoir mesurer simultanément les températures de surface à l’aide capteurs en film mince de nickel, de capteurs de pressions instantanées et de caméra rapide pour la visualisation du phénomène d’ébullition. Le chauffage du fluide a été réalisé à l’aide du dépôt en couche mince d’un film résistif en aluminium directement appliqué à la surface des micro canaux. L’étude expérimentale a permis d’analyser les phénomènes de changement de phase par ébullition, du transfert thermique local ainsi que la chute de pression de l’écoulement. En particulier, le travail expérimental a permis de mettre en évidence les effets sur le transfert, du rapport de forma a, de la température de sous refroidissement du fluide à l’entrée des canaux et de la dynamique de formation et de grossissement des bulles. Des fluctuations importantes de pression et de températures ont été enregistrées pour des températures de surfaces avoisinant les 250 °C. Les micro canaux avec a= 1.5 et Dh =120 μm, correspondent à la configuration la plus performante. Les mesures par thermographie infrarouge (IR) combinées à la visualisation par caméra rapide et aux mesures des fluctuations de pressions par capteurs piézoresitifs, ont été réalisées dans le cas de canaux en Polydimethylsiloxane (PDMS) de grand rapport de forme (a = 22) et de diamètre hydraulique Dh =192 μm. L’objectif était d’identifier des cartographies bidimensionnelles et instationnaires de coefficients d’échanges convectifs dans le cas d’un micro canal utilisant un fluide diélectrique le FC -72. La double visualisation par thermographie infrarouge et par caméra CCD rapide a permis de corréler la dynamique de l’ébullition, et notamment le grossissement des bulles, l’asséchement et ou le mouillage des parois, aux coefficients d’échanges convectifs locaux. / Flow boiling in microchannels is a promising technology for cooling of small-scale devices such as electronic chips, power rectifiers, radar arrays, chemical microreactors that require dissipating heat fluxes of several MW m-2 while maintaining constant temperature at the surface. Although flow boiling in macroscale provides higher performance than single-phase or pool boiling heat transfer, the advantages in microscale have not been yet completely justified. This study aims to assist in the better understanding of some outstanding issues regarding flow instabilities, two-phase heat transfer mechanisms and early dryout that occur in microchannels while increasing their aspect ratios (α). Fully integrated and instrumented silicon multimicrochannel heat sinks of width-to-height aspect ratios from 0.3 to 3 and hydraulic diameters ℎ from 50 to 150 μm were developed in order to fully characterise their local heat transfer performance during flow boiling. Local wall temperature measurements were obtained from five thin nickel film temperature sensors with simultaneous pressure measurements and flow visualisation from the top. Uniform heating was achieved with a thin aluminium heater integrated at the back of the microchannels. The effect of , mass flux, inlet subcooling temperature and bubble dynamics on two-phase flow boiling local heat transfer coefficient and pressure drop were investigated for constant heat fluxes. Severe pressure and temperature fluctuations in excess of 250 °C were measured at high microchannels. The heat sinks with microchannels of = 1.5 and ℎ = 120 μm, achieved the maximum heat transfer performance. High spatial and temporal resolution wall temperature maps were obtained with advanced thermography technique, synchronised with simultaneous high-speed imaging and pressure measurements from integrated miniature piezoresistive pressure sensors inside a high aspect ratio (α= 22) transparent Polydimethylsiloxane (PDMS)-based microchannel of ℎ =192 μm. The aim was to produce accurate two-dimensional (2D) high spatial and temporal resolution two-phase heat transfer coefficient maps across the full domain of a single microchannel using FC-72 dielectric liquid. The novel PDMS based microchannel provided measurements in the vicinity of the wall due to the transparency of PDMS to midwave infrared radiation. Synchronised flow visualisation images were related with liquid-vapour distribution of the channel base and were correlated with the two-phase heat transfer coefficient maps in order to elucidate flow boiling instabilities, film thinning during bubble confinement and wetting / rewetting phenomena during annular flow pattern.

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A metal adiabatic demagnetization cryostat was designed and constructed. The final-temperature dependence of a single-crystal synthetic ruby was investigated as a function of applied magnetic field. The ruby was used both as a thermometer and coolant in superconducting thin-film experiments at temperatures below 1°K. Difficulties encountered in these experiments have been discussed in detail. Further improvements on the system have been suggested in the application of thin-film superconductor studies. / Thesis / Master of Science (MS)

A wearable cooling system was developed in this study for use in elevated temperature environments by military, fire-fighting, chemical-response, and other hazardous duty personnel. Such a system is expected to reduce heat-related stresses, increasing productivity and allowable mission duration, reduce fatigue, and lead to a safer working environment. The cooling system consists of an engine-driven vapor-compression system assembled in a backpack configuration, coupled with a cooling garment containing refrigerant lines worn in close proximity to the skin. A 2.0 L fuel tank in the backpack powers a small-scale engine that runs a compressor modified from the original air compression application to the refrigerant compression application here. A centrifugal clutch and reduction gear train system was designed and fabricated to couple the engine output to the refrigerant compressor and heat rejection fan. The overall cooling system, including the wearable evaporator, had a total mass of 5.31 kg (11.7 lb) and measured 0.318 נ0.273 נ0.152 m (12.5 נ10.75 נ6 inches). Testing was conducted in a controlled environment to determine system performance over a wide range of expected ambient temperatures (37.7-47.5㩬 evaporator refrigerant temperatures (22.2-26.1㩬 and engine speeds (10,500-13,300 RPM). Heat removal rates of up to 300 W, which is the cooling rate established in the literature as being required for maintaining comfort at an activity level comparable to calisthenics or moderate exercise, were demonstrated at a nominal ambient temperature of 43.3㠨110橮 Modeling the fuel as 88 percent methanol (LHV ~ 1.992ױ07 J/kg) and 12 percent oil, the system consumed 1750 W at an average fuel mass flow rate of 0.316 kg/hr to provide a nominal cooling rate of 178 W for 5.7 hrs between refueling.

No description available.

No description available.

The coldness of outer space is a huge thermodynamic resource that can be utilized as an infinite heat sink that helps in cooling terrestrial objects without the need for electrical energy through a phenomenon known as radiative sky cooling. In the last decade, radiative cooling has seen an increasing attention as a sustainable and clean cooling method and many researchers made smart use of it as a thermal management method. One example in the literature is the radiative cooling of solar cells. Like solar cells, Light Emitting Diodes (LEDs) are semiconductor devices that deteriorate because of high temperatures. Specifically, the high temperature in LEDs lowers their efficiency and lifetime. Therefore, reducing the temperature by increasing heat dissipation can help in optimizing the efficiency of the LED. In this work, I investigate a novel low-cost solution that can help in reducing the temperature of outdoor LEDs through radiative cooling. The suggested solution utilizes the coldness of outer space to radiatively cool the LED by using a layer of a visible-reflective-infrared-transparent material, nanoporous polyethylene (nanoPE), as a cover to reflect the visible light back to earth while transmitting infrared radiation to outer space. I theoretically discuss the potential cooling performance of LEDs in the suggested design and estimate a cooling power enhancement by 128 W/m2 in ideal conditions compared to current designs. In addition, I study the fabrication and characteristics of nanoPE and show how it can be used as a reflective/diffusive cover for LEDs. Lastly, I experimentally demonstrate the use of nanoPE as a cover for LEDs and show an LED temperature reduction of 15 ⁰C in the laboratory environment and 4 ⁰C outdoor and calculate a relative LED efficiency increase of 28% in the indoor scenario and 4% in the outdoor scenario. This efficiency increase can result in an energy saving of 2.2 TWh in the United States corresponding to at least 0.44 MMT CO2 emission reduction making this cooling solution attractive due to its low cost and high impact.

Gas turbines are widely used for power generation, producing megawatts of usable energy, but consume fossil fuels in order to do so. With gas prices on the rise, all eyes have turned to operating cost and fuel efficiency. To increase efficiency, manufactures raise the temperature of the gas that is combusted. This temperature is high above the melting point of the turbine components. In order for the gas turbine to work under these conditions, its parts must be protected. This study focuses on two aspects of cooling for turbine components. Over the last decades, researchers have investigated many aspects of film cooling, The present study investigates the impact of the stagnation region created by a downstream airfoil on endwall film cooling effectiveness with and without the presence of wake. Experimental measurements are presented for a single row of cylindrical holes inclined at 35° with hole length to diameter ratio, LID= 7.5, pitch to diameter ratio, Pl/D = 3 with a constant density ratio of 1.26, and with nitrogen as the coolant. Twelve different configurations were studied. The airfoil was positioned at X/D equal to 6.35, 12.7, and 25.4. A wake plate was added upstream of the film holes at -12.7 and -50.8 X/D. The effect of stagnation and wake was combined by placing both the airfoil and the wake plate in the test section, combining all positions of each. Baseline cases for the cooling holes alone, and the cooling holes with the airfoil and wake individually were compared to the combined effects. The experimental data shows that as the airfoil stagnation region inhibits film cooling close to the airfoil, and strong wake decreases film effectiveness. With both stagnation region and wake combined, an overall decrease in film cooling performance is observed. Higher blowing ratio increase lateral spreading of the jet promoting jet to jet interaction and mainstream interaction enhancing mixing. The presence of wake promotes jet mixing with the mainstream resulting in lower film cooling effectiveness. High performance turbine airfoils are typically cooled with a combination of internal cooling channels and impingement/film cooling. In such applications, the jets impinge against a target surface, and then exit along the channel formed by the jet plate, target plate, and side walls. Local convection coefficients are the result of both the jet impact, as well as the channel flow produced from the exiting jets. Numerous studies have explored the effects of jet array and channel configurations on both target and jet plate heat transfer coefficients. However, little work has been done in examining effects of height variation and heating on all channel walls, in which both target wall and side wall data is taken, as was neglected by previous literature. This study examines the local and averaged effects of channel height on heat transfer coefficients for target and side walls. High resolution local heat transfer coefficient distributions were measured using temperature sensitive paint and recorded via a scientific grade CCD camera. Streamwise pressure distributions for both the target and side walls was recorded and used to explain heat transfer trends. Results are presented for average jet based Reynolds numbers 17K to 45K. All experiments were carried out on a large scale single row, 15 hole impingement channel, with X/D of 5, YID of 4, and Z/D of 1, 3 and 5. Providing high quality results will aid in the validation of predictive tools and development of physics-based models.

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