Today nucleate boiling is widely used in numerous industrial applications such as cooling processes because of the high achieved heat transfer rates for low temperature differences. It remains a possible cooling solution for the next generation of central processing units (CPU), which dissipate heat fluxes exceeding the capabilities of today’s conventional forced air cooling. However, nucleate boiling is a very complex and elusive process involving many mechanisms which are not fully understood yet and a comprehensive model is still missing. For this study a new experimental setup was designed, constructed and commissioned to investigate bubble nucleation, growth, departure and interaction during nucleate pool boiling from a silicon device fully immersed in fluorinert FC-72. The location of bubble nucleation is controlled by artificial cavities etched into the silicon substrate. Boiling is initiated with a heater integrated on the back and micro-sensors indicate the wall temperature at the bubble nucleation site. During this work three different silicon test section designs were fabricated and boiling experiments on these substrates successfully conducted. Bubble growth, bubble departure frequencies and bubble departure diameters for different dimensioned artificial cavities, varied pressure and increasing wall temperature were measured from high-speed imaging sequences. Bubble interactions like vertical and horizontal coalescence were visualised and their impact on the boiling heat transfer investigated. The influence of spacing between two neighbouring artificial cavities on bubble nucleation and departure frequencies, vertical coalescence frequencies and departure diameters was analysed. The acquired data are used as input for a numerical code developed by our collaborators (Brunel University, UK and Los Alamos National Laboratories, USA) and are a first step to validate the code. The code studies the interactions between bubble nucleation sites on solid surfaces as a network. The simulations will help design boiling substrates utilised for chip cooling applications with optimal artificial cavity distribution to maximise the cooling heat transfer.

Flow boiling in microchannels, while very promising as a cooling technology in electronics thermal management, is still a subject being explored that requires further investigation. Before applying this technology for high heat flux computer chip cooling, challenging issues such as fully understanding boiling mechanisms in confined spaces, extending and stabilising the nucleate boiling regime, suppressing flow boiling instabilities, maintaining uniform flow distribution among microchannels, have to be addressed. If flow boiling is to be used as a thermal management method for high heat flux electronics it is necessary to understand the behaviour of a non-uniform heat distribution, which is typically the case observed in a real operating computer chip. In this study, flow boiling of deionised water in a silicon microchannel heat sink under uniform and non-uniform heating has been investigated with particular attention to flow boiling instabilities. An experimental system was designed and constructed to carry out the experimental investigations. The experimental heat sink consisting of forty parallel rectangular microchannels with 194 μm hydraulic diameter together with integrated inlet and outlet manifold was fabricated on a silicon wafer using inductive coupled plasma dry etching, in conjunction with photolithographic techniques. A design with integrated temperature sensors made from a thin nickel film allows local temperature measurements with a much faster response time and smaller thermal resistance as compared to temperature measurements using thermocouples. The integrated heater was designed to enable either uniform or non-uniform heating (hotspot investigation) with a low thermal resistance between the heater and the channels. Numerical simulations for single phase flow in adiabatic conditions were used to assist the design of the manifold geometry in the microchannels heat sink. Microfabricated temperature sensors were used together with simultaneous high speed imaging in order to obtain a better insight related to temperature fluctuations caused by two-phase flow instabilities under uniform and non-uniform heating. Two types of two-phase instabilities with flow reversal were identified and classified into flow stability maps. The effect of inlet water temperature on flow boiling instabilities was experimentally studied, with the influence of different subcooling conditions on the magnitude of temperatures as well as the influence on temperature uniformity over the heat sink being assessed. The effect of various hotspot locations on flow boiling instabilities has been investigated, with hotspots located in different positions along the heat sink. Bubble growth and departure size have been experimentally investigated. The results of this study demonstrate that bubble growth in microchannels is different from that in macroscale channels. Furthermore, the effects of bubble dynamics on flow instabilities and heat transfer coefficient have been investigated and discussed.

A novel fabrication process is presented which allows for three-dimensionally independent features to be etched in silicon using SF6 gas in a deep reactive ion etcher (DRIE) after a single etch step. The mechanism allowing for different feature depths and widths to be produced over a wafer is reactive ion etch lag, where etch rate scales with the exposed feature size in the mask. A modified Langmuir model has been developed relating the geometry of the exposed areas in a specific mask pattern as well as the etch duration to the final depth and width of a channel that is produced after isotropic silicon etching. This fabrication process is tailored for microfluidic network design, but the capabilities of the process can be applied elsewhere. A characterization of an Alcatel DRIE tool is also presented in order to enhance RIE lag by varying etch process parameters, increasing the variety of channel sizes that can be fabricated. High values of flow rate, coil power, and pressure were found to produce this effect. The capability of the modeled process for creating a microchip cooling device for high-heat flux applications was also investigated. Using meander channels, heat flux in excess of 100W/cm2 were cooled using 750µL/s flow rate of water through the chip. This single-mask process reduces risk of damage to the chip and provides the capability to cool high-heat-flux microprocessors for the next 10 years, and for an even longer time once the geometry of the channels is optimized. / Master of Science

Abstract Carbon nanotubes (CNTs) have fascinating mechanical, electrical and thermal properties, all of which significantly depend on structural properties such as nanotube length, number of walls, lattice defect densities, impurities and surface functional groups. A number of different applications of carbon nanotubes have been demonstrated during the past two decades including electrical interconnects, transistors, heating and cooling devices, sensors and various actuators. However, further studies on the structure-dependent properties and innovative handling techniques of these materials are needed in order to explore the limitations of use and to be able fully to exploit the advantageous properties of such one-dimensional sp2 hybridized carbon nanomaterials. In this thesis, random networks of single-wall and multi-walled carbon nanotubes (SWCNTs and MWCNTs, respectively) and aligned films of multi-walled carbon nanotubes are studied in the context of three main application fields: gas sensing, electrical interconnects/electrodes and thermal cooling elements. Analyses of associated material properties and some feasible integration techniques are discussed. Single-wall and multi-walled carbon nanotube films cast from aqueous dispersions are shown to be selective nitric oxide sensing components in Taguchi-type sensor devices, in which films based on SWCNTs outperformed those made of MWCNTs. The thickness dependent electrical conduction mechanism of inkjet-printed SWCNT films is also discussed. Robust aligned MWCNT films are demonstrated as soft electrical contact brushes in DC motors and in other moving electrical contacts. The thermal properties of freestanding aligned MWCNT forests are analyzed and shown to be potential alternatives to copper or aluminium in the thermal management of electrical components. / Tiivistelmä Hiilinanoputkien kiehtovat mekaaniset, sähköiset ja lämmönjohto-ominaisuudet ovat kiinnostaneet tutkijoita suuresti viimeisten kahden vuosikymmenen ajan. Monia erilaisia applikaatioita on demonstroitu tänä aikana: mukaan lukien sähköiset kontaktit, transistori-rakenteet, lämmitys- ja jäähdytyslaitteet, anturirakenteet sekä erilaiset aktuaattori-rakenteet. Tämän väitöskirjan päätavoitteena on tutkia hiilinanoputkien toiminnollisuutta ja käytännöllisyyttä erilaisissa sovelluskohteissa. Tässä työssä käytettävät hiilinanoputkirakenteet ovat joko satunnaisjärjestyksessä olevia nanoputkista koostuvia verkostorakenteita tai yhdensuuntaisia, makroskooppisia hiilinanoputkikalvoja. Nanoputkia tutkitaan kolmessa erityyppisessä sovelluskohteessa: kaasuanturisovelluksessa, sähköisissä kontaktirakenteissa sekä jäähdytyselementteinä. Työssä analysoidaan hiilinanoputkirakenteiden ominaisuuksia eri sovelluskohteissa sekä esitetään joitain käyttökelpoisia tekniikoita hiilinanoputkien integroimiseen olemassa oleviin tekniikoihin. Hiilinanoputkien osoitetaan olevan käyttökelpoisia aktiivisia materiaaleja typpioksidille resistiivisessä kaasuanturirakenteessa. Tulosten perusteella yksiseinämäiset hiilinanoputket ovat moniseinämäisiä herkempiä ja parempia kyseisessä sovelluksessa. Lisäksi tutkitaan ja analysoidaan mustesuihku-tulostettujen yksiseinämäisten hiilinanoputkifilmien sähköisten ominaisuuksien riippuvuutta filmin paksuudesta. Vantterien yhdensuuntaisten moniseinämäisten hiilinanoputkirakenteiden osoitetaan toimivan erinomaisesti pehmeinä sähköisinä kontaktielementteinä liikkuvissa sähköisissä kontakteissa. Vapaasti seisovien yhdensuuntaisten, moniseinämäisten hiilinanoputkirakenteiden lämmönjohto-ominaisuuksien tutkiminen ja analysointi osoittaa, että kyseisiä rakenteita voidaan käyttää tehokkaina jäähdytyselementteinä ja mahdollisesti korvaavana vaihtoehtona alumiinille ja kuparille sähköisten komponenttien lämmönhallinta sovelluksissa.

[pt] Diante dos constantes avanços da tecnologia os dispositivos eletrônicos vêm passando por um processo de miniaturização, ao mesmo tempo em que sustentam um aumento de potência. Essa tendência se mostra um desafio para seu gerenciamento térmico, uma vez que os sistemas de resfriamento típicos para eletrônicos utilizam ar como fluido de trabalho, e o seu baixo coeficiente de transferência de calor limita sua capacidade de atender às necessidades térmicas da indústria atual. Nesse sentido, o resfriamento bifásico tem sido considerado uma solução promissora para fornecer resfriamento adequado para dispositivos eletrônicos. Circuitos de termossifão bifásico combinam a tecnologia de resfriamento bifásico com sua inerente natureza passiva, já que o sistema não requer uma bomba para fornecer circulação para seu fluido de trabalho, graças às forças da gravidade e de empuxo. Um dissipador de calor de microcanais, localizado bem em cima do dispositivo eletrônico, dissipa o calor gerado. Isto o torna uma solução de baixo custo e energia. Além disso, ter um circuito de termossifão operando com um refrigerante de baixo GWP, como o R-1234yf, resulta em baixo impacto para o meio ambiente, uma vez que é um refrigerante ecologicamente correto e o sistema tem baixo ou nenhum consumo de energia. Este trabalho fornece um modelo numérico detalhado para a simulação de um circuito de termossifão bifásico, operando em condições de regime permanente. O circuito compreende um evaporador (chip e dissipador de calor de micro-aletas), um riser, um condensador refrigerado a água de tubo duplo e um downcomer. Equações fundamentais e constitutivas foram estabelecidas para cada componente. Um método numérico de diferenças finitas, 1-D para o escoamento do fluido por todos os componentes do sistema, e 2-D para a condução de calor no chip e evaporador foi empregado. O modelo foi validado com dados experimentais para o refrigerante R134a, mostrando uma discrepância em relação ao fluxo de massa em torno de 6 por cento, para quando o sistema operava sob regime dominado pela gravidade. A pressão de entrada do evaporador prevista apresentou um erro relativo máximo de 4,8 por cento quando comparada aos resultados experimentais. Além disso, a maior discrepância da temperatura do chip foi inferior a 1 grau C. Simulações foram realizadas para apresentar uma comparação de desempenho entre o R134a e seu substituto ecologicamente correto, R1234yf. Os resultados mostraram que quando o sistema operava com R134a, ele trabalhava com uma pressão de entrada no evaporador mais alta, assim como, com um fluxo de massa mais alto. Por causa disso, o R134a foi capaz de manter a temperatura do chip mais baixa do que o R1234yf. No entanto, essa diferença na temperatura do chip foi levemente inferior a 1 grau C, mostrando o R1234yf como comparável em desempenho ao R134a. Além disso, o fator de segurança da operação do sistema foi avaliado para ambos os refrigerantes, e para um fluxo de calor máximo do chip de 33,1 W/cm2, R1234yf mostrou um fator de segurança acima de 3. Isso significa que o circuito de termossifão pode operar com segurança abaixo do ponto crítico de fluxo de calor. Dada a investigação sobre a comparação de desempenho dos refrigerantes R134a e R1234yf, os resultados apontaram o R1234yf como um excelente substituto ecologicamente correto para o R134a, para operação em um circuito de termossifão bifásico. / [en] Given the constant advances in technology, electronic devices have been going through a process of miniaturization while sustaining an increase in power. This trend proves to be a challenge for thermal management since commonly electronic cooling systems are air-based, so that the low heat transfer coefficient of air limits its capacity to keep up with the thermal needs of today s industry. In this respect, two-phase cooling has been regarded as a promising solution to provide adequate cooling for electronic devices. Two-phase thermosyphon loops combine the technology of two-phase cooling with its inherent passive nature, as the system does not require a pump to provide circulation for its working fluid, thanks to gravity and buoyancy forces. A micro-channel heat sink located right on top of the electronic device dissipates the heat generated. This makes for an energy and cost-efficient solution. Moreover, having a thermosyphon loop operating with a low GWP refrigerant such as R-1234yf results in low impact for the environment since it is an environmentally friendly refrigerant, and the system has low to none energy consumption. This work provides a detailed numerical model for the simulation of a two-phase thermosyphon loop operating under steady-state conditions. The loop comprises an evaporator (chip and micro-fin heat sink), a riser, a tube-in-tube water-cooled condenser and a downcomer. Fundamental and constitutive equations were established for each component. A finite-difference method, 1-D for the flow throughout the thermoysphon s components and 2-D for the heat conduction in the evaporator and chip, was employed. The model was validated against experimental data for refrigerant R134a, showing a mass flux discrepancy of around 6 percent for when the system operated under gravity dominant regime. The predicted evaporator inlet pressure showed a maximum relative error of 4.8 percent when compared to the experimental results. Also, the chip temperature s largest discrepancy was lower than 1 C degree. Simulations were performed to present a performance comparison between R134a and its environmentally friendly substitute, R1234yf. Results showed that when the system operated with R134a, it yielded a higher evaporator inlet pressure as well as a higher mass flux. Because of that, R134a was able to keep the chip temperature lower than R1234yf. Yet, that difference in chip temperature was slightly lower than 1 C degree, showing R1234yf as comparable in performance to R134a. In addition, the safety factor of the system s operation was evaluated for both refrigerants, and for a maximum chip heat flux of 33.1 W/cm2, R1234yf showed a safety factor above 3. This means the thermosyphon loop can operate safely under the critical heat flux. Given the investigation on the performance comparison of refrigerants R134a and R1234yf, results pointed to R1234yf being a great environmentally friendly substitute for R134a for the two-phase thermosyphon loop.