Analyses of heat and mass transfer between different materials and phases are essential in numerous fundamental scientific problems and practical engineering applications, such as thermal and chemical transport in porous media, design of heat exchangers, dendritic growth during solidification, and thermal/mechanical analysis of additive manufacturing processes. In the numerical simulation, interface treatment can be further divided into sharp interface schemes and diffuse interface schemes according to the morphological features of the interface. This work focuses on the following subjects through computational studies: (1) critical evaluation of the various sharp interface schemes in the literature for conjugate heat and mass transfer modeling with the lattice Boltzmann method (LBM), (2) development of a novel sharp interface scheme in the LBM for conjugate heat and mass transfer between materials/phases with very high transport property ratios, and (3) development of a new diffuse-interface phase-field-lattice Boltzmann method (PFM/LBM) for dendritic growth and solidification modeling. For comparison of the previous sharp interface schemes in the LBM, the numerical accuracy and convergence orders are scrutinized with representative test cases involving both straight and curved geometries. The proposed novel sharp interface scheme in the LBM is validated with both published results in the literature as well as in-house experimental measurements for the effective thermal conductivity (ETC) of porous lattice structures. Furthermore, analytical correlations for the normalized ETC are proposed for various material pairs and over the entire range of porosity based on the detailed LBM simulations. In addition, we provide a modified correlation based on the SS420-air and SS316L-air metal pairs and the high porosity range for specific application. The present PFM/LBM model has several improved features compared to those in the literature and is capable of modeling dendritic growth with fully coupled melt flow and thermosolutal convection-diffusion. The applicability and accuracy of the PFM/LBM model is verified with numerical tests including isothermal, iso-solutal and thermosolutal convection-diffusion problems in both 2D and 3D. Furthermore, the effects of natural convection on the growth of multiple crystals are numerically investigated.

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Thesis (M. Sc. (Applied Mathematics)) -- University of Limpopo, 2022 / Nanofluids are a class of heat transfer fluids created by suspending nanoparticles in base fluids. Due to their enhanced thermal conductivity, nanofluids are fast replacing conventional heat transfer fluids like water, mineral oil, ethylene glycol and others. They contribute to advancement of technology and modernity through pertinent applications in fields such as biomedical, automotive industry, cooling technologies and many others. This study documents a survey of nanofluids and their applications and an investigation of peristaltic nanofluid flow through a two dimensional microchannel with and without slip effects. Peristaltic fluid transport plays an important role in engineering, technology, science and physiology. The Buongiorno model formulation is employed and the governing equations for peristaltic nanofluid flow in a two dimensional microchannel are non-dimensionalised and solved semi-analytically using the Adomian decomposition method. Series solutions for axial velocity, temperature and nanoparticle concentration profiles are coded into symbolic package MATHEMATICA for easy computation of the numerical solutions. The effects of the various parameters embedded in the model are simulated graphically and discussed quantitatively and qualitatively. The results are compared with those in literature that were obtained using other approximate analytical methods and the homotopy analysis method. The study revealed that the Brownian motion, thermophoresis, buoyance and the slip parameters have significant influence on the peristaltic flow axial velocity, temperature and nanoparticle concetration profiles. In the flow without slip, both the Brownian motion and thermophoresis parameters caused a cooling effect around the channel walls and a marginal temperature enhancement in the channel core region and significant flow reversal was noticed in the channel half-space with maximum axial velocity recording in the channel core region. In the slip flow, both Brownian motion and thermophorisis had a retardation effect on the nanoparticle concentration profile.

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This work investigated a novel vapor chamber for efficient heat spreading and heat removal. A vapor chamber acting as a heat spreader enables for more uniform temperature distribution along the surface of the device being cooled. First, a vapor chamber was studied and compared with the traditional copper heat spreader. The thickness of vapor chamber was kept 1.35 mm which was considered to be ultra-thin vapor chamber. Then, a new geometrical model having graphite foam in vapor space was proposed where the graphite foam material was incorporated in vapor space as square cubes. The effects of incorporating graphite foam in vapor space were compared to the vapor chamber without the embedded graphite foam to investigate the heat transfer performance improvements of vapor chamber by the high thermal conductivity graphite foam. Finally, the effects of various vapor chamber thicknesses were studied through numerical simulations. It was found that thinner vapor chamber (1.35 mm thickness) had better heat transfer performance than thicker vapor chamber (5 mm thickness) because of the extreme high effective thermal conductivities of ultra-thin vapor chamber. Furthermore, the effect of graphite foam on thermal performance improvement was very minor for ultra-thin vapor chamber, but significant for thick vapor chamber. The GF could help reduce the junction temperature by 15-30% in the 5-mm thick vapor chamber. Use of GF embedded vapor chamber could achieve 250-400 Watt per Centimeter square local heat removal for power electronics. The application of this is not only limited to electronic devices but actuator and avionics cooling in aircrafts, thermal management of electronics in directed energy weapon systems, battery thermal management for electric and hybrid vehicles, smart phones cooling, thus covering a wide gamut of heat flux applications.

Very limited research studies have been conducted to examine the behaviour of rubberised concrete (RuC) with nano silica (NS) and addressed the acoustic benefits of rubberised concrete. The current research investigates the effect of incorporating colloidal nano silica on the mechanical, thermal and acoustic properties of Rubberised concrete and compares them with normal concrete (NC). Two sizes of rubber were used RA (0.5 – 1.5 mm) and RB (1.5 – 3 mm). Fine aggregate was replaced with rubber at a ratio of 0%, 10%, 20% and 30% by volume, and NS is used as partial cement replacement by 0%, 1.5% and 3%. A constant water to cement ratio of 0.45 was used in all concrete mixes. Various properties of rubberised concrete, including the density, water absorption, the compressive strength, the flexural strength, splitting tensile strength and the drying shrinkage of samples was studied as well as thermal and acoustic properties. Experimental results of compressive strength obtained from this study together with collected comprehensive database from different sources available in the literature were compared to five existing models, namely Khatib and Bayomy- 99 model, Guneyisi-04 model, Khaloo-08 model, Youssf-16 model, and Bompa-17 model. To assess the quality of predictive models, influence of rubber content on the compressive strength is studied. An artificial neural network (ANN) models were developed to predict compressive strength of RuC using the same data used in the existing models. Three ANN sets namely ANN1, ANN2 and ANN3 with different numbers of hidden layer neurons were constructed. Comparison between the results given by the ANN2 model and the results obtained by the five existing predicted models were presented. A finite element approach is proposed for calculating the transmission loss of concrete, the displacement in the solid phase and the pressure in the fluid phase is investigated. The transmission loss of the 50mm concrete samples is calculated via the COMSOL environment, the results from the simulation show good agreement with the measured data. The results showed that, using up to 20% of rubber as fine aggregate with the addition of 3% NS can produce a higher compressive strength than the NC. Experimental results of this research indicate that incorporating nano silica into RuC mixes enhance sound absorption and thermal conductivity compared to normal concrete (NC) and rubberised concrete without nano silica. This work suggests that it is possible to design and manufacture concrete which can provide an improvement to conventional concrete in terms of the attained vibro-acoustic and thermal performance. / Libyan Ministry of Higher Education

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Thermoelectric devices with the ability to convert rejected heat into electricity are widely used in nowadays technology. Several studies have been done to improve the efficiency of these devices. However, because of the strong correlation between thermoelectric properties (electrical conductivity, Seebeck coefficient, and thermal conductivity including lattice and electron counterpart), improving ZT has always been a challenging task. In this study, thermal conductivity of group IV-based binary and ternary alloys such as SiGe, SiSn, GeSn, and SiGeSn has been studied. Phonon Boltzmann Transport Equation has been solved in the relaxation time approximation including intrinsic and extrinsic (in the presence of boundary and interfaces in the low-dimensional material) scattering mechanisms. Full phonon dispersion based on the Adiabatic Bond Charge model has been calculated for Si, Ge, and Sn. Virtual crystal approximation has been adapted to calculate the dispersion of SiGe, SiSn, GeSn, and SiGeSn. Two approaches have been introduced to reduce the lattice thermal conductivity of the materials under study. First, alloying results in a significant reduction of thermal conductivity. But, this reduction has been limited by the mass disorder scattering in the composition range of 0.2 to 0.8. Second, nanostructuring technique has been proposed to further reduce the thermal conductivity. Our study shows that, due to the atomic mass difference which gives rise to the elastic mass scattering mechanism, SiSn has the lowest thermal conductivity among the other materials under study. SiSn achieved the thermal conductivity of 1.18 W/mK at 10 nm at the Sn composition of 0.18, which is the experimentally stable state of SiSn. The results show that SiSn alloys have the lowest conductivity (3 W/mK) of all the bulk alloys, more than two times lower than SiGe, attributed to the larger difference in mass between the two constituents. In addition, this study demonstrates that thin films offer an additional reduction in thermal conductivity, reaching around 1 W/mK in 20 nm SiSn, GeSn, and ternary SiGeSn films, which is close to the conductivity of amorphous SiO$_2$. This value is lower than the thermal conductivity of SiGe at 10 nm which is 1.43 W/mK. Having lattice thermal conductivity reduced, electron transport has been studied by solving Boltzmann Transport Equation under low electric field including elastic and inelastic scattering mechanisms. Rode's iterative method has been applied to the model for obtaining perturbation of distribution function under a low electric field. This study shows that nanostructuring and alloying can reduce $\kappa_{ph}$ without significantly changing the other parameters. This is because of the phonon characteristics in solids in which MFP of phonons is much larger than those of electrons, which gives us the possibility of phonons confinement without altering electrons transport. Thermoelectric properties of SiGe in the bulk and nanostructure form have been studied to calculate ZT in a wide range of temperatures. The results demonstrate that ZT reaches the value of 1.9 and 1.58 at the temperatures of 1200 K and 1000 K respectively, with the Ge composition of 0.2 and carrier concentration of 5$\times$10$^{19}$ cm$^{-3}$ at 10 nm thickness. This model can be applied to SiSn and other binary and ternary alloys, to calculate the improved ZT. Hence, we conclude that group IV alloys containing Sn have the potential for high-efficiency TE energy conversion.

Ett experiment har genomförts på en anisotrop golvvärmeskiva i materialet wellpapp utvecklat av Flooré AB. Detta i bland annat syfte av att ta fram värmekonduktiviteten i skivans olika riktningar. Företaget har en golvvärmeskiva i expanderad polystyrenplast (EPS) sedan tidigare. Det är även av intresse att se om parametrar som framledningstemperatur och energiåtgång skiljer sig på ett gynnsamt vis mellan wellpappen och EPSen. Det vill säga om anisotropin kan vara till fördel. Mätningar har genomförts på flertalet provbitar i materialet wellpapp för att bestämma värmekonduktiviteten i wellpappens olika riktningar. Mätningarna analyserades och λ-värden vid 10 ˚C togs fram. Wellpappens värmekonduktivitet i de olika riktningar var högst i den riktning som har högre hållfasthetsvärde och lägre λ-värde gavs i de svagare riktningarna. Därefter användes värdena från analysen i finita elementprogrammet COMSOL. Resultatet från COMSOL matades in i den applikation i Excel som var framtagen av Flooré. Med hjälp av applikationen erhölls optimala framledningstemperaturer på vattnet och effekt per kvadratmeter i vid tre olika värmeeffektbehov där wellpapp och EPS användes som golvskivematerial. Resultatet från applikationen på de olika golvvärmeskivorna jämfördes därefter med varandra. Slutsatserna som drogs var att lägre framledningstemperatur kunde sättas för wellpappskivan eftersom den distribuerade värmen från vattenledningarna bättre än EPSskivan. / An experiment has been carried out on an anisotropic underfloor heating panel in the material corrugated cardboard developed by Flooré AB. The company already has an underfloor heating panel in expanded polystyrene (EPS). It is also of interest to see if parameters such as supply temperature and energy consumption differ in a favourable way between the corrugated board and the EPS. That is, if the anisotropy can be beneficial.  Measurements have been carried out on several test pieces in the material corrugated cardboard to determine the thermal conductivity in the different directions of the board. The measurements were analysed and λ-values at 10 °C were produced. The thermal conductivity of corrugated board in the different directions was highest in the direction with a higher mechanical strength value and a lower λ-value was given in the weaker directions. The values from the analysis were then used in the finite element program COMSOL. The results from COMSOL were entered into the application in Excel that was developed by Flooré. With the help of the application, optimal water supply temperatures and power per square meter were obtained with three different heat flow requirements. This was done for the different materials. The results from the application were then compared with each other. The conclusions that were drawn were that a lower supply temperature could be set for the corrugated board because it distributed the heat from the water pipes better than the EPS board.

Résumé : L’isolation thermique est essentielle dans de nombreux types de MEMS (micro-systèmes électro-mécaniques). Elle permet de réduire la consommation d’énergie, améliorer leurs performances, ou encore isoler la zone chaude du reste du dispositif, ce qui est essentiel dans les systèmes sur puce. Il existe quelques matériaux et techniques d’isolation pour les MEMS, mais ils sont limités. En effet, soit ils ne proposent pas un niveau d’isolation suffisant, sont trop fragiles, ou imposent des contraintes trop importantes sur la conception du dispositif et sont difficiles à intégrer. Une approche intéressante pour l’isolation, démontrée dans la littérature, est de fabriquer des pores de taille nanométrique dans le silicium par gravure électrochimique. En nanostructurant le silicium ainsi, on peut diviser sa conductivité thermique par un facteur de 100 à 1000, le transformant en isolant thermique. Cette solution est idéale pour l’intégration dans les procédés de fabrication existants des MEMS, car on garde le silicium qui est déjà utilisé pour leur fabrication, mais en le nanostructurant localement, on le rend isolant là où on en a besoin. Par contre sa porosité cause des problèmes : mauvaise résistance chimique, structure instable au-delà de 400°C, et tenue mécanique réduite. La facilité d’intégration des semiconducteurs poreux est un atout majeur, nous visons donc de réduire les désavantages de ces matériaux afin de favoriser leur intégration dans des dispositifs en silicium. Nous avons identifié deux approches pour atteindre cet objectif : i) améliorer le Si poreux ou ii) développer un nouveau matériau. La première approche consiste à amorphiser le Si poreux en l’irradiant avec des ions à haute énergie (uranium, 110 MeV). Nous avons montré que l’amorphisation, même partielle, du Si poreux entraîne une diminution de sa conductivité thermique, sans endommager sa structure poreuse. Cette technique réduit sa conductivité thermique jusqu’à un facteur de trois, et peut être combinée avec une pré-oxydation afin d’atteindre une réduction d’un facteur cinq. Donc cette méthode permet de réduire la porosité du Si poreux, et d’atténuer ainsi les problèmes de fragilité mécanique causés par la porosité élevée, tout en gardant un niveau d’isolation égal. La seconde approche est de développer un nouveau matériau. Nous avons choisi le SiC poreux : le SiC massif a des propriétés physiques supérieures à celles du Si, et donc à priori le SiC poreux devrait conserver cette supériorité. La fabrication du SiC poreux a déjà été démontrée dans la littérature, mais avec peu d’études détaillées du procédé. Sa conductivité thermique et tenue mécanique n’ont pas été caractérisées, et sa tenue en température que de façon incomplète. Nous avons mené une étude systématique de la porosification du SiC en fonction de la concentration en HF et le courant. Nous avons implémenté un banc de mesure de la conductivité thermique par la méthode « 3 oméga » et l’avons utilisé pour mesurer la conductivité thermique du SiC poreux. Nous avons montré qu’elle est environ deux ordres de grandeur plus faible que celle du SiC massif. Nous avons aussi montré que le SiC poreux est résistant à tous les produits chimiques typiquement utilisés en microfabrication sur silicium. D’après nos résultats il est stable jusqu’à au moins 1000°C et nous avons obtenu des résultats qualitatifs encourageants quant à sa tenue mécanique. Nos résultats signifient donc que le SiC poreux est compatible avec la microfabrication, et peut être intégré dans les MEMS comme isolant thermique. // Abstract : Thermal insulation is essential in several types of MEMS (micro electro-mechanical systems). It can help reduce power consumption, improve performance, and can also isolate the hot area from the rest of the device, which is essential in a system-on-chip. A few materials and techniques currently exist for thermal insulation in MEMS, but these are limited. Indeed, either they don’t have provide a sufficient level of insulation, are too fragile, or restrict design of the device and are difficult to integrate. A potentially interesting technique for thermal insulation, which has been demonstrated in the literature, is to make nanometer-scale pores in silicon by electrochemical etching. By nanostructuring silicon in this way, its thermal conductivity is reduced by a factor of 100 to 1000, transforming it into a thermal insulator. This solution is ideal for integration in existing MEMS fabrication processes, as it is based on the silicon substrates which are already used for their fabrication. By locally nanostructuring these substrates, silicon is made insulating wherever necessary. However the porosity also causes problems : poor chemical resistance, an unstable structure above 400◦C, and reduced mechanical properties. The ease of integration of porous semiconductors is a major advantage, so we aim to reduce the disadvantages of these materials in order to encourage their integration in silicon-based devices. We have pursued two approaches in order to reach this goal : i) improve porous Si, or ii) develop a new material. The first approach uses irradiation with high energy ions (100 MeV uranium) to amorphise porous Si. We have shown that amorphisation, even partial, of porous Si leads to a reduction of its thermal conductivity, without damaging its porous structure. This technique can reduce the thermal conductivity of porous Si by up to a factor of three, and can be combined with a pre-oxidation to achieve a five-fold reduction of thermal conductivity. Therefore, by using this method we can use porous Si layers with lower porosity, thus reducing the problems caused by the fragility of high-porosity layers, whilst keeping an equal level of thermal insulation. The second approach is to develop a new material. We have chosen porous SiC: bulk SiC has exceptional physical properties and is superior to bulk Si, so porous SiC should be superior to porous Si. Fabrication of porous SiC has been demonstrated in the literature, but detailed studies of the process are lacking. Its thermal conductivity and mechanical properties have never been measured and its high-temperature behaviour has only been partially characterised. We have carried out a systematic study of the effects of HF concentration and current on the porosification process. We have implemented a thermal conductivity measurement setup using the “3 omega” method and used it to measure the thermal conductivity of porous SiC. We have shown that it is about two orders of magnitude lower than that of bulk SiC. We have also shown that porous SiC is chemically inert in the most commonly used solutions for microfabrication. Our results show that porous SiC is stable up to at least 1000◦C and we have obtained encouraging qualitative results regarding its mechanical properties. This means that porous SiC is compatible with microfabrication processes, and can be integrated in MEMS as a thermal insulation material.