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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.

No description available.

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.

Halbleitermaterialien sind in den elektrischen Anwendungen der heutigen Zeit unerlässlich geworden. In dieser Arbeit wird der Fokus auf die Untersuchung der elektrischen und thermischen Eigenschaften von zwei Halbleiterklassen gelegt. Zum einen wird mit -Ga2O3 ein Mitglied der Klasse der transparenten leitfähigen Oxide untersucht.Hier wurden die elektrischen Eigenschaften von dünnen Schichten (Dicke von 28nm-225nm) und Volumenkristallen temperaturabhängig untersucht.Dabei zeigt sich bei Volumenkristallen und mindestens 150nm dicken Schichten eine Steigerung der elektrischen Leitfähigkeit bis 100K durch die Streuung von Elektronen an Störstellen und bei Temperaturen über 100K wieder ein Abfall der elektrischen Leitfähigkeit durch Elektron-Phonon-Wechselwirkung. Die Untersuchung der thermische Leitfähigkeit von beta-Ga2O3 zeigt ein anisotropes Verhalten mit minimalen Werten in [100]-Richtung und maximalen Werten in [010]-Richtung. Die Temperaturabhängigkeit der thermischen Eigenschaften zeigt eine Verringerung der thermischen Leitfähigkeit und der thermischen Diffusivität mit steigender Temperatur. Eine zweite untersuchte Materialklasse ist die der Kesterite. Zu dieser Kristallstruktur wurden zwei Elementkonfigurationen untersucht, Kupfer-Zink-Zinn-Sulfid und Kupfer- Zink-Zinn-Selenid. Der Transport bei Raumtemperatur und darunter findet über verschiedene Tunnelprozesse lokalisierter Ladungsträger statt. Zusätzlich wird auf die Veränderung der elektrischen Eigenschaften durch die Kristallinität und Komposition eingegangen. Die thermischen Eigenschaften zeigen analog zum beta-Ga2O3 eine Dominanz der Phonon-Phonon-Umklapp-Streuung bei hohen Temperaturen, während bei niedrigen Temperaturen Streuung an Störstellen und Grenzflächen vorherrscht. Methodisch zeigt diese Arbeit unterschiedlichste Messmethoden zur Charakterisierung der elektrischen und thermischen Eigenschaften, welche die Standardmethoden sowohl nutzen, als auch sinnvoll erweitern. / Semiconductors are essential for electronic applications nowadays. Here, the electrical and thermal properties of two semiconductor classes with huge application potential are investigated. As a transparent conducting oxide beta-Ga2O3 is investigated. In this work, the temperature dependent electrical properties were investigated for bulk materials and thin films. An increase in the electrical conductivity until 100K is found through electron-impurity-scattering and a decrease at higher temperatures through electron-phonon-scattering for for films with a thickness of at least 150nm. The investigation of the thermal properties of -Ga2O3 show an anisotropy for the different crystal orientations with minimal primary axis values for the [100]-direction and maximal values for the [010]-direction. The temperature-dependence of the thermal properties shows a decease in conductivity and diffusivity for increasing temperature. For temperatures over 150K phonon-phonon-Umklapp-scattering can explain the measured values. For low temperatures phonon-impurity scattering is most likely the dominant scattering mechanism. A second investigated material class are kesterites. For this crystal structure two configurations were investigated, copper-zinc-tin-sulfide and copper-zinc-tin-selenide. The electrical properties show semiconducting characteristics with p-type conduction. The transport processes are defined through localised thermal activated tunneling within the band gap. Other reductions of the mobility are found by the crystalinity and the composition of the materials. The thermal properties show dominant phonon-phonon- Umklapp-scattering at higher temperatures and phonon-impurity-scattering for lower temperatures in a similar way as in beta-Ga2O3. This work shows new implemented measurement methods for investigating electrical and thermal properties as extentions to common methods.

Efficient thermal management of spacecraft systems is crucial to overcome the extreme thermal conditions in space. Phase change materials (PCMs) offer significant advantages in thermal energy storage (TES) systems due to their high latent heat of fusion, the amount of energy stored or released during phase transitions. However, PCMs face limitations due to low thermal conductivity, especially in microgravity where heat transfer is primarily through conduction. This research addresses this issue by incorporating a 3D-printed aluminum lattice structure to enhance heat transfer to PCMs. Experimental data from sub-orbital flight tests and ground-based experiments, coupled with numerical simulations, are used to analyze the melting process of PCMs in microgravity and the effect on thermal performance of PCMs through the incorporation of high thermal conductivity lattice structure. The results provide a deeper understanding of heat transfer to PCM’s in microgravity, potentially enhancing the performance of TES systems utilizing PCM’s in space environments.

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.