This study consists of an experimental investigation into the fluid flow and heat transfer aspects of microchannels. Rectangular copper microchannels of hydraulic diameters 1.05 mm, 0.85 mm and 0.57 mm were considered. Using water as the working fluid, heat transfer and pressure drop characteristics were determined under a constant surface heat flux for different inlet configurations in the laminar and transitional regimes. Three inlet geometries were experimentally investigated: a sudden contraction inlet, a bellmouth inlet and a swirl-generating inlet. The influence of the inlet conditions on the pressure drop, Nusselt number and critical Reynolds number was determined experimentally. Pressure drop results showed good agreement with existing correlations for adiabatic conditions. Diabatic friction factor results for the sudden contraction and bellmouth inlets were overpredicted when using the friction factor results from literature. It is noted that a relationship between the pressure drop and heat flux existed in the laminar regime, where an increase in the heat input resulted in a decrease in the friction factor. The bellmouth inlet condition showed an enhancement of the heat transfer in the transition regime compared with the sudden contraction inlet. The critical Reynolds number for the onset of transition for the sudden contraction inlet was found to be approximately 1 950, with a sharp rise to the turbulent regime thereafter. The bellmouth inlet influenced the originating point of the transition regime, which commenced at a Reynolds number of approximately 1 600. A smoother and more gradual increase to the turbulent regime was observed as an effect of the bellmouth inlet over the sudden contraction inlet. The swirl-generating inlet condition produced higher friction factor results in all three flow regimes. Transition occurred at a Reynolds number of approximately 1 500 and the turbulent regime was quickly ii reached thereafter. The turbulent regime friction factor was found to be significantly higher with the swirl inlet compared with both the sudden contraction and bellmouth inlets. Nusselt numbers continued to increase until the onset of the transition regime, and did not converge to a constant value as stated in theory. Similar enhancement of the transition regime with the bellmouth inlet was observed for the Nusselt numbers as with the friction factors. The initial turbulent regime results followed the trend of the theory for both the sudden contraction and bellmouth inlet conditions for most of the data sets, with deviation occurring in some of the 0.57 mm test cases. The swirl inlet Nusselt number results were significantly underpredicted by the theory in the early turbulent regime. / Dissertation (MEng)--University of Pretoria, 2014. / gm2014 / Mechanical and Aeronautical Engineering / unrestricted

Red blood cells (RBCs) are the most abundant cells in human blood, representing 40 to 45% of the blood volume (hematocrit). These cells have the particular ability to deform and bridge together to form aggregates under very low shear rates. The theory and mechanics behind aggregation are, however, not yet completely understood. The purpose of this work is to provide a novel method to analyze, understand and mimic blood behaviour in microcirculation. The main objective is to develop a methodology to quantify and characterize RBC aggregates and hence enhance the current understanding of the non-Newtonian behaviour of blood at the microscale. For this purpose, suspensions of porcine blood and human blood are tested in vitro in a Poly-di-methylsiloxane (PDMS) microchannel to characterize RBC aggregates within these two types of blood. These microchannels are fabricated using standard photolithography methods. Experiments are performed using a micro Particle Image Velocimetry ( PIV) system for shear rate measurements coupled with a high speed camera for the flow visualization. Corresponding numerical simulations are conducted using a research Computational Fluid Dynamic (CFD) solver, Nek5000, based on the spectral element method solution to the incompressible non-Newtonian Navier-Stokes equations. RBC aggregate sizes are quantified in controlled and measurable shear rate environments for 5, 10 and 15% hematocrit. Aggregate sizes are determined using image processing techniques. Velocity fields of the blood flow are measured experimentally and compared to numerical simulations using simple non-Newtonian models (Power law and Carreau models). This work establishes for the first time a relationship between RBC aggregate sizes and corresponding shear rates in a microfluidic environment as well as one between RBC aggregate sizes and apparent blood viscosity at body temperature in a microfluidic controlled environment. The results of the investigation can be used to help develop new numerical models for non-Newtonian blood flow, provide a better understanding of the mechanics of RBC aggregation and help determine aggregate behaviour in clinical settings such as for degenerative diseases like diabetes and heart disease.

In recent times, high power density trends and temperature constraints in integrated circuits have led to conventional cooling techniques not being sufficient to meet the thermal requirements. The ever-increasing desire to overcome this problem has led to worldwide interest in micro heat sink design of electronic components. It has been found that geometric configurations of micro heat sinks play a vital role in heat transfer performance. Therefore, an effective means of optimally designing these heat sinks is required. Experimentation has extensively been used in the past to understand the behaviour of these heat extraction devices. Computational fluid dynamics (CFD) has more recently provided a more cost-effective and less time-consuming means of achieving the same objective. However, in order to achieve optimal designs of micro heat sinks using CFD, the designer has to be well experienced and carry out a number of trial-and-error simulations. Unfortunately, this will still not always guarantee an accurate optimal design. In this dissertation, a design methodology which combines CFD with a mathematical optimisation algorithm (a leapfrog optimisation program and DYNAMIC-Q algorithm) is proposed. This automated process is applied to three design cases. In the first design case, the peak wall temperature of a microchannel embedded in a highly conductive solid is minimised. The second case involves the optimisation of a double row micropin-fin heat sink. In this case, the objective is to maximise the total rate of heat transfer with the effect of the thermal conductivity also being investigated. The third case extends the micropin-fin optimisation to a heat sink with three rows. In all three cases, fixed volume constraint and manufacturing restraints are enforced to ensure industrial applicability. Lastly, the trends of the three cases are compared. It is concluded that optimal design can be achieved with a combination of CFD and mathematical optimisation. / Dissertation (MEng)--University of Pretoria, 2011. / Mechanical and Aeronautical Engineering / Unrestricted

Recent technological developments have made it possible to design various microdevices where fluid flow and heat transfer are involved. For the proper design of such systems, the governing physics needs to be investigated. Due to the difficulty to study complex geometries in micro scales using experimental techniques, computational tools are developed to analyze and simulate flow and heat transfer in microgeometries. However, conventional numerical methods using the Navier-Stokes equations fail to predict some aspects of microflows such as nonlinear pressure distribution, increase mass flow rate, slip flow and temperature jump at the solid boundaries. This necessitates the development of new computational methods which depend on the kinetic theory that are both accurate and computationally efficient. In this study, lattice Boltzmann method (LBM) was used to investigate the flow and heat transfer in micro sized geometries. The LBM depends on the Boltzmann equation which is valid in the whole rarefaction regime that can be observed in micro flows. Results were obtained for isothermal channel flows at Knudsen numbers higher than 0.01 at different pressure ratios. LBM solutions for micro-Couette and micro-Poiseuille flow were found to be in good agreement with the analytical solutions valid in the slip flow regime (0.01 < Kn < 0.1) and direct simulation Monte Carlo solutions that are valid in the transition regime (0.1 < Kn < 10) for pressure distribution and velocity field. The isothermal LBM was further extended to simulate flows including heat transfer. The method was first validated for continuum channel flows with and without constrictions by comparing the thermal LBM results against accurate solutions obtained from analytical equations and finite element method. Finally, the capability of thermal LBM was improved by adding the effect of rarefaction and the method was used to analyze the behavior of gas flow in microchannels. The major finding of this research is that, the newly developed particle-based method described here can be used as an alternative numerical tool in order to study non-continuum effects observed in micro-electro-mechanical-systems (MEMS).

The aim of this research was to demonstrate a high current and stable field emission (FE) source based on carbon nanotubes (CNTs) and electron multiplier microchannel plate (MCP) and design efficient field emitters. In recent years various CNT based FE devices have been demonstrated including field emission displays, x-ray source and many more. However to use CNTs as source in high powered microwave (HPM) devices higher and stable current in the range of few milli-amperes to amperes is required. To achieve such high current we developed a novel technique of introducing a MCP between CNT cathode and anode. MCP is an array of electron multipliers; it operates by avalanche multiplication of secondary electrons, which are generated when electrons strike channel walls of MCP. FE current from CNTs is enhanced due to avalanche multiplication of secondary electrons and in addition MCP also protects CNTs from irreversible damage during vacuum arcing. Conventional MCP is not suitable for this purpose due to the lower secondary emission properties of their materials. To achieve higher and stable currents we have designed and fabricated a unique ceramic MCP consisting of high SEY materials. The MCP was fabricated utilizing optimum design parameters, which include channel dimensions and material properties obtained from charged particle optics (CPO) simulation. Child Langmuir law, which gives the optimum current density from an electron source, was taken into account during the system design and experiments. Each MCP channel consisted of MgO coated CNTs which was chosen from various material systems due to its very high SEY. With MCP inserted between CNT cathode and anode stable and higher emission current was achieved. It was ~25 times higher than without MCP. A brighter emission image was also evidenced due to enhanced emission current. The obtained results are a significant technological advance and this research holds promise for electron source in new generation lightweight, efficient and compact microwave devices for telecommunications in satellites or space applications. As part of this work novel emitters consisting of multistage geometry with improved FE properties were was also developed.

La presente tesis se ha llevado a cabo en el Instituto de Ingeniería Energética de la Universitat Politècnica de València y durante una estancia en el National Institute of Standards and Technology (NIST). El objetivo principal de la tesis es desarrollar un modelo de alta precisión para intercambiadores de calor de microcanales (MCHX), que tiene que ser útil, en términos de coste computacional, para tareas de diseño. En la opinión del autor, existen algunos inconvenientes cuando los modelos existentes se aplican a algunos diseños recientes de intercambiador de calor, tales como MCHXs, bien de tubos en serpentín o en paralelo. Por lo tanto, la primera etapa de la tesis identifica los fenómenos que tienen el mayor impacto en la precisión de un modelo para MCHX. Adicionalmente, se evaluó el grado de cumplimiento de varias simplificaciones y enfoques clásicos. Con este fin, se desarrolló el modelo de alta precisión Fin2D como una herramienta para llevar a cabo la investigación mencionada. El modelo Fin2D es una herramienta útil para analizar los fenómenos que tienen lugar, sin embargo requiere un gran coste computacional, y por tanto no es útil para trabajos de diseño. Es por ello que en base a los conocimientos adquiridos con el modelo Fin2D, se ha desarrollado un nuevo modelo, el Fin1Dx3. Este modelo tan sólo tiene en cuenta los fenómenos más importantes, reteniendo casi la misma precisión que Fin2D, pero con una reducción en el tiempo de cálculo de un orden de magnitud. Se introduce una novedosa discretización y un esquema numérico único para el modelado de la transferencia de calor del lado del aire. Este nuevo enfoque permite modelar los fenómenos existentes de forma consistente con mayor precisión y con mucho menos simplificaciones que los modelos actuales de la literatura. Por otra parte, se logra un coste razonable de cálculo para el objetivo fijado. La tesis incluye la validación experimental de este modelo tanto para un condensador y un enfriador de gas. Con e / Martínez Ballester, S. (2012). NUMERICAL MODEL FOR MICROCHANNEL CONDENSERS AND GAS COOLERS WITH AN IMPROVED AIR-SIDE APPROACH [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/17453 / Palancia

This work deals with the fabrication of closed cavities in Low Temperature Co-Fired Ceramics (LTCC). In recent years, the LTCC ceramics have become widely used in the areas of sensors, MEMS and micro-fluidic applications. When fabricating such devices, it is important that the cavities maintain their compactness and dimensions after the manufacturing process . This is achieved primarily with the right choice of lamination process and its parameters and also by appropriate setting of the firing profile. The theoretical part describes the various steps in the production of LTCC structures and the most common technological processes used for creating structures with cavities and micro - channels . In the practical part are selected laminating procedures performed on the test pattern, which contains cavities of different sizes. Emphasis was placed on the correct execution of each method , while gradually modifying the lamination parameters. The achieved results are further discussed in terms of process parameters and their influence on the dimensions of manufactured cavities.

Cryocooling is a continuously developing field of engineering, applied in the fieldsof aerospace, military, and medical sciences among others. There is a demand forsmaller and more efficient cryocoolers for spaceborne low-light observation missions,with many custom cooling systems having completed successful missions. The Stir-ling cycle is the most prevalent refrigeration technique used for space applications,with the pulse-tube, Joule-Thomson or reverse Brayton cycles being used in somespecial cases.A matrix heat exchanger is designed with 3D-printed 17-4 PH stainless steel end capsstreamlined for computer numerical control (CNC) production. The heat exchanger (HX) core consists of 1mm thick stainless steel spacers and 250μm thick copperchips that are tolerance-matched for photo etching, as well as pyrolytic graphitesheets (PGS) of 25μm, the thickest commercially available PGS without addedadhesive film material.The experiments of joining PGS and copper chips with Epo-Tek 301-2 epoxy tocreate a solid core structure for the heat exchanger did not result in a pressure-resistant laminate material. The graphite surface proved difficult to adhere to usingthis epoxy, creating voids, and easily delaminated into separate layers of PGS. Bond-ing the stack together using indium, testing epoxy with a higher ability to permeatethe PGS or diffusion-bonding through other means are presented as options forfurthering the HX development.Pressure testing of a copper-only laminated heat exchanger core showed that theend cap recess adhesion capability is a potential point of failure, as the designedstructure makes it impossible to inspect the results of the bond without curingthe epoxy and pressurising the system. The difficulty in establishing a tight seambetween the main counter-flow channels of the HX is also demonstrated here, asleakage between the channels occurred at pressures in the vicinity of 2

In Chapter 1, a brief introduction of nanoindentation and finite element method is presented. General procedures have been developed based on FEM modeling of nanoindentation data to obtain the mechanical properties of thin films. Selected FEM models are illustrated in detail. In Chapter 2, nanoindentation test is simulated using finite element method based on contact mechanics approach. The relationship between load and indentation depth is obtained. The numerical results show good agreement with experimental data. It is shown that FEM is an effective tool for simulation of nanoindentation tests of metallic films. However, limitations caused by simplification of models and assumptions should not be neglected. In Chapter 3, finite element method is used to analyze bonded repair structure of aluminum plates with Multiple Site Damage (MSD). A 2-D 3-layer technique is used to deal with the damage area. A typical aluminum plate with multiple collinear twin cracks is taken as an example. The effects of relative position of two cracks, patch size, and patch thickness on stress intensity factors are studied in detail. The results reveal that the stress intensity factors at the tips of collinear twin cracks can be reduced greatly through bonded composite repair. In order to increase the performance of the patch repair, the adhesive properties, the patch length and thickness must be optimized. In Chapter 4, finite element method is used for thermo-mechanical analysis of porous coatings in steel micro channels used for catalysis. Thermal stresses in the coating due to temperature changes are obtained. The effects of micro channel geometry on thermal stresses are studied in detail. The results reveal that in order to increase the mechanical performance of the coatings, film thickness and profile geometry must be optimized. Chapter 5 summarizes major results and outlines future work.

Recent technological developments, mostly in the fields of concentrated solar power and microelectronics, have driven heat transfer requirements higher than current heat exchangers are capable of producing. Processing power is increasing, while processor size simultaneously decreases and the heat flux requirements of concentrating solar power plants are being driven up by the high temperatures that produce the best thermal efficiency. Heat transfer in microchannels, specifically when utilising flow boiling, has been shown to produce significantly higher heat fluxes than their macro-scale counterparts and could have a large impact on many industrial fields. This high heat transfer characteristic is caused by a number of factors, including the large difference between the sensible and latent heat of the working fluid and the evaporation of a thin liquid film that forms between the microchannel walls and the vapour bubbles. These phenomena occur at incredibly small scales. Flow visualisations, temperature and pressure measurements are therefore difficult to obtain. Many experiments that cover a wide range of microchannel sizes, shapes and orientations, and utilise different working fluids and heat fluxes have been reported. However, the correlations between confined boiling, heat flux and pressure drop have mostly been produced for macro-scale flow. Many different criteria have been developed to distinguish the macro scale from the micro scale, but the general consensus is that macro-scale heat transfer correlations do not perform well when used in the micro scale. Heat transfer correlations are typically created by performing physical experiments over a wide range of parameters and then quantifying the effect that varying these parameters has on the performance of the system. The small scale and high complexity of microchannel-based heat exchangers make visualising the flow within them difficult and inaccurate because both the working fluid and the microchannel walls distort light. The use of numerical modelling via computational fluid dynamics software allows phenomena that occur within the channel to be simulated, which provides valuable insight into how rapid bubble growth affects the surrounding fluid, which can lead to the design of better heat exchangers. This study focused on numerically modelling the growth of a single bubble during the flow boiling of FC-72 in a microchannel with a hydraulic diameter of 0.9 mm and an aspect ratio of 10. The numerical domain was limited to a 10 mm section of the microchannel where bubble nucleation and detachment were observed in an experimental study on a similar microchannel setup. The high cost of 3D simulations was offset by an interface-tracking mesh refinement method, which refined cells not only at the interface, but also a set distance on either side of the interface. To focus on the effects of gravity, a simplified approach is used, which isolates certain phenomena. Density gradients, material roughness and multiple bubble interaction are ignored so that the effects of buoyancy and bubble detachment can be analysed. Simulations are first performed in a 2D section through the centre of the microchannel, and then in the full 3D domain. In both the 3D numerical and experimental cases (Meyer et al., 2020), the bottom heated case had the lowest maximum temperature and the highest heat transfer characteristics, which were influenced by the detachment of the bubble from the heated surface. This observation indicates that the gravitational orientation of the channel can have a significant effect on the heat transfer characteristics of microchannel-based heat exchangers, and that more investigation is required to characterise the extent of this effect. / Dissertation (MEng)--University of Pretoria, 2019. / Mechanical and Aeronautical Engineering / MEng / Unrestricted