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As the fluidic devices are miniaturized to improve portability, the friction of the microchannel becomes intrinsically high and a high pumping power will be required to drive the fluid. Since the pumping power delivered by portable devices is limited, one method to reduce this is to render the surface to become slippery. This can be achieved by roughening up the microchannel wall and form a bed of air pockets between the roughness elements, which is known as the superhydrophobic Cassie-Baxter state. While the study on superhydrophobic microchannels are focused mainly in maximizing the friction reduction effects and maintaining the stability of the air pockets, less attention has been given to characterizing the microchannel friction under a metastable state, where partial flooding of the micro-textures may be present, and under heated conditions, where the air pockets are trapped between the micro-textures. In order to quantify the frictional characteristics, microchannels with micron-sized trenches on the side walls were fabricated and tested under varying inlet pressures and heating conditions. By measuring the hydrodynamic resistance and comparing with numerical simulations, results suggest that (1) the air-water interface behaves close to a no-slip boundary condition, (2) friction becomes insensitive to the wetting degree once the micro-trenches become highly wetting, (3) the fully wetted micro-trench may be beneficial over the de-wetted ones in order to achieve friction reduction effects and (4) heating the micro-trenches to induce a highly de-wetting state may actually be detrimental to the microchannel flow due the excessive growth of the air layer. As part of the future work to characterize heat transfer in superhydrophobic microchannels, a rectangular microchannel with microheaters embedded close to the side walls was fabricated and the corresponding heat transfer rates were measured through dual fluorescence thermometry. Results suggested that significant heat is lost through the environment despite the high thermal resistance of the microchannel material. An extra insulation is suggested prior to characterizing the convective heat transfer coefficients in the superhydrophobic microchannel flow. / text

Perforated tubes are widely used in industry for various applications. A special application arises when there is fluid injection into the perforated tube. Such cases arise when perforated tubes are used for horizontal oil well drilling and French drains. The behaviour of the flow under these conditions has led to the development of correlations, which consider the effects of the perforations and injection process. However, there are few friction factor correlations in literature, which consider the increased kinetic energy at the perforated tube outlet caused by fluid injection and acceleration. The current study reports experimental correlations for determining the friction factor of perforated tubes when the additional kinetic energy from the injection process is considered. The friction factor measurements were conducted in copper tubes with an internal diameter of 20.8 mm and a wall thickness of 1 mm at three nondimensional pitches of 0.375, 0.75 and 1.5. A perforated length-to-diameter ratio of 40:1 was used for the perforated tubes. A perforation row contained seven small perforation holes with a diameter of 1.5 mm spaced evenly around the perimeter of the tube. These perforation rows were staggered row to row, resulting in triangular perforation patterns. Water was used as a test medium with Reynolds numbers at the tube outlet ranging from 20 000 to 60 000. The injection ratio was varied from 0 to 5% to obtain a total of 135 unique combinations of perforated tube friction factor data at different injection ratios, Reynolds numbers and nondimensional perforation pitches. The experiments were condensed into friction factor correlations, which allow perforated tube parameters to be optimisation for minimising pressure losses encountered in draining operations. Copyright / Dissertation (MEng)--University of Pretoria, 2011. / Mechanical and Aeronautical Engineering / unrestricted

Frictional resistance reduction of liquid flow over surfaces has recently become a more important topic of research in the field of fluid dynamics. Scientific and technological progress and continued interest in nano and micro-technology have required new developments and approaches related to reducing frictional resistance, especially in liquid flow through nano and micro-channels. The application of superhydrophobic surfaces could be very effective in achieving the desired flow through such small channels. Superhydrophobic surfaces are created by intentionally creating roughnesses on the surface and applying a uniform hydrophobic coating to the entire surface. Liquid droplet tests have revealed that because of the trapped air within the cavities such surfaces could have contact angles as high as 179°. Such a property gives superhydrophobic surfaces liquid repelling characteristics making them very suitable for frictional resistance reduction in liquid flow through nano or micro-channels, provided wetting of the cavities could be avoided. This study presents 3-D numerical simulation results of liquid laminar flow over post patterned superhydrophobic surfaces. The research was performed in three phases, 1) pressure-driven flow with square micro-posts, 2) Couette flow with square micro-posts, and 3) pressure-driven flow with rectangular micro-posts at various aspect ratios. In phases (1) and (2) the influences of important parameters such as the cavity fraction, in the range of 0.0-0.9998, and the relative module width, from 0.01 to 1.5, on frictional resistance reduction in the creeping flow regime were explored. Phase (1) also addressed the effect of varying Reynolds number from 1 to 2500 on frictional resistance. Phase (3) was conducted for aspect ratios of 1/8, 1/4, 1/2, 2, 4, and 8 also in the creeping flow regime. The obtained results suggest that important parameters such as cavity fraction (relative area of the cavities), relative module width (combined post and cavity width relative to the channel hydraulic diameter), and the Reynolds number have great influence on the frictional resistance reduction. For pressure-driven flow at cavity fraction 0.9998, reductions as high as 97% in the frictional resistance were predicted compared with the classical channel flow. This reduction is directly related to the significant reduction in liquid-solid contact area. With respect to the effect of relative module width on the overall frictional resistance, a reduction of 93% in the frictional resistance was observed as the relative module width was increased from 0.1 to 1.5. This is indicative of the importance of the relative spacing size of the posts/cavities compared to the channel size in micro-channel liquid flow. The overall frictional resistance for post-patterned superhydrophobic surfaces was found to be independent of the Reynolds number up to a value of nominally 40 after which the non-dimensional frictional resistance increased at high values of the Reynolds number. However, at very high cavity fractions the frictional resistance was independent of Reynolds number only up to about 4. When the driving mechanism was a Couette flow, similar to the pressure-driven flow, as the cavity fraction and the relative module width increased the frictional resistance on the superhydrophobic surface decreased. At a cavity fraction of 0.9998 the reduction in the non-dimensional frictional resistance was approximately 96%, which was only 1% different from the similar pressure-driven scenario. However, a more significant difference was observed between the slip velocities for the two flow types, and it was determined that the pressure-driven flow resulted in greater apparent slip velocities than Couette flow. A maximum difference in normalized slip between the two scenarios of approximately 20% was obtained at relative module width 0.1 and Reynolds number 1. Results for superhydrophobic surfaces with rectangular micro-posts approached those reported in the literature for micro-ribs as the aspect ratio of the posts increased. When the flow was perpendicular to the long side of the posts, and as the aspect ratio increased, the frictional resistance approached previously published transverse rib results. Similarly, when the liquid flow direction was parallel to the long side of the posts, the frictional resistance results also approached those of the previously published longitudinal ribs as the aspect ratio increased.

The purpose of this thesis was to compare graphene nanoplatelets (GNP) and WS2 as solid lubricant additives to aluminum in order to reduce friction and wear. The central hypothesis of this work relied on lubricating properties of 2D materials, which consist layers that slip under a shear force. Two aluminum composites were made (Al-2 vol.% GNP and Al-2 vol.% WS2) by spark plasma sintering. Tribological properties were evaluated by ball-on-disk wear tests at room temperature (RT) and 200°C. WS2 not only presented the lowest COF (0.66) but also improved the wear resistance of aluminum by 54% at RT. Al-2 vol.% GNP composite displayed poor densification (91%) and low hardness resulting in poor wear resistance. The wear rate of Al-2 vol.% GNP composite increased by 233% at RT and 48% at 200°C as compared to pure aluminum. GNP addition also resulted in lower COF (0.79) as compared to pure aluminum (0.87).

This work is concerned with the study of flow stability and turbulence control - two old but still open problems of fluid mechanics. The topics are distinct and are (currently) approached from different directions and with different strategies. This thesis reflects this diversity in subject with a difference in geometry and, consequently, flow structure: the first problem is approached in the study of the flow in a toroidal pipe, the second one in an attempt to reduce the drag in a turbulent channel flow. The flow in a toroidal pipe is chosen as it represents the common asymptotic limit between spatially developing and helical pipes. Furthermore, the torus represents the smallest departure from the canonical straight pipe flow, at least for small curvatures. The interest in this geometry is twofold: it allows us to isolate the effect of the curvature on the flow and to approach straight as well as helical pipes. The analysis features a characterisation of the steady solution as a function of curvature and the Reynolds number. The problem of forcing fluid in the pipe is addressed, and the so-called Dean number is shown to be of little use, except for infinitesimally low curvatures. It is found that the flow is modally unstable and undergoes a Hopf bifurcation that leads to a limit cycle. The bifurcation and the corresponding eigenmodes are studied in detail, providing a complete picture of the instability. The second part of the thesis approaches fluid mechanics from a different perspective: the Reynolds number is too high for a deterministic description and the flow is analysed with statistical tools. The objective is to reduce the friction exerted by a turbulent flow on the walls of a channel, and the idea is to employ a control strategy independent of the small, and Reynolds number-dependent, turbulent scales. The method of choice was proposed by Schoppa &amp; Hussain [Phys. Fluids 10:1049-1051 (1998)] and consists in the imposition of streamwise invariant, large-scale vortices. The vortices are re-implemented as a volume force, validated and analysed. Results show that the original method only gave rise to transient drag reduction while the forcing version is capable of sustained drag reduction of up to 18%. An analysis of the method, though, reveals that its effectiveness decreases rapidly as the Reynolds number is increased. / <p>QC 20161004</p>

Le secteur du transport en Europe dépend à 98% du pétrole, participe pour plus de 30% à la consommation énergétique et représente plus de 20% des émissions totales de gaz à effet de serre. Dans ce contexte, le développement des biocarburants permettrait une diversification énergétique et une diminution des émissions de gaz à effet de serre, les biocarburants ayant un impact environnemental réduit. De plus, le frottement dans les moteurs diesel est responsable de la perte de la consommation totale d'énergie de près de 15%. La plupart de ces pertes sont liées au contact piston-chemise. Développer les biocarburants peut alors contribuer à réduire d’une part la consommation d’énergie et d’autre part les émissions polluantes. L'additivation de biocarburants avec des modificateurs de frottement organiques (MF) peut être l’une des solutions pour répondre à cette problématique. Dans cette thèse, nous avons étudié les propriétés de réduction de frottement de différents types de MF organiques dans les matrices carburant (B0) et biocarburant (B7), en contact acier / acier. Une diminution significative du coefficient de frottement, de plus de 50%, a été observée en présence d'un mélange d'acides gras en matrice B7 à 100 °C. De plus, peu d’usure est causée durant le test en frottement grâce à la formation d'un tribofilm organique. Cette tribofilm semble être amorphe et généré par une réticulation accélérée des acides gras insaturés (MF) en présence d'esters (B7) dans les conditions tribologiques utilisées. Appuyé par des techniques d’analyses de surface, un mécanisme d'action de réduction de frottement par les additifs organiques dans les biocarburants est proposé et de nouvelles pistes de développement sont présentées. / Improve friction efficiency and environmental sustainability of engines have been increasing in importance since the end of 20th century. Indeed, friction in diesel engines is responsible for almost 15% loss of the total energy consumption. Most of these losses are related to the pistonliner contact. The development of new biofuels, allowing an energy diversification and decreasing the emissions of greenhouse gases due to their reduced environmental impact, can contribute to environmental sustainability and reduction in energy costs. The additivation of biofuels with organic friction modifiers additives (FM) can be an essential way to address this problem. In this thesis, we investigated the friction reduction properties of different kinds of organic FM in fuel (B0) and biofuel (B7) matrixes for steel/steel contact. A significant decrease of friction coefficient, more than 50%, was observed in the presence of a mixture of fatty acids blended to the B7 matrix at 100°C. Moreover, almost no wear is caused on the track thanks to the formation of an organic tribofilm. This organic-based tribofilm appears to be amorphous and generated by an easiest crosslinking of the unsaturated fatty acids (FMs) in the presence of esters under tribological conditions. Supported by advances surface analyses, a potential friction reduction action mechanism of acid-based additives in biofuels is proposed and new avenues for development are provided.

The possible use of graphene and its derivatives dissolved in water as a green lubricant is an interesting avenue of research from tribological perspective. In this study, a highly concentrated stock solution of aqueous Graphene employing Polyethylene Glycol(PEG) as surfactant was diluted using proportionate volumes of De-Ionized (D.I.) water to generate target concentrations of Graphene in solution ranging from 15 μg/ml to 350 μg/ml . These samples were then tested for both sliding and rolling contacts. The sliding contact tests included the use of both 4-ball Tribometer test rig and triborheometer. The rolling contact tests were performed on Mini Traction Machine. The tested graphene-PEG-water admixtures held significant friction and wear advantage over D.I. Water and surfactant solutions under the same testing conditions. / Den möjliga användningen av grafen och dess derivat upplöst i vatten som ett grönt smörjmedel är en intressant forskningsaveny ur ett tribologiskt perspektiv. I denna studie späddes en högkoncentrerad grafenlösning med av-joniserat (D.I.) vatten för att alstra mållösningskoncentrationer av greven mellan 15 μg/ml och 350 μg/ml. Den testade grafenlösningen hade polyetylenglykol som ytaktivt ämne Proven testades sedan för både glidande och rullande kontakter. De glidande kontakttesten innefattade användandet av både en 4-kuletribometer och en triborometer. De rullande kontakttesterna utfördes med en Mini-traktionsmaskin. De testade proverna uppvisade signifikanta friktions- och förslitningsfördel jämfört med D.I. vatten och ytaktiva lösningar under samma testförhållanden