Due to the increase of application in a number of emerging technologies, a growing amount of research has focused on the reduction of drag in microfluidic transport. A novel approach reported in the recent literature is to fabricate micro-ribs and cavities in the channel wall that are then treated with a hydrophobic coating. Such surfaces have been termed super- or ultrahydrophobic and the contact area between the flowing liquid and the solid wall is greatly reduced. Further, due to the scale of the micropatterned structures, the liquid is unable to wet the cavity and a liquid meniscus is formed between ribs. This creates a liquid-vapor interface at the cavity regions and renders surfaces with alternating regions of no-slip and of reduced shear on the microscale. This thesis reports the numerical study of hydrodynamically fully-developed laminar and turbulent flows through a parallel plate channel with walls exhibiting micro-ribs and cavities oriented parallel to the flow direction, where fully developed turbulent flow is considered in a time-averaged sense. Three laminar flow models are implemented to investigate the liquid-vapor interface and to account for the effects of the vapor motion in the cavity regions. For each of the laminar flow models, the liquid-vapor interface was idealized as a flat interface. As a benchmark for the proceeding laminar flow models, the first model considers the case of a vanishing shear stress at the interface between the liquid and vapor domains. Effects of the vapor motion in the cavity are then accounted for in a one-dimensional cavity model where the vapor velocity is considered to be dependent on the wall normal coordinate only, followed by a two-dimensional cavity model that accounts for the vapor velocity's dependence on the transverse coordinate as well. The vapor cavity is modeled analytically and is coupled to the liquid domain by equating the fluid velocities and shear stresses at the liquid-vapor interface. In the turbulent flow model the liquid-vapor interface is idealized as a flat interface with a zero shear stress boundary condition. In general the numerical predictions show a reduction in the total frictional resistance as the cavity width is increased relative to the channel width, the channel height-to-width aspect ratio is decreased, and the vapor cavity depth is increased. The frictional resistance is also reduced with increased Reynolds number in the turbulent flow case. In the range of parameters examined for each fluid flow regime, reductions in drag as high as 91% and 90% are reported for the laminar flow and turbulent flow models, respectively. Under similar conditions however, the turbulent flow results indicate a greater reduction in flow resistance than for the laminar flow scenario. Based on an analysis of the obtained data, analytical expressions are proposed for both laminar and turbulent flow which facilitates the prediction of the frictional resistance.
| Identifer | oai:union.ndltd.org:BGMYU2/oai:scholarsarchive.byu.edu:etd-2215 |
| Date | 14 November 2007 |
| Creators | Jeffs, Kevin B. |
| Publisher | BYU ScholarsArchive |
| Source Sets | Brigham Young University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Theses and Dissertations |
| Rights | http://lib.byu.edu/about/copyright/ |
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