Effects of structured roughness on fluid flow at the microscale level

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Title: Effects of structured roughness on fluid flow at the microscale level
Author: Wagner, Rebecca Noelani
Abstract: There are no theoretical differences between microscale and macroscale flows for incompressible liquids. Fluid flow and heat transfer characteristics in microchannels, however, are known to deviate from conventional macroscale theory in both the laminar and turbulent regimes. As the hydraulic diameter of a channel decreases, the effects of inherent surface roughness within the channel becomes more apparent, causing an increase in frictional losses and early transition to turbulence, as well as unpredictable heat transfer performance. Though many have experimentally, analytically, and numerically established that such deviations occur, the hypotheses attempting to characterize the deviations are sometimes contradictory and frequently employ correctional factors. Hence, no concise and conclusive explanation has been given. Difficulties with existing knowledge hinge around defining surface roughness itself. Average roughness amplitude parameters are commonly in use, but do not provide sufficient representation of designed roughness structures. Two-dimensional grooves or ridges can yield the same amplitude values yet exhibit vastly different hydraulic and heat transfer performance. This work aims to characterize structured surface roughness using existing parameters, and form a theoretical model to correlate surface descriptors to fluid performance in rectangular channels of varying aspect ratios and surface geometries. A theoretical model was developed to predict the effect of roughness pitch and height on pressure drop along the channel length, using friction factors for comparison with prior work. Validation of the proposed theory was carried out through experimentation with water flow in channels possessing designed transverse rib roughness. The end goal was to develop a clear understanding of the effect of two-dimensional structured roughness on frictional losses in fully-developed laminar flow, with the potential for extension to analysis of heat transfer and developing flow.
Record URI: http://hdl.handle.net/1850/12996
Date: 2010-11

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