Surface Roughness Effects on Friction Factors and Hydraulic Entry Lengths in Micro-Scale Channels - D. Pence (Mechanical Engineering)

The influence of surface roughness on frictional flow characteristics in circular cross-sectional channels is being characterized. A proprietary technique is used to fabricate channels with uniform surface roughness, and the range of scales over which macro-scale friction factor and hydraulic entry length correlations break down is being studied. For channel diameters below which traditional macro-scale correlations are no longer valid, the effect surface roughness has on friction factors and on hydrodynamic and thermal entry lengths will be investigated.

This project concerns the development of a unique thermal property measurement technique that can simultaneously determine thermal conductivity, specific heat, and thermal diffusivity of a thin film sample in the direction perpendicular to the plane of the sample. A fully developed technique is currently being applied to thin polymeric samples (polyimide films ~ 25 microns thick). Further research now underway will study very thin films grown on silicon substrates. The ultimate goal of the work is to measure thermal conductivity and diffusivity of films in the micron size regime.

A source of energy is needed to drive meso- and micro-devices. At the macro-scale, combustion is an often used source of heat to drive processes such as chemical processing and power production. The Micro Combustion project is a research study on the feasibility of creating micro sources of high temperature heat with energy release rates in the sub-watt range. The concept being used is to employ a microscale counter-flow heat exchanger to feed reactants to a micro combustor. The heat exchanger is designed to isolate the combustor from ambient temperature conditions to minimize heat loss from the system. Also, catalytic processes at the hot end are being studied to promote combustion with the reactants being hydrogen and hydrocarbons mixed with air.

In support of the Stirling cooler project and the general miniaturization of thermomechanical devices, a mesoscopic linear actuator is being developed that can directly drive a displacer, or compression element, in a small-scale device. This is a critical item for many of the devices that are being developed at OSU. Elements making up the linear driver are based on three different effects: a.) thermal expansion mismatch, b.) piezoelectric deformation, and c.) magnetostriction. Modeling, fabrication, and experiments are currently underway to develop these critical components for thermomechanical device actuation.

Mesoscale cryocoolers would have a footprint about the size of a glass microscope slide. Current coolers able to reach cryogenic conditions are rather complex, expensive, and often require large compressors. Stirling machines have been proven as a reliable cooler for many applications and further miniaturization can be achieved. Using fabrication techniques being developed at OSU as part of the MECS initiative, components are being developed for a mesoscale cryocooler. Micro-channel arrrays for regenerators and heat exchangers, small expansion engines, and miniature fluid channels, valves and nozzles are being fabricated for use in the cooler. The device is based on concepts being developed by Prof. Peterson to overcome some the the traditional problems associated with Stirling machines. These concepts should allow a simple cryocooler to be fabricated to meet cooling needs over a large useful temperature range. Furthermore, the new device could be employed for small cooling applications, e.g. sensors and instruments operating at cryogenic temperatures.

Analysis of the flow and heating processes associated with phase change phenomena associated with forced convection heat transfer in enclosed microchannels and enclosed arrays of other microstructructures such as microfins. Advanced flow visualization techniques (particle image velocimetry, laser induced fluorescence) are being used to allow detailed visualization of the flow fields in these micro-geometries to provide both quantitative and qualitative information on single-phase flow and heat transfer behaviour.

In this effort, a number of fluids, including liquid metals, are being used to determine the effects of fluid properties on the flow and heat transfer characteristics in rectangular microchannels of varying aspect ratios.

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