A Fundamental Study on the Effect of Surface Roughness Structures on Fluid Flow and Heat Transfer at the Microscale Level

Rochester Institute Of Tech, Rochester NY

Investigators

Abstract

Proposal Title: A Fundamental Study on the Effect of Surface Roughness Structures on Fluid Flow and Heat Transfer at the Microscale Level Principal Investigator: Kandlikar, Satish G. Institution: Rochester Institute of Tech Proposal No: CBET-0829038 Abstract This project aims at transforming our conventional understanding of roughness in incompressible laminar flow at both microscale and macroscale, and providing new insight in controlling transport processes in microscale devices with designer roughness structures. The work includes developing a theoretical model that is validated through numerical simulation as well as pressure drop and heat transfer experiments with structured roughness incorporated on the walls of rectangular channels that are 10-mm wide with gaps ranging from 0.2 mm to 2 mm. The roughness heights tested cover a relative roughness range (defined as the roughness height to the channel hydraulic diameter) from 0 to 30 percent. Our conventional understanding of roughness was derived mainly from the work of researchers such as Nikuradse and Moody several decades ago on the basis of their studies on large diameter tubes. Fluid flow and pressure drop were believed to be unaffected by the wall surface roughness in the relative roughness range below 5 percent. The large uncertainties in Nikuradse?s experiments are largely responsible for this discrepancy. Microscale fluid flow experiments conducted in the last five years under carefully controlled conditions showed that laminar flow is indeed affected by the wall roughness. Regarding the roughness effects on heat transfer, there is very little data available in the literature and a good understanding is lacking on the roughness effects on this important transport phenomenon. Due to the small hydraulic diameter, laminar and transition flows are often encountered in microchannels and are of particular importance. The changes in the flow structure and instabilities caused by roughness in such channels are not well understood at the present time. Previous studies performed by others as well as at RIT indicate an increase in pressure drop and heat transfer with an increase in relative roughness. Presence of roughness leads to an increase in the surface area to volume ratio, which is already larger at the microscale than in conventional-sized channels. The effects of low relative roughness have been previously modeled at RIT using lubrication theory. For a given roughness profile, this model can predict the resulting effect on pressure drop. Further wall modifications will be necessary in order to accurately model the effects of larger relative roughness. Previous work indicates an earlier transition from laminar to turbulent flow in rough microchannels, and these effects also will be investigated for heat transfer characteristics. The theoretical work will be complemented with numerical simulations and advanced experimental techniques (using water) in the Thermal Analysis and Microfluidics Laboratory at RIT. This work will provide basic characterization of different types of rough surfaces (uniform roughness, cross-hatched, and uniformly spaced ribs), and a fundamental understanding of how surface roughness impacts the fluid flow, heat transfer and laminar/turbulent transition in microchannels and minichannels in the laminar and transition regions. The planned work on low Reynolds number flows with controlled roughness elements will provide a fundamental design tool for microscale fluid flow devices. For example, in electronics cooling applications, although the heat transfer coefficients in microchannels are high, even higher values are desired with lower pressure drop penalties for water as well as air flow (new emphasis on air-cooling in aerospace field). Specially designed roughness elements offer an effective way to improve the performance. Such ?designer? surfaces will also enhance respective transport processes in other applications, e.g. biological and chemical assay systems, micro-total system analysis, micro propulsion systems, on-board microscale devices in space exploration, cooling passages for PEM fuel cells, and micromixers. As a result of this work, an exhaustive set of data will be available from carefully controlled microscale experiments over a wide range of parameters. On the educational front, the work will provide exciting undergraduate research opportunities in this emerging field. It will consolidate microfluidics research infrastructure at RIT through Ph.D. students working with the PI in the Microsystems Engineering program. The results of this work will be presented in multidisciplinary meetings for cross-fertilization of ideas. The project will be featured at the E3 Fair, an annual event heralded by the PI for middle school students since 1991. The PI will continue his strong commitment to diversity, with employment of two minority and five female students (one with hearing disability) so far this year.