Gas flow and heat transfer in microchannels: An experimental investigation of compressibility, rarefaction, and surface roughness.

摘要

This dissertation presents an experimental investigation of laminar gas flow and heat transfer through microchannels. The independent variables: relative surface roughness, Knudsen number, Reynolds number and Mach number were systematically varied to determine their influence on the friction factor and the Nusselt number. The microchannels were etched into silicon wafers, capped with glass, and have hydraulic diameters between 5 and 96 microns. Isothermal gas flow was investigated by measuring the local pressure at seven locations along the channel length. Heat transfer experiments were conducted using thin film sensors developed for direct measurement of the surface temperature.; Eight microchannel test sections were fabricated for flow tests; five with smooth surfaces, three with rough surfaces. Local values of Knudsen number, Mach number and friction factor were determined for laminar gas flow with Reynolds numbers ranging from 0.1 to 1000. The results show agreement within 3% for the friction factor in the limiting case of low Ma and low Kn with the incompressible continuum flow theory. The effect of compressibility is observed to have a mild (8%) increase in the friction factor as Ma approaches 0.35. A 50% decrease in the friction factor was seen as Kn was increased to 0.27. Finally, the influence of surface roughness on the friction factor was shown to be insignificant for both continuum and slip flow regimes.; Heat transfer experiments were conducted in the laminar flow regime with the outlet Ma between 0.10 and 0.42. The experimental measurements of inlet and outlet gas temperature and the microchannel wall temperature were used to validate a computational model. The model was then used to determine local values of Ma, Re, and Nu. The model results show that after the entrance region, Nu approaches 8.23, the fully developed value of Nu for incompressible flow with constant heat flux. Then, the model predicts that Nu increases along the channel length as Re and Ma increase.