AN EXPERIMENTAL AND THEORETICAL INVESTIGATION OF FLUID FLOW AND HEAT TRANSFER IN MICROTUBES

摘要

The fluid flow and heat transfer characteristics of dry nitrogen gas and water in microtubes, with diameters of 19, 52, and 102 micrometers, were investigated for Re ranging from 250 to over 20,000 and Pr ranging from .7 to 5.0. The friction factor results confirmed the findings of earlier investigators who noticed a lower value for the product, f{sup}*Re, for laminar flow in microtubes than for larger tubes (eg., 53 instead of 64). A similar, but smaller reduction in the friction factor f was observed for the transition and turbulent regimes for the microtubes. In the turbulent regime the heat transfer was enhanced and the Nusselt number was considerably higher than would be predicted for larger tubes. The Reynolds analogy does not appear to apply in the channel where the channel dimension is on the order of the turbulent length scale. A theoretical scaling analysis indicated that turbulent momentum and energy transport in the radial direction is significant in the near wall zone in a microtube. By considering the turbulent eddy interacting with the walls as a frequent event, an analogy was developed that could account for the lower friction factors and increased heat transfer in the turbulent flowregime for microtubes.