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Roughness effects on the heat transfer and pressure losses in microscale tubes and channels are investigated through a finite element CFD code. Surface roughness is explicitly modelled through a set of randomly generated peaks along the ideal smooth surface. Different peak shapes and distributions are considered; geometrical parameters are representative of tubes in the diameter range from 50 to 150 µm. The use of a sufficiently fine mesh allows the direct computation of tube performances under the assumption of incompressible, fully developed flow; a comparison with the predictions of simplified models is also presented. As a result, in the present computations a significant increase in Poiseuille number is detected for all the configurations considered, while the effect of roughness on the heat transfer rate is smaller and highly dependent on the tube shape.

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The aim of this study was to compare the measured effectiveness of an air curtain device at different jet velocities against a three-dimensional (3-D) computational fluid dynamics (CFD) model. The air curtain device was not as wide as the entrance and had a geometry that encouraged 3-D flow. By carefully setting up the air curtain an effectiveness of 0.71 was achieved compared to the initial value of only 0.31 as set by the air curtain device installer. The 3-D CFD model predicted the infiltration through the entrance with no air curtain to an accuracy of within 20-32%. The predicted effectiveness, E, of the air curtain at different jet velocities was 0.10-0.15 lower than measured. The shape of the effectiveness curve against jet velocity was well predicted. CFD has shown that the flow from this air curtain cannot be considered as 2-D. The central part of the jet is deflected away from the cold store by the Coanda effect caused by the air curtain device's fan body. The edges of the jet are deflected into the cold store by the stack pressures and turn into the void caused by the deflected central jet. (c) 2006 Elsevier Inc. All rights reserved

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