Flow and heat transfer in rough micro steel tubes

This article investigates the flow and heat transfer characteristics in micro steel tubes with inner diameters of 168 mu m, 399 mu m and relative roughness of 3.5% and 2.7%, respectively, by measuring the friction factors and the Nusselt number from laminar state to transitional state. Experiments show that the experimental Nusselt numbers are less than those predicted by the classical laminar correlation due to the effect of the variation of the thermophysical properties with temperature when Reynolds number is low. As the Reynolds number is higher than 800, the experimental Nusselt number are 25-50% higher than the predictions of the classical laminar and transitional correlations due to the effects of the roughness and the entrance length. The transition from laminar to turbulent flow occurs at the Reynolds number of 1,100-1,500.