EXPERIMENTAL ANALYSIS OF LIQUID FORCED CONVECTION IN ROUGH MICROTUBES

In this paper will be described an experimental campaign aimed to determine the experimental values of the Nusselt number for rough microtubes having a relative roughness up to 4% in laminar and transitional regime. Three microtubes having an inner diameter equal to 440, 280 and 146 m uniformly heated by Joule effect with a constant power DC supply have been tested. The experimental results have evidenced that in laminar regime the average Nusselt number approaches the fully developed constant for uniformly heated tubes when the Reynolds number decreases since the axial conduction along the fluid and the conjugate heat transfer to the walls can be neglected at low Reynolds numbers for the operative conditions adopted during the tests. For higher Reynolds numbers the region of thermal development increases its importance, augmenting the average convective heat transfer coefficient. It has been demonstrated that large values of the microtube roughness influence more strongly the friction factor than the Nusselt number. The experimental results obtained in this work demonstrate that in microtubes having a relative roughness less than 5% the effect of the roughness on the Nusselt number can be considered negligible.