Experimental Analysis of the Convective Heat Transfer in the Transition Region of Microtubes

This paper reports the main findings of an experimental campaign aimed to determine the values of the Nusselt number for liquid flows through microtubes having a relative roughness up to 4% in the laminar and transitional regime. Three microtubes having an inner diameter of 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 under laminar conditions the average Nusselt number approaches the fully developed constant value for uniformly heated tubes when the Reynolds number decreases. It is proven that 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 runs. For higher Reynolds numbers the region of thermal development exerts a stronger influence, increasing the average convective heat transfer coefficient, which becomes a function of the Reynolds number,of the Prandlt number and of the d/L (inner diameter-to-heated length) ratio. The experimental results of this work also demonstrate that in microtubes with a relative roughness lower than 4% the effect of the roughness on the Nusselt number can be neglected in the laminar regime. When the laminar-to-turbulent transition is reached the average value of the Nusselt number steeply increases with the Reynolds number. This behaviour is more pronounced for microtubes than for pipes of conventional size, where the onset of mixed convection smoothes the step variation of the average Nusselt number at the change in the flow regime.