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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.

The present work deals with the compressible flow of nitrogen gas inside microtubes ranging from 30 μm to 500 μm and with different values of the surface roughness (

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.

In questo lavoro viene descritta una campagna sperimentale mirata a determinare il valore assunto dal numero di Nusselt in microtubi rugosi con scabrezza relativa fino al 4% in regime laminare e di transizione. Sono stati analizzati tre microtubi in acciaio con diametri interni di 440, 280 e 146 m, riscaldati uniformemente per effetto Joule tramite un alimentatore stabilizzato in corrente continua. I risultati sperimentali evidenziano che nel regime laminare il numero di Nusselt si avvicina al valore costante per tubi uniformemente riscaldati al decrescere del numero di Reynolds. La conduzione assiale nel fluido e lo scambio termico coniugato nella parete possono essere trascurati nelle condizioni in cui sono state svolte le prove. All’aumentare del numero di Reynolds la regione di sviluppo termico aumenta la sua influenza, incrementando il valor medio del coefficiente di scambio termico che diventa funzione del numero di Reynolds e di Prandlt nonché del rapporto d/L (diametro interno/lunghezza scaldata). I risultati sperimentali ottenuti in questo lavoro dimostrano come in microtubi con scabrezza relativa inferiore al 4% gli effetti della rugosità sul numero di Nusselt sono trascurabili in regime laminare. Quando si raggiunge il numero di Reynolds critico, in corrispondenza del quale si verifica la transizione tra il regime laminare e quello turbolento, il numero di Nusselt subisce un brusco aumento all’aumentare del numero di Reynolds. Tale comportamento risulta particolarmente accentuato nei microtubi rispetto a quanto si registra nei tubi di dimensioni ordinarie dove l’innesco della convezione mista riduce la variazione a gradino di Nusselt al raggiungimento della transizione.