Single-phase convective heat transfer in microtubes was numerically studied with consideration on the heat conduction in the tube wall. It indicates that the Nusselt numbers of the fully developed laminar convective heat transfer in microtubes with convective boundary condition outside the tube vary from 3.66 to 4.36, which represent the conventional results for isothermal and constant heat flux boundaries respectively. The Nusselt number depends on the parameters of thermal conductivity ratio (k*), diameter ratio (D*), and Biot number. One-dimensional thermal resistance model could underestimate the Nusselt number if the axial heat conduction in the wall can not be ignored. Discrepancies between the experimental results for the Nusselt number based on 1-D model and the standard values might be misunderstood as being caused by novel phenomena at microscales.

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