Experiments were carried out to investigate forced convective heat transfer of distilled water flowing through quartz microtubes with inner diameter of 242, 315 and 520 μm. The microtube was heated directly by electrifying a brass wire with diameter 80 μm,which was coiled evenly up around the microtube to keep a constant heat flux.The wall average temperature was obtained according to relationship between wire resistance and temperature and achieved the synchronization of heating and measuring temperature.As the Reynolds number varied in the range from 100 up to 7000,the Nusselt numbers were measured.The experimental results were compared with the classical convective heat transfer correlations in laminar,transitional and turbulent regimes for conventional tubes.The comparison results indicated that the Nusselt number may be less than the classical laminar correlations when Reynolds number is lower.The Nusselt number quickly increases with the increase of Reynolds number. As Reynolds number is larger than 1700～1900 and 4000～5500, the corresponding Nusselt number will reach the value of the transitional correlations or that of the turbulent correlations.
