Friction factors and heat transfer in microtubes
The friction factors, roughness of the inner wall surface of microtubes and heat transfer coefficients for the flow of nitrogen in microtubes have been measured. The diameter of the microtubes were between 3 $\mu$m and 81.2 $\mu$m. The experiment involved both the laminar and turbulent flow regimes. The experimental results for microtubes display characteristics different from those reported in the literature for tubes with large dimensions. The laminar zone extended up to a Reynolds number of 2500, and the product of friction factor and Reynolds number was estimated to be 53 in the laminar zone. In addition, the friction factor of microtubes in the laminar zone was not affected by roughness. In the transient zone, the results show that the friction factors are less than those predicted by the Blasius relationship. The curve of Nusselt number as a function of Reynolds number is steeper than the previous empirical relationships for relatively large tubes in the laminar zone. In the turbulent zone, the experimental results show that the measured Nusselt number is much higher than that predicted by the conventional relationship for large tubes.