No new physics in single-phase fluid flow and heat transfer in mini-and micro-channels-is it a conclusion?

First the flow friction characteristics of nitrogen and helium in stainless steel microtubes, glass microtubes, square glass microchannels, and rectangular silicon microchannels are tested. The data in glass microtubes with diameters from 50 to 201 microns and in square glass channels with characteristic diameters from 52 to 100 microns show that the friction factors are in good agreement with the conventional predictions. The friction factors in stainless-steel tubes with diameters from 119 to 300 microns are much higher than the conventional ones. The results for two of the four silicon microchannels with characteristic diameters from 26-60 microns are in good agreement while those of the other two channels are larger. This discrepancy is resulted from the large relative surface roughness. Smaller friction factors in glass microtubes with diameters from 10 to 20 microns are obtained due to the rarefaction effect. Second the flow friction experimental data for deionized water flow in glass microtubes with diameters from 50 to 530 microns show that friction factors and transition Reynolds numbers are in good agreement with the conventional predictions. However, the friction factors in stainless steel microtubes with diameters from 50-1570 microns are much higher than the conventional predictions. This discrepancy is attributed to the large surface relative roughness or denser roughness distribution. Numerical simulations considering electroviscous effect are carried out. The simulation results show that the electroviscous effect does not play a significant role in the friction factor for channel dimensions of the order of microns though it does affect the velocity profile and hence it could be neglected in engineering applications for channel dimensions of the order of microns. Third the measured local Nusselt number distribution of deionized water along the axial direction of the stainless steel tubes of 373-1570 microns with uniform heat flux do not accord with the conventional results when Reynolds number is low and the relative thickness of the tube wall is high. Numerical study reveals that the large ratio of wall thickness to tube diameter at low Reynolds number causes significant axial heat conduction in the tube wall, leading to a non-linear streamwise distribution of the fluid temperature. The axial wall heat conduction effect is gradually weakened with the increase of Reynolds number and the decrease of the relative tube wall thickness. In conclusion, the conventional fluid flow and heat transfer theories should still be applied for single-phase flow in smooth microchannels. Nevertheless, micro-channels do raise some issues to be paid special attention to when being applied.