The results of the tube with CRs and also the plain tube are also reported, for comparison. The results show that the use of A-CRs and CRs leads to significant increase of friction factor as compared to that of the plain tube. This is because of the dissipation of dynamic pressure of the fluid due to the very high viscosity loss near the tube wall and the act caused by the recirculating flow. Moreover, the friction has the high possibility to occur by the interaction of the pressure forces with inertial forces in the boundary layer. The results also show that all A-CRs cause higher friction loss than CRs and friction factor tends to increase with increasing annulus diameter ratio (d/Do). According to the results, A-CRs with annulus diameter ratios of d/Do = 0.1, 0.15, and 0.2 cause friction losses as high as 1.8, 3.0 and 5.8 times of that caused by CRs, respectively. In addition, the friction factors of the tubes with A-CRs at d/Do = 0.1, 0.15, and 0.2, are around 28, 47 and 90 times of the plain tube, respectively. The higher friction loss caused by the A-CRs with larger annulus diameter ratio is attributed to the stronger turbulence intensity. From experimental results, it can also be observed that the friction factor ratio (f/fp) tends to increase with increasing Reynolds number (Fig. 7(b)). This can be explained that the stronger turbulence intensity at higher Reynolds number causes extra recirculating flow strengthen which is more efficient for interruption of boundary layer along the flow path.