In the present study, the A-CRs were used as the turbulence promoter for enhancing the heat transfer rate in the heat exchanger tubes. The front/isometric view and details of the A-CR are illustrated in Fig. 1. The A-CR was made of thin aluminum sheet with 2 mm uniform thickness and have the geometric dimensions of 64 mm (Do) in outer circular-ring diameter, 51 mm (Di) in inner circular-ring diameter, respectively. The A-CRs were equipped in the uniform heat flux tube arranged at constant space length ratio of s/D = 1.0. The space length ratio is defined here in as the ratio of the space length (s) to the outer circular-ring diameter (Do). The A-CRs were examined at three different ratios of annulus diameter to circular-ring diameter of A-CRs, DR = d/Do = 0.1D, 0.15D, and 0.2D, respectively. In this experimental, the re-circulating flow from A-CR is supposed to be effective in the vicinity of the tube wall, where thermal resistance is high. Its expected roles are to mix and rotate the fluid in this region as well as causing increased viscous dissipation, there by enhancing the heat transfer rate.