Single-Phase Heat Transfer Inside of Internal Helically Ribbed Tubes

This study focuses on experimentally investigating the performance of two generations of internal helically ribbed tubes during single-phase heat transfer of water inside the tube, where boiling or condensation of refrigerant occurs outside the tube. The design parameters of the two generations of reinforced tubes include differences in internal helical rib height, helix angle, number of ribs per turn, and rib base thickness, including the smooth tube (Tube-1) as well as Tube-2 through Tube-8 as the first generation of internal helically ribbed tubes from the 1990s, and Tube-9 through Tube-19 as the second generation of internal helically ribbed tubes that are now popular. The rib heights of the two generations of internal helical rib tubes range from 0 to 0.55 mm, the helix angle varies from 0°to 60°, the number of ribs per turn fluctuates from 0 to 50, and the thickness of the rib base ranges from 0 to 0.79 mm. The results of the study showed that among the first generation of internal helically ribbed tubes, Tube-8 (Ns=18,eh=0.57mm,α=25°) had the lowest heat transfer factor, which was about 1.4 to 1.5 times that of the smooth tube, while Tube-2 (Ns=45,eh=0.36mm,α=45°)had the highest heat transfer factor, which was about 2.21 to 2.37 times that of the smooth tube. On the other hand, in the second generation of internal helically ribbed tubes, Tube-11 (Ns=44,eh=0.67mm,α=40°) has the lowest heat transfer factor, which is about 2.41 to 2.52 times that of the smooth tube, and Tube-15 (Ns=42,eh=0.92mm,α=45°) has the highest heat transfer factor, which is about 4.11 to 4.37 times that of the smooth tube, whereas compared to the Tube-2 in the first generation, the heat transfer factors of Tube-11 and Tube-15 have been improved by a factor of 1.07 to 1.16 times respectively and 1.80 to 1.93 times. With the maturity of the process, the number of ribs per turn and the helix angle of the second-generation internal helically ribbed tubes have been increased, which effectively increases its heat transfer area and enhances the degree of turbulence of the fluid. When the fluid passes through, it will cause more vortex and secondary flow, further enhancing the degree of fluid mixing. The increase in the number of ribs per turn also disturbs the fluid boundary layer, which reduces the thickness of the boundary layer, thereby reducing the thermal resistance and further improving the heat transfer efficiency.