Experimental and numerical analysis of the pressure drop and convective heat transfer in the three-period spirally fluted tubes

This study proposes a unique configuration for a three-period spirally fluted (3PSF) tube that strengthens the flow mixing effect to enhance heat transfer. Conventional spirally fluted tubes exhibit an internal flow with two periodic characteristics that depend on the number of starts and the spiral angle, while the proposed 3PSF tube also periodically varies the depth of the spiral dimple. Experimental and numerical analyses were carried out to investigate the heat transfer and flow characteristics of six types of 3PSF tube, which were compared to a circular tube and a conventional two-period spirally fluted tube. The heat transfer performance of the 3PSF tubes was determined based on the f-factor, Nusselt number, goodness factor, and performance evaluation criteria. For Reynolds numbers from 1,000 to 30,000, the 3PSF tube with three dimple periods per spiral lead had an f-factor and a Nusselt number that were 349–460% and 184–215%, respectively, of that of the two-period spirally fluted (2PSF) tube with the same outermost diameter. The performance evaluation criteria of 3PSF tubes are higher than that of 2PSF tube in the wide range of Reynolds number. In the 3PSF tubes, high velocity swirling strength developed along the main flow direction due to the varying depth of the spiral dimples. Consequently, the flow-mixing effects were enhanced, improving the heat transfer characteristics despite the higher pressure drop. The proposed 3PSF tube design thus demonstrates the potential to be employed in heat exchangers that require a high heat transfer.