Flow Characteristics and Heat Transfer Performance in Tubes with Dimples-protrusions

Dimple and protrusion are microstructures processed on the surface of heat transfer tubes to improve thermal-hydraulic performance (THP). However, the effect and mechanism of composite structures composed of dimples and protrusions (dimples-protrusions) on the THP are still unclear. In this work, heat transfer tubes with simple dimples (DT-SC) and different cross-section dimples-protrusions (DT-RRRS, DT-RCRS, and DT-RTRS) are chosen to explore the impact of dimples-protrusions on THP compared to dimples. Various parameters, such as velocity, three-dimensional streamline, pressure, etc., are used to reveal the mechanism of the influence of dimples-protrusions on THP. The research presents that dimples-protrusions can improve the intensity of the fluid impinges on the wall and expand the area of secondary flow compared to dimples, resulting in a significant increase in THP. The PEC of DT-RRRS is increased by 7.9 % compared with that of DT-RS at Re = 5000. In addition, the dimples-protrusions with different cross-sections have different effects on THP due to differences in the distribution of secondary flow and the impingement of fluid on the wall. The impact of dimples-protrusions depth on THP is closely related to Re. DT-RCRS can achieve a maximum PEC of 1.38 under the conditions of Re = 5,000, H1 = 2 mm, and H2 = 1 mm. This work provides a reference for the design of dimples-protrusions to achieve optimal THP.