A Numerical Study on Heat Transfer Enhancement and Fluid Flow of Enhanced Tube with Ellipsoidal Dimples and Protrusions

Abstract Heat transfer enhancement is an important thermal-hydraulic phenomenon in heat exchangers. Previous investigations mostly focused on unidirectional heat transfer enhancement inside the tube. In order to realize the bidirectional heat transfer enhancement inside and outside the tube, an enhanced tube with ellipsoidal dimples and protrusions is proposed. Flow characteristics and heat transfer performance of the enhanced tube are investigated by numerical method. The distributions of temperature and flow fields are carried out to illustrate the heat transfer enhancement mechanisms. The Nusselt number, friction coefficient, and PEC are presented to reveal thermal-hydraulic performance. Moreover, the effects of the geometric parameters such as dimple depth, dimple pitch, and dimples/protrusions numbers in the circumference on thermal-hydraulic characteristics are investigated. The results show that for the internal flow, the structure of dimples induces intensive impingement and interrupts the boundary layers, improving the heat transfer, whereas the structure of protrusions induces recirculation flows and deteriorates the heat transfer performance. The effects of the structure of dimples/protrusions on heat transfer are opposite for the inside and outside flow. Besides, the Nu increases with depth and dimples/protrusions numbers in the circumference and decreases with the pitch in most cases. Under structure parameters considered, when D = 2mm, P = 25mm, n = 6, and Re = 5000, the largest PEC of 1.40 and 1.42 occurs for the internal and the external flow, respectively.