Heat transfer enhancement for laminar flow in a tube using bidirectional conical strip inserts

In the present work, a novel tube insert (bidirectional conical strip inserts) is proposed, and the heat transfer performance and flow characteristics of this insert are studied numerically. Effects of three geometric parameters (numbers of bidirectional conical strip (n), central angle (a) and pitch ratio (P* = p/D)) are also investigated. The results indicate that cold fluid in the core region and the hot fluid near the tube wall are rapidly exchanged as the fluid flows through the bidirectional conical strip, and multiple longitudinal swirling flows are formed downstream of the bidirectional conical strip. Therefore, the heat transfer (the Nusselt number) is significantly enhanced by 2.35-9.85 times compared to the smooth tube. Moreover, because of the cooperation between the forward and the reverse conical strips, the formation of the dead zone and eddy on the back of the conical strips is inhibited. Thus, the increase in flow resistance is smaller than many other published works, as the friction factor is enhanced to 2.37-21.18 times of the smooth tube. The overall heat transfer performance (PEC value) is located in range of 1.75-3.93. Both the Nusselt number and friction factor increase with the increasing numbers of bidirectional conical strip, central angle and the decreasing pitch ratio. However, the friction factor is more sensitive to geometric parameters, so the maximum overall heat transfer performance (PEC value) is obtained at moderate geometric parameters (n = 3, alpha = 40 and P* = 3). In addition, Correlation formulas for Nusselt number and friction factor are derived. (C) 2018 Elsevier Ltd. All rights reserved.