Numerical study on characteristics of heat transfer and friction factor in a circular tube with central slant rods

In the present work, a numerical study on the characteristics of heat transfer and friction factor of laminar water flow in a circular tube fitted with a new tube insert (central slant rod inserts) is presented. Results reveal that the new inserts can fully disturb the core flow in the tube and create multiple longitudinal vortex structures, which are confirmed to be excellent flow structures for heat transfer enhancement. In addition, we carried out stereoscopic particle image velocimetry (SPIV) measurements in exactly the same facility as that of the numerical study to validate the accuracy of our numerical study. The results show good agreement between the experiment and numerical simulation. Following this, the effects of three parameters of central slant rods, including the slant angle, pitch and length in the radial direction, are discussed numerically. The computational results indicate that a moderate slant angle ranging from 20 to 30 can obtain the largest Nusselt number; however, the effect of slant angle on the friction factor is quite limited. The Nusselt number and friction factor both increase with a decrease in rod pitch and an increase in rod length in the radial direction. The simulation results show that the Nusselt number and friction factor are enhanced by 1.81-5.05 times and 2.49-6.92 times, respectively, to that of a plain tube. The performance ratio R3 values are in the range of 1.74-4.60. Empirical formulas for Nu and f are obtained based on calculation results. All results indicate that the central slant rod is a promising high-performance tube insert for heat transfer enhancement in practical applications. (C) 2016 Elsevier Ltd. All rights reserved.