Numerical Investigation of Thermal-Hydraulic Performance of U-Tubes Enhanced With Ellipsoidal 45 deg Dimples

Abstract

This study aims to numerically investigate thermal–hydraulic performance augmentation of ellipsoidal 45 deg dimpled U-tubes with various bend curvatures subjected to constant external heat flux (q′′=10kW/m2) for a range of Reynolds numbers (5000 ≤ Re ≤ 30,000). Three smooth U bends with curvatures radii of 0.695Dh, 1.5Dh, and 2.0Dh, where Dh is the hydraulic diameter of the smooth tube, are used in both smooth and enhanced tubes. A comparison of thermal-hydraulic characteristics of dimpled and smooth U-tubes is carried out using steady-state Reynolds averaged Navier–Stokes simulations. The analysis shows that the performance of the dimpled U-tube is superior to the smooth tube for all bend curvatures. The swirl flow patterns generated by the dimples induce early flow reattachment in the postbend section of the tube, which enhances its heat transfer rate. The dimpled U-tube having the shortest curvature radius significantly alters Dean vortices, which leads to a substantial improvement in its heat and flow performances. The dimpled U-tube having the shortest curvature radius enhances the thermal–hydraulic performance by 21.4% while for other curvature radii (1.5Dh, and 2.0Dh), the performance augmentations are found to be 10.7% and 8.9%, respectively.