Turbulent flow and heat transfer enhancement for straight circular and twisted oval tubes with different dimple shapes
Kazem Mashayekh, Amin Etminan, Kevin Pope, Yuri S. Muzychka
Abstract
Dimples on a tube surface can cause flow separation and generate secondary flows over the upper half of the dimples. The secondary flows disrupt the velocity and thermal boundary layers, which can enhance heat transfer rates. This work makes several contributions: it numerically investigates the effects of twisting and dimpling a straight smooth circular tube (SSCT) on hydraulic and thermal performance, examines the influence of different dimple geometries on SDCTs and TDOTs, and compares in-line and helical dimple arrangements in SDCTs regarding their thermo-hydraulic characteristics. Therefore, this study aims to examine and compare the hydraulic and thermal performance of SDCTs and TDOTs with three dimple shapes: spherical, elliptical, and teardrop. For SDCTs, spherical and elliptical dimples are implemented in both helical and in-line configurations, while for TDOTs, spherical, elliptical, and teardrop-shaped dimples are employed in an in-line configuration. The study considers Reynolds numbers (Re) from 5,000 to 25,000 at a fixed Prandtl number of 1.7, using the performance evaluation factor (PEF) to compare the thermo-hydraulic performance of various cases with that of an SSCT. According to the simulations, the SDCT with spherical dimples in a helical pattern enhances the convective heat transfer coefficient by as much as 2.1 at Re = 5,000 compared to the SSCT, while the corresponding friction factor ratio increases by only 7.85 at Re = 25,000. In contrast, the SDCT with helically arranged elliptical dimples attains the highest PEF value of 1.15 at Re = 5,000.