Aerothermal performance improvement by cross and X ribbed stripes

Abstract

The incentive to save energy in heat exchangers has led to enhanced thermal performance in ducted flows, achieved via inserts designed to increase the heat transfer efficiency and efficacy. We introduce a novel approach using cross and X stripe inserts, roughened by 90° and 45° ribs, to enhance the structural integrity and heat transfer rate of the duct. Additionally, the incorporation of edged notches aims to improve aerothermal performance by directing flow traction away from the recirculating cell behind a rib, thereby reducing the drag. Utilizing the infrared thermography method, this research quantifies the full-field Nusselt number distributions for 34 enhanced ducts at Reynolds numbers 5,000–25,000. The outcomes of flow simulations elucidate the underlying mechanisms of aerothermal performance enhancement afforded by these inserts. These enhancements result from the combined effects of increased turbulence intensity, fluid mixing, and rib-induced multicellular vortices, which improve the synergy between fluid velocity and temperature gradient vectors, thereby optimizing heat transfer. Quantitatively, the Nusselt numbers (friction factors) for ducts with cross or X ribbed stripes increase significantly. For cross stripes, the ranges increase by 1.25–3.17 times (3.14–21.75 for friction factors), and for X stripes, 1.29–3.39 times (6.58–43.41 for friction factors), compared with the Dittus–Boelter (Blasius) standards. Notably, the aerothermal performance index for ducts featuring notched cross stripes with 45° ribs and a rib pitch-to-height ratio of 10 reaches 1.47 to 1.11, though this decreases with increasing Reynolds number. Furthermore, the study presents empirical correlations to assess the average Nusselt number and friction factor in a square duct equipped with either cross or X ribbed stripes.