Thermal-hydraulic performance enhancement of ellipsoidal dimpled U-tubes with different bend curvatures

Abstract

U-tubes are an integral part of most compact heat exchange systems. The efficiency of these systems is governed by the thermal and fluid flow characteristics of the U-tubes. Dimpled tubes are widely known for the performance improvement in straight tubes. However in U-tubes, with inherent secondary flow and detachment at bends, dimples add complexity to heat and fluid flow. Therefore, this study was focused on the numerical investigation of the thermal-hydraulic performance of ellipsoidal 0-degree, and 45-degree dimpled U-tubes with short and long radius bends, under the constant external heat flux of 10 kW/m², for Reynolds number range of 5000 ≤ Re ≤ 40,000. A steady-state implicit numerical methodology including SST k-ω turbulence model was adopted. It was observed that Ellipsoidal 0-degree and 45-degree dimpled U-tubes altered Dean vortices substantially and induced early attachment in the post-bend sections which enhanced the heat transfer rate significantly. As compared to the smooth tube, the long-radius Ellipsoidal 0-degree dimpled U-tube enhanced thermal-hydraulic performance by 35.3 % while the short-radius U-tube the Ellipsoidal 45-degree improved the heat and fluid flow performance by 45.4 %. The 0-degree dimpled long-radius U-tube performed better at higher Re, while the 45-degree dimpled short-radius U-tube was more effective at lower Re.