Thermodynamic characteristics of a twisted oval tube with dimples at varying positions

Abstract

To further enhance the thermal performance of twisted oval tubes, this paper innovatively installs dimples at different positions on the cross-section of twisted oval tubes and investigates how the dimple position affects the flow and heat transfer characteristics in these tubes. Four dimple configurations—windward, leeward, middle (short-axis), and poles (major-axis)—are evaluated against a smooth twisted oval tube for turbulent flow conditions (Re = 3000-15,000). The velocity and temperature fields, local and average Nusselt number (Nu), friction factor (f), comprehensive performance factor (JF), and field synergy angle are examined in detail. Results indicate that dimple position significantly modulates the secondary flow structures, with distinct physical mechanisms governing each configuration. The windward dimple induces a strong vortex entrainment effect, generating counter-rotating vortices that transport cold fluid from the central region to the windward region. The windward dimple arrangement outperforms all other configurations in terms of overall thermal performance. At Re = 3,000, the windward dimple arrangement yields a 66.21% improvement in Nu and a JF factor of 1.67. Field synergy analysis confirms that the windward dimple configuration provides the minimum field synergy angle, indicating the best alignment between the velocity gradient fields and temperature gradient fields.