Influence of Helical-Wire-Coil Inserts on Swirl-Flow Generation and Heat Transfer Distribution in an Annular Channel

The application of helical-wire-coil as a heat transfer enhancement technique in the annular region of double pipe heat exchangers has received limited attention despite its ease of installation and promising thermal performance. The present study reports a combined numerical and experimental investigation on the impact of helical-circular-wire-coil inserts on the flow dynamics and heat transfer enhancement in an annular channel. The geometric parameters of the wire coil studied are coil pitch to coil diameter ratio (PR = 6, 10, 12, 15, 18) and coil diameter to channel hydraulic diameter ratio (DR = 0.08, 0.1, 0.14), across different Reynolds numbers (Re = 5000, 7500, 10000, 12500). The numerical studies provide an in-depth analysis into local flow structures, turbulence intensity and swirl-induced secondary flows. The generation of swirl-flow is influenced by both coil diameter and pitch, with higher swirl-flow for higher diameter and larger pitch. The development of high-intensity swirl-flows diminishes the recirculation zone downstream to the coil, which hinders the turbulence generation and heat transfer. A novel experimental setup was developed to measure spatially resolved Nusselt number distributions over the inner surface of the annular channel. The wire coil with PR = 12 and DR = 0.08 was identified to be the optimal configuration, offering a notable enhancement in heat transfer performance while maintaining a moderate pressure drop.