Numerical analysis of the laminar forced convective heat transfer in coiled tubes with periodic ring-type corrugation

Wall curvature and wall corrugation represent two of the most used passive techniques to enhance convective heat transfer. The effectiveness of wall curvature is due to the fact that it gives origin to a secondary fluid motion orthogonal to the main flow, while wall corrugation is used to disrupt the development of the boundary layers, by enhancing the convective heat transfer mechanism. The compound use of the two techniques has been investigated in literature, mainly experimentally, but further investigation is still needed. In particular, it has been experimentally observed that this compound enhancement technique brings an additional heat transfer augmentation in the majority of applications whereas in the very low Reynolds number range the surface average performances of corrugated coils are lower than the one shown by smooth wall coils. This paper deepened the knowledge on this phenomenon presenting a numerical investigation of the effect induced by a periodic ring-type corrugation on the laminar convective heat transfer in coiled tubes. The study considered the laminar flow in the Reynolds and Dean number range 25-100 and 6-24 respectively. The investigation was particularly focused on the Dean's vortices destruction mechanism, induced by the wall corrugation and on the consequent breakdown of the average Nusselt number.