Experimental study of the transitional flow regime in coiled tubes by the estimation of local convective heat transfer coefficient

The present work is focused on studying the transition between the laminar and turbulent flow regime in coiled tubes. Wall curvature is a popular heat transfer enhancement technique since it gives origin to a secondary flow in the flowing fluid due to the non-uniformity of the centrifugal force over the cross section. This phenomenon, both in the laminar and the turbulent flow regime, promotes local maxima in the velocity distribution that locally increase the temperature gradient at the wall by enhancing the heat transfer and, at the same time, leading to a significant variation in the convective heat-transfer coefficient along the circumferential angular coordinate. However, this geometry delays the transition from laminar to turbulent regime, transition that, in the majority of the papers available in the scientific literature, has been investigated on the basis of pressure drop data behaviour. In the present work the estimation of the local convective heat transfer coefficient distribution, based on the solution of the inverse heat conduction problem in the tube's wall, is proposed as a complementary and detailed tool to investigate the transitional flow regime. Moreover, the present research, thanks to the application of the proposed approach to an experimental case, gives additional information on the phenomenon of transition in coiled tubes.