Turbulent flow regime in coiled tubes: local heat-transfer coefficient

Wall curvature represents a widely adopted technique for enhancing heat transfer: the fluid flowing inside a coiled pipe experiences the centrifugal force and this phenomenon induces local maxima in the velocity distribution that locally increase the temperature gradients at the wall by enhancing the heat transfer both in the laminar and in the turbulent flow regime. Consequently, the distribution of the velocity field over the cross-section of the tube is strongly uneven thus leading to significant variations along the circumferential angular coordinate of the convective heat-transfer coefficient at the wall internal surface: in particular, it shows higher values at the outer bend side of the coil than at the inner bend side. The aim of the present work is to estimate experimentally the local convective heat-transfer coefficient at the fluid wall interface in coiled tubes when turbulent flow regime occurs. In particular, the temperature distribution maps on the external coil wall are employed as input data of the inverse heat conduction problem in the wall and a solution approach based on the Tikhonov regularisation is implemented. The results, obtained with water as working fluid, are focused on the fully developed region in the turbulent flow regime in the Reynolds number range of 5000 to 12,000. For the sake of completeness, the overall efficiency of the coiled tubes under test is assessed under a first-law performance evaluation criterion.