Influence of thermal boundary conditions on local convective heat transfer in coiled tubes

Helical coil heat exchangers are commonly used in a wide variety of engineering applications because, due to curvature of the tube that generates secondary flows, this kind of geometry can generate an enhancement of the convective heat transfer coefficient. Since the distributions of the fluid velocity and temperature in coiled pipes are strongly asymmetrical over the cross-section of the tube, particularly in laminar flow conditions, it is important to monitor the local heat transfer phenomena, mainly with regard to the applications where the local behaviour plays a leading role, such as in food industry. Indeed, due to the high viscosity of fluid foods or to the low velocity in which they may operate, the flow regime is often laminar in this kind of devices. In laminar flow regime, the thermal boundary conditions considerably affect the convective heat transfer performance. In the present paper, a numerical investigation focused on the evaluation of the effect of the choice of the thermal boundary condition on the local heat transfer phenomena in coiled-tubes, within the laminar regime, is presented. To cover realistic configurations in which the helical coil heat exchangers are commonly used, the conjugate heat transfer problem in the tube was also considered. The momentum and energy equations were solved by means of the open-source CFD software OpenFOAM®, with second-order accurate finite-volume schemes. The here adopted numerical model was validated by comparing the results with local experimental data. The numerical outcomes, obtained for several values of the Prandtl number, revealed that the thermal boundary conditions could significantly change the local distribution of the convective heat transfer coefficient. Finally, the influence of the thermal boundary condition on the average convective heat transfer coefficient were evaluated by considering the two definitions of the circumferentially averaged Nusselt number commonly used in literature.