Swirl-affected turbulent fluid flow and heat transfer in a circular tube

Experiments were performed to study the fluid flow and heat transfer characteristics for turbulent airflow in a tube in which there is a decaying axisymmetric swirl. Measurements were made of the local swirl angle at the tube wall and of the local Nusselt number and friction factor, all as a function of position along the length of the tube. Supplementary flow visualization experiments were performed to establish the axisymmetry of the swirl and to explore the pattern of fluid flow at the inlet of the tube. The swirl angle was found to decay exponentially along the tube, with the decay being more rapid at lower Reynolds numbers. The swirl gave rise to substantial heat transfer enhancement in the initial portion of the tube. The enhancement prevailed over a greater length of the tube at higher Reynolds numbers than at lower Reynolds numbers. Compared with the enhancements encountered in the conventional thermal entrance region in a nonswirling pipe flow, those associated with swirl are substantially greater and longer lived.