A numerical study of swirl flow in U-bends: In-depth understanding of vortex evolution and cooling performance

This work numerically investigated the improvement of flow and heat transfer characteristics (HTC) of swirl flow in U-bends, with the goal of enhancing internal cooling in turbine blades. Simulations were conducted to gain an in-depth understanding of the flow patterns and HTCs of U-bends with a diameter of 40 mm. It studied conditions with curvature ratios of 0.6-1.4, initial swirl numbers of 4.451-14.923, and axial Reynolds numbers of 10 000-50 000. The results show that the curvature ratio has a significant impact on the swirl flow. When the curvature ratio is small, cross flow induced by centrifugal forces causes the swirl flow direction at the outlet to be opposite to that at the inlet. Swirl flow significantly affects the flow and HTC within the U-bends. The essence of enhanced heat transfer in swirl flow within the bend is the interaction among the circumferential vortex, the cross-flow vortex, and the separation vortex, which enhances the fluid disturbance and improves the coordination of velocity and temperature gradients. However, while the HTC is enhanced, it also results in higher pressure loss. Finally, empirical correlations for Nu, S, and f in both axial and swirl flow in the U-bends were derived from the simulations, meeting industrial accuracy requirements. This study uncovers the processes that improve the HTC in swirl flow in U-bends, providing a theoretical foundation for applying swirl flow technology in internal cooling.