Numerical investigation of flow and heat transfer in a swirl tube

•Swirling tube flow is simulated via DES and compared to own experimental data.•The flow field is characterized by a vortex breakdown with an axial backflow.•Turbulent structures show a double helix vortex.•At the swirl inlet Nusselt numbers are eight times higher than an axial tube flow.•The circumferential velocity is the major mechanism for the high heat transfer.

A swirl tube is a promising cooling method for heavily thermally loaded parts like turbine blades due to the additional circumferential velocity and therefore improved turbulent mixing of the fluid. However, the flow and the heat transfer in such a swirl tube are quite complex and not yet fully understood. To gain understanding of the flow structure and the cooling capability, we simulated a swirl tube via Detached Eddy Simulation (DES) and compared it to own experimental data. The numerical method was validated with DNS literature data simulating a turbulent channel flow with a temperature gradient.DES and experiments agreed well for the mean velocity profile. The heat transfer coefficients are underestimated by the simulation near the inlet, but show an agreement further downstream. The results show that the flow field is characterized by a vortex system around the tube axis. Near the tube wall we observed an axial flow towards the outlet with a high circumferential velocity component. In contrast, the vortex core consists of an axial backflow. Additionally, turbulent structures showed double helix vortices especially in the inlet region. Furthermore, heat transfer results elucidate the highest Nusselt numbers at the swirl inlet which are up to eight times higher compared to a smooth tube. The heat transfer then decreases towards the tube exit due to the decay of circumferential velocity and becomes more uniform, but is still higher than the heat transfer in a smooth tube. The circumferential velocity with strong gradients in the wall region is the major mechanism for the high heat transfer in the swirl tube.