Numerical Simulations on Flow Separation Within an Axial Turbine at Very Low Reynolds Number

Reynolds number has significant impact on the aerodynamic performance of axial turbines. The internal flow within the stator of a low pressure turbine could be all laminar when the inlet Reynolds number is very low, and large flow separation may occur on the suction surface of the blade. The separated laminar boundary layer and the wake have strong unsteady interactions with the main flow in rotor passages. In order to investigate the laminar flow separation and the interaction, both laminar flow simulation and detached eddy simulation (DES) have been performed on a low speed axial turbine under a very low Reynolds number condition in this paper. For comparison, fully laminar modeling, transitional modeling and fully turbulent modeling are performed, also. The comparison between the computational results and the experimental results shows that both the laminar modeling and transition modeling can capture the laminar separation on the suction side of the stator blade accurately. The separation region locates in a thin zone strengthening from the blade tip to the hub, which is induced by the tip passage vortex. The separation generates a high turbulence intensity zone at the stator outlet. However, this zone in laminar simulation is smaller than that in the experimental due to the absence of turbulence disturbances. Fully turbulent modeling predicts a delayed separation and a smaller separation region. Detached eddy simulation is performed for single stator row, which gives better predictions for both the flow separation and high turbulent zone. The detailed flow structures of the secondary vortices of the stator, the rotor passage vortex and the tip leakage vortex are illustrated. The simulation results show that the laminar separation has obvious three dimensional behaviour. The radial movement of the horse shoe vortex is the main disturbance to the flow separation.