CFD Simulations On Internal Flow Of A Low Speed Axial Turbine

The performance and operational stability of the axial turbines are closely related to the complex unsteady flow structure of the internal flow. A deep insight into the inherent unsteady interaction between the rotor and the stator is important for axial turbine design. In particular, the internal flow within the stator of a low speed turbine could be mainly laminar when the inlet Reynolds number is very low. Large flow separation may occur on the suction surface of the blade, resulting in the increase of losses and sharply drop of the efficiency. Thus it is necessary to identify the laminar boundary layer separation accurately. Numerical simulations have been performed on a low speed axial turbine to address this problem, and the following conclusions can be drawn:(1) RANS simulations are performed to evaluate the turbulence modeling methods and the independency of the computational grid. Compared with the experimental results, solving the laminar NS equation in the stator passage and solving the RANS equations in the rotor passage, respectively, represents the laminar separation accurately. A grid of 2.13 million nodes can satisfy the precision requirement of the compute.(2) The Non-linear Harmonic method is used for unsteady simulations. Results show that the influence of the stator-rotor interaction on the stator flow is less than that on the rotor flow. Reasons for the strong periodic unsteady characteristics downstream the stator are the periodic chopping on the stator wake and the periodic blocking action on the upstream flows by the rotor. The periodic circumferential movement causing by stator-rotor interaction between the high speed mainstream and the wake zone that consists of the blade tip clearance vortex, the tip passage vortex of the rotor, the boundary layer separation area and the root passage vortex, is the key to the large fluctuations of the downstream rotor parameters.(3) The Detached eddy simulation method is applied for the unsteady numerical simulation on the stator cascade. The separation region is very thin and extends to the trailing edge without reattaching to the suction side of the blade. Compared with the RANS simulation results, the DES method can simulate the narrow low speed high loss zone forming from the laminar flow separation region and the secondary flow on the downstream of the cascade more accurately.