Unsteady 3D Simulation of a Jet Fan With Symmetric Blades

This work develops the numerical modeling of a monoplane axial jet fan with symmetric blades. The goal of the study is the simulation of the flow inside a rotor with elliptic airfoils, where the Kutta condition cannot be satisfied. The unsteady 3D model includes tip clearance gridding and a sliding mesh technique to simulate transient effects. The flow patterns inside the blade passage and the wake-core structure will be studied at design operating conditions. Also, the interaction of the tip leakage flow with time-averaged structures will be analyzed in detail. Therefore, the impact of the tip vortex in the mean time performance of the jet fan will be introduced. The investigation shows how the tip leakage vortex modifies the blade loading on the suction surface. The leakage flow rolls-up in a vortical structure at the suction side, establishing a mixing mechanism that produces a low axial velocity region. As a result, the adverse pressure gradient is enhanced and a major flow separation overcomes. This feature is especially critical in case of a rotor with symmetric blades, where the flow is always detached at the trailing edge. The simulation is carried out using a commercial code, FLUENT, which resolves the Navier-Stokes set of equations. An extremely high dense mesh is introduced in the model, so tip leakage is expected to be well-captured. In addition, fully-developed detachment of the boundary layer requires superior discretizations and high quality meshes, so restrictive y+ criteria have been employed for both endwall boundaries and blade surfaces. Turbulence modeling is closed using URANS models. The Reynolds Stress Model (RSM) has been employed because of its suitable predictions for rotating flow passages. In addition, this model considers anisotropic turbulence, and effects of curvature and rotation are directly addressed in the transport equations. Therefore, swirl effects of the tip vortex are expected to be well-captured. The numerical results are compared with previous experimental data of velocity fields to validate the simulation. Axial and tangential velocity profiles were obtained using a five-hole probe. Complementary, the instantaneous wake flow structure was measured with a dual hot wire anemometer.