Impact of the tip vortex on the passage flow structures of a jet fan with symmetric blades

Abstract

Abstract

The goal of this study is the simulation of the flow inside a rotor with elliptic airfoils, where the Kutta condition cannot be satisfied. This work develops a three-dimensional numerical modelling of a monoplane axial jet fan with symmetric blades. The three-dimensional model includes tip clearance gridding and turbulence modelling based on high-order Reynolds-averaged Navier—Stokes (RANS) schemes. 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 blade-to-blade structures will be analysed in detail. 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 the 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. A high dense mesh is introduced in the model, so tip leakage is expected to be well captured. Different turbulence models have been tested in order to determine the most accurate choice. It is shown that a linear Reynolds stress model provides velocity distributions more adjusted to experimental data. This suitable prediction for rotating flow passages is a consequence of the characteristics of the model: consideration of anisotropic turbulence and direct inclusion of curvature and rotation effects in the transport equations. Therefore, swirl effects of the tip vortex can be modelled correctly. The numerical results are compared with previous experimental data of velocity fields to validate the simulation. In particular, the instantaneous wake flow structure was measured with a two hot-wire anemometer. Axial and tangential velocity profiles were obtained after pitch averaging the time-resolved flow patterns.