Influence of circumferential grooves on the aerodynamic and aeroelastic stabilities of a transonic fan

Casing treatments have been widely used in operation range extension of compressors or fans. However, if the extension is accompanied by aeroelastic instabilities like flutter, no improvement is achieved at all. Heretofore, the influence of casing treatments on aeroelastic stability as well as the underlying mechanism is still in doubt. In this paper, a transonic fan, NASA rotor 67, is used to investigate the influence of circumferential grooves on the aerodynamic and aeroelastic stabilities. The work is carried out numerically by solving three-dimensional Reynolds-averaged Navier-Stokes (RANS) equations. The flutter behavior of the fan is analyzed by performing a unidirectional Fluid-Structure-Interaction (FSI) simulation. Energy method is adopted to obtain the normalized aerodynamic damping at different nodal diameters. The steady-state simulation results of smooth casing are firstly compared with the experiment to validate the accuracy and effectiveness of the Computational Fluid Dynamics (CFD) model. Then the comparison is performed both in aerodynamic and aeroelastic aspects. The results indicate that compared with smooth casing, the installation of circumferential grooves decreases the aeroelastic stability despite increased stall margin. The high-speed flow transported by the grooves from the pressure side of the blade improves the existing low-speed blockage in the blade tip at near stall point, thus delaying stall. However, the impingement of this high-speed flow on the leading edge of the suction side brings additional positive aerodynamic work on the blade, which leads to a decrease in the overall aeroelastic stability. Hence, the aeroelastic stability must be checked when introducing casing treatments for stall margin extension.