Aerodynamics of Sweep in a Tandem-Bladed Subsonic Axial Compressor Rotor

The present numerical investigation aims to understand the effect of the chordwise sweep on the performance of a tandem rotor. In a tandem compressor rotor, diffusion is achieved through two smaller airfoils, which are positioned in a particular fashion to nullify the effect of the boundary layer separation. The forward and aft rotor blades have different blade setting angles from the hub to the tip. Executing a chordwise sweep turns out to be an invalid tandem configuration due to the intersection of the forward and the aft rotor. To overcome this, a certain amount of lean is provided to the aft rotor. Therefore, the configurations selected for the current investigation are blades with a combination of sweep and lean. Two different forward sweep configurations and one backward sweep configuration are investigated. A significant improvement in the stall margin is observed for the forward-swept rotor configuration. However, the stage loading and efficiency of the unswept rotor are found to be higher than the forward-swept rotor case. Forward sweep reduces the tip loading of the forward rotor, which delays the tip stall. Backward sweep appears to be more detrimental in terms of the operating range of the tandem rotor, causing a substantial drop in the stall margin. The performance of the swept and the unswept rotor is compared at the design mass flow and near the stall mass flow rate. The pay-off derived from the chordwise sweep is elucidated with the help of blade loading, tip leakage vortices, blockage, skin friction lines, and various vortex interactions.