Transient analysis of stall onset in highly-loaded tandem rotor configurations for an axial compressor

High thrust-to-weight ratio is crucial in aero-engine design. The adverse pressure gradient in compressors limits the maximum diffusion per stage, necessitating more stages to achieve the desired pressure ratio. Tandem airfoils, with their superior diffusion capability compared to conventional single airfoils, can achieve the required pressure ratio with fewer stages. However, the presence of two tip leakage vortices from both the forward and aft rotors creates a more complex tip region compared to conventional rotors. The design and performance aspects of the tandem rotor have been reasonably well documented. However, the stall characteristics of such rotors are yet to be thoroughly investigated. To better understand the stall phenomenon in a tandem rotor, the role of each tip leakage vortex must be investigated separately. Full annulus unsteady analysis of the highly loaded tandem rotor is carried out using the commercially available software Ansys CFX. As the rotor approaches stall, significant changes occur in the trajectory and strength of these vortices, with increased blockage near the stall point. For the tandem rotor, forward rotor spillage is critical. This spillage increases the local incidence near the tip of the forward rotor, resulting in localized flow separation. Small disturbances arising from the leading-edge separation coalesce, forming a rotating stall cell that grows in strength and size as it rotates in the direction opposite to rotor rotation. Even though the aft rotor encounters tip vortex spillage from the forward and aft rotors of the subsequent passage, the nozzle gap effect effectively mitigates flow separation, ensuring stable operation of the tandem rotor system. The leading-edge separation over the forward rotor suction surface evolves into a tornado vortex, with the suction leg on the forward rotor suction surface and the other end connected to the casing. Apart from the tip leakage vortex of the aft rotor, other vortex structures on the aft rotor are intermittent, with some collapsing and new vortices forming.