Effect of Incoming Wakes on the Stator Performance in a Single-Stage Low Speed Axial Flow Compressor Operating at Design and Near Stall Conditions

Unsteady flow phenomena can significantly influence the performance of turbomachines. The convection of the wake coming from a rotor into a downstream stator is one of these phenomena. In the case of compressors, when the rotor wake is transported through a downstream stator, it undergoes viscous mixing and stretching (Smith 1966), which are two mechanisms responsible for its attenuation. The flow field of a low speed single-stage compressor comprising a rotor and a downstream stator is computed using unsteady CFD simulations at design and near stall conditions. Simulations results are compared to steady and unsteady data obtained from yawmeter and hotwire measurements at both rotor and stator exit. The study focuses on the rotor wake attenuation and the related unsteady total pressure loss generated in the stator passage. The loss due to viscous mixing of the rotor wake is calculated analytically using a wake dissipation model. Based on experimental, numerical and analytical results, a break-down of the unsteady total pressure losses is performed for the two operating conditions. Unsteady total pressure losses are classified into two categories. The first category is the loss generated by viscous mixing of the rotor wake and the second one the loss generated by the interactions between the rotor wake and the stator pressure and suction surfaces boundary layers (interaction loss). Results show that the interactions between the rotor wake and the stator surfaces boundary layers play an important part in the unsteady loss generation process and that the contribution of this interaction loss increases from design to near stall condition.