Unsteady Analysis of Secondary Vortex Formations Within An Axial Compressor Stage with Tandem Rotor

Complex three-dimensional flow features and their interaction with the primary flows have long been a topic of interest for turbomachinery researchers. There are several intricate flow features that are yet to be fully well understood. The present study uses unsteady Reynolds averaged Navier–Stokes-based computations to try and explain the complex flow field in single-stage axial compressor with a tandem-bladed stage. Critical points in the highly three-dimensional flow field are identified and used to highlight the main flow structures among the erratic and chaotic motions of different types of vortices. The findings demonstrate the unsteady behavior of three major secondary flow phenomena within the rotor passage, namely, trailing edge leakage flow, radial migration of boundary layers over the blade surfaces and hub corner separation vortex. The results clearly reveal the flow modifications like circumferential dragging of trailing edge leakage flow and circumferential turning of endwall boundary layer over the rotor hub endwall under the influence of rotor passing effects. Pressure gradient existing across the pressure surface and suction surface is the reason for the formation of trailing edge leakage flow over the rotor hub endwall. Moreover, the oscillating nature of the critical points that represent the base of various secondary vortex structures reveals an interesting aspect of tandem stage aerodynamics. In addition, the hub corner vortex, trailing edge shed vortex, passage vortex and trailing edge wakes, all together create a self-sustaining closed loop that accounts for the flow instabilities within the stator passage. Formation of longitudinal separation vortices within the stator passage and the consequent evolution of “C”-shaped structure due to the discrepancy in the propagation speed possessed by the separation vortices at hub and midspan region are explored using Q-criterion. The results demonstrate that, even under nominal operating conditions, the stator suction surface experiences open corner separation both at its hub and its casing region.