Periodic Unsteady Kinematics of Hub Flows in a Shrouded Multistage Compressor

Abstract

Periodic unsteady flow kinematics in a shrouded multistage low-speed axial compressor has been measured for the first time. Data have been acquired at the inlet and exit of a shrouded third-stage stator with a particular focus on the hub flows. The newly found features of the hub flow in a shrouded multistage compressor are different from those at the midspan or in unshrouded (i.e., cantilevered) compressors. First, the merging of the second-stage stator and third-stage rotor wakes causes positive radial migration near the rotor wake pressure surface at the hub of the third-stage stator inlet. Second, the low-momentum labyrinth seal leakage flow of the third-stage stator merges with the third-stage rotor wake to create streamwise vorticity at the third-stage stator inlet hub. Third, contrary to unshrouded stators, suction side hub corner separation in the shrouded third-stage stator reduces rotor wake stretching. Thus, velocity disturbances are attenuated less, and amplitudes of periodic fluctuations in flow angles are larger at the third-stage stator exit hub than at midspan. The positive radial migration of the rotor wake hub flow and wake stretching reduction are expected to decrease efficiency, whereas streamwise vorticity generation is expected to increase efficiency.