Rotor boundary layer development in a two-stage compressor

Abstract

This experimental study provides striking examples of the complex flow and turbulence structures resulting from blade-wake and wake-wake interactions in a multistage turbomachine. Particle image velocimetry measurements were performed within the second-stage rotors of a two-stage compressor. The first-stage stator wake is distorted and produces a kink structure in the second-stage rotor blades passage. This kink, also called a turbulent hot spot, with concentrated vorticity, high turbulence levels, and high turbulence kinetic energy, is caused by the interaction between the first-stage rotor wake and the stator wake. A high-speed region and a low-speed region are observed around the turbulent hot spot. The perturbation velocity is counterclockwise around the turbulent hot spot, with a magnitude much larger than that in the wake. The turbulent hot spot is more unstable and active than the wake and, thus, might play a pivotal role in the passage flow. The high turbulence and the negative jet behavior of the wake dominate the interaction between the unsteady wake and the separated boundary layer on the suction surface of the blade. When the upstream wake impinges on the blade, the boundary layer thickness first increases owing to the presence of the negative jet, and a thickened boundary layer region in the form of a turbulent spot is formed because of the high turbulence intensity in the wake. Then, the boundary layer gradually becomes thinner because of the presence of a calmed region that follows the thickened boundary layer region. Finally, the boundary layer gradually thickens again and recovers to separation. Thus, the boundary layer thickness is periodic in a wake passing cycle.