An Investigation of the Flow Physics of Vane Clocking Using Unsteady Flow Measurements

Vane clocking, the circumferential indexing of adjacent vane rows with similar vane counts, has been shown to affect stage efficiency in compressors and turbines. Steady flow measurements acquired in the embedded stage of the Purdue 3-Stage Compressor showed a change in stage efficiency with vane clocking, as discussed in a companion paper. The optimum efficiency condition at design loading occurred when the upstream vane wake impinged on the downstream vane, as had been reported by other vane clocking studies. However, at high loading, the impingement of the upstream vane wake triggered a vane suction side boundary layer separation and resulted in the worst efficiency condition. The objective of this research is to experimentally investigate the maximum and minimum efficiency clocking configurations with unsteady flow measurements to illuminate the flow physics associated with the measured changes in Stage 2 performance. Vane exit unsteady total pressure, velocity, and flow angle measurements were acquired at 50 pitchwise locations spanning one vane passage. Fourier decomposition is used to identify the impact of the upstream rotor wake on the shedding characteristics of the Stator 2 boundary layer and how the placement of the upstream vane wake affects this phenomenon. For the clocking configuration that located the Stator 1 wake at the leading edge of the Stator 2 vane at design loading, it dampened the boundary layer response to the fluctuating incidence associated with rotor wake chopping, leading to a reduction in the size of the structures shed in the Stator 2 vane wake. At the high loading condition, the placement of the Stator 1 wake at the leading edge of Stator 2 triggered a suction side boundary layer separation, resulting in an absence of the upstream rotor blade pass frequency in the spectrum measured in the Stator 2 wake.