Sensitivity analysis on the impact of geometrical and operational variations on turbine hub cavity modes and practical methods to control them

This paper presents an experimental investigation on the impact of different design and operational variations on the instabilities induced at the hub cavity outlet of a turbine. The experiments were conducted at the “LISA” test facility at ETH Zurich. The axial gap at the 2nd stage hub cavity exit was varied, and also three different flow deflectors were implemented at the cavity exit to control the cavity modes (CMs). Furthermore, the turbine pressure ratio was altered to mimic the off-design condition and study the sensitivity of the CMs to this parameter. Measurements were performed using pneumatic, and Fast Response Aerodynamic Probes (FRAP) at stator and rotor exit. In addition, unsteady pressure transducers were installed at the cavity exit wall to measure the characteristic parameters of the CMs. For the small axial gap, distinct and strong CMs were generated, which actively interacted with stator and rotor hub flow structures. Increasing the gap damped the fluctuations; however, a broader range of frequencies was amplified. The flow deflectors successfully suppressed the CMs by manipulating the shear layer velocity profile and blocking the growing instabilities. Eventually, the increase in the turbine pressure ratio strengthened the CMs and vice versa.