Sensitivity Analysis on the Impact of Geometrical and Operational Variations on Turbine Hub Cavity Modes and Practical Methods to Control them

Abstract

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 2 nd 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.

Analysis of Wheel Space Ingress in a One-Stage Axial Turbine

“…This analysis, however, shows that they have a major impact on the main annulus gas ingestion. Hence, these results suggest that ingress in this axial turbine stage setup is not driven by the modes with the largest amplitudes of velocity or pressure fluctuations such that a tuning of the wheel space geometry as suggested by (Iranidokht et al, 2021) to reduce these mode amplitudes does not necessarily result in a better cooling effectivenss.…”

“…Outside this range, the fluctuations vanished abruptly. Recently, (Iranidokht et al, 2021) were able to affect the amplitudes and the frequencies of cavity modes observed on their test rig by modifying the wheel space geometry.…”

AIAA Propulsion and Energy 2021 Forum

“…Furthermore, the activity registered in the rim seal cavity generally did not appear in the inner rotor-stator cavity signal. Iranidokht et al (2020) reported experimental evidence of cavity modes and their sensitivities to turbine axial gap and turbine pressure ratio. An increased rotorstator axial gap is found to damp the peak amplitude of cavity modes by approximately a factor of seven.…”