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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. INTRODUCTION Cavity modes (CMs) are large scale nonsynchronous unsteady pressure fluctuations, which are mainly observed at the cavity exit where the cavity discharge flow interacts with the main flow of the turbine. The nonsynchronous term is used due to the different rotational speed of the CMs when compared to the rotor speed. This nonsynchronous behavior makes it difficult to resolve them with section simulation or current sensor technology. In fact, the CMs have frequencies between the rotor and rotor blade passing frequency (BPF). If these unsteady fluctuations are not controlled, they will be convected into the main flow affecting the performance of the turbine in different ways. The CMs impact has been discussed on different topics such as the profiled endwall performance [1], noise [2], sealing effectiveness [3], rotor loss [4] and also hot gas ingestion into the hub cavity and wheel space [5],[6]. Although the first openly published study on the rim seal flow was [7], but the evidence of the large scale nonsynchronous cavity modes was discussed in detail by [8] in early 2000. Recently, [9] carried out an excellent review study of the flow mechanisms on the axial turbine rim sealing with a focus on cavity modes. Schädler et al. [4] performed extensive experimental and full annular computations studying the migration of the cavity modes into the annulus flow under two purge flow rates. They reported that the reduced rotor loss due to lowering the purge flow rate is weakened by the contribution of the cavity modes. The author stresses the importance of considering the cavity modes during the turbine design process in order to reduce the loss and noise emission by stabilizing the cavity dynamics. The stabilizing effect of the increased purge flow is also mentioned in the study of [1], [10]. Also, [11] investigated the interaction between the annulus and rim seal flow by simulating a generic high-pressure turbine with different sealing geometry, specifically the overlap-type. They reported that the frequency of the cavity induced vortices is related to rim seal geometry and suggested the stabilization of the modes by either increasing the sealing mass flow or sealing tangential velocity. CMs are also observed by [6] with the frequency of f/f_bld=0.44. The author relates the generation of these modes to the centrifugal force and pressure gradient inside the hub cavity. More recently, [12] investigated the flow instabilities at the turbine rim seal using both experimental and

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