Abstract
Non-synchronous pressure and temperature fluctuations at the hub cavity of a turbine stage are the main focus of this study. Cavity modes are unsteady fluctuations generated at the cavity exit due to instabilities in this region. The cavity modes carried into the main flow impose an unsteady flow field in the rotor passages, which varies the passage-wise flow parameters considerably. A two-stage axial turbine was designed and tested in the “LISA” test facility at ETH Zurich. A reference case with baseline geometry and a modified case with an axial deflector at the hub cavity exit were tested. Comprehensive unsteady pressure and temperature measurements were performed using fast response aerodynamic (FRAP) and entropy probes (FENT), respectively. In addition, 12 fast response unsteady pressure transducers were mounted on the stationary wall of the cavity exit to measure the main characteristic parameters of the cavity modes. Full annular unsteady simulations were also carried out for both cases to support the experiments. Computational fluid dynamics (CFD) successfully predicted the effect of cavity modes on both frequency and amplitude of the fluctuations. The cavity modes indicated fluctuation amplitudes up to eight times of the blade passing fluctuations at the cavity exit. The analysis shows that the convected cavity modes alter the efficiency of different rotor passages by redistributing the mass flow and the losses resulting in a drop in overall efficiency. This work suggests that implementing a small axial deflector at the hub cavity exit would completely eliminate the cavity modes leading to a reduced pressure unsteadiness and enhanced efficiency.
