This turbine stage consists of a radial stator and a centripetal rotor. Due to specific blade count combinations between the stator and the rotor, an unsteady rotating radial load appears on the rotor blades. The frequency of this load is determined by the Tyler-Sofrin theory. The objective of this CFD study was to compute the amplitude of this rotating load.
A structured, multi-blocks mesh was generated with AutoGrid5™ software. The periodic unsteady flow behavior coming from the stator/rotor interactions was computed with the Non-Linear Harmonic (NLH) method implemented in FINE™/Turbo. This technique, that solves the periodic spatial perturbations in the frequency domain, allowed to significantly reduce the computational cost of the calculation compared to a full 360° unsteady approach. Only one mesh passage was required for the stator and the rotor. The unsteady flow field was reconstructed in time using the amplitude and phase of the solved harmonics. The direction and amplitude of the radial force were computed for each time-step.
Several fluids, operating conditions, blade periodicities and rotational speeds were computed during the project. The analysis of the results showed that a resonance can occur for specific conditions, leading to a significant amplification of the load on the turbine.
The animation gives the instantaneous static pressure on the rotor blades. The picture presents the amplitude and direction of the radial load over a full rotor rotation.