Non-Periodic 3D Diffusers for Mitigating Aerodynamic Exciters
Authors: Grigoriev Mikhail , Hitt James 
Published: January 2015

The great majority of the modern centrifugal stages utilize periodic stationary structures such as inlet guide vanes and/or diffuser vanes. To maximize the aerodynamic performance of the centrifugal stage, these vanes must be positioned at a close proximity of the centrifugal impeller. This arrangement results in a dramatic interaction between rotating impeller and stationary vanes due to reflection of the pressure waves from the periodic vanes back onto the impeller blades. The periodic nature of the reflected pressure waves may lead to an excitation of the impeller blade eigenmodes if the fundamental frequency (or, its multiple) of the external force matches with the natural frequency of the subject impeller. As the impeller blades provide very little to no damping, there is a strong possibility of the high cycle fatigue resonance failure of the impeller blades if the impeller design does not provide with a sufficient separation from the resonance modes. We should note that ensuring such a separation is not straightforward task for many stages with periodic exciters, and may not be even feasible for some practical design cases.

This presentation focuses on a novel way to mitigate possible resonance issues for centrifugal impellers due to pressure reflection waves emanating from the diffuser blades. We propose to utilize non-periodic centrifugal diffuser together with the sculpting leading edges for the three-dimensional diffuser vanes. In order to demonstrate the attractiveness and feasibility of this approach, we have utilized Computational Fluid Dynamics (CFD) tools to perform time-accurate unsteady turbulent flow analyses in centrifugal stages and capture cyclic pressure waves acting on the impeller blades. The present work considers a regular periodic low-solidity diffuser with two-dimensional vanes, a three-dimensional periodic diffuser with a sculpted leading edge, and, finally, a non-periodic three-dimensional diffuser with an unequal, non-repeating stagger.

We have utilized eighteen CFD pressure probes located on the impeller blade pressure and suction sides to monitor temporal variations of the static pressure that capture the pressure reflection waves from the diffuser vanes. The Fourier series decomposition facilitates detailed analyses of the pressure energy distribution over a wide range of frequencies. The results of the numerical studies demonstrate that even the use of the periodic diffuser with 3D sculpted leading edges help reduce the magnitude of the pressure oscillations at the dominant frequency and its integer multiples. However, the pressure energy distribution changes dramatically when using the non-periodic diffuser arrangement together with the sculpted leading edge vanes. The strength of the pressure waves associated with the dominant harmonics and its integer multiples are reduced about 30% to 85% and spread over the frequencies that constitute integer multiples of the fundamental impeller frequency. This pressure energy redistribution of the 3D non-periodic diffuser is a significant aid to the aerodynamicist. By significantly reducing the mechanical constraint compromises, the designer is allowed to focus more on aerodynamic component efficiency.