Unsteady numerical simulations in a centrifugal compressor stage consisting of an impeller followed by a discrete passage diffuser are employed to characterize unsteady impeller blade loading. For a stage typical of modern design, peak dynamic loads can be as high as 3.5 times greater that the peak steady-state loads. Unsteady load fluctuations of 0.2 times the tip dynamic pressure are observed in the forward 30% splitter chord, indicating a significant upstream propagation of the unsteady pressure disturbance. There can thus be an upstream manifestation of a downstream stimulus. The upstream manifestation is the considerable unsteady loading in the splitter blade leading edge while the downstream stimulus is the unsteady interaction between the rotating impeller and the stationary downstream diffuser. Increased relative frame diffusion, characterized by the DeHaller number, increases the sensitivity to downstream disturbances and leads to decreased attenuation of the upstream disturbance. Increased relative Mach number decreases the relative frame disturbance wavelength and thus improves the attenuation of the upstream disturbance. The length scale of spatial variation in unsteady loading is set by the relative Mach number, diffuser passing time, and the acoustic speed.

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