Unsteady Acoustic Forcing on an Impeller Due to Coupled Blade Row Interactions

Abstract

This article contains an investigation of the unsteady acoustic forcing on a centrifugal impeller due to coupled blade row interactions. Selected results from an aeromechanical test campaign on a GE Oil and Gas centrifugal compressor stage with a vaneless diffuser are presented. The most commonly encountered sources of impeller excitation due to upstream wake interaction were identified and observed in the testing campaign. A 30/rev excitation corresponding to the sum of upstream and downstream vane counts caused significant trailing edge vibratory stress amplitudes. Due to the large spacing between the impeller and the return channel vanes, this 30/rev excitation was suspected to be caused by an aero-acoustic excitation rather than a potential disturbance. The origin of this aero-acoustic excitation was deduced from an acoustic analysis of the unsteady compressor flow derived from CFD. The analysis revealed a complex excitation mechanism caused by impeller interaction with the upstream vane row wakes and subsequent acoustic wave reflection from the downstream return channel vanes. The findings show it is important to account for aero-acoustic forcing in the aeromechanical design of low pressure ratio centrifugal compressor stages.

Experimental Measurements of the Natural Frequencies and Mode Shapes of Rotating Disk-Blades-Disk Assemblies from the Stationary Frame

“…In this paper, the type of structure disk-blades-disk, which is used for closed impellers in centrifugal compressors, pump-turbines, centrifugal pumps and low specific speed Francis runners [30,34,39,42], is analyzed in detail.…”

“…The test rig setup consists of a test rig with a disk-blades-disk structure that can rotate in the range of some kind of real turbomachinery (0-15 Hz), which use this kind of assembly. As mentioned before, this kind of assembly is used basically in closed impellers of pump turbines, pumps, centrifugal compressors, and low specific speed Francis runners [30,34,39,42].…”

“…Real engineering cases where disk-blades-disk assemblies are used. (a) Low specific speed Francis Turbine runner[39], (b) Pump-turbine impeller[40], (c) Centrifugal pump impeller[41], (d) Centrifugal compressor impeller[42].…”

Multi-Blade Row Interactions in a Low Pressure Ratio Centrifugal Compressor Stage With a Vaned Diffuser

“…However, they mentioned that the CFD could not resolve any such interaction and that further studies were necessary to understand this phenomenon. Also recently, Richards et al [13] presented a peculiar spinning acoustic mode excitation mechanism in a low pressure ratio compact stage with a vaneless diffuser. The combined experimental and unsteady CFD investigation revealed a complex aero-acoustic interaction between the impeller and the upstream and downstream return channel vanes.…”

“…For the problem of interest, there is no publically available validation of the unsteady CFD solver. It has however, been validated internally on axial machines, and also shown to correlate with measurements in the CVL stage [13]. The unsteady CFD domain comprises the impeller, the upstream deswirler, the downstream vaned diffuser, and the return channel vanes.…”