Subjected to an oscillating flow rate a compressor may feed additional (excitational) energy into the attached piping system. The relation between this additional energy input and the instantaneous behavior of a centrifugal compressor stage is dealt in a first part. Modeling the stage behavior by taking into account either inertia of the enclosed fluid mass or a first-order transient element or transient stall in any component leads to a different energy input.

The energy input at a flow rate oscillation of given frequency and amplitude was calculated as a function of the slope of the characteristic and the reduced frequency applying a previously published model to describe the instantaneous behavior of the stage. In this model transient stall in the diffuser is taken into account. At reduced frequencies above unity the energy input of the diffuser was reduced by a considerable amount due to the specified instantaneous behavior of the diffuser. This indicates a potential to reduce the additional energy input of the diffuser either by increasing the time constant of the stall process or by increasing the mild surge frequency. For the investigated diffuser size the required reduced frequencies imply mild surge frequencies in a range being too high for industrial application (> 200 Hz). Still, this method turned out to give useful insight into the link between the instantaneous behavior of the compressor and its energy input.

In a second part for the same centrifugal compressor the energy contribution of several stage segments during mild surge oscillations was determined from detailed instantaneous measurements. As a result the contribution of each stage segment to the conservation of the mild surge pulsation emerges. Although at the investigated mild surge frequencies the stage segments no longer behave strictly quasi-steadily their contribution to the additional energy input is found to be mainly determined by the slope of their quasi-steady characteristic.

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