Energy input of a centrifugal stage into the attached piping system duringmild surge

Subjected to an oscillating flow rate, a compressor may feed additional (ex citational) energy into the attached piping system. The relation between th is additional energy input and the instantaneous behavior of a centrifugal compressor stage is dealt in a first part. Modeling the Stage behavior by t aking into account either inertia of the enclosed fluid mass or a first-ord er transient element or transient stall in any component leads to a differe nt energy input. The energy input at a flow rate oscillation of given frequ ency, and amplitude was calculated as a function of the slope of the charac teristic 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 u nity 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 b y increasing the rime constant of the stall process or by increasing the mi ld surge frequency. For the investigated diffuser size the required reduced frequencies imply mild surge frequencies in a range being too high for ind ustrial 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 oscill ations was determined from detailed instantaneous measurements. As a result , the contribution of each stage segment to the conservation of the wild su rge pulsation emerges. Although at the investigated mild surge frequencies the stage segments no longer behave strictly quasi-steadily, their contribu tion to the additional energy input is found to be mainly determined by the slope of their quasi-steady characteristic.