Investigation of the mild surge in an axial–centrifugal compressor

Abstract

The performance and structural integrity of compressors are critically impacted by aerodynamic instability. This study examines an unstable phenomenon, known as high-frequency mild surge (HFMS), in axial–centrifugal compressors through an experimental investigation and a low-order model. The results reveal that the pressure signal during HFMS is synchronized along the circumferential and streamwise directions. To elucidate the HFMS mechanism and further explore its influential law, a multi-actuator model is developed, successfully capturing the dynamic instability behavior. As the mass flow decreases, the front axial stage becomes unstable first, while the latter centrifugal stage remains stable with a negative slope near the operating point on the performance curve. The strong pressurization of the centrifugal stage maintains the stability of the entire compression system, increasing the mass flow rate. Subsequently, the pressure ratio of the latter centrifugal stage decreases with increasing mass flow, causing the front axial stage to become unstable again. The axial stage's Helmholtz frequency is consistent with the test HFMS frequency, further proving that it is a local systematic instability dominated by the axial stage. This matching feature is particularly common in combined compressors, making HFMS a typical instability phenomenon in such systems.