Experimental investigations on the frequency of partial surge

Partial surge is characterized by axisymmetric low-frequency disturbance localized in the hub region and with the potential risk of causing full compressor instability in a transonic axial flow compressor. To find out what determines the low frequency of partial surge, most typical vibro-acoustic frequencies in the test compressor system are first examined, and the observed frequency of partial surge is found to be the closest to the Helmholtz frequency estimated with Greitzer's duct-compressor-plenum model. Experiments are then conducted to observe the frequency of partial surge during the evolution of instability in the same compressor system with varied lengths of compressor inlet duct and inlet duct in front of the settling chamber and at the same rotor speed. The result also suggests that the dominant frequency of partial surge can be determined by the Helmholtz frequency of the system. When the length of compressor inlet duct is increased beyond a critical range, a different type of system oscillation is also observed in the form of a disturbance at higher frequency. The occurrence of this disturbance is discussed in this article with an extended theory of system oscillation.