Investigation of surge in a 1.5-stage transonic axial compressor

In this paper we give insight into characteristics of a 1.5-stage transonic axial compressor rig with focus on surge during a stalled operating point. The new compressor rig at TU Darmstadt is representative for the front stage of an industrial gas turbine. Transient throttling maneuvers were conducted for multiple operating points during the first test campaign of the TCD 2 (Transonic Compressor Darmstadt 2), providing an extensive set of unsteady structural and aerodynamic data beyond the stability limit. Enhanced analytical methods allow detailed studies including aerodynamic spectral analysis as well as determination of propagation speed and size of disturbances. The results differ from observations at comparable test rigs, revealing an interesting manifestation of stall: In a wide range of the stability limit it shows a periodicity. The stall emerges and vanishes recurrently, causing strong oscillations of the pressure ratio. Additional unsteady measurements of the mass flow indicate a surge. Regarding the compressor map, this results in staggering operating points, showing a hysteresis. However, due to a rather small plenum and experience with a similar test rig the TCD 2 was not expected to surge. Comprehensive analyses are carried out to characterize this phenomenon.