Flow instability evolution in high pressure ratio centrifugal compressor with vaned diffuser

Compressor stability is an essential issue for developing single-stage high pressure ratio centrifugal compressors. In this paper, a comprehensive stability map of a centrifugal compressor stage with the peak pressure ratio of 6.2 has been illustrated. Fast response pressure transducers are mounted inside the casing or the back-plate wall to measure internal transient behaviors. From Fourier analysis of pressure signals, various instabilities across time and length scales are identified. At low speeds, the impeller inlet rotating instability develops in scale and evolves into the inducer stall, which eventually induces the mild surge and deep surge of the compression system. At middle speeds, the compressor successively experiences stable state, mild surge, rotating instability, and deep surge. The mild surge region coincides with the dip region of the compressor S-shape pressure rise curve, and deep surge occurs when the compressor pressure rise raises to the second peak. At high speeds, mild surge and deep surge abruptly occur without preceding stall or rotating instability. To explain the complex surge behavior, the mechanical analogy between the compression system and the mass-spring-damper system is applied. Both mild surge and deep surge transients are found to belong to the dynamic instability of the mass-spring-damper system, and the exact form of the surge state will be determined by eigenvalues of the system governing equation.