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Incipient Surge Analysis in Time and Frequency Domain for Centrifugal Compressors


 The use of centrifugal compressors has been increasing tremendously in the last decade as they are a key component in the present energy scenario both in the internal combustion engine design and in advanced cycles and innovative plant layouts.
 Instability phenomena limit the operating range of the whole compressor system, especially during fast transients. The target is therefore to extend the minimum flow limit in order to improve the operability of each unit, while avoiding compressor surge operation and guaranteeing safe operation.
 The presented experimental investigation consists of steady state and transient measurements used to characterize and identify compressor behaviour in correspondence of surge inception conditions. The data analysis concentrates on pressure and vibro-acoustic signals. The cross correlation function and wavelet analysis have been identified as techniques to define precursors able to detect incipient surge conditions. Through cross correlation function, it has been possible to find the presence of propagation phenomena and to evaluate how these events become more significant near an unstable low-mass flow rate condition. Additionally, the wavelet transform has been applied to operational signals to show how their time-dependent spectral structure responses can highlight the rise of unstable phenomena. Moreover, system response was studied in high frequency range and through a demodulation technique it was found how blade pass frequency energy content change interacting with rotating stall inception, moving close to surge.
 The obtained results provide an interesting diagnostic and predictive solution to detect compressor instabilities at low mass flow rate operating conditions.