59501 - Incipient Surge Analysis in Time and Frequency Domain for Centrifugal Compressors 

The use of centrifugal compressors has been rapidly increasing in the last decade as they are a key component in the present energy scenario both in the modern IC engine design and in advanced cycles and innovative plant layouts as fuel cell systems.

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 and guaranteeing safe operation.

For this reason, it is necessary to develop a monitoring system capable of preventing surge and extending the operating range of these machines, their performance, and reliability to allow the integration with other plant components.

The experimental investigation, carried out at the University of Genoa turbocharger test facility and presented in this work, consists of steady state and transient measurements used to characterize and identify compressor behaviour in correspondence of surge inception conditions and to determine different signal processing techniques which could indicate the proximity to unstable operation.

The compressor outlet circuit was designed to be modular in terms of outlet pipe length and volume and allows to perform a determined set of tests on a variety of different circuit configuration providing data for both technique development and validation.

The data analysis concentrates on pressure and vibro-acoustic signals by applying different analysis methods in time and frequency domain to classify compressor operation as stable or unstable. The cross correlation function and wavelet analysis have been identified as techniques to define a precursor able to detect incipient surge conditions.

The cross correlation function has been evaluated with reference to a couple of static pressure signals that have been acquired from two distinct points located in the compressor line.

Through cross correlation function analysis, it has been possible to identify the presence of propagation phenomena in the system and to evaluate how these events become more significant near an unstable low-mass flow rate condition. Additionally, the pressure from the impeller inlet zone has been recorded and considered for this specific analysis correlating its trend with the one of other system responses.

Wavelet application, not widely found in literature in relation to centrifugal compressor stability analysis, has been also considered in this work as a way to investigate compressor instability. The continuous wavelet transform has been applied to pressure and vibro-acoustic signals to show how the time-dependent spectral structure of these signals can highlight the rise of the rotating stall phenomenon, not easily identifiable with traditional signal processing techniques. Since the rotating stall phenomenon generally appears during stable compressor operation but close to the surge boundary its detection can help surge identification without risks of damaging the machine.

The sensitivity and reliability proposed methods have been tested and validated against experimental data collected in different circuit configurations.

The results provide an interesting diagnostic and predictive solution to detect compressor instabilities at low mass flow rate operating conditions and to prevent compressor unstable operating conditions which could lead to compressor fails.