Stall inception in a high-speed centrifugal compressor has been examined. The main objective was to find stall precursor and to develop a reliable stall warning method. Eight equally spaced fast-response pressure transducers in the inducer detected the spatial structure of small amplitude perturbations, via spatial Fourier transform, as stall is approached. Near the stall inception point, the phase of spatial Fourier coefficients increased linearly with the speed of impeller rotation for several impeller revolutions at all test speeds, and the spectrum at impeller frequency increased as stall is approached. This is the clear evidence that the impeller frequency participates in the stalling process. For stall warning, a method which uses the spectrum at impeller frequency is suggested. The use of spectrum at impeller frequency as a stall warning method showed a warning time of about two hundreds impeller revolutions. This method uses only one sensor that it has made the stall warning method more useful. And the well-known traveling wave energy method proved to be a good method for stall warning also in a high-speed centrifugal compressor. The warning time was about one hundred impeller revolutions at lower speeds, and about one thousand impeller revolutions at higher speeds. The stall warning methods used here were found to be robust and reliable. Therefore, it seems to be promising to set up a reliable stall avoidance control based on this analysis.

The aim of this paper is to understand the time averaged pressure field and unsteady pressure patterns in a high speed centrifugal compressor channel diffuser. Pressure distributions from the impeller exit to the channel diffuser exit are measured and discussed for various flow conditions. Unsteady pressure signals from six fast-response sensors in the channel diffuser are analyzed by decomposition method and wavelet transform. Measured results are shown for various operating condition from choke to surge that the effect of operating condition is well discussed. The strong non-uniformity in the pressure distribution is obtained over the diffuser shroud wall caused by the impeller-diffuser interaction. As the flow rate increases, flow separation near the throat, due to large incidence angle, increases aerodynamic blockage and reduces the aerodynamic flow area downstream. Thus the minimum pressure location occurs downstream of the geometric throat, and it is named as the aerodynamic throat. And at choke condition, normal shock occurs downstream of this aerodynamic throat. The variation in the location of the aerodynamic throat is discussed. The pressure ratio waveforms by blade passing show regular oscillation not only for the normal but also for the surge conditions and the high frequency fluctuations are superposed on the oscillating pressure waveform as the flow rate increases. Periodic unsteadiness by blade passing does not decay in the diffuser channel. It depends on the operating point and is generally larger in the channel than in the vaneless space. Aperiodic unsteadiness rapidly decrease downstream of diffuser channel. At surge, the spectrum becomes broad banded with peaks at the surge frequency as well as blade passage frequency and the impeller rotating frequency. The surge signal was analyzed using wavelet transform and it is found that surge signal is composed of not only surge scale and blade scale but also multi-scale aperiodic waves. The broadband spectrum in surge condition is due to this multi-scale aperiodic waves.

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This paper presents an experimental and analytical investigation of a rotating stall inception in a vaneless diffuser of a centrifugal compressor. Eight fast-response pressure transducers are equally spaced around the circumference at the inlet and exit of a parallel vaneless diffuser. Instantaneous pressure data is measured near the stall inception point and characteristics of a rotating stall, a stall-initiating mechanism, a stall precursor and its warning schemes are discussed. It is found that one-cell, two-cell and three-cell structures of small amplitude wave grow and decay repeatedly before they are fully developed to a rotating stall, which is named as “pre-cell.” When it appears, the phase of spatial Fourier coefficient increases linearly and the traveling wave energy increases. The pre-cell travels at, or slightly lower than, the speed of the fully developed rotating stall. Its growing-decaying life span is about several decades of the impeller revolution. Pre-cells of one-cell, two-cell, and three-cell structures are found to interact frequently with their growing and decaying mechanism through transferring energy from one structure to another. Two stall warning schemes are used for the stall in the vaneless diffuser. The first scheme is to detect the linear increase region in the phase of the spatial Fourier coefficient from where the according warning time is about 0.3∼1.4 sec. (300∼700 impeller revs.) The second scheme is to detect the increase of traveling wave energy from where the according warning time is about 0.2∼2.3 sec. (200∼1200 impeller revs.) These warning schemes are useful because their warning time is long enough to be applied in active control of a compressor stall.

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