Unsteady Flow Phenomena in a Centrifugal Compressor Channel Diffuser

Abstract

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.

Comparative Study of Unsteady Flows in a Transonic Centrifugal Compressor with Vaneless and Vaned Diffusers

“…The larger difference between the pressure values at the inlet and outlet of the vaned diffuser indicates the better diffusion compared to the vaneless case. The waveform and amplitude of the pressure fluctuation in the vaned diffuser case are similar to what J. Kang and S. Kang [6] observed in their vane-island diffuser. To show the difference of the flow fields inside the compressor with the vaneless and vaned diffusers, the field quantities are studied during both periods at the same moments equally spaced over the periods.…”

“…The degree of unsteadiness for the compressor with vaneless and vaned diffusers at design conditions is calculated using (6). Since the vaneless case is chosen as the reference case, the degree of unsteadiness calculated for the vaned case indicates the difference between the two cases at design conditions.…”

Fast-response total pressure probe for turbomachinery application

“…To analyze the measured instantaneous signal, a decomposition method suggested by Suryavamshi et al [10,11] and modified by Kang et al [12] is used. Each discrete measurement of the total pressure is indexed with i, j, and k. The subscripts i, j, and k represent indices in the ensemble averaging process (i indicates the index of revolution, j the index of the blade, and k the index of the window in the blade passage).…”

Numerical simulation of rotating stall in a centrifugal compressor with vaned diffuser

“…While the successful control of rotating stall to improve compressor stability is based on the detection of early precursors to stall, Lawless [1] experimentally studied the rotating stall acoustic signatures in a low speed centrifugal compressor with vaneless and vaned diffusers. Recently Kang et al [2][3][4] examined the stall inception in a high speed centrifugal compressor through a series of experiments. Both of them wanted to find stall precursors and develop reliable stall warning method.…”