Steady and Unsteady Flow Phenomena in a Channel Diffuser of a Centrifugal Compressor

The aim of this paper is to understand the time averaged pressure distributions and unsteady pressure patterns in a channel diffuser of a centrifugal compressor. Pressure distributions from the impeller exit to the channel diffuser exit are measured and discussed for various flow conditions. And unsteady pressure signals from six fast-response sensors in the channel diffuser are analyzed by decomposition method. 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.