Particle Image Velocimetry (PIV) and pressure fluctuation measurements are used for investigating the onset and development of rotating stall within a centrifugal pump having a vaned diffuser. The experiments are performed in a facility that enables measurements between the diffuser vanes, within part of the impeller, in the gap between them and in the volute. The diffuser is also instrumented with pressure transducers that track the circumferential motion of rotating stall in the stator. The timing of low-pass-filtered pressure signals are also used for triggering the acquisition of PIV images. The data include detailed velocity distributions, instantaneous and phase-averaged, at different blade orientations and stall phases, as well as auto-and cross-spectra of pressure fluctuations measured simultaneously in neighboring vane passages. The cross-spectra show that the stall propagation rate is 0.93 Hz, 6.2 percent of the impeller speed, and that the stall travels from the passages located on the exit side of the volute toward the beginning side, crossing the tongue region in the same direction as the impeller, where it diminishes. Under stall conditions the flow in the diffuser passage alternates between outward jetting, when the low-pass-filtered pressure is high, to a reverse flow, when the filtered pressure is low. Being below design conditions, there is a consistent high-speed leakage flow in the gap between the impeller and the diffuser from the exit side to the beginning of the volute. Separation of this leakage flow from the diffuser vane causes the onset of the stall. The magnitude of the leakage and the velocity distribution in the gap depend on the orientation of the impeller blade. Conversely, the flow in a stalled diffuser passage and the occurrence of stall do not vary significantly with blade orientation. With decreasing flow-rate the magnitudes of leakage and reverse flow within a stalled diffuser passage increase, and the stall-cell size extends from one to two diffuser passages.
The main goal of the present work is to analyse the numerical simulation of a centrifugal pump by solving Navier-Stokes equations, coupled with the ‘standard k-∊’ turbulence model. The pump consists of an impeller having five curved blades with nine diffuser vanes. The shaft rotates at 890r/min. Flow characteristics are assumed to be stalled in the appropriate region of flowrate levels of 1.31-2.861/s. Numerical analysis techniques are performed on a commercial FLUENT package program assuming steady, incompressible flow conditions with decreasing flowrate. Under stall conditions the flow in the diffuser passage alternates between outward jetting when the low-pass-filtered pressure is high to a reverse flow when the filtered pressure is low. Being below design conditions, there is a consistent high-speed leakage flow in the gap between the impeller and the diffuser from the exit side of the diffuser to the beginning of the volute. Separation of this leakage flow from the diffuser vane causes the onset of stall. As the flowrate decreases both the magnitude of the leakage within the vaneless part of the pump and reverse flow within a stalled diffuser passage increase. As this occurs, the stall-cell size extends from one to two diffuser passages. Comparisons are made with experimental data and show good agreement.
Detailed flow measurements have been made in the vaneless diffuser of a large low-speed centrifugal compressor using hot-wire anemometry. The three time mean velocity components and full stress tensor distributions have been determined on eight measurement planes within the diffuser. High levels of Reynolds stress result in the rapid mixing out of the blade wake. Although high levels of turbulent kinetic energy are found in the passage wake, they are not associated with strong Reynolds stresses and hence the passage wake mixes out only slowly. Low-frequency meandering of the wake position is therefore likely to be responsible for the high kinetic energy levels. The anisotropic nature of the turbulence suggests that Reynolds stress turbulence models are required for CFD modeling of diffuser flows.
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