PIV measurements of the unsteady flow structures in a volute centrifugal pump at a high flow rate

An experimental investigation based on PIV measurements is presented on the unsteady flow in a centrifugal pump with vaneless volute. The study has focused on the fluid-dynamic interaction between impeller and volute associated with the blade passage period when the pump operates well above design conditions (150 % of nominal flow rate). The test pump, which is fully transparent, has a 2D-shaped geometry and the impeller has 6 arc-shaped backward-curved blades. The flow in the midplane perpendicular to the pump axis was captured using fluorescent seeding particles and two fields of view (FOV), each with different magnification: a large FOV (low magnification) that covers a large portion of the impeller, volute and discharge duct, and a small FOV (high magnification) to observe the details of flow evolution at the tongue region. Results are presented on the in-plane phase-averaged relative and absolute velocities, turbulent kinetic energy, its production and vorticity fields. Their evolution over time during one blade passage is described, including a frequency analysis based on 32 blade positions. The spatial distribution of the spectral components at the blade-passing frequency and some harmonics reveals that the second harmonic is dominant in the narrow region of the volute at the tongue tip. The data obtained show that the fluid-dynamic blade-tongue interaction is dominated by highvorticity sheets (positive and negative) being shed from the impeller channels, especially from the blade trailing edges, and their impingement on the tongue tip with subsequent cutting and distortion. The turbulence production is seen to be mainly concentrated in the wake regions from the blade leading and trailing edges and from the tongue tip. In particular, it gets maximum behind the blade trailing edge when the blade aligns with the tongue tip.