Influence of Pump-Circuit Coupling on Acoustic Waves in Pipelines and Pump Velocity Fields

Hydraulic piping systems are sensitive to excessive acoustic emissions which can give rise to vibrations or to failure of mechanical elements due to fatigue. Common excitation sources are centrifugal pumps due to the periodic interaction of the impeller with the volute tongue. They radiate pressure fluctuations into the connected circuit at the blade-passing frequency which are reflected in the circuit. The aim of the present investigation was the experimental characterization of the perturbations induced in a piping network as a function of the acoustic impedance of the circuit using fast-response pressure transducers. Three transducers were placed along the discharge pipe to decompose the pressure signal into the radiated and reflected acoustic wave, with amplitude and phase. The speed of sound in the water pipelines was determined experimentally. Results of impedance at the pump tongue were compared with a theoretical approach, using a Transfer Matrix Analysis, where each pipe element is represented by a 2×2-matrix, relating the acoustic pressure and velocity fluctuations at the two ports. Attenuation was considered by using complex wavenumbers. The acoustic impedance was changed by using different rotation speeds and by using a cavity which works as a harmonic oscillator whose resonance frequency can be changed. The results presented for different operating points show the influence of changing impedance on the pressure perturbations due to pump-circuit acoustic coupling. The pump was built with a transparent impeller and volute to conduct Particle Image Velocimetry (PIV) measurements in a plane perpendicular to the pump rotation axis. Phase-averaged velocity fields were obtained at different blade positions in the zone around the tongue. Vorticity fields were derived from it and turbulence is shown by representing in-plane turbulent kinetic energy (TKE). Strong vorticity and turbulence occurred in the volute channel behind the blade and near the tongue tip.