Impact of blade outlet angle on acoustic of centrifugal pump as turbine

Abstract: As a kind of energy saving technology by recovery of residual pressure, the centrifugal pump as turbine (PAT) has been widely used in many fields. To improve the efficiency of energy recovery, the PAT is gradually developing for high power. The flow-induced noise becomes one of the most important issues that cause negative effect on reliability. The flow-induced noise consists of flow-borne noise and flow-induced structure noise from dipole source. In order to identify the effect of blade outlet angle on flow-induced noise of the PAT, a single-stage end-suction centrifugal pump as turbine was chosen as research model. The blade outlet angles were set to 20°, 30° and 40° respectively, while the rest geometric parameters of impeller and volute were kept unchanged. The flow-induced interior/exterior acoustics of the PAT were studied experimentally and numerically. A synchronous acquisition of performance parameters and noise signals were realized on the basis of INV3020C data acquisition system and performance test system in an open test loop. The liquid was pressurized through booster pump, and then impacted the turbine's impeller to make it rotate. The dynamometer consumed and measured the turbine's energy. The operating condition was adjusted by regulating the converter's frequency to change the booster pump's capacity. Experimental studies on acoustic characteristics at downstream of the PAT were carried out on the test bench at variable flow rates. The flow-induced noise signals were collected using hydrophone at a sampling frequency of 25 600 Hz. The signals were amplified and recorded by INV3020C data acquisition system, and Fast Fourier Transform was used to compute the spectra with the Hanning window for reducing the spectrum leakage. The computational fluid dynamics (CFD) numerical simulation was firstly performed to obtain noise-generating fluid forces. In this step, the k-ε turbulence model was used to solve transient flow field in the PAT, and a time series for pressure fluctuations at fluid-wall interface was obtained. Then, the boundary element method (BEM) was applied to study flow-borne noise caused by impeller and casing dipole sources in interior acoustics of the PAT. Meanwhile, the finite element method/automatically matched layer (FEM/AML) technology was used to study exterior acoustics considering the structure's vibration due to casing source. The interior acoustics were calculated and compared with experimental results, showing that the k-ε turbulence model combined with the BEM for flow-borne interior noise computation was verified. And in the validation of finite element model of casing structure, the exterior acoustics were investigated. In order to investigate the sound's spatial distribution around the PAT, 3 monitoring planes and 36 monitoring points were arranged every 1 meter farther from the center of impeller, and noise directivity distribution was obtained by using FEM/AML calculation. Then, the impact of blade outlet angle on interior/exterior acoustics of the PAT was investigated. Results showed that flow-borne noise due to casing source could reflect joint action of multiple sources. Its spectrum curve agreed well with experimental result, with an error of 3.7% at blade passing frequency (BPF) by the BEM. With the increase of outlet angle, the efficiency within overall flow range reduced. Taking one-third octave A-weighted sound pressure level and sound power level as criteria, the outlet angle had a certain influence on interior/exterior acoustics. Considering both hydraulic performance and noise, a suitable blade outlet angle of 30° exists which ensures a better comprehensive performance of the PAT.