Experiment on cavitation erosion mechanism of centrifugal hydraulic cavitation generator

Abstract: The performance of a rotor-stator centrifugal cavitation generator was experimentally studied, aiming at investigating the correlation between its cavitation mechanism and damage distribution. The generator was modified from a centrifugal pump, including a cut impeller, a volute, a rotor and a stator. The rotor and stator consist of 12 teeth each, whereas the stator has wedge grooves on each tooth. Therefore, as the rotor spins, there are 12 nozzles when the rotor tooth overlaps the stator tooth. The experiments were conducted in a closed-loop test rig in the Laboratory for Water and Turbine Machines of University of Ljubljana, Slovenia. The cavitation generator was also used as a flow driver in the test. In order to study the cavitation mechanism, an observation window was mounted on the side of the generator volute. The cavitation evolution was recorded via a high speed camera accordingly. Based on the visualization tests, it is found that there are 3 kinds of cavitation generating mechanisms. One is produced in nozzle tubes formed by the interaction movement of the rotor and stator. The other two are caused by the high speed rotation of the rotor. One happens on the leading edge of the stator tooth as the rotor moves towards it. At the same time, the rotor itself generates bubbles on the leading edge of the tooth. Hence, in one rotor-stator teeth interlacing period, the cavitation generated on the leading edge of the stator (nozzle cavitation and rotating-induced cavitation) has 2 circulations. For detecting the cavitation intensity, the pressure pulsation between the rotor and volute was measured by a hydrophone under different operating conditions. The results show that the pressure pulsation increases as the flow rate increases while keeping the rotating speed constant, but the pressure pulsation cycle remains the same. As the driving frequency is 50 Hz, the pressure amplitudes under 18 and 31.4 m3/h are 1.0×105 and 1.5×105 Pa, respectively, while the cycle is approximately 0.002 s. Additionally, regardless of the flow rate, the dominant frequency is equal to the rotor blade-passing frequency, but not the impeller-passing frequency of the original centrifugal pump. That is to say, the dominant frequency is 12 times shaft frequency. When the flow rate remains the same (18 m3/h), the pressure pulsation rises with the increasing of rotating speed, whereas the cycle declines. As the driving frequency reaches 50 Hz, the pressure pulsations are nearly 2.5 times that when the driving frequency is 30 Hz, but its cycle increases from 0.002 to 0.003 s. And the domain frequencies under each driving frequency are still equal to the rotor blade-passing frequency. Meanwhile, the influence of the distance between rotor and stator on the pressure pulsation was also studied. The distance was adjusted by the shims under the stator. It is found that increasing the distance would slightly decrease the pressure. Furthermore, the oil ink painting approach was employed to investigate the erosion area of the cavitation generator. The result indicates that the rear part and the middle part of the rotor tooth are eroded. For the stator, the damage almost covers the wedge grooves and some rear part of the tooth, which means these parts are the potential erosive area. The nozzle cavitation is the dominant trigger for these damages, since it has the strongest cavitation intensity among the above discussed 3 kinds of cavitation generating mechanisms. When the teeth of the rotor and stator interlace each other, the flow velocity in the gap between the rotor and stator is getting faster, creating stronger cavitation intensity. Hence, the erosion area primarily locates on the wedge groove. Moreover, the nozzle cavitation initiates while the rotor tooth overlaps most part of the stator tooth, and starts to shed off as they begin to stagger, so the shed bubbles collapse downstream, contributing to damages on the rear part of both rotor and stator. It reveals that the cavitation erosion in hydraulic machinery is primarily caused by the collapse of bubbles that are close to the solid wall.