RESEARCH ON THE CHARACTERISTICS UNDER THE CONDITION OF ASYMMETRIC CAVITATION IN A CENTRIFUGAL PUMP

摘要

Major types of cavitation instabilities in hydraulic machineries include cavitation surge (CS), rotating cavitation (RC), asymmetric cavitation (AC) and high-order instabilities. Asymmetric cavitation, in which cavity lengths are unequal on each blade, is known as a source of cavitation induced shaft vibration in turbomachinery. In this paper, the internal flow characteristics and inlet and outlet pressure pulsation characteristics of the centrifugal pump of asymmetric cavitation have been investigated through numerical simulation and experimental method. It could be found that asymmetric and uneven cavity volume distribution is resulted from rotor-stator interaction between impeller and volute tongue, and the uneven distribution of cavity volume between each blade also influence each other, which may induce the asymmetric cavitation. Clear corresponding relationship between the distribution of cavity and the position of vortex in each blade channel can be found. It is obvious that the dominant frequency of pump inlet pressure fluctuation keeps at about 0.6 - 0.8 times (about 30 - 40 Hz) of the shaft frequency, and broadband pulsations distribute between 0 and 20 Hz at the condition of asymmetric cavitation. The generation of flexible vapor bubbles plays an important role in enhancing the buffering effect to the pump inlet liquid. Compared to the effect of rotor-stator interaction between impeller and volute tongue, the collapse of vapor bubbles plays a dominant role to the pump inlet pressure fluctuation in the condition of asymmetric cavitation. Meanwhile, the dominant frequency of pump outlet pressure pulsation is the blade passing frequency, and the generation and collapse of asymmetric cavity in each blade channel produced the broadband pulsation of pump outlet pressure under asymmetric cavitation.