Cavitation characteristics of tip leakage vortex and suction-side-perpendicular vortices in axial flow pump

Abstract: The tip leakage vortex (TLV) cavitation mechanism of axial flow pump was investigated with the results of high speed photography and pressure fluctuation measurement. The tip leakage vortex cavitation morphology and the transient characteristics of the TLV-induced suction-side-perpendicular cavitating vortex (SSPCV) were analyzed under different flow rates and different cavitation numbers. The visualization experimental results were combined with the time domain spectrum of pressure fluctuation to elucidate the relationship between the tip cavitation and pressure fluctuation. The performance curves show a good agreement and the maximum error appears at 1.2 times rated flow condition, and the error value is 2.7%, which proves the reliability of the experiment and stability of the experiment apparatus. The head of the pump at different flow rates increases slightly with the decrease of cavitation number at first, and then decreases sharply. With the decrease of the flow rate, the value of incipient cavitation number increases gradually, which means cavitation is easier to occur under low flow rate. Due to that the angle between the tip leakage vortex and the back of the blade becomes larger with the decrease of the flow rate, the blockage caused by the tip vortex is enhanced. Compared with the condition of 0.8 times rated flow, the heads in 0.6 and 0.7 times rated flow condition are lower, which coincides with the performance test curves. The results of high speed photography show that cavitation inception occurs easier under low flow condition, and with the increase of the flow rate, the primary position gradually moves to the back edge of the blade tip. Reducing the cavitation number, the triangular cloud cavitating area formed in the blade tip grows gradually, and the cavitation structure becomes unstable. In addition, some vertices shed from the trailing edge of the cloud, perpendicular to the suction surface of the blade. At the same flow rate, the size of triangular cloud cavitation increases as the cavitation number decreases. Consequently, the SSPCV generates gradually rearward at the chord of blade tip, which grows in scale and intensity. With the similar cavitation numbers, the shedding of SSPCVs is more intense with the decrease of the flow rate. In the process of shedding, the width of triangular cloud cavitation area reduces sharply, while the SSPCV moves toward the adjacent blade perpendicular to the suction surface, resulting in blockage in the flow passage, accompanied with the drop in hydraulic performance of the pump. The results of the combination of the high speed photography and pressure fluctuation present that the cavitation structure is consistent with the transient pressure of the monitoring points. The corresponding relationship between the transient pressure and the cavitation images is determined by that the suction surface of the blade is the lowest pressure point at the circumference of the monitoring point. From suction side to pressure side of the adjacent blade, the pressure increases continuously. The blade tip cavitation area is the low pressure zone, and under large flow rates, the tip leakage vortex appears to be a long and narrow range of low pressure. With the decrease of flow rate and cavitation number, the tip leakage vortex and blade tip are connected to form a triangle cavitation cloud, which is a broad low pressure area. The SSPCVs shed from the trailing edge of the cloud cavitation have an important influence on the pressure field at the blade tip. The formation of the SSPCVs is at the cost of the reduction of cloud cavitation area, resulting in a decrease in the range of the low pressure. In the process of moving to the adjacent blade pressure surface, the SSPCVs are mixed with the trailing edge of the tip leakage vortex, which causes the fluctuation in the process of pressure recovery. When a SSPCV moves away from the blade tip and dissipates gradually, its influence on the pressure field at the tip of the blade is weakened and eventually it almost no longer has any effect.