Efficiency, reliability and cavitation are three important indicators to evaluate the performance of the centrifugal pump. Thereinto, cavitation is considered as the "cancer" in the pump field which has not been well solved for a long time [1-3]. The essence of cavitation flow is gas-liquid two-phase turbulent flow, including quite complex mass transfer, momentum transfer and energy exchange between bubbles and water. Cavitation of pump not only has great influence on pump efficiency, but also causes serious damage to materials. The most important research work for cavitation of centrifugal pumps is how to evaluate the state. The conventional energy method to judge the cavitation inception of a centrifugal pump is based on the drop of 3% of the head. It is regrettable that there is a great lag in this method. However, other methods of determining cavitation cannot guide the engineering application as well for the low sensitivity and poor universality. With the development of computer technology and computational fluid mechanics, the method of establishing the relationship between cavitation and the distribution of cavitation flow field, the regulation of the generation and the development of bubbles, etc. by CFD has been widely concerned [4-8].
Tan  has studied the internal transient flow of centrifugal pump under cavitation, and found that the motion of cavitation bubbles in impeller presents periodicity, and the dominant frequency of induced pressure pulsation changes from blade frequency to low frequency in comparison with the non-cavitating conditions. MM Athavale  carried out numerical simulation of cavitation-flow in hydraulic jet propulsion pump, centrifugal pump and turbopump, etc. by using the Singhal cavitation model. The results show that the occurrence of cavitation is always on the suction surface at the leading edge of blade. Li et al.  established a closed loop to analyze the relationship between the cavitation performance and the suction pressure signals. The static parameters used can only capture the critical cavitation condition and cavitation damage condition, whereas they are difficult for the detection of incipient cavitation in the pump. In addition, cavitation produces complex vortex structures inside the flow field, which enhance the interaction between turbulent flow and cavitation . Friedrichs and Kosyna  discussed the effects of the cavitation number and the incidence angle of the blade on rotating cavitation through an experimental investigation of two similar centrifugal pump impellers of low specific speed.
The external characteristics of centrifugal pump and the change in the shape of cavitation bubbles attached to the impeller at varied cavitation numbers were conducted by Li et al. . It is pointed out that the size of cavitation bubbles increases with the decrease of cavitation number and so does the decline in performance of a centrifugal pump. The application of the cavitation model has been implemented to investigate the internal relationship between the force of the impeller and cavitation . The obtained results show that the axial force increases with the development of cavitation, however, the radial force changes little with cavitation under rated conditions, instead of reducing in low flow rate. Pouffary et al.  analyzed the variations of head and energy conversion process of the centrifugal pump under cavitation conditions, and found that the head decreases with reduction of power and blade load.
It should be noted that cavitating flow is complex multiphase turbulence , and the vortex structure can enhance the turbulent intensity of flow field . Thus, the complex vortex structure generated by the cavitating flow contributes to the better interaction between turbulent flow and cavitation bubble . Gopalan and Katz  studied enclosed area of the nozzle exit by using Particle Image Velocimetry and high-speed photography system. It is found that the collapse of the vapor cavity in the enclosed area is the main mechanism of vorticity, besides, the hairpin structure vortex generated by collapse of the cavity will dominate the flow in the downstream of the cavitation region. The simulation results of the unsteady flow of cavitation in centrifugal pump show that the cavitation has a pulsating characteristic under off-design conditions. Such phenomenon involves the vortex- induced response and the non-uniform pressure distribution on the volute . Centrifugal pumps easily generate cavitation when running off-design conditions. The development of cavitation has an effect on the load distribution on blade. The head, pressure pulsation and radial force are influenced by the load distribution . In addition, the load distribution on blade also changes the blade angle at the leading edge and affects the cavitation performance of pump . Experimental investigations on pressure pulsation induced by cavitation in double suction centrifugal pumps were carried out by Yao et al. . The results show that the amplitude of pressure pulsation at the shaft frequency and the special low frequency of the impeller increases first and then decreases with the development of cavitation.
Based on the available literatures about cavitation, the researchers were concerned with the following important questions: how to analyze cavitation stages of centrifugal pump, which signal treatment to use (in the frequency or time domain), and which parameter to calculate in order to quantify the degree of erosive cavitation. Few scholars have summarized the characteristics of the unsteady flow of cavitation in centrifugal pump based on a reasonable criterion for cavitation stages. Therefore, the primary work of the paper is to build a visual closed test-bed in order to judge the cavitation stages of centrifugal pump combined with high-speed photography. Then, the most applicable model is selected for the computation of cavitating-flow in centrifugal pump by comparing different turbulence models and cavitation models. Finally, the regularity of distribution for vortex structure, pressure pulsation and radial force distribution on the impeller under different cavitation stages are concentrated on to support the decision of different cavitation stages.