# Dynamic Instability Analysis of a Double-Blade Centrifugal Pump
## 1 Introduction  
Double-blade centrifugal pumps are widely used to transport liquids containing solid particles, for instance, wastewater treatment, chemical process and liquid food transportation. The impeller with two blades can be capable of handling the liquids with large solid particles or long fibers, due to its larger flow passages [1]. However, compared with multi-blade centrifugal pump, the internal flow instability of the double-blade centrifugal pump is more serious, and the corresponding slip is increased, because the flow control capability of the impeller is decreased due to its large passage. From an engineering viewpoint, to extend the operating lifetime of double-blade centrifugal pump, we should focus on lowering the vibration level of the pump. Typically, the vibration generated in a centrifugal pump consists of tonal components and broadband components. Tonal components are often dominated by the tones at blade passing frequency and its harmonics [2; 3; 4]. This is a consequence of the strong interaction between the impeller and tongue of volute [5; 6]. Additionally, broadband vibration is generated by flow separations on the impeller blades or pump volute, turbulence boundary layers, back flow and vortices [7; 8; 9].  
Studies on the flow instability of centrifugal pumps are numerous. In the early stage, [PERSON] et al. [10] pointed out that pressure pulsations in centrifugal pump are created by the unsteady flow, for instance, the wake flow from impeller blade trailing edge, the large-scale turbulence and vortices generated by flow separation and flow recirculation at part load conditions. Afterwards, [PERSON] et al. [11] adopted the numerical methodsto investigate the pressure pulsations and unsteady flow inside the pump, and a better agreement between computational fluid dynamics (CFD) data and experimental data was achieved. [PERSON] et al. [12] carried out the study on flow instability of a centrifugal pump, and a correlation between the fluid dynamics and the emitted noise was investigated. It was identified that a fluid-dynamical origin is connected with the jet-wake phenomenon. Recently, the flow instability of a multistage centrifugal pump at off-design conditions was investigated by [PERSON] et al. [13]. Pressure pulsations at diffuser inlet were measured and rotating stall phenomenon was observed with CFD tools. In addition, some studies were carried out to do the deep analysis on the unsteady flow phenomena in centrifugal pump, for instance, flow separation [14; 15; 16], rotating stall [17; 18; 19] and recirculation [20; 21]. Those directly cause the formation of flow instability, pressure pulsations and pump vibration.  
Although much work has been done on flow instability of centrifugal pump, there is sparse literature available to analyze the flow instability and vibration characteristics of a double-blade centrifugal pump. Thus, the objectives of this study are: (1) identify the unsteady flow phenomena and pressure pulsations of the double-blade centrifugal pump; (2) evaluate the vibration characteristics and its relationship with flow instability; (3) explain the flow instability generating mechanism of double-blade centrifugal pump.