Effect of Discharge Elbow on Performance of the Axial Flow Pump in Saddle Zone

The discharge elbow is an important part for the axial flow pump system, and the saddle area on the performance curve is a special characteristic of the axial flow pump. In order to reveal the influence of discharge elbow on the saddle zone, the saddle zone performance of an axial flow pump with two different discharge elbows (90° and 60°) is described in this paper by simulation and test. First, the effect of discharge elbows on pump energy performance is analyzed. The results indicate that the saddle zone of model pump with different discharge elbows is always between 0.5QBEP and 0.6QBEP (QBEP represents flow rate at the best efficient point), and the head value reaches the minimum at 0.55QBEP. Then, the analysis on pressure pulsation of different models is shown. The results show that compared to 90° discharge elbow model, the peak to peak value of pressure pulsation of 60° discharge elbow model at impeller inlet and pump outlet all reduces significantly under each flow rate. When the discharge elbow is 90°, the main frequency of the pressure pulsation at impeller inlet and pump outlet is the blade passing frequency fp, and many low-frequency pulsation components appear in saddle zone. When the discharge elbow is 60°, the main frequency of pressure pulsation at pump outlet changes from the blade passing frequency fp to the axial passing frequency fn. Finally, a focus by the numerical simulation about the internal characteristics is given. It is found that a small amount of vortex occurs in these two discharge elbows at saddle zone condition while the high speed area appears in pump outlet extend. After the 90° discharge elbow is replaced by 60° discharge elbow, the streamline distribution becomes more uniform and the hydraulic loss caused by the vortex decreases.