The internal flow field of an axial-flow pump under low flow rate conditions is extremely turbulent.The unstable flow region is formed due to the tip leakage and the rotating stall,and is distinguished by a saddle patterned zone in its flow-head curve that demonstrates restrictions in its workable flow range.It is therefore important to understand and improve the operational characteristics of an axial-flow pump under low flow rate conditions.In this study,experiments are performed for the performances of an axial-flow pump at the flow rate in a range between 0.8Qd and 1.2Qd,with the internal flow field measured by the particle image velocimetry (PIV),and with the pump performances and the internal flow field simulated by the Ansys CFX.The simulation results agree well with the experimental results.From the predicted heads at the flow rate in the range between 0.1Qd and 0.7Qd by the numerical simulation,the complete flow-head curves are obtained.The streamlines and the velocity contours in the region in front of the impeller leading edge under different flow conditions are analyzed.By adopting the double-inlet structures,the flow-head characteristics are studied by varying the values of δ and σ respectively,where δ denotes the gap between the inner cylinder end and the impeller leading edge,and σ denotes the gap between the inner cylinder and the outer cylinder.The findings indicate that with the double-inlet design,the velocity distribution can be effectively improved in the region in front of the impeller leading edge,as well as the head performance under the low flow rate conditions,and the positive slope value of the head curve is reduced in the unstable flow range.The optimal δ and σ values are determined.