Design of axial-flow impeller guide cone and simulation on hydraulic performance of its passage

Abstract: Axial flow pumps have advantages of large capacity and low head and the impeller is an important component of axial-flow pump. Guide cone is usually installed on the top of impeller and its appropriate design can enhance flow quality of pump inlet, lower turbulivity, make velocity steady, and so on. To meet with engineering demands, find feasible design and investigate the internal flow characteristics of guide cone, we designed different types of guide cones installed on the impeller. Based on three dimensional incompressible Navier-Stokes equation and k-ε turbulent model, SIMPLEC algorithm was applied to solve a discretization governing equation, five different types of guide cones were designed with contraction curves such as Witozinsky, Bicubic, Fifth degree polynomial curves. The CFD method was used to simulate 3D flow field of guide cone. In order to verify the feasibility of simulation models, the guide cones were installed on the impeller and the simulated head and efficiency values were obtained using simulation method same as the flow field simulation of guide cone. Meanwhile, a laboratory test was performed on a DN200 test bench to measure the pumping head, discharge, and other parameters for calculation of head and efficiency. Results showed that the simulated and measured head and efficiency had relative error less than 4%, indicating the feasibility of the simulation method for flow field simulation of guide cones. Simulation on flow velocity of guide cones suggested three flow processes: 1) flow velocity is even in inlet passage and slightly increased; 2) the flow velocity starts to increase and change its direction in contraction passage of guide cone flow field; and 3) the flow field contraction becomes slow in the outlet passage of guide cone. Hydraulic loss of different guide cones varied. The head loss of circular cone was lower than the others. In the sharp contraction cross section of guide cone passage, the uniformity of axial velocity distribution was low, but the velocity weighted average drift angle and radial velocity gradient was high. The rectification capability was better when the contraction at outlet section of guide cone flow field was slow and gentle. Taking into account of uniformity of axial velocity, velocity weighted average drift angle, and the others, the guide cone based on Witozinsky curve had the best rectification capacity and better flow field uniformity. When the length of guide cone was 0.25-0.8 times as impeller diameter, increasing cone length could decrease the hydraulic loss and velocity weighted average drift angle, and improve flow field quality of cone. The results above in combination with practical application, we suggested that the optimal length of the guide cone was 0.5-0.6 times as impeller diameter. This study is helpful to design hydraulic models of high-efficient axial-flow pumps.