Static direct and cross-coupled forces due to lateral offset in a mixed flow pump

A pump’s impeller could become eccentric relative to its casing and this geometric eccentricity can generate rotordynamic forces. However, studies of this problem in mixed-flow pumps are scarce. In this study, a complete mixedflow pump system, which includes a suction pipe, shrouded impeller, diffuser, and discharge volute, has been investigated using three-dimensional steady Reynolds-averaged Navier-Stokes (RANS) simulations. RANS simulation results with a concentric impeller (Model C) have been validated against experimental data. Two eccentric models - one with eccentric seal and concentric impeller and (Model 1) and another with both eccentric seal and eccentric impeller (Model 2)–have been simulated. Numerical results show that geometric eccentricity leads to a non-axisymmetric pressure distribution inside the seal cavity. For Model 1, the direct force is positive, and its magnitude is approximately 2.5 times larger than that of the cross force. For Model 2, direct stiffness force is also positive, but its magnitude is approximately 6 times larger than that of cross stiffness force. Thus, the direct force of Model 2 is approximately 30% higher than that of Model 1. This difference between the two eccentric models is likely due to the upstream inlet flow redistribution in the tangential direction as well as non-axisymmetric impeller torque in Model 2. Therefore, it is important to consider non-axisymmetric primary flow path as well as non-axisymmetric secondary flow path for rotordynamic analysis of mixed-flow pumps.