Analysis of Rotordynamic Stiffness Forces in a Mixed-Flow Pump With a Shrouded Impeller

Abstract

Abstract. Pump impellers can become eccentric relative to their casings for various reasons, including manufacturing errors and component degradation, and such geometric eccentricity can generate rotordynamic forces. Yet, rotordynamic forces in mixed flow pumps have not been investigated. Therefore, this study investigates a mixed-flow pump—including the suction pipe, shrouded impeller, diffuser, and discharge volute—for the first time using steady 3D Reynolds-averaged Navier–Stokes (RANS) simulations. Concentric RANS simulation results have been validated against experimental data, and two eccentric models—one with eccentric seal but concentric impeller (Model 1) and another with both eccentric seal and eccentric impeller (Model 2)—have been simulated. In both eccentric models, positive cross and direct forces are generated, mostly due to the nonaxisymmetric pressure inside the seal path. The cross forces from both models are comparable in magnitude and arise due to circumferential mass flow redistribution inside the seal. Model 2's direct force is about 1.5 times that of Model 1. This difference in the direct force is due to the tangential flow redistribution downstream of the impeller when the eccentric primary flow path is taken into account (Model 2). Therefore, the nonaxisymmetries in both primary and secondary flow paths need to be simulated to obtain accurate rotordynamic force predictions for more rigorous stability analysis.