Flow Field Instability and Rotordynamic Impedances for an Open Impeller Centrifugal Pump in Transient Four-Quadrant Regimes

Abstract

Abstract The American Petroleum Institute (API) level 2 rotordynamic stability analysis requires determination of possible destabilization forces on a compressor or pump impeller. Dynamic forces in transient regimes are often excluded although a turbomachine impeller may experience transient operation intentionally or accidentally. The centrifugal pump head, flow direction, rotation and torque can be both positive and negative in transient regimes. For example, in a renewable energy application, pump flow direction and rotation are reversed to generate power from the imposed fluid head. The complete characteristics of a centrifugal pump correspond to all four quadrants (4Q) of operation, to encompass all possible operating conditions. It is required to understand centrifugal pump impeller dynamic forces and rotordynamic responses for all 4Q for design, fault diagnostic, instability analysis, upset conditions (such as water hammer, surge etc.) and for reliable operation of high energy density machines. In the open literature, whirling impeller rotordynamic analyses appears only for normal pump operation. Centrifugal pump dynamic forces, rotordynamic impedances and flow instabilities of an open impeller are reported for 4Q operating regimes in this paper. A transient Computational Fluid Dynamics (CFD)-based model is implemented which is applicable to nonaxisymmetric turbomachinery components, such as with a volute and/or vaned diffuser. Whirling motion of the impeller is modeled by imposing mesh deformation at the impeller walls. A phase modulated multi-frequency mesh deformation method is imposed for better numerical efficiency. Reynolds Averaged Navier-Stokes (RANS) equations with the Shear Stress Transport (SST) turbulence model along with a transitional bypass turbulence model are employed for the CFD solution. The results show the underlying flow field instability and stall cells responsible for the impedance shapes. Furthermore, the model is employed for determining the dependence of the outputs on specific speed to extract rotordynamic forces more efficiently. Impeller dynamic forces are found to scale with the size of the impeller for the same eccentricity ratio and the same flow coefficient. Strength of impeller rotating stalls has dependence on whirl frequency ratio.