Rotordynamic Force Prediction of a Shrouded Centrifugal Pump Impeller—Part I: Numerical Analysis
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Computational fluid dynamics (CFD) is employed to calculate the fluid-induced forces in the leakage path of an incompressible shrouded centrifugal impeller. Numerical solutions of the whirling shrouded impeller at the centered position provide the radial and tangential impedances that can be modeled as a quadratic function of whirl frequency. Calculated impedance results on a face-seal impeller can be modeled with a second-order function of whirl frequency and the predicted rotordynamic coefficients show reasonable agreement with the test results by Bolleter et al. (1989, "Hydraulic Interaction and Excitation Forces of High Head Pump Impellers,"Â "Pumping Machinery: Third Joint ASCE/ASME Mechanics Conference, La Jolla, CA, pp. 187-194). However, the present analysis shows that the calculated impedance curves for a conventional wear-ring seal impeller using three-dimensional (3D) numerical approach have "bumps" and "dips" which can be observed in the bulk flow analysis of impeller models by Childs (1989, "Fluid-Structure Interaction Forces at Pump Impeller-Shroud Surfaces for Rotordynamic Calculations," J. Vib., Acoust., Stress, Reliab. Des., 111(3), pp. 216-225). After reviewing previous predictions and experiments that showed the bump and dip in the computed and measured forces, the current study focuses on the estimation of the possible sources causing the peaks. The selected possible sources are inlet tangential velocity at the shroud entrance, flow rate of primary passage, shape of the shroud leakage path, and seal clearance. The fluid-induced forces of the conventional wear-ring seal impeller are calculated and analyses on the effects of these variables are provided.
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