Hydrodynamic forces in centrifugal pump and compressor impellers in volute casings: Measurements using magnetic bearings and CFD simulations

In this thesis the radial impeller forces on centrifugal impellers are examined by direct measurements on a pump using magnetic bearings (as load cells) and CFD simulations where both pumps and compressors are modeled. The hydrodynamic forces result from the interaction effects that occur between the radial exit of a centrifugal impeller and the collector housing into which the fluid is discharged. A magnetic bearing supported centrifugal pump research apparatus and impeller force measurement techniques using the magnetic bearings as active load cells was developed. Detailed magnetic bearing characterizations are documented. Direct impeller force measurements were made for a variety of impeller and casing geometries. The impeller position stiffness (change in force due to change in relative static impeller-to-volute position) was measured for several impeller-volute combinations. The static position stiffness measurements were compared with dynamic stiffness measurements and were found to be substantially equivalent. A whirl orbit controller was developed and implemented on the apparatus for the purpose of imposing precise user specified orbits on the impeller. Impeller interaction forces due to whirl were measured. The computational effort was conducted on the research pump geometry using TASCFlow, a commercially available CFD code. The model was validated and benchmarked against pressure and laser velocimetery test data for incompressible flows. Impeller forces were evaluated using the CFD and compared with the direct force measurements. The working fluid, in the CFD model, was switched to air and a numerical study of impeller forces in a compressor was made to show the effect of compressibility. The simulations showed the impeller forces to be strongly dependent on Mach number.