The fundamental characteristics of rotordynamic fluid force moment on the backshroud of a Francis turbine runner in precession motion were studied using model tests and computations based on a bulk flow model. The runner is modeled by a disk positioned close to a casing with a small axial clearance. An inward leakage flow is produced by an external pump in the model test. The effects of the leakage flow rate, the preswirl velocity at the inlet of the clearance, and the axial clearance on the fluid force moment were examined. It was found that the fluid force moment encourages the precession motion at small forward precession angular velocity ratios and the region encouraging the precession motion is affected by the preswirl velocity. Through the comparisons of the fluid force moment with and without the rotation of the disk, it was found that the normal moment without the disk rotation did not have the effect to encourage the precession motion. Thus, the swirl flow due to disk rotation was found to be responsible for the encouragement of the precession motion.

The rotordynamic fluid forces acting on a closed type impeller in whirling motion were measured and the influence of the clearance geometry on the stability of the impeller was examined. At small positive whirling speed, the rotordynamic forces acted as destabilizing forces for all casings. A small clearance between the shroud of the impeller and the casing caused large fluid force, but did not change the destabilizing region. Radial grooves in the clearance were effective for reducing the fluid forces and destabilizing region due to the reduction of the circumferential velocity without the deterioration of the pump performance. A rotating phenomenon like a rotating stall of the impeller occurred at low flow rate and the resonance between it and the whirling motion led to a sudden increase in force at the whirling speed ratio of 0.7.

The rotordynamic fluid forces on a closed type centrifugal impeller in whirling motion were measured with a vaneless diffuser at various flow rates for three types of casing, with the casing/impeller shroud clearance of 2[mm], 5[mm], and 5[mm] with seven radial grooves to break the flow swirl. The rotordynamic fluid forces are destabilizing within small positive whirl speed ratio Ω/ω, where Ω and ω are a whirling speed and a rotational speed of the shaft, respectively. The radial grooves were effective to reduce the fluid forces and the destabilizing region due to the reduction of the circumferential velocity of the clearance flow. At low flow rates, the fluid forces increased at Ω/ω, ≈ 0.7. By the measurement of unsteady pressure on the casing wall and the CFD analysis of flow field in the pump, it was found that the large fluid forces at Ω/ω ≈ 0.7 were caused by the rotating stall of the impeller. The rotating stall was caused by the backflow along the suction surface of the main blade due to the separation vortices near the middle of the chord.

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