隙間流れによる円柱型ロータの自励振動

抄録

In a Francis turbine, a super-synchronous self-excited vibration of the rotor was observed. According to our previous study, it was considered that the vibration was caused by the inertia of the leakage flow between a front shroud and a casing when a seal was set at the upstream of the leakage flow. In the present study, characteristics of vibrations and the rotordynamic fluid forces acting on the rotor were investigated to clarify an effect of the seal set at the downstream of the leakage flow. Experiments and computations were performed with a columnar rotor modeled on a front shroud of the Francis turbine. In experiments, it was found that a smaller clearance of the downstream seal had a larger damping effect on vibrations. In computations, it was shown that the rotordynamic fluid force had a damping effect on the whirling motion in the case with the downstream seal and the damping effect increased as the clearance of the downstream seal decreased. It was also shown that the absolute value of the tangential fluid force decreased as a width of a side chamber increased.