The pressure fluctuations acting on the impeller and the impeller vibration excited by the fluctuations were investigated in a centrifugal pump with several vaned diffusers. Firstly, the pressures both acting on the impeller and in the volute were measured simultaneously. It was demonstrated that both the volute static pressure and the pressure fluctuations on the impeller are uneven circumferentially (not uniform) under off-design operating conditions. At high flow rates, the pressure fluctuations are large in the front of the volute exit, where the static pressure is low. Secondly, the impeller vibration excited by the fluctuations was measured. It was found that resonance can be excited even when the resonance condition of rotor-stator interaction is not satisfied and sidebands occur in frequency spectra because of the circumferential unevenness of the fluctuations. Resonance can also be excited when a natural frequency of the impeller coincides with the frequencies of sidebands. Furthermore, the vibration wave is a traveling wave in the circumferential direction when resonance of a nodal diameter mode is excited, and the resonant frequency may depend on the traveling direction of the wave due to the inertia effect (added mass) of water.
The pressure fluctuations and the radial fluid forces induced by rotor-stator interaction in a centrifugal pump were measured and their relationship was investigated. Experiments were done for various guide vanes, flow rates, and rotating speeds. It was demonstrated that both the blade pressure fluctuations and the volute static pressures are non-uniform circumferentially (not axisymmetric) under off-design operating conditions and that the two have a strong relationship. At high flow rates, the interaction-induced blade pressure fluctuations are large in areas where the volute static pressure is low. The propagating directions of the pressure fluctuations, the whirling directions of the radial fluid forces acting on the impeller and the dominant frequency components of both the fluctuations and the fluid forces are discussed. When measuring the fluid forces in the rotating frame, other frequency components, in addition to those related to the products of the number of guide vanes and the rotating frequency, may occur due to the circumferential unevenness of the pressure fluctuations.
This paper reports on the complex phenomena of pressure fluctuations and vibrations in a large-capacity drainage pump station. Significant pressure fluctuations were observed in suction water tunnels when an axial flow pump was operated without an actual head at some blade angles. To identify the causes, investigations were done by measuring acoustic natural frequencies and pressure fluctuations in the tunnels, lateral and torsional vibrations of the pump shaft, and pressure fluctuations of the oil in the hydraulic system to control the blade angle. The measurements were taken for different blade angles, rotating speeds, as well as acoustic natural frequencies. The natural frequency was changed by inserting air into the suction tunnels with a compressor and by setting air bags. The results showed that acoustic resonance occurred in the tunnels, but it was not a simple resonance. The dominant frequency, which was neither the blade passing frequency nor its higher harmonics, depended on rotating speed in the reverse way: it decreased when rotating speed increased, and vice versa. Pressure fluctuations in the water tunnels and lateral/torsional vibrations of the pump shaft had a strong relationship. However, they had different dominant frequencies and occurred at different blade angles. Several measurements were made in different seasons and it was found that the phenomena were season-dependent (dependent on quality of water). The causes are discussed. It is believed that the excitation source was vortex shedding from the blades, which locked into the acoustic resonance in the water tunnels via vibration of the blades.
Asymmetric cavitation, in which cavity lengths are unequal on each blade, is known as a source of cavitation induced shaft vibration in turbomachinery. To investigate the relationship of the uneven cavity length and rotordynamic force in a cavitating inducer with three blades, we conducted two experiments. In one, the growth of cavity unevenness at the inception of synchronous rotating cavitation in cryogenic flow was observed, and in the other, the rotordynamic fluid forces in water were examined by using a rotordynamic test stand with active magnetic bearings. Rotordynamic performances were obtained within a wide range of cavitation numbers and whirl/shaft speed ratios included super-synchronous/synchronous rotating cavitation. These experimental results indicate that the shaft vibration due to the rotating cavitation is one type of self-excited vibrations arising from the coupling of cavitation instability and rotordynamics.
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