Abstract
Asymmetric cavitation is known as one type of the sources of cavitation induced vibration in turbomachinery. Cavity lengths are unequal on each blade under condition of synchronous rotating cavitation, which causes synchronous shaft vibration. To investigate the relationship of the cavity length, fluid force, and shaft vibration in a cavitating inducer with three blades, we observed the unevenness of cavity length at the inception of synchronous rotating cavitation. The fluid force generated by the unevenness of the cavity length was found to grow exponentially, and the amplitude of shaft vibration was observed to increase exponentially. These experimental results indicate that the synchronous shaft vibration due to synchronous rotating cavitation is like selfexcited vibrations arising from the coupling between cavitation instability and rotordynamics.
Highlights
For a turbopump, an inducer is placed upstream of the main impeller to obtain high suction performance, and it is one of the key components of a liquid propellant rocket engine
The experiment was conducted in the Cryogenic Inducer Test Facility (CITF) at Kakuda Space Center of the Japan Aerospace Exploration Agency (JAXA)
We focus on the growth of the cavity length at the inception of synchronous rotating cavitation in detail to examine its mechanism
Summary
An inducer is placed upstream of the main impeller to obtain high suction performance, and it is one of the key components of a liquid propellant rocket engine. Under some operating conditions, cavitation instabilities such as rotating cavitation or cavitation surge are observed [1, 2] In addition to these cavitation instabilities, synchronous rotating cavitation with uneven cavities on each blade is often observed in three-bladed inducers at relatively low cavitation numbers. Synchronous rotating cavitation results in severe synchronous shaft vibration and causes degradation of suction performance due to an asymmetric cavity pattern [2,3,4,5]. Fujii et al [7] examined such an unsteady phenomenon at lower cavitation numbers through measurements of the pressure fluctuation and blade stress fluctuation in which the pattern of the cavity on each blade changes irregularly while the overall asymmetric pattern is maintained. We estimated the fluid force on the inducer based on measurement of unsteady pressure distributions and compared it with the amplitude of the shaft vibration
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