Rotordynamic Forces Acting on a Two-Stage Inducer

Rocket engines are required to provide higher power with lighter weight, and thus the rotational speed of turbopumps of the engines generally become higher than the first critical speed of the rotor. Therefore rocket engine turbopumps sometimes caused self-excited shaft vibration which was generated with the rotordynamic forces induced by a whirling motion of the rotor. Because of the higher power level and higher rotational speed, the rotational stability of rocket engine turbopump is an important factor to increase the reliability. Yoshida et al. measured rotordynamic forces acting on three-bladed inducer under supersynchronous/synchronous rotating cavitation[1]. The rotating cavitation generate unbalance of the rotordynamic and cause a vibration on the shaft. The shaft vibration varies the clearance between the blade tip and the inner wall and promotes the cavitation because the tip clearance is one of the factor to increase the cavitation. The phenomena is a problem of coupled hydrodynamics with rotordynamics. Pasini et al. measured rotordynamic forces on a four-bladed inducer which are varied with different operational conditions such as flow coefficient, fluid temperature and cavitation number[2]. Both reports show strong nonlinearity of rotordynamic forces against the vibration frequency because of cavitation occurrence. In this way, the rotordynamic forces on a cavitated inducer caused a shaft vibration and it is difficult to foresee the value because it has a nonlinearity. Establishing a method to measure the rotordynamic forces of a cavitated inducer is a key technology to design the rotor and suppress the shaft vibration. We have developed the equipment which is able to measure the rotordynamic forces. In this report, we present the specification of the equipment and show the measured data of a two-stage inducer as an example.