Naoki Nagao

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.

Effect of microstructure on high-cycle fatigue properties of Alloy718 plates

Effect of microstructure on high-cycle fatigue properties of Alloy718 were investigated at 77 K by using samples with three different microstructures; fine-grained (FG), coarse-grained (CG) and bimodal-grained (BG) ones. The BG sample consisted of FG and CG microstructural regions and grain sizes of those regions were close to those of the FG and the CG samples, respectively. High-cycle fatigue strength of the FG sample was higher than that of the CG sample. High-cycle fatigue strength of the BG sample was clearly lower than that of the FG sample and almost the same as that of the CG one. Flat area (facet) was found at fatigue crack initiation site in all specimens. Facet size was similar to the grain size and found to be almost same in the CG and the BG samples. Observations of the microstructure beneath the fatigue crack initiation site of the BG sample revealed that the facet corresponds to transgranular cracking in the course grain, meaning that fatigue crack initiated at the coarse grain in the BG sample. It is deduced that the high-cycle fatigue strength of Alloy 718 with the BG microstructure is strongly affected by that of the CG region in that material.

Comparison of Rotordynamic Fluid Forces Between Closed Impeller and Open Impeller

In the design of rocket turbopump with high energy density, rotordynamics considerations are very important to stabilize the motion of the shaft. Rotordynamic instabilities generated by rotordynamic fluid forces are often reported. It is recognized that the forces affect the whirling motion of the rotor, and induce the self-excited vibration. To understand the effect of the rotordynamic fluid forces of the impellers on rotor vibration, a tangential fluid force in a forward whirling direction and a normal fluid force in a radial direction were measured by using the experimental apparatus with an active magnetic bearing. As the experimental result for a closed impeller and an open impeller, it was clarified their fluid forces are quite different in the characteristics of destabilizing/damping effects and restoring/inertia effects.© 2012 ASME