• Cavitation
• Cavitation instabilities
• Fluid-induced rotordynamic forces
• Forced whirl
• Inducer
• Turbopump

Liquid-propellant rocket engines need high power density turbopumps to provide pressurized propellant for the combustion and to reduce overall system weight. Operation under limited cavitation and sometimes at supercritical speeds, is usually tolerated in order to maximize the performance, exposing however the pumps to the risk of developing dangerous cavitation-induced fluid dynamic and rotordynamic instabilities and, in turn, possible catastrophic failures. Therefore, inducers are often employed in liquid rocket turbopumps to produce the initial pressure rise necessary to prevent the cavitation in the main stages. Consequently, these components are exposed to cavitation under normal operating conditions, which is an intrinsically unstable phenomena leading to flow instabilities. As a consequence of the complex three-dimensional internal flow, the effects of upstream and downstream flow distortions and cavitation affect the fluid-induced rotordynamic forces, which arise when a whirling inducer interacts with the working fluid. As typically highlighted in the experimental results, the rotordynamic forces in inducers are significantly dependent on the flow coefficient and do not present a quadratic behavior with respect to the whirl ratio, as commonly reported in radial pumps. These forces become destabilizing for both forward and backward whirl and were found to increase with decreasing cavitation number. Therefore the prediciton of rotordynamic forces in high speed turbomachinery is important in order to evaluate the dynamic instabilities of inducers and to avoid failure.
The present research thesis develops an analytical dynamic model of the rotordynamic system for the assessment of the influence of fluid-induced rotordynamic forces on the dynamics response of axial inducers under cavitating/non-cavitating condition. The dynamic model is applied to the Cavitating Pump Rotordynamic Test Facility based in Sitael for the experimental campaign intended to be carried out on the inducer designed by Massachusetts Institute of Technology (MIT). The rotordynamic analysis is performed on two selected rotor configuration A and B through a forced response test, which provides an imposed circular whirl motion to the inducer shaft throughout the range of whirl frequencies. After modeling both rotor systems with Direct Stiffness Method the implementation of fluid-induced forces in the inducer dynamics has been proposed through the so-called F.I.R.F. method. Consequently, the open loop analysis is conducted where whirl natural frequencies and the relative mode shapes are found to be strongly dependent on the bearings stiffness. A parametric study of how critical frequencies vary with respect to bearings siffness is done. The differences found between the free-forced analysis carried out in the dry case and in the wet case, considering the fluid-induced forces, have been summarized in Campbell diagrams and in whirl speed maps. Finally, due to forced motion, a whirl response analysis is conducted in order to estimate the stability and the time position of the rotor system considering all the loading conditions. The time varying lateral deformation of the inducer allows to evaluate the tip clearance and, consequently the possible onset of rotating cavitation instability. The proposed results show how the deformation of the inducer station is greatly influenced by the trend of the fluid-induced forces and, as expected, the greater deformations are evaluated where the tangential and normal component of the rotordynamic forces present the destabilization peaks under cavitating condition.