• Cavitation-Induced Instabilities
• Cavitation
• Turbopump
• Inducer
• Forced Whirl
• Rotordynamics

Turbopumps is a key component of the feeding system for high thrust, liquid rocket engines such as those used in launch vehicles. This component is used mainly because of its capability of generating the large pressure rise requested for feeding the combustion chamber with propellants and because of its high power density capable of meeting the stringent requirements of high performance and low weight. However, turbopumps are prone to develop flow instabilities caused by cavitation which may lead to catastrophic failure of the space vehicle. Axial inducers are the component of a turbopump that operates under cavitating conditions and thus are more subjected to develop cavitation-induced instabilities. Detailed investigation on these instabilities relies inevitably on experimental approaches due to the complexity of the phenomena involved. New experimental methods borrowed from research activity on aeroengine compressors begin to be implemented on cavitating inducers to investigate instabilities with a different point of view.
The present thesis aims at designing a forced response test which utilizes an imposed whirl motion of the inducer shaft, with variable rotating speed, as a way of exciting azimuthally the flow and perform the identification of cavitation-induced rotating instabilities. The design is carried out for the Cavitating Pump Rotordynamic Test Facility based in Sitael for the experimental campaign on a mixed-flow inducer designed at the Massachusetts Institute of Technology. Based on test requirements, a first preliminary design of the test section and sensor arrangement is set up. It is found out that, with the present configuration of the test rig is not possible to match the required cavitation number. Conceptual study and design of a system which provides the right pressure at inducer inlet is therefore carried out. Detection of instabilities will be performed with azimuthal arrays of velocity and pressure sensors placed upstream and downstream of inducer. Fiber film probes are used to measure the velocity field. Estimation of the occurrence of cavitation on these sensors is carried out showing a fully wet flow up to inducer breakdown. Due to large deformation of inducer shaft, different versions of the designed setup are proposed and then subjected to trade-off study. After that, two design configurations are identified for the first implementation of the forced whirl experiment concept. The first one is denoted as the simplest application of the idea of forced experiment, called configuration A, allowing characterization on cavitation instabilities in terms of frequency spectrum and spatial mode. Configuration B extends the capability of investigating cavitation phenomena also from the standpoint of rotordynamics by measuring forces in two-phase and fully-wet flow conditions.
The rotordynamic analysis of the two configurations is carried out to assess nominal operation of the experiment throughout the range of whirl frequencies. The rotor systems are modeled with Direct Stiffness Method and the inclusion of a dynamic model of the dynamometer is carried out. Solutions of the natural whirl frequencies and mode shapes are found to be strongly dependent on bearing stiffness. Parametric study is done to assess subcritical operation of the rotor for both A and B configurations even in presence of parametrically assigned value of bearing stiffness. The models of the rotor system is used also to analytically model the imposed whirl motion needed for successive activities in the dynamic modeling framework of the inducer. The imposed motion is described with use of static constraint modes. The model is then used to preliminary estimate the rotor responses to the various loads and to verify that the inducer displacement stays within the radial tip clearance.