Tesi di dottorato di ricerca
Pumping Performance Similarity, Cavitation-Induced Instabilities and Fluid-Induced Rotordynamic Forces in Tapered Inducers
Settore scientifico disciplinare
Corso di studi
tutor Prof. d'Agostino, Luca
Data inizio appello
A comprehensive experimental campaign has been carried out on two highhead, mixed-flow unshrouded inducers with tapered-hub and variable-pitch. Both the pumps, called DAPAMITO inducers, have been designed by means of a reduced order model and procedure reported in this thesis. The model is based on the incompressible, inviscid, irrotational flow approximation, where the 3D velocity field inside the blade channels is expressed as the superposition of a fully-guided axisymmetric flow with radially uniform axial velocity and a 2D cross-sectional vorticity correction. Boundary layer blockage is estimated by means of a suitable redefinition of the diffusion factor for bladings with non-negligible radial flow and Carter's rule is used to account for flow deviation at the inducer trailing edge. Performance losses are evaluated by adding suitably adapted correlations of turbulent duct losses to the inlet flow incidence losses.<br>The noncavitating performance of the inducers has been investigated at different clearance and temperature. The close matching between the experimental data in cold water and the predicted performance has confirmed both the predictive capability of the reduced order model and the high-head nature of the DAPAMITO inducers.<br>The suction performance of the inducers have been investigated at design and off-design conditions both at high and low clearance in cold and hot<br>water. Several cavitation-induced instabilities has been detected and identified on the DAPAMITO inducers. The experimental evidence suggests that<br>the occurrence of the synchronous rotating cavitation can be associated to a strong performance degradation. Furthermore, the comparison between the results at low and high clearance clearly highlights that a high value of<br>the clearance tends to suppress the rotating instabilities, whereas it seems that axial phenomena are not affected, or at least less affected, by the tip blade clearance. The semi-empirical method proposed by Ruggeri and Moore for scaling the thermal cavitation effects has been successfully applied and, consequently, a further validation of this method has been provided. A new typology of cavitation thermal effects has been identified in the influence of<br>the temperature on the intensity of the performance degradation associated to the attached cavitation instability. In this case the inhibition of the bubble growth due to thermal effects can be detected by the reduction of the<br>performance degradation usually associated to this type of instability. The tests carried out on whirling inducers has investigated the roles of<br>the flow coefficient, cavitation number, liquid temperature and of the imposed whirl motion of the rotor. The spectra, especially the continuous ones obtained by means of a novel data reduction procedure, of the rotordynamic forces measured at several flow coefficients in noncavitating regime clearly show the same qualitative behavior for both the inducers. The occurrence of large destabilizing peaks confirms that the conventional approach which uses the stiffness, damping and inertia matrices is clearly not able to characterize rotordynamic forces on noncavitating/cavitating inducers. For positive whirl frequency ratio, the continuous spectra of the module of the rotordynamic force clearly show a minimum that corresponds to a negligible intensity of the force and a maximum associated to the beginning of a destabilizing range.<br>The intensity of the flow reversal and the magnitude of rotation in the tip clearance flow and backflow affect the strength of the rotordynamic force and the location of the minimum and maximum with respect to the whirl<br>frequency ratio. The experimental results have confirmed that the cavitation has a dramatically clear destabilizing effect on the rotordynamic forces. Under cavitating regime, the minimum usually happens at the same value of<br>the whirl ratio of noncavitating condition while, in general, the maximum is anticipated and its intensity is higher. For negative whirl ratio, the cavitation tends to reduce the stabilizing range to a small interval next to zero. Based<br>on the available experimental evidence, the novel data reduction procedure capable of obtain a continuous spectrum of fluid-induced rotordynamic forces can be very useful to catch the unlikely foreseeable complexity of the rotordynamic<br>forces and their consequences on stability of axial flow inducers. Finally, a brief overview of the laws of similarity in pumps has been provided.<br>The second part of the thesis reports the results of a further research activity carried out during the years of the Ph. D. course. The research deals<br>with a comprehensive experimental campaign on three hydrogen peroxide monopropellant thruster prototypes performed in order to characterize the<br>decomposition capabilities of seven different platinum catalysts on alumina carriers for the decomposition of high grade hydrogen peroxide.