Luca d’Agostino

Thermal Cavitation Experiments on a NACA 0015 Hydrofoil

The present paper illustrates the main results of an experimental campaign conducted in the Thermal Cavitation Tunnel of the Cavitating Pump Rotordynamic Test Facility (CPRTF) at Centrospazio/Alta S.p.A. Experiments were carried out on a NACA 0015 hydrofoil at various incidence angles, cavitation numbers, and freestream temperatures. in order to investigate the characteristics of cavitation instabilities and the impact of thermal cavitation effects. Measured cavity length, surface pressure coefficients, and unsteady pressure spectra are in good agreement with the data available in the open literature and suggest the existence of a strong correlation between the onset of the various forms of cavitation and instabilities, the thermal cavitation effects, and the effects induced by the presence of the walls of the tunnel. Further analytical investigations are planned in order to provide a better interpretation of the above results.

Setup of a high-speed optical system for the characterization of flow instabilities generated by cavitation

The present paper illustrates the setup and the preliminary results of an experimental investigation of cavitation flow instabilities carried out by means of a high-speed camera on a three-bladed inducer in the cavitating pump rotordynamic test facility (CPRTF) at Alta S.p.A. The brightness thresholding technique adopted for cavitation recognition is described and implemented in a semi-automatic algorithm. In order to test the capabilities of the algorithm, the mean frontal cavitating area has been computed under different operating conditions. The tip cavity length has also been evaluated as a function of time. Inlet pressure signal and video acquisitions have been synchronized in order to analyze possible cavitation fluid-dynamic instabilities both optically and by means of pressure fluctuation analysis. Fourier analysis showed the occurrence of a cavity length oscillation at a frequency of 14.7 Hz, which corresponds to the frequency of the rotating stall instability detected by means of pressure oscillation analysis.

On the Preliminary Design and Noncavitating Performance Prediction of Tapered Axial Inducers

A reduced order model for preliminary design and noncavitating performance prediction of tapered axial inducers is illustrated. In the incompressible, inviscid, irrotational flow approximation, the model expresses the 3D flow field in the blade channels by superposing a 2D cross-sectional vorticity correction to a fully guided axisymmetric flow with radially uniform axial velocity. Suitable redefinition of the diffusion factor for bladings with non-negligible radial flow allows for the control of the blade loading and the estimate of the boundary layer blockage at the specified design flow coefficient, providing a simple criterion for matching the hub profile to the axial variation of the blade pitch angle. Carters rule is employed to account for flow deviation at the inducer trailing edge. Mass continuity, angular momentum conservation, and Eulers equation are used to derive a simple second order boundary value problem, whose numerical solution describes the far-field axisymmetric flow at the inducer discharge. A closed form approximate solution is also provided, which proved to yield equivalently accurate results in the prediction of the inducer performance. Finally, the noncavitating pumping characteristic is obtained by introducing suitably adapted correlations of pressure losses and flow deviation effects. The model has been verified to closely approximate the geometry and noncavitating performance of two space inducers tested in Altas Cavitating Pump Rotordynamic Test Facility, as well as the measured pumping characteristics of a number of tapered-hub inducers documented in the literature.

Continuous Spectrum of the Rotordynamic Forces on a Four Bladed Inducer

The paper illustrates the results of an experimental campaign conducted in the CPRTF (Cavitating Pump Rotordynamic Test Facility) at ALTA S.p.A., aimed at characterizing the rotordynamic forces acting on a whirling four-bladed, tapered-hub, variable-pitch inducer, designated as DAPAMITO4. The roles of the imposed whirl motion of the rotor, flow coefficient, cavitation number and liquid temperature have been investigated. A novel experimental technique, consisting in measuring the continuous spectra of the forces as functions of the whirl ratio, has been developed and validated. This technique gives the possibility of extracting valuable information from the experiments by clearly identifying the qualitative and quantitative behavior of the forces, and is therefore useful to catch the unlikely foreseeable complexity of the rotordynamic forces and their consequences on the stability of axial inducers.Copyright

Simultaneous cavitation susceptibility meter and holographic measurements of nuclei in liquids

Cavitation Susceptibility Meter (CSM) and holographic measurements of cavitation nuclei distributions are compared. The CSM optically detects cavitation in water samples flowing through a venturi and relates the unstable nuclei concentration to the applied tension in the fluid. A ruby laser holographic system measures the nuclei size distribution directly. Microbubbles have been used as the dominant nuclei source. The data from the two detection schemes are correlated by accounting for the dynamic response of the cavities in the venturi throat

Dynamic Response of Ducted Bubbly Flows to Turbomachinery-Induced Perturbations

The present work investigates the dynamics of the three-dimensional, unsteady flow of a bubbly mixture in a cylindrical duct subject to a periodic pressure excitation at one end. One of the purposes is to investigate the bubbly or cavitating flow at inlet to or discharge from a pump whose blade motions would provide such excitation. The flow displays various regimes with radically different wave propagation characteristics. The dynamic effects due to the bubble response may radically alter the fluid behavior depending on the void fraction of the bubbly mixture, the mean bubble size, the pipe diamter, the angular speed of the turbomachine and the mean flow Mach number. This simple linearized analysis illustrates the importance of the complex interactions of the dynamics of the bubbles with the average flow, and provides information on the propagation and growth of the turbopump-induced disturbances in the feed lines operating with bubbly or cavitating liquids. Examples are presented to illustrate the influence of the relevant flow parameters. Finally, the limitations of the theory are outlined.

The Different Role of Cavitation on Rotordynamic Whirl Forces in Axial Inducers and Centrifugal Impellers

The linearized dynamics of the flow in cavitating axial helical inducers and centrifugal turbopomp impellers is investigated with the purpose of illustrating the impact of the dynamic response of cavitation on the rotordynamic forces exerted by the fluid on the rotors of whirling turbopumps. The flow in the impellers is modeled as a fully-guided, incompressible and inviscid liquid. Cavitation is included through the boundary conditions on the suction sides of the blades, where it is assumed to occur uniformly in a small layer of given thickness and complex acoustic admittance, whose value depends on the void fraction of the vapor phase and the phase-shift damping coefficient used to account for the energy dissipation. Constant boundary conditions for the total pressure are imposed at the inlet and outlet sections of the impeller blade channels. The unsteady governing equations are written in rotating “body fitted” orthogonal coordinates, linearized for small-amplitude whirl perturbations of the mean steady flow, and solved by modal decomposition. In helical turbopump inducers the whirl excitation and the boundary conditions generate internal flow resonances in the blade channels, leading to a complex dependence of the lateral rotordynamic fluid forces on the whirl speed, the dynamic properties of the cavitation region and the flow coefficient of the machine. Multiple subsynchronous and supersynchronous resonances are predicted. At higher levels of cavitation the amplitudes of these resonances decrease and their frequencies approach the rotational speed (synchronous conditions). On the other hand, application of the same approach indicates that no such resonances occur in whirling and cavitating centrifugal impellers and that the rotordynamic fluid forces are almost insensitive to cavitation, consistently with the available experimental evidence. Comparison with the scant data from the literature indicates that the present theory correctly captures the observed features and parametric trends of rotordynamic forces on whirling and cavitating turbopump impellers. Hence there are reasons to believe that it can usefully contribute to shed some light on the main physical phenomena involved and provide practical indications on their dependence on the relevant flow conditions and parameters.

Two-Phase Flow and Inertial Effects on the Rotordynamic Forces in Whirling Journal Bearings

This paper presents the application of the isenthalpic two-phase flow approximation (Brennen, 1995) to the study of cavitation and ventilation effects in plane journal bearings with whirling eccentricity. A quasi-homogeneous bubbly liquid/vapor model, suitably modified to account for thermal effects (Rapposelli and d’Agostino, 2001), is used to describe the occurrence of flow cavitation. An homogeneous liquid/gas/vapor model without thermal effects is used to describe the simultaneous occurrence of cavitation and ventilation. The proposed model treats the fully-wetted and two-phase portions of the fluid in a unified manner in order to avoid the use of “ad hoc” matching conditions, whose applicability and accuracy is questionable in the presence of significant inertial and/or unsteady effects. A non-linear analysis that accounts for the inertia of the lubricant is used to determine the reaction forces caused by the shaft’s eccentric motion both in the viscosity-dominated regime and at intermediate values of the Reynolds number, where the inertia of the lubricant is no longer negligible. The classical iteration method for the Reynolds lubrication equation (Muster and Sternlicht, 1965; Mori and Mori, 1991; Reinhardt and Lund, 1975) has been extended to the unsteady two-phase flow case in order to account for flow acceleration effects in the presence of cavitation and/or ventilation. Significant deviations from the steady-state case are obtained at moderately high Reynolds numbers (Re = ωRc/νL ≅ 10). Comparisons with the scant experimental data support the validity of the proposed model. Results are shown in a number of representative cases in order to illustrate the influence of the relevant parameters.Copyright

An Introduction to Flow-Induced Instabilities in Rocket Engine Inducers and Turbopumps

The article reviews the main forms of flow-induced instabilities detected in the liquid propellant turbopumps systems of modern rocket engines, with special reference to rotating stall, rotating cavitation, cavitation surge and higher order surge modes, illustrating their characteristics, origin and damage potential.

An Introduction to Cavitation in Inducers and Turbopumps

After a brief review of the fundamental aspects of cavitation relevant to the operation of high-performance inducers and turbopumps, the article summarizes their application to the analysis of pumping systems, illustrates the scaling of cavitation phenomena from model tests to full-scale operation, describes the occurrence of flow-induced instabilities in turbomachinery, and introduces the concepts of static and dynamic instability of pumping systems and their generalization to cavitating turbopump systems.