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Cavitation and Rotordynamic Activities at Centrospazio
Full text linkCentrospazio has long been carrying out an articulated program of experimental, theoretical and numerical research on cavitation and two-phase flow dynamics in connection with liquid propellant rocket fuel feed systems. The areas of involvement dealt with in recent years range from the modeling and simulation of cavitation to the study of rotordynamic fluid forces in whirling and cavitating axial inducers and journal bearings. Specifically, this paper illustrates the development of a new cavitation model accounting in an approximate but physical way for the occurrence of thermal cavitation and liquid quality effects, and the realization of the CPRTF (Cavitating Pump Rotordynamic Test Facility), a water loop for the measurement of rotordynamic fluid forces on whirling and cavitating turbopump impellers, and the TCT (Thermal Cavitation Tunnel), a modified version of the CPRTF for cavitation experiments in fluid dynamic and thermal cavitation similarity. Representative results of the application of cavitation model to journal bearings, hydrofoils and helical inducers and the typical performance of the CPRTF and TCT are presented
Modeling Activities on Cavitating Flows at Centrospazio
Full text linkThe present paper illustrates recent theoretical and numerical activities on cavitating flows carried out at CENTROSPAZIO in the last few years. Specifically, it deals with the development and application of a modified isenthalpic cavitation model that accounts in an approximate but physical way for the occurrence of thermal cavitation effects and the concentration of active cavitation nuclei in the liquid. Expanding on the work of Brennen (1994), the model leads to a quasi-homogeneous barotropic description of cavitating flows, whose sound speed smoothly reduces to that of the liquid in the limit for low void fractions, thus eliminating the inconsistencies of previous formulations. Thermal effects are accounted for by assigning a single parameter expressing the nondimensional thickness of the thermal boundary layer in the liquid surrounding the growing cavities. The value of this parameter is related to the concentration of active cavitation nuclei, whose value can therefore be alternatively used to specify the impact of thermal cavitation effects. Applications to cavitating journal bearings, hydrofoils and helical inducers relevant to space engineering are presented
A Modified Isenthalpic Model of Cavitation in Plane Journal Bearings
No full textThis paper presents the development of a quasi-homogeneous isenthalpic cavitation flow model, suitably modified to account for thermal cavitation, and its application to the study of plane journal bearings with constant eccentricity. The proposed model treats the cavitating and noncavitating portions of the fluid in a unified manner with the aim of avoiding the use of matching conditions at the phase interface, whose accuracy is questionable in the presence of significant inertial and/or unsteady effects. A non-linear analysis which accounts for the inertia of the lubricant is used to determine the reaction forces caused by the shaft eccentricity 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 two-phase flow case in order to account for flow acceleration effects in the presence of cavitation. Comparison with available experimental data are shown in a number of representative cases, in order to illustrate the validity and the capabilities of the proposed model for the analysis of cavitating flows in journal bearings, in view of its extension to the case of whirling loads and eccentricities
A Barotropic Cavitation Model with Thermal Effects
No full textExtending Brennen’s original formulation [1], this paper illustrates the development of a modified bubbly isenthalpic flow model that naturally accounts in an approximate but physical-based way for the effects of thermal cavitation and the concentration of active nuclei. The model allows for the closed form evaluation of the sound speed in the cavitating mixture, with regular transition to the sonic speed in the liquid at zero void fraction. Numerical integration from the saturation conditions then leads to the barotropic relation for the cavitating/fully-wetted flow, depending on the value of the pressure. Speed of sound curves for three different liquids are shown (water, LOX and LN2), highlighting the influence of thermal effects at different temperatures. The model is then applied to the numerical simulation of cavitation on axisymmetric headforms and 2D hydrofoils. The results are compared with available experimental data with the purpose of assessing the potential of the proposed model for engineering simulation of more complex cavitating flows with special emphasis on liquid propellant rockets applications, where thermal effects are often of crucial importance
A New Cavitating Pump Rotordynamic Test Facility
Full text linkThe present paper illustrates the operational characteristics of the CPRTF (Cavitating Pump Rotordynamic Test Facility), an experimental apparatus specifically designed for the measurement of rotordynamic fluid forces acting on turbopump impellers in fluid dynamic and inertial/thermal cavitation similarity conditions. The realization of the CPRTF is currently in progress under ASI (Agenzia Spaziale Italiana) funding and consists in the upgrade of the CPTF (Cavitating Pump Test Facility), already available at Centrospazio, Consorzio Pisa Ricerche, Pisa, Italy. The experimental apparatus, operating in water, will be capable of carrying out the measurement of the steady and unsteady forces exerted by the flow on the impellers of cavitating/noncavitating turbopumps. More generally, the facility is designed as a flexible, versatile and inexpensive apparatus that can be readily be adapted to carry out detailed experimental investigations on practically any kind of fluid dynamic phenomena relevant to high performance turbopumps. The main operational requirements, development choices and design trade-offs that led to the final configuration of the facility are illustrated and its performance in testing of cavitating/noncavitating turbopumps under fluid dynamic and thermal cavitation similarity are discussed. Experimental results from a number of turbopump configurations and operational conditions are presented to illustrate the present capabilities of the facility
A Simplified Model for the Evaluation of the Delta V of Ascent Trajectories
No full textA simplified trajectory model for the evaluation of the performance of planetary ascent vehicles is presented. The model is based on a “linear-sine steering to orbit” approximation. The ascent trajectory is divided in two phases. In the first phase, characterized by a given steering law referred to a ground-fixed coordinate frame, the influence of atmospheric drag can be relevant. The second phase is an elliptical coasting orbit referred to a planet-centered inertial frame, starting from the burn-out point of the first phase and tangent to the final parking orbit, which is assumed to be circular. The paper shows the main results of the application of the proposed model to a sample Mars Ascent Vehicle case. The results obtained are in good accordance with the ones presented in the open literature for similar cases, showing a maximum error in the estimation of the total velocity change not higher than 10%. The effectiveness of the proposed model, despite its simplicity and ease of use, is thus demonstrated
Two-Phase and Inertial Effects on the Rotordynamic Forces in Whirling Journal Bearings
No full textThis 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
Thermal Effects on Cavitation Instabilities in Helical Inducers
No full textThe present paper illusn·ates the main results of an experimental campaign conducted in the CPRTF (Cavitating Pump Rotordynamic Test Facility) at Alta S.p.A. The experiments have been carried out on two really different axial inducers (a 3-bladed aluminium-made inducer of extremely simple helical geometry and a prototype of the axial inducer of the Vulcain MK1 engine LOX turbopump) in order to characterize the instabilities affecting the pumps in a wide range of flow conditions; some experiments have also been carried out at higher temperatures· to investigate the possible influence of the thermal cavitation effects on the observed phenomena. The transparent inlet section of the facility has been instrumented with several piezoelectric pressure transducers located at three axial stations: inducer inlet, outlet and at the middle of the axial chord of the blades. For each axial station at least two transducers were mounted at a given angular spacing, in order to cross-correlate their signals for coherence and phase analysis. The most interesting instabilities have been detected on the 3-bladed inducer, including a cavitation surge, a rotating stall and an auto-oscillation leading to a violent surge-mode instability. Some of these instabilities have also been found to be slightly affected by temperature. On the other band, very few oscillating phenomena have been detected on the MKl inducer, with a practically flat frequency spectrum at flow coefficients near to its nominal operating point. A natural frequency of the facility has also been detected, having a value in good agreement with that evaluated using a simple linear lD model
Thermal Effects on Cavitation Instabilities in Helical Inducers"
No full textThe main results of an experimental campaign conducted in the Cavitating Pump Rotordynamic Test Facility at Alta S.p.A. are presented. The experiments have been carried out on two different axial inducers (a three-bladed aluminum-made inducer of simple helical geometry and a prototype of the axial inducer of the Vulcain MK1 engine liquid oxygen turbopump) to characterize the instabilities affecting the pump on a wide range of flow conditions: some experiments have also been carried out at higher temperatures to investigate the possible influence of the thermal cavitation effects on the observed phenomena. The transparent inlet section of the facility has been instrumented with several piezoelectric pressure transducers located at three axial stations: inducer inlet, outlet, and middle of the axial chord of the blades. For each axial station, at least two transducers were mounted at a given angular spacing to cross-correlate their signals for coherence and phase analysis. The most interesting instabilities have been detected on the three-bladed inducer, including cavitation surge, a rotating stall and an auto-oscillation leading to a violent surge-mode instability. Some of these instabilities have also been found to be slightly affected by temperature. On the other hand, very few oscillating phenomena have been detected on the MK1 Inducer, with a practically flat frequency spectrum at flow coefficients near its nominal operating point
Thermal Cavitation Experiments on a NACA 0015 Hydrofoil
Full text linkThe present paper illustrates the main results of an experimental campaign conducted in the Thermal Cavitation Tunnel of the CPRTF (Cavitating Pump Rotordynamic Test Facility) at Centrospazio. 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 will be carried out in order to provide a better interpretation of the above results
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