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Dynamics of the Blade Channel of an Inducer Under Cavitation-Induced Instabilities
No full textA high-head three-bladed inducer has been equipped with pressure taps on the hub along the blade channels with the aim of more closely investigating the dynamics of cavitation-induced instabilities developing in the impeller flow. Spectral analysis of the pressure signals obtained from two sets of transducers mounted both in the stationary and rotating frames has allowed to characterize the nature, intensity, and interactions of the main flow instabilities detected in the experiments: subsynchronous rotating cavitation (RC), cavitation surge (CS), and a high-order axial surge oscillation. A dynamic model of the unsteady flow in the blade channels has been developed based on experimental data and on suitable descriptions of the mean flow and the oscillations of the cavitating volume. The model has been used for estimating at the inducer operating conditions of interest the intensity of the flow oscillations associated with the occurrence of the CS mode generated by RC in the inducer inlet
Analysis of Flow Instabilities on a Three-Bladed Axial Inducer in Fixed and Rotating Frames
No full textThe paper describes the results of recent experiments carried out in the Cavitating Pump Rotordynamic Test Facility for the dynamic characterization of cavitation-induced flow instabilities as simultaneously observed in the stationary and rotating frames of a high-head, three-bladed axial inducer with tapered hub and variable pitch. The flow instabilities occurring in the eye and inside the blading of the inducer have been detected, identified, and monitored by means of the spectral analysis of the pressure measurements simultaneously performed in the stationary and rotating frames by multiple transducers mounted on the casing near the inducer eye and on the inducer hub along the blade channels. An interaction between the unstable flows in the pump inlet and in the blade channels during cavitating regime has been detected. The interaction is between a low frequency axial phenomenon, which cyclically fills and empties each blade channel with cavitation, and a rotating phenomenon detected in the inducer eye
Analysis of Flow Instabilities on a Three-Bladed Axial Inducer in the Fixed and Rotating Frames
Full text linkThe paper describes the results of an experimental campaign performed on a three bladed axial inducer with tapered hub and variable pitch, named RAPDUD. The tests have been performed at the Cavitating Pump Rotordynamic Test Facility at Sitael. S.p.A. under similarity conditions. The tests aimed at characterizing the flow instabilities arising in the fluid elaborated by the inducer at different flow coefficients through two reference frames: a statoric frame by placing pressure transducers on the casing and a rotoric frame by placing the transducers on the inducer hub. The additional reference frame has allowed a better understanding of the flow phenomena arising inside the inducer
Dynamics of the Blade Channel of an Inducer under Cavitation-Induced Instabilities
Full text linkA high-head three-bladed inducer has been equipped with pressure taps along the blade channels to characterize the dynamics of the impeller in view of a deeper insight of the cavitation-induced instabilities. The spectral analysis of the pressure signals located in both the statoric and rotoric frames has allowed to characterize the nature and intensity of the principal flow instabilities detected during the experimental campaign: a sub-synchronous rotating cavitation, a cavitation surge and a high order axial oscillation. A model of the dynamics of the blade channels, that relies on experimental data and on a suitable modelling of the mean flow and cavitating volume oscillations, has been used to estimate, for all the relevant flow coefficients, the intensity of flow oscillations associated to the cavitation surge generated by the rotating cavitation
Inducer and centrifugal pump contributions to the rotordynamic fluid forces acting on a space turbopump
No full textThe fluid-induced rotordynamic forces acting on a whirling space turbopump composed by an inducer and a radial impeller have been compared to the same forces measured on each single component of the turbomachine (i.e., on the inducer and on the radial impeller). The experimental campaign has been carried out in cold water at design and offdesign conditions (80%, 100%, and 120% of the design flow rate) both in noncavitating (NC) and cavitating regimes. The paper illustrates the different trends of the rotordynamic forces on the axial and radial pumps and highlights their contributions on the overall turbomachine. At positive whirl ratios, the behavior of the inducer is dominant while, at negative ones, both the pumps show the same trends in such a way that the overall behavior is roughly the sum of each single component
Inducer and Centrifugal Pump Contributions to the Rotordynamic Fluid Forces Acting on a Space Turbopump
Full text linkThe experimental characterization of the rotordynamic fluid forces acting on a whirling centrifugal impeller have been investigated at different flow rates and cavitating conditions. The recently developed method for continuously measuring the rotordynamic forces at variable whirl ratios has been readapted and successfully applied for measuring the same forces at constant whirl ratio but variable cavitation number. The flowrate has a major influence on the stability of the rotordynamic forces at positive whirl ratios where a threshold flowrate separates the stable zone from the unstable. At negative whirl ratios, the normal force is typically unstable independently on the flow rate. Cavitation has always a destabilizing effect at positive whirl ratio while it can stabilize the rotordynamic forces at negative whirl ratio
Experimental Characterization of Unsteady Forces Triggered by Cavitation on a Centrifugal Pump
Full text linkThe present paper illustrates the results of recent experiments carried out under ESA funding in the Cavitating Pump Rotordynamic Test Facility (CPRTF) for the dynamic characterization of cavitation-induced flow instabilities as simultaneously observed in the stationary and rotating frames of a typical inducer for space propulsion applications. Specifically, the data refer to a three-bladed, tapered-hub, variable-pitch, "RAPDUD" inducer, designed by means of the reduced order model recently developed by the author and his collaborators for the geometric definition and performance prediction of this kind of hydraulic turbomachinery. Flow instabilities and their dynamic features have been detected, identified and monitored by means of the spectral analysis of the pressure measurements simultaneously generated in the absolute and relative frames by transducers respectively mounted a suitable locations on the casing and the hub of the test inducer. The comparison of the results obtained in the two reference frames over a wide range of flow coefficients and cavitation numbers contributed to better describe and understand the complex flow phenomena occurring in the blade channels of typical space inducers in the presence of cavitation-induced instabilities
Analysis Of Rotordynamic Forces On Cavitating/Noncavitating Turbopumps
No full textThe present work illustrates the main results of an experimental campaign conducted in the Cavitating Pump Rotordynamic Test Facility (CPRTF) at Alta S.p.A., Pisa, on a three bladed, tapered-hub, variable-pitch inducer, named DAPROT3, and on a mixed flow pump, named VAMPDAP. The latter consists of the combination between the DAPROT3 inducer and a centrifugal pump placed downstream, called VAMPIRE. As a consequence of a displacement of the rotation axis, fluid induced rotordynamic forces arise, leading to a secondary motion of the shaft axis here referred as whirl motion. This precession of the shaft can be both unstable or stable depending on the direction of the rotordynamic force and the whirl frequency ratio. The combined effects of rotordynamic fluid forces and cavitation represent the dominant fluid mechanical phenomena that adversely affects the dynamic stability and pumping performance of turbopumps. In the experimental campaign, the rotordynamic forces and the hydraulic efficiency have been measured both in noncavitating and cavitating conditions, at different flow rates and temperatures, by means of a rotating dynamometer placed just behind the inducer. Two different methods have been exploited for the evaluation of fluid-induced rotordynamic forces. Indeed, in addition to the classic approach of discrete tests, a series of continuous tests have been performed in order to obtain a continuous spectra, allowing the possibility of accurately and unambiguously identifying the spectral minima and maxima, which are of special interest in the identification of the most dangerous operational condition of the test item. It has been found that the DAPROT3 inducer presents a different behavior of rotordynamic forces with respect to the typical trend observed in centrifugal pump impellers in presence of a whirl motion concordant with impeller rotation, according to previous experimental campaigns. It has been observed also that cavitation does not affect the rotordynamic forces in a significant way. Nevertheless particular situations may arise in which a certain level of cavitation is capable to alter the stability regions and in which the presence of a cold or hot flow may play an important role. Conversely the flow rate has a great influence and its variation may destabilize a pump that is stable at design condition. An increasing deviation has been obtained for decreasing flow rate as a consequence of the interaction between rotating backflow and whirl motion. The effect of cavitation on hydraulic efficiency has been found to be negligible with variations within 1%. On the other hand, the presence of rotordynamic forces may affect the efficiency up to 2% depending on the whirl speed both for cold and hot flow conditions. For what concerns the VAMPDAP pump, a deviation from the typical trend of fluid-induced rotordynamic forces has been observed as a consequence of the presence of the inducer placed upstream with respect to a radial pump, whose experimental diagrams respect the expectations. The effect of cavitation on this pump is relevant only at high flow rates, where for discordant whirl motion a decrease in rotordynamic force appears, whereas in other operating conditions it can be considered almost negligible. The experimental campaign has included the evaluation of thermal cavitation effect by performing tests at two different temperatures also for VAMPDAP pump. From the results it has been observed that no significant thermal effects are found in terms of rotordynamic forces. Moreover, the hydraulic efficiency is not affected from cavitation since the maximum variations are in the order of 0.9 % only at design and even lower at off-design conditions. At the same time the rotordynamic forces influence the efficiency in the order of 1% depending on whirl speed
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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