1,720,992 research outputs found
Design methodology of multi-hole spargers to prevent steam coalescence at sub-atmospheric pressure
Full text linkn case of an in-vessel Loss Of Coolant Accident (LOCA), flash steam can be released in the Vacuum Vessel (VV) of the International Thermonuclear Experimental Reactor (ITER) causing its pressurization. To avoid this, the safety system named Vacuum Vessel Pressure Suppression System (VVPSS) will intervene sending the steam to four Vapour Suppression Tanks (VSTs) through a multi-hole sparger and condenses via Direct Contact Condensation (DCC). To support the design of the multi-hole sparger, which is a key safety component of VVPSS, at the University of Pisa two testing facilities were designed and built in order to study and qualify the VVPSS, named Small Scale Test Facility (SSTF) and Large Scale Test Facility (LSTF). During the experimental tests performed using LSTF with a VVPSS multi-hole full scale sparger, under certain conditions, the coalescence of the steam jet plumes resulted in the formation and collapse of large, isolated steam bubbles which produced high pressure loads at low frequency on the structure and flow reversal of the pool water inside the sparger. To limit these large pressure loads, a methodology is needed to prevent the coalescence of the steam jet plumes. With this aim, an image analysis of 15 experimental tests performed using SSTF was performed to develop and validate a correlation of the ratio between the maximum radius of the steam jet plumes and the hole diameter. Subsequently, two limiting radii for multi-hole spargers (named r1 and r2) were determined which allow avoiding the partial and the transitional complete coalescence of the steam jet plumes when compared to the maximum radius. The proposed methodology is new and quite innovative, and it was applied and validated by using the several videos recorded during the transient test performed using sparger B, consisting of DN450 pipe with 1000 holes. The correlation estimates that partial coalescence and transitional to complete coalescence regions are avoided when the water subcooling temperature ranges between 37-45 degrees C and 25-31 degrees C, respectively, as observed in the recordings of the cameras. Results allow to identify the sparger design dimensions preventing the steam jet plumes coalescence, and avoiding the onset of excessive dynamic loads
Influence of Multi-Hole Sparger Geometry on the Chugging Regime
No full textSeveral experimental tests have been carried out at the University of Pisa using the Large-Scale Experimental Facility simulating the behaviour of the Vacuum Vessel Pressure Suppression System of ITER fusion reactor.
The experiments were carried out by injecting steam produced by an electric steam generator through a multi-hole sparger into a condensation tank, known as Experimental Test Tank, containing 55 m3 of water. Two spargers called sparger A (a reduced scale of ITER sparger) and sparger B (a full scale of ITER sparger) were used during the experimental campaign. The geometry of the two spargers differs for the arrangement of the holes at the external (injecting) surface.
This study compares the behavior of the two spargers by analyzing two experimental tests. The unstable chugging regime was also considered.
The image analysis of the videos permitted the comparison of the steam bubble dynamics.
The geometry of sparger B leads to the coalescence of the entire steam mass, while the sparger A developed two distinct steam bubbles which do not coalesce together.
The analysis of the strain gauges and accelerometer signal allowed to compare the structural-dynamic response of the steam bubble collapse. This preliminary analysis showed the strong influence of the sparger geometry on the steam bubble dynamics and on the developed load on the sparger
Structural and thermal fluid dynamics analyses of the ITER Pressure Suppression System considering no stable steam condensation regimes
No full textAt the University of Pisa, an experimental campaign was performed with the purpose of qualifying the ITER Vacuum Vessel Pressure Suppression System (VVPSS) and studying the Direct Contact Condensation at sub-atmospheric conditions. With financial support from ITER Organization, a Large-Scale Experimental Facility was designed and built to investigate the steam condensation in the operation conditions of the ITER VVPSS. The experimental tests were performed by injecting steam, produced by an electric steam generator (1.7 MW of power), through a multi-hole sparger into a condensation tank called Experimental Test Tank (ETT). On the sparger support, strain gauges and an accelerometer were installed to analyse the structural behaviour during the experimental tests. Under certain thermal hydraulic conditions (low subcooling and low steam mass flow rate) a large steam pocket can form and collapse causing high loads on the sparger structure. The aim of this study was comparing numerical fluid dynamics and structural analyses with the experimental results in order to estimate the pressure impulse caused by bubble collapse. The bubble collapse was simulated by means of ANSYS Fluent code in order to analyse the bubble dynamics and to determine the pressure impulse. The shape, the dimension and the collapse time of steam pocket were experimentally determined by means of an image analysis of video recorded during the tests. The numerical and experimental maximum steam bubble volume at the final stage of the transient before the detachment shows a good agreement with an error of 6 %. The pressure load was estimated at 5 different points of the simulation domain; the maximum pressure impulse on the sparger is 1.15 bar and has a duration of 40 ms. A dynamic structural FEM analysis was performed applying the pressure impulses on the sparger in order to evaluate the accelerations. The average value of RMS, maximum, minimum and duration of the experimental acceleration signals was obtained and compared to the numerical one. The errors of the RMS, the maximum and the minimum acceleration of the sparger are 16 %, 35 % and 51 %, respectively. The time duration of the oscillations is in good agreement (the error is only 2.54 %)
Influence of Noncondensable Gas–Dust Mixture on Direct Contact Condensation of Steam at Atmospheric Pressure
No full textAt the Department of Civil and Industrial Engineering (DICI) of the University of Pisa, an experimental research program, funded by International Thermonuclear Experimental Reactor (ITER) Organization, concerning steam direct condensation in a flux containing also noncondensable gas and dust, was carried out. This mixture of fluids and dust is injected into the ITER pressure suppression tanks during a loss of coolant accident (LOCA) in the Vacuum Vessel. The aim of the research program is to determine the steam condensation efficiency in such conditions. Experimental tests were performed injecting this mixture in a tank partially filled with water. Alumina was used to simulate the actual dust present in the ITER Vacuum Vessel. Mass flow rates, temperature, and pressure of the different fluids involved were recorded during the tests. The steam condensation into the subcooled water pool at a temperature ranging between 318 and 369 K was investigated to determine the condensation regimes occurring during the mixture injection. The values of the fraction of the energy absorbed by water, dust, and metallic structures of the heat losses and of the average heat transfer coefficient were determined considering pure steam, steam-dust and steam-air-dust injection. The average heat transfer coefficient, determined calculating the steam jet surfaces by means of image elaboration, was compared with empirical correlations
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
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Large-Scale Experimental Analysis of Steam Sub-Atmospheric Condensation for ITER Vacuum Vessel Pressure Suppression System During LOCA Event
No full textThe loss of coolant accidents (LOCA) postulated to occur in the vacuum vessel (VV) of the International Thermonuclear
Experimental Reactor (ITER) entails dangerous VV pressurization, which is foreseen to be mitigated by steam
discharge into the Vacuum Vessel Pressure Suppression System (VVPSS). This safety system realizes steam direct contact
condensation (DCC) in four Vapour Suppression Tanks (VSTs, 100 m3 each) at a very characteristic initial condition: subatmospheric cover gas pressure, for receiving steam from the VV at low pressure (0.2 ÷ 1.5 bar abs). This upper limit was defined to preserve the VV structural integrity.
The scenario of steam DCC at vacuum condition in a VVPSS
tank was extensively experimentally investigated at University of Pisa (UNIPI) Laboratory Guerrini, in a suitable Large-Scale
Test Facility (LSTF), assuming a wide range of water pool temperature, injected steam mass flow rate and water pool
pressure, characteristic for ITER VV postulated LOCA event. In addition, both a scaled and mock-up configuration of the
sparger adopted in the VVPSS were implemented and tested in the LSTF. This activity continues and builds up the UNIPI
research commitment, started with Small-Scale Test Facility projects (SSTF, condensation vessel of 4.5 m3), increasing the
condensation tank volume and steam mass flow rate to 92 m3 and 500 g/s, respectively. This effort aimed to contribute to fill the existing literature gap regarding the steam DCC at subatmospheric pressure.
An overall number of 174 steam DCC experimental tests, subdivided in four campaigns, was carried out in the LSTF for
water pool temperature ranging between 30 and 100°C, steam mass flow rate 50 ÷ 500 g/s and water pool cover gas pressure 20 ÷ 100 kPa abs. Steady state pure steam and steam plus noncondensable (air) tests were carried out with both sparger system A (scaled) and B (mock-up). In addition, pure steam transient tests were also performed adopting both spargers. The experimental results were plotted in the available condensation map, previously elaborated during SSTF activities at subatmospheric condition, verifying the instability regime domains with the support of the LSTF video acquisition system. In addition, the analysis of pressure drops across both spargers was carried out providing a relation with the injected steam mass flow rate. Finally, the condensation efficiency was evaluated for the whole test campaign showing a value next to 100% (complete condensation), with the exception of tests performed with water pool at saturation conditions (92%). This research activity permitted an extensive characterization of steam DCC regimes in vacuum conditions, occurring in ITER VVPSS during postulated LOCA scenarios, and contributed to evaluate and confirm the VVPSS safe functioning
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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