1,720,958 research outputs found
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
Investigation of vibrations caused by the steam condensation at sub-atmospheric condition in VVPSS
No full textFusion technology deployment passes through the design of safety systems aimed to protect the Vacuum Vessel (VV) from pressurizing accidents event such as LOVA (Loss Of Vacuum Accident) or the failure of the Tokamak Water Cooling System (TWCS) causing the LOCA (Loss Of Coolant Accident). One of important safety systems of the ITER plant is the Vacuum Vessel Pressure Suppression System (VVPSS), which is designed to protect the VV by the steam condensation at sub-atmospheric pressure conditions in Suppression Tanks. The aim of this study is to investigate vibrations phenomenon, originated during the steam-water direct contact condensation (DCC) at sub-atmospheric conditions, and determine any correlations between the steam jet dynamic and water temperature (TW), steam mass flux (GS) and downstream pressure (PW). According to the thermal hydraulic conditions characterising the DCC, vibrations may evolve because of the latent heat of the phase change (large amount of heat transmitted quickly to the water). In the paper also presents the numerical investigations performed by means of proper FEM code, the set-up procedure and the experimental activity carried out at Lab. B. Guerrini of DICI- University of Pisa. This latter allowed correlating the different steam condensation regimes (chugging, condensation oscillation, etc.) to the acceleration and force, to be in turn used to determine the strength capacity of the VVPSS
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
A passive method for enhancing the performance of a symmetric diffuser in turbulent flow regime
No full textThe object of the present work is testing the capabilities of a passive method
for improving the efficiency of a plane symmetric diffuser. The passive control
method consists in introducing small localized flow recirculations. These
localized flow separations are obtained modifying the geometry of the diffuser
diverging walls using couples of appropriately contoured cavities. The turbulent
flow regime is considered. The Reynolds number, based on the diffuser
width at the inlet section and the inlet velocity on the axis in a certain section
is Re = 20000. The diffuser area ratio in 4.7 and the divergence angle is 5°.
The inlet flow is fully-developed turbulent. A preliminary mesh sensitivity
analysis for two turbulence models (SST k-ω and RSM) is performed for the
flow inside the reference diffuser configuration (without cavities). Based on
this analysis the SST k-ω model is chosen for an optimization procedure in
order to identify the cavity geometries that maximize the pressure recovery
in the diffuser and minimize the main boundary separation extent. Three
different configuration have been considered: one, two and three couples
of subsequent couples of general contoured cavities. In the optimized diffuser
configuration with one couple of contoured cavities there are not small
localized flow recirculations, while in the other two optimized diffuser configurations
with two and three couples of contoured cavities small localized flow
recirculations are present. The passive control increases the performance of
the diffuser in all three configurations (the maximum pressure recovery is
reached with the optimized diffuser configuration with three couples of cavities),
leading to an increase of the mean pressure recovery coefficient by
reducing the extent of the main flow separation region. In particular, the
success of the passive control is due both to a virtual geometry modification
of the flow inside the diffuser and to a favourable effect of the cavities in
reducing the momentum losses near the wall. A classical shape optimization
with Bézier curve is also carried out in order to mazimixe the pressure recovery.
The resulting optimized diffuser shape with Bézier curve does not show
small local flow recirculations but produces a diverging wall shape similar
to the one optimized with contoured cavities. Since the optimized diffuser
with Bézier curve provides a pressure recovery very similar but not equal to
the one of the optimized diffuser with three couples of cavities it seems that
the "optimum" configuration can be given by the union of a high degrees of
freedom Bézier curve and the presence of small localized flow recirculation placed
at precise locations
Vibration issues caused by steam condensation at sub-atmospheric conditions
No full textDirect Contact Condensation (DCC) of steam is an efficient way to remove heat and mass with passive devices and it is an important process encountered in many engineering situations, especially in the nuclear field. All conventional nuclear fission plants take advantage of this process to suppress pressure at atmospheric pressure conditions. On the other hand, since the ITER (International Thermonuclear Experimental Reactor) reactor operates at high vacuum conditions, it has been designed to withstand a maximum pressure of only 0.15 MPa and it requires suppression and therefore steam condensation at sub-atmospheric pressure conditions. This is an absolute novelty in the nuclear field and deserves to be studied in depth given the growing interest in nuclear fusion reactors. Associated with the DCC of steam, pressure oscillations can arise when several thermo-hydraulic parameters vary, which result in vibrations on the structures, the integrity of which can be endangered depending on their intensity and frequencies. In particular, during unstable condensation regimes, it is possible to observe pressure instabilities associated with oscillations which involve the pool and the surrounding structures. This issue is even more significant when the vibrations occur at low frequencies and approach the natural ones of the structures which can go into resonance. This PhD thesis therefore focuses attention on the vibratory aspects associated with the phenomenon of DCC at sub-atmospheric pressure conditions.
In the introduction chapter the relevant safety and protection system called Vacuum Vessel Pressure Suppression System (VVPSS) of the ITER experimental nuclear fusion plant has been described along with the engineering issues associated with its operability. The VVPSS is mainly constituted by 4 Vapour Suppression Tanks (VSTs) measuring 4.7 m in height, with a diameter of 6.2 m and an inner volume of 100 m3 each partially filled with water, 3 with 60 m3 and one with 40 m3. The 3 VSTs containing 60 m3 of water, also called Large LOCA Tanks (LLTs), are designed to manage bigger LOCA (Loss Of Coolant Accident) events, while the one containing 40 m3, also called Small LOCA Tank (SLT), is designed to manage smaller LOCA events. Each tank is crossed by a sparger immersed in water to release the steam together with the potential presence of non-condensable gases. In the second chapter a summary of the state of the art (scientific literature) regarding pressure oscillations that arise during the DCC process has been illustrated. This literature search showed many discrepancies between the study results. The third chapter contains the description of the small-scale test facility where the superheated steam produced by 137 kW steam generator was conveyed in a 4 m3 condensation tank. In this experimental rig two experimental campaigns were carried out during which the vibrations that involved the sparger due to the steam DCC were studied (even in the presence of non-condensable gas). In the same chapter also a calibration procedure and a numerical analysis have been described. The fourth chapter refers to the Large-Scale Test Facility (LSTF) whose design and realization took up most of the doctoral time. This experimental rig quite faithfully replicates an ITER VST and includes a 92 m3 tank in which the steam produced by a 1750 kW steam generator is condensed. This chapter contains in the first part a detailed description of the facility and in the second part an analysis of a particular condensation situation encountered during the experimental tests performed for ITER organization. In this particular condition heavy vibrations of the whole structure were observed. The last chapter is finally dedicated to the results obtained from these studies and to some perspectives for future research activities
Variations on the Author
Get PDF“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
Get PDFWe 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
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe 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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