1,720,972 research outputs found
Optimization of the flow distribution in a gyrotron cavity using evolutionary CFD simulations driven by a genetic algorithm
Full text linkThe steadily increasing performance requested to gyrotrons, to comply with their function of heating and current drive in fusion reactors, put a progressively increasing burden on the heat removal from the interaction cavity, where the heat flux can easily reach 20 MW/m2 on its inner surface. The cavity is actively cooled by subcooled water in forced flow in an annular region, and the water flow typically enters and exits through a single inlet and outlet. The high-speed flow entering from the inlet potentially drives an inhomogeneous flow azimuthally in the cavity region, but helps in locally bursting the heat removal due to the impinging effect of the cold-water flow. Here a new design of the water inlet in the cavity region is performed through a simplified genetic algorithm, in such a way that the flow homogeneity in the gyrotron cavity is maximized, without reducing the beneficial cooling due to the impact of the water jet on the cavity wall. The presence of multiple holes feeding the cavity, with their azimuthal location driven by the genetic algorithm, is analyzed through Computational Fluid Dynamics (CFD) simulations. Once the multi-inlet location is optimized, the dimension of the inlet holes is tuned to burst the heat transfer effect, reaching a high level of temperature and flow homogeneity in the cavity
Biogeography-Based Optimization of the Resonator Cooling in a MW-Class Gyrotron for Fusion Applications
No full textThe performance of MW-class gyrotrons, candidate technology for the plasma external heating in magnetic-confined fusion machines, crucially depends on the heat sink capability and thermomechanical stability of the resonator cavity. Because of the high and nonlinear heat flux and consequent high temperature reached on the cavity inner surface, high thermomechanical stresses are typically observed, and large displacements jeopardize the operation. An efficient cooling, typically with subcooled water in forced flow, beside reducing the displacement and stress of the cavity, allows to select operative modes with high beam/wave interaction efficiency. An optimized solution for the cavity cooling is addressed here, which should minimize the frequency shift of the radio frequency wave by controlling the displacements on the inner wall of the cavity while respecting the yield strength limit. The optimization study is based on a biogeography-based optimization (BBO) algorithm, which targets the optimal profile of the heat transfer coefficient (HTC) to the coolant, capable to guarantee that the maximum stress in the resonator is lower than its yield strength while minimizing the inner wall displacements. The optimum analytical HTC is translated into an engineering design and its thermal-hydraulic as well as mechanical performance (hot spot temperature, pressure drop, and displacements/stresses) is compared to the current existing cooling solution, showing potential advantages
Two-fluid modelling for poly-disperse bubbly flows in vertical pipes: Analysis of the impact of geometrical parameters and heat transfer
Full text linkThe bubbly flow of air or steam in subcooled water are investigated here in several test cases, characterized by different pipe sizes, bubble dimensions and flow rates, by means of CFD using a Eulerian-Eulerian approach. The performance of models that differ for the turbulence closure in the continuous phase, as well as for the description of the lift force on the dispersed phase, are compared in detail. When air is considered, the space of the experimental parameters leading to a reasonable performance for the selected models are identified and discussed, while the issues left in the modelling of the concurrent condensation are highlighted for the cases where steam is used
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
Development of an Equivalent Porous Medium Model for a Tubular Receiver Equipped With Raschig Rings
Full text linkThe porous insert has become one of the promising methods for heat transfer enhancement in many industrial applications ranging from small electronic devices to nuclear reactors, and large solar fields. For the assessment of such systems, the CFD numerical studies are usually employed by scientists to investigate the heat and mass transfer inside the region in micro or macro scales. Although micro studies are accurate and provide a detailed analysis of the process, they cannot be used for every study due to complex and costly computational resource they may demand for the case under study. Therefore, sometimes macro-scale simulations become more favorable thanks to the reduction in time and cost as well as the simplification over the morphology of the porous medium they offer. For these reasons, this study aims at developing a macro model for a novel porous disc made of Raschig Rings, to be applied to the tubular solar absorber for future simulations. The methodology devised in this study was to exploit detailed micro-scale simulations, achieving the macro properties and then developing a new equivalent macro model of a porous medium, based on the obtained properties. Numerical data indicated that when the developed macro model is compared to the micro simulations, the thermo-hydraulic results are in good agreement. Applying the macro model to a solar absorber working under linear Fresnel heating showed that the proposed porous disc could reduce the temperature rise on the tube wall by 40%
Test and modeling of the hydraulic performance of high-efficiency cooling configurations for gyrotron resonance cavities
Full text linkThe design and manufacturing of different full-size mock-ups of the resonance cavity of gyrotrons, relevant for fusion applications, were performed according to two different cooling strategies. The first one relies on mini-channels, which are very promising in the direction of increasing the heat transfer in the heavily loaded cavity, but which could face an excessively large pressure drop, while the second one adopts the solution of Raschig rings, already successfully used in European operating gyrotrons. The mock-ups, manufactured with conventional techniques, were hydraulically characterized at the Thales premises, using water at room temperature. The measured pressure drop data were used to validate the corresponding numerical computational fluid dynamics (CFD) models, developed with the commercial software STAR-CCM+ (Siemens PLM Software, Plano TX, U.S.A.) and resulting in excellent agreement with the test results. When the validated models were used to compare the two optimized cooling configurations, it resulted that, for the same water flow, the mini-channel strategy gave a pressure drop was two-fold greater than that of the Raschig rings strategy, allowing a maximum flow rate of 1 × 10–3 m3/s to meet a maximum allowable pressure drop of 0.5 MPa
Experimental and numerical investigation of a porous receiver equipped with Raschig Rings for CSP applications
Full text linkIn the context of central solar tower systems, tubular receivers are among the most appealing absorber solutions: the absorbed solar radiation is transferred from the tube external surface to the heat transfer fluid (HTF) flowing within the absorber. In the case of air as HTF, very high temperatures of the coolant can be obtained in principle, thus increasing the efficiency of the downstream thermodynamic cycle. To explore the possible applicability of a porous medium made of Raschig Rings (RRs), already successfully adopted in the heat removal from the resonant cavity of a technological device, the gyrotron, where the heat flux can go up to 20–25 MW/m2 and removed by subcooled water, a mock-up of a planar receiver equipped with RRs has been tested in a solar furnace, using air as coolant. The test results are presented here and analyzed1. Furthermore, a numerical model of the mock-up, where the RRs are modeled in detail by the Discrete Element Method, is presented and its capability to reproduce the measured data demonstrated. The model shows, for the tested configuration, an enhancement of the heat transfer of a factor of ~5 with respect to a plain channel with the same envelope, and a Performance Evaluation Criteria of 2–2.5 when the device is compared to the same receiver configuration, but without RRs
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