1,721,032 research outputs found
An efficient 3D CFD model for the analysis of the flow field around Darrieus rotors
No full textAn efficient numerical technique has been developed in order to investigate flow characteristics and global performance of Darrieus rotors. The interest for this kind of vertical axis wind turbines arises from their great capacity for integration within urban areas and for distributed generation.
The proposed methodology is based on the solution of the steady three-dimensional governing equations, by means of a robust commercial CFD code. Since the effect of the turbine blades on the flow field is simulated through the introduction of momentum sources in the porous shell representing the volume swept by the turbine blades, any expensive refinement of the grid, near the rotor, is avoided. This approach dramatically reduces the computational costs, with respect to conventional unsteady flow simulations. The model efficiency enables the simulation of the flow field around Darrieus rotors considering complex and realistic computational domains, for instance when these turbines are clustered within a wind farm or placed inside urban areas.
The methodology is validated by reproducing the performance of the Sandia 17-meter Darrieus rotor with approximate troposkien shape. Comparisons with other codes are also presented in order to highlight the advantages of the proposed method
Design and CFD performance analysis of a novel impeller for double suction centrifugal pumps
Full text linkDouble suction centrifugal pumps are the main devices involved in the feedwater system of nuclear power plants and they are responsible for a significant share of their energy consumption (by affecting the balance of both the gross and net electrical energy production), hence even an efficiency increase of only a few percentage points could be substantial in their economy. Herein a novel impeller designed for low-medium specific speed double suction centrifugal pumps (nq<60) is proposed showing an efficiency improvement with respect to conventional designs in the order of 1–2% associated to a slip factor increase, secondary losses reduction and impeller outflow homogeneity improvement. The novel double suction impeller is characterized by a new arrangement of its flow channels, which come up alternately on the same circumferential exit even if they start from the two different sides. The flow field through the impeller is investigated via numerical simulations run by means of the open source CFD code OpenFOAM and its performance is compared against experimental results. The CFD model set-up, in terms of grid size and discretization schemes, has been previously assessed against results of consolidated CFX simulations on a conventional centrifugal pump
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
Accurate numerical simulation of a high solidity Wells turbine
No full textThis paper provides an accurate description of the steady three-dimensional flow-field in a high solidity Wells turbine to be used in an oscillatory water column (OWC) device for wave energy conversion. The analysis has been performed by solving numerically the incompressible Navier–Stokes equations in a non-inertial reference frame rotating with the turbine. Turbulence effects are taken into account via a Reynolds-average approach, the turbulent stresses being evaluated by means of the Spalart–Allmaras model. Using a very fine mesh, a quite good agreement with experimental data has been obtained in terms of efficiency, torque, and total pressure drop, at different flow rates. Moreover, the tip clearance has been properly modeled and the effect of the tip-gap on the turbine performance has been analyzed in details
Analysis of the degradation of a Proton Exchange Membrane Fuel Cell for propulsion of a coastal vessel
Full text linkThe International Maritime Organization (IMO) is making efforts on reducing pollutant emissions within port areas in order to achieve the ambitious goal of zero net carbon emissions by 2050. This work analyses the effects of the cell degradation on the performance and thermal management of a hydrogen fueled Proton Exchange Membrane Fuel Cell (PEMFC) power system for ferry electric propulsion. The manuscript firstly describes the proposed models, which simulate the fuel cell system and the battery. Then, the development of both an Energy Management strategy and the optimization framework are shown. In details, the zero-order model for the fuel cell and the methodology to estimate its time degradation are described. The accuracy of the model is established through calibration with the characteristic curves of the Ballard FCvelocityTM HD6 PEMFC (150 kW) and further validated against experimental data. The model also involves energy storage and converters that connect both the fuel cell and battery to the electrical grid. The proposed online strategy for the Energy Management System considers four different operating modes and takes into account the estimated fuel cell degradation. Furthermore, the optimization framework finds the solution to achieve the best performance in terms of stack degradation over one year of ship operation, based on a real mission duty cycle. Overall, the efficiency of the fuel cell system decreases by 5%. The degradation involves an increase in fuel consumption of 14.65%. Furthermore, after the last mission, the cooling efficiency achieved through the proposed thermal management strategy is 90%
CFD modeling of pulverized coal combustion in an industrial burner
No full textThe accurate prediction of pulverized coal combustion in industrial application still remains a great challenge. This is mainly due to the lack of high quality experimental data acquired during the operation of industrial plants.
This work describes the CFD model used in order to numerically simulate the pulverized coal combustion of a full scale, swirl stabilized, aerodynamically staged, industrial burner. In particular, two different combinations of devolatilization and char burnout models were investigated comparing the numerical results with available experimental data obtained during a burner test carried out, in full-scale configuration, in a 50 MWth, fully instrumented, test rig.
In order to avoid any unrealistic assumption on pulverized coal distribution at the burner inlet, the entire primary air duct for pulverized coal transportation has been considered. The main flow is computed solving the steady, incompressible, three-dimensional, Reynolds Averaged Navier-Stokes (RANS) equations, whereas the pulverized coal is simulated as a reacting discrete second phase in a Lagrangian frame of reference, computing the trajectories of the discrete phase entities, as well as heat and mass transfer.
The numerical analysis confirms the very good burner performance obtained during the tests with a very low percentage of fixed carbon left in the ashes
Performance of a small prototype of a high solidity Wells turbine
No full textThis paper presents a numerical and experimental investigation of the performance of a small prototype of a high solidity monoplane Wells turbine without guide vanes. The prototype, with blades of constant chord and NACA0015 profile, has been designed to be matched with the REWEC (Resonant Wave Energy Converter) breakwater located off the beach of Reggio Calabria, which represents an excellent small scale model of full scale sea wave energy conversion devices for oceanic applications. Experimental tests, carried out in steady conditions for different flow rates, provided lower peak efficiency (ηmax = 0.41) in comparison to the performance of geometrically similar turbines with larger diameter or operated at higher rotational speed. The numerical analysis, performed by means of 3D numerical simulations using a code able to solve the steady incompressible RANS equations, discretized by a finite volume approach, showed a satisfactory agreement with experimental results, mainly as far as concerns torque and pressure drop, and confirmed the effects of the small scale of the turbine on the efficiency
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
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