1,722,512 research outputs found
Dissipative Range Scaling of Higher Order Structure Functions for Velocity and Passive Scalars
No full textDifferently to Kolmogorov's second similarity hypothesis, we find that the 2n-th order velocity and scalar structure functions scale with n-th order moment of the energy dissipation and the scalar dissipation, respectively. The origins of this scaling are analyzed by the transport equations of the fourth order velocity and scalar increment moments and by direct numerical simulations
Flamelet modeling of coal particle ignition
No full textThis paper numerically investigates the ignition of a single coal particle during the devolatilization phase in a laminar entrained-flow reactor, for which experimental data are available from Molina and Shaddix [3]. Different numerical approaches are combined to evaluate the non-premixed flamelet approach for coal particle ignition. First, the particle trajectory and the particle heating are simulated with a Lagrangianâ??Eulerian approach using a detailed pyrolysis model. In a second step, these results are used as transient boundary conditions for a simulation fully resolving the flow, the mixing field and the chemical reactions around the particle. Finally, in combination with the boundary conditions the time-dependent scalar dissipation rate profiles from the resolved particle calculation are used in a flamelet calculation for the particle up- and downstream directions. Very good agreement is obtained in terms of ignition delay as well as temperature and chemical species distributions in the mixture fraction space when the resolved particle calculation and the unsteady flamelet calculation are compared in the downstream direction. Good agreement is obtained when the numerical results for the ignition time and the time-averaged OH distribution are compared with the available experimental data. The results show the capability of the laminar flamelet approach to correctly predict coal particle ignition during devolatilization using accurate scalar dissipation rate profiles
Simulation of entrained flow gasification with advanced coal conversion submodels. Part 1: Pyrolysis
No full textCFD modeling results for entrained flow coal gasification using advanced submodels for coal conversion are presented and compared to detailed experimental data. The focus of this investigation is on the accurate modeling of the pyrolysis process. An iterative procedure is proposed and validated to bridge the gap between detailed pyrolysis models such as CPD, FLASHCHAIN or FG-DVC and empirical models based on single- or multiple-step kinetic expressions, which are usually used in CFD. Multiple particle heating rates from the CFD solution are taken to perform detailed pyrolysis calculations and these results are used to find optimal kinetic parameters for the empirical models using an automated procedure. It is shown that the heating rate strongly influences the devolatilization process (rate and yield). CFD simulations are performed for the BYU entrained flow gasifier. Due to the high heating rate in entrained flow gasification, the volatile yield can differ significantly from the proximate analysis value. Accurate pyrolysis modeling is shown to be important to capture coal flame ignition, flame location, species distribution and outlet composition. Since the final volatile yield determines the split in carbon conversion between pyrolysis and the subsequent fast conversion in the gas phase and the heterogeneous char conversion, which is a comparatively slow process under gasification conditions, it also directly influences the overall carbon conversion. Overall, the application of the new comprehensive CFD model including the fitted kinetic rates is shown to give similar results to the full coupling of the CFD and pyrolysis software. The comparison between the simulations and the experiments shows very good agreement for three out of four coals. The fourth coal (lignite with high O/C ratio) is well outside the range for which the detailed models were developed, but reasonable agreement is still obtained
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
twoWayGPBEFoam: An open-source Eulerian QBMM solver for monokinetic bubbly flows
Full text linktwoWayGPBEFoam is an open-source mesoscopic Eulerian QBMM solver for monokinetic bubbly flows. The solver is implemented within the OpenFOAM software framework. Unlike the existing macroscopic two-fluid model (TFM) solver twoPhaseEulerFoam, it can predict the size segregation phenomenon and the size-conditional velocities of the disperse phase, although it will not be able to predict the particle trajectory crossing (PTC). On theoretical grounds, the evolution of the disperse phase in multiphase flows is dictated by the generalized population balance equation (GPBE), which can be transformed into moment transport equations and solved using the finite-volume method with higher-order realizable spatial-discretization schemes and time-integration schemes. In order to address the closure problem of the size-conditional spatial flux, the size-conditional velocities need to be modeled. In many previous works, these are assumed to be identical with the disperse phase velocity predicted by the TFM. In this work, the size-conditional velocities were modeled using the velocity polynomial approximation (VPA), for which the velocity polynomial coefficients (VPCs) can be obtained from the moments themselves. By carrying out several test cases with both one-way and two-way coupling, we show that the results predicted by our solver agree well with the analytical solutions and the existing experimental data. Program summary: Program Title: twoWayGPBEFoam and oneWayGPBEFoam Program Files doi: http://dx.doi.org.ezproxy.biblio.polito.it/10.17632/rzstnw9ytw.1 Licensing provisions: GNU General Public License 3 Programming language: C++ Nature of problem: twoWayGPBEFoam and oneWayGPBEFoam have been developed to help investigate multiphase flows using the Eulerian QBMM. It provides an easily extended, parallelized, Eulerian QBMM environment. Solution method: The continuous phase is solved by the Eulerian approach. The disperse phase is solved by the QBMM. These equations are one-way coupled in oneWayGPBEFoam and two-way coupled in twoWayGPBEFoam. Additional comments including restrictions and unusual features: All appropriate methodological references are contained in the section entitled References
CFD Modeling of Gas-Fuel Interaction and Mixture Formation in a Gasoline Direct-Injection Engine Coupled with the ECN Spray G Injector
No full textThe thorough understanding of the effects due to the fuel direct injection process in modern gasoline direct injection engines has become a mandatory task to meet the most demanding regulations in terms of pollutant emissions. Within this context, computational fluid dynamics proves to be a powerful tool to investigate how the in-cylinder spray evolution influences the mixture distribution, the soot formation and the wall impingement. In this work, the authors proposed a comprehensive methodology to simulate the air-fuel mixture formation into a gasoline direct injection engine under multiple operating conditions. At first, a suitable set of spray sub-models, implemented into an open-source code, was tested on the Engine Combustion Network Spray G injector operating into a static vessel chamber. Such configuration was chosen as it represents a typical gasoline multi-hole injector, extensively used in modern gasoline direct injection engines. Afterwards, the Spray G injector was coupled with the Darmstadt optical engine and full-cycle simulations were carried out for three operating points, combining two engine speeds, respectively equal to 800 rpm and 1500 rpm, and two different engine loads, with pressures of 0.95 bar and 0.4 bar in the intake manifold. The case at 800 rpm and 0.95 bar represented the reference condition. By switching to 1500 rpm and 0.95 bar the effect of the piston speed on the in-cylinder flow and spray evolution was analysed, while the reduction of the intake pressure down to 0.4 bar, coupled with the engine speed of 800 rpm, allowed to study the effects of the engine load on spray evolution and mixture fraction formation. Furthermore, comparisons between the engine cases at 0.95 bar and the simulations in vessel allowed to understand the effects exerted by the turbulence generation on the spray morphology. A detailed post-processing was proposed for each condition. For the vessel, axial vapor and liquid penetrations were assessed, along with spray morphology and liquid mass distribution inside the jet. In the engine, quantities such as in-cylinder gas velocities, mixture fraction distribution and charge homogeneity were investigated. The achieved results demonstrated the potential of the computational fluid dynamics as an effective tool for direct-injection, spark ignition engines optimization towards the goals of emissions reduction and increased efficiency
Numerical Investigation on the Effect of the Oxymethylene Ether-3 (OME3) Blending Ratio in Premixed Sooting Ethylene Flames
Full text linkSynthetic fuels, especially oxygenated fuels, which can be used as blending components, make it possible to modify the emission properties of conventional fossil fuels. Among oxygenated fuels, one promising candidate is oxymethylene ether-3 (OME3). In this work, the sooting propensity of ethylene (C2H4) blended with OME3 is numerically investigated on a series of laminar burner-stabilized premixed flames with increasing amounts of OME3, from pure ethylene to pure OME3. The numerical analysis is performed using the Conditional Quadrature Method of Moments combined with a detailed physico-chemical soot model. Two different equivalence ratios corresponding to a lightly and a highly sooting flame condition have been investigated. The study examines how different blending ratios of the two fuels affect soot particle formation and a correlation between OME3 blending ratio and corresponding soot reduction is established. The soot precursor species in the gas-phase are analyzed along with the soot volume fraction of small nanoparticles and large aggregates. Furthermore, the influence of the OME3 blending on the particle size distribution is studied applying the entropy maximization concept. The effect of increasing amounts of OME3 is found to be different for soot nanoparticles and larger aggregates. While OME3 blending significantly reduces the amount of larger aggregates, only large amounts of OME3, close to pure OME3, lead to a considerable suppression of nanoparticles formed throughout the flame. A linear correlation is identified between the OME3 content in the fuel and the reduction in the soot volume fraction of larger aggregates, while smaller blending ratios may lead to an increased number of nanoparticles for some positions in the flame for the richer flame condition
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