1,720,955 research outputs found
Una metodologia estensiva per simulazioni Lagrangiane 3D-CFD di iniettori benzina e diesel
No full textNell’ambito di simulazioni 3D-CFD interno cilindro, una corretta modellazione del processo di iniezione è fondamentale per ottenere una valida previsione del miscelamento aria-combustibile, ovvero di combustione, autoaccensione, ed emissioni. Lo scopo principale di questa tesi è l’elaborazione di una metodologia estensiva per la caratterizzazione numerica del processo di iniezione per spray benzina e diesel. Tre fenomeni importanti per l’evoluzione dello spray sono stati analizzati: il break-up primario, il break-up secondario, ed il flash-boiling; essi sono stati analizzati ed una metodologia alternativa per la loro modellazione è proposta. Le simulazioni numeriche sono state effettuate con due codici commerciali, STAR-CCM+ e STAR-CD, entrambi rilasciati da Siemens. I risultati sono validati tramite dati sperimentali in termini di portata di iniezione, immagini di spray, curve di penetrazione, ed analisi Phase Doppler Anemometry (PDA). Per il break-up primario, una strategia di atomizzazione è proposta al fine di estendere le capacità predittive delle simulazioni per un’ampia gamma di condizioni operative. A tal scopo sono presentate simulazioni Lagrangiane 3D-CFD per due iniettori multi-foro. Il primo è un prototipo GDI 5-fori, operante a condizioni ambiente, il secondo è l’iniettore SprayG fornito dall’ Engine Combustion Network (ECN), caratterizzato da una maggiore contropressione. Con lo scopo di validare la strategia di break-up primario, è riportata una simulazione interno iniettore per lo SprayG, con la quale si ottengono informazioni sulla velocità ed il diametro della colonna liquida in uscita dal polverizzatore. Per il break-up secondario, un nuovo modello, d’ora in poi indicato come ‘GruMo’, è proposto. Lo scopo è quello di semplificare l’attività di calibrazione per il break-up secondario nelle simulazioni Euleriane-Lagrangiane per spray benzina e diesel. Pertanto, i parametri del modello vengono impostati come funzioni della densità ambiente, che incide direttamente sulla rottura delle gocce liquide. Il set di funzioni per iniettori benzina è calibrato su un iniettore mono-foro GDI per tre condizioni operative. In seguito, il modello viene validato su un differente iniettore mono-foro GDI, e sull’iniettore GDI 5-fori menzionato in precedenza. Per gli iniettori diesel i parametri del modello sono calibrati sul noto SprayA, fornito da ECN, il quale è rappresentativo di applicazioni commerciali leggere. Successivamente, il modello viene validato su due iniettori mono-foro, chiamati SprayC e SprayD, entrambi rappresentativi di applicazioni commerciali pesanti. Il modello ‘GruMo’ fornisce un buon accordo tra dati sperimentali e numerici per tutti gli iniettori testati, senza una calibrazione dedicata. Viceversa, i modelli più diffusi come il Reitz-Diwakar ed il KHRT, adottati per simulare gli stessi iniettori con costanti di calibrazione di default, forniscono risultati che differiscono significativamente dai dati sperimentali. Infine, il fenomeno di flash-boiling viene analizzato. Esso ricopre potenzialmente un ruolo chiave nel conseguimento di una distribuzione di combustibile target all’interno della camera di combustione per un ampio intervallo di condizioni operative. Infatti, in determinate condizioni, il combustibile subisce un break-up estremamente rapido che porta ad un’accelerata vaporizzazione. In questo lavoro di tesi, è presentata l’applicazione di un modello di flash-boiling alternativo, recentemente implementato da Siemens-PLM in STAR-CD, su un iniettore mono-foro da ricerca. Il nuovo approccio consiste in tre parti principali: un modello di atomizzazione che determina le condizioni iniziali delle gocce e l’angolo di cono complessivo; un modello di vaporizzazione ed un modello di break-up secondario.A proper modelling of the injection process is mandatory in 3D-CFD in-cylinder simulations, in order to predict the correct formation of the air-fuel mixture, that directly affects combustion, knock and emissions. Therefore, the main goal of this thesis is the formulation of an extensive methodology for the numerical characterization of the injection process for both gasoline and diesel sprays. In particular, the attention is focused on three different phenomena, namely primary break-up, secondary break-up and flash-boiling, which play a significant role in the spray evolution. In the present work, such aspects are investigated and an alternative solution for their modelling is proposed. Numerical simulations are carried out via commercial codes, i.e. STAR-CCM+ and STAR-CD, both licensed by Siemens. Results are validated using experimental datasets consisting of injection rate, spray imaging, liquid penetration curve, and Phase Doppler Anemometry (PDA) data. As for the primary break-up, an atomization strategy is proposed, aiming at extending simulation predictive capabilities over a wider range of operating conditions. 3D-CFD Lagrangian simulations of two different multi-hole injectors are presented. The first is a 5-hole GDI prototype unit operated at ambient conditions, while the second one is the SprayG injector released by the Engine Combustion Network (ECN), characterized by a higher back pressure. Moreover, to validate the primary break-up strategy adopted for the initialization of the droplets, an internal nozzle flow simulation is carried out on the Spray G injector, able to provide information on both velocity and diameter of the liquid jet at the nozzle exit. As for the secondary break-up, a novel model, hereafter indicated as “GruMo”, is proposed. It aims at minimizing the secondary break-up calibration efforts in Eulerian-Lagrangian simulations for both gasoline and diesel sprays. Model parameters are assumed as functions of the ambient density, which directly impacts the disruption of liquid droplets into smaller ones. The set of functions for gasoline injectors is calibrated on a single-hole GDI injector for three different operative conditions. Thereafter model validation is carried out on two different GDI injectors: the first is again a single-hole while the second is the 5-hole GDI prototype mentioned above. In case of Diesel injection, model parameters are calibrated on the well-known SprayA still provided by the ECN, which can be assumed as representative of injectors for light-duty applications. Afterwards, the model is validated on two different single-hole injectors, namely SprayC and SprayD, both representative of injectors for heavy-duty applications. The new “GruMo” model provides a good agreement between numerical and experimental outcomes for all the tested injectors, without any dedicated tuning. Conversely, the most popular models, such as Reitz-Diwakar and KHRT ones, adopted to simulate the same injectors with default calibration constants, provide results which significantly deviate from the experiments. Finally, the flash-boiling phenomenon is faced. It can potentially play a key role to achieve the required fuel distribution inside the combustion chamber over a wide range of engine operating conditions. In fact, under certain conditions, the fuel undergoes extremely accelerated break-up and quickly evaporates. In the present work, the application of an alternative flash-boiling model, recently implemented by Siemens-PLM in STAR-CD, is shown on a single-hole research injector. The new flash-boiling model consists of three main parts: an atomization model able to compute the droplet initial conditions and the overall spray cone angle; an evaporation model and a droplet secondary break-up model
Evaluation of the single jet flow rate for a multi-hole GDI nozzle
Full text linkFuel injectors featuring differentiated hole-to-hole dimensions improve the fuel distribution in the cylinder ensuring a more efficient and cleaner combustion for GDI (Gasoline Direct Injection) engines. A proper diagnostic system able to detect the actual fuel flow rate exiting each hole of a GDI nozzle is requested in order to optimize the matching between the spray and the combustion chamber. Measuring the spray impact force of a single plume allows the detection of the momentum flux exiting the single hole and, under appropriate hypotheses, the evaluation of the corresponding mass flow rate time-profile. In this paper two methodologies for the hole-specific flow rate evaluation, both based on the spray momentum technique, were applied to two different GDI nozzles, one featuring equal hole dimensions and one with two larger holes. Three different energizing times at 100 bar of fuel pressure were tested in order to cover a wide range of operating conditions. The results were validated in terms of injected mass by means of a proper device able to collect and weigh the fuel injected by each single nozzle hole, and in terms of mass flow rate using a Zeuch-method flow meter as reference. Both the proposed methodologies showed an excellent accuracy in the fuel amount detection with percentage error lower than 5% for standard energizing times and lower than 10% for very short injections working in ballistic conditions. The mass flow rate time-profile proved a good accuracy in the detection of the start and end of injection and the static flow rate level
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
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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