1,721,013 research outputs found
On the Scalability of a Partially Premixed Low NOx Hydrogen Burner
Full text linkIncreasingly stringent regulations on nitric oxide (NOx) emissions led to the development of various low and ultra low NOx combustion technologies over the last decades. These technologies are further challenged by the increasing demand for fuel flexibility and rising contribution of non-carbon fuels to the global fuel mix, such as for example hydrogen. Modern burners incorporate complex designs and flow regimes, to address the high burning velocities and temperatures associated with hydrogen and hydrogen rich fuels, allowing stable combustion and low NOx emissions. The development process of such burners requires the scaling of the burner geometry to facilitate simulations and testing at laboratory scale. Scaling is conducted by employing scaling laws, that aim to preserve relevant burner characteristics.
The present doctoral thesis investigates the effect of the most commonly used scaling laws, constant velocity and constant residence time scaling, on complex burner designs. Both scaling laws are derived based on "simple" flames that can be defined by a single length and velocity scale. Previous studies have, however, indicated that such global scaling laws fail when applied to complex industrial burners, where flow conditions can vary significantly for different burner regions. The main focus of the present study is, therefore, the analysis of local effects on the combustion and NOx characteristics. The analysis was conducted based on a partially premixed bluff body (PPBB) burner, which incorporates an inner and outer recirculation zone, stagnation point flow, staged fuel injection and varying degree of partial premixing; all typical features of complex, modern burner designs.
A numerical model of the PPBB burner was developed and validated against experimental data. The simulation of turbulent combustion processes, including detailed combustion kinetics, heat radiation and NOx formation, at various burner scales is computational demanding. Hence, a compromise between model accuracy and costs is sought after. Considering this, a non reacting flow configuration of the burner was investigated first, which made it possible to conduct the CFD simulations with a wide variety of different turbulence models, ranging from steady state Reynolds averaged (RANS) to transient scale resolving simulations. Varying the complexity of the turbulence model allowed the identification of model requirements and exposed limitations for the cost effective RANS models. The non reactive model was, thereupon, further extended to incorporate species transport, detailed hydrogen combustion kinetics, turbulence chemistry interaction, radiation and NOx formation. The extended model was employed to conduct twelve simulations of different operational conditions, i.e., secondary fuel fraction and thermal loads. Furthermore, a postprocessing routine was developed to incorporate scatter data, obtained from the CFD simulations, in combustion regime diagrams, leading to a more refined representation of the burner characteristics. Finally the burner geometry was scaled based on the constant velocity as well as the constant residence time scaling approach, and CFD simulations at various scales in the range of 10 kW to 500 kW were conducted. The simulations were used to study the effect of the two different scaling laws on the local NOx formation rate and flame characteristics, and to compare the CFD prediction to scaling theory. The fluid domain was, furthermore, decomposed in order to investigate different burner regions, and their contribution to the NOx formation individually. Based on the identified burner regions, work on a reactor network of the PPBB burner was initiated.
The non reacting flow simulations showed, for all investigated turbulence models, good agreement between the simulated and the measured recirculation zone length. However, all RANS models generally overpredicted the velocity magnitudes, especially within the inner recirculation zone, while scaling resolving simulations resulted in a reasonable good agreement between simulation and experiment. The predicted NOx emissions, for a wide range of operational conditions, were between +10% and -20% of the measured data, underpredicting them in average by 7%. The model was, furthermore, able to predict the overall NOx trend for varying thermal loads, while it failed to reproduce the effect of a varying secondary fuel fraction on the NOx emissions at low thermal loads, of 10 kW.
The detailed analysis of the flame characteristics in CFD based combustion regime diagrams revealed that primary fuel burns in a multi regime mode, mainly in the thin reaction zone regime, spanning a wide range of equivalence ratios, while secondary fuel is burned closer to the global and stoichiometric equivalence ratio. The simulations showed, furthermore, a transition from a fuel lean to a fuel rich mixture in the inner recirculation zone, when the secondary fuel fraction is increased from 0% to 30%. The numerical model was utilized to simulate the burner at various scales in the range of 10 kW to 500 kW. Constant velocity scaling leads to lower Karlovitz numbers and ultimately to a transition, for large parts of the flame, from the thin reaction zone regime to the corrugated flamelet regime. Constant residence time scaling, on the other hand, preserves the global Damköhler number. However, a significant part of the flame follows a constant Karlovitz number, close to unity, which is not expected based on scaling theory. Constant velocity scaling leads due to the larger volumes and residence times to overall higher NOx emissions, with exception of the inner recirculation zone, even though constant residence time leads locally to the larger mean volumetric formation rates. An interesting finding, regarding flame stability, was found for constant velocity scaling, which led to the sudden breakup of the inner recirculation zone at a scale in-between 450 kW and 500 kW, while the flame is stable at 500 kW, when constant residence time scaling is applied
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
Author Under Sail The Imagination of Jack London, 1893-1902
No full textIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Intro -- Title Page -- Copyright Page -- Dedication -- Contents -- Acknowledgments -- Introduction -- 1. Spirit Truth -- 2. From Absorption to Theatricality and Back Again -- 3. "I Will Build a New Present" -- 4. Sons as Authors -- 5. Fathers as Publishers -- 6. The Daughter as Author -- 7. Lovers as Authors -- 8. At Sea with the Family -- 9. Yellow News, Yellow Stories -- 10. The Return Home -- Notes -- Bibliography -- Index -- About Jay WilliamsIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Description based on publisher supplied metadata and other sources.Electronic reproduction. Ann Arbor, Michigan : ProQuest Ebook Central, YYYY. Available via World Wide Web. Access may be limited to ProQuest Ebook Central affiliated libraries
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