1,720,956 research outputs found
Renormalization-group techniques for single-field inflation in primordial cosmology and quantum gravity
No full textWe study inflation as a 'cosmic' renormalization-group (RG) flow. The flow, which encodes the dependence on the background metric, is described by a running coupling a, which parametrizes the slow roll, a de Sitter free, analytic beta function and perturbation spectra that areRGinvariant in the superhorizon limit. Using RG invariance as a guiding principle, we classify the main types of flows according to the properties of their spectra, without referring to their origins from specific actions or models. Novel features include spectra with essential singularities in a and violations of the relation r + 8nt = 0 to the leading order. Various classes of potentials studied in the literature can be described by means of the RG approach, even when the action includes aWeyl-squared term, while others are left out. In known cases, the classification helps identify the models that are ruled out by data. The RG approach is also able to generate spectra that cannot be derived from standard Lagrangian formulations
Flashback of H2-enriched premixed flames in perforated burners: Numerical prediction of critical velocity
Full text linkHydrogen has been identified as a potential tool for decarbonizing various industrial sectors by replacing natural gas while maintaining the combustion-based conversion system. However, due to its high reactivity, hydrogen is prone to flashback, making it essential to implement safety measures for its efficient use in practical appliances. This study investigates the flashback of different H2/CH4 mixtures at an equivalence ratio φ = 0.8 and proposes a numerical model to predict the critical flashback velocity for laminar premixed flames. The model considers the conjugate heat transfer between the gas phase and the burner plate. To overcome the computational cost needed to resolve both gas and burner heating, different time steps are employed for the fluid and the solid domains. Two flashback regimes are identified depending on the H2 content. The results suggest that using the laminar flame speed alone is inadequate for designing the safe operation of practical premixed burners
Effects of slit shapes on the flashback limits of premixed hydrogen slit burners
No full textWith a growing interest in hydrogen as a clean fuel for providing heat in residential and commercial sectors, the development of new designs for end-user devices is becoming increasingly necessary to ensure both efficiency and safety. In this work, we use 3D simulations to investigate the effects of slit shape and dimensions on the flash back limits for hydrogen premixed flames in perforated burners, typically employed in end-user devices such as condensing boilers. We compute flashback velocities for different slit lengths and widths and analyze the results in terms of the physical mechanisms involve
The importance of Soret effect, preferential diffusion, and conjugate heat transfer for flashback limits of hydrogen-fueled perforated burners
Full text linkAvoiding flashback is a primary challenge in the development of modern burners, which should be capable of substituting natural gas with hydrogen in domestic end-user devices. The heat exchange between the burner plate and the burned and unburned gases, as well as the effects of preferential diffusion, significantly impact the flashback of such burners fueled with hydrogen. For these effects, the design of the burner plate plays a pivotal role. In this study, three-dimensional simulations with detailed chemistry have been performed to investigate the effect of three competing physical mechanisms, namely, preheating of fresh gases, preferential diffusion, and Soret effect, which drive the flame flashback dependence on the holes/slits size. Two different geometries are considered: circular holes with varying diameters and slits with fixed lengths but different widths. Steady-state simulations with decreasing inlet velocities are employed to estimate the critical inlet velocity for flashback. Conjugate heat transfer (CHT) is considered for the heat exchange between the burner plate and the gases. For circular holes, the enclosed geometry promotes more effective heat transfer, leading to a higher influence of preheating effects for small diameters. This results in a non-monotonic dependence on hole size, with a non-trivial optimum diameter to avoid flashback. This behavior is specific to circular holes and differs from that observed in previously studied infinitely long slits, where a linear dependence on the slit width was found. Additionally, the individual influence of non-unity Lewis numbers and Soret diffusion is analyzed. Notably, the Soret effect, in combination with CHT, is found to instaurate a strong, non-linear, self-accelerating mechanism that has a leading-order effect on the flashback propensity of larger holes. This finding underscores the necessity of including both effects in numerical simulations for accurate estimations of the flashback limits in domestic burners
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
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