1,720,960 research outputs found
Effects of three-dimensional slit geometry on flashback of premixed hydrogen flames in perforated burners
No full textAddressing flashback represents a pivotal challenge in the advancement of innovative perforated burners intended to substitute natural gas with hydrogen in household appliances. Current numerical models, employing 2D configurations to estimate flashback velocities in the slits of such burners, offer valuable insights with reasonable computational costs. However, the inherent complexity of the phenomena suggests that a 2D model may inadequately capture flashback dynamics, resulting in inaccurate estimations of flashback limits. In this study, 3D simulations are employed for the first time to explore the impact of the three-dimensional shape of slits on the flashback limits of hydrogen-premixed flames. Steady-state simulations are conducted to compute flashback limits for different equivalence ratios, investigating slits with fixed width and varying lengths up to 8 mm. Additionally, transient simulations are performed to investigate the flashback dynamics. The results are compared with those from 2D configurations to assess the reliability of the infinite slit approximation. Notably, 2D simulations significantly underestimate flashback limits as the critical initiation region is consistently located at the slit extremities, which are neglected in 2D configurations. For the same reason, the flashback velocity exhibits weak dependence on slit length, since the flashback is consistently initiated at the slit far ends regardless of length. The physical mechanisms driving the initiation of flashback in that zone are identified as preferential diffusion effects, which cause the enrichment of the mixture at the slit extremities, and enhanced heat transfer promoted by the enclosed geometry, which increases the pre-heating of the fresh gases in that regions
The importance of Soret effect, preferential diffusion, and conjugate heat transfer for flashback limits of hydrogen-fueled perforated burners
No full textAvoiding 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.</p
Three-dimensional numerical investigation of flashback in premixed hydrogen flames within perforated burners
No full textPredicting flashback represents a pivotal challenge in the development of innovative perforated burners for household appliances, especially for substituting natural gas with hydrogen as fuel. Most existing numerical studies have utilized two-dimensional (2D) simulations to investigate flashback in these burners, primarily to reduce computational costs. However, the inherent complexity of flashback phenomena suggests that 2D simulations may inadequately capture the flame dynamics, potentially leading to inaccurate estimations of flashback limits. In this study, three-dimensional (3D) simulations are employed to examine the impact of the actual slit shapes on the flashback velocities of hydrogen-premixed flames. Steady-state simulations are conducted to compute flashback velocities for three equivalence ratios (ϕ=0.6, 0.8, and 1.0), investigating slits with fixed width W and varying length L. Additionally, transient simulations are performed to investigate the flashback dynamics. The results are compared with those from 2D configurations to assess the reliability of the infinite slit approximation. For stable flames, 2D simulations underpredict the burner plate temperature compared to slits with lengths typical of practical devices but match the 3D results as L→∞. Conversely, flashback velocities are consistently underpredicted in 2D simulations compared to 3D simulations, even as L→∞. This is due to the critical role of the slit ends in flashback dynamics, where favorable aerodynamics, preferential diffusion, the Soret effect, and higher preheating due to a higher surface-to-volume ratio trigger the initiation of flashback in those regions. These findings underscore the necessity of employing 3D simulations to accurately estimate the flashback velocities in domestic perforated burners. Novelty and significance statement This study presents a novel investigation into how finite slit lengths affect the critical flashback velocities in hydrogen-fueled perforated burners, using three-dimensional simulations. Our findings indicate that two-dimensional configurations, which are widely used in the literature, significantly underpredict flashback velocities because they fail to capture the crucial influence of slit ends. For the first time, we show that in slits of finite length and circular holes, the combined effect of favorable aerodynamic conditions and enhanced preheating, due to the increased surface area available for heat transfer, leads to higher flashback velocities compared to infinite-length slits. Additionally, we provide the first analysis of the temporal evolution of flashback dynamics in a realistic three-dimensional configuration, demonstrating that flashback initiation occurs at the slit ends. These insights are essential for the development of advanced numerical models that can inform the design of innovative perforated burners to prevent flashback effectively.</p
Predizioni Testabili della Gravità Quantistica in Cosmologia Inflazionaria
No full textITALIANO
Lo scopo di questo lavoro di tesi è calcolare gli spettri di potenza e gli indici spettrali delle fluttuazioni primordiali scalari e tensoriali per la nuova teoria della gravità quantistica, formulata da Anselmi nel 2017. Tali quantità sono direttamente collegabili ad odierne osservazioni della radiazione cosmica di fondo, e possono dunque costituire delle predizioni testabili della teoria. Dopo un'introduzione sul concetto di particella fake in teoria dei campi su spazio piatto, studiamo come estendere il concetto alla teoria dei campi su spazio curvo, per le applicazioni alla cosmologia, e usiamo questi risultati per calcolare in modo sistematico gli indici spettrali attraverso uno sviluppo in serie nei parametri di slow-roll attorno al background di de Sitter. Ci concentriamo dapprima sul limite di massa infinita del fakeon, corrispondente alla teoria R+R^2 di Starobinsky, sviluppando un metodo che consente di calcolare le quantità di interesse a qualsiasi ordine perturbativo nei parametri di slow-roll. In seguito, applichiamo tali tecniche alla nuova teoria della gravità quantistica. In questo caso, il calcolo richiede sforzi aggiuntivi a causa della necessità di utilizzare la prescrizione e la proiezione fake: i risultati ottenuti per gli spettri di potenza e gli indici spettrali sono all'ordine next-to-leading nei parametri di slow-roll. La speranza è che, nel vicino futuro, saranno sviluppate nuove tecnologie nell'ambito delle osservazioni della CMB, per aumentare la risoluzione sperimentale e verificare i risultati di questa tesi, che rappresenterebbero una validazione per la nuova teoria della gravità quantistica.
ENGLISH
The purpose of this thesis is to compute the power spectra, the spectral indices and the runnings of the primordial scalar and tensor fluctuations for the new theory of quantum gravity, formulated by Anselmi in 2017. These quantities are directly related to present-day observations of the CMB, and can therefore constitute testable predictions of the theory. After an introduction to fake particles in flat space, we study how to extend the concept to curved space, and we use these results to systematically compute the spectral indices by means of a series expansion in slow-roll parameters around the de Sitter background. We first focus on the limit of infinite fakeon mass, which gives the Starobinsky's R+R^2 theory, developing a procedure that makes it possible to calculate the results at any perturbative order. Next, we apply these techniques to the new theory of quantum gravity. In this case, the calculation requires additional efforts due to the fakeon prescription and projection: the results obtained for the power spectra and the spectral indices are to the next-to-leading order in the slow-roll parameters. We hope that new technologies for CMB observations will be developed in the next future to increase the experimental resolution and verify the results of this thesis, which would be a validation of the new theory of quantum gravity
Flashback dynamics of H2-enriched flames in perforated burners: a numerical and stochastic sensitivity analysis
No full textThe decarbonization of energy systems, particularly within the heating sector, necessitates the deployment of low-emission technologies capable of integrating with existing infrastructure. Hydrogen-enriched combustion offers a promising route to this end; however, the propensity of hydrogen for flashback, defined as the upstream propagation of the flame into premixing regions, poses significant safety challenges in practical applications such as condensing boilers. This thesis presents a comprehensive numerical investigation of flashback and autoignition phenomena in premixed hydrogen-methane-air and hydrogen-air flames stabilized by perforated burner plates, with a specific focus on multi-slit geometries.
From a physical standpoint, the study analyzes how flame dynamics, heat transfer, and species transport influence flashback behavior across different burner configurations. Direct Numerical Simulations (DNS), incorporating conjugate heat transfer, indicate that flashback is strongly affected by thermal feedback mechanisms, preferential diffusion, and the Soret effect. Two distinct flashback regimes are identified, with the transition between them depending on hydrogen content and burner temperature. A strong correlation emerges between these mechanisms and the geometric characteristics of the burner, highlighting how local physical processes are coupled with design features. Three-dimensional simulations are found to be necessary to capture key phenomena, especially the localized interactions at slit ends (absent in traditional two-dimensional models) that enhance flashback velocities through intensified preheating and local fuel enrichment. The analysis also addresses autoignition-driven flashback at elevated wall temperatures, examining critical Damköhler-number regimes and geometric effects on ignition thresholds.
To support practical burner design, the research employs a suite of computational tools for high-dimensional uncertainty quantification and design space exploration. Surrogate models based on generalized Polynomial Chaos (gPC) and sparse grids are developed to approximate DNS-derived quantities such as flashback velocity and critical burner temperature. These models enable rapid parametric sweeps and global sensitivity analyses, quantifying the relative importance of geometrical and operational parameters in shaping flashback behavior. A surrogate-informed sparse grid framework is also proposed to explore the impact of slit geometry with reduced computational cost. These tools are intended to support design decisions under uncertainty for hydrogen-compatible combustion systems.
By combining detailed physical modeling with surrogate-based analysis, the thesis aims to contribute to the understanding and predictive modeling of flashback phenomena, with potential implications for the development of safer and more efficient hydrogen-fueled heating technologies
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
- …
