1,720,975 research outputs found
Analysis of thermodiffusive instabilities in hydrogen premixed flames using a tabulated flamelet model
No full textPreferential diffusion plays a critical role in the evolution of lean premixed hydrogen flames, influencing flame surface corrugation and overall flame behavior. Simulating such flames with tabulated chemistry (TC) methods remains challenging due to the complexity of flame dynamics. A detailed assessment of flamelet-based manifolds for capturing these dynamics is still needed. This work incorporates preferential diffusion via mixture-averaged molecular diffusion within TC to study the propagation and structure of freely propagating hydrogen flames influenced by intrinsic instabilities. Model performance is evaluated against detailed chemistry (DC) calculations, focusing on linear and non-linear regimes and sensitivity to pressure and temperature variations. The impact of mesh resolution on flame response is also examined to assess the method's capabilities without subgrid models. The linear regime is analyzed through the dispersion relation, revealing that higher temperature or pressure extends the range of wave numbers accurately predicted by the model, although some overprediction of flame wrinkling in stable regions is observed. The nonlinear regime is assessed by comparing global flame parameters and flame structure to reference solutions, showing that the model captures key flame descriptors with relative errors under 20%. Overall, the model effectively reproduces key effects governing flames with thermodiffusive instabilities, offering a viable alternative to DC at a significantly reduced computational cost
RANS-FGM simulation of n-heptane spray flame in OpenFOAM: A new implementation of flamelet generated manifold to account for enthalpy loss with detailed reaction mechanisms
No full textThe efficient and clean combustion of liquid fuels is a fundamental requirement in the design of future energy systems. Simulation plays a more and more important role in the design of such burners. In this work the spray combustion simulation approach introduced by Ma (2016) is improved, and validated against the CORIA Jet Spray Flame database (Verdier et al., 2017). The database presents droplet temperatures measured by global rainbow refractometry technique, which gives a unique insight in the flame structure. The two phase flow is treated with an Eulerian-Lagrangian approach. Flamelet Generated Manifold (FGM) is used to model the gas phase combustion. The RANS equations are solved using final volume method, with standard k − ε turbulence modelling. The turbulence-chemistry interaction is addressed with assumed probability density function method. The spray cloud is modelled with the Lagrangian transport of droplets including heat and mass transfer. Ma (2016) developed a solver based on the OpenFOAM 2.3.x libraries. His development is complemented in this work with a novel spray model. The improved spray modelling allows the treatment of droplet evaporation in the context of FGM without limiting the complexity of the chemical mechanism. This improvement is crucial for the modelling of complex fuels and the correct prediction of emissions. The modelling concept is rather light-weight considering the RANS approach. Despite the low computational expenses, most of the results agree fairly well with the measurement data. However the correct prediction of droplet temperature remains an an unresolved problem.Ma, L. (2016). Computational modelling of turbulent spray combustion. PhD Thesis, Delft University of Technology.Verdier, A., Santiago, J. M., Vandel, A., Saengkaew, S., Cabot, G., Grehan, G., and Renou, B. (2017). Experimental study of local flame structures and fuel droplet properties of a spray jet flame. Proceedings of the Combustion Institute , 36(2):2595-2602.Mechanical Engineering | Sustainable Process & Energy Technolog
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
High-fidelity numerical simulations of reacting flows with tabulated chemistry
No full textA la portada BSC(English) In the transition to net-zero carbon emission technologies, turbulent combustion is expected to retain an important role in various applications. In particular, the aviation industry is projected to widely adapt the usage of biofuels and hydrogen-based fuels in the coming decades, while the regulations concerning non-CO2 pollutant emissions are becoming more stringent. This transition entails both the gradual evolution of existing burner technologies, and the exploration of revolutionary new concepts. Large-eddy simulation of these combustion systems provides valuable insight into complex dynamic phenomena, such as flame stability and pollutant formation, thus it is an increasingly more important component of the engine development process. Such high-fidelity simulation approaches need to be further developed, gaining confidence in their applicability and exploring their limitations, while keeping in mind the complexity of available high-performance computing resources. This dissertation presents the development of computational tools for the study of multiphase reacting flows, and their application on model aero-engine combustors.
Specifically, the present work applies tabulated chemistry methods for combustion modeling, which allow for the representation of complex chemical phenomena while keeping the computational cost feasible. Furthermore, liquid fuel droplets are modeled in a Lagrangian manner, giving an intuitive representation of sprays. Stand-alone computational tools are created to facilitate the user friendly flamelet calculation and tabulation in a unified framework, and to increase confidence in droplet evaporation models through single droplet simulations. Meanwhile, a high-fidelity simulation method is implemented in the multi-physics simulation code: Alya, utilizing the various preexisting code elements, and exploiting the ongoing development efforts of a large team. In particular, the low-dissipation Navier-Stokes solver of Alya is extended here to variable density flows under low Mach number conditions. This Navier-Stokes solver is coupled with arbitrarily complex thermo-chemical tables in a new framework.
The developed stand-alone tools are used in this work to analyze the single droplet behavior, and to explore steady and temporally evolving laminar flamelets, and their applicability to chemistry tabulation. Subsequently the new framework of Alya is validated extensively and it is used in the analysis of tabulated chemistry methods. The turbulent combustion process in model aero-engines is studied with the developed method at atmospheric pressure. The flame stabilization phenomenon is analyzed in a swirl stabilized technically premixed hydrogen burner approaching flashback conditions. This turbulent hydrogen flame is studied using a perfectly premixed assumption and considering mixture fraction stratifications as well. Subsequently, the Cambridge swirl bluff-body burner is studied under different conditions. A non-premixed gaseous methane flame and two spray flames using n-heptane and n-dodecane fuels are simulated close to the lean blow-out limit, studying the shear induced localized extinction in detail. The prediction of this finite rate chemistry effect is a challenging task for the applied tabulated chemistry methods, however valuable insight is gained on the applicability of different thermo-chemical manifolds, and on the effect of the liquid fuel behavior.
Overall, the implemented low-dissipation finite element method is capable of predicting complex unsteady combustion phenomena, despite the simplicity of the tabulated chemistry methods. The work presented herein has been published in two peer reviewed journal articles and several conference contributions. The developed simulation framework enables the further study of industrially relevant combustion systems using high-performance computing resources.(Català) En la transició cap a tecnologies de zero emissions de carboni, es preveu que la combustió turbulenta mantingui un paper important en diverses aplicacions. En el cas de la indústria de l'aviació, s'espera un ús ampli de biocombustibles i combustibles basats en l'hidrogen en les properes dècades, mentre que les regulacions relatives a les emissions de contaminants no CO2 es tornen més estrictes. Aquesta transició implica tant l'evolució gradual de les tecnologies de combustió existents com l'exploració de nous conceptes revolucionaris. L'aplicació de la tècnica de 'large-eddy simulation' per simular aquests sistemes de combustió proporciona informació valuosa dels fenòmens dinàmics complexos, com l'estabilitat de la flama i la formació de contaminants, convertint-se cada vegada en una part més important del procés de desenvolupament del motor. Aquest tipus de simulacions d'alta fidelitat segueixen encara en desenvolupament, buscant millorar la seva fiabilitat i explorant les seves limitacions, tenint en compte la complexitat dels recursos de computació d'alt rendiment disponibles. Aquesta tesi presenta el desenvolupament d'eines computacionals per l'estudi de fluxos reactius multifàsica i la seva aplicació en models de combustors de motors d'aviació.
En concret, aquest estudi aplica mètodes de química tabulada per a la modelització de la combustió, que permeten la representació de fenòmens químics complexos, al mateix temps que mantenen un cost computacional factible. Les gotes de combustible líquid són modelades de manera Lagrangiana, donant una representació intuïtiva dels esprais. Eines computacionals són creades per facilitar el càlcul i la tabulació de 'flamelets' d'una manera senzilla, i per augmentar la confiança en els models d'evaporació de gotes. Al mateix temps, un mètode de simulació de alta fidelitat s'implementa en el codi de simulació multifísica Alya, utilitzant els diversos elements de codi preexistents i aprofitant els esforços d'un gran equip de programadors. En concret, el mòdul de Navier-Stokes de baixa dissipació d'Alya s'adapta aquí a fluxos de densitat variable en condicions de baix número de Mach. Aquest codi està acoblat amb taules termoquímiques en un nou marc de treball.
Els codis desenvolupades en aquest treball s'utilitzen per analitzar el comportament de gotes, i per explorar 'flamelets' estables i evolutius, així com la seva aplicabilitat a la tabulació química. El nou marc de treball d'Alya s'ha validat àmpliament i s'ha utilitzat en l'anàlisi de mètodes de tabulació química. El procés de combustió turbulent en motors d'aviació ha estat estudiat amb el mètode desenvolupat a pressió atmosfèrica. El fenomen d'estabilització de la flama s'ha analitzat en un cremador pre-mesclat d'hidrogen que s'apropa a les condicions de retrocés de flama. Aquesta flama d'hidrogen turbulent s'ha estudiat tan assumint una pre-mescla perfecta com considerant estratificacions de fracció de mescla. Posteriorment, s'ha estudiat el cremador de Cambridge tipus 'swirl bluff-body', en diferents condicions. S'han simulat una flama de metà no pre-mesclat i dues flames de pulverització utilitzant combustibles de n-heptà i n-dodecà, estudiant l'extinció localitzada amb detall. La predicció d'aquest efecte de química de taxa finita és una tasca difícil per als mètodes de tabulació química aplicats, però s'obté una idea sobre l'aplicabilitat de diferents mètodes i sobre l'efecte del comportament del combustible líquid.
En general, el mètode d'elements finits de baixa dissipació implementat és capaç de predir fenòmens de combustió complexos, malgrat la simplicitat dels mètodes de tabulació química. El treball presentat s'ha publicat en dos articles de revistes revisades per experts i en diverses contribucions a conferències. El marc de simulació desenvolupat permetrà ampliar l'estudi de sistemes de combustió industrialment rellevants utilitzant recursos de computació d'alt rendiment.DOCTORAT EN CIÈNCIA I TECNOLOGIA AEROESPACIALS (Pla 2013
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe 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
Analysis of thermodiffusive instabilities in hydrogen/air premixed flames using a tabulated flamelet model
No full textIn this work, a comprehensive formulation including detailed transport effects through mixture-averaged molecular diffusion in the context of tabulated chemistry is applied to the study of the propagation and structure of freely propagating hydrogen flames where intrinsic instabilities play an important role. The performance of the tabulated approach is evaluated by comparing its predictions with those from detailed chemistry calculations. The analysis focuses on two key aspects: the model's behaviour in both linear and non-linear regimes, and its sensitivity to pressure and temperature variations. Additionally, the impact of mesh resolution on the flame response is examined to assess the capabilities of the proposed method to recover the fundamental aspects of the flames. The analysis begins by examining the linear regime through the dispersion relation. The results indicate that thermodynamic conditions significantly influence the wavenumber range predicted by the tabulated model. Specifically, increasing temperature or pressure extends the model's predictive capability—either by reducing flame instability (at higher temperature) or by producing a thinner flame front (at higher pressure). However, some discrepancies in the dispersion relation within the linear regime, particularly for the stable range, are observed, revealing a slight tendency of the tabulated model to overpredict flame wrinkling. Subsequently, the non-linear regime is analysed by computing global flame parameters and comparing the flame structure with the reference solutions. The results show that the model accurately captures global flame descriptors for the three conditions investigated with relative errors of less than 10%. Considering the complexity of the physical and chemical phenomena involved, it can be concluded that the model successfully reproduces the most relevant effects governing flames exhibiting thermodiffusive instabilities and offers a reliable alternative to detailed chemistry with notably lower computational cost. Novelty and significance statement This research work contributes to delimiting the capabilities of a new formulation for a flamelet tabulated method that includes preferential diffusion through mixture-averaged diffusion to predict intrinsic instabilities in premixed hydrogen flames. To this end, several operating conditions are simulated to understand the influence of pressure and temperature on the accuracy of the model's response. The linear and non-linear regimes are studied and compared with the detailed chemistry solutions to provide an integral description. The novelty of this investigation lies in the demonstration that such formulation can recover significant characteristics of the flame that exhibits thermodiffusive instabilities for representative operating conditions.</p
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe 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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