1,720,971 research outputs found
Transported PDF Modelling of Soot-Radiation Turbulence-Chemistry Interactions
Get PDFThis thesis concerns modelling of diesel-relevant spray combustion, with a focus on the interconnected aspects of turbulence, chemistry, radiation and pollutant formation, particularly formation of soot. The approach taken to the modelling exploits the composition transported probability density function (TPDF) method, which has particular advantages for the consideration of pollutant formation and radiation that are not straightforwardly available in other methods. In particular, predictions of pollutant formation are facilitated by the chemical reaction source term appearing in closed form in the equations, while for predictions of radiation, emission source terms also appear in closed form. Focussing on a series of experimental target flames of the Engine Combustion Network (ECN) involving spray combustion of n-dodecane in constant volume chambers (known as spray A and its parametrics) the thesis makes contributions in assessing the TPDF methodology in terms of its ability to predict soot, radiation, and effects of multiple injections. In the approach adopted, soot is modelled with a two-equation acetylene precursor model, while where radiation is considered the discrete ordinates method is adopted and compared to an optically thin model. The TPDF mixing model employed is interaction by exchange with the mean. A particular focus is applied on the effects of turbulent fluctuations, known as turbulence-chemistry interactions (TCI) and turbulence-radiation interactions (TRI), which are assessed by comparison to simple well mixed (WM) models which ignore fluctuations. The key aspects of the study are as follows. First, a study of flame structure and soot formation is conducted for spray A (with a single long injection) under various ambient conditions. A wide array of chemical mechanisms is tested and evaluated, with clear conclusions about the advantages and limitations of the available mechanisms being established. For two selected reduced mechanisms, comparisons to experiment for overall flame structure as revealed by 355 nm planar laser-induced fluorescence (PLIF, imaging formaldehyde and polycyclic aromatic hydrocarbons) and shlieren images is reported, with generally good agreement. A detailed analysis of soot formation processes is presented and quantitative comparisons to experimental soot measurements reported. Soot is predicted well in the baseline spray A condition during the quasi-steady phase, but the initial transient phase needs further work and some off-baseline conditions are not predicted particularly well. TCI effects on soot were found in the present study to be relatively minor with the present choices for mixing model and other parameters. Next, the effects of radiation and turbulence-radiation interactions (TRI) are assessed and evaluated in terms of effects on pollutant formation. Radiation and TRI were found to play relatively minor roles in terms of both soot and NO; however, in contrast to soot, NO was significantly affected by TCI. Finally, cases having multiple injections are studied. The results are compared to experimental 355nm PLIF and reveal good agreement in qualitative trends to the experiments, with key differences between single and multiple injection cases being reproduced. The results are analysed further with respect to presumed probability density functions (PDFs). Overall the work progresses the TPDF approach both in terms of incorporating additional physics and by providing significantly enhanced comparisons to experiment compared with previous work for diesel sprays
On the Flame Shape in a Premixed Swirl Stabilised Burner and its Dependence on the Laminar Flame Speed
No full textGas turbines for power generation are optimised to run with fossil fuels but as a response to tighter pollutant regulations and to enable the use of renewable fuels there is a great interest in improving fuel flexibility. One interesting renewable fuel is syngas from biomass gasification but its properties vary depending on the feedstock and gasification principle, and are significantly different from conventional fuels. This paper aims to give an overview of the differences in combustion behaviour by comparing numerical solutions with methane and several different synthesis gas compositions. The TECFLAM swirl burner geometry, which is designed to be representative of common gas turbine burners, was selected for comparison. The advantage with this geometry is that detailed experimental measurements with methane are publicly available. A two-stage approach was employed with development and validation of an advanced CFD model against experimental data for methane combustion followed by simulations with four syngas mixtures. The validated model was used to compare the flame shape and other characteristics of the flow between methane, 40% hydrogen enriched methane and four typical syngas compositions. It was found that the syngas cases experience lower swirl intensity due to high axial velocities that weakens the inner recirculation zone. Moreover, the higher laminar flame speed of the syngas cases has a strong effect on the flame front shape by bending it away from the axial direction, by making it shorter and by increasing the curvature of the flame front. A hypothesis that the flame shape and position is primarily governed by the laminar flame speed is supported by the almost identical flame shapes for bark powder syngas and 40% hydrogen enriched methane. These gas mixtures have almost identical laminar flame speeds for the relevant equivalence ratios but the heating value of the syngas is more than a factor of 3 smaller than that of the hydrogen enriched methane. The syngas compositions used are representative of practical gasification processes and biomass feedstocks. The demonstrated strong correlation between laminar flame speed and flame shape could be used as a rule of thumb to quickly judge whether the flame might come in contact with the structure or in other ways be detrimental to the function of the combustion system.Validerad;2022;Nivå 2;2022-03-08 (hanlid);Funder: Swedish Gasification Centre (SFC);For correction, see: Papafilippou, N., Chishty, M.A. & Gebart, R. Correction to: On the Flame Shape in a Premixed Swirl Stabilised Burner and its Dependence on the Laminar Flame Speed. Flow Turbulence Combust (2021). https://doi.org/10.1007/s10494-021-00287-6</p
Systematic Assessment of the Two-Step, One-Way Coupled Method for Computational Fluid Dynamics
No full textThis paper assesses the validity of the Two-Step, One-Way (TSOW) coupled method for computational fluid dynamics, which splits a complicated geometry into an upstream and a downstream part. The problem is solved in two steps: first, the upstream part using approximate downstream boundary conditions, followed by a solution of the downstream flow where the inlet boundary conditions are extracted from the upstream solution. The method is based on two assumptions: first, the solution for the upstream part should be identical in the common domain to a complete solution. Second, the solution for the downstream part should be identical in the common domain to a complete solution. The resulting agreement between the upstream solution and the full solution was excellent, except in the vicinity of the outflow boundary. For the assessment of the second assumption, the downstream flow was simulated with two sets of boundary conditions, one that was extracted from the full simulation, and one that came from the upstream part solution. The two solutions in the downstream geometry with slightly different boundary conditions agreed excellently with each other but exhibited small differences from the full solution. Overall, the difference to the full solution is judged to be acceptable for many engineering design situations. The solution time for the TSOW method was about 23 h faster than the full solution, which took about 85 h on the same hardware. For additional design iterations, where the same upstream geometry can be used, a 30-h gain would be obtained for each step.Godkänd;2023;Nivå 0;2023-04-14 (hanlid);</p
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
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
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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