1,721,015 research outputs found
Population balance modeling and large-eddy simulation of soot formation and oxidation
No full textTo meet the increasing global demand of usable energy and mobility, the combustion of fossil as well as renewable fuels in multifarious applications such as power plants, vehicles, airplanes, industrial furnaces, and household heating systems, is of utmost importance. Pivotal targets for the development of next-generation, high-fidelity combustion devices are the enhancement of the energy efficiency, applicability to alternative fuels and combustion strategies, and the reduction of pollutant emissions, in particular soot emissions due to their adverse impact on the environment as well as human health.Computational Fluid Dynamics simulations have been established as an integral part of the development process of modern combustion applications. With increasing computing capacities and focus on pollutant emissions, theinterest in Large-Eddy Simulations (LES) is growing. In particular, the accuracy of simulations of soot formation in turbulent flames largely benefits from the resolution of large-scale turbulent fluctuations in LES. However, a furtherexploitation of the predictive potential of LES requires the improvement of a variety of submodels that an integral soot model is composed of as well as appropriate coupling strategies for these model components. Furthermore,methodologies are needed for the systematic analysis of integrated models to identify critical modeling assumptions regarding the overall performance.The present thesis aims at contributing to both the systematic analysis of integrated LES models for soot evolution and the development of submodels of such integrated modeling suites. The first part of the thesis focuses on theassessment and development of moment methods for the solution of population balance equations governing the soot particle dynamics. A combined a-priori and a-posteriori analysis of different interpolation-based moment methods led to recommendations for an optimal choice of interpolation functions and orders to achieve highly accurate soot predictions for a variety of laminar and turbulent flame conditions. In addition, an advanced quadrature-based moment method, which also provides a reconstruction of an approximation of the soot Number Density Function (NDF), was coupled with a method to describe the disappearance of particles during soot oxidation in a mathematically rigorous manner. Furthermore, a multivariate physico-chemical soot model accounting for the diversity of the chemical composition of soot particles was formulated and validated for the evolution of the NDF in alaminar flame.Next, the propagation of errors related to flamelet assumptions into soot quantities was investigated taking advantage of large-scale Direct Numerical Simulation (DNS) data of a sooting turbulent jet flame. Various terms coupling flamelet-based combustion models with soot models were first analyzed a-priori. Then, a partial a-posteriori analysis was performed, where the soot evolution along Lagrangian trajectories was computed using the flow field and selected thermodynamic properties extracted from the DNS. The resulting error decomposition and quantification identified the modeling of polycyclic aromatic hydrocarbon-based soot growth processes to be associated with uncertainties of leading order. Eventually, LES of a model aircraft combustor were performed employing a flamelet combustion model and a state-of-the-art soot model based on a bivariate description of the soot particles
Large eddy simulation of soot formation in compression ignition engines
No full textThe urgent need to mitigate climate change demands effective strategies for rapidly reducing greenhouse gas (GHG) emissions. Alternative fuels derived from renewable energy offer a promising approach to reduce GHG emissions while utilizing existing infrastructure. Furthermore, these fuels can be tailored for specific applications to reduce the formation of pollutants, such as soot. To fully exploit their potential, a detailed understanding of the complex pollutant formation process is required. However, the available models are not accurate enough, especially when predicting soot. A major challenge is modeling turbulence-chemistry interactions (TCIs). Since the gas-phase chemistry is significantly influenced by the local mixing field, TCIs are often considered using mixture fraction-based models, which have relatively low computational costs. These models solve for the mixture evolution in physical space, while reactive scalars are calculated based on mixture fraction. Conventional models, however, often disregard that scalar transport in mixture fraction space must be formulated consistently with the mixture fraction evolution in physical space.This thesis demonstrates that this inconsistency leads to significant mass conservation errors for certain scalars, such as soot mass. Another contribution of this work is the development of two analytically consistent mixture fraction-based models, which exhibit better predictive accuracy than conventional approaches, particularly regarding soot formation. These new models are applied in Large Eddy Simulations (LES) of auto-igniting spray flames. It is shown that using dissipative numerical methods in the LES solver also causes significant scalar mass conservation errors, which can be substantially reduced by employing low-dissipative methods or discretely consistent approaches. The revised combustion model is finally applied to simulate a compression ignition engine running on biofuels. The results, showing good agreement with experimental data, provide detailed insights into the soot formation process
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
Eulerian models for disperse multiphase flows
No full textTalk given at the von Karman Institute (VKI)
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
Hyperbolic quadrature method of moments
No full textTalk given at the Workshop on Moment Methods in Kinetic Theory III
Quadrature-based moment methods for kinetic models
No full textTalk given at the Workshop on Moment Methods in Kinetic Theory II
Quadrature-based moment methods in chemical engineering
No full textTalk given at Mathematics in (bio)Chemical Kinetics and Engineering 201
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