1,721,116 research outputs found
Intrinsic thermoacoustic instability in hydrogen enriched partially premixed flames
No full textThe influence of hydrogen addition on the intrinsic thermoacoustic instability in swirl stabilized partially premixed methane flames is investigated using large eddy simulation (LES) and reduced order modeling (ROM). The LES results compare well with the measurements. Hydrogen addition results in a significant deviation of the fundamental frequency from the cavity acoustic modes. The thermoacoustic behavior is analyzed using an ROM with the classical n- τ model for flame dynamics, where the interaction index n and time delay τ are deduced from the LES results. A parametric sweep of n and τ with ROM reveals a broad variation of the acoustic eigenvalues in the eigenspectrum. The eigenspectrum shows two distinct regions corresponding to the classical acoustic and intrinsic modes. These intrinsic modes are characterized by a distinct change in the sign of axial pressure gradient across the flame. In the hydrogen-enriched cases, the pressure mode shapes obtained from the ROM and LES analyses show this change in the axial pressure gradient across the flame zone. This is due to flame-induced pulsation dominating the near-field acoustics, resulting in the emergence of intrinsic modes. Further, general results obtained using the one-dimensional acoustic model on the influence of geometry on the stability of acoustic and intrinsic modes are discussed.</p
Analyse laseroptischer Messprinzipien zur Untersuchung der Transportprozesse während der Sprühstrahl-Wandfilm-Interaktion
No full textIn the present work, two laser optical measurement systems for the investigation of spray-wall-film-interactions are developed, analyzed, and evaluated with respect to their suitability for the application. There is a lack of measurement systems that are capable to distinguish between spray and wall-film fluid to measure their mass fractions. Therefore, the wall film fluid discharged into the atmosphere during spray-wall-film-interaction is not quantified so far. The two systems are adapted to a test bench to investigate the interaction of a Diesel spray with a thin wall film. The first developed system measures temporally and spatially resolved the film thickness based on the principles of a Fizeau-interferometer. The second system uses planar laser-induced fluorescence for spatially resolved and time-averaged measurements of the film thickness and fluid concentration. The technical and physical limitations are analyzed, and the estimated measurement accuracy are determined. The technical applicability of the measurement systems is evaluated with a first application on the test bench. Although the Fizeau interferometer provides better measurement accuracy and temporal resolution, planar laser-induced fluorescence is the more suitable measurement system for the application. The interferometer is only able to measure thicknesses for surface angles smaller than 4° and in addition the measurement field is very limited. Thus, the interferometer is only able to determine the film thickness 10 millisecond after the interaction finished. However, temporal resolution is then no longer necessary. Laser-induced fluorescence is limited in the use of liquids, and both quantities can only be determined by averaging over many injections. Nevertheless, the measurement system determines film thicknesses and fluid concentrations after the interaction for the first time. Thereby, complete displacement of the wall film in the spray impingement zone is observed. In addition, for investigations at high injection pressures a significant wall film removal is measured and emphasized the necessity of quantitative film thickness and concentration measurements
Laseroptische Untersuchungen von Verbrennungsprozessen bio-hybrider Kraftstoffe
No full textA reliable energy supply and the independence of energy and fuels of certain countries became even more relevant for society and the economy in the year 2022, especially in the European Union. Conventional energy sources such as oil and gas must therefore be replaced by regenerative forms of energy, not only from an ecological but also from a political point of view. Regenerative forms of energy provide up to 40% of the electrical energy supply of Germany and consist of 70% of wind- and solar power in 2021. However, wind and solar energy are volatile due to the altitude of the sun, the weather, and the season and thus less predictable and base-load capable than previous, conventional forms of energy. Smoothing the volatility of these energy supplies can be overcome by storing the energy in liquid fuels using their high energy density compared with easy storing and transportation capabilities. The stored energy can then be transformed back using combustion. To reduce emissions resulting from combustion, improved combustion process need to be invented. Additionally, combustion processes are not fully understood. Optical combustion diagnostics can help to provide data such as molecule concentrations and temperatures spatially- and temporally resolved inside a combustion to develop highly efficient and low-emission combustion technologies. In this work, a novel combustion technique of a reactivity-controlled compression ignition (RCCI) with crossing fuel jets of different reactivity is analyzed using diethoxymethane and ethyl acetate. Therefore, mixing, ignition, and combustion of both fuels are investigated by using sodium as a tracer to determine the injection parameters of the delay time between low- and high-reactivity fuel injection as well as the injection duration of the low-reactivity fuel. This tracer technique is applied for the first time to an RCCI combustion and can resolve the combustion zones of both fuels to determine homogeneous combustion conditions. Based on the determined injection parameters, the RCCI-combustion is then analyzed with two-photon laser-induced fluorescence (TPLIF) to provide emission data in an early combustion state, the main combustion and in the exhaust phase after the combustion ended. Therefore, the molecules of C2, as an indicator of unburned hydrocarbons, as well as carbon monoxide are investigated during early and main combustion and after reactions have finished, similar to an exhaust gas phase. Here the feasibility of the application of TPLIF to the RCCI combustion with crossing jets is demonstrated and the reduction potential of diethoxymethane and ethyl acetate compared to diesel and ethanol is investigated. Determining molecule concentrations requires precise temperature measurements. To estimate nitric oxides in the combustion of the two-stage combustion of n-pentane, accurate temperature measurements are performed in this work using the Raman scattering technique. The measured temperature data provides first insights into the temperature spatially resolved during the first stage of the ignition which is highly relevant for simulation models. Additionally, high-speed chemiluminescence across the reactors cross section underlines the higher temperatures with higher measured reactivity
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
kHz-rate planar imaging diagnostics for turbulent combustion research: recent advances and development
No full textTurbulent combustion research at the DLR Institute for Combustion Technology - An Overview of recent highspeed measurement campaigns
No full textHighspeed laser diagnostics applied in lean, partically-premixed gas turbine model combustors
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