1,720,970 research outputs found
Impact of oxygen enrichment on ammonia combustion in spark-ignition engines under partial load conditions
No full textNH3-fueled internal combustion engines are a promising technology in the perspective of decarbonization. However, when NH3 is used as fuel, its high minimum ignition energy and slow flame propagation severely limit operating conditions, making its application in internal combustion engines challenging. The low reactivity of NH3/air mixtures results in high unburned NH3 emissions and low combustion efficiency. In this context, oxygen-assisted combustion is a promising technique for improving both the NH3/O2/N2 mixture reactivity and the performance of internal combustion engines. This study experimentally explores the feasibility of using NH3 in a fully premixed spark ignition single-cylinder engine with a compression ratio of 12:1 at 1000 rpm under both medium and low load conditions. The oxygen (O2) content in the intake gas mixture was varied to achieve stable combustion without any misfires. At medium load, i.e. with an indicated mean effective pressure of 5.8 bar, stable combustion was achieved with 22.4% vol. of O2 (relative to the O2/N2 mixture) and a significant reduction in ignition delay time and combustion duration was observed. 1-D numerical simulations of adiabatic, unstretched, freely propagating flat premixed flame were performed, using Ansys Chemkin-Pro 2024, implementing the chemical kinetic mechanism for NH3 oxidation (31 species and 203 reactions) proposed by Stagni et al. 2023, to investigate the influence of O2 additions on the laminar flame speed of NH3/O2/N2 mixtures under spark timing thermodynamic conditions. The addition of O2 enhances the laminar flame speed by 47% when O2 concentration is 26.8% in the O2/N2 mixture. Its role in the chain-branching reaction, O2 + H ↔ O + OH, is crucial for the production of O and OH radicals. Moreover, H-abstraction involving the OH radical (NH3 + OH ↔ NH2 + H2O) plays a key role in NH3 oxidation even when O2 is added to the mixture. Finally, to further investigate the role of NH3 combustion in cycle-to-cycle variability, the intake pressure was reduced to reach lower load, resulting in highly unstable combustion in the absence of O2 enrichment. As expected, the O2 enrichment stabilizes the combustion process. However, as the engine load decreases until 4.3 bar, the amount of O2 needed to achieve stable combustion increases significantly, up to 32.5% vol
Lagrangian CFD modeling of ammonia sprays: A correlation across flash boiling and evaporative conditions
No full textSimulations of liquid ammonia spray are performed for different ambient pressures to investigate the transition between flash-boiling and non-flashing regimes, through Computational Fluid Dynamics (CFD). The Lagrangian particle method, within the Reynolds Averaged Navier Stokes (RANS) approach for turbulence is used. Numerical results are compared with experimental liquid and vapor tip penetrations, spray morphology and Sauter Mean Diameter (SMD) measurements, for a multi-hole injector. An adaptation of Kelvin-Helmholtz - Rayleigh-Taylor (KH-RT) breakup model constants and prescribed jet cone angle is necessary for each regime, as standard values used for traditional fuels, e.g., gasoline, appear not to work with ammonia. Capturing local spray details and SMD values across all regimes with a single model setup is very challenging, especially with a new fuel such as ammonia, whose properties differ by a large amount from more established values for hydrocarbons. In this study a correlation for ammonia is proposed for the KH-RT breakup model constants and jet cone angle as a function of operating conditions across flash-boiling and non-flashing regimes. In addition, local temperature predictions are extensively discussed, for both liquid and gaseous phases, highlighting and quantifying the strong cooling effect that ammonia produces during the phase change process
Low- and intermediate-temperature ammonia/hydrogen oxidation in a flow reactor: Experiments and a wide-range kinetic modeling
Full text linkUnderstanding the chemistry behind the oxidation of ammonia/hydrogen mixtures is crucial for ensuring the flexible use of such mixtures in several applications, related to propulsion systems and power generation. In this work, the oxidation of ammonia/hydrogen blends was investigated through an experimental and kinetic-modeling study, where the low- and intermediate-temperature conditions were considered. An experimental campaign was performed in a flow reactor, at stoichiometric conditions and near-atmospheric pressure (126.7 kPa). The mole fraction of fuels, oxidizer and final products was measured. At the same time, a comprehensive kinetic model was set up, following a modular and hierarchical approach, and implementing the recently-available elementary rates. Such a model was used to interpret the experimental results, and to extend the analysis to literature data, covering several oxidation features. The reactivity boost provided by H2 addition was found to be approximately linear with its mole fraction in both flow- and jet-stirred-reactor conditions (except for the smallest H2 amounts in the flow reactor), in contrast with the more-than-linear increase in the laminar flame speed. The key role of HO2 in regulating fuel conversion and autoignition at low temperature was confirmed for binary mixtures, with H2NO being the bottleneck to the low-temperature oxidation of NH3-rich blends. On the other hand, the nitrogen fate was found to be mostly regulated by NHx + NO propagation and termination channels
Modeling of ammonia under-expanded jets for application in advanced propulsion systems
No full textThe adoption of ammonia as a fuel is gaining significance in various applications, including propulsion systems, gas turbines, and burners. In particular, the combustion of liquid ammonia presents an attractive option due to its low cost and its capacity to reduce local temperatures, thereby limiting thermal NOx emissions. In this context, investigating ammonia injection processes is relevant for optimizing and improving ammonia-fueled combustion systems. Thus, this work presents a combined experimental and numerical analysis of ammonia jets for applications in advanced propulsion systems, where the multiphase flow resulting from ammonia injection was investigated using a purposely developed CFD code characterized by low numerical dissipation and the adoption of real-fluid properties. The simulation methodology is validated against experiments performed at the Université d’Orléans, including optical measurements based on Schlieren and Diffuse Back-Illumination (DBI) extinction techniques, as well as local temperature measurements. This study provides insights into jet morphology and offers a quantitative assessment of parameters — including liquid penetration and local temperature — that are crucial for advancing the design of ammonia combustion systems. The outcomes offer an enhanced representation of jet structure, highlighting the presence of under-expanded jets and Mach discs when ammonia is injected at high pressures. They also demonstrate that under-expanded shock structures are strongly dependent on the temperature of liquid fuel injection
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
Computational studies on HCCI engines.
Full text linkEmission legislation is changing in an effort to reduce exhaust emissions from
the IC engines. Research and development has been focused on improving
both spark-ignition and compression ignition engines to make them more
efficient. Over the last decade, research has been increasing in Homogeneous
Charge Compression Ignition (HCCI) engines. This alternative combustion
process has the potential to combine the best of spark-ignition engines, namely
the cleaner combustion with virtual no emission of NOx and soot, with the best
of compression ignition, the increased engine efficiency with low fuel
consumption and consequently lower carbon dioxide emissions.
In this thesis, the effect upon HCCI combustion of different additives has been
investigated. These additives can be used, to control the start of auto-ignition
and/or to extend the load limits in which HCCI operation is possible.
As part of this work, the HCCI combustion capability of running with different
fuels was investigated. In order to study these effects on HCCI combustion, a
detailed chemistry fuel oxidation mechanism was used together with a powerful
chemical kinetic modelling tool, CHEMKIN. Simulations of the HCCI combustion
were performed using a single-zone zero dimensional model and later a simpler
multi-zone model comprising three different zones.
Using the single-zone CHEMKIN model, additives influence on combustion was
studied.
From this study more complex mixtures were defined and more simulations
were done, this time including different fuels such as ethanol, iso-octane and
mixtures of both.
A more complex model other than the single-zone tool available at CHEMKIN
was developed and comparisons between the two simulation methods were
performed.
The results obtained from the simulations performed confirmed the fuel
tolerance of HCCI combustion and the capability of running water diluted
mixtures
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
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