An a priori thermodynamic data analysis based chemical lumping method for the reduction of large and multi-component chemical kinetic mechanisms

A chemical species lumping approach for reduction of large hydrocarbons and oxygenated fuels is presented. The methodology is based on an a priori analysis of the Gibbs free energy of the isomer species which is then used as main criteria for the evaluation of lumped group. Isomers with similar Gibbs free energy are lumped assuming they present equal concentrations when applied to standard reactor conditions. Unlike several lumping approaches found in literature, no calculation results from the primary mechanism have been employed prior to the application of our chemical lumping strategy. An 807 species and 7807 individual reactions detailed mechanism comprising n-decane, alpha-methylnaphthalene and methyl decanoate has been used. The thermodynamic data have been analyzed and 74 isomer groups have been identified within the oxidation of n-decane and methyl decanoate. The mechanism reduction has led to a mechanism size of 463 species and 7600 reactions. Thereafter the lumped mechanism has been checked under several reactor conditions and over a broad range of temperature, pressure, and equivalence ratio in order to quantify the accuracy of the proposed approach. In all cases, very good agreement between the predictions obtained using the lumped and the detailed mechanism has been observed with an overall absolute error below 12%. Effects of the lumping procedure on sensitivities and on isomer concentrations were considered to further demonstrate the validity of the proposed approach

Supercharged Homogeneous Charge Compression Ignition

The Homogeneous Charge Compression Ignition (HCCI) is the third alternative for combustion in the reciprocating engine. Here a homogeneous charge is used as in a spark-ignited engine, but the charge is compressed to autoignition as in a diesel. The main difference compared with the Spark Ignition (SI) engine is the lack of flame propagation and hence the independence from turbulence. Compared with the diesel engine, HCCI has a homogeneous charge and hence no problems associated with soot and NOdx formation. Earlier research on HCCI showed high efficiency and very low amounts of NOdx, but HC and CO were higher than in SI mode. It was not possible to achieve high IMEP values with HCCI, the limit being 5 bar. Supercharging is one way to dramatically increase IMEP. The influence of supercharging on HCCI was therefore experimentally investigated. Three different fuels were used during the experiments: iso-octane, ethanol and natural gas. Two different compression ratios were used, 17:1 and 19:1. The inlet pressure conditions were set to give 0, 1, or 2 bar of boost pressure. The highest attainable IMEP was 14 bar using natural gas as fuel at the lower compression ratio. The limit in achieving even higher IMEP was set by the high rate of combustion and a high peak pressure. Numerical calculations of the HCCI process have been performed for natural gas as fuel. The calculated ignition timings agreed well with the experimental findings. The numerical solution is, however, very sensitive to the composition of the natural gas

Development of a Computationally Efficient Tabulated Chemistry Solver for Internal Combustion Engine Optimization Using Stochastic Reactor Models

The use of chemical kinetic mechanisms in computer aided engineering tools for internal combustion engine simulations is of high importance for studying and predicting pollutant formation of conventional and alternative fuels. However, usage of complex reaction schemes is accompanied by high computational cost in 0-D, 1-D and 3-D computational fluid dynamics frameworks. The present work aims to address this challenge and allow broader deployment of detailed chemistry-based simulations, such as in multi-objective engine optimization campaigns. A fast-running tabulated chemistry solver coupled to a 0-D probability density function-based approach for the modelling of compression and spark ignition engine combustion is proposed. A stochastic reactor engine model has been extended with a progress variable-based framework, allowing the use of pre-calculated auto-ignition tables instead of solving the chemical reactions on-the-fly. As a first validation step, the tabulated chemistry-based solver is assessed against the online chemistry solver under constant pressure reactor conditions. Secondly, performance and accuracy targets of the progress variable-based solver are verified using stochastic reactor models under compression and spark ignition engine conditions. Detailed multicomponent mechanisms comprising up to 475 species are employed in both the tabulated and online chemistry simulation campaigns. The proposed progress variable-based solver proved to be in good agreement with the detailed online chemistry one in terms of combustion performance as well as engine-out emission predictions (CO, CO2, NO and unburned hydrocarbons). Concerning computational performances, the newly proposed solver delivers remarkable speed-ups (up to four orders of magnitude) when compared to the online chemistry simulations. In turn, the new solver allows the stochastic reactor model to be computationally competitive with much lower order modeling approaches (i.e., Vibe-based models). It also makes the stochastic reactor model a feasible computer aided engineering framework of choice for multi-objective engine optimization campaigns

Insights into nitromethane combustion from detailed kinetic modeling – Pyrolysis experiments in jet-stirred and flow reactors

International audienceThe pyrolysis of nitromethane highly diluted in helium was studied in a plug flow reactor and in a jet-stirred reactor at 1.07 bar and over the temperature range from 500 to 1100 K. Mole fraction profiles of major products and of intermediates were identified with gas chromatography and Fourier transform infrared spectroscopy. Using these experimental data, as well as published ones, we have developed a newly compiled model for the prediction of the pyrolysis and of the oxidation of nitromethane in jet-stirred and flow reactors, freely propagating, and burner-stabilized premixed flames, as well as in shock-tubes. The experimental results from the present work and from the literature are interpreted with the help of the kinetic model derived here. This study mainly focuses on the analysis of speciation in different reactors. Among the nitrogenous species, NO is found to be a major product for pyrolysis and oxidation. The model suggests that for nitromethane pyrolysis and oxidation the thermal dissociation channel to CH3 and NO2 is the main reaction path for the nitromethane degradation followed by the H-atom abstraction channel. The most sensitive reactions for nitromethane pyrolysis in a flow reactor and during pyrolysis and oxidation in a jet-stirred reactor are found to be CH3NO2(+M) ⇋ CH3 + NO2(+M) and CH3 + NO2 ⇋ CH3O + NO. The reaction CH3 + NO2 ⇋ CH3O + NO is found to be the most important reaction for all conditions studied. In a burner-stabilized premixed flame, as the mixture gets richer, the thermal dissociation channel CH3NO2(+M) ⇋ CH3 + NO2(+M) becomes more important as the contribution of the H-atom abstraction channel is decreased. Furthermore, in the burner-stabilized premixed flames, it was found that NO is mainly formed via NO2: NO2 + H ⇋ NO + OH, NO2 + CH3 ⇋ CH3O + NO. The model provided an overall reasonable agreement with the experimental data. However, for pyrolysis conditions, future work is desirable to improve predictions of intermediate species. This work extends the kinetic database and helps to improve the understanding of nitromethane chemistry. The kinetic model presented in this work can serve as a base model for hydrocarbons and oxygenated fuels higher than C2 and nitrogen-containing compounds higher than C1 as well as for pure nitrogen compounds

Counterparts: Clothing, value and the sites of otherness in Panapompom ethnographic encounters

This is an Author's Accepted Manuscript of an article published in Anthropological Forum, 18(1), 17-35, 2008 [copyright Taylor & Francis], available online at: http://www.tandfonline.com/10.1080/00664670701858927.Panapompom people living in the western Louisiade Archipelago of Milne Bay Province, Papua New Guinea, see their clothes as indices of their perceived poverty. ‘Development’ as a valued form of social life appears as images that attach only loosely to the people employing them. They nevertheless hold Panapompom people to account as subjects to a voice and gaze that is located in the imagery they strive to present: their clothes. This predicament strains anthropological approaches to the study of Melanesia that subsist on strict alterity, because native self‐judgments are located ‘at home’ for the ethnographer. In this article, I develop the notion of the counterpart as a means to explore these forms of postcolonial oppression and their implications for the ethnographic encounter

What's in a copy?

ABSTRACTI will answer the question “What’s in a copy?” by considering three sets of related issues: the importance of copies in academia; in cultural life; and in the economic world. In academia the current capability of making copies is challenging pedagogical practices and the trust of its members, plagiarism being the most immediate problem. The notion of authorship is also undergoing changes provoked by a proliferation of authors and new possibilities opened up by cyberspace. In cultural life, imitation and mimesis have long been fundamental engines of socialization. Our enhanced capacity of copying problematizes, with new intensity, the relationships between homogeneity and heterogeneity, between the genuine and the spurious. In the economic world, the digital era is threatening some of the fundamental tenets of capitalism, especially of its variant called the “knowledge society”, regarding the control of intellectual property rights. The gap between normativity and social practices is widening. The many dilemmas and tensions identified in the text are understood as symptoms of two major characteristics of the current times: hyperfetishism and hyperanimism. ________________________________________________________________________________ RESUMOResponderei à pergunta “O que existe em uma cópia?” considerando três conjuntos de questões relacionadas: a importância das cópias na academia, na vida cultural, no mundo econômico. Na academia a presente capacidade de fazer cópias está desafiando práticas pedagógicas e a confiança dos seus membros, o plágio sendo o problema mais imediato. A noção de autoria também está sofrendo mudanças provocadas por uma proliferação de autores e novas possibilidades abertas pelo ciberespaço. Na vida cultural, a imitação e a mimese de há muito são importantes motores de socialização. A nossa capacidade ampliada de fazer cópias problematiza, com nova intensidade, as relações entre homogeneidade e heterogeneidade, entre o genuíno e o espúrio. No mundo econômico, a era digital ameaça algumas das premissas fundamentais do capitalismo, especialmente da sua variante “sociedade do conhecimento”, no tocante aos direitos de propriedade intelectual. Cresce a distância entre normatividade e práticas sociais. Os muitos dilemas e tensões identificados no texto são compreendidos como sintomas de duas grandes características do presente: o hiperfetichismo e o hiperanimismo

2023 Roadmap on ammonia as a carbon-free fuel

The 15 short chapters that form this 2023 ammonia-for-energy roadmap provide a comprehensive assessment of the current worldwide ammonia landscape and the future opportunities and associated challenges facing the use of ammonia, not only in the part that it can play in terms of the future displacement of fossil-fuel reserves towards massive, long-term, carbon-free energy storage and heat and power provision, but also in its broader holistic impacts that touch all three components of the future global food-water-energy nexus

Detailed mass spectrometric and modeling study of isomeric butene flames

Schenk M, Leon L, Moshammer K, et al. Detailed mass spectrometric and modeling study of isomeric butene flames. COMBUSTION AND FLAME. 2013;160(3):487-503

Chemical kinetic modeling of combustion in internal combustion engines using reduced chemistry

A general method for automatically reducing detailed kinetic mechanisms for complex fuels is applied. The method is based on the simultaneous use of sensitivity, reaction-flow, and extended lifetime analyses. The sensitivity analysis detects species to which the overall combustion process is sensitive. Small inaccuracies in calculating these species result in large errors in the characteristic behavior of the chemical scheme. Redundant species are detected by applying a simultaneous reaction-flow and sensitivity analyses. The sensitivities are transported through the reacting flow and each species is assigned an importance according to the importance of the species itself and the flow of atoms to and from the important species. The redundant species are removed from the detailed mechanism (74 species and 510 reactions) resulting in a skeleton mechanism (63 species and 386 reactions). The skeleton mechanism is in turn the object for a further reduction by applying extended species sensitivity and lifetime analyses. These analyses are the basis for a reduction by means of a quasi-steady-state assumption. By introducing the quasi-steady-state assumption, the skeleton mechanism is reduced further to 19 species and 16 global reactions. The skeleton and reduced mechanisms generated using different cutoff levels of "relative species importance" and "lifetime," respectively, are validated against the detailed mechanism to find the final skeleton and reduced mechanisms. Gradually increasing cutoff levels results in a correspondingly gradual increase in the difference between reduced, skeleton, and detailed mechanisms. The skeleton and reduced mechanisms are valid for the predetermined parameter ranges of initial and boundary conditions, depending on experimental conditions to be modeled