11 research outputs found
Global uncertainty and sensitivity analysis of a reduced chemical kinetic mechanism of a gasoline, n-butanol blend in a high pressure rapid compression machine
No full textA detailed evaluation of a recently developed combined n-butanol/toluene reference fuel (TRF) reduced chemical kinetic mechanism (Agbro, 2017) describing the low temperature oxidation of n-butanol, gasoline and a gasoline/n-butanol blend was performed using both global uncertainty and sensitivity methods with ignition delays as the predicted output for the temperature range 678 - 858 K, and an equivalence ratio of 1 at 20 bar. A global sampling technique was applied in the simulations in order to quantify the uncertainties of the predicted ignition delays when incorporating the effects of uncertainties in forward rate constants in the simulations. In addition, a variance-based global sensitivity analysis using a high dimensional model representation (HDMR) method was carried out to understand and rank the parameters responsible for the predicted uncertainties. The results showed that uncertainties in predicting key target quantities for the various fuels studied are currently large but driven by few reactions. Global sensitivity analysis of the mechanism based on predicted ignition delays of stoichiometric TRF mixtures, showed the toluene + OH route = phenol + CH3 to be among the most dominant pathways in terms of the predicted output uncertainties but an update on the mechanism based on recent data from the study of Seta led to the toluene + OH hydrogen abstraction reaction becoming the most dominant reaction as expected. For the TRF/n-butanol blend, hydrogen abstraction reactions by OH from n-butanol appear to be key in predicting the effect of blending. Uncertainties in the temperature dependence of relative abstraction rates from the ? and ? sites may still be present within current mechanisms, and in particular may affect the ability of the mechanisms to capture the low temperature delay times for n-butanol. Further studies of the product channels for n-butanol + OH for temperatures of relevance to combustion applications could help to improve current mechanisms. At higher temperatures, the reactions of HO2 and that of formaldehyde with OH also became critical and attempts to reduce uncertainties in the temperature dependent rates of these reactions would be useful
Global uncertainty and sensitivity analysis of a reduced chemical kinetic mechanism of a gasoline, n-butanol blend in a high pressure rapid compression machine.
Get PDFA detailed evaluation of a recently developed combined n-butanol/toluene reference fuel (TRF) reduced chemical kinetic mechanism describing the low temperature oxidation of n-butanol, gasoline and a gasoline/n-butanol blend was performed using both global uncertainty and sensitivity methods with ignition delays as the predicted output for the temperature range 678 - 858 K, and an equivalence ratio of 1 at 20 bar. The results obtained when incorporating the effects of uncertainties in forward rate constants in the simulations, showed that uncertainties in predicting key target quantities for the various fuels studied are currently large but driven by few reactions. Global sensitivity analysis of the mechanism based on predicted ignition delays of stoichiometric TRF mixtures, showed the toluene + OH route = phenol + CH3 to be among the most dominant pathways in terms of the predicted output uncertainties but an update on the mechanism based on data from a recent study led to the toluene + OH hydrogen abstraction reaction becoming the most dominant reaction as expected. For the TRF/n-butanol blend, hydrogen abstraction reactions by OH from n-butanol appear to be key in predicting the effect of blending. Uncertainties in the temperature dependence of relative abstraction rates from the α and γ sites may still be present within current mechanisms, and in particular may affect the ability of the mechanisms to capture the low temperature delay times for n-butanol. Further studies of the product channels for n-butanol + OH for temperatures of relevance to combustion applications could help to improve current mechanisms. At higher temperatures, the reactions of HO2 and that of formaldehyde with OH also became critical and attempts to reduce uncertainties in the temperature dependent rates of these reactions would be useful
Experimental and Chemical Kinetic Modelling Study on the Combustion of Alternative Fuels in Fundamental Systems and Practical Engines
Get PDFIn this work, experimental data of ignition delay times of n-butanol, gasoline, toluene reference fuel (TRF), a gasoline/n-butanol blend and a TRF/n-butanol blend were obtained using the Leeds University Rapid Compression Machine (RCM) while autoignition (knock) onsets and knock intensities of gasoline, TRF, gasoline/n-butanol and TRF/n-butanol blends were measured using the Leeds University Optical Engine (LUPOE). The work showed that within the RCM, the 3-component TRF surrogate captures the trend of gasoline data well across the temperature range. However, based on results obtained in the engine, it appears that the chosen TRF may not be an excellent representation of gasoline under engine conditions as the knock boundary of TRF as well as the measured knock onsets are significantly lower than those of gasoline. The ignition delay times measured in the RCM for the blend, lay between those of gasoline and n-butanol under stoichiometric conditions across the temperature range studied and at lower temperatures, n-butanol acts as an octane enhancer over and above what might be expected from a simple linear blending law. In the engine, the measured knock onsets for the blend were higher than those of gasoline at the more retarded spark timing of 6 CA bTDC but the effect disappears at higher spark advances. Future studies exploring the blending effect of n-butanol across a range of blending ratios is required since it is difficult to conclude on the overall effect of n-butanol blending on gasoline based on the single blend that has been considered in this study.
The chemical kinetic modelling of the fuels investigated has also been evaluated by comparing results from simulations employing the relevant reaction mechanisms with the experimental data sourced from either the open literature or measured in-house. Local as well as global uncertainty/sensitivity methods accounting for the impact of uncertainties in the input parameters, were also employed within the framework of ignition delay time modelling in an RCM and species concentration prediction in a JSR, for analysis of the chemical kinetic modelling of DME, n-butanol, TRF and TRF/n-butanol oxidation in order to advance the understanding of the key reactions rates that are crucial for the accurate prediction of the combustion of alternative fuels in internal combustion engines. The results showed that uncertainties in predicting key target quantities for the various fuels studied are currently large but driven by few reactions. Further studies of the key reaction channels identified in this work at the P-T conditions of relevance to combustion applications could help to improve current mechanisms.
Moreover, the chemical kinetic modelling of the autoignition and species concentration of TRF, TRF/n-butanol and n-butanol fuels was carried out using the adopted TRF/n-butanol mechanism as input in the engine simulations of a recently developed commercial engine software known as LOGEengine. Similar to the results obtained in the RCM modelling work, the knock onsets predicted for TRF and TRF/n-butanol blend under engine conditions were consistently higher than the measured data. Overall, the work demonstrated that accurate representation of the low temperature chemistry in current chemical kinetic models of alternative fuels is very crucial for the accurate description of the chemical processes and autoignition of the end gas in the engine
Chemical Kinetic Modelling Study on the Influence of n-butanol blending on the Combustion, Autoignition and Knock Properties of Gasoline and its Surrogate in a Spark Ignition Engine
Get PDFThe ability of a mechanism describing the oxidation kinetics of toluene reference fuel (TRF)/n-butanol mixtures to predict the impact of n-butanol blending at 20% by volume on the autoignition and knock properties of gasoline has been investigated under conditions of a strongly supercharged spark ignition (SI) engine. Simulations were performed using the LOGEengine code for stoichiometric fuel/air mixtures at intake temperature and pressure conditions of 320 K and 1.6 bar, respectively, for a range of spark timings.
At the later spark timing of 6 °CA bTDC, the predicted knock onsets for a gasoline surrogate (toluene reference fuel, TRF) and the TRF/n-butanol blend are higher compared to the measurements, which is consistent with an earlier study of ignition delay times predicted in a rapid compression machine (RCM, Agbro et al., Fuel, 2017, 187:211-219). The discrepancy between the predicted and measured knock onsets is however quite small at higher pressure and temperature conditions (spark timing of 8 °CA bTDC) and can be improved by updating a key reaction related to the toluene chemistry. The ability of the scheme to predict the influence of n-butanol blending on knock onsets requires improvement at later spark timings. The simulations highlighted that the low-intermediate temperature chemistry within the SI engine end gas, represented by the presence of a cool flame and negative temperature coefficient (NTC) phase, plays an important role in influencing the high temperature heat release and consequently the overall knock onset. This is due to its sensitisation effect (increasing of temperature and pressure) on the end gas and reduction of the time required for the high temperature heat release to occur. Therefore, accurate representation of the low-intermediate temperature chemistry is crucial for predicting knock. The engine simulations provide temperature, heat release and species profiles that link conditions in practical devices and ignition delay times predicted in an RCM. This facilitates a better understanding of the chemical processes affecting knock onsets predicted within the engine and the main reactions governing them
Chemical Kinetic Modelling Study on the Influence of n-butanol blending on the Combustion, Autoignition and Knock Properties of Gasoline and its Surrogate in a Spark Ignition Engine.
No full textThe ability of a mechanism describing the oxidation kinetics of toluene reference fuel (TRF)/n-butanol mixtures to predict the impact of n-butanol blending at 20% by volume on the autoignition and knock properties of gasoline has been investigated under conditions of a strongly supercharged spark ignition (SI) engine. Simulations were performed using the LOGEengine code for stoichiometric fuel/air mixtures at intake temperature and pressure conditions of 320 K and 1.6 bar, respectively, for a range of spark timings. At the later spark timing of 6 °CA bTDC, the predicted knock onsets for a gasoline surrogate (toluene reference fuel, TRF) and the TRF/n-butanol blend are higher compared to the measurements, which is consistent with an earlier study of ignition delay times predicted in a rapid compression machine (RCM, Agbro et al., Fuel, 2017, 187:211-219). The discrepancy between the predicted and measured knock onsets is however quite small at higher pressure and temperature conditions (spark timing of 8 °CA bTDC) and can be improved by updating a key reaction related to the toluene chemistry. The ability of the scheme to predict the influence of n-butanol blending on knock onsets requires improvement at later spark timings. The simulations highlighted that the low-intermediate temperature chemistry within the SI engine end gas, represented by the presence of a cool flame and negative temperature coefficient (NTC) phase, plays an important role in influencing the high temperature heat release and consequently the overall knock onset. This is due to its sensitisation effect (increasing of temperature and pressure) on the end gas and reduction of the time required for the high temperature heat release to occur. Therefore, accurate representation of the low-intermediate temperature chemistry is crucial for predicting knock. The engine simulations provide temperature, heat release and species profiles that link conditions in practical devices and ignition delay times predicted in an RCM. This facilitates a better understanding of the chemical processes affecting knock onsets predicted within the engine and the main reactions governing them.The authors would like to thank COST (European Cooperation in Science and Technology www.cost.eu) for providing financial support for scientific exchange visits to LOGE Lund Combustion Engineering under the COST Action SMARTCATs (CM 1404). We also thank Inna Gorbatenko for valuable discussions and contributions. We also wish to acknowledge the Tertiary Education Trust Fund (TETFUND), Nigeria, for scholarship funding for E. Agbro. The work at King Abdullah University of Science and Technology (KAUST) was funded under the Clean Combustion Research Center (CCRC) Future Fuels program
Experimental Study on the Influence of n-butanol Blending on the Combustion, Autoignition and Knock Properties of Gasoline and its Surrogate in a Spark Ignition Engine
Get PDFThe impact of n-butanol blending on the combustion, autoignition and knock properties of gasoline has been investigated under supercharged spark ignition engine conditions for stoichiometric fuel/air mixtures at intake temperature and pressure conditions of 320 K and 1.6 bar, respectively, for a range of spark timings. A toluene reference fuel (TRF) surrogate for gasoline containing toluene, n-heptane and iso-octane has been tested experimentally in the Leeds University Ported Optical Engine (LUPOE) alongside a reference gasoline and their blends (20 % n-butanol and 80 % gasoline/TRF by volume). Although the gasoline/n-butanol blend displayed the highest burning rate, and consequently the highest peak pressures compared to gasoline, TRF and the TRF/n-butanol blend, it exhibited the least propensity to knock, indicating that addition of n-butanol provides an opportunity for enhancing the knock resistance of gasoline as well as improving engine efficiency via the use of higher compression ratios. The anti-knock enhancing quality of n-butanol on gasoline was however observed to weaken at later spark timings. Hence, whilst n-butanol has shown some promise based on the current study, its application as an octane enhancer for gasoline under real engine conditions may be somewhat limited at the studied blending ratio. As expected based on previous ignition delay studies, the TRF showed an earlier knocking boundary than the rest of the fuels, which may possibly be attributed to the absence of an oxygenate (ethanol or n-butanol) as present in the other fuels and a lower octane index. Overall, the TRF mixture gave a reasonable representation of the reference gasoline in terms of the produced knock onsets at the later spark timings for the pure fuels. However, on blending, the TRF did not reproduce the trend for the gasoline at later spark timings which can be linked to difficulties in capturing the temperature trends in ignition delays around the negative temperature coefficient region observed in previous work in a rapid compression machine (Agbro et al., Fuel, 2017, 187:211-219)
Global Uncertainty and Sensitivity Analysis of a Reduced Chemical Kinetic Mechanism of a Gasoline, N-Butanol Blend in a High Pressure Rapid Compression Machine
Get PDFA detailed evaluation of a recently developed combined n-butanol/toluene reference fuel (TRF) reduced chemical kinetic mechanism (Agbro, 2017) describing the low temperature oxidation of n-butanol, gasoline and a gasoline/n-butanol blend was performed using both global uncertainty and sensitivity methods with ignition delays as the predicted output for the temperature range 678 - 858 K, and an equivalence ratio of 1 at 20 bar. A global sampling technique was applied in the simulations in order to quantify the uncertainties of the predicted ignition delays when incorporating the effects of uncertainties in forward rate constants in the simulations. In addition, a variance-based global sensitivity analysis using a high dimensional model representation (HDMR) method was carried out to understand and rank the parameters responsible for the predicted uncertainties. The results showed that uncertainties in predicting key target quantities for the various fuels studied are currently large but driven by few reactions. Global sensitivity analysis of the mechanism based on predicted ignition delays of stoichiometric TRF mixtures, showed the toluene + OH route = phenol + CH3 to be among the most dominant pathways in terms of the predicted output uncertainties but an update on the mechanism based on recent data from the study of Seta led to the toluene + OH hydrogen abstraction reaction becoming the most dominant reaction as expected. For the TRF/n-butanol blend, hydrogen abstraction reactions by OH from n-butanol appear to be key in predicting the effect of blending. Uncertainties in the temperature dependence of relative abstraction rates from the α and γ sites may still be present within current mechanisms, and in particular may affect the ability of the mechanisms to capture the low temperature delay times for n-butanol. Further studies of the product channels for n-butanol + OH for temperatures of relevance to combustion applications could help to improve current mechanisms. At higher temperatures, the reactions of HO2 and that of formaldehyde with OH also became critical and attempts to reduce uncertainties in the temperature dependent rates of these reactions would be useful
Evaluation of combustion mechanisms using global uncertainty and sensitivity analyses: A case study for low-temperature dimethyl ether oxidation
Get PDFA global uncertainty analysis is performed for three current mechanisms describing the low-temperature oxidation of dimethyl ether (Aramco Mech 1.3, Metcalfe et al., Int J Chem Kinet 2013, 45, 638-675; Zheng et al., Proc Combust Inst 2005, 30, 1101-1109; Liu et al., Combust Flame 2013, 160, 2654-2668) with application to simulations of species concentrations (CH2O, H2 O2 ,CH3 OCHO) corresponding to existing data from an atmospheric pressure flow reactor and high-pressure ignition delays. When incorporating uncertainties in reaction rates within a global sampling approach, the distributions of predicted targets can span several orders of magnitude. The experimental profiles, however, fall within the predictive uncertainty limits. A variance-based sensitivity analysis is then undertaken using high dimensional model representations. The main contributions to predictive uncertainties come from the CH3 OCH2 + O2 system, with isomerization, propagation, chain-branching, secondary OH formation, and peroxy-peroxy reactions all playing a role. The response surface describing the relationship between sampled reaction rates and predicted outputs is complex in all cases. Higher order interactions between parameters contribute significantly to output variance, and no single reaction channel dominates for any of the conditions studied. Sensitivity scatter plot
The influence of n-butanol blending on the ignition delay times of gasoline and its surrogate at high pressures
No full textThe influence of blending n-butanol at 20% by volume on the ignition delay times for a reference gasoline was studied in a rapid compression machine (RCM) for stoichiometric fuel/air mixtures at 20 bar and 678-858 K. Delay times for the blend lay between those of stoichiometric gasoline and stoichiometric n-butanol across the temperature range studied. At lower temperatures, delays for the blend were however, much closer to those of n-butanol than gasoline despite n-butanol being only 20% of the mixture. Under these conditions n-butanol acted as an octane enhancer over and above what might be expected from a simple linear blending law. The ability of a gasoline surrogate, based on a toluene reference fuel (TRF), to capture the main trends of the gasoline/n-butanol blending behaviour was also tested within the RCM. The 3-component TRF based on a mixture of toluene, n-heptane and iso-octane was able to capture the trends well across the temperature range studied. Simulations of ignition delay times were also performed using a detailed blended n-butanol/TRF mechanism based on the adiabatic core assumption and volume histories from the experimental data. Overall, the model captured the main features of the blending behaviour, although at the lowest temperatures, predicted ignition delays for stoichiometric n-butanol were longer than those observed. A brute-force local sensitivity analysis was performed to evaluate the main chemical processes driving the ignition behaviour of the TRF, n-butanol and blended fuels. The reactions of fuel + OH dominated the sensitivities at lower temperatures, with H abstraction from n-butanol from a and 7 sites being key for both the n-butanol and the blend. At higher temperatures the decomposition of H2O2 and reactions of HO2 and that of formaldehyde with OH became critical, in common with the ignition behaviour of other fiiels. Remaining uncertainties in the rates of these key reactions are discussed. Crown Copyright (C) 2016 Published by Elsevier Ltd. All rights reserved.The authors would like to thank Dr. Roger Cracknell and Shell Global Solutions for the provision of fuels. The work performed by the Clean Combustion Research Center was supported by competitive research funding from King Abdullah University of Science and Technology (KAUST). Thanks also to Dr. Myeji Materego, Professor Tiziano Faravelli, Dr. Kieran Somers and Dr. Stephen Dooley for helpful discussions
INTERROGATION DE LA DEVIATION COMME TENDANCE POSTMODERNE CHEZ MABANCKOU SUR LE PLAN STYLISTIQUE ET THEMATIQUE DANS VERRE CASSE / INTERROGATION OF DEVIATION AS A POSTMODERN TREND AT MABANCKOU ON THE STYLISTIC LEVEL AND THEMATIC IN BROKEN GLASS
No full textLa littérature africaine d’expression française avait attiré l’attention globale dans le monde de la scolarité avec les approches divers employées par les critiques dès sa naissance. Une lecture panoramique de Verre Cassé (2005), traduit en anglais comme Broken Glass (2010) est capable de convaincre un lecteur même dans une seule fois de l’exceptionnalité stylistique de Mabanckou. L’effort de l’écrivain d’aller plus loin est évident au niveau du thème et du style dès le commencement du roman selon l’emploi stylistique d’une graphie exceptionnellement qui attire l’attention du lecteur. La nature audacieuse du texte résume la pensée postmoderniste à travers l’usage de langage par l’auteur pour la présentation de son message. En conséquence, cet article vise à examiner et élucider soigneusement les tendances postmodernes dans ce roman spécial qui attire l’attention mondiale de plus en plus dans la décennie récente. L’explication de texte estadoptée comme la méthodologie de recherche pour examiner la naissance d’une écriture différente à celle conventionnelle, résumé comme écriture postmoderne. Cette recherche termine avec la réitération des outils stylistiques et postmodernistes employés par l’écrivain de la manière qu’ils exposent les problèmes contemporains de l’individu et des vices sociaux capables d’infliger et ruiner l’individu, la conjugalité et la société en générale en soulignant la rupture d’une société moderne et celle postmoderne. Nous proposons donc l’adoption du postmodernisme pour la création de l’écriture vraisemblance comme elle permet l’écrivain de s’exprimer librement à la compréhension de l’audience évitant en même temps l’ambiguïté. African francophone literature in recent years has attracted global recognition in literary discourse with several approaches adopted by critics since its inception. A panoramic reading of Verre Cassé (2005), translated as Broken Glass (2010) will convince an avid reader, even for a single reading of the stylistic exceptionality of Mabanckou. The additional effort of the writer’s thematic and stylistic presentation is underlined from the commencement of the text through the employment of exceptional graphic properties that attract readers. The daring nature of the text could be summarized to be of postmodernist school of thoughts following the use of language in the projection of the author’s message. Hence, this paper tries to examine critically and elucidate the postmodernist tendencies employed by the author in this unique novel that has attracted critics from all over the world in the last decade. The “French Explication de texte” is adopted as methodology for this research in investigating the birth of a new form of writing that differs from the conventional, known as postmodernist writing. This paper concludes by reiterating the various stylistic and postmodernist tools used by the author as he lays bare contemporary individual and societal vices, capable of hurting and ruining the individual, conjugal settings and the society at large, by underlining the dichotomy of a modern and postmodern society. We thus propose the adoption of postmodern writing as it encourages free flow of thoughts by the author and easy comprehension for readers with absence of all forms of ambiguity. Article visualizations
