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Kinetic Mechanisms for Hydrocarbon Ignition

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Kinetic Mechanisms for Hydrocarbon Ignition Kinetic mechanisms for Hydrocarbon Ignition Von der Fakult¨at fur¨ Maschinenwesen der Rheinisch-Westf¨alischen Technischen Hochschule Aachen zur Erlangung des akademischen Grades eines Doktors der Ingenieurwissenschaften genehmigte Dissertation vorgelegt von Georgios Bikas aus Gutersloh¨ Berichter: Univ.-Prof. Dr.-Ing. Dr. h.c. N. Peters Univ.-Prof. Dr.rer.nat. K. Kohse-H¨oinghaus Tag der mundlic¨ hen Prufung:¨ 17. Dezember 2001 Diese Dissertation ist auf den Internetseiten der " Hochschulbibliothek online verfugbar¨ " View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Publikationsserver der RWTH Aachen University i Dedicated to Evangelia, and to my Grandmother ii Vorwort Die vorliegende Arbeit ist w¨ahrend meiner T¨atigkeit als wissenschaftlicher Mitarbeiter im Institut fur¨ Technische Mechanik (ITM) an der Rheinisch- Westf¨alischen Technischen Hochschule Aachen entstanden. Vor allem danke ich Herrn Prof. Dr.-Ing. Dr. h.c. N. Peters fur¨ seine stetige Unterstutzung¨ und die vermittelte Inspiration und Leidenschaft wissenschaftlich zu arbeiten. Ebenso bin ich Frau Prof. Dr. rer.nat. K. Kohse-H¨oinghaus fur¨ ihr Interesse an meiner Arbeit und fur¨ die Ub¨ ernahme des Korreferats, wie auch Herrn Prof. Dr. rer.nat. G. Grunefeld¨ fur¨ die Ub¨ ernahme des Vorsitzes der Prufungsk¨ ommission zu Dank verpflichtet. Weiterhin m¨ochte ich Herrn Dr. D. Goussis, Dr. E. Mastorakos und Herrn D. Diamantis, fur¨ die hervorangende Zusammenarbeit am ICEHT-FORTH in Patras w¨ahrend meines wissenschaftlichen Aufenthaltes im Sommer 2000 danken. Bei Prof. Seshadri an UCSD in San Diego, und Dr. Seiser bedanke ich mich fur¨ die Gastfreundschaft und den wissenschaftlichen Rat, w¨ahrend meines Aufenthaltes in San Diego im Herbst 1999. Ich m¨ochte mich bei den jetzigen und ehemaligen Mitarbeitern am ITM fur¨ die stets freundschaftliche Unterstutzung¨ bedanken. Im Besonderen gilt mein Dank Peter Spiekermann, Christoph Kortschik, Jost Weber, Markus Bollig, Hardo Barths und Heinz Pitsch fur¨ die viele interessante und anregende Diskussionen. Ich bin den Herren Dr. Ingo Mikulic, und Gun¨ ter Paczko fur¨ die intellektuellen Analysen der politischen, wirtschaftlichen und sozialen Problemen unserer Gesellschaft, neben und nicht zum Nachteil unserer Forschungsaktivit¨aten, zu Dank verpflichtet. Herrn D. Osthoff und die komplette Werkstatt-Mannschaft danke ich ganz herzlich fur¨ die stetige Unterstutzung¨ bei der Konstruktion der Gegenstromanordnung. Meinen Kollegen bei Ford m¨ochte ich ganz herzlich fur¨ ihre Unterstutzung¨ bei der Vorbereitung meiner Promotionsprufung¨ danken. Georgios Bikas Aachen im M¨arz 2002 Kinetische Mechanismen zur Zundung¨ von Kohlenwasserstoffen Zusammenfassung Verbrennungsmotoren leisten den gr¨oßten Beitrag zur Befriedigung der Nach- frage nach Mobilit¨at. Die letzte geh¨ort zu den wichtigsten Gutern¨ der modernen Industriegesellschaften. Obwohl abnehmende Reserven an nicht regenerativen En- ergiequellen, zunehmende Belastung der Umwelt durch Luftverschmutzung und Treibhauseffekt eine langfristige Erschließung alternativer Techniken erfordern, bleibt die derzeitige Verbrennungstechnik unverzichtbar. Daher stellen die Opti- mierung der Effizienz und Umweltvertr¨aglichkeit die wichtigsten gesellschaftlichen Forderungen an die Weiterentwicklung dieser Technik dar. Der direkteinspritzende Pkw-Dieselmotor erlebt in den letzten Jahren eine Re- naissance wegen seiner Vorteile gegenub¨ er dem Benzinmotor bezuglic¨ h Wirkungs- grad, Verbrauch und Emissionen. Trotz signifikanter technischer Fortschritte haben Dieselmotoren immer noch den Nachteil, bedeutende Konzentrationen von Ruß und Stickoxiden zu emittieren. Um Strategien zur Vermeidung dieser Schad- stoffe aufzuzeigen, braucht man ein tiefes Verst¨andnis der relevanten physikali- schen und chemischen Prozesse. Mit steigender Leistungsf¨ahigkeit der Daten- verarbeitung und st¨andiger Verbesserung der Modellierung wird es m¨oglich, die wichtisten Prozesse bei der dieselmotorischen Verbrennung zu simulieren. Die wichtigsten, durch chemische Vorg¨ange bestimmten Einzelprozesse bei der diesel- motorischen Verbrennung sind die Selbstzundung,¨ der chemische Umsatz und damit die W¨armefreisetzung und die Bildung von Schadstoffen, wobei Ruß und NOx die bedeutendsten Komponenten darstellen. In der vorliegenden Arbeit werden Modelle entwickelt, die die Voraussetzun- gen fur¨ Verbrennungssimulationen darstellen. Dazu geh¨oren vor allem chemisch- kinetische Reaktionsmechanismen, die in Verbindung mit Verbrennungsmodellen iii iv und Computerprogrammen zur numerischen Str¨omungsmechanik die Simulation technischer Verbrennungsvorg¨ange erm¨oglichen. Ausgehend von einfachen Kon- figurationen null- und eindimensionaler Flammen und verfugbarer¨ Experimente werden Mechanismen aufgebaut und validiert, die vor allem fur¨ die motorische Verbrennung relevant sind. Besonders Augenmerk wird auf die Beschreibung und Modellierung von Zundv¨ org¨ange sowie als auch die Beschreibung der Stickoxidbil- dung und -reduzierung gelegt. Die entwickelten Mechanismen werden ub¨ er einen weiten Parameterbereich analysiert und wichtige Reaktionspfade ermittelt. Nach einer Einleitung in die Thematik, stellt Kapitel 2 die Erhaltungsgle- ichungen fur¨ alle verwendeten eindimensionalen Flammenkonfigurationen vor. Das Flamelet Model wird vorgestellt und das RIF-Konzept (Repr¨asentatives Interaktives Flamelet) erl¨autert. Das dritte Kapitel besch¨aftigt sich mit der Entwicklung von Reaktionsmechanis- men fur¨ n-Heptan, n-Dekan, Toluol, und 1,2,4-Trimethylbenzol. Durch Verglei- ch mit experimentellen Ergebnissen aus laminaren Flammen und Ruhrreaktoren¨ werden die Mechanismen analysiert und im begrenzten Bereich validiert. In einer Gegenstromanordnung sind Selbstzundv¨ ersuche durchgefuhrt¨ worden. Die dadurch erfaßten experimentellen Ergebnisse werden im Vergleich mit Simulationsergebnisse dargestellt. Im vierten Kapitel werden verschiedene Methoden zur Reduzierung von chemi- schen Mechanismen vorgestellt und Ergebnisse pr¨asentiert, die mit der Hilfe von Computational Singular Perturbation (CSP) gewonnen wurden. Das funfte¨ Kapitel beschreibt ein neues Verbrennungskonzept, das so gennante Homogeneous Charge Compression Ignition (HCCI) Konzept. Es wird sowohl mit Hilfe einfacher Modellen als auch mit Hilfe des RIF-Modells analysiert und die Vorteile bezuglic¨ h Ruß und NOx Emissionen gegenub¨ er dem Dieselmotor vorgestellt. Contents 1 Introduction 1 2 Mathematical Models 6 2.1 Governing Equations for Reacting Flows . 6 2.2 Premixed Flames . 8 2.3 Stationary Diffusion flames - Eigenvalue Formulation . 10 2.4 Instationary Diffusion flames - Eigenvalue Formulation . 12 2.5 Sensitivity Analysis . 13 2.6 Laminar Flamelets . 13 2.6.1 Flamelet Equations - Flamelet Parameters . 16 2.7 CFD-Code Conservation Equations . 20 2.7.1 RIF Concept . 23 3 Hydrocarbon Oxidation and Autoignition 25 3.1 Autoignition Phenomena . 25 3.1.1 Characteristics of Autoignition . 25 3.2 Kinetic Modelling of Autoignition . 27 3.2.1 Comprehensive Models . 27 3.2.2 Reduced Models . 28 3.2.3 Simplified Models . 28 3.3 Basis Mechanism for C1-C2 . 31 3.4 NOx-Model . 37 3.5 n-Heptane . 39 3.6 n-Decane . 42 3.7 Toluene . 63 v CONTENTS vi 3.8 1,2,4-Trimethylbenzene . 71 3.9 Counterflow Ignition . 75 4 Reduced Chemical Mechanisms 85 4.1.1 Partial-Equilibrium Assumptions . 85 4.1.2 Steady-State Assumptions for Radicals . 85 4.1.3 Mathematical Reduction Technique . 86 4.2 Mathematical Reduction Technique with CSP Data . 86 4.3 Construction of global reduced mechanisms . 89 4.4 Reduced Mechanism for n-Heptane . 92 5 Application to Homogeneous Charge Compression Ignition (HCCI) 98 5.1 Introduction . 98 5.2 Potential of HCCI . 99 5.2.1 Oxides of Nitrogen (NOx) . 99 5.2.2 Particulate Matter (PM) . 99 5.2.3 Hydrocarbons (HC) and Carbon Monoxide (CO) . 99 5.3 Modelling and Simulation results on HCCI . 100 5.3.1 Kinetic Modelling and engine data . 101 5.3.2 Effect of EGR . 102 6 Summary and Conclusions 110 References 111 A The Chemical Mechanisms 125 B The Global Rates 149 C Species list & Thermodynamic Properties 152 D Nomenclature 162 Chapter 1 Introduction Flames and fires have played a very intimate role in man's life since ages unknown. Our civilization is bred on combustion of fuel for useful heat. Power production by coal and oil burning power plants is yet a predominant feature of our society and seems like we ought to learn efficient ways of this production. Pyrometallurgy is the back-bone of our industrial development. Internal combustion engines con- stitute a large and integral fraction of our economy. Of course, there are many unwanted effects of combustion. Motor vehicles -cars, trucks, and busses- are a major source of air pollution. For more than 35 years we have been both learning about the problem and attempting to control vehicle emissions. In the 1950s through studies in Los Angeles, it became clear that emissions from automobiles were a major contributor to urban air pollution. This smog, formed in the atmosphere as a result of complex photochemistry between hydrocarbons -often called volatile organic compounds (HC or VOC), and oxides of nitrogen (NOx)- on warm spring, summer and fall days, results in high ambient levels of ozone and other oxidants. In addition, automobiles are the dominant source of carbon monoxide (CO). It is not just cars: Light trucks, heavy trucks, and off- road vehicles also contribute significantly. So do stationary combustion systems. Even natural (i.e biogenic) hydrocarbon emissions are important. Starting in the
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