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Chemkin 4.1.1 Chapter 5: Chemical Mechanism Analysis changes, (3) options associated with modeling plasma reactions, (4) the Chebyshev polynomial option for describing pressure-dependent gas-phase reactions, the (5) Landau-Teller rate formulation used for energy transfer processes, or (6) multiple materials. If such options are included in a reaction mechanism, they will be ignored. 5.1.1 Background Information 5.1.1.1 Bath Gas and Carrier Gas The concept of a “bath gas” is used throughout the Mechanism Analyzer. The specification of a bath gas consists of a characteristic temperature, pressure, and composition at which quantities are to be evaluated by default. Composition, here, refers to the default composition for all phases defined in the mechanism. Reaction rate information is evaluated at the bath gas conditions, unless it is tabulated as a function of a system parameter, such as temperature. In this case, all other parameters are fixed at the bath gas conditions in the table. The default temperature for the bath gas is 298.15 K. The default pressure is 1 atmosphere, and the default composition is an equimolar composition in each phase. All defaults can be overridden by selections in the CHEMKIN Interface input panels. The concept of a “carrier gas” is also used in the Mechanism Analyzer. Unless overridden, the carrier gas is assumed to be the gas component having the largest mole fraction. CHEMKIN calculates a single number for the characteristic time scale of diffusion (to be compared with the characteristic time scale of reaction). To make this comparison, the diffusion coefficient is calculated for a specified “major” species in the carrier gas. For example, the diffusion of the major species CH 4 in the carrier gas H 2 is used to calculate the characteristic diffusion time scale. Unless overridden, the major species in the gas phase is assumed to be the gas component having the second largest mole fraction. 5.1.1.2 Uniform-dimensional and Non-dimensional Reaction Rate Information It is often useful to know, in some sense, which reactions in a mechanism are “fast” and which are “slow.” It is difficult or misleading to simply compare rate constants, which can have different units depending on the molecularity of the reaction. In order to compare the rates of reactions in the mechanism, we define a quantity Equation 5-1 k f * = ∏ k f [ G] g [ Sn] sn n © 2007 Reaction Design 162 RD0411-C20-000-001
Chapter 5: Chemical Mechanism Analysis <strong>Tutorials</strong> <strong>Manual</strong> which we call the “uniform-dimensional” rate constant. Regardless of the order of reaction, it will have units of mole • cm -3 •sec -1 for a gas-phase reaction or mole • cm -2 •sec -1 for a surface reaction. In this expression, k f is the rate constant for the forward reaction, [G] is the total concentration of gas-phase species determined at the bath gas conditions (in mole • cm -3 ), g is the sum of the stoichiometric coefficients of all gas-phase species appearing as reactants in the reaction, [Sn] is the total site density of surface phase n determined at the bath gas conditions, Sn is the sum of the stoichiometric coefficients of all surface species in phase n participating as reactants in this reaction. Using the usual rate constant, one calculates the forward reaction rate as k f times the product of the concentrations of the reactant species (in mole • cm -3 for gas species, or mole • cm -2 for surface species) raised to the power of their stoichiometric coefficients. With the uniform-dimensional rate constant, one calculates the same * reaction rate as k f multiplied by the (dimensionless) species mole fractions (gasphase reactants) or site fractions (surface species) raised to the power of their stoichiometric coefficients. Thus, independent of the molecularity of the reaction, the * reaction rate is k f times quantities that have maximum values on the order of unity (the mole and site fractions), and it is easier to compare one reaction to another. * The quantity k f just discussed can point out which reactions are fast relative to one another. It can also be of interest to know if a reaction is “fast” relative to a competing process like molecular transport. The Damköhler number for gas-phase reactions, Da, allows for such a comparison. Equation 5-2 Da = k f * ---------------------- DG [ ] ⁄ L 2 In Equation 5-2, D is a diffusion coefficient and L is a characteristic length scale for diffusion; for example, a boundary-layer thickness or a characteristic reactor dimension. The Damköhler number is a dimensionless number that is a measure of the relative importance of gas-phase kinetics versus molecular mass transport. If Da is much greater than 1, then a reaction is fast relative to transport; if it is much less than 1, then transport processes occur on a shorter time scale than kinetic processes. RD0411-C20-000-001 163 © 2007 Reaction Design
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Chemkin 4.1.1 Contents Table of Con
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Chemkin 4.1.1 Tables List of Tables
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Tutorials Manual 4.1.1 Figures List
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List of Figures Tutorials Manual 2-
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List of Figures Tutorials Manual 4-
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Chemkin 4.1.1 1 1 Introduction The
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Chapter 1: Introduction Tutorials M
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Chemkin 4.1.1 2 2 Combustion in Gas
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Chapter 2: Combustion in Gas-phase
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Chemkin 4.1.1 3 3 Catalytic Process
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Tutorials Manual

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