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Development and reduction of chemical kinetic schemes: establishment of a hierarchical, structured reaction model based on sub-models for the prediction of combustion processes

Developing surrogate fuels as well as PAH or soot formation models requires detailed chemical kinetic models of all the initial species, intermediate species and reaction products. These detailed mechanisms covering the entire parameter range of technical combustion devices form the essential basis for any reduced kinetic schemes that are sufficiently small to be incorporated in state-of-the-art numerical tools.

Abb. 1: Diagramm zur Erstellung eines global reduzierten Mechanismus

The main focus of our current work is the development of chemical kinetic mechanisms which are able to predict the heat release and exhaust gas emissions from modern gas turbines. The methodology of this development effort is shown in Fig. 1. The consistent kinetic scheme with sub-models for H₂, CO, CH₄, CH₃OH, C₂H₄, C₂H₅OH, C₇H₈, cy-C₆H₁₂, n-C₇H₁₆, i-C₈H₁₈ and n-C₁₀H₂₂ is based on the following principles:

• only well-referenced thermo-chemical data are applied,
• the scheme has a hierarchical structure for molecules and reactions, and
• the kinetic properties of the various hydrocarbons are continuously adapted, validated and optimised.

This scheme is able to predict heat release, ignition delay, laminar flame velocity and PAH formation (see soot formation) for every single species mentioned previously, as well as for mixtures of those species (see Figs. 2 and 3).

Abb. 2: Zündverzugszeit von n-C₇H₁₆-Luft ; Φ=1,0

Abb. 3: Laminare Flammengeschwindigkeiten von Methan mit Luft und Sauerstoff-Helium-Gemisch bei Raumtemperatur und einem Druck von 10 bar

The RedMaster software program has been specially developed to automatically reduce large detailed mechanisms to skeletal schemes, while still maintaining their predictive capabilities.

The program eliminates all species and reactions which have been consistently identified as being unimportant for the entire parameter range of interest. The code is able to:

• initiate CHEMKIN to predict ignition delays for the selected boundary conditions automatically,
• select the necessary types of sensitivity analyses and predict the appropriate sensitivity coefficients,
• determine automatically the required time steps for which such sensitivity analyses must be performed,
• select species and reactions which have been identified as unimportant for all time steps mentioned previously,
• collect all information about unimportant species and reactions for selected conditions,
• eliminate species and reactions from the scheme which have been identified as unimportant, and, finally,
• to produce an immediately applicable reduced mechanism once all the steps mentioned above have been performed successfully.