The optimization of thermodynamic and rate constant rules to describe the oxidation of large alkane fuels

In this study, a consistent set of rate constant rules have been developed to describe the pyrolysis and oxidation of large alkanes. Initially, the rate rules were developed based on 2,2,3-, 2,3,4-, and 2,2,4-trimethylpentane, chosen for their complex molecular structures. These rate rules were then applied to construct a combustion mechanism for n-decane. However, due to the limited structural variety (only three branched octanes) used in the rate rule development, further refinements are necessary to improve the predictive accuracy for n-decane. To develop more transferable and broadly applicable rate rules, a greater diversity of fuels is needed for the optimization process. The present work includes various pentane, hexane, heptane isomers, iso-octane, and large n-alkanes (C8–C12). However, due to the large number of fuels, an automated optimization methodology was developed to enhance efficiency. This automated approach was tested on the three pentane isomers and further refined through experimental investigations of first-stage ignition behaviour measured in rapid compression machine (RCM) experiments. To address the “global optimized” challenge of optimization for a large set of rate rules, a two-stage optimization strategy was implemented: (a) optimizing alkanes with only primary, secondary, and quaternary carbon atoms, followed by (b) optimizing those containing tertiary carbon atoms. The optimization was carried out using experimental ignition delay time data measured in shock tubes and in RCMs, and species concentration versus time/temperarture data measured in jet-stirred reactors, together with advanced computational techniques such as PCA-SUE and Optima++. The final optimized rate rules were successfully applied to establish combustion chemistry models for and n-undecane and 2-methyl decane. Experimental validation demonstrated accurate predictions of ignition delay times and species concentration profiles, confirming the effectiveness of the rate rules developed. This work provides a systematic approach for refining alkane combustion mechanisms and contributes to an improved understanding of large-alkane oxidation chemistry.

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University of Galway

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CC BY-NC-ND

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University of Galway Theses (PhD Theses)