Reduced-Order Chemical Mechanisms for Oxygen-Enriched Combustion of Methane and N-decane Fuel (original) (raw)

Chemical mechanism reductions are becoming new frontier in computational simulations. Physically accurate chemical combustion mechanisms can include tens of thousands of individual reactions and hundreds of species. Vast amounts of computational resources are required to analyze these sets. Reduced-order chemical mechanisms are a possible alternative being developed for the laminar, premixed combustion of methane and n-decane hydrocarbon fuels. Insight gained into this field could find a wide array of applications in areas such as economic, space, and other stochastic models. In this study, well-researched, large scale combustion chemical mechanisms are first tested using CHEMKIN analysis software for a range of equivalence ratios at two levels of oxygen enrichment, 21% and 25%. The results are compared to experimental data collected for a range of equivalence ratios (Ф) from Ф =0.8 to 1.2 using a flat-flame burner. Strong agreement between the experimental results and numerical laminar flame speed predictions verify the applicability of the computational models. The data collected is used a frame of reference for the reduction of the chemical mechanism to begin. A reduction method is designed to maintain laminar flame speed. A method is developed using tools provided by Chemical Workbench software that is able to reduce the methane chemical mechanism from 325 reactions to 47 reactions. Using three analytical techniques, Direct Relational Graphing (DRG), Computational Singular Perturbation (CSP), and Direct Sensitivity Analysis (SA), executed sequentially reduced mechanisms for methane and decane were carefully tailored preserve laminar flame speed at varying equivalence ratios. The reduced mechanisms were tested and proven to support laminar flame speed predictions within 5-10% maximum deviations for both baseline and experimental data. This warranted the application of the reduction method to the n-decane mechanism. The same process was implemented to reduce the n-decane mechanism from 638 reactions to 220 reactions. More importantly, both of the models were reduced to less than 50 chemical species, which is the current maximum allowed for simulations using ANSYS Fluent Computational Fluid Dynamics (CFD) Software. With the numerical models tested and verified, a simple two-dimensional CFD simulation was performed which modeled premixed combustion in a swiss-roll microcombustor. Usage statistics were gathered after convergence of the simulations. The reduced-order chemical mechanism for methane showed an 81.46% decrease in iv computational time compared to the full-scale mechanism with scalable results held within 5% in the key area of interest. The reduced n-decane mechanism boasted a 79.54% reduction in computational resources and maintained a similar margin of error. The findings show that by selectively reducing comprehensive and computational expensive chemical mechanism, a much smaller model can be tailored for use in a specific application thereby increasing efficiency through speed and reduced need for computational resources. v