Boulder Fluid and Thermal Sciences Seminar Series

Tuesday, November 29, 2016

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3:30pm-4:30pm (refreshments at 3:15pm)
Bechtel Collaboratory in the Discovery Learning Center (DLC)
University of Colorado at Boulder

“Minor” Molecular Thermochemistry Effects Significantly Alter Predictions of Global Combustion Phenomena

Nicole Labbe University of Colorado, Boulder

CH₃, the simplest alkyl radical, is an extremely stable intermediate with a C-H bond energy of D₀(H-CH₂) = 109.28 ± 0.03 kcal/mol, according to Active Thermochemical Tables (ATcT). Consequently, it persists in large concentrations even in high temperature chemically reacting systems such as flames. The dominant process for CH₃ removal in combustion and flame processes is through radical-radical recombination, either with H-atoms (R1: CH₃ + H --> CH₄ ; ∆H_0K = -103.34 ± 0.02 kcal/mol, ATcT) or with itself (R2: CH₃ + CH₃ --> C₂H₆; ∆H_0K = -87.92 ± 0.05 kcal/mol, ATcT). For reaction (R1), direct measurements for k₁ are largely limited to low temperatures, while direct shock tube measurements for k_-1 span a temperature range from 1700 to 4500 K. Reconciling the apparently disparate experimental databases on k₁ and k_-1 requires accurate equilibrium constants for reaction (R1) spanning an extended range of temperatures (300-4500K). While several thermochemical databases have accounted for anharmonic effects in calculations of CH₄ thermochemistry, analogous corrections have not been included in CH₃ radical thermochemistry. As a consequence, existing literature assessments of the equilibrium and rate constants for recombination/dissociation reaction (R1) have notable inaccuracies.

In the present work, we include all relevant anharmonic corrections to calculate an accurate ATcT partition function for CH₃. The resulting nonrigid-rotator-anharmonic-oscillator partition function is used to determine thermochemical parameters over a wide temperature range (200-6000 K) of relevance to atmospheric and combustion chemistry. The new ATcT thermochemistry is also used to prescribe K_eq values for reaction R1. Furthermore, literature experiments and theory (based on two-dimensional master equation calculations with a first principles energy and angular momentum transfer kernel) are used to obtain an accurate representation of the kinetics for k_1,-1(T,P) for subsequent use in combustion modeling. Theory is also used to predict the effects of bath-gas colliders on k_1,-1(T,P). Lastly, we assess the impact of incorporating anharmonic thermochemistry for CH₃ and the updated fits for k_1,-1 in current widely used literature models for simulations of CH₄-air laminar flame speeds.