posted on 2015-07-16, 00:00authored byMichael P. Burke, C. Franklin Goldsmith, Stephen J. Klippenstein, Oliver Welz, Haifeng Huang, Ivan O. Antonov, John D. Savee, David
L. Osborn, Judit Zádor, Craig A. Taatjes, Leonid Sheps
The
present paper describes further development of the multiscale informatics
approach to kinetic model formulation of Burke et al. (Burke, M. P.;
Klippenstein, S. J.; Harding, L. B. Proc. Combust. Inst.2013, 34, 547–555) that directly
incorporates elementary kinetic theories as a means to provide reliable,
physics-based extrapolation of kinetic models to unexplored conditions.
Here, we extend and generalize the multiscale informatics strategy
to treat systems of considerable complexityinvolving multiwell
reactions, potentially missing reactions, nonstatistical product branching
ratios, and non-Boltzmann (i.e., nonthermal) reactant distributions.
The methodology is demonstrated here for a subsystem of low-temperature
propane oxidation, as a representative system for low-temperature
fuel oxidation. A multiscale model is assembled and informed by a
wide variety of targets that include ab initio calculations
of molecular properties, rate constant measurements of isolated reactions,
and complex systems measurements. Active model parameters are chosen
to accommodate both “parametric” and “structural”
uncertainties. Theoretical parameters (e.g., barrier heights) are
included as active model parameters to account for parametric uncertainties
in the theoretical treatment; experimental parameters (e.g., initial
temperatures) are included to account for parametric uncertainties
in the physical models of the experiments. RMG software is used to
assess potential structural uncertainties due to missing reactions.
Additionally, branching ratios among product channels are included
as active model parameters to account for structural uncertainties
related to difficulties in modeling sequences of multiple chemically
activated steps. The approach is demonstrated here for interpreting
time-resolved measurements of OH, HO2, n-propyl, i-propyl, propene, oxetane, and methyloxirane
from photolysis-initiated low-temperature oxidation of propane at
pressures from 4 to 60 Torr and temperatures from 300 to 700 K. In
particular, the multiscale informed model provides a consistent quantitative
explanation of both ab initio calculations and time-resolved
species measurements. The present results show that interpretations
of OH measurements are significantly more complicated than previously
thoughtin addition to barrier heights for key transition states
considered previously, OH profiles also depend on additional theoretical
parameters for R + O2 reactions, secondary reactions, QOOH
+ O2 reactions, and treatment of non-Boltzmann reaction
sequences. Extraction of physically rigorous information from those
measurements may require more sophisticated treatment of all of those
model aspects, as well as additional experimental data under more
conditions, to discriminate among possible interpretations and ensure
model reliability.