D. Kim, F. Rizzi, K.-W. Cheng, J. Han, F. Bisetti, O.M. Knio
Combust. Flame, 162:7, 2904-2915, (2015)
Chemi-ionization, Ion chemistry, Electrons, Uncertainty quantification, Polynomial chaos, Sparse-adaptive sampling
Uncertainty quantification (UQ) methods are implemented to obtain a
quantitative characterization of the evolution of electrons and ions
during the ignition of methane–oxygen mixtures under lean and
stoichiometric conditions. The GRI-Mech 3.0 mechanism is combined with
an extensive set of ion chemistry pathways and the forward propagation
of uncertainty from model parameters to observables is performed using
response surfaces. The UQ analysis considers 22 uncertain rate
parameters, which include both chemi-ionization, proton transfer, and
electron attachment reactions as well as neutral reactions pertaining to
the chemistry of the CH radical. The uncertainty ranges for each rate
parameter are discussed. Our results indicate that the uncertainty in
the time evolution of the electron number density is due mostly to the
chemi-ionization reaction CH + O ⇌ HCO+ + E− and to the main CH consumption reaction CH + O2 ⇌ O + HCO. Similar conclusions hold for the hydronium ion H3O+, since electrons and H3O+
account for more than 99% of the total negative and positive charge
density, respectively. Surprisingly, the statistics of the number
density of charged species show very little sensitivity to the
uncertainty in the rate of the recombination reaction H3O+ + E− → products,
until very late in the decay process, when the electron number density
has fallen below 20% of its peak value. Finally, uncertainties in the
secondary reactions within networks leading to the formation of minor
ions (e.g., C2H3O+, HCO+, OH−, and O−) do not play any role in controlling the mean and variance of electrons and H3O+,
but do affect the statistics of the minor ions significantly. The
observed trends point to the role of key neutral reactions in
controlling the mean and variance of the charged species number density
in an indirect fashion. Furthermore, total sensitivity indices provide
quantitative metrics to focus future efforts aiming at improving the
rates of key reactions responsible for the formation of charges during
hydrocarbon combustion.