TY - JOUR
T1 - Effects of non-equilibrium excitation on methane oxidation in a low-temperature RF discharge
AU - Sun, Jintao
AU - Chen, Qi
AU - Yang, Xiaofang
AU - Koel, Bruce E.
N1 - Funding Information:
This work was supported by the Fundamental Research Funds for the Central Universities (2018YJS141), the National Natural Science Foundation of China (No. 21676024), the Beijing Natural Science Foundation (No. 3182029), and the Advance Space Propulsion Laboratory of BICE and Beijing Engineering Research Center of Efficient and Green Aerospace Propulsion Technology (LabASP-2018-10).
Publisher Copyright:
© 2019 IOP Publishing Ltd.
PY - 2020
Y1 - 2020
N2 - The kinetic effects of non-equilibrium excitation by direct electron impact on low-temperature oxidation of CH4 were investigated by experiment and simulation. We focused on the vibrational-electronic-chemistry coupling of methane and oxygen molecules under conditions of immediate reduced electric field strengths of 30-100 Td in an RF dielectric barrier discharge. A detailed plasma chemistry mechanism governing the oxidation processes in an He/CH4/O2 combustible mixture was proposed and studied by including a set of electron impact reactions, dissociative recombination reactions, reactions involving vibrationally- and electronically- excited species, and important three-body recombination reactions. A linear increase in reactant consumption with an increase in plasma power was observed experimentally. This suggested the presence of decoupling between the molecular excitation by plasma and the low-temperature chemistry. However, CO formation showed a non-linear trend, with its formation increasing with lower energy inputs and decreasing at higher energy inputs. By modelling the chemical kinetic sensitivity and reaction pathways, we found that the formation of radicals via the chain propagation reactions CH4 + O(1D) → CH3 + OH, and O2(a1Δg) + H → O + OH was mainly accelerated by the electronically excited species O(1D) and O2(a1Δg). The numerical simulation also revealed that under conditions of incomplete relaxation, the vibrational species CH4(v) and O2(v) enhanced chain propagating reactions, such as CH4(v) + O → CH3 + OH, CH4(v) + OH → CH3 + H2O, O2(v) + H → O + OH, thus stimulating the production of active radicals and final products. Specifically, for an E/N value of 68.2 Td in a stoichiometric mixture (0.05 CH4/0.1 O2/0.85 He), O(1D), CH4(v13), and O2(v) were estimated to contribute to 12.7%, 3.6%, and 3.8% of the production of OH radicals respectively. The reaction channel CH4(v13) + OH → H2O + CH3 was estimated to be responsible for 1.6% of the H2O formation. These results highlight the strong roles of vibrational states in a complex plasma chemistry system and provide new insights into the roles of excited species in the low-temperature oxidation kinetics of methane.
AB - The kinetic effects of non-equilibrium excitation by direct electron impact on low-temperature oxidation of CH4 were investigated by experiment and simulation. We focused on the vibrational-electronic-chemistry coupling of methane and oxygen molecules under conditions of immediate reduced electric field strengths of 30-100 Td in an RF dielectric barrier discharge. A detailed plasma chemistry mechanism governing the oxidation processes in an He/CH4/O2 combustible mixture was proposed and studied by including a set of electron impact reactions, dissociative recombination reactions, reactions involving vibrationally- and electronically- excited species, and important three-body recombination reactions. A linear increase in reactant consumption with an increase in plasma power was observed experimentally. This suggested the presence of decoupling between the molecular excitation by plasma and the low-temperature chemistry. However, CO formation showed a non-linear trend, with its formation increasing with lower energy inputs and decreasing at higher energy inputs. By modelling the chemical kinetic sensitivity and reaction pathways, we found that the formation of radicals via the chain propagation reactions CH4 + O(1D) → CH3 + OH, and O2(a1Δg) + H → O + OH was mainly accelerated by the electronically excited species O(1D) and O2(a1Δg). The numerical simulation also revealed that under conditions of incomplete relaxation, the vibrational species CH4(v) and O2(v) enhanced chain propagating reactions, such as CH4(v) + O → CH3 + OH, CH4(v) + OH → CH3 + H2O, O2(v) + H → O + OH, thus stimulating the production of active radicals and final products. Specifically, for an E/N value of 68.2 Td in a stoichiometric mixture (0.05 CH4/0.1 O2/0.85 He), O(1D), CH4(v13), and O2(v) were estimated to contribute to 12.7%, 3.6%, and 3.8% of the production of OH radicals respectively. The reaction channel CH4(v13) + OH → H2O + CH3 was estimated to be responsible for 1.6% of the H2O formation. These results highlight the strong roles of vibrational states in a complex plasma chemistry system and provide new insights into the roles of excited species in the low-temperature oxidation kinetics of methane.
KW - RF discharge
KW - low-temperature chemistry
KW - non-equilibrium excitation
KW - plasma-assisted combustion
KW - sensitivity analysis
KW - vibrational excitation
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U2 - 10.1088/1361-6463/ab57dc
DO - 10.1088/1361-6463/ab57dc
M3 - Article
AN - SCOPUS:85077776732
SN - 0022-3727
VL - 53
JO - Journal Physics D: Applied Physics
JF - Journal Physics D: Applied Physics
IS - 6
M1 - 064001
ER -