Skeletal Methane–Air Reaction Mechanism for Large Eddy Simulation of Turbulent Microwave-Assisted Combustion

Abstract

Irradiating a flame via microwave radiation is a plasma-assisted combustion (PAC) technology that can be used to modify the combustion chemical kinetics in order to improve flame stability and to delay lean blow-out. One practical implication is that combustion engines may be able to operate with leaner fuel mixtures and have an improved fuel flexibility capability including biofuels. Furthermore, this technology may assist in reducing thermoacoustic instabilities, which is a phenomenon that may severely damage the engine and increase NO X production. To further understand microwave-assisted combustion, a skeletal kinetic reaction mechanism for methane−air combustion is developed and presented. The mechanism is detailed enough to take into account relevant features, but sufficiently small to be implemented in large eddy simulations (LES) of turbulent combustion. The mechanism consists of a proposed skeletal methane−air reaction mechanism accompanied by subsets for ozone, singlet oxygen, chemionization, and electron impact reactions. The baseline skeletal methane−air mechanism contains 17 species and 42 reactions, and it predicts the ignition delay time, flame temperature, flame speed, major species, and most minor species well, in addition to the extinction strain, compared to the detailed GRI 3.0 reaction mechanism. The amended skeletal reaction mechanism consists of 27 species and 80 reactions and is developed for a reduced electric field E/N below the critical field strength (of ∼125 Td) for the formation of a microwave breakdown plasma. Both laminar and turbulent flame simulation studies are carried out with the proposed skeletal reaction mechanism. The turbulent flame studies consist of propagating planar flames in homogeneous isotropic turbulence in the reaction sheets and the flamelets in eddies regimes, and a turbulent low-swirl flame. A comparison with experimental data is performed for a turbulent low-swirl flame. The results suggest that we can influence both laminar and turbulent flames by nonthermal plasmas, based on microwave irradiation. The laminar flame speed increases more than the turbulent flame speed, but the radical pool created by the microwave irradiation significantly increases the lean blow-out limits of the turbulent flame, thus making it less vulnerable to thermoacoustic combustion oscillations. Apart from the experimental results from low-swirl flame presented here, experimental data for validation of the simulated trends are scarce, and conclusions build largely on simulation results. Analysis of chemical kinetics from simulations of laminar flames and LES on turbulent flames reveal that singlet oxygen molecule is of key importance for the increased reactivity, accompanied by production of radicals such as O and OH.

Reduced Chemical Kinetic Reaction Mechanism for Dimethyl Ether-Air Combustion

“…The reactions in the H/C 1 /O submechanism are the same 42 irreversible reactions (reactions 28-69 in Table 3) as in previously presented work by Larsson et al [35], and then later by Zettervall et al [36,41,43,[49][50][51], but with tuning of some rate constants, as noted in Table 3. These adjustments, applied on ten reactions, are mainly made in order to adjust the production and consumption of the radical pool (O, H, OH).…”

“…The reduced reaction mechanism presented here builds on the well-proven mechanism build-up concept used by Zettervall et al [36] on hydrogen (H 2 ), Larsson et al [35] on methane (CH 4 ), and Zettervall et al [49] on ethylene (C 2 H 4 ), propane (C 3 H 8 ) [43] and kerosene (C 12 H 23 ) [41,50,51].…”

“…They have been validated for both laminar flames [32] and shock tube experiments [32,33]. Ever increasing development in computational capacities has enabled the use of Large Eddy Simulations (LES) to become a viable simulation option for a wide range of applications [34][35][36]. That said the computational capacity still represents a major limiting factor with geometric representation, applied simulation models, number of mesh cells and the complexity and numerical stiffness of the chemistry determining the computational cost.…”

“…The high computational cost effectively excludes use of any of the detailed reaction mechanisms due to their high computational cost, creating a demand for smaller reaction mechanisms. Several high-fidelity combustion LES studies, incorporating reduced kinetic mechanisms of hydrogen and a range of hydrocarbons, have been published by Bulat et al [40], Zettervall et al [36,[41][42][43], Liu et al [44], Fureby [45], Larsson et al [35], Vincent-Randonnier et al [46,47] and Danel et al [48].…”

A Combined Experimental and Computational Study of the LAPCAT II Supersonic Combustor

“…The H 2 -air combustion process is here modeled using a novel H 2 -air reaction mechanism that employs the H 2 -O 2 chemical structure from [39], with three additional fuel breakdown reactions from [40][41][42], creating a reaction mechanism called Z22, of 9 species and 22 irreversible reactions, listed in Table 2. Hereafter, all reactions are referenced with their reaction numbers in Table 2.…”