This work aims at comparing the accuracy and overall performance of a low-Mach CFD solver and a fully-compressible CFD solver for direct numerical simulation (DNS) of nonequilibrium plasma assisted ignition (PAI) using a phenomenological model described in Castela et al. [1]. The phenomenological model describes the impact of nanosecond pulsed plasma discharges by introducing source terms in the reacting flow equations, instead of solving the detailed plasma kinetics at every time step of the discharge. Ultra-fast gas heating and dissociation ofO2 are attributed to the electronic excitation ofN2 and the subsequent quenching to ground state. This process is highly exothermic, and is responsible for dissociation of O2 to form O radicals; both of which promote faster ignition. Another relatively slower process of gas heating associated with vibrational-to-translational relaxation is also accounted for, by solving an additional vibrational energy transport equation. A fully-compressible CFD solver for high Mach (M>0.2) reacting flows, developed by extending the default rhoCentralFoam solver in OpenFOAM, is used to perform DNS of PAI in a 2D domain representing a cross section of a pin-to-pin plasma discharge configuration. The same case is also simulated using a low-Mach, pressure-based CFD solver, built by extending the default reactingFoam solver. The lack of flow or wave dominated transport after the plasma-induced weak shock wave leaves the domain causes inaccurate computation of all the transport variables, with a rather small time step dictated by the CFL condition, with the fully-compressible solver. These issues are not encountered in the low-Mach solver. Finally, the low-Mach solver is used to perform DNS of PAI in lean, premixed, isotropic turbulent mixtures of CH4-air at two different Reynolds numbers of 44 and 395. Local convection of the radicals and vibrational energy from the discharge domain, and straining of the high temperature reaction zones resulted in slower ignition of the case with the higher Re. A cascade effect of temperature reduction in the more turbulent case also resulted in a five - six times smaller value of the vibrational to translational gas heating source term, which further inhibited ignition. Two pulses were sufficient for ignition of the Re = 44 case, whereas three pulses were required for the Re = 395 case; consistent with the results of Ref. [1].
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Numerical Investigation of Ignition Kernel Development with Nanosecond Pulsed Plasma in Quiescent and Flowing Mixtures
Nanosecond Pulsed High Frequency Discharges (NPHFD) are gaining popularity over conventional spark and arc discharges as they have been shown to increase energy efficiency, enhance ignition probability and sustained kernel growth, and offer more flexibility and control for ignition applications under various conditions. Hence, it is important to determine the impact of different factors such as the optimal pulse energy, background flow conditions, inter-pulse time, mixture equivalence ratio, etc. on the success of ignition of premixed mixtures with NPHFD. This work presents a numerical investigation of the morphology of ignition kernel development with both single-pulse and multiple-pulse discharges. Nanosecond non-equilibrium plasma discharges are modeled between pin-pin electrodes in a subsonic ignition tunnel with quiescent and flowing premixed mixtures of methane and air. Large eddy simulations (LES) are conducted to investigate the reasons for successful and failed ignition in different scenarios. A single pulse discharge in the presence of electrodes, in a quiescent medium, elucidates the gas recirculation pattern caused by the plasma pulse which results in a separated toroidal kernel from the primary ignition kernel between the electrodes. Convection heat loss to the mean flow results in quenching of the high temperature, radical-rich hot-spots creeping on the electrode walls, and leaving only the semi-toroidal kernel to propagate downstream. Finally, simulations with multiple pulses with different inter-pulse times have been conducted to analyze the synergistic effect of overlapping kernels with high temperature and OH concentration, which has been attributed as the primary reason for higher ignition probabilities in the “fully coupled” regime reported in the experiments. Successful ignition kernel formation is reported with 3 pulses at a pulse repetition frequency of 300 kHz in the fully coupled regime. This kernel volume was almost 4 times, and develops in two-thirds the time, compared to the ignition kernel volume formed by the single pulse discharge with the same total energy. Ten pulses with twice as much total energy were deposited at a much lower frequency of 2 kHz, which resulted in disjoint hot-spots that fail to form an ignition kernel in the decoupled regime.
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- Award ID(s):
- 2002635
- NSF-PAR ID:
- 10481919
- Publisher / Repository:
- American Institute of Aeronautics and Astronautics
- Date Published:
- Journal Name:
- AIAA SCITECH 2023 Forum
- Format(s):
- Medium: X
- Location:
- National Harbor, MD & Online
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.2514/6.2023-0749
