An official website of the United States government
Some of the popular and successful atmospheric pressure fuel/air plasma-assisted combustion methods use repetitive ns pulsed discharges and dielectric-barrier discharges. The transient phase in such discharges is dominated by transport under strong space charge from ionization fronts, which is best characterized by the streamer model. The role of the nonthermal plasma in such discharges is to produce radicals, which accelerates the chemical conversion reaction leading to temperature rise and ignition. Therefore, the characterization of the streamer and its energy partitioning is essential to develop a predictive model. We examine the important characteristics of streamers that influence combustion and develop some macroscopic parameters. Our results show that the radicals’ production efficiency at an applied field is nearly independent of time and the radical density generated depends only on the electrical energy density coupled to the plasma. We compare the results of the streamer model to the zero-dimensional uniform field Townsend-like discharge, and our results show a significant difference. The results concerning the influence of energy density and repetition rate on the ignition of a hydrogen/air fuel mixture are presented.
more »
« less
Effects of spatiotemporal plasma power distribution on the modeling of ignition kernel evolution in quiescent and turbulent methane/air mixtures
Abstract The present work improves a phenomenological plasma-assisted combustion model by integrating the spatiotemporal distribution of plasma power density, thereby considering the evolution of plasma streamers in the modeling, and subsequently, better predicting the ignition kernel evolution. The improved phenomenological model is validated against experiments representing the plasma discharge and post-discharge ignition kernel evolution. Specifically, the new model demonstrates a more accurate prediction of ultrafast gas heating and O2dissociation during the plasma discharge, compared to the original model. In addition, the new model is found to closely match the experimental pressure wave and heated channel profiles post-discharge without the need for tuning the energy deposition (unlike the original model), highlighting its accuracy of post-discharge ignition kernel dynamics. The improved phenomenological model is then employed to investigate ignition kernel evolution for a stoichiometric methane-air discharge across various discharge gap configurations. Simulations reveal a non-uniform temperature and streamer distribution progressing from the electrode tips toward the center, contrasting uniform cylindrical discharges previously described in the original model. Streamer propagation is observed to be faster for larger gaps when maintained at the same average electric field for different discharge gaps. The tendency of smaller gaps to produce detached toroidal ignition kernels is observed, while larger gaps promote cylindrical and attached ignition kernels. Interactions between successive ignition kernels from consecutive discharges varied significantly, with the smallest gap (1 mm) promoting the quenching of the preceding ignition kernel due to the initial kernel–kernel separation. The intermediate gap (2 mm) promotes detached kernel growth. In contrast, in the largest gap (4 mm), kernels consistently combine and expand attached to electrodes. The impact of homogeneous isotropic turbulence is also explored, showing the persistence of ignition kernels early on but eventually quenching due to enhanced radical and heat losses with pronounced turbulence intensity.
more »
« less
- Award ID(s):
- 2002635
- PAR ID:
- 10533185
- Publisher / Repository:
- IOP Publishing
- Date Published:
- Journal Name:
- Journal of Physics D: Applied Physics
- Volume:
- 57
- Issue:
- 45
- ISSN:
- 0022-3727
- Format(s):
- Medium: X Size: Article No. 455201
- Size(s):
- Article No. 455201
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
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.more » « less
-
The impacts of the pulse repetition frequency (PRF), number of pulses, and energy per pulse in a train of nanosecond discharge pulses on the ignition of a flowing lean premixed methane–air mixture are investigated using numerical simulations. A phenomenological plasma model coupled with a compressible reacting flow solver is used for these simulations. The simulation strategy has been well validated by comparing the experimental schlieren and OH planar laser induced fluorescence (PLIF) results with the numerical schlieren (i.e., density gradient) and OH density profiles, respectively. The characteristics of the ignition kernels produced by each discharge pulse and their interaction with each other as functions of the PRF are investigated. Three regimes were defined in the literature based on this interaction of the ignition kernels — fully coupled, partially coupled, and decoupled. This study uses numerical simulations to probe into the constructive and destructive effects, that ultimately determine ignition success, in these different regimes. The complete overlap of kernels and the complete lack of synergy between kernels produced by consecutive pulses are attributed to the success and failure of ignition and flame propagation in the fully coupled and decoupled regimes, respectively. In the partially coupled regime, the convection heat loss driven by the shock-turned-acoustic wave of the next discharge pulse, on the kernel produced by the previous discharge pulse, in addition to diffusion losses, contribute to ignition failure. However, the expansion of the next kernel in a region of higher average temperature and radical concentration created by the previous kernel could help to bridge the gap between the two kernels and result in successful ignition. The important parameters of energy per pulse, number of pulses, and equivalence ratio affect the competition between these constructive and destructive effects, which eventually determines the ignition success in this regime. Finally, the change in the nature of interaction between consecutive kernels from decoupled to partially coupled, at the same frequency but with different energies per pulse, is also shown.more » « less
-
Natural gas contains a significant fraction of methane, a strong greenhouse gas besides being a potent hydrogen carrier. Thus, reforming methane to a more reactive gas mixture could potentially abate the associated greenhouse heating by depleting methane and provide a pathway to generate hydrogen. The present study investigates the non-equilibrium plasma-assisted reforming of methane to produce hydrogen and reactive alkenes using repetitive nanosecond pulse discharges. A detailed gas-phase chemical kinetics mechanism along with plasma reforming kinetics derived from our previous work are used to perform 0D calculations to obtain the energy fractions for various plasma processes. A phenomenological model for the plasma-assisted reforming of methane/nitrogen mixtures is developed by considering the vibrational energy transport equations of both methane and nitrogen separately. The energy fractions involved in various plasma processes, such as ultra-fast gas heating and ultra-fast gas dissociation due to the electron excitation reactions, and slow gas heating due to the relaxation of vibrational excitation modes of methane and nitrogen, are accounted for in our new phenomenological model using energy fractions derived earlier. The newly developed phenomenological model is then used to perform 3D direct numerical simulation (DNS) of methane reforming diluted with 60% nitrogen in a pin-to-pin electrode configuration with a discharge gap of 1 mm. The effect of pulsing on the evolution of reformed mixture kernels is investigated by comparing two cases: a single-pulsed case with a pulse energy of 0.8 mJ, and another case using 4 pulses at 200 kHz, with a per pulse energy of 0.2 mJ. The single-pulsed case was observed to promote kernel separation and higher fractions of reformed products, while the multiple-pulsed case resulted in a more diffused kernel.more » « less
-
Abstract First‐principles plasma fluid modeling is used for investigation of electrical gas discharges ignited by a configuration of two approaching conducting hydrometeors with typical radii on the order of several millimeters under thunderstorm conditions (i.e., at an elevated location in the Earth's atmosphere corresponding to half of air density at ground level and at applied electric field approximately half of that required for avalanche multiplication of electrons in air). It is demonstrated that ultraviolet photons produced by the electrical discharges developing due to the electric field enhancement in the gap between two hydrometeors and resultant photoionization in the discharge volume lead to much less stringent conditions for conversion of these discharges to a filamentary streamer form than in the case not accounting for the effects of photoionization. It is also demonstrated that this photoionization feedback is critical for understanding and correct description of the subsequent streamer discharges developing on the outer periphery of two hydrometeors whose potential is equalized due to the electrical connection established by the initial streamer discharge between them. The initial streamer ignition between the hydrometeors can be preceded by the corona development, which can have detrimental effect on the ignition. However, it is demonstrated that for hydrometeors approaching with a speed of10 m/s the effect of this onset corona is small.more » « less
