An official website of the United States government
Plasma-assisted combustion of ammonia leverages non-equilibrium electrical discharges to modify flame dynamics and emissions. In this work, we perform a numerical investigation using a one-dimensional model to examine the influence of nanosecond-repetitively pulsed discharges on the propagation of stoichiometric ammonia-air flames at atmospheric conditions. The model incorporates detailed plasma chemistry solved with ZDPlasKin. In particular, we look into the influence of pulse repetition frequency on laminar flame speed and NOx emissions. The simulations reveal unexpected behavior in the spatial distribution of plasma energy deposition within the flame. The plasma is found to significantly speed up the flame, up to +140%, although some numerical challenges prevented us from exploring operation at higher frequencies. The chemical kinetics used also predict a small decrease in 𝑁𝑂 in the product region of the flame.
more »
« less
Laminar flame speed modification by nanosecond repetitively pulsed discharges, Part II: Experiments
This paper systematically evaluates how, and to what extent, nanosecond repetitively pulsed (NRP) discharges modify the laminar flame speed of methane–air mixtures at ambient conditions, using both experiments in a narrow-channel quartz burner and a one-dimensional plasma-combustion model described in an accompanying paper (Part I). By varying the discharge location relative to the flame, four actuation strategies were explored at variable pulse repetition frequency: (i) discharges far ahead of the flame, (ii) pre-treatment of fresh reactants, (iii) direct (insitu) plasma–flame overlap, and (iv) a combination of pre-treatment and insitu interaction. Results show that acoustic waves produced by upstream discharges can reduce flame speed by as much as 30%–40%; while partially overlapping the discharge with the flame significantly accelerates it, with measured enhancements of up to 50% in both model and experiment. Flame speed modification by plasma increased with pulse repetition frequency, so that the envelope of performance enhancement reported here is limited by the highest frequencies tested (8kHz). The model captures these trends by attributing the detrimental effects to pressure-wave disturbances and the beneficial effects to radical-seeding and mild heat addition in, and close to, the reaction zone. These observations may help shed light on previously reported experiments and are here presented in a unified manner by focusing on a fundamental combustion metric (laminar flame speed), to give generality to the results obtained in laminar flames. The results demonstrate how spatio-temporal positioning of the discharge governs whether plasma aids or hinders the flame, ultimately guiding the design of optimal plasma-assisted combustion strategies.
more »
« less
- Award ID(s):
- 2339518
- PAR ID:
- 10661569
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Combustion and Flame
- Volume:
- 282
- Issue:
- C
- ISSN:
- 0010-2180
- Page Range / eLocation ID:
- 114475
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Plasma-assisted combustion (PAC) offers significant potential to enhance combustion processes by modifying thermal, kinetic, and transport properties. Despite progress in the field, challenges remain in reconciling disparate experimental results and understanding the mechanisms of plasma-flame interaction. This work develops a numerical modeling framework to systematically evaluate the impact of Nanosecond Repetitively Pulsed Discharges (NRPDs) on PAC systems. The focus of this contribution is modeling laminar premixed flames; and the main metric to assess the impact of plasma on flame is the laminar flame speed. The model is exercised on a stoichiometric methane/air flame. A combined 0D plasma-combustion model, PlasmaChem, is presented, enabling accurate energy tracking and coupling of detailed plasma and combustion mechanisms. The model is extended to 1D to incorporate compressible fluid dynamics, capturing the interaction between plasma and flame propagation. The results reveal distinct phases of plasma-flame interaction, demonstrating both beneficial effects, such as increased laminar flame speed due to radical production, and adverse effects, including flame deceleration from pressure disturbances. The model is compared to experiments in an accompanying paper, Part II of this work.more » « less
-
Recent efforts to reduce emissions of NOx, N2O, and NH3-slip in ammonia-methane (NH3/CH4) combustion using Nanosecond Repetitively Pulsed Discharges (NRPD) have relied largely on complex plasma-assisted combustion experiments in three-dimensional, turbulent, swirl-stabilized burners. In this work, we introduce a modular experimental platform based on a flat-flame McKenna burner to isolate and study the fundamental plasma-flame interactions that govern NRPD-assisted combustion of NH3/CH4/air mixtures. The burner-plasma assembly supports both nanosecond-spark and uniform-discharge operating modes when generating the plasma in the hot exhaust gases. Preliminary NO measurements, with NRPD applied either in the post-flame region or directly within the reaction zone, indicate a net NO penalty under the conditions tested. These results highlight the need for further mechanistic investigation of NRPD-driven chemistry in ammonia-methane flames and its implications for emissions reduction strategies.more » « less
-
Abstract The next generation of advanced combustion devices is being developed to operate under ultra-high-pressure conditions. However, under such extreme conditions, flame tends to become unstable and measurement of fundamental properties such as the laminar flame speed becomes challenging. One potential method to resolve this issue is measuring the ignition-affected region during spherically expanding flame experiments. The flame in this region is more resistant to perturbations and remains smooth due to the high stretch rates (i.e. small radii). Stable flame propagation allows for improved flame measurement, however, the experimentally observed kernel propagation is a function of both inflammation and ignition plasma. Therefore, the goal of the present study is to better understand the plasma formation and propagation during the ignition process, which would allow for reliable laminar flame speed measurements. To accomplish this goal, thermal plasma operating at high pressures is studied with emphasis on the spark energy effects on the formation of the ignition kernel. The thermal effect of the plasma is experimentally observed using a high-speed Schlieren imaging system. The energy dissipated within the plasma is measured with the use of voltage and current probes with a measurement of plasma sheath voltage drop as an input to numerical modeling. The measured kernel propagation rate is used to assess the accuracy of the model. The experiments and modeling are conducted in dry air at 1, 3, and 5 atm as well as in CH 4 -N 2 mixtures at 1 atm, and kernel radius, temperature, and mass are reported. The voltage-drop (as a non-thermal loss) is measured to be approximately 330 ± 5 V (dry air at 1 atm) for glow plasma with a large dependency on pressure, gas composition, electrode surface quality, electrode geometry, electrode shape, and current density. The same loss within the arc plasma is measured to be 15 ± 5 V, however the arc phase loss which agrees with arc propagation is significantly higher (∼45 V) which suggest additional unaccounted for phenomena occurring during the arc phase. With these losses, the modeling results are shown to predict the final kernel radius within 10%–20% of the observed kernel size. The difference found between the modeling and experimental results is determined to be a result of assuming that the primary loss mechanism (voltage drop across sheath formation) remains constant for the duration of glow discharge. The discrepancy for arc discharge is discussed with several potential sources, however, additional studies are required to better understand how the arc formation affects the kernel propagation.more » « less
-
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
- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1016/j.combustflame.2025.114475
