More Like this

1. 1D Simulation of Avalanche to Streamer to Spark Transition of Plasma Discharge in Ammonia-Air Combustion

   https://doi.org/10.2514/6.2024-2607  

   Taneja, Taaresh Sanjeev; Yang, Suo; Sitaraman, Hariswaran (January 2024, American Institute of Aeronautics and Astronautics)

   The backward problem of plasma assisted combustion emphasizes evaluating the effect of the evolving thermochemical state on the plasma discharge. This paper investigates the dependence of avalanche to streamer to spark formation dynamics and kinetics on the gas composition and temperature at different points in an ammonia-air premixed laminar flame using a self-consistent multigrid-based 1D plasma solver. Different values of alpha, the coefficient for effective ionization events per unit length, have been reported for electron avalanches in air and stoichiometric NH3-air mixtures. The streamer inception has been shown to obey the Meek’s criterion. An exponential reduction in streamer and spark formation time has been observed from plasma simulations at different points in the unburnt, pre-heat zone, reaction zone and the fully burnt regions of the premixed flame. While the enhancement of the reduced electric field with increasing temperature affects effective ionization, there exists a minimum breakdown field for streamer formation, which does not vary proportionally with the changing number density of the gas. The change in the mixture from reactants (NH3, O2, N2) to products of complete combustion of ammonia in air (N2, H2O) has also been shown to affect the streamer and spark formation. Finally, the major pathways during the streamer and spark phases which are responsible for producing important radicals used in combustion of NH3 are also discussed. 

   more » « less
2. Plasma Assisted Emission Control of Hydrocarbon Gas Flares: A 0D Feasibility Study

   https://doi.org/10.2514/6.2023-2060  

   Johnson, Praise Noah; Taneja, Taaresh Sanjeev; Yang, Suo (January 2023, AIAA SCITECH 2023 Forum)

   Natural gas associated with oil wells and natural gas fields is a significant source of greenhouse gas emissions and airborne pollutants. Flaring of the associated gas removes greenhouse gases like methane and other hydrocarbons. The present study explores the possibility of enhancing the flaring of associated gas mixtures (C1 – C4 alkane mixture) using nanosecond pulsed non-equilibrium plasma discharges. Starting with a detailed chemistry for C0 – C4 hydrocarbons (Aramco mechanism 3.0 – 589 species), systematic reductions are performed to obtain a smaller reduced mechanism (156 species) yet retaining the relevant kinetics of C1 – C4 alkanes at atmospheric pressure and varying equivalence ratios (φ = 0.5 – 2.0). This conventional combustion chemistry for small alkanes is then coupled with the plasma kinetics of CH4, C2H6, C3H8, and N2, including electron-impact excitations, dissociations, and ionization reactions. The newly developed plasma-based flare gas chemistry is then utilized to investigate repetitively pulsed non-equilibrium plasma-assisted reforming and subsequent combustion of the flare gas mixture diluted with N2 at different conditions. The results indicate an enhanced production of hydrogen, ethylene and other species in the reformed gas mixture, owing to the electron-impact dissociation pathways and subsequent H-abstractions and recombination reactions, thereby resulting in a mixture of CH4, H2, C2H4, C2H2, and other unsaturated C3 species. The reformed mixture shows an enhanced reactivity as exhibited by their shorter ignition delays. The reformed mixture is also observed to undergo increased methane destruction and higher equilibrium temperatures compared to the original mixture as the gas temperature increases, thereby exhibiting a potential for reducing the unburnt emissions of methane and other hydrocarbons. 

   more » « less
3. Plasma-based global pathway analysis to understand the chemical kinetics of plasma-assisted combustion and fuel reforming

   https://doi.org/10.1016/j.combustflame.2023.112927  

   Johnson, Praise N.; Taneja, Taaresh S.; Yang, Suo (July 2023, Combustion and Flame)

   Pepiot, Perrine (Ed.)

   The Global Pathway Analysis (GPA) algorithm helps analyze the chemical kinetics of complex combustion systems by identifying important global reaction pathways connecting a source species to a sink species through various important intermediate species (i.e., hub species). The present work aims to extend GPA algorithm to plasma-assisted combustion and fuel reforming systems to identify the dominant global pathways in such systems at various conditions. In addition, the present study extends the ability of GPA algorithm to identify reaction cycles involving the excitation of high-concentration species (e.g., O2, N2, and fuel) to their vibrational and electronic states and the subsequent de-excitation to their ground state, based on their significance on the reactivity of plasma-assisted systems in terms of gas heating and radical production. Provisions are made in the GPA algorithm to evaluate the reactivity of identified reaction pathways and cycles based on the element-flux transfer (i.e., dominance), heat release, and radical production rate. The newly developed Plasma-based Global Pathway Analysis (PGPA) algorithm is then used to analyze the plasma-assisted combustion of ammonia and reforming of methane. The PGPA analyses elucidated the significance of vibrational-translational cycles on the reactivity of NH3/air mixtures. Further, analyses on the production of NO ascribed the early reforming of NH3 to N2 and H2 in impeding the production of NO during plasma-assisted NH3 ignition. Lastly, the enhanced reforming of CH4/N2 mixtures using plasma has been attributed to electron impact dissociation of CH4 when compared to thermal reforming. In contrast, conventional path-Flux analysis (PFA) was found to require significant manual effort and pre-analysis intuitions from expert knowledge, making it arduous to provide valuable insights into plasma chemistry. The user-friendly and automated nature of PGPA thus provides a valuable tool for assessing the kinetics of plasma-assisted systems helpful in analyzing and, further, a foundation in reducing plasma-assisted chemistry, without the needs of expert knowledge. 

   more » « less
4. Plasma Chemistry and Applications of Atmospheric Pressure Transient Electrical Discharges

   Dhali, Shirshak K (July 2025, Springer Series on Atomic, Optical, and Plasma Physics; Springer: Berlin/Heidelberg, Germany.)

   Guharay, S; Wada, M (Ed.)

   At or near atmospheric pressure, overvolted gas breakdown results in a streamer formation. In many applications of non-thermal plasma where efficient excited species generation is critical, the streamers are quenched to prevent it from reaching the arc phase. This can be achieved by repetitive nano second pulsing or dielectric barrier discharges were the dielectric charging quenches the arc formation. In such discharges, the plasma characteristics such as electron and ion densities and the production of excited species is determined by the streamer properties. Over the past five decades, a vast amount of experimental and computational work has been accumulated to establish a well-accepted theory of streamer formation and propagation. In this article we discuss the fluid models for streamers and quantify some macroscopic properties which can inform specific applications. We discuss in detail the fluid equations needed to model streamers and several schemes of parametrization of the transport and electron collisional processes. From an application point of view, the steamer simulations are used to quantify the excited species production by electron impact. This information is used to predict the specific outcomes via the plasma chemical conversion pathways. We present results of streamer discharges for three applications which are of technological importance to illustrate this approach: Plasma-assisted combustion, remediation of toxic gases, and plasma medicine. For plasma-assisted combustion the results of hydrogen ignition are discussed since non-hydrocarbon-based fuels such as hydrogen and ammonia are potential fuel candidates to reduce greenhouse gases. For the remediation of toxic gases, we discuss the removal of SOX/NOx from flue gas. Plasma medicine is a relatively new field and repetitive nano-second pulsed discharges in a helium gas carrier shows promise as a reactive plasma source for treating biological material. We discuss the helium metastable production in a streamer discharge since this species leads to the production of OH radicals which plays an important role. 

   more » « less
5. Simulating the Effect of Electric Bias Voltages on the Electrical Characteristics of Oxyfuel Preheat Flame Using Reduced Combustion Mechanism

   https://doi.org/10.1115/1.4062963  

   Rahman, S. M.; Warrier, Rohith; Untaroiu, Alexandrina; Martin, Christopher R. (November 2023, Journal of Fluids Engineering)

   Abstract A three-dimensional computational model is presented in this paper that illustrates the detailed electrical characteristics, and the current–voltage (i–v) relationship throughout the preheating process of premixed methane-oxygen oxyfuel cutting flame subject to electric bias voltages. As such, the equations describing combustion, electrochemical transport for charged species, and potential are solved through a commercially available finite volume computational fluid dynamics (CFD) code. The reactions of the methane-oxygen (CH4–O2) flame were combined with a reduced mechanism, and additional ionization reactions that generate three chemi-ions, H3O+, HCO+, and e−, to describe the chemistry of ions in flames. The electrical characteristics such as ion migrations and ion distributions are investigated for a range of electric potential, V ∈ [−5 V, +5 V]. Since the physical flame is comprised of twelve Bunsen-like conical flames, inclusion of the third dimension imparts the resolution of fluid mechanics and the interaction among the individual cones. It was concluded that charged “sheaths” are formed at both torch and workpiece surfaces, subsequently forming three distinct regimes in the i–v relationship. The i–v characteristics obtained from this study have been compared to the previous experimental and two-dimensional computational model for premixed flame. In this way, the overall model generates a better understanding of the physical behavior of the oxyfuel-cutting flames, along with more validated i–v characteristics. Such understanding might provide critical information toward achieving an autonomous oxyfuel-cutting process. 

   more » « less