A coupled 2D computational model of nanosecond pulsed plasma induced flame ignition and combustion for a lean H 2 -air mixture (dry air) in a high pressure environment is discussed. The model provides a full fidelity description of plasma formation, combustion ignition, and flame development. We study the effects of three important plasma properties that influence combustion ignition and flame propagation, namely (a) plasma gas temperature, (b) plasma produced primary combustion radicals O, OH and H densities, and (c) plasma generated charged and electronically excited radical densities. Preliminary 0D studies indicate that plasma generated trace quantities of O, OH and H radicals drastically reduce the ignition delay of the H 2 -air mixture and become especially important for high pressure lean conditions. Multi-dimensional simulations are performed for a lean H 2 -air mixture (ϕ = 0.3) at 3.3 atm and an initial temperature of 1000 K. The plasma is accompanied by fast gas heating due to N 2 metastable quenching that results in uniform volumetric heating in the interelectrode gap. The spatial extent of the high temperature region generated by the plasma is a key parameter in influencing ignition; a larger high temperature region being more effective at initiating combustion ignition. Plasma generation of even trace quantities (~0.1%) of primary combustion radicals, along with plasma gas heating, results in a further fifteen-fold reduction in the ignition delay. The radical densities alone did not ignite the H 2 -air mixture. The generation of other plasma specific species results only in a slight ~10% improvement in the ignition delay characteristics over the effect of primary combustion radicals, with the slow decaying ions (H + 2 , O − 2 , O − ) and oxygen metastable species (O a1 2 , O b1 2 , O * 2 ) primarily contributing to combustion enhancement. These species influence the ignition delay, directly by power deposition due to quenching, attachment and recombination reactions, and indirectly by enhancing production of primary combustion radicals.
Recent experiments by Loebner et al. [IEEE Trans. Plasma Sci. 44, 1534 (2016)] studied the effect of a hypervelocity jet emanating from a coaxial plasma accelerator incident on target surfaces in an effort to mimic the transient loading created during edge localized mode disruption events in fusion plasmas. In this paper, we present a magnetohydrodynamic (MHD) numerical model to simulate plasma jet formation and plasma-surface contact in this coaxial plasma accelerator experiment. The MHD system of equations is spatially discretized using a cell-centered finite volume formulation. The temporal discretization is performed using a fully implicit backward Euler scheme and the resultant stiff system of nonlinear equations is solved using the Newton method. The numerical model is employed to obtain some key insights into the physical processes responsible for the generation of extreme stagnation conditions on the target surfaces. Simulations of the plume (without the target plate) are performed to isolate and study phenomena such as the magnetic pinch effect that is responsible for launching pressure pulses into the jet free stream. The simulations also yield insights into the incipient conditions responsible for producing the pinch, such as the formation of conductive channels. The jet-target impact studies indicate the existence of two distinct stages involved in the plasma-surface interaction. A fast transient stage characterized by a thin normal shock transitions into a pseudo-steady stage that exhibits an extended oblique shock structure. A quadratic scaling of the pinch and stagnation conditions with the total current discharged between the electrodes is in qualitative agreement with the results obtained in the experiments. This also illustrates the dominant contribution of the magnetic pressure term in determining the magnitude of the quantities of interest.
We present a magnetohydrodynamic (MHD) numerical simulation to study the physical mechanisms underlying plasma acceleration in a coaxial plasma gun. Coaxial plasma accelerators are known to exhibit two distinct modes of operation depending on the delay between gas loading and capacitor discharging. Shorter delays lead to a high velocity plasma deflagration jet and longer delays produce detonation shocks. During a single operational cycle that typically consists of two discharge events, the plasma acceleration exhibits a behavior characterized by a mode transition from deflagration to detonation. The first of the discharge events, a deflagration that occurs when the discharge expands into an initially evacuated domain, requires a modification of the standard MHD algorithm to account for rarefied regions of the simulation domain. The conventional approach of using a low background density gas to mimic the vacuum background results in the formation of an artificial shock, inconsistent with the physics of free expansion. To this end, we present a plasma-vacuum interface tracking framework with the objective of predicting a physically consistent free expansion, devoid of the spurious shock obtained with the low background density approach. The interface tracking formulation is integrated within the MHD framework to simulate the plasma deflagration and the second discharge event, a plasma detonation, formed due to its initiation in a background prefilled with gas remnant from the deflagration. The mode transition behavior obtained in the simulations is qualitatively compared to that observed in the experiments using high framing rate Schlieren videography. The deflagration mode is further investigated to understand the jet formation process and the axial velocities obtained are compared against experimentally obtained deflagration plasma front velocities. The simulations are also used to provide insight into the conditions responsible for the generation and sustenance of the magnetic pinch. The pinch width and number density distribution are compared to experimentally obtained data to calibrate the inlet boundary conditions used to set up the plasma acceleration problem.
Coaxial plasma accelerators are electromagnetic acceleration devices that employ a self-induced Lorentz force to produce collimated plasma jets with velocities ~50 km s −1 . The accelerator operation is characterized by the formation of an ionization/thermalization zone near gas inlet of the device that continually processes the incoming neutral gas into a highly ionized thermal plasma. In this paper, we present a 1D non-equilibrium plasma model to resolve the plasma formation and the electron-heavy species thermalization phenomena that take place in the thermalization zone. The non-equilibrium model is based on a self-consistent multi-species continuum description of the plasma with finite-rate chemistry. The thermalization zone is modelled by tracking a 1D gas-bit as it convects down the device with an initial gas pressure of 1 atm. The thermalization process occurs in two stages. The first is a plasma production stage, associated with a rapid increase in the charged species number densities facilitated by cathode surface electron emission and volumetric production processes. The production stage results in the formation of a two-temperature plasma with electron energies of ~2.5 eV in a low temperature background gas of ~300 K. The second, a temperature equilibration stage, is characterized by the energy transfer between the electrons and heavy species. The characteristic length scale for thermalization is found to be comparable to axial length of the accelerator thus putting into question the equilibrium magnetohydrodynamics assumption used in modeling coaxial accelerators.
We report on transient generation of highly ionized (ionization degree ~10%) argon microplasma using a self-consistent fluid plasma model coupled with the compressible Navier-Stokes equations. The plasma is generated within a micrometer size cathode spot immediately after the onset of intense secondary electron emission from the cathode and exists over a relatively short duration of ~10 ns. We observe the electron pressure within this microplasma exceeding the background gas pressure by a few times and discuss the mechanisms of the energy transfer from this plasma to the heavy species. The localized gas heating generates a compression wave that propagates from the cathode to the anode.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.