Fast Gas Heating Research Articles

ChemPlasKin: A general-purpose program for unified gas and plasma kinetics simulations

This work introduces ChemPlasKin, a freely accessible solver optimized for zero-dimensional (0D) simulations of chemical kinetics of neutral gas in non-equilibrium plasma environments. By integrating the electron Boltzmann equation solver, CppBOLOS, with the open-source combustion library, Cantera, at the source code level, ChemPlasKin computes time-resolved evolution of species concentration and gas temperature in a unified gas–plasma kinetics framework. The model allows high fidelity predictions of both chemical thermal effects and plasma-induced heating, including fast gas heating and slower vibrational–translational relaxation processes. Additionally, a new heat loss model is developed for nanosecond pulsed discharges, specifically within pin–pin electrode configurations. With its versatility, ChemPlasKin is well-suited for a wide range of applications, from plasma-assisted combustion (PAC) to fuel reforming. In this paper, the reliability, accuracy and efficiency of ChemPlasKin are validated through a number of test problems, demonstrating its utility in advancing gas–plasma kinetic studies.

Quantifying the thermal effect and methyl radical production in nanosecond repetitively pulsed glow discharges applied to a methane-air flame

In this work, we investigated non-equilibrium plasma produced by nanosecond repetitively pulsed glow discharges applied across a lean premixed methane-air flame. The flame is stationary, axisymmetric, and laminar. The discharges are applied on the symmetry axis crossing the reactant gases, flame front, and product gases, allowing phase-locked averaged measurements and comparisons with axisymmetric numerical simulations. The thermal effect and methyl radical production are quantified in the discharge in the reactant gas region. One-dimensional, two-beam, hybrid, femtosecond-picosecond, coherent anti-Stokes Raman scattering is used to acquire spatial and temporal profiles of temperature and oxygen-to-nitrogen concentration ratio. Photo-fragmentation laser-induced fluorescence is used to acquire quantitative two-dimensional profiles of methyl radicals in the discharge providing the first quantitative imaging of methyl produced ahead of a flame by plasma-induced methane dissociation. The spatial profiles of temperature and oxygen-to-nitrogen concentration ratio are in steady state, indicating that individual discharges have an insignificant heating effect. Upper and lower bounds of the produced mole fraction of methyl radicals in the plasma are obtained due to uncertainties in the collisional quenching rates of excited state methylidyne radicals in the plasma. The discharges produce a maximum of 600–1100 ppm of methyl radicals upstream of the flame front within 25 ns. This amount is similar to the predicted methyl mole fraction for the flame without plasma and thus represents a significant chemical perturbation to the reactants upstream of the flame front. The produced methyl follows an exponential decay in the first microsecond after the discharge with a decay constant of 8 µs close to the flame, and 0.8 µs further from the flame. The decay then deviates from the exponential curve and the methyl persists for tens of microseconds. The results suggest that for the tested configuration, the thermal effect of individual discharges through fast gas heating is negligible, while active chemical species are produced in large quantities in the reactant gases, upstream of the flame front.

Pulse-to-pulse coupling in cylindrical discharges

Several filamentary discharges can be applied to a combustible mixture, which can then ignite. The energy density of this discharge is a vital parameter, as it directly influences the local temperature rise and radical production. The goal of this article is to investigate how a previous discharge affects the energy density of a second discharge. To investigate the pulse-to-pulse coupling of filamentary discharges a one-dimensional numerical model is developed. In the developed model, the compressible Navier–Stokes equations are coupled to a plasma model. The plasma model is used to estimate the local energy density, while the compressible Navier–Stokes equations model the reactive flow. As a first step, skeletal air plasma chemistry is used, which includes fast gas heating, slow gas heating and the rapid generation of radicals. The skeletal plasma chemistry is combined with a detailed hydrogen combustion mechanism. Simulations in both air and hydrogen/air are conducted at several discharge energies and pressures. From the analysis of these results, we conclude that the main mechanism of pulse-to-pulse coupling is the reduction in molar density due to temperature rise.

Disorder-induced heating as a mechanism for fast neutral gas heating in atmospheric pressure plasmas

Recent findings suggest that ions are strongly correlated in atmospheric pressure plasmas if the ionization fraction is sufficiently high ( ≳10−5 ). A consequence is that ionization causes disorder-induced heating (DIH), which triggers a significant rise in ion temperature on a picosecond timescale. This is followed by a rise in the neutral gas temperature on a longer timescale of up to nanoseconds due to ion–neutral temperature relaxation. The sequence of DIH and ion–neutral temperature relaxation suggests a new mechanism for ultrafast neutral gas heating. Previous work considered only the case of an instantaneous ionization pulse, whereas the ionization pulse extends over nanoseconds in many experiments. Here, molecular dynamics simulations are used to analyze the evolution of ion and neutral gas temperatures for a gradual ionization over several nanoseconds. The results are compared with published experimental results from a nanosecond pulsed discharge, showing good agreement with a measurement of fast neutral gas heating.

On a Possible Mechanism of Space Stem Formation in Negative Long Sparks

AbstractTo study the mechanism of space stem formation, the high‐speed direct imaging technique and Schlieren photography are used to simultaneously observe the space stems in a 1.35‐m air gap under the lightning impulse voltage. It is found that the average length and diameter of the space stem is approximately 5 and 0.5 mm, which is about 1/3 of the length and 1/10 of the diameter of the local luminous region respectively. Moreover, the space stem exhibits a non‐uniform distributed temperature with a maximum value of several hundred Kelvin for tens of microseconds. Accordingly, a possible mechanism is proposed that the space stem is formed at the primary corona streamer boundary as a result of the propagation of a secondary ionization wave from the HV electrode and survives long enough (to finish the polarization) due to the fast gas heating and production of atomic oxygen.

Mode transition (α–γ) and hysteresis in microwave-driven low-temperature plasmas

We discovered a hysteresis in a microwave-driven low-pressure argon plasma during gas pressure change across the transition region between α and γ discharge modes. The hysteresis is manifested in that the critical pressure of mode transition depends on the direction of pressure change. As a corollary, the plasma would attain different discharge properties under the same operating parameters (pressure, power, and gas composition), suggesting a bi-stability or existence of memory effect. Analysis of the rotational and vibrational temperatures measured from the OH (A–X) line emissions shows that the hysteresis is mainly due to the fast gas heating in the γ-mode leading to a smaller neutral density than that of the α-mode. When increasing the gas pressure, the γ-mode discharge maintains a relatively higher temperature and lower neutral density, and thus, it requires a higher operating pressure to reach the α-mode. On the other hand, decreasing the pressure while maintaining α-mode, the transition to γ-mode occurs at a lower pressure than the former case due to a relatively higher neutral density of α-mode discharge. This interpretation is supported by the fact that the hysteresis disappears when the plasma properties are presented with respect to the neutral gas density instead of pressure.

Numerical analysis of structural change process in millimeter-wave discharge at subcritical intensity

Plasma-front propagation processes of 170 GHz millimeter-wave discharge were investigated under subcritical incident electric field intensity by using a one-dimensional model. The discharge structure was numerically reproduced at more than 0.2 MV/m by introducing the detailed chemical reaction and radiation transport processes into the conventional model. The results revealed that the propagation mechanism of the plasma front in the millimeter-wave discharge changes depending on the incident electric field intensity. At intensities greater than 1.4 MV/m, the plasma front propagated at supersonic speed, while forming a discrete structure, which has intervals of 1/4 wavelength of the millimeter wave. This structure was generated by electron-impact ionization and photoionization processes. At the intermediate intensities, the plasma front propagated continuously rather than discretely because the gas expansion increased the reduced electric field and induced electron-impact ionization. The dominant heating process at the plasma front was fast gas heating. At intensities less than 0.3 MV/m, the plasma front propagated continuously, but the dominant heating process changed to vibrational–translational relaxation. The discharge was maintained by thermal ionization and associative ionization. The simulation results were in good agreement with the past millimeter discharge experiments at this intensity.

Effect of plasma-flow coupling on the ignition enhancement with non-equilibrium plasma in a sequential combustor

The effect of the regime of nanosecond repetitively pulsed discharges (NRPDs) on ignition and stabilization of a natural-gas/hydrogen/air flame in the sequential stage of a lab-scale atmospheric pressure sequential combustor is investigated experimentally. Electrical parameters of the NRPDs are characterized by measuring voltage, current, and deposited energy. Fast gas heating (FGH) of the nanosecond discharges is measured in a single pulse regime and validated by means of 0D kinetic modelling. It was found that the conventional scheme for energy release from internal degrees of freedom adequately describes the dynamics of FGH in vitiated hot environment diluted with air. Short-gated ICCD imaging and spatially-resolved emission spectroscopy are used to identify the coupling between the NRPDs and the vitiated hot flow. The effectiveness of the NRPDs actuation is assessed through the chemiluminescence images of the sequential flame. The distance of the center of gravity of the sequential flame to the outlet of the mixing channel is evaluated, with and without plasma actuation. The effect of fuel reactivity on plasma effectiveness is also studied by varying the fraction of hydrogen in the fuel blend of the second stage of the combustor. The results show that the glow NRPDs regime allows strengthening the flame anchoring for the most reactive blends considered in this work, while the spark NRPDs is required for the ignition and prevention of lean blow-out of the flame for the least reactive fuel blends which exhibit low fractions of hydrogen.

Numerical study of plasma assisted combustion in a sequential combustor

The effects of Nanosecond Repetitively Pulsed Discharges (NRPDs) on the second stage flame of a sequential combustor is investigated using numerical tools supported by experiments. In this work, Large Eddy Simulations (LESs) with an accurate description of the combustion chemistry and a simplified model of the plasma kinetics are performed and successfully reproduce experimental observations of the combustion enhancement obtained with NRPDs. Detailed plasma kinetics calculations and experimental data demonstrate that the simplified plasma modelling retrieves the main energetic routes for the dissipation of the electron energy during the discharges. This model initially developed for discharges in pure air, is still valid for the highly air-diluted mixture considered in this study. The results of the LESs are analyzed to explain the underlying physics. Chemical Explosive Mode Analysis (CEMA) is used to identify the combustion modes of the ignition kernels, which result from the fast gas heating and the radical production by the NRPDs. These ignition kernels quickly give rise to propagating flames, advected through the sequential burner. Moreover, it is shown that the oxygen atoms produced by the Non-Equilibrium Plasma (NEP) dramatically accelerate the oxidation of the fuel.

Sensitive and single-shot OH and temperature measurements by femtosecond cavity-enhanced absorption spectroscopy.

In many low-temperature plasmas (LTPs), the OH radical and temperature represent key properties of plasma reactivity. However, OH and temperature measurements in weakly ionized LTPs are challenging, due to the low concentration and short lifetime of OH and the abrupt temperature rise caused by fast gas heating. To address such issues, this Letter combined cavity-enhanced absorption spectroscopy (CEAS) with femtosecond (fs) pulses to enable sensitive single-shot broadband measurements of OH and temperature with a time resolution of ∼180 ns in LTPs. Such a combination leveraged several benefits. With the appropriately designed cavity, an absorption gain of ∼66 was achieved, enhancing the actual OH detection limit by ∼55× to the 1011 cm-3 level (sub-ppm in this work) compared with single-pass absorption. Single-shot measurements were enabled while maintaining a time resolution of ∼180 ns, sufficiently short for detecting OH with a lifetime of ∼100 μs. With the broadband fs laser, ∼34,000 cavity modes were matched with ∼95 modes matched on each CCD pixel bandwidth, such that fs-CEAS became immune to the laser-cavity coupling noise and highly robust across the entire spectral range. Also, the broadband fs laser allowed simultaneous sensing of many absorption features to enable simultaneous multi-parameter measurements with enhanced accuracies.

Fast gas heating and kinetics of electronically excited states in a nanosecond capillary discharge in CO2

Fast gas heating in a pulsed nanosecond capillary discharge in pure CO2 under the conditions of high specific deposited energy (around 1.2 eV/molecule) and high reduced electric fields (150–250 Td) has been studied experimentally and numerically. Specific deposited energy, reduced electric field and gas temperature have been measured as functions of time. The radial distribution of the electron density has been analyzed experimentally. The role of quenching of O(1D), O(1S) and CO(a3Π) excited atoms and molecules leading to heat release at sub-microsecond time scale have been analyzed by numerical modeling in the framework of 1D axial approximation.

Plasma assisted combustion of methane-air mixtures: Validation and reduction

For several years now plasma assisted combustion has been the subject of intense research due to stabilization effects a plasma can have on flames. Particularly, experiments have shown the promising impact of Nanosecond Repetitively Pulsed discharges on combustion while not exceeding an energy consumption of a few percent of the flame power. In this work, an incremental methodology with a step-by-step approach has been used to build a single plasma mechanism upon which combustion is added using the GRI 3.0 and Konnov v0.6. The methodology focuses on three key aspects of plasma assisted combustion: fast gas heating, slow gas heating and radical production. Selected experiments focusing on one or more of these aspects allow to validate the mechanism in large ranges of temperature (300-1500 K) and pressure (0.1-1 bar) in air, methane-air and argon diluted mixtures using glow and spark discharges. These experiments include a plasma assisted ignition case on which the ignition delay time is well captured by the mechanism. Slow gas heating has been modeled using a vibrational relaxation model validated against a detailed vibrational description. Discussions on ambiguous rates for critical reactions of excited nitrogen quenching are made in the light of their impact on the results on the chosen experiments. Finally, the resulting 100-species GRI 3.0-based and 264-species Konnov v0.6-based plasma mechanisms are reduced to make them suitable for multi-dimensional simulations. The DRGEP reduction method, based on plasma experiments and canonical combustion cases, is applied allowing to reduce the number of species by a factor larger than two. For the GRI-3.0 plasma mechanism, the reduced mechanism contains 47 species and 429 reactions. Hence significant performance is gained, opening the way to multi-dimensional simulations of plasma assisted combustion.

A Computational Study of the Thermodynamic Conditions Leading to Autoignition in Nanosecond Pulsed Discharges

Abstract Nanosecond pulsed discharges have attracted the attention of engine manufacturers due to the possibility of attaining distributed ignition sites that accelerate burn rates while resulting in very little electrode erosion. Multidimensional modeling tools currently capture the electrical structure of such discharges accurately, but resolving the chemical structure remains a challenging problem owing to the disparity of time-scales in streamer propagation (nanoseconds) and ignition phenomena (microseconds). The purpose of this study is to extend multidimensional results toward resolving the chemical structure in the wake of streamers (or the afterglow) by using a batch reactor model (BRM). This can afford the use of very detailed chemical kinetic information. The full nonequilibrium nature of the electrons is taken into account, along with fast gas heating, shock wave propagation, and thermal diffusion. The results shed light on ignition phenomena brought about by such discharges.

Numerical investigations of nanosecond surface streamers at elevated pressure

Single-pulse nanosecond surface dielectric barrier discharges operated in synthetic air and pure nitrogen at elevated pressure (6 bar) have been numerically studied by the classical fluid method. The aim of this work is to provide the necessary basis for analyzing the surface streamer-to-filament transition phenomenon. The electrical parameters, discharge morphology and propagation dynamics, as well as the possible influence of photoionization, kinetics and gas heating on the surface streamer stage at elevated pressure are discussed in a combined numerical-analytical way. A good agreement between measurement, numerical simulation and analytical estimation is achieved. The streamer thickness is derived to be inversely proportional to the pressure, the average reduced electric field in the streamer channel ranges from 75–150 Td, and the average electron density in the channel is ∼1022 m−3 for both polarities. The characteristic time of electron decay in the positive streamer channel is only on the order of ∼1 ns due to high charge exchange and electron-ion recombination rates. Photoionization provides an ionization source of 1027–1028 m−3 s−1 in front of the streamer head for both gases. Stepwise ionization/dissociation significantly affects the discharge dynamics of negative polarity SDBD, while for positive polarity this influence is negligible. The fast gas heating could lead to ±0.15 cm change of the positive streamer length. The secondary surface ionization wave is numerically observed, providing a time duration 0.25–0.5 ns of high electric field (230 Td), making it possible for streamer-to-filament transition kinetics to take place.

Modeling of fast ionization waves in pure nitrogen at moderate pressure

The fast ionization wave (FIW) discharges in pure nitrogen in a capillary tube at 27 mbar, initiated by positive polarity, high-voltage nanosecond pulses, are numerically studied by coupled two-dimensional plasma fluid modeling. The 2D fluid code based on local mean energy approximation is validated and used, an extended three-exponential Helmholtz photoionization model is proposed for pure nitrogen. The development and structure of the nitrogen FIW is analyzed, the electric field and current are calculated compared with experimental measurements. The evolution of radial distribution of electrons and N2(C 3Π u ) during the FIW development stage and in the afterglow are analyzed, the radial profiles of electron density show a ‘hollow’ structure in the FIW development stage, the temporal-spatial evolution of N2(C 3Π u ) is dominated by the competition between the pooling reaction of and the quenching by electrons. The role of photoionization on the nitrogen FIW radial morphology is discussed, the equivalent background electron density of photoionization in nitrogen discharge is suggested to be (4–6) × 1013 m−3. Spatial distribution of specific energy deposition (SED), fast gas heating (FGH) energy and temperature rise are obtained, heating efficiency varies with electric field E/N and SED and tends to be 10% at high SED. The dominating reactions responsible for FGH and their fractional contribution in space and time are analyzed, in the near axis region, pooling reactions of and N2(B 3Π g ) contribute up to 80% FGH energy, electron impact dissociation of molecular nitrogen contributes about 10%, while e- recombination and quenching of N(2 D) atoms by N2 molecules contribute rest.

Thermal instability and volume contraction in a pulsed microwave N2 plasma at sub-atmospheric pressure

We studied the evolution of an isolated pulsed plasma in a vortex flow stabilised microwave (MW) discharge in N2 at 25 mbar via the combination of 0D kinetics modelling, iCCD imaging and laser scattering diagnostics. Quenching of electronically excited N2 results in fast gas heating and the onset of a thermal-ionisation instability, contracting the discharge volume. The onset of a thermal-ionisation instability driven by vibrational excitation pathways is found to facilitate significantly higher N2 conversion (i.e. dissociation to atomic N2) compared to pre-instability conditions, emphasizing the potential utility of this dynamic in future fixation applications. The instability onset is found to be instigated by super-elastic heating of the electron energy distribution tail via vibrationally excited N2. Radial contraction of the discharge to the skin depth is found to occur post instability, while the axial elongation is found to be temporarily contracted during the thermal instability onset. An increase in power reflection during the thermal instability onset eventually limits the destabilising effects of exothermic electronically excited N2 quenching. Translational and vibrational temperature reach a quasi-non-equilibrium after the discharge contraction, with translational temperatures reaching ∼1200 K at the pulse end, while vibrational temperatures are found in near equilibrium with the electron energy (1 eV, or ∼11 600 K). This first description of the importance of electronically excited N2 quenching in thermal instabilities gives an additional fundamental understanding of N2 plasma behaviour in pulsed MW context, and thereby brings the eventual implementation of this novel N2 fixation method one step closer.

Flow Separation Control of Nacelle in Crosswind by Microsecond Pulsed Surface Dielectric Barrier Discharge Plasma Actuator

Flow separation under crosswind conditions seriously jeopardizes the quality of the nacelle’s flow field. In this paper, microsecond pulsed surface dielectric barrier discharge (μSDBD) is used to suppress the flow separation and reduce the crosswind distortion of the nacelle. The flow structure induced by the μSDBD is first explored by a high-speed schlieren system. The pressure waves composed of a cylindrical wave surrounding the electrodes and a flat wave at the top of the cylindrical one can be perceived, which indicates the fast gas heating produced by the μSDBD. A set of wind tunnel tests are then conducted to verify the ability of μSDBD to suppress the nacelle flow separation and study the influence laws of pulse frequency, coverage area, and the actuator layout on the flow control effects. Results show that plasma actuation can not only improve the total pressure at the exit of the nacelle but also suppress the flow distortion caused by the crosswind. The best flow control effect can be achieved at the pulse frequency of 500 Hz, with the value of sectional distortion coefficient reduced by 57.76% compared with the baseline condition. The flow control effect with the plasma actuator covering 120° of the nacelle perimeter is better than that of 60° and 180° coverage, showing the highest flow control efficiency in the 120° condition. The μSDBD can improve mixing between the boundary layer and the main flow, enhancing the ability of the boundary layer to resist the adverse pressure gradient, which is beneficial to flow separation control.

Numerical Investigation on the Effects of Chemical Reactions on the Discharge Characteristics and Energy Balance of a Nanosecond Repetitive Pulsed Dielectric Barrier Discharge

Numerical investigation on a nanosecond repetitively pulsed dielectric barrier discharge (NS-DBD) in air is a temporal and spatial multi-scale problem involving a large number of species and chemical reactions. To know the effects of the species and chemical reactions on the discharge characteristics and energy balance, a high voltage repetitive plane to plane NS-DBD is numerically studied. Four groups of species and the corresponding chemical reactions are adopted in the investigation. The most complex one has 31 species and 99 chemical reactions that contains all reaction types, in particular, the vibrational-translational relaxation reactions, whereas the simplest one has only 4 species and 4 reactions, which represents the main kinetic processes. The others are in between. The discharge energy reaches to a periodic phase equality state after the second pulse in the repetitive pulses, and the present analysis is focused on the 7th pulse. All the N 2 / O 2 mixture reaction models predict almost the same discharge energies, which are qualitatively similar with that in the simplified 4-species model. The prediction of the discharge energy is determined by the electronic excitation and the energy gain by ions, but the vibrational excitation, negative ions, associative ionization, dissociation of nitrogen and oxygen molecules have very weak effects. The gas heating is determined by the exothermic reactions and the ions. The main processes in the fast and slow gas heating are the energy release of ions and the exothermic reactions, respectively. The negative ions, vibrational excitation, and associative ionization have very weak effects on the gas heating during the high voltage pulse, but they have considerable effects at a larger time scale. The magnitudes of the fast gas heating efficiency ( η G H ) are in the range of 41%∼47% in the N 2 / O 2 mixture reduced kinetic models, but η G H is higher in the 4-reaction model.

Flow control on a high-lift wing with microsecond pulsed surface dielectric barrier discharge actuator

In this study, microsecond pulsed surface dielectric barrier discharge (mSDBD) is utilized to improve the aerodynamic characteristics of a high-lift wing. First, the characteristics of the mSDBD were tested qualitatively by a Schlieren system, revealing that the thermal perturbation induced by rapid heating is the main flow feature of the mSDBD at low working frequency. Subsequently, wind tunnel experiments were conducted at a free-stream speed of 45 m/s (Re=1.34×106) to investigate the effects of discharge frequency on the aerodynamic performance of the wing. Results indicate that there is an optimal excitation frequency or frequency band, at which the flow instability can be effectively excited. In this optimal situation, the relative improvement of the maximum lift coefficient reaches 9.1% and the stall angle is delayed by 4°. Besides, the flow control mechanisms with mSDBD actuation were analyzed by Large-eddy simulation (LES). The large-scale vortex induced by fast gas heating creates two negative pressure zones near the wing root and tip, both of which are crucial for lift augmentation. Moreover, when plasma actuation is applied to the case of small flap deflection, comparable or even better flow control effects are obtained, in reference to the case of large flap deflection but without plasma actuation. Based on these experimental and simulation results, a novel concept of plasma-actuator-enhanced high-lift wing was proposed.

Numerical Investigation on the Effects of Dielectric Barrier on a Nanosecond Pulsed Surface Dielectric Barrier Discharge.

In order to understand the impacts of dielectric barrier on the discharge characteristics of a nanosecond pulsed surface dielectric barrier discharge (NS-DBD), the effects of dielectric constant and dielectric barrier thickness are numerically investigated by using a three-equation drift–diffusion model with a 4-species 4-reaction air chemistry. When the dielectric constant increases, while the dielectric barrier thickness is fixed, the streamer propagation speed (V), the maximum streamer length (L), the discharge energy (), and the gas heating () of a pulse increase, but the plasma sheath thickness (h), the fast gas heating efficiency , and the charge densities on the wall surface decrease. When the dielectric barrier thickness increases, while the dielectric constant is fixed, V, L, , and of a pulse decrease, but h, , and the charge densities on the wall surface increase. It can be concluded that the increase of the dielectric constant or the decrease of the dielectric barrier thickness results in the increase of the capacitance of the dielectric barrier, which enhances the discharge intensity. Increasing the dielectric constant and thinning the dielectric barrier layer improve the performance of the NS-DBD actuators.