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This study investigates the plasma properties and chemical kinetics of plasma-assisted methane reforming in a He diluted nanosecond-pulsed plane-to-plane dielectric barrier discharge (ns-DBD) through the combination of time-resolved in situ laser diagnostics and a 1-D numerical model. Plasma-assisted fuel reforming kinetic mechanisms have predominantly been evaluated on the basis of matching reactant conversion and syngas production to steady-state measurements, which cannot describe the full range of chemistry and physics necessary to validate the model. It was found that adding 1% CH4 to a pure He ns-DBD led to a faster breakdown along the rising edge of the applied voltage pulse, thereby lowering the reduced electric field (E/N), electron number density, and electron temperature. Further addition of CH4 did not continue to alter the E/N in the model. Laser absorption spectroscopy was used to measure gas temperature, C2H2, H2O, and CH2O in a CH4/CO2/He discharge to serve as validation targets for the predicted reaction pathways. CH2O was predicted within 25% of the measured value, while H2O and C2H2 were under-predicted by a factor of two and three, respectively. From path flux analysis, the major pathway for CH2O formation was through the reaction between CH3 and O, while C2H2 formation had multi-step pathways that relied on ions like CH 3 + and C2H 5 + . The path flux analysis also shows that CH2 is a significant intermediate for production of both CH2O and C2H2, and increased CH2 concentration could improve model predictions. The results show that the use of reaction rate constants with lower uncertainties and inclusion of He 2 + are needed to improve the predictions. Finally, varying the ”equivalence ratio”, defined by the CH4 dry reforming reaction to H2 and CO, from 0.5 to 2 was shown to have a weak effect on measured product species and experimental trends were explained based on pathways extracted from the model.

The present study investigates the kinetics of low-temperature pyrolysis and oxidation of n-dodecane/O2/N2 mixtures in a repetitively-pulsed nanosecond discharge experimentally and numerically. Time-resolved TDLAS measurements, steady-state gas chromatography (GC) sampling, and mid-IR dual-modulation Faraday rotation spectroscopy (DM-FRS) measurements are conducted to quantify temperature as well as species formation and evolution. A plasma-assisted n-dodecane pyrolysis and oxidation kinetic model incorporating the reactions involving electronically excited species and NOx chemistry is developed and validated. The results show that a nanosecond discharge can dramatically accelerate n-dodecane pyrolysis and oxidation at low temperatures. The numerical model has a good agreement with experimental data for the major intermediate species. From the pathway analysis, electronically excited N 2 * plays an important role in n-dodecane pyrolysis and oxidation. The results also show that with addition of n-dodecane, NO concentration is reduced considerably, which suggests that there is a strong NO kinetic effect on plasma-assisted low-temperature combustion via NO-RO2 and NO2-fuel radical reaction pathways. This work advances the understandings of the kinetics of plasma-assisted low-temperature fuel oxidation in N2/O2 mixtures.

Understanding the multi-channel dynamics of O(1D) reactions with unsaturated hydrocarbon molecules in low temperature reaction kinetics is critically important in stratospheric chemistry, plasma chemistry, plasma assisted fuel reforming, materials synthesis, and plasma assisted combustion. A photolysis flow reactor coupled with highly selective mid-infrared Faraday Rotation Spectroscopy (FRS) and direct ultraviolet-infrared (UV-IR) absorption spectroscopy (DAS) techniques was developed for the first time to study the multi-channel dynamics of excited singlet oxygen atom O(1D) reactions with C2H2 and the kinetics of subsequent reactions. Time-resolved species concentrations of OH, HO2 and H2O were obtained and used to develop a validated kinetic model of O(1D) reactions with C2H2. The branching ratios of O(1D) reaction with C2H2 and subsequent HO2 kinetics were also quantified. It is found that, contrary to O(1D) reactions with saturated alkanes, OH formation via direct H abstraction by O(1D) is negligible. The results revealed that two chain-branching and propagation reactions via direct O(1D) insertion are the major pathways for radical production. The present study clearly demonstrated the advantage of radical detection and kinetic studies using FRS in the effective suppression of absorption interference from non-paramagnetic hydrocarbons.

The present work reports on the effects of controlled non-equilibrium excitation of reactant molecules on low temperature H2/O2/He ignition by numerically modeling a hybrid repetitive nanosecond (NSD) and DC discharge at atmospheric pressure. At first, a detailed plasma-combustion kinetic model of H2/O2/He, including non-equilibrium excitation, is developed and validated by experimental data of a repetitively-pulsed nanosecond discharge. Then, the effects of ignition enhancement by NSD and a hybrid NSD/DC discharge, with controlled electron energy distribution for selective non-equilibrium excitation of vibrationally excited H2(v) and O2(v) as well as electronically excited O2(a1Δg) and O(1D), are compared. The results show that H2(v1) contributes significantly to the H production and OH consumption in the hybrid plasma discharge. Moreover, O2(a1Δg) and O2(v1−4) also contribute to the production O and OH. Uncertainty analysis of H2(v) and O2(a1Δg) elementary reactions on ignition delay time is conducted by using several different kinetic models. The comparison of ignition delay time using different plasma kinetic models indicates the selection of accurate rate constants involving excited species is important for plasma assisted ignition modeling. The results of hybrid discharge assisted H2/O2 ignition show that the optimized ignition enhancement is achieved when both excited species and radicals are produced efficiently at an appropriate DC electric field strength. The present modeling provides useful insight into the plasma-combustion model development and the development of controlled plasma discharge to achieve efficient ignition with optimized non-equilibrium excitation of reactants.

Recently acquired high-resolution images of nanosecond laser ablation plumes suggest a strong correlation between the internal plume structure and the type of material being ablated. However, the details of this relation are currently not well understood. In this work, we attempt to explore this correlation using a 2D radiation hydrodynamics model to study the dependence of internal plume structure formation on the ablation material. Spatio-temporal emission maps and plume expansion velocities from experimental measurements are compared with the model predictions, including synthetic emission maps. The shape and expansion rate of an outer air plume region are found to be in good agreement for both carbon and aluminum, as are the inner material plume dynamics for carbon ablation. The largest disagreement is observed in the case of a polished aluminum target, where the chaotic inner plume features seen in the experimental images are not observed in the model. The possible physical mechanisms responsible for this discrepancy are discussed. This effort constitutes a continued development toward a predictive model of ablation plume dynamics and chemistry for various materials in extreme environments.

The accelerating chemical effect of ozone addition on the oxidation chemistry of methyl hexanoate [CH3(CH2)4C(═O)OCH3] was investigated over a temperature range from 460 to 940 K. Using an externally heated jet-stirred reactor at p = 700 Torr (residence time τ = 1.3 s, stoichiometry φ = 0.5, 80% argon dilution), we explored the relevant chemical pathways by employing molecular-beam mass spectrometry with electron and single-photon ionization to trace the temperature dependencies of key intermediates, including many hydroperoxides. In the absence of ozone, reactivity is observed in the so-called low-temperature chemistry (LTC) regime between 550 and 700 K, which is governed by hydroperoxides formed from sequential O2 addition and isomerization reactions. At temperatures above 700 K, we observed the negative temperature coefficient (NTC) regime, in which the reactivity decreases with increasing temperatures, until near 800 K, where the reactivity increases again. Upon addition of ozone (1000 ppm), the overall reactivity of the system is dramatically changed due to the time scale of ozone decomposition in comparison to fuel oxidation time scales of the mixtures at different temperatures. While the LTC regime seems to be only slightly affected by the addition of ozone with respect to the identity and quantity of the observed intermediates, we observed an increased reactivity in the intermediate NTC temperature range. Furthermore, we observed experimental evidence for an additional oxidation regime in the range near 500 K, herein referred to as the extreme low-temperature chemistry (ELTC) regime. Experimental evidence and theoretical rate constant calculations indicate that this ELTC regime is likely to be initiated by H abstraction from methyl hexanoate via O atoms, which originate from thermal O3 decomposition. The theoretical calculations show that the rate constants for methyl ester initiation via abstraction by O atoms increase dramatically with the size of the methyl ester, suggesting that ELTC is likely not important for the smaller methyl esters. Experimental evidence is provided indicating that, similar to the LTC regime, the chemistry in the ELTC regime is dominated by hydroperoxide chemistry. However, mass spectra recorded at various reactor temperatures and at different photon energies provide experimental evidence of some differences in chemical species between the ELTC and the LTC temperature ranges.

The reaction network of the simplest Criegee intermediate (CI) CH2OO has been studied experimentally during the ozonolysis of ethylene. The results provide valuable information about plasma- and ozone-assisted combustion processes and atmospheric aerosol formation. A network of CI reactions was identified, which can be described best by the sequential addition of CI with ethylene, water, formic acid, and other molecules containing hydroxy, aldehyde, and hydroperoxy functional groups. Species resulting from as many as four sequential CI addition reactions were observed, and these species are highly oxygenated oligomers that are known components of secondary organic aerosols in the atmosphere. Insights into these reaction pathways were obtained from a near-atmospheric pressure jet-stirred reactor coupled to a high-resolution molecular-beam mass spectrometer. The mass spectrometer employs single-photon ionization with synchrotron-generated, tunable vacuum-ultraviolet radiation to minimize fragmentation via near-threshold ionization and to observe mass-selected photoionization efficiency (PIE) curves. Species identification is supported by comparison of the mass-selected, experimentally observed photo-ionization thresholds with theoretical calculations for the ionization energies. A variety of multi-functional peroxide species are identified, including hydroxymethyl hydroperoxide (HOCH2OOH), hydroperoxymethyl formate (HOOCH2OCHO), methoxymethyl hydroperoxide (CH3OCH2OOH), ethoxymethyl hydroperoxide (C2H5OCH2OOH), 2-hydroxyethyl hydroperoxide (HOC2H4OOH), dihydroperoxy methane (HOOCH2OOH), and 1-hydroperoxypropan-2-one [CH3C([double bond, length as m-dash]O)CH2OOH]. A semi-quantitative analysis of the signal intensities as a function of successive CI additions and temperature provides mechanistic insights and valuable information for future modeling work of the associated energy conversion processes and atmospheric chemistry. This work provides further evidence that the CI is a key intermediate in the formation of oligomeric species via the formation of hydroperoxides.

Ignition enhancement using a hybrid repetitive nanosecond and DC discharge is studied in CH4/O2/He mixtures at atmospheric pressure by using a hybrid ZDPlasKin-CHEMKIN method. Special attention is placed on the control of vibrational and electronic excitations of methane and oxygen using different electric field strengths. A plasma-ignition kinetic mechanism incorporating the reactions involving both vibrational and electronic excitations of CH4 and O2 as well as the low temperature methane oxidation pathways of O2(a1Δg) is developed and validated. The results show that the hybrid non-equilibrium plasma excitation is much more effective in ignition enhancement than thermal heating and the nanosecond discharge alone. O2(a1Δg) is generated more efficiently in the hybrid discharge and shortens the ignition delay time more effectively than O(1D) and O produced in the nanosecond discharge. Vibrationally excited species CH4(ν) and O2(ν) are also produced but mainly contribute to ignition enhancement via energy relaxation and gas heating. The results also show that e + O2 → e + O2(a1Δg) and e + O2 → e + O + O(1D) reactions compete with each other for oxygen consumption and play opposite roles in ignition enhancement in a hybrid NSD/DC discharge with a given plasma energy. For a given repetitive nanosecond discharge, there is an optimum DC electric field strength which has the minimum ignition delay time due to the selective production of excited species and the difference in electron density. This work provides fundamental understanding for the design of a hybrid discharge to optimize low temperature ignition enhancement and fuel reforming.

The present study explores the chemical kinetics of low temperature oxidation and pyrolysis of pentane/O2/He mixtures in a nanosecond repetitively-pulsed DBD discharge. Time-dependent TDLAS measurements and steady state GC sampling are conducted to quantify species evolution in a plasma discharge. An improved kinetic model of plasma assisted combustion and pyrolysis of pentane is developed with updated electron impact dissociation reactions and low temperature reactions involving excited species. This kinetic model is then validated against the experimental data. The results show that a nanosecond plasma discharge causes significant low temperature fuel pyrolysis and oxidization as well as fast gas heating. The original kinetic model fails to predict many intermediate species such as CH4, C2H2, and CH2O due to missing corresponding reaction pathways and inaccuracies in electron impact cross section areas. The new model shows a dramatic improvement in modeling both pentane pyrolysis and oxidation, with good prediction of the time histories of pentane, CH2O, C2H2 and H2O, and slight under-prediction of CH4. There is still a large discrepancy in OH prediction and measurement in fuel oxidation. The results show that direct electron impact dissociation pathways play a critical role in plasma assisted fuel pyrolysis and oxidation and that plasma generated radicals and excited species such as O and O(1D) enhance low temperature fuel oxidation.

The present study seeks to explore the parametric effects of oxygen concentration, argon dilution and plasma discharge frequency on pyrolytic and oxidative reaction pathways of n-heptane assisted by non-equilibrium plasma discharge. Low temperature reaction pathways of n-heptane/O 2 /Ar mixtures with a nanosecond repetitively pulsed plasma discharge are experimentally investigated in both in situ time-dependent TDLAS and steady state gas sampling diagnostics. Fuel consumption is found to be more effective in higher argon than higher oxygen concentrations, indicating higher electron number densities with argon dilution is more effective than direct electron impact dissociation of oxygen. Steady state sampling results suggest a linear trend of n-heptane dissociation and product species formation with increasing plasma frequency, with different major product species for oxidation and pyrolysis. In the time-dependent measurements, the comparison between experiments and numerical modeling show that formation of a major intermediate species, formaldehyde, is significantly under-predicted while fuel and water production are over-predicted. This discrepancy suggests missing reactions in the current model, possibly involving excited alkyl peroxide radicals.

Faraday rotation spectroscopy (FRS) employs the Faraday effect to detect Zeeman splitting in the presence of a magnetic field. In this article, we present system design and implementation of radical sensing in a photolysis reactor using FRS. High sensitivity (100 ppb) and time resolved in situ HO2 detection is enabled with a digitally balanced acquisition scheme. Specific advantages of employing FRS for sensing in such dynamic environments are examined and rigorously compared to the more established conventional laser absorption spectroscopy (LAS). Experimental results show that FRS enables HO2 detection when LAS is deficient, and FRS compares favorably in terms of precision when LAS is applicable. The immunity of FRS to spectral interferences such as absorption of hydrocarbons and other diamagnetic species absorption and optical fringing are highlighted in comparison to LAS.

An instability in a nanosecond pulsed dielectric barrier discharge plasma occurring in methane–oxygen–argon mixtures is experimentally observed and measured by 1D time-resolved in situ electric field measurements. This instability, which seems to be created by the positive feedback between plasma kinetics and plasma-assisted low temperature fuel oxidation, is studied using electric field induced second harmonic generation and direct ICCD imaging. The rapid formation of streamers from an originally uniform discharge appears to be caused by the chemical kinetics of plasma-assisted low temperature methane oxidation, which may be resulting in a new type of plasma instability: a thermal-chemical instability. The results also revealed that the occurrence of this possible thermal-chemical instability in a reactive flow drastically changes the plasma properties by forming multiple secondary discharges and possibly leads to micron-sized non-uniform electric distributions. Single shot uncalibrated measurements of the electric field of the micron sized streamers appears to show much greater strengths than the average electric field. Furthermore, one-dimensional data analysis shows the positive feedback loop between the streamers and the low temperature plasma assisted oxidation chemistry in the plasma thermal-chemical instability. The present finding advances the understanding plasma instability growth and provides a new way to control plasma uniformity in plasma-assisted combustion and plasma fuel reforming.

The kinetic acceleration of deflagration-to-detonation transition (DDT) of acetylene/oxygen mixtures at lean conditions in a 1 mm2 microchannel is investigated using ozone addition. Equivalence ra...

Non-equilibrium plasmas for plasma-assisted combustion, pollutant remediation, fuel reforming, and catalysis rely on the production of energetic electrons that ionize, dissociate, and excite the fuel and oxidizer molecules. Experimental characterization of the electron temperature, electron density, and vibrational temperature are necessary to validate and improve plasma kinetic models. An experimental apparatus capable of Thomson scattering and vibrational Raman scattering measurements in the same discharge with molecular admixtures was developed. Both diagnostics are necessary to study the induced vibrational non-equilibrium from electron impact. Thomson scattering spectra were resolved by placing a physical mask at the output of a single grating spectrometer. The electron temperature and density and the impact of hydrocarbon addition was measured for a 60 Torr CH4/He nanosecond pulsed plane-to-plane dielectric barrier discharge with 0%-2% CH4 addition. Electron densities as low as 1 x 10(12) cm(-3) and electron temperatures ranging from 0.5 eV to 9 eV were observed. A decrease in the electron temperature and density was observed even with 1% H-4 addition. Moreover, the addition of N-2 to the discharge enabled vibrational Raman scattering and quantification of the first level vibrational temperature starting from 75 ns after the voltage pulse. The electron temperature and density were also measured in this CH4/N-2/He mixture by Thomson scattering. Addition of N-2 led to a faster electron temperature decay than in the original CH4/He mixture. The advantages and disadvantages of this detection scheme for Thomson scattering over the triple grating spectrometer and the volume Bragg grating notch filter is discussed.

Ethylene oxidation initiated by ozone addition (ozonolysis) is carried out in a jet-stirred reactor from 300 to 1000 K to explore the kinetic pathways relevant to low-temperature oxidation. The temperature dependencies of species' mole fractions are quantified using molecular-beam mass spectrometry with electron ionization and single-photon ionization employing tunable synchrotron-generated vacuum-ultraviolet radiation. Upon ozone addition, significant ethylene oxidation is found in the low-temperature regime from 300 to 600 K. Here, we provide new insights into the ethylene ozonolysis reaction network via identification and quantification of previously elusive intermediates by combining experimental photoionization energy scans and ab initio threshold energy calculations for isomer identification. Specifically, the C2H4 + O3 adduct C2H4O3 is identified as a keto-hydroperoxide (hydroperoxy-acetaldehyde, HOOCH2CHO) based on the calculated and experimentally observed ionization energy of 9.80 (±0.05) eV. Quantification using a photoionization cross-section of 5 Mb at 10.5 eV results in 5 ppm at atmospheric conditions, which decreases monotonically with temperature until 550 K. Other hydroperoxide species that contribute in larger amounts to the low-temperature oxidation of C2H4, like H2O2, CH3OOH, and C2H5OOH, are identified and their temperature-dependent mole fractions are reported. The experimental evidence for additional oxygenated species such as methanol, ketene, acetaldehyde, and hydroxy-acetaldehyde suggest multiple active oxidation routes. This experimental investigation closes the gap between ozonolysis at atmospheric and elevated temperature conditions and provides a database for future modeling.

We report measurements of HO2 radicals using wavelength modulated Faraday Rotation Spectroscopy in a multi-pass photolysis reactor. The current setup enables line-locked measurements to improve sensitivity and time resolution compared to the line-scanning system.

We perform Faraday rotation spectroscopy at 7.2 μm to quantify HO 2 formation from the reaction between excited singlet O(1D) atoms and C 2 H 2 . A digitally balanced detection scheme is employed to suppress spectral interferences from diamagnetic C 2 H 2 .

A study of the low temperature chemistry of methane oxidation in a nanosecond repetitively pulsed (NRP) discharge is undertaken. Both laser absorption spectroscopy measurements of temperature and formaldehyde, as well as gas chromatograph sampling of major products are used to evaluate the kinetic mechanism of a stoichiometric methane/oxygen mixture with 75% helium dilution. In addition, a predictive tool for calculating electron collision reactions, excited and ionized species reactions, and combustion reactions has been built. A model for methane, oxygen, and helium plasma has been assembled for use with this new tool, and is incorporated with a low temperature combustion mechanism. Comparisons between the model and the measured species are in agreement for CH4 and O2 consumption, as well as production of H2O, CO, CO2, and H2. However, CH2O, CH3OH, C2H6, C2H4, and C2H2 are not accurately predicted. Path flux analysis reveals that methane oxidation proceeds through one of two intermediates: CH3 or CH2, which are created primarily by electron collision reactions and H-abstraction reactions by plasma-generated radicals. The major species resulting from CH2 oxidation are generally well predicted, while those resulting from methyl oxidation are poorly predicted. Thus, further investigation of low temperature (400-500 K) methyl reactions, particularly the consumption pathways of CH3O2, are needed to bring the model into agreement with the present measurements.

Speciation and temperature measurements of methane oxidation during a nanosecond repetitively pulsed discharge in a low-temperature flow reactor have been performed. Measurements of temperature and formaldehyde during a burst of pulses were made on a time-dependent basis using tunable diode laser absorption spectroscopy, and measurements of all other major stable species were made downstream of a continuously pulsed discharge using gas chromatography. The major species for a stoichiometric methane/oxygen/helium mixture with 75% dilution are H 2 O, CO, CO 2 , H 2 , CH 2 O, CH 3 OH, C 2 H 6 , C 2 H 4 and C 2 H 2 . A modelling tool to simulate homogeneous plasma combustion kinetics is assembled by combining the ZDPlasKin and CHEMKIN codes. In addition, a kinetic model for plasma-assisted combustion (HP-Mech/plasma) of methane, oxygen and helium mixtures has been assembled to simulate the measurements. Predictions can accurately capture reactant consumption as well as production of the major product species. However, significant disagreement is found for minor species, particularly CH 2 O and CH 3 OH. Further analysis revealed that the plasma-activated low-temperature oxidation pathways, particularly those involving CH 3 O 2 radical reactions and methane reactions with O( 1 D), are responsible for this disagreement.