Argon Bath Gas Research Articles

Experimental temperature- and pressure-dependent absorbance cross sections and a pseudo-line-list model for methyl formate near [formula omitted]

We first present quantitative, broadband absorbance cross sections of the C=O stretch fundamental rovibrational band of methyl formate in the 1670–1850 cm−1 range at temperatures of 300, 600, 800, and 1000 K and pressures of 1–2 atm. The title molecular spectra were additionally studied across the pressure range of 1 to 35 atm at room temperature. Elevated temperature measurements were taken in argon bath gas behind the reflected shock wave of a shock tube by a rapid-tuning, broad-scan external cavity quantum cascade laser. The room temperature cross section of methyl formate was collected to validate our method and agreed well with previous room-temperature measurements by Sharpe et al. Then, to extend the utility of the collected data across intermediate temperatures, a pseudo-line-list (PLL) absorbance model was generated through a simultaneous fit of line-by-line intensities and lower-state energies to the measured cross sections. This PLL model was able to replicate the measurements within 3% across most of the spectral range and within 8% around the extremely sharp Q-branch feature present at room temperature. Cross-validation was then performed by re-fitting the PLL excluding the 800 K data set and demonstrated that the PLL approach is effective in interpolating between measured absorption cross sections at discrete temperatures. Finally, the additional room-temperature measurements from 1 to 35 atm were collected in a high-pressure static cell in nitrogen bath gas. These measurements revealed a notable Q-branch cross section pressure dependence while the P- and R-branch wings remained largely pressure independent across this wide pressure range. The collected methyl formate cross sections comprise the latest addition to the Stanford ShockGas-IR database for mid-infrared polyatomic absorption cross sections at elevated temperatures.

High-temperature absorption cross-sections and interference-immune sensing method for formaldehyde near 3.6-µm

Absorption cross-sections of formaldehyde (CH2O) were measured at elevated temperatures near 3.6 µm, a region of strong intensity due to the overlapping QQ5 branch of the v1 band and the PQ7 branch of the v2+v4 combination band. 1,3,5-trioxane was shock-heated in an argon bath gas to produce controlled mixtures of formaldehyde over a range of temperatures (900–1500 K) and pressures (0.4–4.8 atm). Spectrally-resolved absorption measurements of formaldehyde were obtained using a rapidly-tunable distributed-feedback interband cascade laser scanned at 40 kHz over 2778.1–2778.9 cm−1 behind reflected shock waves. The resulting absorption cross-sections were fit with functions reflecting the temperature and pressure dependence of the targeted features, enabling spectral reconstruction at any temperature and pressure within the range of the study. By exploiting the strong temperature and pressure sensitivity of the cross-sections, it is demonstrated that CH2O concentration, temperature, and pressure can be inferred from a measured absorbance spectrum. Cross-sections of common combustion species were also measured over the selected wavelength range to validate that the region is largely free of narrow-band interference. Further, the multi-parameter spectral fitting method is shown to correct for baseline interference and demonstrated in experiments involving transient gas properties.

Line mixing and broadening of carbon dioxide by argon in the v3 bandhead near 4.2 µm at high temperatures and high pressures

Temperature-dependent line mixing and line broadening parameters were empirically-determined for rovibrational transitions (J = 99–145) in the (0000 → 0001) and (0110 → 0111) bandheads of carbon dioxide near 4.2 µm. Collisional effects by argon on the high rotational energy lines (E″ = 3920–8090 cm−1) in the R-branch were studied over a range of temperatures from 1200–3000 K in a shock tube. Measured absorption spectra comprising the target lines in an argon bath gas at near-atmospheric pressures were fit with Voigt profiles to determine line-broadening coefficients, with temperature dependence accounted by a power law. With line broadening established, line-mixing effects were examined at elevated pressures up to 58 atm and similar temperatures, reflecting conditions in high-pressure combustion environments. A modified exponential gap model for line mixing was developed to capture the pressure and temperature dependence of collisional transfer rates for the bandhead region using the relaxation matrix formalism.

Kinetic study of the OH + ethylene reaction using frequency‐modulated laser absorption spectroscopy

AbstractThe pressure dependence of the OH + C2H4 addition reaction has been investigated using frequency‐modulated laser absorption spectroscopy to monitor OH kinetics. Bimolecular rate coefficients for the title reaction are reported in argon bath gas at room temperature and total pressures ranging from 2 to 361 Torr. The pressure‐dependent rate coefficients measured here agree well with the majority of published kinetic studies under similar conditions. Previous high‐level ab initio calculations have identified a prereaction complex on the OH + C2H4 potential energy surface. The influence of this complex on the OH + C2H4 kinetics has been investigated using one‐dimensional master equation analyses of the current and previous experimental measurements.

High-sensitivity 308.6-nm laser absorption diagnostic optimized for OH measurement in shock tube combustion studies

We report the development of a high-sensitivity laser absorption diagnostic optimized for measurement of the hydroxyl radical (OH) at temperatures relevant to combustion studies, and demonstrated here in shock tube experiments. This diagnostic utilizes a narrow-linewidth CW UV laser that is tunable over the A2Σ – X2Π (0,0) band of the OH rovibronic transitions. First, we identified the strongest absorption transition of OH, over the current temperature range of interest, to be the Q1(5) transition near 308.61 nm. We then measured the OH absorption coefficients behind reflected shock waves over temperatures of 1656–2993 K and pressures of 0.88–4.09 atm, and determined the pressure-broadening and pressure-shifting coefficients in argon bath gas. Compared to the previous diagnostic targeting the OH R1(5) transition, the current diagnostic has approximately 2.2 times the sensitivity. Finally, we demonstrated the excellent sensitivity of the current OH diagnostic in a set of highly-diluted C3H8 oxidation experiments in a shock tube, where a 1 − σ detection limit of less than 0.15 ppm OH was successfully achieved.

Theoretical Study of the Ti–Cl Bond Cleavage Reaction in TiCl4

Abstract In this work the kinetics of the TiCl4 ⇌ TiCl3 + Cl reaction is studied theoretically. A variable-reaction coordinate transition-state theory (VRC-TST) is used to calculate the high-pressure limit rate coefficients. The interaction energy surface for the VRC-TST step is sampled directly at the CASPT2(6e,4o)/cc-pVDZ level of theory including an approximate treatment of the spin-orbit coupling. The pressure-dependence of the reaction in an argon bath gas is explored using the master equation in conjunction with the optimised VRC-TST transition-state number of states. The collisional energy transfer parameters for the TiCl4–Ar system are estimated via a “one-dimensional minimisation” method and classical trajectories. The Ti–Cl bond dissociation energy is computed using a complete basis set extrapolation technique with cc-pVQZ and cc-pV5Z basis sets. Good quantitative agreement between the estimated rate constants and available literature data is observed. However, the fall-off behaviour of the model results is not seen in the current experimental data. Sensitivity analysis shows that the fall-off effect is insensitive to the choice of model parameters and methods. More experimental work and development of higher-level theoretical methods are needed to further investigate this discrepancy.

Experimental and modeling study of styrene oxidation in spherical reactor and shock tube

The oxidation of styrene, one of the main stable intermediates from the oxidation of large alkylated aromatic hydrocarbons, has been investigated in the present work both experimentally and numerically. Experiments were performed using two complementary techniques, spherical bomb for laminar flame speed studies and shock tube for ignition delay time measurements. In particular, the laminar flame speeds of styrene/air mixtures were measured at three different initial temperatures (342 K, 373 K, and 405 K), over a wide range of equivalence ratios (0.75–1.45), for an initial pressure of 100 kPa. In addition, the autoignition of styrene/O2 mixtures in argon bath gas (ϕ = 0.5, 1.0, and 1.5) was investigated over a wide range of temperatures (1390–1990 K), at highly diluted conditions (94.3%–99% argon), and for pressures between 110 and 200 kPa. A detailed chemical kinetic model, based on the toluene chemistry by Metcalfe et al. (2011), was developed and validated against the newly obtained experimental results and the flow reactor data available in the literature (Litzinger et al., 1986). Sensitivity and rate of production analyses were performed and showed that, at the conditions studied herein for the flame speed investigation, the main fuel consumption pathways include the reaction of the fuel with H atoms to form phenyl radical and ethylene or benzene and vinyl radical, with O to form benzyl radical + HCO, and the H-abstraction reactions on both the vinyl moiety and the ring. On the other hand, the analyses performed at the high-temperature, highly-diluted conditions encountered in the shock tube study highlighted the importance of the fuel thermal decomposition steps for the simulation of the ignition delay time measurements. The model was also tested against the low-pressure flame and jet stirred reactor data by Yuan et al. (2015). The results highlight the need for future modifications in the benzene chemistry by Metcalfe et al. and inclusion of pressure dependent rate parameters in order to improve the prediction capabilities of the model.

Measurements of Propanal Ignition Delay Times and Species Time Histories Using Shock Tube and Laser Absorption

ABSTRACTPropanal is an aldehyde intermediate formed during the hydrocarbon combustion process. Potentially, the use of oxygenated biofuels reduces greenhouse gas emissions; however, it also results in increased toxic aldehyde by‐products, mainly formaldehyde, acetaldehyde, acrolein, and propanal. These aldehydes are carcinogenic, and therefore it is important to understand their formation and destruction pathways in combustion systems. In this work, ignition delay times were measured behind reflected shock waves for stoichiometric (Φ = 1) mixtures of propanal (CH3CH2CHO) and oxygen (O2) in argon bath gas at temperatures of 1129 K < T < 1696 K and pressures around 1 and 6 atm. Measurements were conducted using the kinetics shock tube facility at the University of Central Florida. Current results were compared to available data in the literature as well as to the predictions of three propanal combustion kinetic models: Politecnico di Milano (POLIMI), National University of Ireland at Galway, and McGill mechanisms. In addition, a continuous wave‐distributed feedback interband cascade laser centered at 3403.4 nm was used for measuring methane (CH4) and propanal time histories behind the reflected shock waves during propanal pyrolysis. Concentration time histories were obtained at temperatures between 1192 and 1388 K near 1 atm. Sensitivity analysis was carried for both ignition delay time and pyrolysis measurements to reveal the important reactions that were crucial to predicting the current experimental results. Adjustments to the POLIMI mechanism were adopted to better match the experimental data. Further research was suggested for the H abstraction reaction rates of propanal. In addition to extending the temperature and pressure region of literature ignition delay times, we provide the first high‐temperature species concentration time histories during propanal pyrolysis.

Experimental investigation of the reaction of HCFC-22 and methane in a dielectric barrier discharge non-equilibrium plasma

A dielectric barrier discharge non-equilibrium plasma was employed to study the reaction of HCFC-22 (CHClF2) with methane (in an argon bath gas) at atmospheric pressure and in the absence of oxygen and nitrogen. The reaction produced a fluorine-containing polymeric product, as well as gaseous species including H2, HF, HCl, CHF3, C2H3F, CH3Cl, CH2ClF, C2HClF4, CHCl2F, CH2Cl2 and CCl2F2. While the main polymeric fraction is non-crosslinked, a small portion of the solid product is crosslinked. The polymers contain a wide range of functional groups including CH3, CH2, CHCl, CHF, CHClF, CHF2, CF2 and CF3. The conversion level of CHClF2 increased from 53% to 78%, with an increment in input energy density from 3kJL−1 to 13kJL−1. A reaction mechanism is proposed explaining the formation of gaseous as well as polymeric products.

Experimental Study on the Reaction of CCl3F and CH4 in a Dielectric Barrier Discharge Nonequilibrium Plasma Reactor

The reaction of CCl3F (CFC-11) with CH4 (in an argon bath gas) in a dielectric barrier discharge nonequilibrium plasma was examined. Oxygen and nitrogen were excluded from the feed stream and the reactions resulted in the production of fluorine-containing polymers, as well as a range of gaseous products including H2, HCl, HF, C2H3F, C2H3Cl, C2H2ClF, CHCl2F, CCl2F2, CH3Cl, CH2Cl2, CHCl3, and C2Cl4. The polymeric material synthesized during reaction is characterized as being non-cross-linked and random in nature, containing functional groups including CH3, CH2, CHCl, CHF, CF2, and CF3. The conversion level of CCl3F increased from 37% to 63% as the input energy density increased from 3 to 13 kJ L–1 (the applied voltage range was 14.1 to 15.2 kV, peak–peak). The electrical discharge was characterized and found to be a slight modification of filamentary discharge toward a diffuse discharge due to the presence of the relatively low concentration of CCl3F and CH4 (less than 2% each) in argon. A reaction mechanis...

Shock tube ignition delay times and methane time-histories measurements during excess CO2 diluted oxy-methane combustion

The combustion of methane in air results in large amounts of CO2 and NOX emissions. In order to reduce the NOX emissions, one possible solution is the oxy-methane combustion with large CO2 dilution so that the combustion products can be reduced mainly to CO2 and H2O. However, there are very few studies on the chemical kinetics of oxy-methane combustion in a CO2 diluted environment. In this study, methane time-histories, CH* emission profiles, and pressure time-histories measurements were conducted behind reflected shock waves to gain insight into the effects of CO2 dilution of the gas mixtures on the ignition of methane. The measurements were carried out for mixtures of CH4, CO2 and O2 in argon bath gas at temperatures of 1577–2144 K, pressures of 0.53–4.4 atm, equivalence ratios (Φ) of 0.5, 1, and 2, and CO2 mole fractions (XCO2) of 0, 30, and 60%. The laser absorption measurements were conducted using a continuous wave distributed feedback interband cascade laser (DFB ICL) centered at 3403.4 nm. The results showed the decrease of activation energy and the increase of ignition delay time as the amount of CO2 dilution was increased. However, the changes were minor and within the experimental uncertainties of the measurements. Also, the results were compared to the predictions of two different natural gas mechanisms: GRI 3.0 and AramcoMech 1.3 mechanisms. In general the predictions were reasonable when compared to the experimental data; however, there were discrepancies at some conditions. Three different influences of CO2 addition to the argon bath gas in regards to chemistry, collision efficiencies, and heat capacities were examined. In addition, the present study included experimentally obtained correlations for absorption cross sections of methane for its P(8) line in the v3 band in argon bath gas with and without carbon-dioxide dilutions at temperatures between 1200 < T < 2000 K and pressures between 0.7 < P < 1.2 atm.

Effect of methane on the conversion of HFC-134a in a dielectric barrier discharge non-equilibrium plasma reactor

The effect of methane on the conversion of HFC-134a (CF3CH2F) in a dielectric barrier discharge non-equilibrium plasma reactor was examined. Reactions were conducted in an argon bath gas and in the absence of oxygen and nitrogen. The products of the reaction include H2, HF, CHF3, CH2F2, C2H6, C3H8, C2H3F, CHF2CHF2, C2H4F2, C3H7F as well as a polymeric solid deposit. The polymeric materials are predominantly fluorine containing random copolymers, which can be categorised as fluoropolymers, constituted from various functional groups including CF3, CF2, CHF, CHF2, CH2F, CH2 and CH3. While the presence of methane in the feed stream reduces the conversion level of CF3CH2F, it also modifies the polymer architecture. The addition of 1.25% methane with 12.5% CF3CH2F in an argon bath gas at 100cm3min−1 feed rate reduces the conversion of CF3CH2F from 74.5% to 46.8% and increases the formation of HF from 1500μmolh−1 to 2640μmolh−1. The effect of methane addition on the electrical discharge and the reaction pathways are discussed.

The Reaction Kinetics of Amino Radicals with Sulfur Dioxide

Abstract Application of the laser photolysis–laser-induced fluorescence method to the reaction NH2 + SO2 in argon bath gas yields pressure-dependent, third-order kinetics which may be summarized as k = (1.49 ± 0.15) × 10−31 (T/298 K)−0.83 cm6 molecule –2 s –1 over 292–555 K, where the uncertainty is the 95% confidence interval and includes possible systematic errors. The quenching of vibrationally excited NH2 is consistent with a high-pressure limit for NH2 + SO2 of (1.62 ± 0.25) × 10−11 cm3 molecule –1 s –1 over the temperature range 295–505 K, where again the 95% confidence interval is shown. Ab initio analysis yields a H2N–SO2 dissociation enthalpy of 73.5 kJ mol –1, and comparison with RRKM theory and the exponential down model for energy transfer yields 〈ΔE〉down = 350 cm –1 for Ar at room temperature.

Conversion of natural gas into the gaseous constituents and nano-graphene in the presence of chlorine as homogeneous promoter by DC-spark discharge

This paper describes the results of an experimental study on the usage of high-frequency, high-voltage, low direct current spark discharge plasma to examine the catalytic role of chlorine (Cl2) gas on the selective and rapid conversion of light hydrocarbons in the natural gas stream. Our results indicate that the cold DC-spark discharge method in the presence of small amount of chlorine gas is a useful method for decomposition of methane into gaseous constituents and carbon at relatively low power consumption (10 W). Presence of Cl2 boosts the conversion rate of methane (the main constituent of natural gas) to C2 hydrocarbons, as well as formation of nano-graphene, and apparently, molecular hydrogen. Our results indicate that conversion of methane increases by a factor of ~3 in the presence of 6–8 % of chlorine gas. This method is adopted as an efficient and selective routine for the formation of nano-carbons such as graphene using natural gas along with a trace amount of Cl2 in an inert atmosphere such as argon bath gas.

Characterization of Polymer Synthesized from the Nonequilibrium Plasma Conversion of CFC-12 and Methane in a Dielectric Barrier Discharge Reactor

A dielectric barrier discharge (DBD) nonequilibrium plasma reactor was employed to polymerize CFC-12 (CCl2F2, dichlorodifluoromethane) at atmospheric pressure. The plasma polymerization of this saturated halogenated hydrocarbon was conducted in the presence of methane as reactant, in an argon bath gas and where the reaction environment was free from oxygen and nitrogen. The reaction resulted in the formation of non-cross-linked polymer product and whereby the non-cross-linked nature of the polymer enabled its characterization by solution state 13C and 19F nuclear magnetic resonance (NMR) spectroscopic analysis. The generated polymer was also analyzed by Fourier transform infrared (FTIR) spectrometry, and the spectra thus obtained were consistent with the analysis by NMR. The analyses of NMR and FTIR spectroscopy reveal the formation of fluoropolymers from the conversion of CFC-12.

Nonequilibrium Plasma Polymerization of HFC-134a in a Dielectric Barrier Discharge Reactor: Polymer Characterization and a Proposed Mechanism for Polymer Formation

Nonequilibrium plasma polymerization of hydrofluorocarbon HFC-134a (CF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> CH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> F) in argon bath gas has been studied in a dielectric barrier discharge reactor at atmospheric pressure and in the absence of oxygen and nitrogen. The reaction resulted in the formation of a polymeric solid fraction and the noncrosslinked properties of this material assisted in its characterization by solution state <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> C and <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> F nuclear magnetic resonance spectroscopy. Gel permeation chromatography revealed that the polymers include low (number average molecular weight, Mn values between 900 and 3000 g mol <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ) and high (Mn approximately 60000 g mol <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ) molecular weight fractions. A detailed polymerization mechanism is proposed, based on the published literature and the findings of the current investigation.

Autoignition of n-Decane/n-Butylbenzene/n-Propylcyclohexane Mixtures and the Effects of the Exhaust Gas Recirculation

In the present work, the autoignition of single-component and binary mixtures composed of n-decane, n-butylbenzene, and/or n-propylcyclohexane has been investigated using shock-tube techniques. In addition, the effects of the presence of the exhaust gas recirculation (EGR) components on the autoignition behavior of a 4:3:3 molar n-decane: n-butylbenzene:n-propylcyclohexane surrogate mixture have been investigated experimentally. The experiments have been performed at highly diluted conditions in argon bath gas, over a wide range of temperatures (1250–1750 K), equivalence ratios (Φ = 0.2–1.5), and nominal pressures of 10–20 bar. A chemical kinetic model was developed to simulate the newly obtained experimental data by blending sub-models from the literature. In particular, the experimental and numerical analyses suggest that the observed increase in the ignition delay times in the presence of EGR is mainly due to the dilution levels and not to the chemistry of the fuel. Additional kinetic analyses were performed to compare the high-temperature autoignition properties of the different components considered herein.

Kinetic studies of the reaction NH2 + H2S

Amidogen (NH2) radicals were generated by pulsed laser photolysis of ammonia precursor at 193nm, and monitored by laser-induced fluorescence excited at 570.3nm. Experiments were conducted in argon bath gas with traces of hydrogen sulfide (H2S) at pressures of 9–25mbar and temperatures of 294–820K. The NH2+H2S rate constant was found to be (4.30±0.25)×10−13cm3molecule−1s−1 independent of temperature. The rate constant for removal of vibrationally-excited NH2 (0, 1, 0) by H2S was estimated as (1.5±0.2)×10−11cm3molecule−1s−1, which includes both physical quenching and chemical reaction.

Comparative Study on Cyclohexane and Decalin Oxidation

Fuel surrogates are mixtures of a few single aliphatic and aromatic compounds representative of the main classes of hydrocarbons present in the corresponding fuels. Among these hydrocarbon classes, the cycloalkanes constitute one of the key components in several surrogate formulations. In the present investigation, the oxidation of cyclohexane and decalin has been studied based on new experimental results. The experiments were obtained using two different techniques. A heated shock tube was used to conduct ignition delay time measurements at different stoichiometric conditions, nominal pressure of around 10 bar, and temperatures between 1090 and 1860 K. Ignition delay times in the range between 10 and 1300 μs were measured from both the OH* and the CH* emission signals behind reflected shock waves for highly diluted mixtures (99% argon bath gas). Experiments were also conducted varying the dilution from 99.5% to 92.7% for specific stoichiometric conditions. In addition to the ignition studies, flame speed...

Study on the Reaction of CCl2F2with CH4in a Dielectric Barrier Discharge Nonequilibrium Plasma

The reaction of CCl2F2 (CFC-12) with methane (in an argon bath gas) in a dielectric barrier discharge (DBD) non-thermal plasma reactor at atmospheric pressure has been investigated. The reaction (where oxygen and nitrogen are excluded from the feed gas) produced a range of gas phase products, including H2, HCl, HF, CHF3, CF3CH2F, CHClF2, CH3Cl, CH2ClF, CH2Cl2, CHCl2F and CHCl3 as well as solid products which were primarily non-crosslinked polymers. The conversion of CCl2F2 increased, from 44 to 71%, when the input energy density of the plasma was raised from 3 to 13 kJ · L−1 (applied voltage range is 13.4–14.8 kV, peak-peak). The relatively low input feed concentration of CCl2F2 and methane in the argon bath gas modifies the strong filamentary appearance of the pure argon discharge to a more diffuse appearance.