CH3 Radicals Research Articles

Implication of Sensitive Reactions to Ignition of Methyl Pentanoate: H-Abstraction Reactions by H and CH3 Radicals

Implication of Sensitive Reactions to Ignition of Methyl Pentanoate: H-Abstraction Reactions by H and CH3 Radicals

Combustion chemistry of n-heptane/ethanol blends: a ReaxFF study

ABSTRACT The effect of ethanol on n-heptane oxidation was studied using the reactive molecular dynamics (RMD) method. With different ethanol concentrations, simulations were performed under fuel-rich conditions from 2000 −2500 K. The results showed that ethanol addition had little effect on the n-heptane oxidation rate. However, the presence of ethanol inhibited the formation of major species (e.g. Ethylene and Formaldehyde) in different ways. The amount of C2H4 during n-heptane oxidation was clearly greater than that during n-heptane/ethanol oxidation, indicating that the dissociation of ethanol (C2H5OH →C2H4 + H2O) was not an effective pathway to produce C2H4. Conversely, ethanol addition had a two-fold impact on CH2O formation at high temperatures. Ethanol promoted the formation of the C2H5O radical, which was the main intermediate to produce CH2O. However, ethanol addition reduced the amount of n-heptane in the system, lowered the rate of CH3 radical formation, and subsequently decreased CH2O formation, consistent with previous experiments.

A new instrument for kinetics and branching ratio studies of gas phase collisional processes at very low temperatures.

A new instrument dedicated to the kinetic study of low-temperature gas phase neutral-neutral reactions, including clustering processes, is presented. It combines a supersonic flow reactor with vacuum ultra-violet synchrotron photoionization time-of-flight mass spectrometry. A photoion-photoelectron coincidence detection scheme has been adopted to optimize the particle counting efficiency. The characteristics of the instrument are detailed along with its capabilities illustrated through a few results obtained at low temperatures (<100 K) including a photoionization spectrum of n-butane, the detection of formic acid dimer formation, and the observation of diacetylene molecules formed by the reaction between the C2H radical and C2H2.

Photodissociation of the methyl radical: the role of nonadiabatic couplings in enhancing the variety of dissociation mechanisms.

The nonadiabatic photodissociation dynamics of the CH3 (and CD3) radical from the 3pz and 3s Rydberg states is investigated by applying a one-dimensional (1D) wave packet model that uses recently calculated ab initio 1D electronic potential-energy curves and nonadiabatic couplings. Calculated predissociation lifetimes are found to be too long as compared to the experimental ones. The 1D dynamical model, however, is able to predict qualitatively and explain the fragmentation mechanisms that produce the hydrogen-fragment translational energy distributions (TED) measured experimentally for the ground vibrational resonance of the 3pz and 3s Rydberg states (CH3(v = 0, 3pz) and CH3(v = 0, 3s)). The CH3(v = 0, 3pz) TED found experimentally displays a rather large energy spreading, while the experimental CH3(v = 0, 3s) TED is remarkably more localized in energy. The present model also predicts a widely spread CH3(v = 0, 3pz) TED, produced by a complex dissociation mechanism which involves predissociation to one dissociative valence state through a nonadiabatic coupling, as well as transfer of population to a second valence state through three conical intersections. Also in agreement with experiment, the model predicts a rather localized CH3(v = 0, 3s) TED because the conical intersections no longer operate in this photodissociation process, and predissociation occurs only into a single valence state. Another complex dissociation mechanism is predicted by the model for initial CH3(v > 0, 3s) and CD3(v > 0, 3s) resonances. In this case the mechanism is gradually activated, as vibrational excitation increases, by the interplay between the two nonadiabatic couplings connecting the 3s and 3px,y Rydberg states with the dissociative 2A1 valence state, and produces complex TEDs with signals from several resonances of both 3s and 3px,y. Thus the present 1D quantum model reveals a rich photodissociation dynamics of methyl, where a variety of complex fragmentation mechanisms is favored by the presence of different nonadiabatic couplings between the electronic states involved.

Theoretical analysis of the mechanisms of reactions of unsaturated C&lt;sub&gt;4&lt;/sub&gt;H&lt;sub&gt;6 &lt;/sub&gt;hydrocarbons with a highly reactive CH radical

Theoretical analysis of the mechanisms of reactions of unsaturated C&lt;sub&gt;4&lt;/sub&gt;H&lt;sub&gt;6 &lt;/sub&gt;hydrocarbons with a highly reactive CH radical

Spectroscopic characterization and photochemistry of the vinylsulfinyl radical.

The simplest α,β-unsaturated sulfinyl radical CH2[double bond, length as m-dash]C(H)SO˙ has been generated in the gas phase by high-vacuum flash pyrolysis (HVFP) of sulfoxide CH2[double bond, length as m-dash]C(H)S(O)CF3 at ca. 800 °C. Two planar cis and trans conformers of CH2[double bond, length as m-dash]C(H)SO˙ were isolated in cryogenic matrixes (N2, Ne, and Ar) and characterized with IR and UV/Vis spectroscopy. In addition to the photo-induced cis ⇋ trans conformational interconversion, CH2[double bond, length as m-dash]C(H)SO˙ displays complex photochemistry. Upon irradiation with a purple light LED (400 nm), CH2[double bond, length as m-dash]C(H)SO˙ isomerizes to novel radicals CH3SCO˙, ˙CH2SC(O)H, and ˙CH2C(O)SH with concomitant dissociation to a caged molecular complex CH3S˙CO. Subsequent UV-laser (266 nm) irradiation causes fragmentation to ˙CH3/OCS and additional formation of an elusive carbonyl radical CH3C(O)S˙, which rearranges to ˙CH2C(O)SH upon further UV-light irradiation (365 nm). The vibrational data and bonding analysis of the two conformers of CH2[double bond, length as m-dash]C(H)SO˙ suggest that both are floppy radicals in which the unpaired electron conjugates with the vicinal π(C[double bond, length as m-dash]C) bond, leading to significant contribution of the canonical resonance form of ˙CH2-C(H)SO. The mechanism for the isomerization of CH2[double bond, length as m-dash]C(H)SO˙ is discussed based on the observed intermediates along with a computed potential energy profile at the CCSD(T)-F12a/aug-cc-pVTZ//B3LYP/6-311++G(3df,3pd) level of theory.

Atmospheric oxidation chemistry of hexafluoroisobutylene initiated by OH radical: Kinetics and mechanism

Kinetics and mechanism for the gas-phase reaction of (CF3)2C = CH2 and OH radicals in the absence and presence of O2/NO have been investigated by G3(MP2)//B3PW91/6-311++G(d,p) method. The results showed the OH radical favored to attack the CC bond in (CF3)2C = CH2 molecule, and formed stable intermediates IM4[(CF3)2C(OH)CH2)] and IM9[(CF3)2CC(OH)H2)]. Other accessible products in the absence of O2/NO were (CF3)2C = CH and H2O via the H-extraction reaction. In the atmosphere, IM4 and IM9 could further react with O2/NO to form the likely main products (CF3)2CO, HCHO, (CF3)2C = CH, HO2 and (CF3)2C(OH)COH, in which (CF3)2CO and HCHO had been observed by experiment. The sole oxygen atom in (CF3)2CO or (CF3)2COCH2 was supplied by O2 in atmosphere rather than OH radical. The rate constants for the feasible channels were in good agreement with the experimental datum. At 298 K, the atmospheric lifetime of ~50 days for (CF3)2C = CH2 demonstrates that (CF3)2C = CH2 has atmospherically persistent.

Self-oscillations in solid methane irradiated by electrons

The formation of self-oscillations of temperature and concentration of radicals in an electron-irradiated methane film at low temperatures has been investigated experimentally and theoretically. Self-oscillations arise due to the activation nature of diffusion and radical recombination processes. Self-oscillations were studied experimentally by measuring the desorption of particles from an irradiated sample and theoretically by solving the kinetic equations for defects in a methane sample. Concentration self-oscillations of two types of particles have been found and investigated; namely, hydrogen atoms and CH3 radicals formed during the irradiation of methane by electrons. It is shown that with an increase in the irradiation intensity, the oscillation periods decrease, and the calculation value are of the order of magnitude observed in the experiment. A model of a manifestation of the self-oscillation of hydrogen molecule concentration during desorption is presented.

CH and NO planar laser-induced fluorescence and Rayleigh-scattering in turbulent flames using a multimode optical parametric oscillator.

An optical parametric oscillator (OPO) is developed and characterized for the simultaneous generation of ultraviolet (UV) and near-UV nanosecond laser pulses for the single-shot Rayleigh scattering and planar laser-induced-fluorescence (PLIF) imaging of methylidyne (CH) and nitric oxide (NO) in turbulent flames. The OPO is pumped by a multichannel, 8-pulse Nd:YAG laser cluster that produces up to 225 mJ/pulse at 355 nm with pulse spacing of 100 µs. The pulsed OPO has a conversion efficiency of 9.6% to the signal wavelength of ∼430nm when pumped by the multimode laser. Second harmonic conversion of the signal, with 3.8% efficiency, is used for the electronic excitation of the A-X (1,0) band of NO at ∼215nm, while the residual signal at 430 nm is used for direct excitation of the A-X (0,0) band of the CH radical and elastic Rayleigh scattering. The section of the OPO signal wavelength for simultaneous CH and NO PLIF imaging is performed with consideration of the pulse energy, interference from the reactant and product species, and the fluorescence signal intensity. The excitation wavelengths of 430.7 nm and 215.35 nm are studied in a laminar, premixed CH4-H2-NH3-air flame. Single-shot CH and NO PLIF and Rayleigh scatter imaging is demonstrated in a turbulent CH4-H2-NH3 diffusion flame using a high-speed intensified CMOS camera. Analysis of the complementary Rayleigh scattering and CH and NO PLIF enables identification and quantification of the high-temperature flame layers, the combustion product zones, and the fuel-jet core. Considerations for extension to simultaneous, 10-kHz-rate acquisition are discussed.

STRUCTURE OF IONS AND THE ENERGY OF THEIR FORMATION IN SATURATED ALUMINUM TRIS- DIPIVALOYLMETHANATE VAPOR UPON ELECTRON IONIZATION

The DFT/B3LYP method is used to study geometric and electronic structures of ions formed in the mass spectrum upon electron ionization of molecular forms in the vapor of aluminum tris-dipivaloylmethanate (Al(C11H19O2)3, Al(dpm)3). It is shown that the most energetically favorable processes upon dissociative ionization of molecular form AlL3 (L = dpm) are related to the detachment of radicals CH3•, t-Bu• and the anionic form of the ligand dpm–. It is established that one chelate ring is broken upon the formation of ions [AlL3–t-Bu]+, [AlL3–2t-Bu]+, [AlL3–6t-Bu]+ and the structure of the latter contains one monodentate ligand and two bidentate ligands. In the ions with compositions [AlL2]+, [AlL2–CH3]+, [AlL2–2CH3]+, [AlL2–4CH3]+, and [AlL2–2t-Bu–CH3]+, the cores of two bidentate ligands are located in mutually perpendicular planes. The energies of reactions leading to the formation of the above ions are calculated. The energies of frontier orbitals of these ions are considered; the reasons of high-intensity [AlL2]+ current are revealed. Detailed examination of ions in the mass spectrum of aluminum tris-dipivaloylmethanate (a typical representative of tris-β-diketonate complexes) reveals a number of structural features that can be used to predict the structure of such ions in the mass spectra of numerous similar complexes belonging to the ML3 type.

Reactivity of CO/H2/CH4/Air Mixtures Derived from In-Cylinder Fuel Reformation Examined by a Micro Flow Reactor with a Controlled Temperature Profile

ABSTRACT The effects of CO/H2/CH4 mixture compositions on the reactivity are investigated based on weak flame responses in a micro flow reactor with a controlled temperature profile (MFR). The ratios of H2 and CH4 are varied widely at a constant fraction of CO and the effects of variations in the compositions on the reactivity of the stoichiometric CO/H2/CH4 mixtures are evaluated both experimentally and computationally at atmospheric pressure. The results show that a weak flame is stabilized at the higher temperature region in MFR as the CH4 mole fraction increases, which indicates that the reactivity of the CO/H2/CH4 mixtures decreases with the increase of the CH4 mole fraction. Ignition delay times of the CO/H2/CH4 mixtures are also computed using a detailed chemical reaction model. The computed ignition delay times of the CO/H2/CH4 mixtures largely increase as the CH4 mole fraction increases, which is consistent with the variation in the reactivity shown by weak flames in MFR. Rate-of-production analyses using a detailed chemical reaction model show that the OH radical consumption by CH4 increases and that by H2 decreases as the CH4 fraction increases. Consequently, the production rate of CH3 radicals, which are less reactive than H radicals, increases and the production rate of H radicals decreases. This can suppress the OH formation from H radicals. Therefore, the primary factor of the significant reduction of the reactivity of the CO/H2/CH4 mixtures seems to be the increase of the OH radical consumption by CH4 and the CH3 radical production due to the increase in the CH4 fraction.

Quantum chemical study on the formation of isopropyl cyanide and its linear isomer in the interstellar medium

AbstractThe formation mechanism of linear and isopropyl cyanide (hereafter n-PrCN and i-PrCN, respectively) in the interstellar medium (ISM) has been proposed from the reaction between some previously detected small cyanides/cyanide radicals and hydrocarbons/hydrocarbon radicals. n-PrCN and i-PrCN are nitriles therefore, they can be precursors of amino acids via Strecker synthesis. The chemistry of i-PrCN is especially important since it is the first and only branched molecule in ISM, hence, it could be a precursor of branched amino acids such as leucine, isoleucine, etc. Therefore, both n-PrCN and i-PrCN have significant astrobiological importance. To study the formation of n-PrCN and i-PrCN in ISM, quantum chemical calculations have been performed using density functional theory at the MP2/6-311++G(2d,p)//M062X/6-311+G(2d,p) level. All the proposed reactions have been studied in the gas phase and the interstellar water ice. It is found that reactions of small cyanide with hydrocarbon radicals result in the formation of either large cyanide radicals or ethyl and vinyl cyanide, both of which are very important prebiotic interstellar species. They subsequently react with the radicals CH2 and CH3 to yield n-PrCN and i-PrCN. The proposed reactions are efficient in the hot cores of SgrB2 (N) (where both n-PrCN and i-PrCN were detected) due to either being barrierless or due to the presence of a permeable entrance barrier. However, the formation of n-PrCN and i-PrCN from the ethyl and vinyl cyanide always has an entrance barrier impermeable in the dark cloud; therefore, our proposed pathways are inefficient in the deep regions of molecular clouds. It is also observed that ethyl and vinyl cyanide serve as direct precursors to n-PrCN and i-PrCN and their abundance in ISM is directly related to the abundance of both isomers of propyl cyanide in ISM. In all the cases, reactions in the ice have smaller barriers compared to their gas-phase counterparts.

A nanozyme-based enhanced system for total removal of organic mercury and SERS sensing

Total removal of organic mercury in industrial wastewater is a crucially important task facing environmental pollution in the current world. Herein, we demonstrate the fabrication of Au-NiFe layered double hydroxide (LDH)/rGO nanocomposite as not only an efficient nanozyme with oxidase-like activity but also an efficient surface-enhanced Raman spectroscopy (SERS) substrate to determine organic mercury, with the minimum detection concentration as low as 1 × 10−8 M. According to the binding energy of X-Ray photoelectron spectrometer (XPS) and the free radicals of electron paramagnetic resonance (EPR) spectra, the mechanism of catalytic enhanced degradation is the production of Au-amalgam on Au surface, accelerating the electron transfer and the generation of O2•– radicals from oxygen molecules and •CH3 radicals from the methyl group in MeHg to participate the oxidase-like reaction. Furthermore, the Au-NiFe LDH/rGO nanocomposite is able to degrade and remove 99.9% of organic mercury in two hours without the secondary pollution by Hg2+. In addition, the material can be used for the multiple degradation-regeneration cycles in actual applications, which is significant in terms of the environmental and economic point of view. This work may open a new horizon for both highly sensitive detection and thorough degradation of organic mercury in environmental science and technology.

Prediction of the product channels in the reaction of the methyl radical with fulminic acid

AbstractThe reaction of CH3 + HCNO was theoretically studied by both density functional theory and molecular orbital calculations and analyzed by quantum statistical methods. The potential energy surface was constructed at the UCCSD(T)//B3LYP/6‐311++G(3df,2p) + ZPE level. Four entrance channels are opened relating to the interaction of the CH3 radical with each of the four atoms of the HCNO molecule, giving rise to 18 different product sets. The predicted geometries and heats of reaction are in good agreement with available experimental data. The major pathways involve addition of CH3 to the C atom of HCNO with a small energy barrier of ∼4 kcal/mol forming both Z and E‐HC(CH3)NO (IS1/IS1b) intermediates lying 45 or 44 kcal/mol, respectively, below the reactants. The nascent intermediates can collisionally be deactivated and subsequently decomposed into H + CH3CNO or isomerized prior to decomposition giving other products. Kinetics calculations covering the temperature range of 400‐2500 K, under pressures of 7.6 × 10–1 to 7.6 × 105 Torr for N2, He, and Ar as the third bodies show that at 760 Torr N2, the adducts including both IS1 and IS1b are the major products at temperature below 600 K, whereas H + CH3CNO and CNO + CH4 are the major products at T ≥ 1500 K. The total high‐pressure rate coefficient can well be expressed by the following three‐parameter equation: k(T) = 7.75 × 10–16 T1.69 exp (−1480 K/T) cm3 molecule–1 s–1.

Measurements of the laminar burning velocities and NO concentrations in neat and blended ethanol and n-heptane flames

Adiabatic laminar burning velocities and post-flame NO mole fractions for neat and blended ethanol and n-heptane premixed flames were experimentally determined using a heat flux burner and laser-induced fluorescence. The flames were stabilized at atmospheric pressure and at an initial temperature of 338 K, over equivalence ratios ranging from 0.6 to 1.5. These experiments are essential for the development, validation and optimization of chemical kinetic models, e.g. for the combustion of gasoline-ethanol fuel mixtures. It was observed that the addition of ethanol to n-heptane leads to an increase in laminar burning velocity that is not proportional to the ethanol content and to a decrease of NO formation. Such a NO reduction is due to the slightly lower flame temperatures of ethanol, which decrease the production of thermal-NO at 0.6 < Φ < 1.2, while under fuel-rich conditions this behavior is due to the lower concentrations of CH radicals, which decrease the production of prompt-NO. At Φ > 1.3, the lower NO formation through the prompt mechanism in the ethanol flames is partially offset by a lower rate of NO consumption through the reburning mechanism. New experimental results were compared with predictions of the POLIMI detailed chemical kinetic mechanism. An excellent agreement between measurements and simulated results was observed for the laminar burning velocities over the equivalence ratio range investigated; however, discrepancies were found for the NO mole fractions, especially under rich conditions. Further numerical analyses were performed to identify the main causes of the observed differences. Differences found at close-to stoichiometric conditions were attributed to an uncertainty in the thermal-NO mechanism. In addition, disagreement under rich conditions could be explained by the relative importance of reactions in hydrogen cyanide consumption pathways.

Investigation of nitric oxide formation in methane, methane/propane, and methane/hydrogen flames under condensing gas boiler conditions

In this study, NOx formation in lean premixed burner-stabilized flames was investigated under condensing gas boiler operating conditions for pure methane, methane/propane, and methane/hydrogen mixtures. Temperature and NOx species profiles were experimentally obtained from thermocouple and wet chemiluminescence detector measurements using in-situ extracted gas samples. Resolved simulations utilizing different NO sub-mechanisms from the GRI 2.11, the GRI 3.0, the CRECK mechanism, and a recent mechanism from Glarborg and co-workers were performed, where the mechanism from Glarborg showed the best agreement with the measured NOx concentrations. While the other mechanisms also agreed fairly well with the experiments, a pathway analysis revealed that this results from error compensation, where the prompt pathway was underestimated, which was balanced by an overestimated contribution of the NNH pathway. Interestingly, the prompt pathway was found to be the major NOx formation route for condensing gas boiler conditions despite the lean mixture. While the temperature level in the post-flame region was too low for the thermal pathway to become dominant, the NNH and N2O pathways reached their chemical equilibrium just downstream of the flame front and therefore did not further contribute to the overall NOx emissions.The partial substitution of methane by hydrogen slightly reduced NOx emissions, which was similarly found in experiment and simulation. The post-flame temperature for methane/hydrogen remained nearly unchanged, because the higher adiabatic flame temperature compared to pure methane was compensated by increased heat losses due to a higher laminar burning velocity. The addition of hydrogen decreased the amount of CH radicals in the flame front, which led to a lower contribution of the most important prompt pathway.Substituting methane by propane increased the temperature level due to a higher adiabatic flame temperature and a slightly smaller laminar burning velocity. While increasing NOx emissions were found in the experiment, the simulation predicted a minimally smaller NOx level compared to pure methane. The higher temperature caused increasing contributions of the thermal, NNH, and N2O pathways. For the prompt pathway, which is again the most important NOx formation route, the temperature trend was superimposed by a decreasing CH concentration in the flame front.

On the meaning of “collision rate constants” for ion-molecule reactions: Association of hydrogen atoms with C6H5+ and small alkyl radicals with C7H7+ ions

Limiting high pressure rate constants for the association of H with C6H5+ and of CH3, C2H5, and n-C3H7 radicals with C7H7+ molecular ions are analyzed in terms of “rigidity factors” arising from the anisotropy of the interaction potentials and “collision rate constants” in the absence of anisotropy (the latter corresponding to phase space theory, PST). Model calculations based on ab initio potential energy surfaces show that the PST rate constants kPST(T) exceed collision rate constants from conventional ion-molecule capture theory (in this case given by Langevin rate constants kL). They can be represented by kPST(T) = kL + kh.sph. where kh.sph. denotes hard-sphere collision numbers with collision radii r0. The r0 derived from the modelled kPST(T) are related to properties of the interaction potentials. Applications to other ion-molecule reactions are proposed.

Kinetic study of reaction C2H5 + HO2 in a photolysis reactor with time-resolved Faraday rotation spectroscopy

The rate constant and branching ratios of ethyl reaction with hydroperoxyl radical, C2H5 + HO2 (1), a key radical-radical reaction for intermediate temperature combustion chemistry, were measured in situ for the first time in a photolysis Herriott cell by using mid-IR Faraday rotation spectroscopy (FRS) and UV-IR direct absorption spectroscopy (DAS). The microsecond time-resolved diagnostic technique in this work enabled the direct rate measurements of the target reaction at 40 and 80 mbar and reduced the experimental uncertainty considerably. C2H5 and HO2 radicals were generated by the photolysis of (COCl)2/C2H5I/CH3OH/O2/He mixture at 266 nm. By direct measurements of the transient profiles of C2H5, HO2 and OH concentrations, the overall rate constant for this reaction at 297 K was determined as k1(40 mbar) = (3.8 ± 0.8) × 10−11 cm3 molecule−1 s−1 and k1(80 mbar) = (4.1 ± 1.0) × 10−11 cm3 molecule−1 s−1. The direct observation of hydroxyl radical (OH) indicated that OH formation channel was the major channel with a branching ratio of 0.8 ± 0.1.

Role of N2O and equivalence ratio on NOx formation of methane/nitrous oxide premixed flames

Nitrous oxide (N2O) is often used as an oxidizer in rocket technology with plenty of potential advantages. However, the N2O/hydrocarbon combustion results in the staggering increase of NOx emission. To scrutinize the role of N2O and equivalence ratio on NO formation characteristics of N2O/CH4 premixed flame, mixtures of N2O/CH4 and N2/O2/CH4 were investigated by employing one-dimensional opposed-jet model with detailed reaction mechanism. The flame structures, NO reaction pathways, and NO formation routes were discussed. In the heat release rate analysis, the heat release is dominated by N2O route, resulting in high flame temperature in N2O/CH4 premixed flame. Regarding the mechanism of NO formation, NO is mainly produced from N2 through the thermal routes and from HNO through the HNO-intermediate routes for the N2/O2/CH4 flame but mainly comes from N2O through the N2O and HNO-intermediate routes in the N2O/CH4 flame. The thermal routes alter to the primary chemical mechanism of NO consumption. The NO production in fuel-lean N2O/CH4 flames is enhanced through the N2O route due to the increase of the mole fraction of O atom. The thermal route becomes forward reactions to produce NO for the extreme fuel-rich N2O/CH4 flame. Regarding the reaction pathways of the prompt route, it can be approximately divided into three categories: (1) N2 → CN → NCN, NO (in terminal) → N2; (2) CN → NCN, NO (in terminal) → NO, CN; (3) N2 → CN → NCN, NO (in terminal) → N → N2. In NO production rate analysis, the main NO formation comes from the N2O and HNO-intermediate routes, while NO consumption associates with the thermal and NNH-intermediate routes. In addition, the prompt-NO formation is dominated by species of N2O instead of CH radical in the extreme fuel-rich N2O/CH4 flames.

Chemiluminescence of electronically excited species during the self-ignition of acetylene behind reflected shock waves

Acetylene is an effective additive to various rocket fuels. Acetam, an equilibrium highly concentrated solution of acetylene in liquid ammonia, is a promising fuel for use in liquid−propellant jet engines. To gain insights into the processes occurring in such systems and provide means for predicting their characteristics, the self-ignition of model acetylene−oxygen−argon mixtures is experimentally and theoretically studied. The experiments are performed behind reflected shock waves in a temperature range from 1100 to 1870 K at a total gas concentration of [M] ≈ 1.0 × 10−5 mol/cm3, with chemiluminescence signals from electronically excited C2* (λ ≅ 516 nm), CH* (429 nm), CO2*(363 nm), and OH* (308 nm) recorded. Numerical simulations within the framework of various detailed kinetic mechanisms closely reproduce the measured temperature dependences of the ignition delay time for the tested mixtures, but in some cases fail to satisfactorily describe the time profiles of the emission signals from C2*, CH*, CO2*, and OH*. The performed numerical simulations make it possible to explain the observed characteristic features of the emission profiles of electronically excited C2*, CH*, CO2*, and OH* during the self-ignition of C2H2–O2−Ar mixtures. Sensitivity analysis demonstrates that the existence and characteristics of the preignition stage in the CO2* and OH* emission signals are largely determined by the kinetic behavior of intermediate species, such as the ketene molecule and CH2 radical. Based on the results obtained, a new detailed kinetic mechanism has been proposed for describing processes in the combustion products of acetylene-containing propellants, in particular chemiluminescent glow, which are of considerable interest for the development of new technologies for astronautics and aeronautics.