Prompt NO formation in flames: The influence of NCN thermochemistry

The influence of the route via the NCN radical on NO formation in flames was examined from a thermochemistry and reaction kinetics perspective. A detailed analysis of available experimental and theoretical thermochemical data combined with an Active Thermochemical Tables analysis suggests a heat of formation of 457.8 ± 2.0 kJ/mol for NCN, consistent with carefully executed theoretical work of Harding et al. (2008) [5]. This value is significantly different from other previously reported experimental and theoretical values. A combination of an extensively validated comprehensive hydrocarbon oxidation model extended by the GDFkin3.0_NCN-NO_x sub-mechanism reproduced NCN and NO mole fraction profiles in a recently characterized fuel-rich methane flame only when heat of formation values in the range of 445–453 kJ/mol are applied. Sensitivity analysis revealed that the sensitivities of contributing steps to NO and NCN formation are strongly dependent on the absolute value of the heat of formation of NCN being used. In all flames under study the applied NCN thermochemistry highly influences simulated NO and NCN mole fractions. The results of this work illustrate the thermochemistry constraints in the context of NCN chemistry which have to be taken into account for improving model predictions of NO concentrations in flames.     

A detailed high-pressure oxidation study of di-isopropyl ether

The oxidation of di-isopropyl-ether (DIPE) was studied in a jet-stirred reactor. Fuel-lean, stoichiometric, and fuel-rich mixtures (φ = 0.5–4) were oxidized at a constant fuel mole fraction of 1000 ppm, at temperatures ranging from 500 to 1160 K, at 10 atm, and constant residence time of 0.7 s. The chosen conditions are consistent with our previous studies on ether oxidation. Mole fraction profiles were obtained through sonic probe sampling, and analyzed by gas chromatography and Fourier transform infrared spectrometry. As opposed to our previous studies on ethers (S. Thion et al. 2017, Z. Serinyel et al. 2018 and 2020), DIPE showed no low-temperature reactivity under the same experimental conditions. Oxidation of the rich mixture showed similarities to pyrolysis producing important quantities of propene and isopropanol, while no isopropanol is observed under lean conditions. In terms of overall reactivity, DIPE showed smaller fuel conversion compared to other symmetric ethers previously studied. The present data and literature experiments were simulated with our ether oxidation mechanism showing good agreement.     

The influence of pressure and equivalence ratio on the NTC behavior of methane

Methane based polygeneration processes in piston engines offer the possibility of a controllable and flexible conversion of energy, to up-convert low value chemicals and to store energy. These processes preferably take place under fuel-rich conditions and at high pressures. Under fuel-rich conditions, there was one experimental report that a distinctive negative temperature coefficient (NTC) behavior occurs in methane oxidation (Petersen et al., 1999). To design a polygeneration process, reliable kinetic models are required to capture the impact of pressure and equivalence ratio variations on reactivity of the gas mixtures. Here, the experimental basis for methane oxidation is expanded to high pressures and very fuel-rich conditions and compared to literature models, both with special emphasis on the NTC behavior. The oxidation of methane/oxygen mixtures at 2 ≤ Φ≤ 20 and pressures ranging from 1 to 20 bar is investigated. The literature reaction mechanisms are assessed with respect to their ability to predict this phenomenon and used to identify reaction pathways. It is found that NTC behavior occurs in a temperature range between 700 and 1000 K and at pressures higher than 5 bar. The lower temperature limit is slightly shifted towards higher temperatures with decreasing equivalence ratio. In addition, the higher the equivalence ratio, the broader the pressure range, in which the NTC behavior is observed. In general, predictions of some models are in good agreement with the experimental data. Reaction path analyses reveal that the competition between oxidation and recombination pathways are responsible for the NTC region in methane oxidation.     

Chemical structure of atmospheric pressure premixed laminar formic acid/hydrogen flames

The work presents an experimental and kinetic modeling study of laminar premixed formic acid [HC(O)OH]/H₂/O₂/Ar flames at different equivalence ratios (φ=0.85, 1.1 and 1.3) stabilized on a flat burner at atmospheric pressure, as well as laminar flame speed of HC(O)OH/O₂/Ar flames (φ=0.5–1.5) at 1 atm. Flame structure as well as laminar flame speed were simulated using three different detailed chemical kinetic mechanisms proposed for formic acid oxidation. The components in the fuel blends show different consumption profiles, namely, hydrogen is consumed slower than formic acid. According to kinetic analysis, the reason of the observed phenomenon is that the studied flames have hydrogen as a fuel but also as an intermediate product formed from HC(O)OH decomposition. Comparison of the measured and simulated flame structure shows that all the mechanisms satisfactorily predict the mole fraction profiles of the reactants, main products, and intermediates. It is noteworthy that the mechanisms proposed by Glarborg et al., Konnov et al. and the updated AramcoMech2.0 adequately predict the spatial variations in the mole fractions of free radicals, such as H, OH O and HO₂. However, some drawbacks of the mechanisms used were identified; in particular, they predict different concentrations of CH₂O. As for laminar flame speed simulations, the Konnov et al. mechanism predicts around two times higher values than in experiment, while the Glarborg et al. and updated AramcoMech2.0 show good agreement with the experimental data.     

Flame studies of C2 hydrocarbons

The oxidation characteristics of C₂ hydrocarbons were revisited in flames established in the counterflow configuration. Laminar flame speeds of ethane/air, ethylene/air, and acetylene/oxygen/nitrogen mixtures as well as extinction strain rates of non-premixed ethane/air flames were measured using digital particle image velocimetry. The experiments were modeled using three different kinetic models. While the experimental and computed laminar flame speeds agreed closely for all C₂ hydrocarbons under fuel-lean conditions, notable discrepancies were identified under fuel-rich conditions. Using the computed flame structures, insight was provided into the controlling mechanisms that could be responsible for the observed discrepancies. More specifically, the uncertainties associated with the kinetics of the thermal decomposition of the ethyl radical were found to be a potential source of the observed discrepancies for ethane flames. It was shown also by using alternative rate constants for the ethyl radical decomposition, the rate of flame propagation and the extinction propensity are affected notably. Furthermore, the values of the branching ratio of acetylene consumption reactions involving atomic oxygen were found to have a significant effect on the propagation of rich acetylene flames.     

Concentration and temperature measurement in fuel-rich flames

Soot formation is still one of the most pressing problems in combustion. Chemical mechanisms have been established which need to be examined in detail under laboratory conditions. Some of the main pathways concerning the formation of soot precursors are still under debate. While it seems commonly accepted that some of the dominant routes may be fuel-specific, experimental data and their comparison with kinetic models for fuels with more than 3 C atoms are scarce. This article will present an overview of the work pursued in Bielefeld on the characterisation of fuel-rich flames by a combination of laser spectroscopy and mass spectrometry.     

Experimental and kinetic modelling studies of flammability limits of partially dissociated NH3 and air mixtures

This paper presents a set of experimental and kinetic modelling studies of the flammability limits of partially dissociated NH₃ in air at 295 K and 1 atm. The experiments were carried out using a Hartmann bomb apparatus. The kinetic modelling was performed using Ansys Chemkin-Pro with opposed-flow premixed flame model employing three detailed reaction mechanisms, namely, the Mathieu and Petersen, Otomo et al., and Okafor et al. mechanisms. The degree of NH₃ dissociation was varied from 0 to 25% (0 to 20%v/v H₂ in the fuel mixture with a fixed H₂/N₂ ratio of 3). It was found that the lower (LFL) and upper (UFL) flammability limits of pure NH₃ in air were 15.0%v/v and 30.0%v/v, respectively, consistent with the literature data. The flammability limits of the mixture widened significantly with increasing the degree of NH₃ dissociation. At 25% NH₃ dissociation, LFL decreased to 10.1%v/v and UFL increased to 36.6%v/v. All tested mechanisms were able to predict the extinction characteristics exhibited by the lean and rich mixtures of partially dissociated NH₃ in air with non-unity Lewis numbers. While all three mechanisms predict well LFL, the Otomo et al. mechanism showed the best agreement with the experimental data of UFL. The rate of production of radicals, sensitivity, and reaction path analyses were performed to identify the key elementary reactions and radicals during combustion of partially dissociated NH₃. The production of key radicals including OH, H, O, and NH₂ was enhanced in the presence of H₂ and thus the conversion of NH to NO and then NO to N₂ near LFL and the conversion of NH₂ and NO to N₂ near UFL leading to wider flammability limits.     

Experimental and kinetic modeling study of the positive ions in premixed ethylene flames over a range of equivalence ratios

Understanding the ion chemistry in flames is crucial for developing ion sensitive technologies for controlling combustion processes. In this work, we measured the spatial distributions of positive ions in atmospheric-pressure burner-stabilized premixed flames of ethylene/oxygen/argon mixtures in a wide range of equivalence ratios ϕ = 0.4÷1.5. A flame sampling molecular beam system coupled with a quadrupole mass spectrometer was used to obtain the spatial distributions of cations in the flames, and a high mass resolution time-of-flight mass spectrometer was utilized for the identification of the cations having similar m/z ratios. The measured profiles of the flame ions were corrected for the contribution of hydrates formed during sampling in the flames slightly upstream the flame reaction zone. We also proposed an updated ion chemistry model and verified it against the experimental profiles of the most abundant cations in the flames. Our model is based on the kinetic mechanism available in the literature extended with the reactions for C₃H₅⁺ cation. Highly accurate W2-F12 quantum chemical calculations were used to obtain a reliable formation enthalpy of C₃H₅⁺. The model was found to reproduce properly the measured relative abundance of the key oxygenated cations (viz., CH₅O⁺, C₂H₃O⁺) in the whole range of equivalence ratios employed, and the C₃H₅⁺ cation abundance in the richest flame with ϕ=1.5, but significantly underpredicts the relative mole fraction of C₃H₃⁺, which becomes a key species under fuel-rich conditions. Apart from this, several aromatic and cyclic C_xH_y cations dominating under fuel-rich conditions were identified. We also considered the most important directions for the further refinement of the mechanism.     

Radial distribution of dose within heavy charged particle tracks – Models and experimental verification using LiF:Mg,Cu,P TL detectors

A new method of experimental verification of radial dose distribution models using solid state thermoluminescent (TL) detectors LiF:Mg,Cu,P has been recently proposed. In this work the method was applied to verify the spatial distribution of energy deposition within a single ¹³¹Xe ion track. Detectors were irradiated at the Department of Physics of the University of Jyväskylä, Finland. The obtained results have been compared with theoretical data, calculated according to the Zhang et al., Cucinotta et al. and Geiss et al. radial dose distribution (RDD) models. At the lowest dose range the Zhang et al. RDD model exhibited the best agreement as compared to experimental data. In the intermediate dose range, up to 10⁴ Gy, the best agreement was found for the RDD model of Cucinotta et al. The probability of occurrence of doses higher than 10⁴ Gy within a single ¹³¹Xe ion track was found to be lower than predicted by all the studied RDD models. This may be a result of diffusion of the charge, which is then captured by TL-related trapping sites, at the distances up to dozens of nanometers from the ionization site.     

The discrimination algorithms for overlapped MPSK and MQAM modulations using higher-order cumulants

Automatic Modulation Classification (AMC) is responsible for detecting the correct modulation types in the intelligent receivers. AMC performance degrades when the signal-to-noise ratio (SNR) decreases because of the overlapping among the digital modulation types’ features, and this performance worsens under fading channel conditions. This paper proposes two new algorithms that improve the AMC performance accuracy of the overlapped digital modulations in feature space by improving their discrimination. These algorithms are named temporal Fisher discriminant analysis (TFDA) and supervised Fisher discriminant analysis (SFDA). The simulation results show that TFDA improves AMC performance accuracy up to 19.01% compared with the reference paper (Ge et al., 2021) and up to 38.15% compared with the reference paper (Teng et al., 2018). In contrast, SFDA improves AMC performance accuracy up to 23.12 % compared with the reference paper (Ge et al., 2021) and up to 49.025% compared with the reference paper (Teng et al., 2018).     

Photoionization mass spectrometry and modeling studies of the chemistry of fuel-rich dimethyl ether flames

Reaction paths are identified for dimethyl ether (DME) combustion using modeling of new data from fuel-rich DME flat flames. A molecular-beam flame-sampling photoionization mass spectrometer, employing VUV synchrotron radiation, is applied to the measurement of mole fractions for 21 flame species in low-pressure premixed fuel-rich (Φ = 1.2, 1.68) DME/oxygen/argon flat flames. This approach is capable of resolving and identifying isomers and other flame species of near equal masses with ionization thresholds that differ by as little as 0.1 eV. The measurements agree well with flame modeling predictions, using a recently revised high-temperature DME kinetic mechanism, which identify reaction paths quite analogous to alkane combustion. They further reveal the presence of ethyl methyl ether, a molecule previously unobserved in flames and not included in present flame models.     

Experimental and numerical study of product gas characteristics of ammonia/air premixed laminar flames stabilized in a stagnation flow

Ammonia has widely attracted interest as a potential candidate not only as a hydrogen energy carrier but also as a carbon free fuel for internal combustion engines, such as gas turbines. Because ammonia contains a nitrogen atom in its molecule, nitrogen oxides (NO_x) and other pollutants may be formed when it burns. Therefore, understanding the fundamental product gas characteristics of ammonia/air laminar flames is important for the design of ammonia-fueled combustors to meet stringent emission regulations. In this study, the product gas characteristics of ammonia/air premixed laminar flames for various equivalence ratios were experimentally and numerically investigated up to elevated pressure conditions. In the experiments, a stagnation flame configuration was employed because an ammonia flame can be stabilized by using such a configuration without a pilot flame. The experimental results showed that the maximum NO mole fraction was about 3,500 ppmv, at an equivalence ratio of 0.9 at 0.1 MPa. The NO mole fraction decreased as the equivalence ratio increased. In addition, the maximum value of the NO mole fraction decreased with an increase in mixture pressure. Furthermore, it was experimentally clarified that the simultaneous reduction of NO and unburnt ammonia can be achieved at an equivalence ratio of about 1.06, which is the target equivalence ratio for emission control in rich-lean two-stage ammonia combustors. Comparison of experimental and numerical results showed that even though the reaction mechanisms employed have been optimized for predicting the laminar burning velocity of ammonia/air flames, they failed to satisfactorily predict the measured species in this study. Sensitivity analysis was used to identify elementary reactions that control the species profiles but have negligible effects on the burning velocity. It is considered that these reaction models need to be updated for accurate prediction of product gas characteristics of ammonia/air flames.     

Formation and destruction of nitric oxide in methane flames doped with NO at atmospheric pressure

A study of formation and destruction of NO in adiabatic laminar premixed flames of CH₄ + O₂ mixtures diluted with N₂ or Ar (with various dilution ratios) in a range of equivalence ratios at atmospheric pressure is presented. Nitric oxide was seeded into the flames using mixtures of diluent gas + 100 ppm of NO. The heat flux method was employed to measure adiabatic burning velocities of these flames. Nitric oxide concentrations in the post-flame zone at 10, 15 and 20 mm above the burner surface were measured using probe sampling. Burning velocities and NO concentrations simulated using a previously developed chemical kinetic mechanism were compared with the experimental results. The conversion ratio of NO seeded into the flames was determined. The kinetic mechanism accurately predicts burning velocities over the range of equivalence ratios and NO conversion in the rich flames. Significant discrepancies between measured and calculated NO conversion in the lean and near-stoichiometric flames were observed and discussed.     

NOx formation and flame velocity profiles of iso- and n-isomers of butane and butanol

NO formation and flame propagation are studied in premixed flames of iso- and n-isomers of butane and butanol through experimental measurements and direct simulation of experimental profiles. The stabilized flame is realized through the impingement of a premixed combustible jet from a contraction nozzle against a temperature-controlled plate. The velocity field is obtained by means of Particle Image Velocimetry (PIV) and nitric oxide concentration profiles are measured using Planar Laser Induced Fluorescence (PLIF), calibrated using known NO seeding levels. It is found that NO formation in n- and iso-isomers is comparable under the conditions considered, except for rich butanol mixtures, whereby NO formation is higher for iso-butanol. Generally, less NO is formed in butanol flames than in the butane flames. The experiment is simulated by a 1D chemically reacting stagnation flow model, using literature models of C1–C4 hydrocarbons [Wang et al., 2010] and butanol combustion chemistry [Sarathy et al., 2009, 2012]. NO prediction is tested using two of these mechanisms with a previously-published NO_x submechanism added into the butane and butanol models. While a good level of agreement is observed in the velocity field prediction under lean and stoichiometric conditions, discrepancies exist under rich conditions. Greater discrepancies are observed in NO prediction, except for the C1–C4 mechanism which shows good agreement with the experiment under lean and stoichiometric conditions. The current study provides data for further development of mechanisms with NO_x prediction capabilities for the fuels considered here.     

Laser diagnostics of NO reburning in fuel-rich propene flames

Absolute NO concentrations were measured by laser-induced fluorescence (LIF) in three different fuel-rich non-sooting propene flames (φ=1.5, 1.8 and 2.3). The experiments were performed in low-pressure premixed propene flames with 0.2%-1% NO added. Laser diagnostics was applied as a tool for investigating reburn chemistry. The Q₂(25.5) line in the A-X(0,0) band was excited because of the small temperature dependence of its ground state population. The NO fluorescence lifetimes were measured directly and compared to theoretical values. The initial NO levels are strongly reduced in all three flames. According to modeling results, the HCN mole fraction increases strongly with stoichiometry. As guidelines for laser diagnostics applications in such systems, the modeling results were analyzed with respect to the main reaction channels and reaction partners in fuel-rich flames. Received: 1 March 2000 / Revised version: 20 April 2000 / Published online: 20 September 2000     

LES of oxy-fuel jet flames using the Eulerian Stochastic Fields method with differential diffusion

The Eulerian Stochastic Fields (ESF) Monte Carlo method to solve the transported PDF (TPDF) equation is extended to account for differential diffusion effects by incorporating species individual molecular diffusivities. The method has been applied in Large Eddy simulation (LES) to non-piloted oxy-fuel jet flames at different Reynolds numbers experimentally investigated by Sevault et al. [1]. Due to the high H₂ content in the fuel stream and CO₂ in the oxidizer these flames pose new challenges to combustion modeling as the flame structures are different compared to CH₄/air flames. The simulations show very good agreement with the experiments in terms of mixture fraction conditional mean values for temperature and mayor species on the fuel lean side and the reaction zone, deviations on the fuel rich side are discussed. The trend and location of localized extinction is reproduced well in the simulations, as well as differential diffusion effects in the near field. Additionally, it is shown that a neglect of differential diffusion in the combustion model leads to a lifted flame.     

Kinetic effects of NO addition on n-dodecane cool and warm diffusion flames

The kinetic effects of NO addition on the flame dynamics and burning limits of n-dodecane cool and warm diffusion flames are investigated experimentally and computationally using a counterflow system. The results show that NO plays different roles in cool and warm flames due to their different reaction pathway sensitivities to the flame temperature and interactions with NO. We observe that NO addition decreases the cool flame extinction limit, delays the extinction transition from warm flame to cool flame, and promotes the ignition transition from warm flame to hot flame. In addition, jet-stirred reactor (JSR) experiments of n-dodecane oxidation with and without NO addition are also performed to develop and validate a n-dodecane/NO_x kinetic model. Reaction pathway and sensitivity analyses reveal that, for cool flames, NO addition inhibits the low-temperature oxidation of n-dodecane and reduces the flame temperature due to the consumption of RO₂ via NO+RO₂?NO₂+RO, which competes with the isomerization reaction that continues the peroxy radical branching sequence. The model prediction captures well the experimental trend of the inhibiting effect of NO on the cool flame extinction limit. For warm flames, two different kinds of warm flame transitions, the warm flame extinction transition to cool flame and the warm flame reignition transition to hot flame, were observed. The results suggest that warm extinction transition to cool flame is suppressed by NO addition while the warm flame reignition transition to hot flame is promoted. The kinetic model developed captures well the experimentally observed warm flame transitions to cool flame but fails to predict the warm flame reignition to hot flame at similar experimental conditions.     

Ethanol ignition in a high-pressure shock tube: Ignition delay time and high-repetition-rate imaging measurements

Ethanol is known to be prone to pre-ignition in internal combustion engines under high-load conditions and its ignition shows large deviations from ideal, spatially, and temporally-homogeneous ignition in shock tubes at moderate temperatures (800–950 K). In this context, the ignition of stoichiometric ethanol/O₂ mixtures with various levels of inert gas dilution was investigated in a high-pressure shock tube at ?20 bar between 800 and 1250 K. Ignition delay times were determined from spatially integral detection of chemiluminescence emission. Additionally, high-repetition-rate color imaging enabled the differentiation of the luminescence in time, space, and spectral range between various ignition modes. In the low-temperature range (800–860 K), different inhomogeneous ignition modes were identified. The addition of small amounts of helium into the undiluted fuel/air mixture was found to be efficient to mitigate pre-ignition, attributed to a variation in heat transfer and thus suppression of the build-up of local temperature inhomogeneities. The experiments in case of spatially homogeneous ignition show very good agreement with the predictions based on three detailed kinetics mechanisms (Zhang et al., CNF 190 (2018) 74, Frassoldati et al., CNF 159 (2012) 2295, and Zhou et al. CNF 197 (2018) 423), inhomogeneities, however, resulted in a shortening of the ignition delay times up to a factor of 2.6.     

An experimental and numerical investigation of the hetero-/homogeneous combustion of fuel-rich hydrogen/air mixtures over platinum

The hetero-/homogeneous combustion of hydrogen/air mixtures over platinum was investigated experimentally and numerically in a channel-flow configuration at fuel-rich equivalence ratios ranging from 2 to 7, pressures up to 5 bar and wall temperatures 760–1200 K. Experiments involved in situ one-dimensional Raman measurements of major gas-phase species concentrations over the catalyst boundary layer and planar laser induced fluorescence (LIF) of the OH radical, while simulations included an elliptic 2-D model with detailed heterogeneous and homogeneous reaction mechanisms. The employed reaction schemes reproduced the measured catalytic reactant consumption, the onset of homogeneous ignition, and the post-ignition flame shapes at all examined conditions. Although below a critical pressure, which depended on temperature, the intrinsic gas-phase kinetics of hydrogen dictated lower reactivity for the fuel-rich stoichiometries when compared to fuel-lean ones, homogeneous ignition was still more favorable for the rich stoichiometries due to the lower molecular transport of the deficient oxygen reactant that resulted in modest catalytic reactant consumption over the gaseous induction zone. Above the critical pressure, the intrinsic gaseous hydrogen kinetics yielded higher reactivity for the rich stoichiometries, which resulted in vigorous gaseous combustion at pressures up to 5 bar, in contrast to lean stoichiometry studies whereby homogeneous combustion was altogether suppressed above 3 bar. Computations at fuel-rich stoichiometries in practical channel geometries indicated that homogeneous combustion was not of concern for reactor thermal management, since the larger than unity Lewis number of the deficient oxygen reactant confined the flames to the core of the channel, away from the solid walls.     

An improved detailed chemical kinetic model for C3-C4 linear and iso-alcohols and their blends with gasoline at engine-relevant conditions

Propanol and butanol isomers have received significant research attention as promising fuel additives or neat biofuels. Robust chemical kinetic models are needed that can provide accurate and efficient predictions of combustion performance across a wide range of engine relevant conditions. This study seeks to improve the understanding of ignition and combustion behavior of pure C3-C4 linear and iso-alcohols, and their blends with gasoline at engine-relevant conditions. In this work, a kinetic model with improved thermochemistry and reaction kinetics was developed based on recent theoretical calculations of H-atom abstraction and peroxy radical reaction rates. Kinetic model validations are reported, and the current model reproduces the ignition delay times of the C3 and C4 alcohols well. Variations in reactivity over a wide range of temperatures and other operating conditions are also well predicted by the current model. Recent ignition delay time measurements from a rapid compression machine of neat iso-propanol and iso-butanol [Cheng et al., Proc. Combust Inst. (2020)] and blends with a research grade gasoline [Goldsborough et al., Proc. Combust Inst. (2020)] at elevated pressure (20–40 bar) and intermediate temperatures (780–950 K) were used to demonstrate the accuracy of the current kinetic model at conditions relevant to boosted spark-ignition engines. The effects of alcohol blending with gasoline on the autoignition behavior are discussed. The current model captures the suppression of reactivity in the low-temperature and negative-temperature-coefficient (NTC) region when either isopropanol and isobutanol are added to a research grade gasoline. Sensitivity and reaction flux analysis were performed to provide insights into the relevant fuel chemistry of the C3-C4 alcohols.