Mole Fraction Profiles Of Species Research Articles

Investigating the oxidation characteristic of a hydro-processed bio-jet fuel: Experimental and modeling study

A rapid transition from conventional jet fuels to sustainable aviation fuels (SAFs) is imperative in order to reduce carbon emissions. Hydro-processed esters and fatty acids synthetic paraffinic kerosene (HEFA-SPK), as a type of SAF, exhibits broad applications. In this study, a new HEFA-SPK named ZH-HEFA was investigated. The fuel comprises 14% n-alkanes, 85% iso-alkanes and only 1% cycloalkanes by weight, with the majority of alkanes ranging from C9 to C17. Oxidation experiments of the fuel were conducted using an atmospheric pressure flow reactor at temperatures ranging from 550 K to 1075 K under three equivalence ratios (0.5, 1.0 and 1.5). Species mole fraction profiles were measured by an on-line gas chromatographic (GC). For comparison purposes, an experiment was also performed on RP-3, a conventional jet fuel commonly used in China, under the equivalence of 0.5. Compared to RP-3, ZH-HEFA exhibited significantly stronger low temperature reactivity and higher combustion conversion rates while demonstrating considerably lower yields of aromatics at high temperatures. The kinetic simulation of ZH-HEFA was achieved by proposing two surrogates and their corresponding kinetic models. Surrogate S-1 consisted solely of n-dodecane, while S-2 comprised 35% n-dodecane and 65% 2,6,10-trimethyl dodecane by weight. Both surrogate models were validated by the experimental data. S-1 exhibited a closer resemblance to the global oxidation characteristics of ZH-HEFA, whereas S-2 demonstrated improved accuracy in predicting the formation of small hydrocarbon intermediates during the fuel oxidation. Rate of production analysis revealed that the branched alkane component in S-2 possessed more pathways and greater capability than S-1 in generating C3 intermediates, which are important for the generation of aromatics. Furthermore, both models displayed good predictive performance for the auto-ignition properties of HEFA-SPK fuels.-

Chemical insights into plasma-assisted dry reforming of methane in a nanosecond discharge

The present study explores the chemical kinetics of plasma-assisted dry reforming of methane (340 K, 30 Torr) in a flow reactor activated by a nanosecond pulsed dielectric barrier discharge (DBD). Experiments are conducted with the inlet CO2/CH4 mol ratios varying from 0.4 to 2.8 in CH4/CO2/Ar mixture. Synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) is employed for detailed species measurement of the ions, radicals, and molecules in this system. Eight ions are observed, including CH3+, O+, CH4+, Ar2+, CO+, C2H4+, Ar+, and CO2+. The identification of several intermediate species and products are achieved based on the photoionization efficiency (PIE) spectra, such as methyl radical (CH3), water (H2O), acetylene (C2H2), carbon monoxide (CO), ethylene (C2H4), formaldehyde (CH2O), methanol (CH3OH), ketene (CH2CO), and acetone (CH3COCH3). Species mole fraction profiles as a function of inlet CO2 concentrations are obtained. The present work provides a unique dataset and enriches our understanding of the kinetics of plasma-assisted dry reforming of methane. A kinetic mechanism incorporating plasma reactions and combustion reactions is developed for this system, and its prediction performance of the product selectivity is validated against the experimental data. The main reaction pathways for the consumption of CH4 and CO2 and the formation of products and intermediates are indicated based on the rate of production (ROP) analysis. The ionic reactions are predominant for CH4 consumption, leading to CH4+ and CH5+, while CH4 reactions with electrons and Ar* contribute to CH2 and CH3 formation. CO2 mainly reacts with electrons and Ar*, producing CO, and can also undergo reactions with CH4+ and CH5+, forming HCO2+. The principal formation pathways for H2, CO, CH2O, and CH3OH are discussed. A competition between the discharge pathways and oxygen-containing pathways in CH4/CO2/Ar plasma is proposed.

Experimental and kinetic modeling study of C5 cyclic hydrocarbons pyrolysis at elevated pressures: Effect of unsaturation on the reactivity and PAH formation

To further explore the decomposition behaviors of C5 cyclic hydrocarbons, important components in the transportation fuels and pivotal intermediates in high-density liquid fuels, the pyrolysis of cyclopentane, cyclopentene and cyclopentadiene with different unsaturation degrees were investigated. The experiments of cyclopentane and cyclopentene were performed at 1.0, 30.0 atm and 773–1173 K in a flow tube reactor. The mole fraction profiles of pyrolysis species, involving C1-C5 small hydrocarbons and monocyclic, bicyclic as well as polycyclic aromatic hydrocarbons (MAHs, BAHs and PAHs), were provided. A universal kinetic model for three C5 cyclic fuels pyrolysis chemistry was proposed and reasonably predicted the speciation profiles newly reported in this work and literature, where rate coefficients of the crucial reactions were compared and upgraded. The differences among three fuels specific pathways were evaluated and the effects of unsaturation on the pyrolysis characteristics of intrinsic reactivity and aromatics formation were revealed. Cyclopentene has the highest pyrolysis reactivity, followed by cyclopentane and cyclopentadiene. Simultaneously formed fuel radicals and key intermediates in three C5 cyclic fuels influence the formation channels of small products, MAHs and BAHs. The saturated cyclopentane is easily occur the ring-opening reaction to generate more C2 and C3 species, whereas the increase in unsaturation promotes MAHs and BAHs formation in cyclopentene and cyclopentadiene pyrolysis, especially the toluene, styrene, indene and naphthalene. The rate of production analysis shows the formations of these MAHs and BAHs are strongly dependent on the concentration levels of cyclopentadienyl resonance stabilized radical (RSR). Differently, the dominated growth pathways of three- and four-ring PAHs are barely influenced by the specific fuel chemistry and indenyl plays a crucial role via the typical RSR chain reactions at high pressures.

A dedicated reduced kinetic model for ammonia/dimethyl-ether turbulent premixed flames

Ammonia (NH3) as a promising energy vector receives growing interest to reduce carbon emissions in combustion applications. Co-firing with dimethyl ether (DME) is an outstanding route to enhance the combustion properties of ammonia. In this study, a reduced model for NH3/DME blend fuels made up of 48 species and 294 reactions was developed starting from a detailed kinetic mechanism. The overall agreement of the reduced model compared with both experimental data and predictions from the original one are good, in terms of ignition delay times, laminar flame speeds, species mole fraction profiles, and S-curves for a variety of NH3/DME mixtures. Additionally, the fidelity of the reduced model has been further evaluated by comparing with other detailed kinetic models from the literature. Using this reduced mechanism, a parametric analysis of one-dimensional flames reveals a trade-off in terms of emissions (NOx vs. CO2), equivalence ratios, and flame propagation characteristics. The ultimate objective of this study is to investigate ignition and turbulent flame dynamics. As a first step in this direction, a turbulent premixed flame of a rich NH3/DME mixture with 25% DME content is investigated by direct numerical simulation (DNS) using the reduced mechanism, and conditional averages are analyzed. This work paves the way for future systematic studies of turbulent NH3/DME flames by DNS.

Resonance enhanced multiphoton ionization detection of aromatics formation in fuel-rich flames

Polycyclic aromatic hydrocarbons (PAHs) are formed in fuel-rich and sooting hydrocarbon flames. These compounds and smaller aromatic flame constituents, such as benzene, toluene and naphthalene can be detected by flame-sampling molecular beam mass spectrometry (MBMS). They are readily ionized by photoionization with UV light, either in resonant or non-resonant ionization processes. The multiphoton ionization of benzene, toluene, and styrene with MBMS detection was investigated as a function of laser power and wavelength and the quantitative relation between ion signal and mole fractions in the flames was established. These measurements were prerequisites for the measurement of quantitative mole fraction profiles of these species in flames of various fuels. In addition, the measurements provide relative mole fraction profiles of various other aromatic flame species. The results are compared to measurements of aromatics in butadiene, propyne, and allene flames by single-photon ionization with vacuum ultraviolet radiation from a synchrotron source and other literature data.

Combustion of spherical PMMA samples in still air simulated using a skeletal chemical kinetic mechanism

The combustion of spherical samples of polymethyl methacrylate (PMMA) under natural convection was studied experimentally and by CFD simulation. Species mole fraction and temperature profiles in the flame were measured by microprobe mass spectrometry and microthermocouples. Gas flow around a solid sphere was modeled using the full Navier–Stokes equations for a multicomponent mixture, with a skeletal chemical kinetic mechanism taking into account heat transfer, convection, and radiation. A kinetic model has been developed that includes the surface pyrolysis of PMMA and a skeletal kinetic mechanism for the gas-phase combustion of its monomer, methyl methacrylate (MMA). Satisfactory agreement between the experimental data and the results of 3D modeling for the chemical and thermal structure of the flame has proven the efficiency of the proposed chemical kinetic model. An analysis of the obtained data has shown that in the diffusion combustion of the spherical samples, the region of the stoichiometric composition of the gas mixture does not coincide with the region of maximum temperature. It has been found that under the conditions studied, gas-phase MMA decomposition proceeds mainly through radical reactions and unimolecular decomposition plays only a minor role.

Interactions in Ammonia and Hydrogen Oxidation Examined in a Flow Reactor and a Shock Tube

Ammonia (NH3) is a promising fuel, because it is carbon-free and easier to store and transport than hydrogen (H2). However, an ignition enhancer such as H2 might be needed for technical applications, because of the rather poor ignition properties of NH3. The combustion of pure NH3 and H2 has been explored widely. However, for mixtures of both gases, mostly only global parameters such as ignition delay times or flame speeds were reported. Studies with extensive experimental species profiles are scarce. Therefore, we experimentally investigated the interactions in the oxidation of different NH3/H2 mixtures in the temperature range of 750-1173 K at 0.97 bar in a plug-flow reactor (PFR), as well as in the temperature range of 1615-2358 K with an average pressure of 3.16 bar in a shock tube. In the PFR, temperature-dependent mole fraction profiles of the main species were obtained via electron ionization molecular-beam mass spectrometry (EI-MBMS). Additionally, for the first time, tunable diode laser absorption spectroscopy (TDLAS) with a scanned-wavelength method was adapted to the PFR for the quantification of nitric oxide (NO). In the shock tube, time-resolved NO profiles were also measured by TDLAS using a fixed-wavelength approach. The experimental results both in PFR and shock tube reveal the reactivity enhancement by H2 on ammonia oxidation. The extensive sets of results were compared with predictions by four NH3-related reaction mechanisms. None of the mechanisms can well predict all experimental results, but the Stagni et al. [React. Chem. Eng. 2020, 5, 696-711] and Zhu et al. [Combust. Flame 2022, 246, 115389] mechanisms perform best for the PFR and shock tube conditions, respectively. Exploratory kinetic analysis was conducted to identify the effect of H2 addition on ammonia oxidation and NO formation, as well as sensitive reactions in different temperature regimes. The results presented in this study can provide valuable information for further model development and highlight relevant properties of H2-assisted NH3 combustion.

Exploring the Effect of Different Reactivity Promoters on the Oxidation of Ammonia in a Jet-Stirred Reactor

The low reactivity of ammonia (NH3) is the main barrier to applying neat NH3 as fuel in technical applications, such as internal combustion engines and gas turbines. Introducing combustion promoters as additives in NH3-based fuel can be a feasible solution. In this work, the oxidation of ammonia by adding different reactivity promoters, i.e., hydrogen (H2), methane (CH4), and methanol (CH3OH), was investigated in a jet-stirred reactor (JSR) at temperatures between 700 and 1200 K and at a pressure of 1 bar. The effect of ozone (O3) was also studied, starting from an extremely low temperature (450 K). Species mole fraction profiles as a function of the temperature were measured by molecular-beam mass spectrometry (MBMS). With the help of the promoters, NH3 consumption can be triggered at lower temperatures than in the neat NH3 case. CH3OH has the most prominent effect on enhancing the reactivity, followed by H2 and CH4. Furthermore, two-stage NH3 consumption was observed in NH3/CH3OH blends, whereas no such phenomenon was found by adding H2 or CH4. The mechanism constructed in this work can reasonably reproduce the promoting effect of the additives on NH3 oxidation. The cyanide chemistry is validated by the measurement of HCN and HNCO. The reaction CH2O + NH2 ⇄ HCO + NH3 is responsible for the underestimation of CH2O in NH3/CH4 fuel blends. The discrepancies observed in the modeling of NH3 fuel blends are mainly due to the deviations in the neat NH3 case. The total rate coefficient and the branching ratio of NH2 + HO2 are still controversial. The high branching fraction of the chain-propagating channel NH2 + HO2 ⇄ H2NO + OH improves the model performance under low-pressure JSR conditions for neat NH3 but overestimates the reactivity for NH3 fuel blends. Based on this mechanism, the reaction pathway and rate of production analyses were conducted. The HONO-related reaction routine was found to be activated uniquely by adding CH3OH, which enhances the reactivity most significantly. It was observed from the experiment that adding ozone to the oxidant can effectively initiate NH3 consumption at temperatures below 450 K but unexpectedly inhibit the NH3 consumption at temperatures higher than 900 K. The preliminary mechanism reveals that adding the elementary reactions between NH3-related species and O3 is effective for improving the model performance, but their rate coefficients have to be refined.

Propylene Oxide Addition Effect on the Chemical Speciation of a Fuel-Rich Premixed n-Heptane/Toluene Flame.

1,2-Propylene oxide (PO, C3H6O) is considered as a promising agent for improving fuel. In this work, the effect of PO additives on the species pool in a premixed burner-stabilized fuel-rich (ϕ = 1.6) flame fueled by n-heptane/toluene mixture (7/3 by volume of liquids) at atmospheric pressure is studied by the flame-sampling molecular beam mass spectrometry and numerical modeling in order to get insight into the chemical aspects of the influence of oxygenates with an epoxy group on the formation of abundant intermediates (including PAH precursors) during combustion of fossil fuels. The flames with various loadings of PO in the fuel blend (from 0 to 16.3% in mole basis) are examined, and detailed kinetic mechanisms available in the literature are validated against the measurements of mole fraction profiles of reactants, major products, and many intermediate species. A higher reactivity of the fresh mixture and a reduction in the peak mole fractions of intermediates playing an important role in PAH formation (benzene, styrene, ethylbenzene, phenol, acetylene, diacetylene, etc.) are observed when PO is added. This was found to be due to simultaneously two factors: the partial replacement of "sooting" fuel (toluene, which is the main precursor of these species) with oxygenated additive, and the changes in the flame radical pool caused by PO addition. Propylene oxide additive was found to change the ratio between H, OH, O, and CH3 toward an increase in the proportion of O and CH3. The detailed kinetic mechanisms considered in the work are found to overpredict the peak mole fraction of acetylene, a key species playing a crucial role in PAH growth. Its chemistry is revisited in order to provide a better prediction of C2H2 and, as a result, PAHs.

Experimental and numerical study on the laminar flame characteristics for PODE3 and PODE3/iso-octane blends under elevated and sub-ambient initial pressures

• The laminar burning speeds for PODE 3 under different initial pressures are tested. • The laminar burning speeds for PODE 3 / iso -octane blends are investigated. • The accuracy of mechanism for PODE 3 and PRF/PODE n is validated. • The Markstein length and laminar flame instability are analyzed in detail. Polyoxymethylene dimethyl ether (PODE n ) is a promising alternative fuel for diesel due to its high oxygen contents (>40%) and high cetane number, thereby improving the thermal efficiency and reducing the pollutant emissions caused by the internal combustion engine. This study investigated the laminar burning speeds and Markstein length for PODE 3 and PODE 3 / iso -octane blends at wide equivalence ratios (0.7–1.6) and initial temperature of 408 K under elevated and sub-ambient initial pressures (0.5–4 bar) in a cylindrical constant-volume combustion vessel with high-speed Schlieren photography method combined with the nonlinear extrapolation model. The numerical simulation was also carried out to validate the accuracy of the mechanisms of PODE 3 and primary reference fuel (PRF)/PODE n blends and analyze the reaction sensitivities and the mole fraction profiles of key species. In addition, flame instability was discussed experimentally and theoretically. The results show that the laminar burning speed for PODE 3 decreases with the initial pressure, and the laminar burning speed for PODE 3 / iso -octane basically accords with the linear relationship to the PODE 3 volume fractions. The findings indicate that the kinetic effect has a dominant impact on the laminar burning speed for PODE 3 when compared with the thermal effect. The results also reveal that the Markstein length is negative only at an equivalence ratio between 1.5 and 1.6 under P = 2 bar, indicating that PODE 3 has more enhanced flame stability. Furthermore, the flame instability decreases with the increase of PODE 3 volume fractions, increases with the increase of initial pressure, and increases with the increase of the equivalence ratio. The differences between experimental results and theoretical results including the Markstein length, the critical radius and the critical Peclet number might be explained by the stretch rate results.

Effects of diethyl ether addition on sooting transition in iso-octane counterflow diffusion flames

This study studied the effects of the addition of diethyl ether (DEE) in the fuel stream on sooting transition process in iso-octane counterflow diffusion flames with optical and chemical diagnostics. Compared with the baseline iso-octane flame, two fuel/oxygen/nitrogen flames were studied with a 20 % DEE/iso-octane and a 40 % DEE/iso-octane mixture as the fuel, while holding constant fuel mole fraction. Optical diagnosis was used for the investigation of the effect of DEE addition on macroscopic sooting transition point with a high-resolution digital camera based on image processing and transition point extraction methods. Online gas chromatograph (GC) was used for all flames to investigate the flame structure and measure quantitative mole fraction profiles of stable and reactive species formed in the sooting transition process, especially the changes caused by partial substitution of iso-octane by DEE. Meanwhile, the experimental results were compared with predictions from a model suitable for both iso-octane and DEE. Sooting transition point was delayed with the increase of DEE addition. Although the overall flame chemical structure did not change significantly with the addition of DEE, it was found that some primary decomposition products and precursors had more significant effects. In most cases, experimental trends could be predicted and explained well by the model. It was noted that the dilution of carbon flow caused by partial substitution was the main reason for the inhibition of soot formation and the delay of sooting transition point. Secondly, on the chemical effect, DEE addition could produce a large amount of C2H2 which could contribute to the production of C3H3 and the formation of large PAHs.

Molecular beam mass spectrometry study on plasma-assisted low-temperature oxidation of ethylene

The chemical kinetics of plasma-assisted low-temperature oxidation of ethylene (C2H4) is explored by molecular beam mass spectrometry and kinetic modeling. In the experiments, a dielectric barrier discharge (DBD) flow reactor coupled with photoionization and electron ionization molecular beam mass spectrometry (PI-MBMS/EI-MBMS) is employed to provide detailed speciation information, including ions, radicals, and molecules in C2H4/O2/Ar systems (∼340 K, 30 Torr). A series of hydrocarbons and oxygenated intermediates including ethenol, methyl hydroperoxide, ethyl hydroperoxide, diacetylene, vinylacetylene, and hydroxyacetaldehyde is detected for the first time in this system. A kinetic mechanism that contains both combustion and plasma reaction networks is developed. The path fluxes of fuel consumption and oxygenates formation are obtained based on the rate of production analysis. Based on our experimental observations and modeling analysis, new insights into the kinetics of C2H4/O2/Ar plasmas are provided. The fuel is mainly consumed through OH addition, dissociation after e/Ar* collision, reactions with O/O(1D), and dissociative ionization. H abstraction reactions that are favored in alkane plasmas contribute little to the fuel consumption of ethylene. Formation pathways of C1 oxygenates are mainly initiated by CH3 produced by O/O(1D) reactions with fuel, and the subsequent pathways of OH/H addition to the fuel are responsible for the formation of most C2 oxygenates. Oxygen addition-elimination reactions of C2H4OH isomers are the sources of ethenol formation. The concentration profiles for some of the species under different oxygen mole fractions are measured. The two distinct patterns of species mole fraction profiles with increasing oxygen concentrations indicate the different dominating formation pathways, namely oxidation pathways and non-oxidation pathways. And the competition between oxidation effects and plasma effects is displayed by the non-monotonic mole fractions profiles of oxidation products.

Turbulence-induced bias in time-averaged laser absorption tomography of correlated concentration and temperature fields with a first-order correction

The influence of correlated scalar fluctuations on time-averaged laser absorption tomography measurements of temperature and species in a piloted turbulent premixed flame was examined using a coupled spectroscopic and fluid-dynamic analysis. To understand bias associated with turbulence, spatio-temporally resolved temperature and species mole fraction profiles predicted by large eddy simulations (LES) were used to synthetically generate time-resolved line-of-sight absorption measurements at short time scales (microsecond) to reflect the unsteady nature of a canonical jet burner across various transverse measurement planes. Inversion methods were employed on the time-averaged line-of-sight data to produce radially-resolved temperature and mole fraction profiles, analogous to those produced by laser absorption tomography performed on a time-averaged axisymmetric flowfield. It is shown that bias in the measurements compared to true time-averaged scalar fields is a function primarily of temperature dependence in absorptivity and non-zero correlation between temperature and species concentration scalars. A first-order correction to tomography measurements is proposed to account for the bias based on estimated correlations and the known spectroscopic parameters of the probed absorption transitions.

A detailed uncertainty analysis of EI-MBMS data from combustion experiments

Electron ionization molecular-beam mass spectrometry (EI-MBMS) is a widely used analytical method for studying the chemistry of combustion processes and for providing quantitative validation targets for kinetic model development often in form of species mole fraction profiles. The present study focuses on the determination of the uncertainties of such data, which result from the mass spectrometric measurements as well as from the data evaluation procedure, in particular through the use of different calibration methods, i.e., through a direct calibration of the instrument, a convolution procedure, and through the method of Relative Ionization Cross Sections (RICS). A series of calibration experiments were carried out using two independent time-of-flight mass spectrometers in order to check the influence of experimental and instrumental parameters on the uncertainties. The data obtained reveal several key contributors to the uncertainty, which are, among others, the signal intensity of a species in the mass spectrum, the instability of instrumental parameters, the species itself, and the calibration method.The propagation of these uncertainties from the different calibration methods as well as from the instrumental inconstancies into the final uncertainties of the mole fractions is further demonstrated using a gaussian error propagation and provision of sensitivities for different uncertainty sources is given. It was found that most of the uncertainties of the mole fractions reported in the literature are smaller than those reported here and presumably only consider the reproducibility of the signal.The use of two independent mass spectrometers for the experiments also revealed that the uncertainties determined for one instrument cannot be directly transferred to another especially for the method of direct calibration due to differences in the signal reproducibility, but that the uncertainties only change marginal for the semi-theoretical approaches.

Pyrolysis and oxidation of a light naphtha fuel and its surrogate blend

An experimental and kinetic study on pyrolysis and oxidation of a real light naphtha fuel and its surrogate blend were conducted. Based on the chemical functional group method, a three-component surrogate blend for the target naphtha was formulated, which contains 64.2 mol% iso-pentane, 21.0 mol% n-hexane, and 14.8 mol% methylcyclopentane. The pyrolysis and oxidation characteristics of the target naphtha fuel and the formulated surrogate were compared using a jet-stirred reactor (JSR) at the equivalence ratios of 0.5, 1.0, 2.0 and ∞, across the temperature range from 700 to 1100 K, and at atmospheric pressure. Mole fractions of the three components, oxygen, hydrogen, CO, CO2, and C1-C4 hydrocarbons were measured by gas chromatograph. Similar global reactivities between the two test fuels were observed in both pyrolysis and oxidation experiments. In addition, a detailed chemical kinetic model was constructed and validated against the species mole fraction profiles measured in JSR experiments. The present model can provide reasonable prediction of the experimental measurements of the species mole fractions in pyrolysis and oxidation of the surrogate blend. Uncertainty-weighted sensitivity analysis indicates that the model prediction of the consumption of the three components of the surrogate blend in pyrolysis are dominated by the rate constants for H-abstraction reactions of isopentane, n-hexane, methylcyclopentane and propene by H atom and methyl radical. The model predictions of the oxidation reactivity of the surrogate blend mainly depend on H-abstraction reactions from the three surrogate components by HO2 radical at 800 K, while the H-abstraction reactions from aldehyde and alkene intermediates become more significant at 950 K.

Experimental and kinetic modelling studies on di-n‑butyl ether (DBE) low temperature auto-ignition

Di-n‑butyl ether (DBE), mainly derived from lignocellulosic biomass, is a long-chain linear ether that is regarded as a promising alternative for diesel in compression-ignition engines. Therefore, understanding of the auto-ignition characteristics of DBE is of great importance. In this work, ignition delay times (IDTs) of DBE/O2/N2 mixtures were experimentally measured on a rapid compression machine (RCM). The experiments were conducted at the compressed temperatures ranging from 525 to 707 K, the compressed pressures at 7 and 10 bar, and the equivalence ratios of 0.7, 1.0 and 1.4. A negative temperature coefficient (NTC) behavior was observed for the total IDTs in the tested temperature range. The effects of compressed temperature, compressed pressure, fuel fraction and oxygen fraction, on both the first-stage and total IDTs, were also discussed. Validated against the experimental measurements, two existing DBE chemical kinetic models were found to have less satisfactory performances for the IDT prediction. Therefore, a tuned model based on the Thion model was presented, and it was found to well capture the fuel NTC behaviors and improve the IDTs prediction at the test conditions. Moreover, the tuned model was tested against the measurements on IDTs and species mole fraction profiles in the literature. The tuned model reasonably predicts the measured IDTs on a shock tube at high temperatures and on a flow reactor at low temperatures. The double-NTC behavior of DBE oxidation observed in jet-stirred reactor (JSR) and plug flow reactor (PFR) experiments can be also reasonably predicted.

Experimental and kinetic study on low temperature oxidation and pyrolysis of iso-octane and gasoline

A comprehensive experimental and kinetic study on the low temperature oxidation and pyrolysis of iso-octane and CNPC 92# was conducted. Species profiles measurements were performed in a jet-stirred reactor (JSR) at the temperature range of 500–1300 K under atmospheric pressure, with a fixed residence time of 2.0 s and for the equivalence ratios ranging from 0.5 to ∞ (pyrolysis). Three kinetic models were adopted to simulate mole fraction profiles of major species in the oxidation and pyrolysis of iso-octane, and sensitivity analysis was also conducted using the model which preformed best. In addition, a detailed gasoline surrogate kinetic model was developed including five representative fuels in gasoline. This kinetic model and a TRF model containing three individual fuels from KAUST were verified by low temperature oxidation and pyrolysis experiment data of CNPC 92#. The results show that prediction profiles by the five-component model and measured mole fraction of major intermediates are in a better agreement than TRF model.

Insight into the Ozone-Assisted Low-Temperature Combustion of Dimethyl Ether by Means of Stabilized Cool Flames.

The low-temperature combustion kinetics of dimethyl ether (DME) were studied by means of stabilized cool flames in a heated stagnation plate burner configuration using ozone-seeded premixed flows of DME/O2. Direct imaging of CH2O* chemiluminescence and laser-induced fluorescence of CH2O were used to determine the flame front positions in a wide range of lean and ultra-lean equivalence ratios and ozone concentrations for two strain rates. The temperature and species mole fraction profiles along the flame were measured by coupling thermocouples, gas chromatography, micro-chromatography, and quadrupole mass spectrometry analysis. A new kinetic model was built on the basis of the Aramco 1.3 model, coupled with a validated submechanism of O3 chemistry, and was updated to improve the agreement with the obtained experimental results and experimental data available in the literature. The main results show the efficiency of the tested model to predict the flame front position and temperature in every tested condition, as well as the importance of reactions typical of atmospheric chemistry in the prediction of cool flame occurrence. The agreement on the fuel and major products is overall good, except for methanol, highlighting some missing kinetic pathways for the DME/O2/O3 system, possibly linked to the direct addition of atomic oxygen on the fuel radical, modifying the product distribution after the cool flame.

Experimental and numerical study on sooting transition process in iso-octane counterflow diffusion flames: Diagnostics and combustion chemistry

The present work focused on the combustion chemistry in sooting transition process in iso-octane counterflow diffusion flames. A series of iso-octane flames ranging from sooting-free to heavy-sooting were investigated by stepwise increasing the oxygen concentration (XO) in the oxidizer stream. In particular an optical approach with two different algorithms based on the three wavelengths distribution of flame images captured by digital single lens reflex (DSLR) camera was proposed to identify the critical condition for the sooting transition process. The results showed that the sharp increase in signal ratio of red luminosity with simultaneously obvious decrease in signal rations of blue luminosity could be used as an effective indicator of sooting onset in flames. On-line gas chromatography (GC) measurements were used to investigate the flame chemistry and determine quantitative mole fraction profiles of stable and reactive species formed in the combustion process. Particular attention was given to the combustion chemistry changes caused by the occurrence of sooting transition. Numerical kinetic simulations were performed along with the comparisons with experimental results to obtain deep insights into the sooting transition process, with focus on benzene chemistry considering the critical role of benzene formation in the subsequent large aromatic species/soot formation process. In most cases, the trend observed in the experiments could be satisfactorily reproduced and explained by the models. For major species and C1–C4 species, it can be seen that the concentration increased with the increase of XO in sooting transition process. For the decomposition of fuel and the formation of benzene, the influence in sooting transition process was mainly reflected in the contributions of different pathways due to temperature effects. The impact of sooting transition on flame chemical structure was mainly reflected in the growth of PAHs. The increasing trend for the PAHs growth caused by the increase of oxygen concentration in the transition process was more and more obvious.

Numerical Simulations of the Influence of Inert Gases (N2/CO2) on Combustion Characteristics of Laminar-Premixed Biosyngas Flame.

In this work, the influence of different N2/CO2 contents (up to 60% in fuel volume) on combustion features of laminar-premixed CO/CH4/H2 flame with various equivalence ratios (0.6–1.6) at standard conditions was numerically calculated using ANSYS CHEMKIN-PRO with the GRI-Mech 3.0 mechanism. The mole fraction profiles of the major species and the rate of production of dominant elementary reactions in the flames of CO/CH4/H2/N2/CO2/air were obtained. The effect of inert gas addition on the formation of NOX, H, O, and OH was analyzed, and the sensitivity coefficient of the active radical mole fraction was obtained. The results suggest that the addition of inert gas of the fuel mixture with various equivalence ratios reduces laminar burning velocity and adiabatic temperature, which have always had a good positive correlation and the maximum peak point shifted left. CO2 has obvious inhibitory effect on the formation of NO by reducing the amount of O radicals and obstructing the conduct of the reaction of NNH + O ⇔ NH + NO, but it promotes the formation of NO2 mainly through the reaction HO2 + NO ⇔ NO2 + OH. The reactions H + O2 + H2O ⇔ HO2 + H2O, H + O2 ⇔ O + OH, and OH + CO ⇔ H + CO2 are three very important reactions for the molar fractions of H, O, and OH that decrease significantly with an increase of inert gas concentration.