Laminar Flow Reactor Research Articles

First direct kinetic measurement of i-C4H5 (CH2CHCCH2) + O2 reaction: Toward quantitative understanding of aromatic ring formation chemistry

The kinetics of the i-C4H5 (buta-1,3-dien-2-yl) radical reaction with molecular oxygen has been measured over a wide temperature range (275–852 K) at low pressures (0.8–3 Torr) in direct, time-resolved experiments. The measurements were performed using a laminar flow reactor coupled to photoionization mass spectrometer (PIMS), and laser photolysis of either chloroprene (2-chlorobuta-1,3-diene) or isoprene was used to produce the resonantly stabilized i-C4H5 radical. Under the experimental conditions, the measured bimolecular rate coefficient of i-C4H5 + O2 reaction is independent of bath gas density and exhibits weak, negative temperature dependency, and can be described by the expression k3 = (1.45 ± 0.05) × 10−12 × (T/298 K)−(0.13±0.05) cm3 s−1. The measured bimolecular rate coefficient is surprisingly fast for a resonantly stabilized radical. Under combustion conditions, the reactions of i-C4H5 radical with ethylene and acetylene are believed to play an important role in forming the first aromatic ring. However, the current measurements show that i-C4H5 + O2 reaction is significantly faster under combustion conditions than previous estimations suggest and, consequently, inhibits the soot forming propensity of i-C4H5 radicals. The bimolecular rate coefficient estimates used for the i-C4H5 + O2 reaction in recent combustion simulations show significant variation and are up to two orders of magnitude slower than the current, measured value. All estimates, in contrast to our measurements, predict a positive temperature dependency. The observed products for the i-C4H5 + O2 reaction were formaldehyde and ketene. This is in agreement with the one theoretical study available for i-C4H5 + O2 reaction, which predicts the main bimolecular product channels to be H2CO + C2H3 + CO and H2CCO + CH2CHO.

Low- and high-temperature study of n-heptane combustion chemistry

n-Heptane has been used extensively in various fundamental combustion experiments as a prototypical hydrocarbon fuel. While the formation of polycyclic aromatic hydrocarbon (PAH) in n-heptane combustion has been studied preferably in premixed flames, this study aims to investigate the combustion chemistry of n-heptane in less-studied diffusion flame and highly rich high-temperature homogeneous oxidation configurations by using a counterflow burner and a flow reactor, respectively. This work addresses the formation of higher-molecular species in the mass range up to about 160 u in both configurations. Samples are analyzed by time-of-flight (TOF) molecular beam mass spectrometry (MBMS) using electron-impact (EI) and single-photon ionization (PI). Highly resolved speciation data are reported. Laminar flow reactor experiments cover a wide temperature range. Especially the measurements at low temperatures provide speciation data of large oxygenates produced in the low-temperature oxidation of n-heptane, which are scarce in the literature. Important precursor molecules for PAH and soot formation, such as C9H8, C10H8, C11H10, and C12H8, are formed during the high-temperature combustion process in the counterflow flame, while oxygenated growth species are observed under low-temperature conditions, even at the fuel-rich equivalence ratio of ϕ=4.00.Numerical modeling for both conditions is performed by using a newly developed kinetic model of n-heptane, which includes the n-heptane and PAH formation chemistry with state-of-the-art kinetic knowledge. Good agreement between model predictions and experimental data of counterflow flame and flow reactor is observed for the major species and some intermediates of n-heptane oxidation. While the concentrations of benzene and toluene measured in the counterflow burner are well-reproduced, the numerical results for flow reactor data are not satisfactory. Differences are found between the formation pathways of fulvene, from whose isomerization benzene is produced in diffusion flame and flow reactor.

Numerical investigation of pulverized coal particle group combustion using tabulated chemistry

The ignition and combustion of coal particle groups are investigated numerically in a laminar flow reactor. The Flamelet Generated Manifold method is extended to account for the complex mixture of gases being released during devolatilization, which is calculated with a competing two-step model. A second mixture fraction is introduced to include the mixing with the second methane fuel stream. The interactions of the gas phase with particles are modeled within a fully coupled Euler-Lagrange framework. To investigate the influence of particle groups on ignition and combustion, successively increasing densities of particle streams have been analyzed. The ignition delay time is increased significantly by higher particle densities. This delay is validated successfully with the available measurements. Moreover, the shape of the volatile flame was found to be strongly influenced by the particle number density inside the flame. A transition from spherical flames around single particles to a conical flame around the particle cloud could be found in numerical results as well as in experiments. As the primary mechanism for the substantial ignition delay and the formation of the flame, the increased heat transfer from the gas-phase to the particle group, resulting in lower gas-phase temperatures, was identified.

Ammonia oxidation features in a Jet Stirred Flow Reactor. The role of NH2 chemistry.

This work focuses on the characterization of the oxidation process of NH3/O2/N2 mixtures in a quartz Jet Stirred Flow Reactor (JSFR). Experimental tests are run as a function of mixture pre-heating temperatures (900–1350 K) and different equivalence ratios (Φ = 0.8, 1, 1.2), while keeping constant pressure, mixture residence time and mixture dilution level (d = 86%).Three different oxidation characteristic regimes are experimentally recognized as a function of the mixture pre-heating temperature (Tin), namely low (Tin < 1100 K), intermediate (1100 < Tin < 1225 K) and high (Tin > 1225 K) temperatures. Ammonia reactivity exhibits no-dependency on the mixture equivalence ratio for low-intermediate temperatures, while it is strongly dependent on it at higher ones.Some effort is devoted to evaluate the heterogeneous surface effects on the homogeneous ammonia oxidation process comparing the previous results with further tests obtained passivating the inner surface of the JSFR with water and comparing experimental results from two tubular Laminar Flow Reactors (LFR) made of different materials (quartz and alumina). Results suggest surface reactions mainly affect NO profiles, with no remarkable influence of key species concentrations.Numerical simulations show that tested models are not able to carefully reproduce the experimental results, specially within the low-intermediate temperature oxidation regimes. The analysis of the Reaction Rates highlights that ammonia oxidation is strongly dependent on the NH2 reaction routes at different temperatures. The discrepancy between the considered kinetic schemes can be ascribed to the different description of NH2 chemistry

Nitrate radical generation via continuous generation of dinitrogen pentoxide in a laminar flow reactor coupled to an oxidation flow reactor

Abstract. Oxidation flow reactors (OFRs) are an emerging tool for studying the formation and oxidative aging of organic aerosols and other applications. The majority of OFR studies to date have involved the generation of the hydroxyl radical (OH) to mimic daytime oxidative aging processes. In contrast, the use of the nitrate radical (NO3) in modern OFRs to mimic nighttime oxidative aging processes has been limited due to the complexity of conventional techniques that are used to generate NO3. Here, we present a new method that uses a laminar flow reactor (LFR) to continuously generate dinitrogen pentoxide (N2O5) in the gas phase at room temperature from the NO2 + O3 and NO2 + NO3 reactions. The N2O5 is then injected into a dark Potential Aerosol Mass (PAM) OFR and decomposes to generate NO3; hereafter, this method is referred to as “OFR-iN2O5” (where “i” stands for “injected”). To assess the applicability of the OFR-iN2O5 method towards different chemical systems, we present experimental and model characterization of the integrated NO3 exposure, NO3:O3, NO2:NO3, and NO2:O2 as a function of LFR and OFR conditions. These parameters were used to investigate the fate of representative organic peroxy radicals (RO2) and aromatic alkyl radicals generated from volatile organic compound (VOC) + NO3 reactions, and VOCs that are reactive towards both O3 and NO3. Finally, we demonstrate the OFR-iN2O5 method by generating and characterizing secondary organic aerosol from the β-pinene + NO3 reaction.

Comprehensive Data Set of Single Particle Combustion under Oxy-fuel Conditions, Part I: Measurement Technique

ABSTRACT An improved experimental methodology is presented that provides combustion data of single pulverized coal and biomass particles with a high level of detail. This is the first part of a two-article series. A stereoscopic imaging system based on four intensified CCD-cameras is calibrated for in-situ measurements of temperature, size, shape, and velocity of solid fuel particles in the diameter range of 30–300 µm. An elaborate approach for 3D shape reconstruction from orthogonal projections of single particles shows significantly improved accuracy, which is validated against particle samples being collected with a suction probe. The close-meshed combination of imaging pyrometry and shadowgraphy is a further novelty. The parallel application of both techniques provides results for both self-luminous “hot” (imaging pyrometry) and “cold” (shadowgraphy) particles. This enhances information on particle ignition (particles switch from cold to hot) and duration of char burn-out (particles switch from hot to cold). Selected experimental results are presented which demonstrate the informative power of data sets formed by this approach. Torrefied miscanthus is burned in a laminar flow reactor. The particle size and aspect ratio are evaluated for the in-situ measuring method as well as for the collected samples. The results show a good correlation between both analysis routines, indicating the progress in solid fuel characterization by the improved optical technique in combination with particle sampling.

Experimental and modeling study of the low to high temperature oxidation of the linear pentanone isomers: 2-pentanone and 3-pentanone

Short-carbon-chain ketones are known for their high octane numbers, their knock resistance, and their low soot emission. However, studies on the combustion behavior of ketones, particularly in the low temperature regime, are sparse, and small ketones as acetone and butanone show limited low temperature chemistry (LTC). Therefore, saturated linear five-carbon ketones are good candidates to better understand the LTC of ketones and the effect of the functional carbonyl group. In this study, the ignition delay times of stoichiometric non-diluted 2- and 3-pentanone-oxygen-inert gas mixtures were measured in a rapid compression machine at pressures between 20 and 40 bar and temperatures ranging between 650 and 950 K. Furthermore, the mole fraction profiles of fuel, oxygen, intermediate species, and products were measured in a laminar flow reactor coupled with molecular-beam mass spectrometry at 0.97 bar, an equivalence ratio of 0.8 and temperatures ranging from 800 to 1050 K, to achieve a better understanding of the oxidation process of linear pentanones. A consistent detailed kinetic model was developed and validated against the newly measured experimental data for 2-pentanone and 3-pentanone to provide insight into the oxidation mechanism of pentanones. The model shows also good agreement with available data from the literature, including ignition delay times measured in a shock tube, speciation in a flat flame burner and laminar burning velocities.

Overcoming diffusion limitations in electrochemical microreactors using acoustic streaming

We provide a theoretical background on laminar flow electrochemical reactors and rationally map typically encountered operating regimes in literature. At the highest (mass transfer limited) current densities attainable, the Fourier number (Fo) and the Sherwood number (Sh) are identified as important dimensionless groups for design considerations. The critical Fourier number places a severe constriction on the production capacity of a microreactor, as it determines the maximum throughput for a reactor of fixed length and inter-electrode distance. This critical Fourier number indicates that the absence of convective mixing and the reliance on diffusion of the electroactive species is the key problem to address for further intensifying electrochemical microreactors. Computer simulations are then used to illustrate the steady-state as well as transient behavior of generic case studies. These simulations also show that the transient response to a voltage step can be used to experimentally verify that the absence of mixing is a major bottleneck of laminar flow reactors. A versatile tubular electrochemical reactor was developed and applied to an acetonitrile solution containing ferrocene and benzylbromide, which is a simple model system for investigating self-supported synthesis (i.e., electrosynthesis without intentionally added supporting electrolyte). The reactor was then fitted with piezoelectric elements that excite a resonating standing wave in the flow direction, resulting in pairs of counter-rotating vortices in the fluid (acoustic streaming). The sonicated experiments prove that ultrasound assisted mixing reduces the critical Fourier number, enabling self-supported synthesis at elevated flow rates and throughput.

Effects of CO2 on soot formation in ethylene pyrolysis

Effects of CO2 on soot formation during ethylene pyrolysis were investigated in a laminar flow reactor with the addition of various amounts of CO2 (0–99.5% in mole fraction). Based on a quantitative dilution sampling technique and a scanning mobility particle sizer, soot particle size distributions and the associated global properties, including soot volume fraction, number density, and soot induction delay time, were examined. Results show that while addition of a small amount of the CO2 (0 - 10%) tends to promote soot formation as the total number and volume of soot particles increase and the soot induction delay time decreases, its further increase, from 10% to 99.5%, leads to an obvious reduction of the soot nucleation and mass growth rates. Subsequent kinetic modeling of the gas-phase chemistry showed that with increasing CO2 concentration, the corresponding concentrations of the soot precursors, namely benzene and pyrene, first increase and then decrease, which is consistent with the observed trend in soot formation. Further sensitivity and reaction path analyses of benzene formation indicate that CO2 addition produces more hydroxyl radicals, such that while the presence of a small amount of hydroxyl radicals increases the propargyl concentration and thereby promotes the formation of soot precursors, excessive hydroxyl radicals lead to more oxidation and hence inhibit soot formation.

Development and validation of surrogates for RP-3 jet fuel based on chemical deconstruction methodology

In this work, surrogates for RP-3 jet fuel have been developed based on chemical deconstruction methodology. In surrogate formulation, eight constraints have been taken into consideration, of which four factors are related to molecular structure, such as carbon number, hydrogen number, hydrogen/carbon ratio and degree of unsaturation and four others related to chemical composition, including concentrations of n-alkane, iso-alkane, cyclo-alkane and aromatics. In consideration of different weight coefficients, two kinds of surrogates S1 and S2 have been proposed. S1 surrogate is developed in particular consideration of the molecular structures while S2 focuses more on chemical compositions. Therefore, S1 surrogate is comprised of five components including n-dodecane, 2,2,4,4,6,8,8-heptamethylnonane, 2-methylheptane, decalin and o-xylene (0.058/0.342/0.019/0.418/0.163 by mole fraction) while S2 has the same components but in different proportions (0.171/0.184/0.038/0.497/0.11). In order to validate the S1 and S2 surrogates, comprehensive verification tests, including those on physical and chemical properties, have been conducted. Results showed that both surrogates had good performance in predicting the physical properties such as density, viscosity and surface tension. In terms of chemical characteristics, point-to-point validations of oxidation properties of surrogate and target fuels were conducted in a laminar flow reactor. Besides, autoignition properties of surrogates have been compared with experimental data of RP-3 jet fuel in a heated shock tube and rapid compression machine. It can be concluded that both of the S1 and S2 surrogates can well reproduce the oxidation properties of the target fuel, whereas S2 surrogate had better performance in predicting autoignition properties, especially in NTC region. Finally, surrogate models have been constructed and validated, including 2851 species and 12,242 reactions. These high-fidelity surrogate models have potentials in predicting combustion characteristics of RP-3 jet fuel.

Investigation of flame retarded polypropylene by high-speed planar laser-induced fluorescence of OH radicals combined with a thermal decomposition analysis

The combustion of micrometer-sized polypropylene (PP) particles is analyzed in situ using a combination of high-speed planar laser-induced fluorescence of the OH radical (OH-PLIF) and a thermal decomposition analysis. The gas phase is investigated by multiple analytical techniques to gain comprehensive knowledge on the decomposition products of flame retardants and their effect on the combustion process. Neat PP is compared with a formulation consisting of 10 wt% of a phosphorus-containing flame retardant (pentaerythritol spirobis(methylphosphonate), PSMP) which is known to provide gas phase activity. The decomposition of the neat flame retardant, PP and the flame retardant formulation is investigated using a simultaneous analysis (STA) consisting of a thermal gravimetric analysis and a differential thermal analysis device which is coupled to Fourier-transform infrared spectroscopy and mass spectrometry devices. By this, the release of decomposition products of the flame retardant additive can be determined. The excitation of OH radicals is used to temporally track the diffusion flame surrounding the particles during combustion in a laminar flow reactor. The radial distance to the peak reactivity zone of flame retardant containing particles increased by about 70 µm compared with neat PP particles. Tracking the peak OH signal in the diffusion flame, during ignition and the early phase of combustion, a decrease in the peak intensity is observed for flame retardant polymer particles. Additionally, cone calorimeter tests are used to evaluate the combustion behavior as a standard test in flame retardancy.

Oxidation and pyrolysis of ammonia mixtures in model reactors

The present work was devoted to an experimental characterization of NH3 oxidation and pyrolysis processes in model reactors. The oxidation tests were performed in a Jet Stirred Flow Reactor (JSFR) for stoichiometric ammonia/oxygen/argon and ammonia/oxygen/argon/water mixtures, as a function of mixture inlet temperature (Tin), at fixed pressure and residence time. Following, the behavior of ammonia/oxygen/argon mixture was analyzed in two tubular laminar flow reactors (LFRs) made of different materials (quartz and alumina), to evaluate potential surface heterogeneous effects.Pyrolysis tests were performed in both the reference model reactors, by substituting oxygen with argon.Experimental results realized in the JSFR allowed to identify three different kinetics regimes in ammonia oxidation: low (Tin < 1130 K), intermediate (1130 < Tin < 1250 K) and high temperatures (Tin > 1250 K). Surface heterogeneous reactions seemed to be negligible on ammonia reactivity and hydrogen profiles, but some effect could be observed on NOx concentration in the low-intermediate temperature range. Experiments realized in presence of water suggested that such species may passivate the surface, minimizing heterogeneous effects.Experimental tests realized in the LFRs confirmed these results, as hydrogen and oxygen profiles were independent of the reactor material, while NOx concentrations exhibited significant changes. Experimental results suggested that heterogeneous reactions were more relevant under pyrolytic conditions.Numerical simulations were performed with different kinetic mechanisms available in literature to value their capability to describe ammonia chemistry. None of them could accurately reproduce the experimental data for ammonia oxidation process, while they completely failed to predict ammonia pyrolysis features.

Effect of Methyl Group Substitution on the Kinetics of Vinyl Radical + O2 Reaction.

The kinetics of (CH3)2CCH + O2 (1) and (CH3)2CCCH3 + O2 (2) reactions have been measured as a function of temperature (223-600 K) at low pressures (0.4-2 Torr) using a tubular laminar flow reactor coupled to a photoionization mass spectrometer (PIMS). These reactions are important for accurate modeling of unsaturated hydrocarbon combustion. Photolysis of a brominated precursor by a pulsed excimer laser radiation at 248 nm wavelength along the flow reactor axis was used for the production of radicals. The measured bimolecular rate coefficient of reaction 1 shows a negative temperature dependence over the temperature range 223-384 K and becomes temperature independent at higher temperatures. The bimolecular rate coefficient of reaction 2 exhibits a negative temperature dependence throughout the experimental temperature range. The bimolecular rate coefficients of reactions 1 and 2 are expected to be at the high-pressure limit under the current experimental conditions, and the following values are obtained at 298 K: k1(298 K) = (4.5 ± 0.5) × 10-12 cm3 s-1 and k2(298 K) = (8.9 ± 1.0) × 10-12 cm3 s-1. The observed products for reactions 1 and 2 were CH3COCH3 and CH3 + CH3COCH3, respectively. Substituting both β-hydrogens in the vinyl radical (CH2CH) with methyl groups decreases the rate coefficient of the CH2CH + O2 reaction by about 50%. However, the rate coefficient of the triply substituted (CH3)2CCCH3 radical reaction with O2 is almost identical to the CH2CH + O2 rate coefficient under the covered temperature range.

층류 유동반응기를 이용한 프로핀 반응 실험 및 축소 메커니즘의 검증

층류 유동반응기를 이용한 프로핀 반응 실험 및 축소 메커니즘의 검증

Predictions of oxidation and autoignition of large methyl ester with small molecule fuels

Biodiesel contains large methyl esters including methyl palmitate (MP), methyl oleate (MO), methyl linoleate (ML), etc. Long alkyl chains and asymmetric structures of these large methyl esters lead to complexity in development of kinetic models. In this paper, surrogate fuels for large methyl esters were proposed and developed. Surrogate models can be used to replace the detailed mechanisms of large methyl esters in prediction of oxidation and autoignition properties. Key characteristics in combustion have non-linear relationships with fuel components in a neural network diagram, which indicates that surrogates with the same functional groups can exhibit similar combustion properties as large methyl esters. In formulation of surrogate fuels, candidates consist of small species such as methyl decanoate, n-hexadecane, methyl trans-3-hexenoate, and 1, 4-hexadiene. To compare the oxidation properties, point-to-point experimental validations were conducted in laminar flow reactor. Oxidation properties of surrogate and target fuels were in satisfactory agreements. Surrogate fuels not only successfully captured the discrepancies in reactivity of different methyl esters, but also reproduced species concentrations of intermediate and products at various equivalence ratios. To test the performance in predicting autoignition characteristics, simulations based on surrogate models were compared with previous experimental data. Surrogate models not only predicted ignition delay times at various pressure and equivalence ratios, but also reproduced the trends of apparent activated energy under different conditions. To sum up, oxidation and autoignition characteristics of large methyl esters can be predicted by the combined kinetic model of small molecule compounds, which will have great potentials in the development of kinetic models for fuels with complicated molecular structures.

High-pressure pyrolysis and oxidation of DME and DME/CH4

The pyrolysis and oxidation of dimethyl ether (DME) and its mixture with methane were investigated at high pressure (50 and 100 bar) and intermediate temperature (450–900 K). Mixtures highly diluted in nitrogen with different fuel–air equivalence ratios (Φ=∞, 20, 1, 0.06) were studied in a laminar flow reactor. At 50 bar, the DME pyrolysis started at 825 K and the major products were CH4, CH2O, and CO. For the DME oxidation at 50 bar, the onset temperature of reaction was 525 K, independent of fuel–air equivalence ratio. The DME oxidation was characterized by a negative temperature coefficient (NTC) zone which was found sensitive to changes in the mixture stoichiometry but always occurring at temperatures of 575–625 K. The oxidation of methane doped by DME was studied in the flow reactor at 100 bar. The fuel–air equivalence ratio (Φ) was varied from 0.06 to 20, and the DME to CH4 ratio changed over 1.8–3.6%. Addition of DME had a considerable promoting effect on methane ignition as the onset of reaction shifted to lower temperatures by 25–150 K. A detailed chemical kinetic model was developed by adding a DME reaction subset to a model developed in previous high-pressure work. The model was evaluated against the present data as well as data from literature. Additional work is required to reconcile experimental and theoretical work on reactions on the CH3OCH2OO PES with ignition delay measurements in the NTC region for DME.

Measurement and modelling of the laminar burning velocity of methane-ammonia-air flames at high pressures using a reduced reaction mechanism

Ammonia and blends of ammonia with methane are gaining increased interest as fuels for gas turbine applications and hence optimized reduced reaction mechanisms for the fuels are required for the development of combustors. However, there is a scarcity of measured data on the laminar burning velocity of the fuels for optimizing and validating reaction mechanisms especially at high pressures. In this study, an extensive set of measurements of the unstretched laminar burning velocity and Markstein lengths of CH4NH3-air flames at high pressures are reported for the first time and an optimized reduced reaction mechanism is proposed. The experiments were conducted in a constant volume chamber for various ammonia heat fractions in the fuel ranging from 0 to 0.30, equivalence ratios ranging from 0.7 to 1.3, and mixture pressures ranging from 0.10 to 0.50 MPa. The reduced reaction mechanism was developed from a detailed reaction mechanism for CH4NH3-air flames by Okafor et al., and optimized against the present measurements and data in the literature. It is shown that the reduced mechanism models the unstretched laminar burning velocity of NH3/air and CH4NH3-air flames with high fidelity at all studied conditions. It also models satisfactorily NH3, NO and CO concentration in CH4NH3O2N2 oxidation in a laminar flow reactor. Furthermore, the reduced mechanism is demonstrated to predict NO, OH, and NH profiles in a premixed stagnation NH3-air flame satisfactorily. The experimental measurements were also used to validate selected detailed reaction mechanisms. It was found that the significant over-prediction of NO production from NH3 oxidation by GRI Mech 3.0 is primarily due to the influence of the reaction NH+H2OHNO+H2, which in fact may not be important in fuel NO chemistry.

Detailed kinetic modeling of dimethoxymethane. Part II: Experimental and theoretical study of the kinetics and reaction mechanism

In this study (Part II), the oxidation of dimethoxymethane (DMM) is investigated and a detailed chemical reaction model developed for a comprehensive description of both high- and low-temperature oxidation processes. The sub-mechanism of DMM is implemented using AramcoMech2.0 as the base mechanism. Rate coefficients are based on analogies with those for dimethyl ether, diethyl ether, and n-pentane oxidation. Furthermore, theoretical studies from recent works are also included in the present model and new calculations for the dissociation kinetics of Q˙OOH radicals have been carried out at the CCSD(T)/CBS(aug-cc-pVXZ; X = D, T) // B2PLYP-D3/6-311 + + G(d,p) level of theory. For validation, new ignition delay time experiments have been performed in a shock tube (ST), a rapid compression machine (RCM), and in a laminar flow reactor covering a wide range of conditions (p = 1–40 bar, T = 590–1215 K, φ = 1). In addition, the kinetic model is validated against laminar burning velocities, jet-stirred reactor, plug flow reactor and further ST and RCM experimental datasets from the literature. Pathway and sensitivity analyses were used to identify critical reaction pathways in the DMM oxidation mechanism. These show that the reactivity of DMM at intermediate temperatures is controlled by the branching between pathways initiated on the primary or secondary fuel radical. While primary fuel radicals eventually lead to chain branching, secondary fuel radical consumption is controlled by fast β-scission over a wide range of temperatures, which inhibits reactivity.

Laminar flow reactor experiments for ignition delay time and species measurements at low temperatures: Linear alkanes and dimethyl ether

A laminar flow reactor has been developed for measurements of first stage ignition delay times and intermediate species at low temperatures. The design concept, technical details, and data evaluation schemes are presented and discussed in detail. Ignition delays of up to one second can be measured in the laminar flow reactor, which extends the measurement range of rapid compression machines and shock tubes significantly. First stage ignition delay time measurements were performed for a wide variety of fuels with well-validated low temperature chemistry to provide the initial classification of the laminar flow reactor. Further, the low temperature asymptote was clearly measured for the investigated fuels. For the linear alkanes n-pentane, n-hexane, n-heptane, and n-decane, as well as for dimethyl ether, ignition delay times were obtained at stoichiometric condition and atmospheric pressure. These ignition delays were compared against recent kinetic models and experimental data from the literature. Very good agreement was found for n-pentane, n-decane, and dimethyl ether. Interestingly, the agreement is less satisfactory for n-hexane, whose model (Zhang et al., 2016) is hierarchically built on the n-pentane model (Bugler et al., 2015) and for n-heptane, which is modeled using the same n-alkane reaction mechanism (Cai et al., 2016) as for n-decane. For n-heptane, the agreement with the ignition data of Campbell et al. (2015) is far better than the agreement with the model.

Kinetics of the Methyl-Vinyl Radical + O2 Reactions Associated with Propene Oxidation.

The bimolecular rate coefficients of reactions CH3CCH2 + O2 (1) and cis/trans-CH3CHCH + O2 (2a/3a) have been measured using a tubular laminar flow reactor coupled with a photoionization mass spectrometer (PIMS). These reactions are relevant in the combustion of propene. Pulsed excimer laser photolysis of a ketone or a bromide precursor molecule at 193 or 248 nm wavelength was used to produce radicals of interest homogeneously along the reactor. Time-resolved experiments were performed under pseudo-first-order conditions at low pressure (0.3–1.5 Torr) over the temperature range 220–660 K. The measured bimolecular rate coefficients were found to be independent of bath gas concentration. The bimolecular rate coefficients possess negative temperature dependence at low temperatures (T < 420 K) and appear to be independent of temperature at high temperatures (T > 420 K). Observed products of the reaction CH3CCH2 + O2 were CH3 and H2CO, while for the reaction cis/trans-CH3CHCH + O2, observed products were CH3CHO and HCO. Current results indicate that the reaction mechanism of both reactions is analogous to that of C2H3 + O2. Methyl substitution of the vinyl radical changes its reactivity toward O2 upward by ca. 50% if it involves the α-position and downward by ca. 30% if the methyl group takes either of the β-positions, respectively.