Negative Temperature Coefficient Region Research Articles

A shock tube study of the ignition delay time of DME/ammonia mixtures: Effect of fuel blending from high temperatures to the NTC regime

An experimental and chemical kinetic investigation has been conducted to understand the effect of dimethyl ether (DME) blending on the ignition delay time of ammonia (NH3) from high temperatures to the negative temperature coefficient (NTC) regime in a shock tube with the DME fractions of 0 %, 5 %, 25 %, 50 %, and 100 %, temperatures of 690–1810 K, pressures of 1.2 and 10 atm, and equivalence ratio of 1.0. Four representative literature chemical kinetic models were validated against our measurements, however none of the above models can provide reasonable predictions of the experimental data in the NTC regime. A chemical kinetic model was developed in this study, the model was also been validated by the ignition delay times, laminar flame speeds and specie profiles data of DME/ammonia mixtures in the literature. Experimental results show that the ignition delay times of NH3 can be obviously shorten by 5 % DME addition in both high temperature and the NTC region. Chemical kinetic analyses reveal that the addition of 5 % DME can rapidly generate OH radical through the reaction pathway of HO2 → H2O2 → OH at 1250 K. At 800 K, the blended DME can be rapidly consumed via the reaction pathway of R → RO2 → QOOH → OOQOOH to promote the production of OH radical in the early stage, thus lead to the reduced ignition delay time of DME/NH3 mixture.

On the NTC behaviors in explosion limits of C1 to C3 n-alkane/air mixtures

In this study, the explosion limits of methane, ethane and propane with air under various equivalence ratios were investigated. For the methane-air mixture, a Negative Temperature Coefficient (NTC) response progressively appears with an increase in equivalence ratio. The NTC response for ethane-air mixture remains relatively constant, and the explosion boundary shifts to high pressure in the high-temperature region. In propane-air mixtures, both the explosion limit curve in the NTC response section and the high-temperature section shift to high pressure, causing a double NTC response under high equivalence ratios. Detailed reaction path analysis confirmed that the primary reaction path of methyl radicals shifted from oxygen addition reactions to recombination reactions, leading to the NTC response and reduced reactivity. The decomposition of C2H5O2H promotes the reactivity of the ethane-air mixture, thereby generating the NTC response. Under high equivalence ratios, both the reaction path of the C3H7 radical via secondary oxygenation at low temperatures and the oxygen addition reactions of small alkane at high temperatures are hindered, leading to the generation of a double NTC response. Temperature and species profiles are compared under the NTC region for these fuels. These findings provide valuable insights and guidance for applications involving fuel-rich hydrocarbon combustion processes.

Formulation of a Jet Fuel Surrogate and Its Kinetic Chemical Mechanism by Emulating Physical and Chemical Properties of Real Jet Fuel

The application of jet fuel in gas turbines and diesel engines adheres to the Army’s single-fuel forward policy, streamlining supply chains. To ensure precise engine combustion numerical studies, surrogate fuels and mechanisms should faithfully replicate real fuel properties and combustion traits. In this work, a new four-component jet fuel surrogate containing 39.05% n-dodecane/21.79% isocetane/11.49% decalin/27.67% toluene by mole fraction is formulated based on a property optimizer. The new-formulated fuel surrogate can satisfactorily emulate the chemical and physical properties of real jet fuel, including cetane number (CN), threshold sooting index (TSI), molecular weight (MW), lower heating value (LHV), the ratio of hydrogen and carbon (H/C), liquid density, viscosity, and surface tension. Furthermore, a reduced and robust kinetic chemical mechanism (containing 124 species and 590 reactions) that could be directly employed in practical engine combustion simulations has also been developed for the proposed surrogate jet fuel. The mechanism is validated through comprehensive experimental data, including ignition delay time (IDT) determined in shock tubes and rapid compression machines (RCMs), species mole fractions measured in premixed flames and jet stirred reactors (JSRs), and laminar flame speeds. Generally, the property deviations of the jet fuel surrogate are less than 2% except for MW (10.73%), viscosity (5.88%), and surface tension (8.71%). The comparison results between the predictions and measurements are in good agreement, indicating that the current kinetic mechanism is capable of reflecting the oxidation process of real jet fuel. The current mechanism can accurately capture variations in the ignition delay time in the negative temperature coefficient (NTC) region as well. In the future, the proposed surrogate jet fuel could be applied in practical engine computational fluid dynamic (CFD) simulations.

Understanding the reversal dilution effect of the species sampling in the negative temperature coefficient region

Species sampling during fuel autoignition can provide valuable insights in studying the chemical kinetic mechanism. Various species sampling techniques have been developed for species quantification in different reactors. However, unavoidable dilution in the physical sampling brings uncertainty to the measurements. Particularly in the negative temperature coefficient (NTC) region, the effect of dilution remains unclear. To address this issue and improve measurement accuracy, numerical species sampling during the autoignition of n-butane mixture within the NTC region was studied in a rapid compression machine reactor. Results show that greater fuel consumption and intermediate generation were observed in regions with lower temperature instead of the hot core zone. Consequently, the dilution effect on the sampling results is reversed inside the NTC region. Furthermore, results from two typical sampling methods were compared with the target core profiles to determine their uncertainties. The feasibility of applying dilution ratio calibration on the sampling results of these two methods was also tested, which further indicates the necessity of considering the NTC effect, particularly for intermediates with lower concentrations. Alternatively, increasing the sampling duration of the fast-sampling method effectively reduces the uncertainty of species concentration and enhances the robustness of mechanism validation regardless of the NTC behavior.

On the Three-Stage Explosion Characteristics of Diethyl Ether-Oxygen Mixtures

ABSTRACT In this article, the three-stage explosion limits of diethyl ether (a typical alternative fuel with a high cetane number and high energy density) are taken as the research object. A comprehensive and in-depth analysis of the double negative temperature coefficient (NTC) behavior of the cool flame region and hot ignition region, through the detailed kinetic analysis, have been carried out. The results show that with the increase of the equivalence ratio, the double NTC phenomenon becomes more remarkable. In very lean conditions, such as the equivalence ratio of less than 0.5, the boundary between the first NTC region and the second NTC region becomes obscure, and a deeper single NTC phenomenon is formed as these two NTC regions merge. The whole explosion limit curve shifts to higher pressure conditions with the increase of the addition of inert gas. The NTC phenomenon in the moderate temperature region becomes more significant and the boundary between these two NTC regions is more obvious. In addition, the hot flame explosion limit also shows such two NTC response. The heat release rate (HRR) profile exhibits three-stage, two-stage, single-stage ignition, or non-ignition depending on the initial conditions. To elucidate the differences in the ignition process between the different explosion limit regimes, the sensitivity and reaction pathway analyses are performed. This study not only provides insight into the oxidation characteristics of ether fuels, but also provides theoretical support for high-performance engines.

Combustion characteristics of waste styrene–butadiene rubber oil derived from thermal pyrolysis process: experiment and numerical simulation

Owing to the similar carbon range in components, the pyrolytic oil derived from waste styrene–butadiene rubber (SBR) is preferred in aviation, for achieving waste-to-energy and fly net zero missions. The present study investigated the combustion and emission characteristics of SBR using a constant volume combustion chamber (CVCC) spray at 673–873 K initial temperature (Tini), 10 and 15 bar initial pressure (Pini), and 0.5 equivalence ratio (φ). SBR combustion properties were benchmarked with conventional jet propellant 5 (JP-5), and JP-5 and SBR were blended at a 50% volume ratio (JP-5-SBR-50). SBR has a shorter ignition delay (ID) and combustion delay (CD) at 15 bar Pini, compared to 10 bar. SBR exhibits negative temperature coefficient (NTC) behavior starting from Tini = 773–823 K, which was more significant at 10 bar. In the NTC region, at Tini = 723 and 823 K, the ID nearly has a 3-fold variation between 10 and 15 bar, and CD has 4- and 1.7-fold variations at Tini = 723 and 823 K, respectively. Finally, τ has a 3- and 1.5-fold variation at Tini = 723 and 823 K. JP-5 has a shorter ID and CD, followed by JP-5-SBR-50 and SBR. At Tini = 673 K, the addition of JP-5 to SBR significantly shortened ID and CD by nearly 24% and 17.5%. JP-5 has higher CO2, thermal NOx, and unburned HC emissions, whereas those were reduced with SBR addition to JP-5. Moreover, CO emissions at the SBR NTC region (773–823 K) gradually increased more than at any other tested Tini. Meanwhile, the measured ID periods and NTC characteristics for SBR were replicated using a reduced mechanism of 106 species and 2,162 reactions. The reduced mechanism also predicted the measured SBR inhibition effect on the ID periods for JP-5.

Weighted Network Degree Screening Method for Low-Temperature Combustion Mechanism Reduction.

A new method is proposed for the reduction mechanism used in the low-temperature negative temperature coefficient region on the basis of the statistical degree screening (SDS) method. Dynamic information is used to redefine network structure and exclude the influence of very weak interactions on node degree according to the statistics character of their distribution as edge weight. Representative low-temperature conditions are used to set weight thresholds to redefine the network structure so that an effective low-temperature oxidation mechanism is covered while negligible interactions are overlooked. Then, the reduction mechanism is obtained by the SDS method through screening out the redundant species and corresponding reactions according to the scale-free character of the degree distribution. This weighted network degree screening (WNDS) method is demonstrated in the n-heptane system. The performance of the reduced mechanism is evaluated in a closed homogeneous reactor for the fuel over T = 600-1000 K, P = 1-30 atm, and φ = 0.5-2. Results show WNDS yields a skeletal mechanism with comparable or even better prediction ability over a wide parameter range than those generated by directed relation graph. WNDS is a novel statistical property-based reduction method that is suitable for low-temperature oxidation reduction. Its good reduction application indicates a brand-new angle for large combustion mechanism reduction.

Oxidation of Methane/n-Heptane Mixtures in a High-Pressure Flow Reactor

To evaluate the use of diesel as a pilot fuel in natural gas combustion in two-stroke maritime engines, research on high-pressure oxidation of methane/n-heptane mixtures is of interest. In this study, laminar flow reactor experiments were conducted at pressures of 21 and 100 bar, temperatures in the range 450–900 K, and under stoichiometric and fuel-lean conditions. For all conditions, the n-heptane conversion starts below 600 K. A pronounced negative temperature coefficient region was observed at 21 bar. The n-heptane was depleted at all conditions before reaching 900 K. Methane oxidation was initiated once n-heptane was consumed. Compared to the oxidation of pure n-heptane and NH3/n-heptane mixtures, the presence of methane promoted n-heptane oxidation in the full temperature range. The model from Zhang et al. provided overall a good agreement with the experimental data. However, the low-temperature conversion of n-heptane at 21 bar and stoichiometric conditions is underpredicted, possibly because some chemical coupling between n-heptane and methane is missing in the model.

Dual-camera high-speed imaging of n-hexane oxidation in a high-pressure shock tube

Shock tubes are widely used in the study of chemical kinetics. Its benefits rely on the almost ideal shock-heating process that provides high temperatures and pressures to a chemical system for a limited test time. Just like any reactor, shock tubes are not immune to non-ideal effects. The study of conditions that might deviate experiments from ideal conditions is thus of the utmost importance. High-speed imaging has been proven to be a powerful tool to analyze non-ideal / non-homogenous combustion in shock tubes. In this work, dual-camera high-speed imaging experiments were performed at 10, 15 and 20 bar in a high-pressure shock tube (HPST). An optical section was designed as an extension of the HPST which enabled simultaneous visualization from the endwall and the sidewall of the driven section of the shock tube. n-Hexane, a fuel with a negative temperature coefficient (NTC) behavior that has been identified as prone to non-homogenous ignition, is used as a test fuel. Reactive mixtures and thermodynamic conditions were selected to visually analyze ignition processes at the high-temperature, NTC and low-temperature regimes. Non-homogeneous ignition was observed mostly at the local maximum of the IDT, which is comprised by the high-temperature and NTC regions. Stoichiometric n-hexane mixture with high fuel loading (5% n-hexane) presented the highest deviation from constant volume chemical kinetic simulations. The inclusion of helium as a bath gas to mitigate preignition was tested and it showed to improve the susceptibility of the mixtures to develop reaction fronts. The modified Sankaran criterion for the identification of ignition regimes in shock tubes was tested and it showed an overall good agreement against the experimental observations.

A computational and experimental study of the effects of thermal boundary layers and negative coefficient chemistry on propane ignition delay times

Experimental measurements of low-temperature ignition often exhibit higher uncertainties and scatter compared with ignition measurements at higher temperatures. One source of experimental variability and uncertainty is the effect of negative temperature coefficient (NTC) chemistry in thermal boundary layers at low-temperature conditions. The objective of the current work was to evaluate the effects of NTC chemistry and thermal boundary layers on propane ignition delay times using a combined modeling and experimental approach. A two-zone “balloon” model was developed where the core zone represented the high-temperature region of an experimental facility and an annulus zone represented the cooler thermal boundary-layer region. Independent initial conditions and elementary reaction chemistry was used in each zone. The effects of boundary-layer size were evaluated using different volume fractions (Vcore/Vtotal). At high initial temperatures (Tcore > 1000 K), there was negligible effect of NTC chemistry on the predicted ignition delay time for propane compared with a single-zone model, and the core region dominated the ignition characteristics for all sizes of thermal boundary layer. However, for lower initial temperature conditions (e.g., Tcore = 800 K), the thermal boundary layer was predicted to ignite first and all ignition delay times, regardless of the size of the boundary layer, were faster than the single-zone prediction for 800 K by up to 25%. Based on the results of the modeling study, propane ignition experiments were conducted using two rapid compression facilities (Vcore/Vtotal ≅ 0.9). at core temperatures of 744–1044 K and pressures of 9.8–25.4 atm for stoichiometric propane-air mixtures. While the measured ignition delay times were within the uncertainties of the both the single- and two-zone model predictions, high-speed imaging showed clear transition from ignition initially in the annulus region at lower temperatures to ignition in the core region at higher temperatures. The results of the study provide important new insights into the effects of NTC chemistry on ignition studies and support the use of diagnostics that can resolve core and boundary layer ignition behavior, particularly for experiments conducted in the NTC region. The two-zone model is also a valuable new tool for assessing the impact of NTC chemistry on previous and planned ignition delay time measurements.

Influence of noble metals on thermoacoustic oscillations and boundaries of the region of negative temperature coefficient during combustion of n-pentane – air mixtures

Using the combustion of n-pentane – air mixtures as an example, it was shown that thermoacoustic regimes of ignition disappear in the presence of a platinum surface, which generates catalytic centers propagating into the volume. Thus, the platinum catalyst eliminates a certain stage of kinetic mechanism, and the negative temperature coefficient phenomenon vanishes. In the presence of a palladium catalytic surface that does not generate catalytic centers propagating into the volume, the phenomenon of a negative temperature coefficient occurs.

N‐Heptane oxidation in a high‐pressure flow reactor

AbstractThe oxidation properties of n‐heptane have been investigated by conducting experiments in a laminar flow reactor at pressures of 21 and 100 bar, near‐stoichiometric () and fuel‐lean conditions (), and temperatures of 450–900 K. At 21 bar and stoichiometric conditions, a negative temperature coefficient (NTC) region was detected around 600 K. At increased pressure or oxygen concentration, the NTC behavior was much less pronounced. The observed concentration profiles for the major species were compared to predictions with selected literature mechanisms. While all models provided a satisfactory agreement for the n‐heptane profiles at 21 bar fuel lean conditions and at 100 bar, only the mechanisms from NUI and Polimi captured the NTC behavior at 21 bar and stoichiometric conditions. The model from Zhang et al. provided the best overall agreement and was selected for detailed comparison with species concentrations and used in analysis of reaction paths and reaction sensitivity under the present conditions.

Ethanol blending effects on auto-ignition and reaction wave propagation under engine-relevant conditions

Ethanol as an attractive oxygenated fuel is often blended with gasoline to yield beneficial anti-knocking performance. Due to the distinctions in fuel chemistry, the combustion behavior of gasoline-ethanol blends is not fully understood. This study investigated ethanol blending effects on auto-ignition and reaction wave propagation under engine-relevant conditions covering the negative temperature coefficient (NTC) region. Three ethanol blending levels (i.e., E10, E20, and E30 on a volume basis) were introduced into gasoline surrogates. Characteristic timescale analysis for homogeneous auto-ignition was conducted, and localized hotspot auto-ignition and reaction wave propagation was investigated for different gasoline-ethanol blends. Meanwhile, ethanol blending effects on detonation development were comparatively explored. The results show that ethanol addition significantly suppresses fuel reactivity for homogeneous scenarios, especially in low-temperature regions, manifesting prolonged timescales and diminishing NTC behavior. Despite this, different reaction wave propagation modes can be identified during transient auto-ignition and reaction wave propagation, including deflagration, detonation development, and thermal explosion. The detonation wave is always at an underdeveloped status under engine-relevant conditions, and the induced pressure characteristics are sensitive to initial temperatures, hotspot sizes, and fuel reactivity. Meanwhile, ethanol addition tends to promote transient detonation development by elevating the reaction front propagation speed inside the hotspot and enhancing the deflagration to detonation transition outside the hotspot. Such observations are not contrary to that ethanol yields beneficial anti-knocking performance because only chemical effects of ethanol blending are addressed, without considering vaporization cooling effects. The present work demonstrates that ethanol addition globally suppresses engine knocking, but it may induce stronger knocking intensity once end-gas auto-ignition and detonation development are initiated. The current study provides useful insights into ethanol perturbative effects on auto-ignition and reaction wave propagation from chemical perspectives.

Experimental Investigation on Auto-Ignition Characteristics of Kerosene Spray Flames

To facilitate the better use of RP-3 kerosene in compression ignition engines, the auto-ignition behaviors of RP-3 kerosene spray were experimentally investigated in an optical rapid compression machine. Results show that most of the tests have good ignitability and combustion performance. For all the successful ignited cases, the flame kernel was found to be formed before the steep rise of pressure, which explained that image-based ignition delay time is always shorter than the pressure-based ignition delay time. The effects of ambient environment, injection pressure, and injection delay time on the pressure history, ignition intensity, combustion duration, heat release rate, and other parameters were investigated individually. The ambient environment has a strong influence on ignition delay time by accelerating the chemical reactions, whereas the high injection pressure helps the better vaporization of fuel spray. The effect of injection delay time is non-monotonic as the trade-off relation between heat loss and blending of fuel and oxygen. The heat release rate histories under different conditions were compared and analyzed, and the two-stage heat release phenomenon was observed in the negative temperature coefficient region. The ignition intensity region was determined based on the measured ignition delay times of RP-3 kerosene spray, and multiple linear regression correlation was used to study the ignition delay time sensitivity to multi-factors.

An experimental and modeling study of acetylene-dimethyl ether mixtures oxidation at high-pressure

The oxidation of acetylene (as soot precursor) and dimethyl ether (DME, as a promising fuel additive) mixtures has been analyzed in a tubular flow reactor, under high-pressure conditions (20, 40 and 60 bar), in the 450–1050 K temperature range. The effect of varying the air excess ratio (λ≈0.7, 1 and 20) and the percentage of DME with respect to acetylene (10 and 40%) has been analyzed from both experimental and modeling points of view. The addition of DME modifies the composition of the radical pool, increasing the production of OH radicals which cause a shift in the onset temperature for C2H2 conversion to lower temperatures; the higher the amount of DME, the lower the temperature. The presence of DME favors the oxidation of C2H2 towards products such as CO and CO2, eliminating carbon from the paths that lead to the formation of soot. On the other hand, in the presence of C2H2, DME begins to be consumed at temperatures higher than those required for the high-pressure oxidation of neat DME, around 175–200 K more. Consequently, the negative temperature coefficient (NTC) region characteristic of this compound at low temperatures is not observed under those conditions. However, an additional analysis of the influence of DME inlet concentration (at 20 bar and λ=1) indicates that, if the amount of DME in the mixture is increased to 500 ppm and more (700 or 1000 ppm), the reaction pathways responsible for this high DME reactivity at low temperatures become more relevant and the NTC region can now be observed.

Effect of Ammonia Addition on the Ignition Delay Mechanism of Methyl Decanoate

In this study, the effect of mixing a small amount of ammonia on the ignition delay time of methyl decanoate under different conditions was studied from the perspective of the combustion mechanism. The effect of adding ammonia on the ignition delay time of methyl decanoate at different pressures and temperatures was studied by means of simulation calculations and numerical comparison. Integrating the detailed mechanism and reaction path of methyl decanoate, the sensitivity of the ignition delay time was investigated. Analyses of the ignition delay time and rate of production were conducted to explore the transformation and influence of ammonia on the oxidation/decomposition process of the main elementary reaction during the ignition of methyl decanoate. The research illustrated that the ignition delay time of methyl decanoate increased with the number of moles of mixed ammonia at a certain temperature range, and in the negative temperature coefficient region, the effect of ammonia on the ignition delay time was the greatest. In addition, the susceptibility and yield analysis of methyl decanoate showed that the addition of ammonia had a weakening effect on the elementary reactions that originally promoted and inhibited methyl decanoate, and its consumption and production rates were reduced.

Study on Chemical Kinetics Mechanism of Ignition Characteristics of Dimethyl Ether Blended with Small Molecular Alkanes

Dimethyl ether (DME)/C1-C4 alkane mixtures are ideal fuel for homogeneous charge compression ignition (HCCI) engines. The comparison of ignition delay and multi-stage ignition for DME/C1-C4 alkane mixtures can provide theoretical guidance for expanding the load range and controlling the ignition time of DME HCCI engines. However, the interaction mechanism between DME and C1-C4 alkane under engine relevant high-pressure and low-temperature conditions remains to be revealed, especially the comprehensive comparison of the negative temperature coefficient (NTC) and multi-stage ignition characteristic. Therefore, the CHEMKIN-PRO software is used to calculate the ignition delay process of DME/C1-C4 alkane mixtures (50%/50%) at different compressed temperatures (600–2000 K), pressures (20–50 bar), and equivalence ratios (0.5–2.0) and the multi-stage ignition process of DME/C1-C4 alkane mixtures (50%/50%) over the temperature of 650 K, pressure of 20 bar, and equivalence ratio range of 0.3–0.5. The results show that the ignition delay of the mixtures exhibits a typical NTC characteristic, which is more prominent at a low equivalence ratio and pressure range. The initial temperature of DME/CH4 mixtures of the NTC region is the highest. In the NTC region, the ignition delay DME/CH4 mixtures are the shortest, whereas DME/C3H8 mixtures are the longest. At low-temperature and lean-burn conditions, DME/C1-C4 alkane mixtures exhibit a distinct three-stage ignition characteristic. The time corresponding to heat release rate and pressure peak is the shortest for DME/CH4 mixtures, and it is the longest for DME/C3H8 mixtures. Kinetic analysis indicates that small molecular alkane competes with the OH radical produced in the oxidation process of DME, which inhibits the oxidation of DME and promotes the oxidation of small molecular alkane. The concentration of active radicals and the OH radical production rate of elementary reactions are the highest for DME/CH4 mixtures, and they are the lowest for DME/C3H8 mixtures.

Constant volume combustion chamber (CVCC) investigations of aerospace F-24 and Jet-A in low-temperature heat release and negative temperature coefficient regions

A study was conducted to determine the characteristics of Low Temperature Heat Release, Derived Cetane Number, and the Vibrations, produced by the combustion of aerospace F24 using Jet-A as a baseline in a Constant Volume Combustion Chamber. The Ignition Delay and Combustion Delay for F-24 were found to be 4.04 ms and 5.71 ms with a DCN resulting at 43.77. Jet-A was found to have a larger DCN of 47 with an ignition delay and combustion delay of 3.35 ms and 5.14 ms. F24 has an extended region of cool flame and releases more of its energy during LTHR than Jet-A, however has a much shorter NTCR than that of Jet-A. From the NVH analysis it was found that there exists a strong correlation between the ID and CD phases and their respective NVH spectrum. It was found that the magnitude of the ringing for Jet-A was 3x higher than that of F24. Meanwhile, the magnitude of vibrations produced by Jet A was approximately 0.5 g greater than those produced by F24′s combustion which correlates well with the higher peak value of AHRR for Jet A and also with higher ringing produced by Jet A compared with F24.

On the controlling mechanism of the lower turning point in the negative temperature coefficient regime

In this study, a comprehensive computational investigation is conducted to observe the negative temperature coefficient (NTC) behavior of the explosion limit of propane explored with detailed chemical kinetics, to find on the controlling mechanism of the lower turning point in the negative temperature coefficient region. The results show that the global behavior of the lower turning point exhibits distinct thermodynamic and chemical kinetic characteristics under different equivalence ratios from those thermodynamic and chemical kinetic characteristics under different inert gas ratios. Further chemical analyses demonstrate that the lower turning point is determined by the competition between chain termination reactions and chain branching/propagation reactions in the low-temperature reactions. Therefore, the ratio of the hydroxyl radical (OH) production rate to the total production rate of the alkyl radical (R) is constructed, which captures the turnover state of the lower turning point in the NTC regime.

A detailed experimental and kinetic modeling study on pyrolysis and oxidation of oxymethylene ether-2 (OME-2)

The application of oxymethylene ethers as an alternative fuel (additive) produced via carbon capture and utilization can lead to lower CO2 and particulate matter emissions compared to fossil fuels. To improve the understanding of the pyrolysis and oxidation chemistry of oxymethylene ether-2 (OME-2), a combined experimental and kinetic modeling study has been carried out. Pyrolysis experiments were performed using a quartz reactor over a broad temperature range, from 373 to 1150 K, to elucidate both the primary and secondary pyrolysis chemistry. The thermal decomposition of OME-2 is initiated via a dominant formaldehyde elimination reaction. Radical chemistry becomes only significant at higher temperatures (>800 K) and competes with unimolecular decomposition. Radicals originate mainly from the decomposition of carbenes. Important intermediate products formed during pyrolysis are dimethoxymethane, formaldehyde, methane and methyl formate. The formation of products with carbon-carbon bonds is minor since only carbon-oxygen bonds are present in OME-2. The oxidation chemistry was investigated between 600 and 715 K by ignition delay time measurements in a rapid compression machine for OME-2/air mixtures with an equivalence ratio ϕ of 0.5. No negative temperature coefficient region is observed. An elementary step kinetic model is constructed with the automatic kinetic model generator Genesys starting from the base mechanism AramcoMech 1.3. Important thermodynamic parameters and reaction rate coefficients to describe the low- and high-temperature decomposition chemistry are obtained from quantum chemical calculations. The new kinetic model satisfactorily reproduces the measured ignition delay times, as well as major product mole fractions from the pyrolysis experiments within the experimental error margin of 10% on average, without fitting thermodynamic or kinetic parameters. Finally, rate of production analyses reveal the important decomposition pathways to methyl formate, formaldehyde and others under pyrolysis and low-temperature oxidation conditions.