Measured Ignition Delay Research Articles

Role of volatile secondary char on the combustion behavior of cellulose-based hydrochars

Hydrothermal carbonization (HTC) can transform a wide range of biomass into renewable biofuels. Hydrochar, the solid fuel resulting from HTC, has a similar energy density to low-rank coals. Despite an extensive body of literature detailing the thermogravimetric analysis (TGA) of hydrochar under slow pyrolysis and oxidation, there remains a limited understanding of hydrochar's behavior in realistic combustion settings. In this work, we integrate combustion experiments and TGA to understand the combustion characteristics of a model hydrochar fuel. Cellulose-based hydrochars produced at different temperatures along with solvent-extracted chars are tested in a Hencken burner across varying temperatures and oxygen concentrations in the oxidizer gas. Simultaneous CH* chemiluminescence, particle image velocimetry, and two-color pyrometry are used to measure ignition delay time and identify homogeneous/heterogeneous ignition mechanisms and combustion phases. Incorporating TGA with combustion results shows that the ignition mode and combustion processes are strong functions of surrounding gas temperature and oxygen mole fraction. The level of carbonization of the hydrochar dictates the ignition delay time and combustion modes. Further, the presence of a tar-like secondary char on the as-carbonized hydrochars leads to more rapid ignition due to high volatility.

Exploring intermediate temperature reactivity: Experimental and kinetic modeling insights into 50/50% blend of methyl butanoate and methyl crotonate

In this work, an experimental and kinetic modeling study has been conducted to bridge the knowledge gap pertaining to the reactivity of blends of saturated and unsaturated methyl esters at intermediate temperatures. We consider two well-known biodiesel surrogates: the saturated ester methyl butanoate and the unsaturated ester methyl crotonate. These compounds were considered due to the wealth of experimental and modeling data available in the literature for the molecules. We conducted experiments to measure ignition delay times of mixtures of methyl butanoate and methyl crotonate using a Rapid Compression Machine (RCM) across different equivalence ratios (0.5, 1.0, 1.5) and at pressures of 20 and 30 bar. Furthermore, we developed an improved kinetic model that thoroughly validates the newly obtained RCM experimental data for the blend of methyl butanoate and methyl crotonate, in addition to existing experimental studies for the individual components. Present kinetic modeling work involved the incorporation of several new reaction pathways and rates, resulting in improved predictions of combustion characteristics. This study offers insights into the various reaction pathways that influence the reactivity of the blend as well as explains the interactions between methyl butanoate and methyl crotonate based on the underlying kinetics.

Experimental and data-driven chemical kinetic modeling study of alcohol-to-jet (ATJ) synthetic biofuel for sustainable aviation fuels

This work assesses the chemical ignition delay behavior of an isoparaffinic alcohol-to-jet (ATJ) fuel, one of the certified sustainable aviation fuels (SAFs). Chemical ignition delay measurements of ATJ were performed at a compressed pressure of PC = 2 MPa and three equivalence ratios (phi = 0.5, 1.0, & 1.3) in synthetic dry air, between 667 K and 1250 K. The unique chemical structures of isoalkanes result in high thermal decomposition rate, but low chemical oxidation reactivity. The new lumped mechanism (ARLMech-HC-ATJ) is developed using a genetic algorithm based on empirical ignition delays. The mechanism shows good performance over the temperature range from 667 K to 1250 K and varied equivalence ratios. Using the suggested mechanism, this study presents sensitivity analysis for the temperature range to understand chemical kinetics of ATJ fuel. The results show the impact of using ATJ modeling to design and optimize propulsion systems operating in the negative temperature coefficient (NTC) or low temperature chemistry (LTC) regime in the future and a methodology that can be adapted to other novel fuels.

Experimental measurements of ultra-lean hydrogen ignition delays using a rapid compression machine under internal combustion engine conditions

In order to mitigate greenhouse gas and pollutant emissions from the transportation sector, the use of green e-fuels is planned. Hydrogen spark ignition engines (H2ICEs) are one of the most important technologies for reducing dependence on fossil fuels. However, these combustion engines are prone to some abnormal combustion phenomena, such as knocking, and require further investigation to get a better understanding of hydrogen combustion properties. This study used a rapid compression machine to present experimental ignition delay measurements of ultra-lean hydrogen/air mixtures under internal combustion engine conditions. The fuel–air ratio ranged from 0.2 to 0.5, the compression temperature varied from 900 to 1030 K, and the compression pressure ranged from 20 to 60 bar. A comparison with often-used and up-to-date hydrogen kinetic mechanisms was performed. The experimental results were consistent and showed an overall good agreement with numerical simulations. The impact of 2 HO2 ⇌ 2 OH + O2 addition on recent kinetic mechanisms was also investigated and presented an average decrease of 20% in ignition delay. Finally, new third-body efficiencies of H2O were evaluated and showed no impact on prediction. The aim of this approach was to provide new experimental data for kinetic mechanism validation and optimization.

High temperature oxidation of residual oil pyrolysis intermediates for modeling gasification and combustion processes

Developing chemical kinetic models for residual oil pyrolysis, gasification, and combustion is very challenging due to the large number of components and molecular complexity. This study provides experimental data for model development and validation for two residual oils, heavy fuel oil (HFO) and vacuum residual oil (VRO). This study aims to identify and quantify the HFO and VRO pyrolysis intermediates and validate an oxidation chemical kinetic model of the pyrolysis intermediates to provide data for future residual oil model development using hybrid chemistry (HyChem) and functional groups for mechanism development (FGMech) approaches. The pyrolysis experiments were conducted in a bench-scale tubular reactor at atmospheric pressure and a temperature of 1273 K. Detailed compositions of the yields from the pyrolysis of HFO and VRO were analyzed using gas chromatography-flame ionization detection (GC-FID) for the gas yields and gas chromatography-mass spectrometry/flame ionization detection (GC–MS/FID) for the liquids. Elemental analysis was conducted for the solid residuals. The experiment results show similar pyrolysis products from HFO and VRO. A surrogate of major components of the gas products and representatives of benzene and naphthalene derivatives (major compounds identified in liquid products) of the liquid products was formulated and its chemistry was studied by measuring ignition delay times (IDTs) in a shock tube. IDT measurements were conducted for stoichiometric and rich (ϕ = 1 and 3) mixtures, pressures of 20 and 40 bar, and diluents of nitrogen and carbon dioxide. Brute force IDT sensitivity analyses were used to study the effect of pressure, equivalence ratio and diluents on the reactivity of the mixture.

Kinetic model-based group contribution method for derived cetane number prediction of oxygenated fuel components and blends

A four-step autoignition model was used to derive an expression for the ignition delay (ID) measured in ignition quality testers (IQT) with the derived cetane number (DCN) determined from this ID using the ASTM D6890 standard correlation. The model predicts DCN for individual compounds and blends as a function of each compound's global initiation and net chain branching rate constants. Expressions for these values were determined assuming they could be related to the functional groups present in each compound. Measurement data for 125 compounds and 94 binary and ternary blends (including oxygenates: alcohols, aldehydes, esters, ethers, and ketones), from literature and from measurements performed in our lab, were used to obtain the dependence of the measured ignition delay on each functional group. The new kinetic model-based group contribution method was able to predict the ignition delay of both pure compounds and blends, with an average DCN error of 4.4 (19%) and 2.8 (11%), respectively. The blend model was also used to develop an ID mixing rule by incorporating existing IQT ignition delay data for each compound. Use of the mixing rule gave an average DCN error of 3.6 (16%) for blends. Both the blend model and mixing rule were found to be superior compared to standard linear by volume fraction or mole fraction mixing rules commonly used to estimate the DCN of mixtures.

High‐Speed, microwave ignition: Antenna igniters with frequency and bandwidth selectivity

AbstractThis effort demonstrates the development of (1) dielectric breakdown igniters and (2) bridgewire igniters powered by half‐wavelength dipole microwave microstrip antennas with bandwidth selectivity for use in standoff ignition applications. Dielectric igniters utilized a dielectric epoxy filled with up to 30 wt. % nanothermite cast between the gap of two quarter wavelength microstrips. Bridgewire igniters employed a wire soldered across the gap of two quarter wavelength microstrips. A variety of different bridgewire materials were tested in diameters ranging from 76 μm to 203 μm. The ignition delay of resonant microwave igniters is measured from high‐speed video observation of the microwave illumination process within a free‐space anechoic microwave cavity driven by a 2 kW continuous magnetron source. Dielectric breakdown igniters produced ignition delays ranging from 400 ms to 850 ms. Ignition delay decreased with higher loading of thermite. Bridgewires were found to be highly efficient and measured ignition delays ranged from 17 ms to 271 ms (24.4 J to 457.1 J ignition energy). Smaller diameter bridgewires and high resistivity metals were found to produce the shortest ignition delays. We demonstrate through S‐parameter measurements the bandwidth selectivity of bridgewire igniters and show that control of dipole geometry enables tuning of igniter center frequency and bandwidth and demonstrate experimentally the ability of bridgewire igniters to reject accidental ignition from an off frequency high power microwave field. The tunability, bandwidth selectivity, and low energy requirements, in particular of bridgewires, make them attractive for a number of new and challenging ignition applications.

Ignition delays of two ADN/GAP mixtures irradiated by a CO2 laser: atmospheric experiments and numerical calculations

The ignition experiments of two ADN/GAP mixtures irradiated by a CO2 laser were conducted to investigate the dominant mechanism of ignition induced process. The power law exponents of experimental curves for ignition delay versus laser power density are all closed to -1. Two reduced numerical models were employed to calculate the ignition delay times against the experimental data. The analysis suggested that the ignition delays of ADN/GAP mixtures related to the corresponding burning rates and the ignition delay behaviours followed a fixed energy pattern approximately. The calculation results indicated that the measured ignition delay time might be mainly dominated by the formation of preheating layer for steady state burning.

A wide range experimental and kinetic modeling study of the oxidation of 2,3-dimethyl-2-butene: Part 1

2,3-Dimethyl-2-butene (TME) is a potential fuel additive with high research octane number (RON) and octane sensitivity (S), which can improve internal combustion engine performance and efficiency. However, the combustion characteristics of TME have not been comprehensively investigated. Thus, it is essential to study the combustion characteristics of TME and construct a detailed chemical kinetic model to describe its combustion. In this paper, two high-pressure shock tubes and a constant-volume reactor are used to measure ignition delay times and laminar flame speeds of TME oxidation. The ignition delay times were measured at equivalence ratios of 0.5, 1.0, and 2.0 in “air”, at pressures of 5 and 10 bar, in the temperature range of 950 – 1500 K. Flame speeds of the TME/ “air” mixtures were measured at atmospheric pressure, at a temperature of 325 K, for equivalence ratios ranging from 0.78 to 1.31. Two detailed kinetic mechanisms were constructed independently using different methodologies; the KAUST TME mechanism was constructed based on NUIGMech1.1, and the MIT TME mechanism was built using the Reaction Mechanism Generator (RMG). Both mechanisms were used to simulate the experimental results using Chemkin Pro. In the present work, reaction flux and sensitivity analyses were performed using the KAUST mechanism to determine the critical reactions controlling TME oxidation at the conditions studied.

Effects of natural-gas blending on ignition delay and pollutant emission of diesel fuel for the condition of homogenous charge compression ignition engine

• Effects of blending of natural gas (NG) with diesel fuel were investigated. • Major properties of diesel were measured at 40 bar and its surrogate fuel was formulated. • Blending of NG reduces pollutant emission with slight decrease in heat production. • Ignition timing is changed by NG blending, but, it is not significant. • The optimal mixture ratio of NG in the dual fuel is suggested in HCCI engines. Burning of dual fuel of diesel and natural gas is a promising technique in compression ignition engine in the aspects of fuel economy and reduced emission. Effects of blending of natural gas with diesel fuel are investigated numerically and experimentally with respect to ignition delay, heat production, and pollutant emission. First, major properties of diesel are measured at high pressure of 40 bar, which is an engine-relevant condition, and thereby, a surrogate fuel with higher accuracy is formulated, which has properties closer to diesel than the other existing surrogate fuels. Especially, as one of key properties, high molecular weight of diesel is considered critically in formulating a surrogate fuel. Ignition delay times of several mixtures of diesel and natural gas (NG), of which mixture ratios of 100/0%, 60/40%, and 20/80%, are measured by a shock tube in the ranges of temperature from 770 K to 1,000 K and of equivalence ratios from 0.5 to 2.0. Measured ignition delay is compared with that calculated by numerical simulations with the surrogate fuel formulated in this study for its validation. Finally, combustion simulations of HCCI (homogeneous charge compression ignition) are conducted with the mixture of diesel and NG and compared with available experimental data. And thereby, optimal mixture ratio of the dual fuel is suggested in terms of ignition time and pollutant emission acceptable in HCCI engines.

The influence of thermochemistry on the reactivity of propane, the pentane isomers and n-heptane in the low temperature regime

The influence of thermochemistry on the reactivity of fuels at low temperatures (600–1000 K) is studied here. Specifically, the effect of different sets of thermochemistry on chemical model predictions is explored, where various sets are calculated at different levels of theory in addition to recently updated group additivity values. Experimentally measured ignition delay times for propane, the pentane isomers and n-heptane are simulated using NUIGMech1.2 and replacing the thermochemistry of the low-temperature species with the calculated values. For propane, three different thermochemistry sets were calculated, namely CCSD(T)-F12/TZ-F12//B2PLYP-D3/TZ//B2PLYP-D3/TZ (QM1), CCSD(T)-F12/TZ-F12//B2PLYP-D3/TZ//ωB97X-D/TZ (QM2) and B2PLYP-D3/TZ/ωB97X-D/6-31G*//ωB97X-D/6-31G* (QM3). The QM2 results provide parameters to optimize new group additivity (NGA) values which are used to calculate the fourth set of thermochemistry. The model predictions using these four sets are compared to those using NUIGMech1.2 for propane. As the QM1 and QM2 calculations are expensive, the thermochemistry calculated from the QM3 and NGA calculations are used in the pentane isomer and n-heptane models. For all of the models, it is found that the thermochemistry of the species involved in the low-temperature reaction sequence (RH, Ṙ, RO2H, RȮ2, Q˙OOH and Ȯ2QOOH species) significantly affect fuel reactivity. The NGA values were developed based on all of these species except Ȯ2QOOH radicals. The thermochemistry of Ȯ2QOOH species cannot be accurately calculated with the NGA representations due to the importance of non-next-nearest neighbor interactions of –OOH substitution. Further development of the NGA method to capture such interactions is in progress. Overall, the model developed using the NGA thermochemistry shows better agreement with experimental data than the model using thermochemistry from affordable and prominent QM methods, such as QM3. Based on the results presented for propane, the pentane isomers and n-heptane, the thermochemistry calculated using the NGA method can be used to model the oxidation of higher order hydrocarbons at low temperatures.

Investigation of cyclohexene thermal decomposition and cyclohexene + OH reactions

Cyclohexene is a common intermediate in the oxidation of cyclohexane and alkyl-substituted cyclohexanes. It also plays an important role in the formation of polycyclic aromatic hydrocarbons (PAHs) by providing a route for the first aromatic ring. Two types of reactions are highly important in cyclohexene kinetics, i.e., H-abstraction by OH radicals and cyclohexene thermal decomposition. In this work, we have investigated these two reactions experimentally by employing sensitive UV diagnostics of OH radicals and 1,3-butadiene. We conducted rate coefficient measurements of cyclohexene + OH reaction (k1) over 933–1259 K and 1–4 bar using a narrow-linewidth UV absorption diagnostic of OH radicals near 306.67 nm. The carefully designed test conditions minimized the influence of secondary reactions such as cyclohexene thermal decomposition. Our determined OH + cyclohexene high-temperature rate coefficients show a positive temperature dependence and a negligible pressure dependence. Our rate values may be fitted with a two-parameter Arrhenius expression (units of cm3molecule−1s−1):k1=1.28×10−10e(−1225T)We also investigated channel-specific competition in OH + cyclohexene reaction. H-abstraction by OH from allylic (kc), alkylic (kb), and vinylic (ka) CH bonds contributes roughly 60%, 35%, and 5% over our temperature range. Our determined kc agrees excellently with literature rate coefficient of H-abstraction from the allylic site of 1-butene.We studied cyclohexene thermal decomposition reaction (k2) by tracing the product 1,3-butadiene near 212.5 nm over 1092–1361 K and 0.82–1 bar. Compared to literature works, our highly sensitive UV diagnostic of 1,3-butadiene enabled time-resolved measurements with low cyclohexene concentration, which guaranteed nearly isothermal conditions despite reaction endothermcity. Our measured rate coefficients may be expressed as (unit of s−1):k2=3.68×1014e(−31,562T)We implemented our determined rate coefficients (k1 and k2) in a literature cyclohexene kinetic model and also updated the 1,3-butadiene sub-mechanism. The updated model shows improved predictions of measured ignition delay times of cyclohexene.

Experimental and chemical kinetic modeling study of trimethoxy methane combustion

The use of oxymethylene ethers (OMEs) as alternative fuels or fuel-additives has motivated the present study on the combustion behavior of trimethoxy methane (TMM), which is a branched version of OMEs. A detailed chemical kinetic model for TMM combustion is provided, which is based on a recent model for the structurally similar dimethoxy methane (DMM) and recent elementary reaction kinetics studies. This model is validated against TMM ignition delay times measured in a shock tube at 20 and 40 bar, under fuel lean, stoichiometric, and fuel rich conditions. Further, the TMM model is validated against laminar burning velocities, which were measured in a heat flux setup for a variation of fuel/air ratios. A good agreement between the TMM model predictions and the measured ignition delay times and laminar burning velocities is observed. At fuel rich conditions in the shock tube, however, the model underestimates the reactivity of TMM by up to 60%, which could not be compensated by physically meaningful modifications of the TMM model. The analysis of the TMM model shows that at low temperatures, TMM is primarily consumed via H-atom abstraction by ȮH radicals, followed by classical β-scission and low-temperature chemistry. At higher temperatures, a significant fraction of TMM is consumed via unimolecular decomposition, leading to a higher reactivity compared to OME2. The present model and experiments lay the foundation for future kinetic modeling of TMM and other branched OME-like compounds.

Hydrogen/PRF skeletal mechanism study based on shock tube experiments and kinetic analysis

In the present work, the experiments of measuring ignition delay times (IDTs) of the hydrogen/n-heptane mixture at high temperature (1130–1485 K) and low pressure (1.25 atm) by a shock tube is implemented. The results show that mixtures with higher hydrogen proportion and lower equivalence ratio present shorter IDT. In the case of high hydrogen proportion, the n-heptane addition causes an obvious inhibition effect on the ignition of the mixture. Whereas, as the hydrogen proportion decreases, such inhibition effect weakens. As the experimental values are consistent with the predicted IDTs by NUI (National University of Ireland) mechanism, the NUI mechanism was selected to conduct the kinetic study. The consumption of n-heptane at the initial stage of reaction is earlier than that of hydrogen because the activities of n-heptane’s H-abstraction and decomposition reactions are relatively high. With shock tube experimental data as the calibration basis, a skeletal hydrogen/PRF (Primary Reference Fuel) mechanism, was updated from a skeletal PRF mechanism. The skeletal hydrogen/PRF mechanism, consisting of 49 species and 162 reactions had been validated against shock tube experiment of hydrogen, n-heptane, isooctane pure fuel, PRF, and hydrogen/n-heptane mixture.

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.

An experimental and kinetic modeling study of auto-ignition and flame propagation of ethyl lactate/air mixtures, a potential octane booster

Spark ignition engines are one of the main technologies in the transport sector. The improvement and optimization of the fuels used to empower these engines are of vital importance, both for economic and environmental reasons. In particular, one of the main issues of spark ignition engines is the knock phenomenon; new formulations of fuels are being studied in order to overcome this problem. In this study, a possible innovative anti-knock, octane booster additive is considered: ethyl lactate. This molecule is almost unknown in combustion literature, as it has been used only as green solvent and food additive. The first experimental results under combustion conditions are presented, together with a kinetic mechanism. Two set-ups have been employed: a rapid compression machine, to measure ignition delay times, and an innovative spherical bomb, OPTIPRIME, to obtain laminar flame speeds. The results are encouraging for the expected application and the mechanism shows good performance. Ignition delay times at all conditions are well predicted by the mechanism and, when compared to ethanol, they are longer, implying a greater anti-knock capability. A rate of production analysis has been performed, where the unimolecular reaction leading to ethylene and lactic acid has been proved to be quite important at high temperatures and lean conditions. For laminar flame speeds, the agreement between model and experiments is good, with some discrepancies at lean conditions and high pressures. Compared to ethanol, at rich and stoichiometric conditions ethyl lactate flame speeds are slightly slower except at lean conditions, indicating that under some conditions this molecule could provide better performances than ethanol as an octane booster additive.

Simultaneous side-wall-schlieren and -emission imaging of autoignition phenomena in conventional and constrained-reaction-volume shock-tube experiments

Simultaneous schlieren and emission imaging through the side wall of a round shock tube is reported for application to experimental autoignition studies. A round, optically accessible shock tube featuring windows designed as aberration-corrected cylindrical lenses provides a large side-wall field of view compatible with both emission and schlieren imaging. Autoignition experiments are reported for non-dilute propane–oxygen–argon mixtures (ϕ=1 C3H8 in 21% O2, 79% Ar) at temperatures (T5) spanning the range 1,203–1,438 K and pressures near 1 atm. Experiments are performed using both conventional and constrained-reaction-volume (CRV) filling, allowing for the most detailed comparison to date of ignition dynamics between these two types of experiments. When T5 exceeds 1,290 K, strong ignition is observed to initiate in the immediate proximity of the end wall regardless of the filling strategy. In lower-T5 experiments, mild ignition is observed to initiate spontaneously at remote locations, which often exceeding the imaging FOV in conventional experiments but are constrained within about 5 cm of the end wall in CRV experiments. Measured ignition delay times (IDTs) are compared to predictions from three kinetic mechanisms. IDTs simulated using the San Diego and USC Mech II mechanisms are of the same magnitude as the measurements but exhibit a lower activation energy; the propane submechanism from NUIG 1.1, meanwhile, systematically over predicts observed IDTs by roughly a factor of two. Comparisons are also made between the experimental measurements and predictions from a recently introduced 1-D axial-temperature-gradient model describing remote ignition in shock tubes. Considering the effect of a supposed dT5/dz=150 K/m gradient, the 1-D model functionally predicts the remote ignition observed in T5< 1,290 K experiments and suggests the use of CRV filling may significantly reduce the error introduced into IDT measurements by remote ignition at low-T5 conditions.

Laminar burning velocities of cyclopropane flames

Cyclopropane, c-C3H6, the simplest cycloalkane, is seldom included in detailed kinetic mechanisms for hydrocarbons, though it may exhibit unusual kinetic features yet to be analysed due to a lack of studies of its combustion characteristics. In this work, laminar burning velocities of cyclopropane flames have been determined using the heat flux method at atmospheric pressure and an initial gas mixture temperature of 298 K. The fuel consisting of 50% c-C3H6 + 50% N2 was burnt with air covering the range of equivalence ratios 0.6 – 1.5. The detailed kinetic model of the authors was extended by the reactions of cyclopropane and cyclopropyl radical, c-C3H5, with the rate constants selected from the literature. This mechanism, as well as the most recent mechanisms for c-C3H6 of Wang et al. (2022) and of Lei et al. (2022), have been compared with the burning velocities of propylene + air flames and with the new experimental results for cyclopropane flames. The model of Lei et al. (2022) significantly underpredicts the burning velocities for both fuels, on the other hand, good agreement with predictions of the present model and of Wang et al. (2022) was observed at 1 atm. However, further sensitivity and rate-of-production analyses revealed important differences in the pathways of c-C3H6 oxidation predicted by the two mechanisms. The present kinetic model was also tested using all available measurements of cyclopropane ignition delays in shock tubes, which combined cover the range of equivalence ratios from 0.33 to 3, at pressures 1 - 10 atm, and temperatures 1100 – 2100 K. Overall good performance of the model was demonstrated across these ranges of conditions and compositions of the mixture. A direct comparison of the experimental data shows that ignition delays of propylene are slightly longer than those of cyclopropane, yet in most cases within the overlapping uncertainties or scattering between different experimental facilities. The laminar burning velocities of c-C3H6 + air are slightly higher than those of C3H6 + air at least according to the predictions of the present mechanism and the model of Wang et al. (2022).

Influence of NOx chemistry on the prediction of natural gas end-gas autoignition in CFD engine simulations

Natural gas (NG) represents a promising low-cost/low-emission alternative to diesel fuel when used in high-efficiency internal combustion engines. Advanced combustion strategies utilizing high EGR rates and controlled end-gas autoignition can be implemented with NG to achieve diesel-like efficiencies; however, to support the design of these next-generation NG ICEs, computational tools, including single- and multi-dimensional simulation packages will need to account for the complex chemistry that can occur between the reactive species found in EGR (including NOx) and the fuel. Research has shown that NOx plays an important role in the promotion/inhibition of large hydrocarbon autoignition and when accounted for in CFD engine simulations, can significantly improve the prediction of end-gas autoignition for these fuels. However, reduced NOx-enabled NG mechanisms for use in CFD engine simulations are lacking, and as a result, the influence of NOx chemistry on NG engine operation remains unknown. Here, we analyze the effects of NOx chemistry on the prediction of NG/oxidizer/EGR autoignition and generate a reduced mechanism of a suitable size to be used in engine simulations. Results indicate that NG ignition is sensitive to NOx chemistry, where it was observed that the addition of EGR, which included NOx, promoted NG autoignition. The modified mechanism captured well all trends and closely matched experimentally measured ignition delay times for a wide range of EGR rates and NG compositions. The importance of C2-C3 chemistry is noted, especially for wet NG compositions containing high fractions of ethane and propane. Finally, when utilized in CFD simulations of a Cooperative Fuels Research (CFR) engine, the new reduced mechanism was able to predict the knock onset crank angle (KOCA) to within one crank angle degree of experimental data, a significant improvement compared to previous simulations without NOx chemistry.

Shock-tube study of the ignition and product formation of fuel-rich CH4/ozone/air and natural gas/ozone/air mixtures at high pressure

The influence of ozone on the ignition delay times and product formation of methane and natural gas was determined in a high-pressure shock tube at fuel-rich conditions (ϕ = 2 and 10), ozone concentrations of 550 ppm, temperatures between 850 and 1700 K and pressures of about 30 bar. The addition of ozone causes a strong reduction of the ignition delay times for temperatures below 1100 K. Product concentrations were determined with fast sampling and GC/MS analysis for the experiments at ϕ = 10. Experiments and simulations show that CO, H2, H2O, C2H2, C2H4, C2H6, CO2, C3H6, and benzene are the main products. A comparison to similar studies with additives such as n-heptane, dimethyl ether, diethyl ether, dimethoxymethane and experiments without additives shows that C2H2, C2H4, and benzene concentrations are decreased, whereas CO concentrations are increased when ozone is used as additive. The experimental results were compared to simulations with chemical kinetics mechanisms from literature. The measured ignition delay times agree well with the simulations. A comparison of measured and simulated product distributions shows that the temperature dependence of the product formation is predicted well but that there are deviations of the absolute concentrations up to a factor of two for acetylene and benzene contrary to studies with other additives.