High-temperature Ignition Research Articles

Effects of NOx addition on autoignition and detonation development in DME/air under engine-relevant conditions

Exhaust gas recirculation (EGR) technology can be used in internal combustion engines to reduce NOx emission and improve fuel economy. However, it also affects the end-gas autoignition and engine knock since NOx in EGR can promote ignition. In this study, effects of NOx addition on autoignition and detonation development in dimethyl ether (DME)/air mixture under engine-relevant conditions are investigated. Numerical simulation considering both low-temperature and high-temperature chemistry is conducted. First the kinetic effects of NOx addition on the negative temperature coefficient (NTC) regime are assessed and interpreted. It is found that NOx addition greatly promotes both low-temperature and high-temperature ignition stages mainly through increasing OH production. Then the autoignitive reaction front propagation induced by either local NO accumulation or a cold spot within NTC regime with different amounts of NO addition is investigated. For the first time, supersonic autoignition modes including detonation induced by local NO accumulations are identified. This indicates that local accumulation of NOx in end gas might induce super-knock in engines with EGR. A new parameter quantifying the ratio of sound speed to average reaction front propagation speed is introduced to identify the regimes for different autoignition modes. Compared to the traditional counterpart parameter used in previous studies, this new parameter is more suitable since it yields a detonation development regime in a C-shaped curve which is almost unaffected by the initial conditions. The results in this study may provide fundamental insights into knocking mechanism in engines using EGR technology.

Investigations on the effects of premixed ethanol/air atmosphere on combustion characteristics of n-heptane spray in a combustion vessel

In ethanol/diesel dual-fuel engines, premixed ethanol/air atmosphere could affect the ignition delay of diesel spray as well as the following soot formation. In this paper, a reduced mechanism describing the chemistry of ethanol and n-heptane was composed and validated with various experimental data. Based on this mechanism, simulations were conducted to investigate the combustion characteristics of n-heptane spray under premixed ethanol/air atmosphere in a combustion vessel. The effects of equivalence ratio of premixed ethanol/air atmosphere, ambient temperature and charge cooling effect of ethanol vaporization were investigated. Under premixed ethanol/air atmosphere, the results showed that high temperature ignition first occurred in the regions with mixture near stoichiometric ratio, rather than regions with largest n-heptane content (i.e. highest fuel reactivity). This is mainly because the vaporization of direct injected n-heptane decreased the gas temperature and consequently restricted the low temperature reaction. While the equivalence ratio of premixed ethanol/air atmosphere was further increased, the mixture near stoichiometric ratio exhibited lower fuel reactivity, which consequently resulted in prolonged ignition delay. It was also found that premixed ethanol could inhibit the soot formation of n-heptane spray under low ambient temperatures while contributed to significantly increased soot emission under high ambient temperatures. Moreover, the simulated results showed that the cooling effect of ethanol vaporization also played an important role in soot reduction of dual-fuel combustion.

Structure of tetrabrachial flames in non-premixed autoigniting dimethyl ether/air mixtures

Structures of tetrabrachial flames in two-stage autoigniting dimethyl ether (DME)/air mixture under diesel engine relevant conditions are investigated by direct numerical simulation. Three cases with different initial turbulent integral length scales are studied. Results show that the first stage of autoignition initiates in lean mixtures, and subsequently develops into a diffusion-supported cool flame propagating into rich mixtures; the second-stage autoignition features spatially distributed kernels in fuel-rich mixtures, followed by hybrid autoignition/tetrabrachial flames. The detailed chemical structures of the tetrabrachial flames are analyzed in terms of reactant concentrations and the reaction rate profiles. The cool flame branch is dominated by low temperature reactions, while the other branches are mainly involved in high temperature oxidation of the remaining fuel and intermediate species. The excess DME is consumed in the premixed flame branches and decomposed into more stable fuels including H2, CH4 and CO in the trailing diffusion flame, where H2 and CO are mainly oxidized by intermediate species OH and O. The structures and reaction rates in the tetrabrachial flame exhibit significant asymmetry, which is more distinct in the mixture fraction-temperature phase space. Effects of turbulence on the timing and location of two-stage ignition are then studied. In this study, turbulence tends to advance ignition compared with laminar cases, while the first high-temperature ignition time is similar for the three cases with different initial turbulence integral length scales.

Ignition properties of lean DME/H2 mixtures at low temperatures and elevated pressures

Ignition delays of lean dimethyl ether (DME)/hydrogen mixtures with hydrogen mole fraction ranging from 0 to 85% were measured using a rapid compression machine with equivalence ratios from 0.3 to 0.5, temperatures from 621 to 852 K and pressures from 7 to 22 bar. Numerical analysis was performed based on the NUI Aramco Mech 2.0 and, in general, good agreement between measurements and predictions was demonstrated. Both measurements and predictions show that H2 addition increases the ignition delay of DME/H2 mixtures non-linearly. Moreover, the effect of H2 addition on ignition delay becomes more pronounced at H2 mole fraction above 70% and lower pressure. The dilution and chemical effects of H2 addition were quantified, and the dilution effect decreases whereas the chemical effect increases with the increase of H2 mole fraction. It is also found that DME/H2 mixtures show a three-stage heat release behavior at very lean condition (φ = 0.3), namely, a low temperature heat release (LTHR) stage followed by two high temperature heat release (HTHR) stages. The first- and second-stage HTHR are caused by the conversion of HCHO to CO and the production of final reaction products CO2 and H2O respectively. Overall, DME is mainly consumed during the LTHR and first-stage HTHR whereas H2 is mainly consumed during the second-stage HTHR. The two-stage HTHR behavior of DME/H2 mixtures occurs when H2 and HCHO produced during DME’s LTHR stage are present in a proper proportion prior to high-temperature ignition and the temperature after LTHR has reached a proper level.

A physics-based approach to modeling real-fuel combustion chemistry - I. Evidence from experiments, and thermodynamic, chemical kinetic and statistical considerations

Real distillate fuels usually contain thousands of hydrocarbon components. Over a wide range of combustion conditions, large hydrocarbon molecules undergo thermal decomposition to form a small set of low molecular weight fragments. In the case of conventional petroleum-derived fuels, the composition variation of the decomposition products is washed out due to the principle of large component number in real, multicomponent fuels. From a joint consideration of elemental conservation, thermodynamics and chemical kinetics, it is shown that the composition of the thermal decomposition products is a weak function of the thermodynamic condition, the fuel-oxidizer ratio and the fuel composition within the range of temperatures of relevance to flames and high temperature ignition. Based on these findings, we explore a hybrid chemistry (HyChem) approach to modeling the high-temperature oxidation of real, distillate fuels. In this approach, the kinetics of thermal and oxidative pyrolysis of the fuel is modeled using lumped kinetic parameters derived from experiments, while the oxidation of the pyrolysis fragments is described by a detailed reaction model. Sample model results are provided to support the HyChem approach.

A phenomenological HCCI combustion model in 0D and 3D-CFD

A phenomenological combustion model for HCCI applications – the 3-Stage heat release model – which predicts low, intermediate and high temperature heat release (LTR, ITR and HTR) is developed and validated for six automotive surrogate fuels. The contribution of all three combustion phases to the overall heat release and their durations are modeled based on rapid compression and expansion (RCEM) single-zone adiabatic simulations with detailed chemical kinetic mechanisms for all six fuels. This allows “pure” chemical effects on combustion to be included in the proposed 3-Stage heat release model, avoiding any combustion chamber related uncertainties such as blow-by, heat losses or trapped mass in crevices. Agreement between the 3-Stage heat release model and virtual RCEM (VRCEM) reference data is good. In a second step, a zero-dimensional HCCI model (0D model) is presented. To predict the onset of combustion, the low temperature ignition term of a previously developed and validated 3-Arrhenius auto-ignition model is employed in conjunction with the ignition integral of Livengood and Wu. Thereafter the 3-Stage heat release model predicts chemical heat release. Results show that the 0D model gives excellent predictions of ignition delays and combustion progress compared to the single-zone RCEM simulations using detailed chemical reaction mechanisms. Furthermore, including ITR in the 0D model is shown to be of essential importance to correctly predict high temperature ignition delay. As a final step a 3D HCCI model (3D model) is presented. The onset of combustion is again predicted by the low temperature ignition term of the 3-Arrhenius auto-ignition model, implemented via the source term of the transport equation into the 3D-CFD code. To predict heat release, the source term of the enthalpy equation is used to incorporate the 3-Stage heat release model into the 3D-CFD code. A simplified RCEM geometry is used for the simulations. Validation of the 3D model is made by a comparison to a chemical kinetic mechanism that is directly integrated into the 3D-CFD code. Combustion progress, temperature stratification in the combustion chamber and ignition delay times agree well with the reference data. The combination of the 3-Arrhenius auto-ignition model and 3-stage heat release model, their integration into 0D or 3D-CFD as a fully predictive HCCI model and their very low computational cost suggest the proposed model constitutes a promising approach for the development of future HCCI combustion engines.

Direct numerical simulation of a temporally evolving air/n-dodecane jet at low-temperature diesel-relevant conditions

We present a direct numerical simulation of a temporal jet between n-dodecane and diluted air undergoing spontaneous ignition at conditions relevant to low-temperature diesel combustion. The jet thermochemical conditions were selected to result in two-stage ignition. Reaction rates were computed using a 35-species reduced mechanism which included both the low- and high-temperature reaction pathways. The aim of this study is to elucidate the mechanisms by which low-temperature reactions promote high-temperature ignition under turbulent, non-premixed conditions. We show that low-temperature heat release in slightly rich fuel regions initiates multiple cool flame kernels that propagate towards very rich fuel regions through a reaction-diffusion mechanism. Although low-temperature ignition is delayed by imperfect mixing, the propagation speed of the cool flames is high: as a consequence, high-temperature reactions in fuel-rich regions become active early during the ignition transient. Because of this early start, high-temperature ignition, which occurs in fuel-rich regions, is faster than homogeneous ignition. Following ignition, the high-temperature kernels expand and engulf the stoichiometric mixture-fraction iso-surface which in turn establish edge flames which propagate along the iso-surface. The present results indicate the preponderance of flame folding of existing burning surfaces, and that ignition due to edge-flame propagation is of lesser importance.. Finally, a combustion mode analysis that extends an earlier classification [1] is proposed to conceptualize the multi-stage and multi-mode nature of diesel combustion and to provide a framework for reasoning about the effects of different ambient conditions on diesel combustion.

Large eddy simulation of the low temperature ignition and combustion processes on spray flame with the linear eddy model

Large eddy simulation coupled with the linear eddy model (LEM) is employed for the simulation of n-heptane spray flames to investigate the low temperature ignition and combustion process in a constant-volume combustion vessel under diesel-engine relevant conditions. Parametric studies are performed to give a comprehensive understanding of the ignition processes. The non-reacting case is firstly carried out to validate the present model by comparing the predicted results with the experimental data from the Engine Combustion Network (ECN). Good agreements are observed in terms of liquid and vapour penetration length, as well as the mixture fraction distributions at different times and different axial locations. For the reacting cases, the flame index was introduced to distinguish between the premixed and non-premixed combustion. A reaction region (RR) parameter is used to investigate the ignition and combustion characteristics, and to distinguish the different combustion stages. Results show that the two-stage combustion process can be identified in spray flames, and different ignition positions in the mixture fraction versus RR space are well described at low and high initial ambient temperatures. At an initial condition of 850 K, the first-stage ignition is initiated at the fuel-lean region, followed by the reactions in fuel-rich regions. Then high-temperature reaction occurs mainly at the places with mixture concentration around stoichiometric mixture fraction. While at an initial temperature of 1000 K, the first-stage ignition occurs at the fuel-rich region first, then it moves towards fuel-richer region. Afterwards, the high-temperature reactions move back to the stoichiometric mixture fraction region. For all of the initial temperatures considered, high-temperature ignition kernels are initiated at the regions richer than stoichiometric mixture fraction. By increasing the initial ambient temperature, the high-temperature ignition kernels move towards richer mixture regions. And after the spray flames gets quasi-steady, most heat is released at the stoichiometric mixture fraction regions. In addition, combustion mode analysis based on key intermediate species illustrates three-mode combustion processes in diesel spray flames.

Experimental and numerical investigation of atmospheric laminar premixed n-butane flames in sooting conditions

Experimental and numerical investigation of atmospheric laminar premixed n-butane/O2/N2 flames operating at two equivalence ratios (ϕ=2.16 and 2.32) has been performed. Both flames produce soot particles. Mole fraction of reactants, stable intermediate and combustion products have been obtained using a quartz microprobe coupled to gas chromatography and infrared spectroscopy technique analysis. The temperature profiles were obtained by Laser induced fluorescence of nitric oxide. The experimental results showed that a weak variation of equivalence ratio in sooting conditions introduces an increase of benzene and some unsaturated aliphatic species peak mole fraction. Except for benzene, the peak species mole fraction increase does not exceed a factor of two. A new reaction kinetic model is proposed to predict gaseous species mole fraction. In addition to the good agreement observed with our experimental data base, the proposed mechanism predicts well the detailed data obtained in low pressure rich n-butane premixed flames and high temperature ignition delay times reported in the literature.Reactions path analysis revealed that the first aromatic ring is controlled by C4+C2 (nC4H5+C2H2=C6H6+H) and C3+C3 (self recombination of C3H3 radicals) reactions routes. C4+C2 reaction path predominates in the reaction zone while C3+C3 path tends to predominate in the post flame region. The impact of the equivalence ratio on both reactions and the main differences between our and three selected literature models are discussed.

Theoretical Kinetics Analysis for Ḣ Atom Addition to 1,3-Butadiene and Related Reactions on the Ċ4H7 Potential Energy Surface

The oxidation chemistry of the simplest conjugated hydrocarbon, 1,3-butadiene, can provide a first step in understanding the role of polyunsaturated hydrocarbons in combustion and, in particular, an understanding of their contribution toward soot formation. On the basis of our previous work on propene and the butene isomers (1-, 2-, and isobutene), it was found that the reaction kinetics of Ḣ-atom addition to the C═C double bond plays a significant role in fuel consumption kinetics and influences the predictions of high-temperature ignition delay times, product species concentrations, and flame speed measurements. In this study, the rate constants and thermodynamic properties for Ḣ-atom addition to 1,3-butadiene and related reactions on the Ċ4H7 potential energy surface have been calculated using two different series of quantum chemical methods and two different kinetic codes. Excellent agreement is obtained between the two different kinetics codes. The calculated results including zero-point energies, single-point energies, rate constants, barrier heights, and thermochemistry are systematically compared among the two quantum chemical methods. 1-Methylallyl (Ċ4H71-3) and 3-buten-1-yl (Ċ4H71-4) radicals and C2H4 + Ċ2H3 are found to be the most important channels and reactivity-promoting products, respectively. We calculated that terminal addition is dominant (>80%) compared to internal Ḣ-atom addition at all temperatures in the range 298-2000 K. However, this dominance decreases with increasing temperature. The calculated rate constants for the bimolecular reaction C4H6 + Ḣ → products and C2H4 + Ċ2H3 → products are in excellent agreement with both experimental and theoretical results from the literature. For selected C4 species, the calculated thermochemical values are also in good agreement with literature data. In addition, the rate constants for H atom abstraction by Ḣ atoms have also been calculated, and it is found that abstraction from the central carbon atoms is the dominant channel (>70%) at temperatures in the range of 298-2000 K. Finally, by incorporating our calculated rate constants for both Ḣ atom addition and abstraction into our recently developed 1,3-butadiene model, we show that laminar flame speed predictions are significantly improved, emphasizing the value of this study.

Correlations for the ignition characteristics of six different fuels and their application to predict ignition delays under transient thermodynamic conditions

The ignition characteristics of six different fuels have been correlated as a function of the temperature, pressure, equivalence ratio and oxygen molar fraction in this investigation. More specifically, the ignition delay referred to cool flames, the high-temperature ignition delay and the critical concentrations and ignition times of HO2 and CH2O have been parameterized for n-dodecane, PRF0, PRF25, PRF50, PRF75 and PRF100. To do so, a wide database of ignition data of the aforementioned fuels has been generated by means of chemical simulations in CHEMKIN, solving a detailed mechanism for PRF mixtures and a reduced mechanism for n-dodecane. In fact, in-cylinder engine-like conditions reached in a Rapid Compression Expansion Machine (RCEM) have been replicated. The mathematical correlations have shown a relative deviation around 20% with the database in the low-temperature, low-pressure zone, which is the typical accuracy of usual correlations for the ignition delay. Finally, the ignition delay under transient conditions measured in the RCEM has been predicted by means of different integral methods coupled to both the proposed correlations and the generated database. I t has been found that deviations between the predictions obtained with the correlations or with the database are lower than 1%. This means that the correlations are accurate enough to predict the ignition time in spite of showing high deviation with the database, since the low-temperature, low-pressure zone has a minor contribution to the ignition delay.

The influence of pilot injection on high-temperature ignition processes and early flame structure in a high-speed direct injection diesel engine

Simultaneous high-speed natural luminosity and OH* chemiluminescence imaging is used to characterize high-temperature ignition processes in conventional diesel combustion with a pilot-main injection strategy in a single-cylinder, light-duty optical diesel engine. High-speed imaging provides temporally and spatially resolved information in terms of high-temperature ignition processes and flame structure during the combustion. Using these imaging measurements, the high-temperature inflammation and the diffusion flame development processes are analyzed. The chemiluminescence signal shows a hot, reactive mixture, which gradually decreases after the peak release of the pilot combustion and lasts long after the apparent heat release has ended. Therefore, when the reactive pilot mixture exists near the main injection jets, the high-temperature ignition of the main injection is apparently initiated through interactions with the reactive pilot mixture. High-temperature autoignition, another process by which ignition of the main injection occurs, is observed in main injection plumes where the chemiluminescence signal of the reactive pilot mixture becomes very weak or is absent at the start of main injection. As the reaction of the main injection continues, the non-premixed main injection jet structure is developed and the high-temperature reacting region expands throughout the jet.

Ignition delay time correlation of fuel blends based on Livengood-Wu description

In this work, a universal methodology for ignition delay time (IDT) correlation of multicomponent fuel mixtures is reported. The method is applicable over wide ranges of temperatures, pressures, and equivalence ratios. n-Heptane, iso-octane, toluene, ethanol and their blends are investigated in this study because of their relevance to gasoline surrogate formulation. The proposed methodology combines benefits from the Livengood-Wu integral, the cool flame characteristics and the Arrhenius behavior of the high-temperature ignition delay time to suggest a simple and comprehensive formulation for correlating the ignition delay times of pure components and blends. The IDTs of fuel blends usually have complex dependences on temperature, pressure, equivalence ratio and composition of the blend. The Livengood-Wu integral is applied here to relate the NTC region and the cool flame phenomenon. The integral is further extended to obtain a relation between the IDTs of fuel blends and pure components. Ignition delay times calculated using the proposed methodology are in excellent agreement with those simulated using a detailed chemical kinetic model for n-heptane, iso-octane, toluene, ethanol and blends of these components. Finally, very good agreement is also observed for combustion phasing in homogeneous charge compression ignition (HCCI) predictions between simulations performed with detailed chemistry and calculations using the developed ignition delay correlation.

Sensitivity analysis and validation of a predictive procedure for high and low-temperature ignition delays under engine conditions for n-dodecane using a Rapid Compression-Expansion Machine

A predictive procedure for cool flames and high-temperature ignition delays based on the accumulation and consumption of chain carriers has been validated for n-dodecane under engine conditions. To do so, an experimental parametric study has been carried out in a Rapid Compression-Expansion Machine, measuring the ignition times for different compression ratios (14 and 19), initial temperatures (from 403K to 463K), O2 molar fractions (from 0.21 to 0.16) and equivalence ratios (from 0.4 to 0.7). The measured ignition delays have been compared to results from chemical kinetic simulations performed in CHEMKIN using a 0-D reactor that replicates the experimental conditions by solving five different chemical kinetic mechanisms, as a way to evaluate the mechanisms accuracy and variability. In general, all chemical kinetic mechanisms are able to accurately replicate the experimental ignition delays, being the mean relative deviation lower than 1.9% and 1.6% for both ignition stages, cool flames and the high-temperature ignition respectively. Furthermore, small differences have been appreciated between mechanisms in terms of ignition delay. Then, the predictive method has been applied using different databases obtained from each mechanism and a sensitivity analysis has been performed in order to evaluate the effects of the selected database on the predicted ignition delay. It has been found that while cool flames seems to be independent on the selected mechanism, the predicted high-temperature ignition delay is very sensitive to the species selected as chain carrier. Thus, if formaldehyde is assumed as ignition tracer, the predicted ignition time can vary up to 3%, while this percent decreases up to 1.3% when hydrogen peroxide takes the role of chain carrier.

Ignition features of methane and ethylene fuel-blends in hot and diluted coflows

Turbulent flames in the moderate or intense low oxygen dilution (MILD) combustion regime have previously exhibited less susceptibility to lift-off than conventional autoignitive flames in a jet-in-hot-coflow (JHC) burner. This has been demonstrated through laser-based diagnostics and examination of CH* chemiluminescence. New experimental observations are presented of turbulent flames of natural gas, ethylene and blends of the two fuels, in coflows with temperatures from 1250–1385K and oxygen concentrations from 3–11% (vol./vol.). Zero- and one-dimensional simulations, as well as turbulent flame modelling, are used to explain the trends seen experimentally with different coflows and fuels. Numerical simulations using simplified batch reactors and opposed-flow flames demonstrate that blending of methane and ethylene fuels is most significant near 1100K. Near this temperature, pure ethylene exhibits a transition between high and low temperature ignition pathways. Further analyses show that a 1:1 methane/ethylene blend behaves more like ethylene near MILD combustion conditions, and more like methane in conventional autoignition conditions. Two-dimensional modelling results of the turbulent flames are then discussed and explained in the context of the simplified reactor results. The flames confined by the lean flammability-limit in the coflow and high strain-rates in jet shear layer, in agreement with previous work using a semi-empirical jet model. The two-dimensional modelling is additionally able to qualitatively replicate the trends in lift-off height, with normalised heat release rate profiles reproducing, and serving to explain, the effects seen in experimental campaigns.

Experimental validation and analysis of seven different chemical kinetic mechanisms for n-dodecane using a Rapid Compression-Expansion Machine

Seven different chemical kinetic mechanisms for n-dodecane, two detailed and five reduced, have been evaluated under Engine Combustion Network (ECN) thermodynamic conditions by comparison to experimental measurements in a Rapid Compression-Expansion Machine (RCEM). The target ECN conditions are imposed at Top Dead Center (TDC), which cover a wide range of temperatures (from 850 K to 1000 K), oxygen molar fractions (0.21 and 0.15) and equivalence ratios (0.8, 0.9 and 1), while the pressure is fixed to keep a constant density at TDC equal to 22.8 kg/m3. The results obtained have been used to validate the chemical kinetic simulations, which have been performed with CHEMKIN, by comparing both cool flames and high temperature ignition delays, as well as the heat released in each stage of the combustion process in case of having a two-stage ignition pattern. The experimental results show good agreement with the chemical kinetic simulations. In fact, the mean relative deviation in ignition delay between experiments and simulations among all the chemical mechanisms is equal to 18.0% (3 CAD) for both cool flames and high temperature ignition. In general, closer correspondence has been obtained for the ignition delay referred to the high-temperature stage of the process, being the cool flames phenomenon more difficult to reproduce. Moreover, the differences between the reduced mechanisms and the most detailed one have been analyzed, concluding that the enhanced specific reaction rates of the most reduced mechanisms cause differences not only on the ignition delays, but also on the Negative Temperature Coefficient (NTC) behavior and on the heat released during cool flames.

Shock Tube Ignition Delay Data Affected by Localized Ignition Phenomena

ABSTRACTShock tubes have conventionally been used for measuring high-temperature ignition delay times of approximately O(1 ms). In the last decade or so, the operating regime of shock tubes has been extended to lower temperatures by accessing longer observation times. Such measurements may potentially be affected by some non-ideal phenomena. The purpose of this work is to measure long ignition delay times for fuels exhibiting negative temperature coefficient and to assess the impact of shock tube non-idealities on ignition delay data. Ignition delay times of n-heptane and n-hexane were measured over the temperature range of 650–1250 K and pressures near 1.5 atm. Driver gas tailoring and long length of shock tube driver section were utilized to measure ignition delay times as long as 32 ms. Measured ignition delay times agree with chemical kinetic models at high (>1100 K) and low (<700 K) temperatures. In the intermediate temperature range (700–1100 K), however, significant discrepancies are observed between the measurements and homogeneous ignition delay simulations. It is postulated, based on experimental observations, that localized ignition kernels could affect the ignition delay times at the intermediate temperatures, which lead to compression (and heating) of the bulk gas and result in expediting the overall ignition event. The postulate is validated through simple representative computational fluid dynamic simulations of post-shock gas mixtures, which exhibit ignition advancement via a hot spot. The results of the current work show that ignition delay times measured by shock tubes may be affected by non-ideal phenomena for certain conditions of temperature, pressure, and fuel reactivity. Care must, therefore, be exercised in using such data for chemical kinetic model development and validation.

Di-tertiary-butyl Peroxide Decomposition and Combustion with Air: Reaction Mechanism, Ignition, Flame Structures, and Laminar Flame Velocities

This study investigates the ignition/combustion of di-tertiary-butyl peroxide (DTBP) over a wide range of fuel/air ratios using numerical flame calculations to gain insight into species profiles and ignition/combustion characteristics of DTBP/air mixtures. The results are flame zone and reaction zone structures as well as the corresponding laminar flame speeds and propagation speeds of reaction zones. Several detailed reaction mechanisms are evaluated in the flame simulations, and sensitivity of the computed results to important reaction steps was evaluated. The ignition of DTBP in the absence of oxygen belongs in the category of thermal runaway because there is no chain branching associated with the early stages of the reaction; i.e., decomposition steps lead to acetone and ethane. In the presence of oxygen, the development of a flame is more complex and exhibits a two-stage ignition, where the first stage, at lower temperatures, involves the thermal decomposition of DTBP to acetone, which further oxidizes to the final combustion products. This second high-temperature ignition process involves chain-branching reactions for the combustion of acetone and ethane and the usual chain-branching reactions of the H2/O2 system. The calculations reproduce this two-stage ignition behavior for fuel-rich flames, and the calculated flame structures reflect the interaction of decomposition of DTBP to acetone and ethane and the subsequent combustion. DTBP has a relatively weak RO–OR bond and readily undergoes dissociation at higher temperatures to two RO• radicals, and it is widely used as a radical source, reaction initiator. Five reaction mechanisms (M1–M5) were employed in this study for analysis of the reaction system, which differ mainly in the reaction rates for the initial DTBE decomposition reaction. When using relative high reaction rates for the initial decomposition reactions (M1 and M2), the flame velocities of DTBP/air mixtures exhibit two maxima over the entire range of the mixture fraction. The first maximum is located at stoichiometric conditions, while the second prevails under fuel-rich conditions. Decelerating the primary decomposition reactions (M3–M5) in favor of the combustion reactions of the DTBP decomposition products shows the second maximum disappearing as result of the relative low propagation speed of the reaction zone. The first maximum is only slightly altered by varying the reaction rates, because the decomposition step of DTBP and the combustion of the primary decomposition products merge at stoichiometric conditions and temperatures are high enough to result in high decomposition rates.

High temperature ignition delay time of DME/n-pentane mixture under fuel lean condition

Ignition delay times were measured behind reflect shock waves for the lean DME/n-pentane fuel blends with DME blending ratios from 0% to 100% over a wide range of conditions: pressures from 2 to 20atm, temperatures from 1100 to 1600K and equivalence ratio of 0.5. Correlations of ignition delay times of DME/n-pentane were deduced from the experimental data using multiple linear regression. Comparison between measured ignition delay times and numerical predictions by five available kinetic mechanisms, namely NUI Galway pentane isomer model, JetSurF2.0 model, LLNL n-alkanes Model, NUI Galway Mech_56.54 Model and Wang’s DME model, were conducted. NUI Galway pentane isomer model shows good predictions for ignition delay times of DME, n-pentane and their blends which was conducted in this study to interpret the effect of DME addition on the ignition chemistry of n-pentane.

A direct numerical simulation of cool-flame affected autoignition in diesel engine-relevant conditions

In diesel engines, combustion is initiated by a two-staged autoignition that includes both low- and high-temperature chemistry. The location and timing of both stages of autoignition are important parameters that influence the development and stabilisation of the flame. In this study, a two-dimensional direct numerical simulation (DNS) is conducted to provide a fully resolved description of ignition at diesel engine-relevant conditions. The DNS is performed at a pressure of 40 atmospheres and at an ambient temperature of 900 K using dimethyl ether (DME) as the fuel, with a 30 species reduced chemical mechanism. At these conditions, similar to diesel fuel, DME exhibits two-stage ignition. The focus of this study is on the behaviour of the low-temperature chemistry (LTC) and the way in which it influences the high-temperature ignition. The results show that the LTC develops as a “spotty” first-stage autoignition in lean regions which transitions to a diffusively supported cool-flame and then propagates up the local mixture fraction gradient towards richer regions. The cool-flame speed is much faster than can be attributed to spatial gradients in first-stage ignition delay time in homogeneous reactors. The cool-flame causes a shortening of the second-stage ignition delay times compared to a homogeneous reactor and the shortening becomes more pronounced at richer mixtures. Multiple high-temperature ignition kernels are observed over a range of rich mixtures that are much richer than the homogeneous most reactive mixture and most kernels form much earlier than suggested by the homogeneous ignition delay time of the corresponding local mixture. Overall, the results suggest that LTC can strongly influence both the timing and location in composition space of the high-temperature ignition.