Dependence Of Ignition Delay Time Research Articles

Ignition enhancement and NOx formation of NH3/air mixtures by non-equilibrium plasma discharge

This work computationally studies plasma assisted low temperature NH3/air ignition and NOx formation in a repetitively-pulsed nanosecond discharge at atmospheric pressure by using an experimentally-validated plasma-combustion kinetic model. The results show that plasma discharge significantly enhances low temperature NH3 ignition. Compared with thermal ignition, the ignition delay time is shortened by 2–5 orders of magnitude due to the kinetic enhancement of excited species and radicals. The radicals (NH2, NH, H and O) produced through electron impact dissociation and quenching of electronically excited N2*, O(1D) and N(2D) promote OH production and further accelerate NH3 oxidation. The results show that there exists a non-monotonic dependence of ignition delay time on the reduced electric field strength. The optimum ignition enhancement is achieved at 250 Td at which the production of electronically excited species and radicals is most efficient. The vibrationally excited species produced at lower electric fields (<100 Td) are less effective in enhancing ignition because they only induce gas heating through the fast vibrational-translational relaxation by NH3 and H2O. At a higher electric field, although the efficient production of NH2, NH, H, O and OH by plasma creates new low temperature reaction pathways in enhancing low temperature NH3 ignition, the ignition is inhibited through NH + NO = N2O + H and chain-termination reaction NH2 + HO2 = NH3 + O2. The ignition delay times at different equivalence ratios show that the ignition enhancement by plasma is more effective at fuel-lean conditions due to the faster generation of N2(B), O(1D) and O from air, leading to accelerated NH3 oxidation via O(1D) + NH3→ NH2 + OH, NH3 + O = NH2 + OH and NH2 + O = NH + OH. The sensitivity analysis shows that the reactions involving O and O(1D) production are more effective on NH3 ignition enhancement than the fuel dissociation by electrons. Moreover, the ignition is also enhanced by NOx formation in plasma via reactions NH2 + NO = NNH + OH and NO + HO2 = NO2 + OH. This work advances the understanding of non-equilibrium excitation and NOx formation by plasma discharge on low temperature NH3 ignition.

Experimental and kinetic modeling study of tetramethylethylenediamine: A promising green propellant fuel

Tetramethylethylenediamine (TMEDA) is a promising green propellant fuel and reactivity promoter. However, the reactions between TMEDA and O2 are usually overlooked under ignition conditions. In this study, significant low-temperature reactivity was observed for 2%TMEDA/O2 mixtures, and autoignition could occur even at 470 K. To probe the chemical kinetics of TMEDA/O2, ignition delay times were measured in a rapid compression machine and a high-pressure shock tube. TMEDA pyrolysis products were also obtained in a single pulse shock tube. The autoignition of TMEDA/O2 in rapid compression machine experiments showed multi-stage heat release, a characteristic which became more obvious at oxygen-lean conditions. During the oxidation experiments, a non-Arrhenius temperature dependence of ignition delay time was observed. A chemical kinetic model of TMEDA was developed hierarchically, based on current reaction kinetics knowledge about hydrocarbons, small amines and NOx. Ignition delay times and pyrolysis products measured in this study, along with experimental data on small amines from the literature, were used to validate the kinetic model, which generally produced good predictions across different temperature, pressure and equivalence ratio conditions.

Laminar flame speed and autoignition characteristics of surrogate jet fuel blended with hydrogen

Hydrogen combustion has emerged as one promising option toward the achievement of carbon-neutral in aviation. In this study, the effects of hydrogen addition on laminar flame speeds, autoignition, and the coupling of autoignition and flame propagation for surrogate jet fuel n-dodecane are numerically investigated at representative engine conditions to elucidate the potential challenges for flame stabilization and the autoignition risks in combustor design. Results show that the normalized flame speed increases almost linearly with hydrogen addition for fuel-lean conditions, while for fuel-rich conditions it increases nonlinearly and can be up to 20. This poses great challenges for avoiding flameholding and flashback, particularly for fuel-rich mixtures. Results further show that flame speed enhancement due to the increased flame temperature can be neglected under fuel-lean conditions, but not for fuel-rich mixtures. For the dependence of ignition delay time on temperature, there exists a unique intersection between pure n-dodecane/air and H2/air mixtures. Near the intersection temperature, there exists subtle kinetic coupling of the two fuels, leading to different H2 roles, e.g., accelerator or inhibitor, for the autoignition process of n-dodecane/H2/air mixtures. With this intersection temperature, the diagram for autoignition risks is constructed, which demonstrates that H2 acts as an inhibitor under subsonic cruise conditions while either an inhibitor or an accelerator under supersonic cruise conditions depending on the combustor inlet temperature and the amount of hydrogen addition. With the potential coupling of autoignition and flame propagation, the 1-D autoignition-assisted flame calculations show that hydrogen addition can alleviate or even eliminate the two-stage ignition characteristics for pure n-dodecane/air flames. For n-dodecane blended with hydrogen, the autoignition-assisted flame propagation speed, as well as the global transition from flame propagation to autoignition, can still be described by an analytic scaling parameterized by the ignition Damkӧhler number.

A Study on Fundamental Combustion Properties of Oxymethylene Ether-2

Abstract Oxymethylene ethers (OMEn, n = 1–5) are a promising class of synthetic fuels that have the potential to be used as additives or substitutes to diesel in compression ignition engines. A comprehensive understanding of their combustion properties is required for their safe and efficient utilization. In this study, the results of a combined experimental and modeling work on oxidation of OME2 are reported: (i) Ignition delay time measurements of stoichiometric OME2/synthetic air mixtures diluted 1:5 with nitrogen using the shock tube method at pressures of 1, 4, and 16 bar, and (ii) laminar flame speeds of OME2/air mixtures using the cone angle method at atmospheric and elevated pressures of 3 and 6 bar. The experimental data sets obtained have been used for validation of three detailed reaction mechanisms of OME2 obtained from literature. The results of ignition delay time measurements showed that ignition of OME2 is characterized by pre-ignition activity at the low temperature side of the measurements regardless of the pressure. Regarding the performance of the different reaction mechanisms, the model from Cai et al. (2020, “Auto-Ignition of Oxymethylene Ethers (OMEn, n = 2–4) as Promising Synthetic e-Fuels From Renewable Electricity: Shock Tube Experiments and Automatic Mechanism Generation,” Fuel, 264, p. 116711) best predicted the temperature and pressure dependence of ignition delay times. For laminar flame speeds, the experimental data were well matched by the mechanism from Ren et al. (2019) at p = 1, 3, and 6 bar and for all equivalence ratios considered. From sensitivity analyses calculations, it was observed that chain reactions involving small radicals, i.e., H, O, OH, HO2, and CH3 control the oxidation of OME2. The comparison of the results of this work and our previous work (Ngugi et al. (2021)) on OME1 show that these two fuels have similar oxidation pathways. The results obtained in this work will contribute to a better understanding of the combustion of oxymethylene ethers, and thus, to the design and optimization of burners and engines as well.

Dependence of Biofuel Ignition Chemistry on OH-Initiated Branching Fractions

Abstraction of hydrogen by ȮH is the dominant initiation step in low-temperature oxidation of biofuels. Theoretical chemical kinetics calculations for such reactions provide a direct means of quantifying rates of abstraction, which are critical to modeling biofuel combustion. However, in several cases and despite agreement on total rate coefficients, branching fractions (i.e. the distribution of initial radicals) can vary depending on the level of theory, which leads to variations in ignition delay time predictions. To examine the connection between branching fractions and ignition delay time predictions, simulations were conducted for 1-butanol, cyclopentanone, and methyl propanoate at 10 atm and from 500–1000 K. For each case, the simulations utilized recent combustion mechanisms to produce an initial set of ignition delay time trends. H-abstraction rates were then replaced using rates from the literature to examine the effects of ȮH-initiated branching fractions on ignition chemistry. Branching fractions were found to significantly influence ignition chemistry, specifically in the case of 1-butanol, even when total rate coefficients were relatively consistent. From comparison of site-specific rates in the literature, branching fractions for initiation of 1-butanol and methyl propanoate are not consistent, which resulted in ignition delay times differing by factors of up to 6.3 and 1.2 respectively. Conversely, in the case of cyclopentanone, for which both the total and the site-specific rate coefficients agree, ignition delay times were unaffected. From the observed dependence of ignition delay times on ȮH-initiated branching fractions, an intermediate step in the development of combustion mechanisms is necessary to validate site-specific rate coefficients and ensure accurate model predictions. Speciation measurements are one example that can provide a critical link to radical-specific, fundamental chemical pathways and determine accurate branching fractions.

Ignition Studies of C1–C7 Natural Gas Blends at Gas-Turbine-Relevant Conditions

Abstract New ignition delay time measurements of natural gas mixtures enriched with small amounts of n-hexane and n-heptane were performed in a rapid compression machine to interpret the sensitization effect of heavier hydrocarbons on auto-ignition at gas-turbine relevant conditions. The experimental data of natural gas mixtures containing alkanes from methane to n-heptane were carried out over a wide range of temperatures (840–1050 K), pressures (20–30 bar), and equivalence ratios (φ = 0.5 and 1.5). The experiments were complimented with numerical simulations using a detailed kinetic model developed to investigate the effect of n-hexane and n-heptane additions. Model predictions show that the addition of even small amounts (1–2%) of n-hexane and n-heptane can lead to an increase in reactivity by ∼40–60 ms at compressed temperature (TC) = 700 K. The ignition delay time (IDT) of these mixtures decreases rapidly with an increase in concentration of up to 7.5% but becomes almost independent of the C6/C7 concentration beyond 10%. This sensitization effect of C6 and C7 is also found to be more pronounced in the temperature range 700–900 K compared to that at higher temperatures (&amp;gt;900 K). The reason is attributed to the dependence of IDT primarily on H2O2(+M) ↔ 2ȮH(+M) at higher temperatures while the fuel-dependent reactions such as H-atom abstraction, RȮ2 dissociation, or Q˙OOH + O2 reactions are less important compared to 700–900 K, where they are very important.

An experimental and kinetic modeling study of gas-phase autoignition of n-Pentadecane at low-to-high temperatures and elevated pressures

In this study, low-to-high temperature oxidation of n-pentadecane (nC15H32) is investigated in a rapid compression machine and a heated shock tube. The experiments are conducted at equivalence ratios of Φ = 0.5–2.0 with air as an oxidizer for the fuel-lean condition while diluted fuel mixtures for other equivalence ratios. A temperature range of 650–1450 K is investigated at a pressure of 6 and 10 bar in a rapid compression machine and a heated shock tube while a temperature range of 900–1350 K is investigated at a pressure of 15 bar in a heated shock tube. The fuel showed a two-stage ignition behavior which is a typical characteristic of large hydrocarbons and a negative temperature coefficient regime. The dependence of ignition delay times on pressure, equivalence ratio, and oxygen mole fraction are separately presented and discussed. The experimental results revealed the sensitivity of ignition delay times to all these factors. The available experimental data of C10-C16 n-alkanes are compared which showed a small effect of chain length on the oxidation of alkanes. An updated reaction mechanism is presented after revising the rate constants of various reaction classes. The kinetic modeling predictions are compared and noticeable improvements are observed in the performance of the updated mechanism especially at low temperatures. The flux analysis is carried out to observe the fuel consumption along major low-temperature pathways. The sensitivity analysis is then performed to analyze the performance of the updated mechanism. To our knowledge, this is the first study on the autoignition of n-pentadecane.

An extended methodology for automated calculations of non-Boltzmann kinetic sequences: H + C2H2 + X and combustion impact

It is generally assumed in phenomenological kinetic models that bimolecular reactions only occur between species whose rovibrational energy follows a Boltzmann (thermal) distribution. That is, any complexes initially formed in non-Boltzmann distributions are assumed to be thermalized by energy-transferring collisions prior to bimolecular reactions. Given the high mole fractions of reactive species, X, in combustion environments, reactive collisions of the complexes with X often occur on the same timescale as energy-transferring collisions – yielding sequences proceeding through non-Boltzmann intermediates across multiple potential energy surfaces. Recent studies have shown that such non-Boltzmann kinetic sequences can have substantial impact on the global reactivity in combustion systems. Simulations of these non-Boltzmann reaction sequences, which can be described in phenomenological kinetic models via chemically termolecular reactions, require that rovibrational excitation from one potential energy surface be carried over to the next. This paper presents an extended theoretical and computational methodology that couples multiple master equations and derives rate constants for phenomenological reactions describing the conversion of thermal reactants to thermal products for use in phenomenological kinetic schemes. The methodology is then implemented using in-house scripts for non-Boltzmann sequences involving C2H3* + X (with X = O2, H, and OH) where C2H3* is formed via H + C2H2 association – which were identified as having strong potential for influencing combustion predictions in a recent study. The results reveal that non-Boltzmann reaction sequences for X = O2 (the primary focus of this paper) significantly alters the total conversion rate from H + C2H2 to products and product branching fractions from those of thermal sequential pathways. Furthermore, the present results demonstrate that non-Boltzmann reaction sequences have significant impact – as high as an order of magnitude – on predicted ignition delay times. Similarly, they yield significantly different dependence of ignition delay times with temperature and O2 mole fraction – yielding signatures that are likely observable experimentally.

The effect of ammonia addition on the low-temperature autoignition of n-heptane: An experimental and modeling study

Ammonia (NH3) is receiving increasing attention as an alternative engine fuel due to its carbon-free nature. However, its fundamental combustion characteristics, such as higher autoignition temperature and lower burning velocity compared to conventional hydrocarbons, limit its direct use in traditional engines. To confront this dilemma, hydrocarbon/NH3 blending fuels are seen as one of the most appropriate ways to exploit its advantage and offset its weaknesses. Using n-heptane as a representative of hydrocarbons, this study investigated the effect of NH3 addition on the low-temperature autoignition of n-heptane. The ignition delay times of five n-heptane/NH3 mixtures with NH3 fractions of 0%, 20%, and 40% were measured in a rapid compression machine at temperatures of 635–945 K, pressures of 10 and 15 bar, and equivalence ratios of 1.0 and 2.0. Experimental results show that the n-heptane/NH3 blending fuels exhibit pronounced low-temperature reactivity, and both the total and the first-stage ignition delay times increase with the increase of NH3 fraction. A blending mechanism of n-heptane/NH3 was compiled based on the existing n-heptane mechanism and NH3 mechanism. It is found that the blending mechanism is capable to predict the inhibition effect of NH3 addition on n-heptane autoignition and qualitatively capture the dependence of ignition delay time on equivalence ratio and oxygen mole fraction. Nevertheless, there are significant discrepancies between the experiments and the simulation. Furthermore, kinetic analyses, including species evolution, rate of production and sensitivity of OH radical were conducted sequentially to reveal the autoignition kinetics of NH3/n-heptane blends and the interaction between n-heptane and NH3. Suggestions are provided for the further development of the blending mechanism.

The autoignition of iso-dodecane in low to high temperature range: An experimental and modeling study

2,2,4,6,6-pentamethylheptane, a highly branched alkane, is a promising component candidate of branched alkanes in the surrogates for jet fuels and a major component in alternative fuels. In spite of its great relevance and importance in real fuels, it has only attracted very little interest. This work provides a new set of experimental data and a newly proposed detailed kinetic mechanism for this hydrocarbon. In the experiments, the autoignition characteristics of 2,2,4,6,6-pentamethylheptane, was measured in a rapid compression machine and a shock tube spanning over the equivalence ratios of 0.5, 1 and 1.5, pressures of 15 and 20 bar and temperature range of 603–1376 K. The dependence of ignition delay times on pressure, mole fraction of fuel, mole fraction of oxygen and dilution ratio were systematically investigated. Negative temperature coefficient (NTC) behavior was observed during the autoignition event, but the NTC temperature window was found to be much lower than the counterpart normal paraffin. A detailed kinetic model containing 729 species and 3390 reactions was proposed to describe the combustion behavior of this compound over low-to-high temperature range. The model was validated against the present experimental data, as well as the limited datasets in literature. Good agreements were observed between the experimental data and the predictions from the proposed model. Kinetic analyses, including sensitivity analysis and reaction pathway analysis, were carried out to provide insight into the combustion characteristics from the kinetic perspective. Comparisons were also carried out with the datasets for other branched alkanes and normal-paraffin with same carbon numbers over the same conditions to reveal the effects of the molecular structure.

Gas-phase autoignition of diesel/gasoline blends over wide temperature and pressure in heated shock tube and rapid compression machine

Diesel/gasoline blended fuel, also named as dieseline, has attracted worldwide interests in varied advanced engine combustion modes dominated by mixing time scale and chemical reaction time scale. However, the research on ignition properties and chemical reaction mechanism of diesel/gasoline blends over wide operating conditions is still lacking. In current study, ignition delay time measurement of diesel/gasoline blends with different diesel proportion (30%, 50%, and 70% by volume) were conducted in a heated shock tube and a heated rapid compression machine (RCM) covering a broad range of temperatures between 636 and 1317 K at pressures between 6 and 20 bar for gas-phase dieseline/O2/N2 mixture at varying equivalence ratios from 0.5 to 1.5. The experimental results revealed different dependence of ignition delay time on pressure, equivalence ratio and composition at different temperatures and varying negative-coefficient (NTC) behaviors were also observed at intermediate-temperature regime. The increase in proportion of diesel led to shorter ignition delay times under all conditions, though the ignition data at high temperature only showed marginal discrepancies. Besides, a crossover of ignition data occurred at the highest temperature for varying equivalence ratios. A tertiary surrogate for diesel and a tertiary surrogate for gasoline were chosen to calculate conditions after reflected shock in shock tube and compression in RCM. Then, simulations of ignition delay time for diesel/gasoline blends were conducted using a detailed model. Finally, kinetic analyses were performed to gain insight into the ignition characteristic of diesel/gasoline blends.

The gel fuel ignition at local conductive heating

An experimental study of ignition of a gel fuel at local conductive heating has been performed. The initial temperatures of the fuel samples were 120, 260, and 290 K. Metal particles in the shape of a sphere, a disk, and a parallelepiped with a characteristic size of 10 mm were used as ignition sources. The initial temperature of the heating sources varied in the range of 1050–1350 K. The use of software and hardware high-speed video recording (4200 frames per second at a maximum resolution of 1280 × 800 pixels) allowed analyzing the regularities of physical and chemical processes, occurring during the induction period. Limit conditions necessary for stable ignition of a gel fuel and dependences of ignition delay time on parameters of a local heating source have been established. A low-inertia (sampling rate of 100 Hz) measuring system based on miniature thermocouples and an oscilloscope served to establish the velocities of the gel fuel melting front motion from the contact boundary between the metal particle and the fuel in the direction of the thermal wave propagation during induction period and under stationary combustion. The mathematical model of ignition of the gel fuel has been developed within the framework of the mathematical apparatus of continuum mechanics and the theory of chemical kinetics. The results of numerical simulation are in good agreement with the experimental data obtained. The applicability limits of the developed model within which it reliably describes characteristics of ignition have been established. The mathematical model may be used to develop advanced energy-efficient technologies for ignition of new fuels in wide ranges of parameters of the system gel fuel/local heating source.

Ignition delay times of decalin over low-to-intermediate temperature ranges: Rapid compression machine measurement and modeling study

Autoignition characteristics of decalin, a bicyclic alkane, were investigated in a heated rapid compression machine (RCM) over a wide range of conditions. Ignition delay time (IDT) was measured at compressed pressures of 10, 15 and 20 bar, for equivalence ratios of 0.5, 1.0, 1.5 and 2.0, and at temperatures in the range 631−930 K. Negative temperature coefficient (NTC) behavior of decalin ignition delay time was observed within the temperatures of 750−860 K, in which the ignition delay time increases with rising temperature. The dependence of ignition delay time on compressed pressure, equivalence ratio, and oxygen concentration was systemically studied. A reasonable modification was made to a literature mechanism. The simulation results using the tuned mechanism are found to well capture the dependence of the measured ignition delay time on temperautre, pressure, equivalence ratio, and oxygen concentration over the entire temperature range. Correlation formulas of the simulated and measured ignition delay times were proposed to quantitatively reveal the ignition delay time dependence and to evaluate the mechanism performance. A reaction pathway analysis was carried out at low temperature (700 K), NTC temperature (850 K), and high temperature (1000 K), respectively, to identify the dominant reaction pathways consuming decalin and intermediate species. A sensitivity analysis was also performed at different temperatures and equivalence ratios to find out the important reactions that promote and inhibit decalin autoignition.

Experimental and kinetic study on ignition delay times of lean n-butane/hydrogen/argon mixtures at elevated pressures

Measurements on ignition delay times of n-butane/hydrogen/oxygen mixtures diluted by argon were conducted using the shock tube at pressures of 2, 10 and 20 atm, temperatures from 1000 to 1600 K and hydrogen fractions (XH2) from 0 to 98%. It is found that hydrogen addition has a non-linear promoting effect on ignition delay of n-butane. Results also show that for XH2 less than 95%, ignition delay time shows an Arrhenius type dependence and the increase of pressure and temperature lead to shorter ignition delay times. However, for XH2 = 98% and 100% mixtures, non-monotonic pressure dependence of ignition delay time were observed. The performances of the Aramco2.0 model, San Diego 2016 model and USC2.0 model were evaluated against the experimental data. Only the Aramco2.0 model gives a reasonable agreement with all the measurements, which was conducted in this study to interpret the effect of pressure and hydrogen addition on the ignition chemistry of n-butane.

Studying solid fuel ignition by CO2-laser

The ignition data on Siberian bituminous coal and lignite by a continuous CO 2 -laser were presented in current work. Measurements were carried out at the heat flux density range of 90–150 W/cm 2 in air. Dependences of ignition delay time on heat flux density were obtained along with the surface temperatures at the moment of fuel sample ignition. Ignition delay time was found to be in the range 50–520 ms for lignite. For bituminous coal ignition the delay time values are larger 1.5–2 times. However, fuel surface temperature in the moment of ignition is the same for both fuels, i.e. 640–680 °C.

Shock tube study of n-nonane/air ignition over a wide range of temperatures

Ignition delay experiments for gas phase n-nonane/air mixtures have been performed behind reflected shock waves over a wide temperature range of 684–1448K, pressures of 2.0–15.0atm, and equivalence ratios of 0.5, 1.0, and 2.0. Ignition delay times were determined using electronically excited CH emission and reflected shock pressure signals monitored at the sidewall of the shock tube. A negative-temperature-coefficient (NTC) behavior of n-nonane/air ignition was observed at temperatures of 800–950K. Dependence of ignition delay time upon temperature, pressure, and equivalence ratio was investigated systematically. High temperature (T>1000K) results show that the effect of equivalence ratio on ignition delay times is different at low and high pressures. In the NTC region, ignition delay times are highly sensitive to equivalence ratio and pressure. Thepresentignitiondata are in satisfactory agreement with predictions of two widely used chemical kinetic mechanisms. Sensitivity and reaction pathway analyses reveal that the dominating reactions affecting ignition delay times and reaction pathways during ignition process for n-nonane/air are quite different at high and low temperatures. Comparison of n-nonane/air ignition delay times with those of other larger n-alkanes (n-heptane, n-octane, n-decane, n-dodecane, and n-tetradecane) indicates that the length of n-alkanes chain influences little on ignition delay times of n-alkanes. The present results are useful for understanding ignition characteristics of n-nonane and providing experimental data to validate chemical kinetic mechanisms for n-nonane.

Numerical research of heat and mass transfer at the ignition of system “fabric – combustible liquid – oxidant” by the local energy source

A numerical research was executed for macroscopic regularities determination of heat and mass transfer processes under the conditions of phase transformation and chemical reaction at the ignition of vapour coming from fabrics impregnated by typical combustible liquid into oxidant area at the local power supply. Limit conditions of heterogeneous system “fabric – combustible liquid – oxidant” ignition at the heating of single metal particle was established. Dependences of ignition delay time on temperature and rates of local power source were obtained.

Ignition of Coal Dust from the Tomsk Region Talovsky Deposit by Air Flow

This paper focuses on the experimental studies of the ignition characteristics of brown coal dust particles from the Tomsk region Talovsky deposit under the conditions of convective heating. The boundary conditions of combustion initiation have been established. The approximation dependence of ignition delay time from the temperature of a heat source has been found.

Ignition in compositionally and thermally stratified n-heptane/air mixtures: A direct numerical simulation study

This paper presents a direct numerical simulation (DNS) study of ignition characteristics in compositionally and thermally stratified n-heptane/air mixtures, relevant to homogeneous charge compression ignition (HCCI) engines. Nine two-dimensional constant volume cases are simulated. For each of three different mean temperatures, all within the negative temperature coefficient (NTC) region, three cases having different levels of mixture stratification are considered. Local temperature fluctuations, negatively correlated with mixture fraction fluctuations are also considered. It is shown that as the level of stratification increases, the heat release rate has a lower peak in the first stage for the mean temperature of 850K. An opposite behaviour is observed for the mean temperature of 950K. Inspection of the homogeneous ignition delay suggests that the distribution of ignition delay time at the initial conditions leads to the observed behaviour.In the second stage of ignition, stratification reduces the ignition delay time and the peak heat release rate relative to a homogeneous case for all three mean temperatures. However, a non-monotonic behaviour for the peak heat release rate in the second stage is observed for the mean temperatures of 850 and 950 K. This behaviour for cases with the mean temperature of 950K can be attributed to the mixture-fraction dependence of ignition delay times, which shows a local minimum at mixtures which are richer than the mean mixture fraction. Due to this effect, increased levels of stratification results in the introduction of a larger range of mixture fractions having similar ignition delay times. However, for cases with the mean temperature of 850K, the distribution of homogeneous ignition delay time does not explain the non-monotonic behaviour. For these cases, the analysis suggests that molecular mixing plays an important role in the steepening of the heat release rate at higher stratification levels.

Effect of pressure and equivalence ratio on the ignition characteristics of dimethyl ether-hydrogen mixtures

Experimental and numerical study on the effect of pressure and equivalence ratio on the ignition delay times of the DME/H2/O2 mixtures diluted in argon were conducted using a shock tube and CHEMKIN II package at equivalence ratios of 0.5–2.0, pressures of 1.2–10 atm and hydrogen fractions of 0–100%. It was found that the measured ignition delay times of the DME/H2 mixtures demonstrate three ignition regimes. For the DME/H2 mixture at XH2 ≤80%, the ignition is controlled by the DME chemistry and ignition delay times present a typical Arrhenius pressure dependence and weak equivalence ratio dependence. For the DME/H2 mixture at 80% < XH2 < 98%, the ignition is controlled by the combined chemistries of DME and hydrogen, and the ignition delay times give higher ignition activation energy at higher pressures and a typical Arrhenius equivalence ratio dependence. However, for the DME/H2 mixture at XH2≥98%, the ignition is controlled by the hydrogen chemistry and ignition delay time shows complex pressure dependence and weak equivalence ratio dependence. Comparison of the measurements of neat DME and neat hydrogen with the calculations using three generally accepted mechanisms, NUIG Aramco Mech 1.3 [1], LLNL DME Mech [2–4] and Princeton-Zhao Mech [5], shows that NUIG Aramco Mech 1.3 gives the best predictions and can well capture the pressure and equivalence ratio dependence at various hydrogen fractions. The sensitivity and normalized H-radicals consumption analysis were performed using NUIG Aramco Mech 1.3 and the key reactions that control the ignition characteristics of DME/H2 mixtures were revealed. Further chemical kinetic analysis was made to interpret the ignition delay time dependence on pressure and equivalence ratio at varied hydrogen fractions.