A wide-range experimental and kinetic modeling study of the pyrolysis and oxidation of 2-butyne

To reduce particulate emissions leading to a cleaner environment, it is important to understand how polycyclic-aromatic hydrocarbons (PAHs) and their precursors are formed during combustion. 2-butyne can decompose to propargyl and allyl radicals. These radicals can produce benzene and other PAHs, leading to the formation of soot. In the present study, pyrolysis, oxidation, and laminar flame speed experiments were performed for 2-butyne. The pyrolysis experiments were conducted in a single-pulse shock tube at 2 bar in the temperature range 1000 – 1500 K. Ignition delay times for 2-butyne/‘air’ mixtures were measured in the pressures range 1 – 50 bar, over the temperature range 660 – 1630 K, at equivalence ratios of 0.5, 1.0, and 2.0 using rapid compression machines and shock tubes. Moreover, laminar flame speed (LFS) experiments were performed at ambient temperature, at p = 1 – 3 atm, over an equivalence ratio range of 0.6 – 1.8. A new, detailed chemical kinetic model for 2-butyne has been developed and widely validated against the data measured in this study and those available in the literature. The significant reactions for 2-butyne pyrolysis, ignition, and oxidation are identified and discussed using flux and sensitivity analyses.     

A wide-range experimental and kinetic modeling study of the pyrolysis and oxidation of 1-butyne

Allyl and propargyl radicals are involved in the production of the first aromatic ring, which is considered a crucial process in forming polycyclic-aromatic hydrocarbons and soot. 1-Butyne decomposes to propargyl radicals during its pyrolysis and oxidation. To improve our knowledge of the kinetics of 1-butyne, its pyrolysis, oxidation, and laminar flame speed properties have been measured. Pyrolysis experiments were performed in a single-pulse shock tube at 2 bar in the temperature range 1000 – 1600 K. Ignition delay times for 1-butyne/‘air’ mixtures were measured at pressures of 1, 10, 30, and 50 bar, in the temperature range 680 – 1580 K, at equivalence ratios of 0.5, 1.0, and 2.0 using rapid compression machines and shock tubes. Furthermore, laminar flame speeds were measured at ambient temperature, at p = 1, 2, 3 atm, over an equivalence ratio range of 0.6 – 1.9. A new detailed mechanism for 1-butyne has been developed and widely validated using the new experimental data and those available in the literature. Important reactions of 1-butyne pyrolysis and oxidation are determined through flux and sensitivity analyses.     

Detailed and simplified kinetic models of n-dodecane oxidation: The role of fuel cracking in aliphatic hydrocarbon combustion

A detailed kinetic model is proposed for the combustion of normal alkanes up to n-dodecane above 850 K. The model was validated against experimental data, including fuel pyrolysis in plug flow and jet-stirred reactors, laminar flame speeds, and ignition delay times behind reflected shock waves, with n-dodecane being the emphasis. Analysis of the computational results reveal that for a wide range of combustion conditions, the kinetics of fuel cracking to form smaller molecular fragments is fast and may be decoupled from the oxidation kinetics of the fragments. Subsequently, a simplified model containing a minimal set of 4 species and 20 reaction steps was developed to predict the fuel pyrolysis rate and product distribution. Combined with the base C₁-C₄ model, the simplified model predicts fuel pyrolysis rate and product distribution, laminar flame speeds, and ignition delays as close as the detailed reaction model.     

Experimental and modeling study of the low to high-temperature oxidation of the methyl isopropyl ketone in O2/N2/Ar and O2/CO2/Ar atmospheres

Few studies on the low-temperature combustion behavior of MIPK, being a promising fuel additive, have been conducted. In this work, ignition delay times (IDTs) of MIPK were measured in the temperatures ranging between 780–910 K and pressures of 20 and 25 bar using a rapid compression machine (RCM). Oxy-fuel combustion combined with biofuel could remove CO₂ from the atmosphere. The IDTs of MIPK were measured in the temperatures ranging between 1125–1600 K under the O₂/CO₂ atmosphere at the pressures of 1 and 10 bar using a shock tube. A low to high-temperature MIPK kinetic model (HUST-MIPK model) was proposed, in which the low-temperature sub-model consists of 19 low-temperature reaction classes and was constructed by analogy-based method, the high-temperature sub-model was adapted from the works of Cheng et al.. The predictions of HUST-MIPK model are in good agreement with the present low-temperature IDTs, high-temperature O₂/CO₂ atmosphere IDTs, and the literature experimental data. The negative temperature coefficient (NTC) behavior was not observed in the temperature range from 790 to 910 K in the present RCM experiments, but was observed for methyl propyl ketone (MPK) and diethyl ketone (DEK) under similar conditions. The low-temperature chemistry of three pentanone isomers (MIPK, MPK, and DEK) was compared using the flux and sensitivity analysis. The comparison of the experimental high-temperature IDTs between O₂/CO₂ and O₂/Ar atmospheres indicates the IDTs of MIPK under O₂/CO₂ atmosphere are longer than those under O₂/Ar atmosphere at 1 bar, and the effects of CO₂ are almost independent of the pressure. The physical and chemical effects of CO₂ on the ignition were studied in detail.     

Combustion behavior of ammonia blended with diethyl ether

Ammonia (NH₃) is recognized as a carbon-free hydrogen-carrier fuel with a high content of hydrogen atoms per unit volume. Recently, ammonia has received increasing attention as a promising alternative fuel for internal combustion engine and gas turbine applications. However, the viability of ammonia fueling future combustion devices has several barriers to overcome. To overcome the challenge of its low reactivity, it is proposed to blend it with a high-reactivity fuel. In this work, we have investigated the combustion characteristics of ammonia/diethyl ether (NH₃/DEE) blends using a rapid compression machine (RCM) and a constant volume spherical reactor (CVSR). Ignition delay times (IDTs) of NH₃/DEE blends were measured using the RCM over a temperature range of 620 to 942 K, pressures near 20 and 40 bar, equivalence ratios (Φ) of 1 and 0.5, and a range of mole fractions of DEE, χ_DEE, from 0.05 to 0.2 (DEE/NH₃ = 5 – 20%). Laminar burning velocities of NH₃/DEE premixed flames were measured using the CVSR at 298 K, 1 bar, Φ of 0.9 to 1.3, and χ_DEE from 0.1 to 0.4. Our results indicate that DEE promotes the reactivity of fuel blends resulting in significant shortening of the ignition delay times of ammonia under RCM conditions. IDTs expectedly exhibited strong dependence on pressure and equivalence ratio for a given blend. Laminar burning velocity was found to increase with increasing fraction of DEE. The burnt gas Markstein length increased with equivalence ratio for χ_DEE = 0.1 as seen in NH₃-air flames, while the opposite evolution of Markstein length was observed with Φ for 0.1 < χ_DEE ≤ 0.4, as observed in isooctane-air flames. A detailed chemical kinetics model was assembled to analyze and understand the combustion characteristics of NH₃/DEE blends.     

Measurements of the critical initiation radius and unsteady propagation of n-decane/air premixed flames

Unsteady flame propagation, the critical radius for flame initiation, and multiple flame regimes of n-decane/air mixtures are studied experimentally and computationally using outwardly propagating spherical flames at various equivalence ratios and pressures. The transient flame speeds, trajectories, and critical radius are measured. The experimental results are compared with direct numerical simulations using detailed high temperature kinetic models. Both experimental and numerical results show that there exist multiple flame regimes in the unsteady spherical flame initiation process. The transition between the flame regimes depends strongly on the mixture equivalence ratio (or Lewis number). It is found that there is a critical flame radius and that it increases dramatically as the mixture equivalence ratio and pressure decrease. The large increase of critical flame radius leads to a dramatic increase of the minimum ignition energy. Furthermore, the flame thickness and the radical pool concentration change significantly during the transition from the ignition flame regime to the self-sustained propagating flame regime. For the same steady state flame speeds, the predicted unsteady flame speeds and the critical flame radius differ significantly from the experimental results. Moreover, different chemical kinetic mechanisms predict different unsteady flame speeds. The existence of multiple flame regimes and the large critical radius of lean liquid fuel mixtures make the ignition of lean mixtures at low pressure and the development of a validated kinetic model more challenging. The unsteady flame regimes, speeds, and critical flame radius should be included as targets of future kinetic model development for turbulent combustion modeling.     

Experimental characterization of spark ignited ammonia combustion under elevated oxygen concentrations

Due to its nature as a carbon free fuel and carrying hydrogen energy ammonia has received a lot of attention recently to be used as an alternative to fossil fuel in gas turbine and internal combustion engines. However, several barriers such as long ignition delay, slow flame speed, and low reactivity need to be overcome before its practical applications in engines. One potential approach to improve the ignition can be achieved by using oxygen enriched combustion. In this study, oxygen-enriched combustion of ammonia is tested in a constant volume combustion chamber to understand its combustion characteristics like flame velocity and heat release rates. With the help of high speed Schlieren imaging, an ammonia-oxygen flame is studied inside the combustion chamber. The influence of a wide range of oxygen concentrations from 15 to 40% are tested along with equivalence ratios ranging from 0.9 to 1.15. Ammonia when ignited at an oxygen concentration of 40% with an equivalence ratio of $\varphi =$ 1.1 at 10 bar has a maximum flame velocity of 112.7 cm/s. Reduced oxygen concentration also negatively affects the flame velocity, introducing instabilities and causing the flame to develop asymmetrically due to buoyancy effects inside the combustion chamber. Heat release rate (HRR) curves show that increasing the oxygen concentration from 21 to 35% of the mixture can help reduce the ignition delays. Peak HRR data shows increased sensitivity to air fuel ratios with increased oxygen concentrations in the ambient gas. HRR also shows an overall positive dependence on the oxygen concentration in the ambient gas.     

Effect of ignition energy on the uncertainty in the determination of laminar flame speed using outwardly propagating spherical flames

The initial propagation processes of expanding spherical flames of CH₄/N₂/O₂/He mixtures at different ignition energies were investigated experimentally and numerically to reduce the effect of ignition energy on the accurate determination of laminar flame speeds. The experiments were conducted in a constant-volume combustion bomb at initial pressures of 0.07???0.7?MPa, initial temperatures of 298???398?K, and equivalence ratios of 0.9???1.3 with various Lewis numbers. The A-SURF program was employed to simulate the corresponding flame propagation processes. The results show that elevating the ignition energy increases the initial flame propagation speed and expands the range of flame trajectory which is affected by ignition energy, but the increase rates of the speed and range decrease with the ignition energy. Based on the trend of the minimum flame propagation speed during the initial period with the ignition energy, the minimum reliable ignition energy (MRIE) is derived by considering the initial flame propagation speed and energy conservation. It is observed that MRIE first decreases and then increases with the increasing equivalence ratio and monotonously decreases with increasing initial pressure and temperature. As the Lewis number rises, MRIE increases. The results also suggest that during the data processing of the spherical flame experiment, the accuracy of determination of laminar flame speeds can be enhanced when taking the flame radius influenced by MRIE as the lower limit of the flame radius range. Then the flame radius influenced by MRIE was defined as RFR. It can also be found that there exist nonlinear relationships between RFR and the equivalence ratio and Lewis number, and the RFR decreases with increasing initial pressure and temperature.     

Laminar flame speed and shock-tube multi-species laser absorption measurements of Dimethyl Carbonate oxidation and pyrolysis near 1 atm

Dimethyl Carbonate (DMC) is a carbonate ester that can be produced in an environment-friendly way from methanol and CO₂. DMC is one of the main components of the flammable electrolyte used in Li-ion batteries, and it can also be used as a diesel fuel additive. Studying the combustion chemistry of DMC can therefore improve the use of biofuels and help developing safer Li-ion batteries. The combustion chemistry of DMC has been investigated in a limited number of studies. The aim of this study was to complement the scarce data available for DMC combustion in the literature. Laminar flame speeds at 318 K, 363 K, and 463 K were measured for various equivalence ratios (ranging from 0.7 to 1.5) in a spherical vessel, greatly extending the range of conditions investigated. Shock tubes were used to measure time histories of CO and H₂O using tunable laser absorption for the first time for DMC. Characteristic reaction times were also measured through OH* emission. Shock-tube spectroscopic measurements were performed under dilute conditions, at three equivalence ratios (fuel-lean, stoichiometric, and fuel-rich) between 1260 and 1660 K near 1.3 ± 0.2 atm, and under pyrolysis conditions (98%+dilution) ranging from 1230 to 2500 K near 1.3 ± 0.2 atm. Laminar flame speed experiments were performed around atmospheric pressure. Detailed kinetics models from the literature were compared to the data, and it was found that none are capable of predicting the data over the entire range of conditions investigated. A numerical analysis was performed with the most accurate model, underlining the need to revisit at least 3 key reactions involving DMC.     

Experiments and modeling of ignition delay times, flame structure and intermediate species of EHN-doped stoichiometric n-heptane/air combustion

The combustion of stoichiometric Ethyl-hexyl-nitrate (EHN)-doped n-heptane/oxygen/argon and (EHN)-doped n-heptane/air mixtures, respectively, was investigated in a low-pressure burner with a molecular-beam mass spectrometer and ignition delay-time (τ_ign) measurements were performed in a high-pressure shock tube. The experiments with the low-pressure flame were used for the determination of the flame structure including concentration profiles of reactants, products and important intermediates in the flame. The shock tube experiments provided τ_ign for a temperature range of 690 K ? T ? 1275 K at a pressure of 40 ± 2 bar for stoichiometric and lean mixtures under engine relevant conditions. A chemical mechanism for n-heptane/EHN mixtures was developed from an automatically generated mechanism for n-heptane by manually adding reactions to describe the influence of EHN. This mechanism was validated against the shock-tube data for various temperatures, levels of EHN-doping and equivalence ratios by homogeneous reactor calculations.The ignition delay times predicted by the model agree well with the shock tube results for a large range of temperatures, equivalence ratios and EHN concentrations. The influence of EHN onto ignition delay was largest in the low-temperature regime (770-1000 K).Numerical analysis suggests that the prevalent reason for the ignition-enhancing effect of EHN is the formation of highly reactive heptyl radicals by thermal decomposition of EHN. Due to this comparatively simple and generic mechanism, EHN is expected to have a similar ignition-enhancing effect also for other hydrocarbon fuels.     

Insights into the oxidation of propylene oxide through the analysis of experiments and kinetic modeling

Cyclic ethers are important intermediates in the oxidation of hydrocarbons and biofuels. Studying the oxidation and pyrolysis of cyclic ethers will help in improving our understanding of this functional group and provide consistency to the base mechanism where they play an important role. In this aspect, propylene oxide has been investigated in this study by obtaining ignition delay time measurements in the rapid compression machine and shock tube. The experiments were performed in a range of pressures varying from 10 to 40 bar at different equivalence ratios (0.5–2.0) and dilution percentages. Additionally, speciation measurements in the shock tube at pyrolysis conditions have been performed at a pressure of 40 bar to explore the isomerization pathways. A detailed kinetic mechanism was developed to describe both the oxidation and pyrolysis chemistry of propylene oxide. The mechanism is not only able to predict the data obtained from this study but also reproduces the data from the literature in a consistent trend. For a better understanding of the oxidation and pyrolysis chemistry of propylene oxide, the kinetic analyses were performed using the developed mechanism to comprehend the important reaction pathways and sensitive reactions. At the investigated regime, the consumption of propylene oxide through its isomerization channels is the critical pathway that controls the reactivity of the fuel.     

A kinetics and dynamics study on the auto-ignition of dimethyl ether at low temperatures and low pressures

Though the combustion chemistry of dimethyl ether (DME) has been widely investigated over the past decades, there remains a dearth of ignition data that examines the low-temperature, low-pressure chemistry of DME. In this study, DME/‘air’ mixtures at various equivalence ratios from lean (0.5) to extremely rich (5.0) were ignited behind reflected shock waves at a fixed pressure (3.0 atm) over the temperature range 625–1200 K. The ignition behavior is different from that at high-pressures, with a repeatable ignition delay time fall-off feature observed experimentally in the temperature transition zone from the negative temperature coefficient (NTC) regime to the high-temperature regime. This could not be reproduced using available kinetic mechanisms as conventionally homogeneous ignition simulations. The fall-off behavior shows strong equivalence ratio dependence and disappears completely at an equivalence ratio of 5.0. A local ignition kernel postulate was implemented numerically to quantifiably examine the inhomogeneous premature ignition. At low temperature, no pre-ignition occurs in the mixture. A conspicuous discrepancy was observed between the measurements and constrained UV simulations at temperatures beyond the NTC regime. A third O₂ addition reaction sub-set was incorporated into AramcoMech 3.0, together with related species thermochemistry calculated using the G3/G4/CBS-APNO compound method, to explore the low-temperature deviation. The new reaction class does not influence the model predictions in IDTs, but the updated thermochemistry does. Sensitivity analyses indicate that the decomposition of hydroperoxy-methylformate plays a critical role in improving the low-temperature oxidation mechanism of DME but unfortunately, the thermal rate coefficient has never been previously investigated. Further experimental and theoretical endeavors are required to attain holistic quantitative chemical kinetics based on our understanding of the low-temperature chemistry of DME.     

Chemical effects of ferrocene and 2-ethylhexyl nitrate on a low-octane gasoline: An experimental and numerical RCM study

An in-depth understanding of fuel additives chemical effects is crucial for optimal use or additive design dedicated to more efficient and cleaner combustion. This study aims at investigating the effect of an organometallic octane booster additive named ferrocene on the combustion of a low-octane gasoline at engine-relevant conditions. Rapid compression machine experiments were carried out at 10 bar, from 675 to near 1000 K for stoichiometric (Φ = 1) and lean (Φ = 0.5) mixtures. The neat surrogate fuel was a blend of toluene and n-heptane whose research octane number was 84. The doping level of additive was set at 0.1% molar basis. Ferrocene does not show a remarkable effect on the 1st- stage ignition but presents a strong inhibiting effect on the main ignition of the surrogate fuel at both equivalence ratios. The inhibiting effect increases with temperature within the investigated range. The negative temperature coefficient (NTC) behavior of the surrogate fuel is enhanced by ferrocene. A kinetic model developed by literature data assembly as well as a novel sub-mechanism involving the formation of alcohols from the reactions of iron species is proposed. The kinetic model developed simulates the inhibiting effect of ferrocene reasonably well at both equivalence ratios. Thanks to the validated kinetic model, the chemical effect of ferrocene on the fuel combustion is explored and compared with 2-ethylhexyl nitrate (EHN), which is a conventional reactivity enhancer. Three major differences between the two additives were identified: the high-temperature stability of the fuel additive, the influence of additive on the toluene reactivity and the effect of the additive on the NTC behavior. The results presented in this study contribute to the in-depth comprehension of chemical effect of two fuel additives (ferrocene and EHN) having opposite effects on fuel reactivity.     

超声速预混可燃气流的点火与燃烧

在激波风洞一激波管组合设备上开展了碳氢燃料超声速预混可燃气流的点火与燃烧实验研究。实验结果表明:利用激波对燃料进行预热,并以高温燃气作为引导火焰,可以有效缩短汽油空气超声速可燃混气的点火延迟时间,使之缩短到 0.2 ms以下。利用纹影照片对超声速燃烧流场结构作出了分析;研究了超声速预混可燃气流的温度以及当量比对超声速燃烧流场结构、点火与火焰传播特性的影响。     

Isomer-specific influences on ignition and intermediates of two C5 ketones in an RCM

Ketones have been considered as potential biofuels and main components of blend stock for internal engines. To better understand the chemical kinetics of ketones, ignition delay times of 2-pentanone (propyl methyl ketone, PMK) and 3-pentanone (diethyl ketone, DEK) were measured at temperatures of 895–1128 K under 10 and 20 bar, at equivalence ratios (?) of 0.5 and 1.0 in a rapid compression machine (RCM). To explore the impact of carbonyl functionality and resonance stabilized structures of fuel radicals on their combustion kinetics, high-temperature pyrolysis at 1130 K and relatively low-temperature oxidation at 950 K studies were performed in an RCM, and the time-resolved species concentration profiles under these two conditions were quantified using a fast sampling system and gas chromatography (GC). A new kinetic model containing low-temperature reactions was built aiming at predicting the pyrolysis and oxidation behaviors of both ketones. The consumption pathways of the resonance stabilization fuel radicals through oxygen addition and following reactions are promoted since the decomposition rates of these radicals are about 4 orders magnitudes lower than regular fuel radicals. The occurrences of the so-called “addition-dissociation reactions”, i.e., ketones reacting with a hydrogen yielding aldehyde or reacting with a methyl radical yielding shorter-chain-length ketones, are verified in pyrolysis experiments. Based on experiments and model analysis, the carbonyl functionality in both ketones is preserved during the process of β-scissions of fuel radicals and α-scissions of fuel-related acyl radicals, resulting in the direct formation of CO and ketene. However, the position of carbonyl functionality has a significant impact on the species pools.     

Experimental and kinetic modeling studies on the auto-ignition of methyl crotonate at high pressures and intermediate temperatures

Biofuels, including biodiesel have the potential to partially replace the conventional diesel fuels for low-temperature combustion engine applications to reduce the CO₂ emission. Due to the long chain lengths and high molecular weights of the biodiesel components, it is quite challenging to study the biodiesel combustion experimentally and computationally. Methyl crotonate, a short unsaturated fatty acid methyl ester (FAME) is chosen for this chemical kinetic study as it is considered as a model biodiesel fuel. Auto-ignition experiments were performed in a rapid compression machine (RCM) at pressures of 20 and 40 bar under diluted conditions over a temperature range between 900 and 1074 K, and at different equivalence ratios (? = 0.25, 0.5 and 1.0). A chemical kinetic mechanism is chosen from literature (Gaïl et al. 2008) and is modified to incorporate the low-temperature pathways. The mechanism is validated against existing shock tube data (Bennadji et al. 2009) and the present RCM data. The updated mechanism shows satisfactory agreement with the experimental data with significant improvements in low-temperature ignition behavior. The key reactions at various combustion conditions and the improved reactivity of the modified mechanism are analyzed by performing sensitivity and path flux analysis. This study depicts the importance of low-temperature pathways in predicting the ignition behavior of methyl crotonate at intermediate and low temperatures.     

Experimental and modeling study on the ignition delay times of ammonia/methane mixtures at high dilution and high temperatures

Ammonia is a promising alternative clean fuel due to its carbon-free character and high hydrogen density. However, the low reactivity of ammonia and the potential high NO_x emissions hinder its applications. Blending methane into ammonia can effectively improve the reactivity of pure NH₃. In addition, lean combustion, as a high-efficiency and low-pollution combustion technology, is an effective measure to control the potential increase in NO_x emissions. In the present work, the ignition delay times (IDTs) of NH₃/CH₄ mixtures highly diluted in Ar (98%) with CH₄ mole fractions of 0%, 10%, and 50% were measured in a shock tube at an equivalence ratio of 0.5, pressures of 1.75 and 10 bar and a temperature range of 1421 K - 2149 K. A newly comprehensive kinetic model (named as HUST-NH₃ model) for the NH₃/CH₄ mixtures oxidation was developed based on our previous work. Four kinetic models, the HUST-NH₃ model, Glarborg model [19], Okafor model [7], and CEU model [10], were evaluated against the ignition delay times, laminar flame speeds, and species profiles of pure ammonia and ammonia/methane mixtures from the present work and literature. The simulation results indicated that the HUST-NH₃ model shows the best performance among the above four models. Kinetic analysis results indicated that the absence of NH₃ + M = NH₂ + H + M (R819) and N₂H₂ + M = H + NNH + M (R902) in the CEU model and Okafor model cause the deviations between the experimental and simulation results. The overestimation of the rate constants of NH₂ + NO = NNH + OH (R838) in the Glarborg model is the main reason for the overprediction of the NH₃ laminar flame speeds.     

Flame kernel characterization of laser ignition of natural gas–air mixture in a constant volume combustion chamber

In this paper, laser-induced ignition was investigated for compressed natural gas–air mixtures. Experiments were performed in a constant volume combustion chamber, which simulate end of the compression stroke conditions of a SI engine. This chamber simulates the engine combustion chamber conditions except turbulence of air–fuel mixture. It has four optical windows at diametrically opposite locations, which are used for laser ignition and optical diagnostics simultaneously. All experiments were conducted at 10 bar chamber pressure and 373 K chamber temperature. Initial stage of combustion phenomena was visualized by employing Shadowgraphy technique using a high speed CMOS camera. Flame kernel development of the combustible fuel–air mixture was investigated under different relative air–fuel ratios (λ=1.2?1.7) and the images were interrogated for temporal propagation of flame front. Pressure-time history inside the combustion chamber was recorded and analyzed. This data is useful in characterizing the laser ignition of natural gas–air mixture and can be used in developing an appropriate laser ignition system for commercial use in SI engines.     

A study of ignition and combustion of liquid hydrocarbon droplets in premixed fuel/air mixtures in a rapid compression machine

The combustion of two fuels with disparate reactivity such as natural gas and diesel in internal combustion engines has been demonstrated as a means to increase efficiency, reduce fuel costs and reduce pollutant formation in comparison to traditional diesel or spark-ignited engines. However, dual fuel engines are constrained by the onset of uncontrolled fast combustion (i.e., engine knock) as well as incomplete combustion, which can result in high unburned hydrocarbon emissions. To study the fundamental combustion processes of ignition and flame propagation in dual fuel engines, a new method has been developed to inject single isolated liquid hydrocarbon droplets into premixed methane/air mixtures at elevated temperatures and pressures. An opposed-piston rapid compression machine was used in combination with a newly developed piezoelectric droplet injection system that is capable of injecting single liquid hydrocarbon droplets along the stagnation plane of the combustion chamber. A high-speed Schlieren optical system was used for imaging the combustion process in the chamber. Experiments were conducted by injecting diesel droplet of various diameters (50 µm < d_o < 400 µm), into methane/air mixtures with varying equivalence ratios (0 < ϕ < 1.2) over a range of compressed temperatures (700 K < T_c < 940 K). Multiple autoignition modes was observed in the vicinity of the liquid droplets, which were followed by transition to propagating premixed flames. A computational model was developed with CONVERGE™, which uses a 141 species dual-fuel chemical kinetic mechanism for the gas phase along with a transient, analytical droplet evaporation model to define the boundary conditions at the droplet surface. The simulations capture each of the different ignition modes in the vicinity of the injected spherical diesel droplet, along with bifurcation of the ignition event into a propagating, premixed methane/air flame and a stationary diesel/air diffusion flame.     

Effects of fuel stratification on ignition kernel development and minimum ignition energy of n-decane/air mixtures

Fuel-stratified combustion has broad application due to its promising advantages in extension of lean flammability limit, improvement of flame stabilization, enhancement of lean combustion, etc. In the literature, there are many studies on flame propagation in fuel-stratified mixtures. However, there is little attention on ignition in fuel-stratified mixtures. In this study, one-dimensional numerical simulation is conducted to investigate the ignition and spherical flame kernel propagation in fuel-stratified n-decane/air mixtures. The emphasis is placed on assessing the effects of fuel stratification on the ignition kernel propagation and critical ignition condition. First, ignition and flame kernel propagation in homogeneous n-decane/air mixture are studied and different flame regimes are identified. The minimum ignition energy (MIE) of the homogeneous n-decane/air mixture is obtained and it is found to be very sensitive to the equivalence ratio under fuel-lean conditions. Then, ignition and flame kernel propagation in fuel-stratified n-decane/air mixture are investigated. The inner equivalence ratio and stratification radius are found to have great impact on ignition kernel propagation. The MIEs at different fuel-stratification conditions are calculated. The results indicate that for fuel-lean n-decane/air mixture, fuel stratification can greatly promote ignition and reduce the MIE. Six distinct flame regimes are observed for successful ignition in fuel-stratified mixture. It is shown that the ignition kernel propagation can be induced by not only the ignition energy deposition but also the fuel-stratification. Moreover, it is found that to achieve effective ignition enhancement though fuel stratification, one needs properly choose the values of stratification radius and inner equivalence ratio.