正辛醇/正丁醚骨架机理的构建及验证

采用定向关联图误差传播敏感度分析法(DRGEPSA)、同分异构体合并和峰值浓度分析法对正丁醚和正辛醇的详细机理进行简化,并将简化后的正丁醚和正辛醇机理合并,获得了包含117个组分和601个基元反应的骨架机理.利用正丁醚和正辛醇详细机理的着火延迟、正丁醚层流火焰速度的试验数据和正辛醇在射流搅拌反应器(JSR)中组分摩尔浓...     

An experimental investigation of ethylene/O2/diluent mixtures: Laminar flame speeds with preheat and ignition delays at high pressures

The atmospheric pressure laminar flame speeds of premixed ethylene/O₂/N₂ mixtures were experimentally measured over equivalence ratios ranging from 0.5 to 1.4 and mixture preheat temperatures varying from 298 to 470 K in a counterflow configuration. Ignition delay measurements were also conducted for ethylene/O₂/N₂/Ar mixtures using a rapid compression machine at compressed pressures from 15 to 50 bar and in the compressed temperature range from 850 to 1050 K. The experimental laminar flame speeds and ignition delays were then compared to the computed values using two existing chemical kinetic mechanisms. Results show that while the laminar flame speeds are reasonably predicted at room temperature conditions, the discrepancy becomes larger with increasing preheat temperature. A comparison of experimental and computational ignition delay times was also conducted and discussed. Sensitivity analysis further shows that the ignition delay is highly sensitive to the reactions of the vinyl radical with molecular oxygen. The reaction of ethylene with the HO₂ radical was also found to be important for autoignition under the current experimental conditions.     

A comprehensive combustion chemistry study of 2,5-dimethylhexane

Iso-paraffinic molecular structures larger than seven carbon atoms in chain length are commonly found in conventional petroleum, Fischer–Tropsch (FT), and other alternative hydrocarbon fuels, but little research has been done on their combustion behavior. Recent studies have focused on either mono-methylated alkanes and/or highly branched compounds (e.g., 2,2,4-trimethylpentane). In order to better understand the combustion characteristics of real fuels, this study presents new experimental data for the oxidation of 2,5-dimethylhexane under a wide variety of temperature, pressure, and equivalence ratio conditions. This new dataset includes jet stirred reactor speciation, shock tube ignition delay, and rapid compression machine ignition delay, which builds upon recently published data for counterflow flame ignition, extinction, and speciation profiles. The low and high temperature oxidation of 2,5-dimethylhexane has been simulated with a comprehensive chemical kinetic model developed using established reaction rate rules. The agreement between the model and data is presented, along with suggestions for improving model predictions. The oxidation behavior of 2,5-dimethylhexane is compared with oxidation of other octane isomers to confirm the effects of branching on low and intermediate temperature fuel reactivity. The model is used to elucidate the structural features and reaction pathways responsible for inhibiting the reactivity of 2,5-dimethylhexane.     

Numerical investigations on tri-fuel chemical kinetics of hydrogen + Methane +LPG/air mixtures using reduced skeletal mechanism

The combustion and ignition characteristics of three fuels with different reactivities have been investigated by a reduced chemical kinetic model. In the present work, the chemical kinetics of conventional single fuel and binary fuel, relevant to gas-turbine engines, are extended and attempted to explore in the tri-fuel (TF) context, with the help of TF blends of LPG + CH₄+H₂ at the pressure and temperature range of 1–20 atm and 900–2000 K, respectively. The blending of hydrogen with hydrocarbon fuels improves flame propagation, reduces emissions, and increases the combustion performance of the engine. A detailed study is conducted to explore the characteristics of TF mixture over a wide range of operating conditions by considering eight different test mixtures (M1-M8). The test mixtures (M2 to M4) contain higher hydrogen content and thus hydrogen kinetics will tend to dominate, while test mixtures (M6 to M8) contain a higher concentration of hydrocarbons, thus the methyl radical chemistry plays a prominent role in the oxidation process. Such contrasting trends were further explored by extensive chemical kinetic modeling with the help of the reduced USC Mech_50 species model from our previous work [1] to analyze the ignition delay time, laminar flame speed, flame temperature, and heat release rate characteristics. In addition, the reaction pathway analysis through sensitivity analysis of OH and CO radical, and flow rate sensitivity analysis has also been conducted to highlight the essential chemical reactions which play a crucial role in auto-ignition, combustion, and emissions characteristics of TF blends.     

Experimental and kinetic modeling study of pyrolysis and oxidation of n-decane

The pyrolysis of n-decane was investigated in a flow reactor at 5, 30, 150 and 760 Torr, and the oxidation of n-decane at equivalence ratios of 0.7, 1.0 and 1.8 was studied in laminar premixed flames at 30 Torr. In both experiments, synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) was used to identify combustion species and measure their mole fraction profiles. A new detailed kinetic model of n-decane with 234 species and 1452 reactions was developed for applications in intermediate and high temperature regions, and was validated against the experimental results in the present work. The model was also validated against previous experimental data on n-decane combustion, including species profiles in pyrolysis and oxidation in high pressure shock tube and atmospheric pressure flow reactor, jet stirred reactor oxidation, atmospheric pressure laminar premixed flame, counterflow diffusion flame and global combustion parameters such as laminar flame speeds and ignition delay times. In general, the performance of the present model in reproducing these experimental data is reasonably good. Sensitivity analysis and rate of production analysis were conducted to understand the decomposition processes of n-decane.     

Shock tube studies on ignition delay and combustion characteristics of oxygenated fuels under high temperature

A comparative study on ignition delay time and combustion characteristics of four typical oxygenated fuel/air mixtures of dimethyl ether (DME), diethyl ether (DEE), ethanol and E92 ethanol gasoline was conducted through the chemical shock tube. The fuel/air mixtures were measured under the ignition temperature of 1100 to 1800 K, initial pressure of 0.3 MPa and the equivalence ratios of 0.5, 1.0 and 1.5. The experimental results show that the ignition delay time of these four oxygenated fuels satisfies the Arrhenius relation. The reaction H + O₂ = OH + O has a high sensitivity in four fuel/air mixtures during high-temperature ignition, which makes the ignition delay lengthen with the increase of the equivalence ratios. By comparing the ignition delay of four fuels, ether fuels have excellent ignition performance and ether functional group has better ignition promotion than hydroxyl group. Moreover, the carbon chain length also significantly promotes the ignition. Due to the accumulation of a large number of active intermediates and free radicals during the long ignition delay time before ignition, the four fuels all have intense deflagration and generate the highest combustion peak pressure at the relatively low ignition temperature (1150-1300 K). For DME, DEE and ethanol, due to the high content of oxygen in their molecules, the combustion peak pressure and luminous intensity increased with the equivalence ratio, and the combustion is intense after ignition. E92 ethanol gasoline with low oxygen content has a lower combustion peak pressure and a longer combustion duration than the other three fuels, and its highest combustion peak pressure appears in the stoichiometric ratio. The combustion process of E92 ethanol gasoline is more oxygen-dependent than the other three fuels.     

Laminar flame speeds and kinetic modeling of hydrogen/chlorine combustion

In this study, laminar flame speeds at atmospheric pressure are accurately measured for H₂/Cl₂/N₂ mixtures at different equivalence ratios and N₂ mole fractions by the counterflow flame technique. A kinetic mechanism based on recently published and evaluated rate constants is developed to model these measured laminar flame speeds as well as the literature data on the concentrations of H₂, Cl₂, and HCl species in flat-burner flames and the ignition delay times from shock tube experiments. The kinetic model yields satisfactory comparison with these experimental data, and suggests that the reactions involving excited HCl(v) species and energy branching are not of substantial significance in combustion situations, and that the use of accurate elementary rate constants is instead crucial to the accuracy of the reaction mechanism.     

Combustion properties of n-heptane/hydrogen mixtures

The possibility to operate current diesel engines in dual-fuel mode with the addition of hydrogen can be limited by the variation in the combustion properties of the fuel mixture. In the present work, n-heptane was selected as a representative fuel to test the effects of hydrogen addition on the laminar flame speeds and ignition delay times. The spherical bomb technique was used to derive the laminar flame speeds of (n-heptane + hydrogen)/air mixtures (0%, 25%, and 50% hydrogen in the fuel) for an initial temperature of 294 K, pressure of 1 bar, and for equivalence ratios between 0.8 and 1.35. The results showed that average increases of 3% and 10% in the flame speeds were obtained with 25% and 50% hydrogen-enrichment, respectively, while a slight decrease of the Markstein length was obtained. Similar laminar flame speed results were predicted numerically with two kinetic models available in the literature with remarkable accuracy, especially for the Cai and Pitsch model [Cai L, Pitsch H. Combust Flame 2015; 162:1623–37]. The kinetic model was subsequently used to perform additional sensitivity and reaction pathway analyses that showed how the chemistry of n-heptane is not substantially influenced by the presence of hydrogen; while the increase in the flame speed is mainly due to the higher concentrations of radical intermediates. The ignition delay times were measured using the reflected shock tube technique for equivalence ratios equal to 0.832, 1.000, and 1.248, initial nominal pressure of 20 bar, temperatures between 730 K and 1200 K, and for different percentages of hydrogen in the fuel (20%, 50%, and 75%). The Cai and Pitsch model once again did a good job of reproducing the experimental data, indicating how at high temperatures the addition of hydrogen does not significantly affect the ignition delay; and in the NTC region (810 K–920 K) the mixtures composed of (50% n-heptane + 50% hydrogen) and (25% n-heptane + 75% hydrogen) are considerably slower than the reference n-heptane case. This is linked to the concentration of the alkane component and the related low temperature chemistry.     

Flame propagation and counterflow nonpremixed ignition of mixtures of methane and ethylene

The ignition temperature of nitrogen-diluted mixtures of methane and ethylene counterflowing against heated air was measured up to five atmospheres. In addition, the stretch-corrected laminar flame speeds of mixtures of air, methane and ethylene were determined from outwardly-propagating spherical flames up to 10 atmospheres, for extensive range of the lean-to-rich equivalence ratio. These experimental data, relevant to low- to moderately-high-temperature ignition chemistry and high-temperature flame chemistry, respectively, were subsequently compared with calculations using two detailed kinetic mechanisms. A chemical explosive mode analysis (CEMA) was then conducted to identify the dominant ignition chemistry and the role of ethylene addition in facilitating nonpremixed ignition. Furthermore, the hierarchical structure of the associated oxidation kinetics was examined by comparing the sizes and constituents of the skeletal mechanisms of the pure fuels and their mixtures, derived using the method of directed relation graph (DRG). The skeletal mechanism was further reduced by time-scale analysis, leading to a 24-species reduced mechanism from the detailed mechanism of USC Mech II, validated within the parameter space of the conducted experiments.     

An experimental and modeling study of 2-methyl-1-butanol oxidation in a jet-stirred reactor

In an effort to understand the oxidation chemistry of new generation biofuels, oxidation of a pentanol isomer (2-methyl-1-butanol) was investigated experimentally in a jet-stirred reactor (JSR) at a pressure of 10 atm, equivalence ratios of 0.5, 1, 2 and 4 and in a temperature range of 700–1200 K. Concentration profiles of the stable species were measured using GC and FTIR. A detailed chemical kinetic mechanism including oxidation of various hydrocarbon and oxygenated fuels was extended to include the oxidation chemistry of 2-methyl-1-butanol, the resulting mechanism was used to simulate the present experiments. In addition to the present data, recent experimental data such as ignition delay times measured in a shock tube and laminar flame speeds were also simulated with this mechanism and satisfactory results were obtained. Reaction path and sensitivity analyses were performed in order to interpret the results.     

Detailed combustion chemical mechanism for surrogates of representative jet fuels

Surrogate fuels are useful for computational fluid dynamics (CFD) simulations. However, there are two main challenges in surrogate fuel: defining an appropriate surrogate and designing compact and reliable kinetic models. Many works of defining an appropriate surrogate have been done in our previous work. In this study, we focus on obtaining compact and reliable kinetic models for jet surrogate fuels. A detailed mechanism was developed by combining n-dodecane, 2,5-dimethylhexane and toluene sub-mechanisms with AramcoMech 2.0 C₀–C₄ mechanism to mimic the combustion properties of three representative jet fuels. Extensive validation of the component mechanisms in surrogate is performed using abundant experimental data sets. Thereafter, various combustion properties of three jet fuels like ignition delay times, species concentrations in flow reactor and laminar flame speeds are simulated by using this surrogate kinetic mechanism and previous surrogate formulations. The simulations are compared with other well-known surrogate models and a wide range of experimental data. The comparisons show that the developed mechanism is substantially improved prediction accuracy of most considered combustion properties for different jet fuels in a wide range of conditions.     

Modelling of combustion process in a spark ignited hydrogen engine

This paper reports the relative merits of the Eddy Entrainment model over the Reynolds Parameter model in describing the combustion process in a spark ignition engine using hydrogen as fuel. The relative performance of each model is analysed with reference to the experimental data obtained from a single-cylinder engine. From the present investigation and in comparison of the performance of these models with other fuels it is found that both the models are capable of satisfactorily predicting the performance of hydrogen fuel engines due to (a) higher laminar flame speeds and (b) small quench distance.     

A comprehensive chemical kinetic combustion model for the four butanol isomers

Alcohols, such as butanol, are a class of molecules that have been proposed as a bio-derived alternative or blending agent for conventional petroleum derived fuels. The structural isomer in traditional “bio-butanol” fuel is 1-butanol, but newer conversion technologies produce iso-butanol and 2-butanol as fuels. Biological pathways to higher molecular weight alcohols have also been identified. In order to better understand the combustion chemistry of linear and branched alcohols, this study presents a comprehensive chemical kinetic model for all the four isomers of butanol (e.g., 1-, 2-, iso- and tert-butanol). The proposed model includes detailed high-temperature and low-temperature reaction pathways with reaction rates assigned to describe the unique oxidation features of linear and branched alcohols. Experimental validation targets for the model include low pressure premixed flat flame species profiles obtained using molecular beam mass spectrometry (MBMS), premixed laminar flame velocity, rapid compression machine and shock tube ignition delay, and jet-stirred reactor species profiles. The agreement with these various data sets spanning a wide range of temperatures and pressures is reasonably good. The validated chemical kinetic model is used to elucidate the dominant reaction pathways at the various pressures and temperatures studied. At low-temperature conditions, the reaction of 1-hydroxybutyl with O₂ was important in controlling the reactivity of the system, and for correctly predicting C₄ aldehyde profiles in low pressure premixed flames and jet-stirred reactors. Enol–keto isomerization reactions assisted by radicals and formic acid were also found to be important in converting enols to aldehydes and ketones under certain conditions. Structural features of the four different butanol isomers leading to differences in the combustion properties of each isomer are thoroughly discussed.     

A skeletal n-butane mechanism with integrated simplification method

A new skeletal mechanism of n-butane is developed for describing its ignition and combustion characteristics applicable over a wide range of conditions: initial temperature 690–1430 K, pressure 1–30 atm, and equivalence ratio 0.5–2.0. Starting with a detailed chemical reaction kinetic model of 230 species and 1328 reactions (Healy et al., Combust. Flame, 2010), the directed relation graph method is applied as the first step to derive a semi-detailed mechanism with 134 species. Then, the reaction path analysis in conjunction with temperature sensitivity analysis is used to remove the redundant species and reaction paths simultaneously under the condition of low-temperature and moderate-to-high temperatures, respectively. Finally, a skeletal n-butane mechanism consisting of 86 species and 373 reactions can be obtained. Mechanism validation indicates that the new developed skeletal mechanism is in good agreement with the detailed mechanism in predicting the global ignition and combustion characteristics. The new skeletal mechanism is further validated using extensive available literature data including rapid pressure machine ignition delay time, shock-tube ignition delay time, laminar flame speed, and jet-stirred reaction oxidation, covering a large range of temperatures, pressures, and equivalence ratios. The comparison results demonstrate that a satisfactory agreement between predictions and experimental measurements is achieved.     

A computational study and correlation of premixed isooctane–air laminar reaction front properties under spark ignited and spark assisted compression ignition engine conditions

To address the need for reliable premixed laminar burning velocity and thickness information within the spark assisted compression ignition (SACI) combustion regime, a large dataset of simulated reaction fronts has been generated in this work. A transient one dimensional premixed laminar flame simulation was applied to isooctane–air mixtures using a 215 species chemical kinetic mechanism. The simulation was exercised over fuel–air equivalence ratios, unburned gas temperatures and pressures ranging from 0.1 to 1.0, 298 to 1000 K and 1 to 250 bar, respectively, a range that extends beyond that of previous researchers. Steady reaction fronts with burning velocities in excess of 5 cm/s could not be established under all of these conditions, especially when burned gas temperatures were below 1500 K and/or when characteristic reaction front times were on the order of the unburned gas ignition delay. Steady premixed laminar burning velocities were correlated using a modified two-equation form based upon the asymptotic structure of a laminar flame, which produced an average error of 2.5% between the simulated and correlated laminar burning velocities, with a standard deviation of 3.0%. Additional correlations were constructed for reaction front thickness and adiabatic flame temperature. The resulting premixed laminar burning velocity correlation showed good agreement with experiments and existing correlations within the spark-ignited (SI) regime. Analysis of the simulated characteristic reaction front times and ignition delays suggests that homogeneous SACI combustion is most useful under medium and high load operating conditions.     

Chemical kinetic modeling of hydrocarbon combustion

Chemical kinetic modeling of high temperature hydrocarbon oxidation in combustion is reviewed. First, reaction mechanisms for specific fuels are discussed, with emphasis on the hierarchical structure of reaction mechanisms for complex fuels. The concept of a comprehensive mechanism is developed, requiring model validation by comparison with data from a wide range of experimental regimes. Fuels of increasing complexity from hydrogen to n-butane are described in detail, and further extensions of the general approach to other fuels are discussed.Kinetic modification to fuel oxidation kinetics is considered, including both inhibition and promotion of combustion. Simplified kinetic models are then described by comparing their features with those of detailed kinetic models. Finally, application of kinetic models to study real combustions systems are presented, beginning with purely kinetic-thermodynamic applications, in which transport effects such as diffusion of heat and mass can be neglected, such as shock tubes, detonations, plug flow reactors, and stirred reactors. Laminar flames and the coupling between diffusive transport and chemical kinetics are then described, together with applications of laminar flame models to practical combustion problems.     

Laminar flame speeds of primary reference fuels and reformer gas mixtures

The laminar flame speeds of neat primary reference fuels (PRFs), n-heptane and iso-octane, PRF blends, reformer gas, and reformer gas/iso-octane/air mixtures are measured over a range of equivalence ratios at atmospheric pressure, using counterflow configuration and digital particle image velocimetry (DPIV). PRF blends with various octane numbers are studied. The synthetic reformer gas mixture employed herein has a composition that would be produced from the partial oxidation of rich iso-octane/air mixture into CO and H₂, namely, 28% H₂, 25% CO, and 47% N₂. Computationally, the experimentally determined laminar flame speeds are simulated using the detailed kinetic models available in the literature. Both experimental and computational results demonstrate that the flame speeds of hydrocarbon/air mixtures increase with addition of a small amount of reformer gas, and the flame speeds of reformer gas/air mixtures are dramatically reduced with addition of a small amount of hydrocarbon fuel. Furthermore, the number density effect of seeding particles on flame speed measurement is assessed, and the experimental uncertainties associated with the present DPIV setup as well as the linear extrapolation method employed herein are discussed.     

Modelling of hydrogen-blended dual-fuel combustion using flamelet-generated manifold and preferential diffusion effects

In the present study, Reynolds-Averaged Navier-Stokes simulations together with a novel flamelet generated manifold (FGM) hybrid combustion model incorporating preferential diffusion effects is utilised for the investigation of a hydrogen-blended diesel-hydrogen dual-fuel engine combustion process with high hydrogen energy share. The FGM hybrid combustion model was developed by coupling laminar flamelet databases obtained from diffusion flamelets and premixed flamelets. The model employed three control variables, namely, mixture fraction, reaction progress variable and enthalpy. The preferential diffusion effects were included in the laminar flamelet calculations and in the diffusion terms in the transport equations of the control variables. The resulting model is then validated against an experimental diesel-hydrogen dual-fuel combustion engine. The results show that the FGM hybrid combustion model incorporating preferential diffusion effects in the flame chemistry and transport equations yields better predictions with good accuracy for the in-cylinder characteristics. The inclusion of preferential diffusion effects in the flame chemistry and transport equations was found to predict well several characteristics of the diesel-hydrogen dual-fuel combustion process: 1) ignition delay, 2) start and end of combustion, 3) faster flame propagation and quicker burning rate of hydrogen, 4) high temperature combustion due to highly reactive nature of hydrogen radicals, 5) peak values of the heat release rate due to high temperature combustion of the partially premixed pilot fuel spray with entrained hydrogen/air and then background hydrogen-air premixed mixture. The comparison between diesel-hydrogen dual-fuel combustion and diesel only combustion shows early start of combustion, longer ignition delay time, higher flame temperature and NOx emissions for dual-fuel combustion compared to diesel only combustion.     

The combustion chemistry of a fuel tracer: Measured flame speeds and ignition delays and a detailed chemical kinetic model for the oxidation of acetone

Acetone ignition delay and stretch-free laminar flame speed measurements have been carried out and a kinetic model has been developed to simulate these and literature data for acetone and for ketene, which was found to be an important intermediate in its oxidation. The mechanism has been based on one originally devised for dimethyl ether and modified through validation of the hydrogen, carbon monoxide and methane sub-mechanisms. Acetone oxidation in argon was studied behind reflected shock waves in the temperature range 1340-1930 K, at 1 atm and at equivalence ratios of 0.5, 1 and 2; it is also shown that the addition of up to 15% acetone to a stoichiometric n-heptane mixture has no effect on the measured ignition delay times. Flame speeds at 298 K and 1 atm of pure acetone in air were measured in a spherical bomb; a maximum flame speed of ∼35 cm s⁻¹ at ?=1.15 is indicated.