Predicted Ignition Delay Times Research Articles

A chemical mechanism for ignition and oxidation of multi-component gasoline surrogate fuels

A chemical kinetic mechanism consisting of 89 species and 355 elementary reactions is developed for the prediction of ignition and oxidation behaviors of gasoline surrogate fuels composed of five components, namely, iso-octane, n-heptane, ethanol, toluene, and diisobutylene. Model validation results show that the predicted ignition delay times and laminar flame speeds of the mechanism are consistent with the experimental results, not only for pure mono-component fuels but also for mixed multi-component gasoline surrogate fuels and real gasoline. The mechanism developed here can correctly describe the auto-ignition and combustion characteristics of real gasoline.

Further study on the ignition delay times of propane–hydrogen–oxygen–argon mixtures: Effect of equivalence ratio

Using a shock tube facility, measurements on ignition delay times of propane/hydrogen mixtures (hydrogen fraction XH2 is from 0% to 100%) were conducted at equivalence ratios of 0.5, 1.0 and 2.0. Results show that when XH2 is less than 70%, ignition delay time shows a strong Arrhenius temperature dependence, and the ignition delay time increases with the increase of equivalence ratio. When XH2 is larger than 95%, the ignition delay times do not retain an Arrhenius-like temperature dependence, and the effect of equivalence ratio is very weak when the hydrogen fraction is further increased. Numerical studies were made using two selected kinetic mechanisms and the results show that the predicted ignition delay times give a reasonable agreement with the measurements under all test conditions. Both measurements and predictions show that for mixtures with XH2 less than 70%, the ignition delay time is only moderately decreased with the increase of XH2, indicating that hydrogen addition has a weak effect on the ignition enhancement. Sensitivity analysis reveals the key reactions that control the simulation of ignition delay time. Further investigation of the H-atom consumption is made to interpret the ignition delay time dependence on equivalence ratio and XH2.

Comprehensive chemical kinetic modeling of toluene reference fuels oxidation

Important reactions in the toluene sub-mechanism of a chemical kinetic model for toluene reference fuels oxidation have been revised based on recent measurements and quantum chemical calculations. The revised model has been checked for validity against a wide range of experimental data including ignition delay in shock tube and rapid compression machine, species profiles in flow- and perfectly stirred reactors and laminar burning velocity. The model which consists of 635 reactions among 137 species, improves overall model predictions, especially at lean fuel–air ratios and flow- and perfectly stirred reactor conditions for both neat toluene and toluene in fuel blends. By using an appropriate gasoline surrogate fuel composition, the model has shown to successfully predict ignition delay times and laminar burning velocity for real gasoline at engine-relevant conditions. This suggests that mixtures with toluene and primary reference provide enough complexity for practical purposes.

Oxidation of a Coal-to-Liquid Synthetic Jet Fuel: Experimental and Chemical Kinetic Modeling Study

The kinetics of oxidation of a Coal-to-Liquid (CtL) Fully Synthetic Jet Fuel (FSJF) was studied using three complementary experiments operating over a wide range of conditions: a jet stirred reactor (p = 10 bar), constant mean residence time of 1 s, over the temperature range 570–1070 K, and for equivalence ratios φ = 0.5, 1.0, and 2.0; a shock-tube (p ∼ 16 bar, temperature range between 900 and 1400 K, φ = 0.5 and φ = 1), and a conical flame burner (preheat temperature T0 = 473 K, and for two pressure regimes, p = 1 bar for equivalence ratios ranging from 0.95 to 1.4, and p = 3 bar for equivalence ratios ranging from 0.95 to 1.3). Concentration profiles of reactants, stable intermediates, and final products in the jet stirred reactor were obtained by probe sampling followed by online and off-line gas chromatography analyses and online Fourier transform infrared spectrometry. Ignition delay times were determined behind reflected shock waves by measuring time-dependent CH* emission at 431 nm. Flame speeds were determined by applying the cone angle method. Comparison with corresponding results for Jet A-1 was performed showing similar combustion properties. The oxidation of the CtL-fuel under these conditions was modeled using a detailed kinetic reaction mechanism consisting of 8217 reactions and 2185 species and a 4-component surrogate fuel mixture (n-decane, iso-octane, n-propylcyclohexane, and n-propylbenzene). A reasonable representation of the kinetics of oxidation of this FSJF was obtained. The model showed good agreement with concentration profiles measured in a jet stirred reactor at 10 bar over a range of temperatures (550–1150 K) and equivalence ratios (0.5–2). Acceptable agreement between measured and predicted ignition delay times was found for the investigated fuel air mixtures, with significantly longer ignition delay times predicted. Also, the ignition behavior of the surrogate is mainly influenced by the n-alkane and not by the addition of iso-alkanes, naphthenes, and aromatics. In general, a reasonable agreement between predicted and measured burning velocity data exists, with larger deviations at higher pressure. No deviation is to be seen between burning velocity data for Jet A-1 and CtL, within the uncertainty range. Within the parameter range studied, the measured data of burning velocity and ignition delay time agree with data obtained earlier for petrol-derived kerosene. Our findings support the potential of the CtL investigated to serve as an alternative aviation fuel.

Shock tube measurements of ignition delay times and OH time-histories in dimethyl ether oxidation

Ignition delay times and OH concentration time-histories were measured in DME/O2/Ar mixtures behind reflected shock waves. Initial reflected shock conditions covered temperatures (T5) from 1175 to 1900K, pressures (P5) from 1.6 to 6.6bar, and equivalence ratios (ϕ) from 0.5 to 3.0. Ignition delay times were measured by collecting OH∗ emission near 307nm, while OH time-histories were measured using laser absorption of the R1(5) line of the A–X(0,0) transition at 306.7nm. The ignition delay times extended the available experimental database of DME to a greater range of equivalence ratios and pressures. Measured ignition delay times were compared to simulations based on DME oxidation mechanisms by Fischer et al. [7] and Zhao et al. [9]. Both mechanisms predict the magnitude of ignition delay times well. OH time-histories were also compared to simulations based on both mechanisms. Despite predicting ignition delay times well, neither mechanism agrees with the measured OH time-histories. OH Sensitivity analysis was applied and the reactions DME ↔ CH3O+CH3 and H+O2 ↔ OH+O were found to be most important. Previous measurements of DME ↔ CH3O+CH3 are not available above 1220K, so the rate was directly measured in this work using the OH diagnostic. The rate expression k[1/s]= 1.61×1079T−18.4 exp(−58600/T), valid at pressures near 1.5bar, was inferred based on previous pyrolysis measurements and the current study. This rate accurately describes a broad range of experimental work at temperatures from 680 to 1750K, but is most accurate near the temperature range of the study, 1350–1750K. When this rate is used in both the Fischer et al. and Zhao et al. mechanisms, agreement between measured OH and the model predictions is significantly improved at all temperatures.

Trajectory generated low-dimensional manifolds generated using the stochastic particle model

The Trajectory Generated Low-Dimensional Manifold (TGLDM) is a way to reduce detailed chemistry based on the separation of time-scales in the chemical reaction system. At low temperatures the TGLDM is difficult to compute because of extremely low reaction rates early in the induction process. The Stochastic Particle Model (SPM) uses a probabilistic description of the reaction system and evolves the reaction system through a Markov process resolving down to the time difference between individual reactions and thus is immune to problems with stiffness in the reaction mechanism and to initial reaction rates being extremely low. Because the SPM uses a Monte Carlo method, every time a trajectory is computed, the resulting trajectory takes a different path through the composition space and a different time to move from the boundary of the composition space to equilibrium. Two different methods for incorporating trajectories calculated using the SPM into a TGLDM have been used in this work. First, many realisations have been averaged together. At high temperatures, where a trajectory can be computed using the more usual continuum approach, the average of the SPM trajectories converges to the continuum solution. Using the TGLDM that results from using the SPM to calculate the average autoignition trajectory for a stoichiometric mixture of methane and air, a significant improvement in the prediction of a perfectly stirred reactor is obtained. Second, a time filter has been applied to individual realisations to eliminate the small time-scale fluctuations. Even after the time-filtering, there is still significant variation between realisations. When different realisations are incorporated into the TGLDMs and these are used in the prediction of autoignition of a turbulent methane jet using Conditional Source-term Estimation in Reynolds averaged Navier–Stokes, there are significant variations in the predicted ignition delay times. These fluctuations are still several times smaller in magnitude than those seen in experiments, which suggests that turbulent fluctuations (which cannot be accounted for in the RANS paradigm) also have a significant influence on the fluctuations in the ignition delay time.

Modeling and experimental investigation of methylcyclohexane ignition in a rapid compression machine

A new chemical kinetic reaction mechanism has been developed for the oxidation of methylcyclohexane (MCH), combining a new low temperature mechanism with a recently developed high temperature mechanism. Predictions from this kinetic model are compared with new experimentally measured ignition delay times from a rapid compression machine. Computed results were found to be particularly sensitive to isomerization rates of methylcyclohexylperoxy radicals. Three different methods were used to estimate rate constants for these isomerization reactions. Rate constants based on comparable alkylperoxy radical isomerizations corrected for the differences in the structure of MCH and the respective alkane, predicted ignition delay times in very poor agreement with the experimental results. The most significant drawback was the complete absence of a region of negative temperature coefficient (NTC) in the model results using this method, although a prominent NTC region was observed experimentally. Alternative estimates of the isomerization reaction rate constants, based on the results from previous experimental studies of low temperature cyclohexane oxidation, provided much better agreement with the present experiments, including the pronounced NTC behavior. The most important feature of the resulting methylcyclohexylperoxy radical isomerization reaction analysis was found to be the relative rates of isomerizations that proceed through 5-, 6-, and 7-membered transition state ring structures and their different impacts on the chain branching behavior of the overall mechanism. Theoretical implications of these results are discussed, with particular attention paid to how intramolecular H atom transfer reactions are influenced by the differences between linear alkane and cycloalkane structures.

A rapid compression facility study of OH time histories during iso-octane ignition

Iso-octane ignition delay times ( τ ign ) and hydroxyl (OH) radical mole fraction ( χ OH ) time histories were measured under conditions relevant to homogeneous charge compression ignition engine operating regimes using the University of Michigan rapid compression facility. Absolute quantitative OH mole fraction time histories were obtained using differential narrow-line laser absorption of the R 1(5) line of the A 2 Σ + ← X 2 Π i ( 0 , 0 ) band of the OH spectrum ( ν 0 = 32606.56 cm −1 ). Ignition delay times were determined using pressure and OH data. Diluted iso-octane/argon/nitrogen/oxygen mixtures were used with fuel/oxygen equivalence ratios from ϕ = 0.25 to 0.6 for τ ign measurements and from ϕ = 0.25 to 0.35 for χ OH measurements. The pressures and temperatures after compression ranged from 8.5 to 15 atm and from 945 to 1020 K, respectively, for the combined τ ign and χ OH data. The maximum mole fraction of OH during ignition and the plateau value of OH after ignition are compared with model predictions using different iso-octane oxidation mechanisms. Sensitivity and rate of production analyses for OH identify reactions important in iso-octane ignition under these lean, intermediate-temperature conditions. The OH time histories show significant sensitivity to the OH + OH + M = H 2O 2 + M, CH 3 + HO 2 = CH 3O + OH, and CH 3 + HO 2 = CH 4 + O 2 reactions, which have rate coefficients with relatively high uncertainties. Improved predictions of the OH time histories can be achieved by modifying the rate coefficient for these reactions. The enthalpy of formation used for OH also has a significant effect on the predicted ignition delay times.

Effects of multicomponent diffusion on predicted ignition characteristics of an n-heptane diffusion flame

Flamelet models for turbulent combustion typically employ the assumption of unity Lewis number, i.e., equal thermal and species diffusivities. These models have been employed to predict ignition delay times and ignition location in combusting sprays. However, there is the interesting question: what would be the effects of including multicomponent species diffusion on the ignition predictions? In this work, a one-dimensional n-heptane–air diffusion flame is chosen to study the effects of multicomponent diffusion on predicted ignition characteristics. The ambient conditions selected include typical in-cylinder conditions of a medium-duty diesel engine: pressure 10–40 bar and air temperature 850–1000 K. The ignition and oxidation of n-heptane are predicted using a reaction mechanism consisting of 34 species and 56 steps. The mixture fraction is computed separately as a passive species, the diffusion coefficient, of which is equal to the local thermal diffusion coefficient. From these computations, the transient structure of the flamelet, including ignition, is obtained. The results are compared with those obtained with the unity Lewis number assumption. The implications of the unity Lewis number assumption on the predicted ignition characteristics are discussed.

1717 数値流体シミュレーションを用いたディーゼル燃焼の着火遅れに関する研究

A numerical study was carried out to predict ignition delay times by using KIVA-3 combined with the ignition model, which is based on Livengood-Wu theory. N-heptane was selected on the basis of similarity of cetane number to diesel oil. The experimental data for n-heptane^<(1)> was referred to certify the adjustment of this model. The computational results show relatively good agreement between computational predictions and experiments.

Radiation Induced Ignition of Porous Solid Fuels

Abstract Abstract–The present study reports that radiation absorption in gas-phase and natural convection play significant roles in gas-phase ignition of porous solid fuels exposed to an external radiation heating. While gas-phase radiation absorption shortens ignition delay times, natural convection delays gas-phase ignition. A theoretical model for a transient one-dimensional analysis includes radiation absorption and scattering by the fibrous structure of the fuel, radiation absorption in gas-phase, natural convection, and internal convection due to the porous structure. Predicted ignition delay times are found to be in excellent agreement with experimental results. A parametric study shows that ignitions under unfavorable conditions to attain a thermal runaway are induced mainly by radiation absorption in gas-phase.

Calculations of the auto-ignition of liquid hydrocarbon mixtures as single droplets

A mathematical model for the auto-ignition of single droplets of multicomponent liquid hydrocarbon mixtures is described, and applied to predict ignition delay times as a function of composition for mixtures approximating commercial diesel fuels. The mixtures devised for the calculations allowed independent variation of the distillation properties and chemical make-up. The results show that the distillation curve of the fuel is in itself no indicator of ignition behaviour; rather, ignition is governed primarily by the chemical nature and boiling points of the most volatile constituents of the fuel.

Spheroidal evaporation and ignition of fuel droplets on a hot surface

An analytical model is presented for the spheroidal fuel droplet evaporation and ignition on a hot surface. The model predicts the average and surface temperature histories of the droplet, rate of evaporation, fluid mechanics of the vapor underneath the droplet and vapor concentration around the droplet as well as the rate of heat transfer from the hot surface to the droplet by conduction and radiation. The transient convective-diffusion equation is solved analytically to obtain an expression for the fuel vapor concentration profiles around the droplet. Numerical predictions of the developed model for the droplet surface temperature, rate of evaporation and evaporation lifetime are compared to experimental data from the literature. Satisfactory agreement is obtained for various diameters of n-hexadecane and n-heptane droplets. A procedure is outlined to estimate the total ignition delay of a droplet on a hot surface at elevated pressures. The predicted ignition delay times for n-hexadecane droplets agree reasonably well with the experimental data for pressures lower than 1500 kPa.