Ignition delay times of low-vapor-pressure fuels measured using an aerosol shock tube

Gas-phase ignition delay times were measured behind reflected shock waves for a wide variety of low-vapor-pressure fuels. These gas-phase measurements, without the added convolution with evaporation times, were made possible by using an aerosol shock tube. The fuels studied include three large normal alkanes, n-decane, n-dodecane and n-hexadecane; one large methyl ester, methyl decanoate; and several diesel fuels, DF-2, with a range of cetane indices from 42 to 55. The reflected shock conditions of the experiments covered temperatures from 838 to 1381 K, pressures from 1.71 to 8.63 atm, oxygen concentrations from 1 to 21%, and equivalence ratios from 0.1 to 2. Ignition delay times were measured using sidewall pressure, IR laser absorption by fuel at 3.39 μm, and CH^* and OH^* emission. Measurements are compared to previous studies using heated shock tubes and current models. Model simulations show similar trends to the current measurement except in the case of n-dodecane/21% O₂/argon experiments. At higher temperatures, e.g. 1250 K, the measured ignition delay times for these mixtures are significantly longer in lean mixtures than in rich mixtures; current models predict the opposite trend. As well, the current measurements show significantly shorter ignition delay times for rich mixtures than the model predictions.     

A shock tube and chemical kinetic modeling study of the pyrolysis and oxidation of butanols

The pyrolysis and oxidation of all four butanols (n-, sec-, iso- and tert-) have been studied at pressures from 1 to 4 atm and temperatures of 1000–1800 K behind reflected shock waves. Gas chromatographic sampling at different reaction times varying from 1.5 to 3.1 ms was used to measure reactant, intermediate and product species profiles in a single-pulse shock tube. In addition, ignition delays were determined at an average reflected shock pressure of 3.5 atm at temperatures from 1250 to 1800 K. A detailed chemical kinetic model consisting of 1892 reactions involving 284 species was constructed and tested against species profiles and ignition delays. The little-known chemistry of enols is included in this work to explain the temperature dependence of acetaldehyde produced in the thermal decomposition of isobutanol.     

Combustion characteristics of micron-sized aluminum particles in oxygenated environments

Experimental data describing combustion of micron-sized aluminum particles as a function of their size are limited. Often combustion characteristics are derived indirectly, from experiments with aerosolized powder clouds. In a recently developed experiment, micron-sized particles cross two laser beams. When each particle crosses the first, low-power laser, it produces a scattered light pulse proportional to the particle diameter. The second, powerful CO₂ laser beam ignites the particle. The optical emission pulse of the burning particle is correlated with its scattered light pulse, so that the combustion characteristics are directly correlated with the size for each particle. In this work, emission signatures of the ignited Al particles are recorded using an array of filtered photomultipliers to enable optical pyrometry and evaluate the molecular AlO emission. Processing of the generated data for multiple particles is streamlined. Experiments are performed with spherical aluminum powder burning in atmospheric pressure O₂/N₂ gas mixtures with the oxygen concentrations of 10%, 15%, and 21% (air). In air, the AlO emission peaks prior to the maximum in the overall emission intensity, and the latter occur before the maximum of the particle temperature. The temperatures at which particles burn steadily increase with particle size for particles less than 7.4 μm. For coarser particles, the flame temperature remains constant at about 3040 K. In the gas mixture with 15% O₂, the flame temperatures are observed to increase with particle size for the entire range of particle sizes considered, 2–20 μm. At 10% O₂, the flame temperatures are significantly lower, close to 2000 K for all particles. The intensity of AlO emission decays at lower oxygen concentrations; however, it remains discernible for all environments. The results of this study are expected to be useful for constructing the Al combustion models relaxing the assumption of the steady state burning.     

A shock tube study of iso-octane ignition at elevated pressures: The influence of diluent gases

The ignition of iso-octane/air and iso-octane/O₂/Ar (∼20% O₂) mixtures was studied in a shock tube at temperatures of 868-1300 K, pressures of 7-58 atm, and equivalence ratios Φ=1.0, 0.5, and 0.25. Ignition times were determined using endwall OH^∗ emission and sidewall piezoelectric pressure measurements. Measured iso-octane/air ignition times agreed well with the previously published results. Mixtures with argon as the diluent exhibited ignition times 20% shorter, for most conditions, than those with nitrogen as the diluent (iso-octane/air mixtures). The difference in measured ignition times for mixtures containing argon and nitrogen as the diluent gas can be attributed to the differing heat capacities of the two diluent species and the level of induction period heat release prior to ignition. Kinetic model predictions of ignition time from three mechanisms are compared to the experimental data. The mechanisms overpredict the ignition times but accurately capture the influence of diluent gas on iso-octane ignition time, indicating that the mechanisms predict an appropriate amount of induction period heat release.     

Char oxidation at elevated pressures

Approximately 100 char oxidation experiments were performed at atmospheric and elevated pressures, with two sizes (70 and 40 μm) of Utah and Pittsburgh bituminous coal chars at 1, 5, 10, and 15 atm total pressure. Reactor temperatures were varied between 1000 and 1500 K with 5% to 21% oxygen in the bulk gas, resulting in average particle temperatures ranging from 1400 to 2100 K and burnoff from 15% to 96% (daf). Independently determined particle temperature and overall reaction rate allowed an internal check on the data consistency and provided insight into the products of combustion. The chars burned in a reducing density and reducing diameter mode in an intermediate regime between the kinetic and pore diffusion zones, irrespective of total pressure. Significant surface CO₂ formation occurred at particle temperatures below about 1800 K over the entire pressure range. Particle temperatures were strongly dependent on the oxygen and total pressures; increasing oxygen pressure at constant total pressure resulted in substantial increases in particle temperature, while increasing the total pressure at constant oxygen pressure led to substantial decreases in particle temperature. Increasing total pressure from 1 to 5 atm in an environment of constant gas composition led to modest increases in the reaction rate; the rate decreased with further increases in pressure. Results indicate that ambient pressure global model kinetic parameters cannot be accurately extrapolated to elevated pressures. The apparent reaction rate coefficients (based on the partial pressure form of the nth-order rate equation) showed significant pressure dependence, since both the activation energy and frequency factor decreased with increasing pressure. This suggests that the empirical nth-order rate equation is not valid over the range of pressures encountered in the experiments. However, simulations indicate that the global model can be used to model elevated pressure char oxidation provided pressure-dependent kinetic parameters are used.     

1-Butanol ignition delay times at low temperatures: An application of the constrained-reaction-volume strategy

Ignition delay times behind reflected shock waves are strongly sensitive to variations in temperature and pressure, yet most current models of reaction kinetics do not properly account for the variations that are often present in shock tube experiments. Particularly at low reaction temperatures with relatively long ignition delay times, substantial increases in pressure and temperature can occur behind the reflected shock even before the main ignition event, and these changes in thermodynamic conditions of the ignition process have proved difficult to interpret and model. To obviate such pressure increases, we applied a new driven-gas loading method that constrains the volume of reactive gases, thereby producing near-constant-pressure test conditions for reflected shock measurements. Using both conventional operation and this new constrained-reaction-volume (CRV) method, we have collected ignition delay times for 1-butanol/O₂/N₂ mixtures over temperatures between 716 and 1121 K and nominal pressures of 20 and 40 atm for equivalence ratios of 0.5, 1.0, and 2.0. The equivalence ratio dependence of 1-butanol ignition delay time was found to be negative when the oxygen concentration was fixed, but positive when the fuel concentration was held constant. Ignition delay times with strong pre-ignition pressure increases in conventional-filling experiments were found to be significantly shorter than those where these pressure increases were mitigated using the CRV strategy. The near-constant-pressure ignition delay times provide a new database for low-temperature 1-butanol mechanism development independent of non-idealities caused by either shock attenuation or pre-ignition perturbations. Comparisons of these near-constant-pressure measurements with predictions using several reaction mechanisms available in the literature were performed. To our knowledge this work is first of its kind that systematically provides accurate near-constant-enthalpy and -pressure target data for chemical kinetic modeling of undiluted fuel/air mixtures at engine relevant conditions.     

Measurement and modeling of shock-tube ignition delay for propene

Propene ignition was studied behind reflected shock waves at postshock temperatures ranging from 1270 to 1820 K and postshock pressures from 0.95 to 4.7 atm. Reactant concentrations were varied from 0.8 to 3.2% propene and from 3.6 to 15.1% oxygen diluted in argon, giving equivalence ratios ranging from 0.5 to 2.0. The pressure-based ignition delay correlation equation τ(s) = 4.2 × 10⁻¹⁵ [C₃H₆]^0.378[O₂]^−1.043 exp(48800/RT₅) for mol/cm³ and cal units was derived. The data could be accounted for using a reaction mechanism with 463 elementary reactions.     

Analysis of the spatial uniformity of the combustion of a gaseous mixture initiated by a nanosecond discharge

The spatial uniformity of combustion in a gas mixture initiated by a high-voltage nanosecond volume discharge has been investigated at gas pressures of 0.3-2.4 atm and temperatures of 1100-2250 K. The experiments were carried out behind a reflected shock wave propagating in a mixture of methane and air diluted with argon. The autoignition time and the time of discharge-induced ignition were determined. It was found that, at relatively low pressures (∼0.5 atm), the discharge significantly decreased the ignition temperature (by 600 K). At higher pressures (1.5-2 atm), the ignition temperature fell by only 100 K. The emission from the discharge and combustion were registered with a nanosecond ICCD camera under various experimental conditions. Comprehensive measurements of the deposited energy and the waveforms of the discharge voltage and current with a nanosecond time resolution made it possible to determine the efficiency of this type of discharge for igniting combustible mixtures.     

The autoignition of C8H10 aromatics at moderate temperatures and elevated pressures

The autoignition of C₈H₁₀ aromatic/air mixtures (ortho-xylene, meta-xylene, para-xylene, and ethylbenzene in air) has been studied in a shock tube at temperatures of 941-1408 K, pressures of 9-45 atm, and equivalence ratios of Φ=1.0 and 0.5. Ignition times were determined using electronically excited OH emission and pressure measurements. The measurements illustrate the differences in reactivity for the C₈H₁₀ aromatics under the studied conditions. Ethylbenzene was by far the most reactive C₈H₁₀ aromatic with ignition times a factor of two to three shorter than the xylenes. The xylene isomers exhibited ignition times that were similar, with o-xylene the most reactive, p-xylene the least reactive, and m-xylene just slightly more reactive than p-xylene. The p-xylene ignition times are almost identical to previous measurements for toluene at the same conditions. The differences in reactivity can be attributed to the C-H and C-C bond strengths in the alkyl side chains and the proximity of the methyl groups in the xylenes. These results represent the first ignition measurements for C₈H₁₀ aromatics at the elevated-pressure moderate-temperature conditions studied, providing needed targets for kinetic modeling at engine-relevant conditions. Kinetic modeling illustrates the importance of the methylbenzyl + HO₂ reaction and indicates further study of this reaction is warranted.     

An experimental and modeling study of iso-octane ignition delay times under homogeneous charge compression ignition conditions

Autoignition of iso-octane was examined using a rapid compression facility (RCF) with iso-octane, oxygen, nitrogen, and argon mixtures. The effects of typical homogeneous charge compression ignition (HCCI) conditions on the iso-octane ignition characteristics were studied. Experimental results for ignition delay times, τ_ign, were obtained from pressure time-histories. The experiments were conducted over a range of equivalence ratios (?=0.25-1.0), pressures (P=5.12-23 atm), temperatures (T=943-1027 K), and oxygen mole fractions (χO₂=9-21%), and with the addition of trace amounts of combustion product gases (CO₂ and H₂O). It was found that the ignition delay times were well represented by the expression

     

Methane/propane mixture oxidation at high pressures and at high, intermediate and low temperatures

The oxidation of methane/propane mixtures in “air” has been studied for blends containing 90% CH₄/10% C₃H₈ and 70% CH₄/30% C₃H₈ in the temperature range 740-1550 K, at compressed gas pressures of 10, 20 and 30 atm, and at varying equivalence ratios of 0.3, 0.5, 1.0, 2.0 and 3.0 in a high-pressure shock tube and in a rapid compression machine. These data are consistent with other experiments presented in the literature for other alkane fuels in that, when ignition delay times are plotted as a function of temperature, a characteristic negative coefficient behavior is observed, particularly for mixtures containing 30% propane. In addition, the results were simulated using a detailed chemical kinetic model. It was found that qualitatively, the model reproduces correctly the effect of change in equivalence ratio and pressure, predicting that fuel-rich, high-pressure mixtures ignite fastest while fuel-lean, low-pressure mixtures ignite slowest. Moreover, the reactivity as a function of temperature is well captured with the model predicting negative temperature coefficient behavior similar to the experiments. Quantitatively the model is faster than experiment for all mixtures at the lowest temperatures (740-950 K) and is also faster than experiment throughout the entire temperature range for fuel rich mixtures.     

Simplified correlation models for CO/H2 chemical reaction times

This paper provides simplified correlation models for CO/H₂ chemical reaction times. The procedure used for the CO/H₂ simplified modeling utilized the full chemical kinetics mechanism run over a range of temperatures from 700 to 1800 K, pressures from 0.5 to 50 atm, mixtures from 0% to 95% CO, and equivalence ratios from 0.2 to 2.0 to determine ignition (or reaction) time. The correlations for ignition times are given in formulas as functions of equivalence ratio, temperature, and pressure. Two different forms of correlations were obtained, one being a single, overall correlation and the other a two-stage correlation representing regions of high and low temperatures. These correlations are shown to work well over a range of chemical time scales spanning ten orders of magnitude. The correlations are also compared with measured data from the literature.     

A high pressure shock tube study of n-propylbenzene oxidation and its comparison with n-butylbenzene

Ignition delay times have been measured for mixtures of n-propylbenzene in air (≈21% O₂, ≈79% N₂) at equivalence ratios of 0.29, 0.48, 0.96 and 1.92 and at reflected shock pressures of 1, 10 and 30 atm in a heated high-pressure shock tube over a wide temperature range (1000–1600 K). The effects of reflected shock pressure and of equivalence ratio on ignition delay time were determined and common trends highlighted. Simulations were carried out using the n-propylbenzene sub-mechanism contained in an n-butylbenzene reaction mechanism available in the literature. This kinetic model was improved by including pressure dependent reactions which were not in place previously and the addition of the NUI Galway C₀–C₄ sub-mechanism. These simulations showed very good agreement with the experimental data. Additionally a comparison is made with experimental data previously obtained and published for n-butylbenzene over the same range of conditions and common trends are highlighted.     

Vibrational nonequilibrium and reaction heat effect in diluted hydrogen-oxygen mixtures behind reflected shock waves at 1000 < T < 1300 K

Vibrationally nonequilibrium model of kinetics in the reacting mixture H₂ + O₂ + Ar behind the reflected shock wave is formulated as a non-isothermal process occurring adiabatically at a constant volume. The model takes into account the vibrational nonequilibrium for the starting (primary) H₂ and O₂ molecules, as well as the molecular intermediates HO₂, OH, O₂(¹Δ), and the main reaction product H₂O. Calculation results that simulate experimental data on the ignition induction time measurements in the hydrogen oxygen mixtures behind reflected shock waves by the methods of absorption spectroscopy (monitoring the OH(²Π) radical) and emission spectroscopy (monitoring the OH*(²Σ⁺) radical) at temperatures of 1000 < T < 1300 K and pressures p < 3 atm are compared with experimental data and analyzed. It has been shown that the vibrational nonequilibrium determines the mechanism and rate of the process as a whole. The self-heating effect in diluted reacting mixtures at concentrations of the reacting additive ≤5% is demonstrated and discussed.     

Experimental and kinetic study on ignition delay times of DME/H2/O2/Ar mixtures

Ignition delay times of dimethyl ether (DME)/hydrogen/oxygen/argon mixtures (hydrogen blending ratio ranging from 0% to 100%) were measured behind reflected shock waves at pressures of 1.2–10 atm, temperature range of 900–1700 K, and for the lean (? = 0.5), stoichiometric (? = 1.0) and rich (? = 2.0) mixtures. For more understanding the effect of initial parameters, correlations of ignition delay times for the lean mixtures were obtained on the basis of the measured data (X_H2 ? 95%) through multiple linear regression. Ignition delay times of the DME/H₂ mixtures demonstrate three ignition regimes. For X_H2 ? 80%, the ignition is dominated by the DME chemistry and ignition delay times show a typical Arrhenius dependence on temperature and pressure. For 80% ? X_H2 ? 98%, the ignition is dominated by the combined chemistries of DME and hydrogen, and ignition delay times at higher pressures give higher ignition activation energy. However, for X_H2 ? 98%, the transition in activation energy for the mixture was found as decreasing the temperature, indicating that the ignition is dominated by the hydrogen chemistry. Simulations were made using two available models and different results were presented. Thus, sensitivity analysis was performed to illustrate the causes of different simulation results of the two models. Subsequently, chemically interpreting on the effect of hydrogen blending ratio on ignition delay times was made using small radical mole fraction and reaction pathway analysis. Finally, high-pressure simulations were performed, serving as a starting point for the future work.     

An experimental and modeling study of surrogate mixtures of n-propyl- and n-butylbenzene in n-heptane to simulate n-decylbenzene ignition

This paper presents experimental data for the oxidation of two surrogates for the large alkylbenzene class of compounds contained in diesel fuels, namely n-decylbenzene. A 57:43 molar% mixture of n-propylbenzene:n  -heptane in air (21% _O2${\mathrm{O}}_2$, 79% _N2${\mathrm{N}}_2$) was used in addition to a 64:36 molar% mixture of n-butylbenzene:36% n-heptane in air. These mixtures were designed to contain a similar carbon/hydrogen ratio, molecular weight and aromatic/alkane ratio when compared to n-decylbenzene. Nominal equivalence ratios of 0.3, 0.5, 1.0 and 2.0 were used. Ignition times were measured at 1 atm in the shock tube and at pressures of 10, 30 and 50 atm in both the shock tube and in the rapid compression machine. The temperature range studied was from approximately 650–1700 K. The effects of reflected shock pressure and equivalence ratio on ignition delay time were determined and common trends highlighted. It was noted that both mixtures showed similar reactivity throughout the temperature range studied. A reaction mechanism published previously was used to simulate this data. Overall the reaction mechanism captures the experimental data reasonably successfully with a variation of approximately a factor of 2 for mixtures at 10 atm and fuel-rich and stoichiometric conditions.     

A shock tube and kinetic modeling study of n-butanal oxidation

n-Butanal is a key stable intermediate during the combustion of n-butanol, and as such strongly affects its chemical kinetics. In this study, ignition delay times of n-butanal/oxygen diluted with argon were measured behind reflected shock waves in the temperature range of 1100–1650 K, at pressures of 1.3, 5 and 10 atm, and equivalence ratios of 0.5, 1.0 and 2.0. An n-butanal sub-model was developed on the basis of literature review, and exhibits fairly good agreement with the experimental results under all test conditions. Reaction pathway and sensitivity analysis were conducted to gain an insight into the controlling reaction pathways and reaction steps.     

Micro-alumina particle volatilization temperature measurements in a heterogeneous shock tube

Peak flame temperatures in aluminum particle combustion should approach the volatilization temperature of the product alumina. References are divided in assigning this temperature anywhere between 3200 and 4000 K, which can provide significant uncertainty not only in numerical models for combustion but also in the interpretation of flame structure from temperature measurements. We present results in the controlled conditions of the UIUC heterogeneous shock tube of volatilization temperature, made by measuring the extinction of light by nano- and micro-alumina particles at non-resonant wavelengths at different ambient temperatures. At 10 atm, there is a sharp cutoff at 3860 K beyond which nano-particles volatilize and stop extinguishing within the shock tube test time. Numerical modeling of the evaporation rate of these particles is used to assign a volatilization temperature of 4340 K at 10 atm. Similarly, a volatilization temperature of 4260 K at 3 atm is measured. From our analysis, the best estimate for the volatilization temperature at 1 atm was 4189 ± 200 K, which is consistent with the high range of volatilization temperature reported in the literature.