Shock Tube Ignition Delay Times Research Articles

Evaluation of high-pressure syngas ignition under high-CO2 dilution in shock tubes

Due to its importance as a candidate fuel for power cycles for future energy production, there have been active efforts to improve our understanding of the chemical kinetics of the Allam-Fetvedt cycle that utilizes syngas highly diluted with CO2. Shock-tube ignition delay times (IDT) of syngas-CO2 mixtures over a range of pressures, syngas blends, and equivalence ratios are now available in the literature, but their unique behavior deserves further study before the resulting data can be used to modify existing syngas kinetics models. In this study, an analysis of CO2-diluted syngas ignition behind reflected shock waves was conducted at the High-Pressure Shock Tube facility at Texas A&M University using an endwall imaging system. A syngas blend of 1:1 H2:CO at an equivalence ratio of 1 was studied at pressures around 12 atm for CO2 dilutions of 85, 88, and 94 % by volume. The combined results of the high-speed OH* imaging through the endwall and OH* time histories from sidewall windows indicate that the initial ignition event appears to be homogeneous and occurs first near the endwall, with no apparent premature ignition or localized flame kernels. An important finding of this investigation is that the existing syngas-CO2 IDT data in the literature are likely free from shock-tube ignition inhomogeneities. Therefore, they can be used to improve existing chemical kinetics mechanisms, which currently have trouble modeling the existing literature data over the range of temperatures of the shock-tube studies (∼1000 – 1400 K). Additionally, the lack of a measured post-combustion pressure rise makes it necessary to use a constant-pressure-enthalpy constraint when modeling the shock-tube IDT event.

Measurements of laminar burning velocities and an improved kinetic model of methyl isopropyl ketone

Methyl isopropyl ketone (MIPK) is the simplest branched ketone and a promising biofuel. In this work, laminar burning velocities (SL) of MIPK + air flames were measured using the heat flux method at atmospheric pressure, over initial mixture temperatures of 298–358 K and equivalence ratios of 0.7–1.4. With the help of the temperature dependence of the SL, data inconsistency between the present measurements and the experimental data reported by Li et al. (Proc. Combust. Inst. 38 (2021) 2135) was demonstrated. Moreover, existing kinetic models for MIPK combustion notably deviate from the present SL measurements. Therefore, the MIPK model suggested by Lin et al. (Proc. Combust. Inst. 39 (2023) 315) was updated by revisiting the MIPK H-abstraction reactions and methyl isopropenyl ketone sub-model. Furthermore, a new di-methyl ketene (critical intermediate during MIPK oxidation) sub-model was constructed and integrated into the MIPK model. Flux and sensitivity analyses revealed that integration of the new di-methyl ketene model improves predictions of the laminar burning velocities as well as shock tube ignition delay times over the pressures of 1–40 bar due to converting di-methyl ketene into C3H5-T (CH2 = C˙CH3) rather than C3H6 or C3H5-S (C˙H = CHCH3) predicted by other MIPK models from the literature. Updates of the MIPK H-abstraction reactions yield more reasonable products branching ratios of formation of the primary fuel radicals, and improve prediction of the SL. It was also found that the rate constants of the MIPK decomposition reaction (MIPK (+M) = CH3CO + IC3H7 (+M)) in the model proposed by Li et al. (Proc. Combust. Inst. 38 (2021) 2135) are significantly underestimated, resulting in underestimation of the present SL measurements and significant overprediction of the ignition delay times.

A Novel Reduced Reaction Mechanism for Diesel/2,5-Dimethylfuran Engine Application

The application of 2,5-dimethylfuran (DMF) as an alternative fuel for internal combustion engines has been gaining popularity. However, it has rarely been studied in previous research on the chemical kinetics of DMF for engine combustion simulations. In the present study, a reduced n-heptane/toluene/DMF-polycyclic aromatic hydrocarbon (PAH) reaction mechanism containing only 78 species amongst 190 reactions was proposed and applied to predict the combustion and emissions of a diesel engine using diesel/DMF blend fuel. First, a detailed reaction mechanism for DMF from the literature was chosen and reduced using combined mechanism reduction methods under engine-relevant conditions. Second, the reduced mechanism of DMF was incorporated into an existing reduced n-heptane/toluene-PAH mechanism to establish a three-component chemistry mechanism. Third, the predictive capability of the combined mechanism was improved by adjusting the rate constants of selected gas-phase reactions. Subsequently, the proposed three-component mechanism was compared and validated with experimental measurements of shock tube ignition delay times and premixed flame species profiles acquired from published papers. Moreover, new experimental data from a conventional diesel engine were used to evaluate the developed mechanism. Overall, the predicted results obtained by this proposed reduced n-heptane/toluene/DMF-PAH mechanism are in reasonable agreement with the available experiments.

NOx formation from ammonia, and its effects on oxy-combustion of hydrocarbon fuels under supercritical-CO2 conditions

NOx formation from ammonia impurities and its effects on fuel oxidation is not well understood for high-pressure supercritical CO2 oxy-combustion conditions. This effect is investigated computationally and experimentally in the present work. A chemical kinetic analysis revealed that the reaction between NH radical and CO2 plays an important role in determining the rate of NOx formation at these conditions. It was also found that a significant reduction in the NOx formation from ammonia oxidation occurs with sCO2 oxy-combustion at 300 atm when compared to traditional gas turbine conditions. A chemical reactor network simulation of realistic sCO2 cycle conditions confirmed the reduced NOx emissions. Monte Carlo simulations used to study the sensitivity of model input variables on the emissions showed that CO2 cooling impacts the CO emissions while most of the NOx are generated in the flame zone. Uncertainty analysis showed that the reaction between NH and CO2 to form HNO radical is an important contributor to the model uncertainty under sCO2 oxy-combustion conditions.The presence of combustion-generated NOx in the recycled-CO2 can impact the fuel oxidation in the primary zone of the combustor. Hence, to understand the effect of NOx on ignition, high-pressure shock tube ignition delay time experiments were performed at conditions relevant to sCO2 oxy-combustion. The ignition delay time measurements were made for syngas and CH4 fuels with and without NO addition under supercritical conditions using CO2 as the bulk diluent at nominal pressures around 100 atm. Experimental data showed that the presence of NO promotes ignition at these conditions. The effect is more pronounced for CH4 compared to syngas. Nitric oxide acts as a chemical catalyst to promote ignition by increasing the combustion radical pool. The catalytic cycle involves the conversion of NO to NO2 which also contributes to CH4 oxidation by H-atom abstraction to generate CH3 radical.

An optimized kinetic model for H[formula omitted]/CO combustion in CO[formula omitted] diluent at elevated pressures

Oxy-fuel combustion is a promising strategy for carbon capture. Existing syngas combustion models overpredicted recently measured ignition delay time data in high-level CO2 diluent and elevated pressures (40, 80, 100 and 200 atm). A trial model was constructed by updating the Kéromnès mechanism (Combust. Flame, 2013, 160, 995–1011) with some recent reaction rate studies and the addition of O+OH+M = HO2+M. Among all 33 reactions, 23 influential reactions were selected for optimization based on sensitivity results. 787 syngas shock-tube ignition delay time data and 1900 laminar flame speed measurements were considered. The optimized model was computed from the trial model using an efficient two-stage optimization strategy. Temperature-dependent initial uncertainty was implemented and all Arrhenius parameters were optimization simultaneously. The optimized model significantly improves the prediction accuracy for the ignition delay time targets, especially those in CO2 diluent, and produces comparable prediction performance for the laminar flame speed targets to the other models. Most of the reaction rate adjustments were well within the corresponding uncertainty (≤30%). Five reactions with relatively large rate adjustment were discussed. Sensitivity and reaction path analysis were conducted to examine the kinetic difference between the optimized model and the Kéromnès model at relevant conditions. Chemical and physical effects of CO2 as a diluent were investigated.

Shock tube ignition delay time measurements for methyl propanoate and methyl acrylate: Influence of saturation on small methyl ester high‐temperature reactivity

AbstractWe report ignition delay time measurements for methyl propanoate (MP) and methyl acrylate (MA), carried out in a high‐pressure shock tube. Experiments were performed behind reflected shock waves across a temperature range of 989‐1 367 K, for fuel‐air mixtures at equivalence ratios of ϕ = 0.5, 1.0, and 2.0, and nominal pressures of 1 and 4 MPa. Ignition delay times were found to decrease with increasing temperature, equivalence ratio, and pressure, and are well described with correlations involving Arrhenius temperature dependence and power‐law dependence on equivalence ratio and pressure. Ignition delay times are compared with model predictions from literature kinetic models, with the models of Zhang et al (Energy & Fuels 2014; 28(11): 7194‐7202) and Bennadji et al (International Journal of Chemical Kinetics 2011; 43(4): 204‐218.) in good agreement with measured ignition delay times for MP and MA, respectively. Kinetic sensitivity analysis shows that the reactions most important for modeling ignition fall into two categories: initiation reactions (ie, decomposition and H‐atom abstraction) and C0‐C1 chemistry controlling the pool of small radicals. The unsaturated MA was observed to have lower reactivity than MP, due to its greater bond strengths for C─C and C─H bonds, resulting in slower rates for initiation reactions.

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.

Reduced mechanism generation for methanol-based toluene reference fuel with combined reduction methods

TRF is a very competitive substitute fuel of gasoline, and methanol as a high octane oxygenated fuel has been attracted widely attention. In this study, a new reduced mechanism for methanol-based toluene reference fuel was developed based on the sensitivity analysis (SA), and reaction pathways analysis methods. At first, reduced mechanisms of methanol (including 24 species and 57 elementary reaction) and TRF (including 98 species and 264 elementary reaction) were developed and vilified separately. Then, coupling methanol and TRF reduced mechanisms that have been validated in above section, a new reduced of mechanism for methanol blending with TRF fuel was proposed that including 100 species and 290 elementary reactions. Extensive validations were performed by comparing experimental data of shock tube ignition delay times, flow reactor species profiles, pressure profiles with simulation results. High accuracy is demonstrated in wide ranges of temperature (T = 300–2500 K) and equivalence ratio (Φ = 0.375–2), which indicates that the present methanol blending with TRF fuel reduced mechanism can well predict the experimental data.

Chemical kinetic modelling of ammonia/hydrogen/air ignition, premixed flame propagation and NO emission

This work reports on a study of chemical kinetic modelling of ammonia/hydrogen/air ignition, premixed flame propagation and NO emission. A survey of chemical mechanisms available in the literature was first conducted, and the performance of 10 mechanisms was analysed in terms of the prediction of shock tube ignition delay times, laminar flame speeds and NOx concentrations for NH3/air and NH3/H2/air flames for a wide range of operating conditions. Then, three of these mechanisms were reduced and their performance compared against the behaviour of the original mechanisms. The results confirm that pure NH3 flames have high ignition delay times and rather low flame speeds, and that the addition of H2 to NH3 flames increases exponentially the flame speed, and significantly the NOx emission. The currently available chemical kinetic mechanisms predict rather scattered ignition delay times, laminar flame speeds, and NOx concentrations in NH3 flames, indicating that improvements in the sub-mechanisms of NH3 and NH3/H2 oxidation are still needed. Sensitivity analysis for NO formation indicates that NO formation in NH3 flames is mainly produced through the NH3/O2 chemical process, and sensitivity analysis for flame speed reveals that the differences among mechanisms are due to the relative importance of the reactions of the NNH and HNO sub-mechanisms. The reduced mechanisms show high fidelity when compared with the original ones, despite some discrepancies at high pressures.

Shock tube study of normal heptane first-stage ignition near 3.5 atm

Shock tube ignition delay times and species time history measurements for Primary Reference Fuels (PRFs) such as normal heptane provide targets for the validation of combustion models, which in turn are used to develop more fuel-efficient engines that have smaller environmental footprints. However, a review of the literature has revealed that most of the shock tube ignition delay time and species measurement data for normal heptane have been obtained at elevated pressures, rather than at relatively low pressures where many other important experimental techniques such as jet-stirred reactors and flow reactors can provide corroborating results. One central problem preventing previous shock tube studies from examining first-stage ignition at these lower pressures was that ignition times were too long under these conditions to be measured within the available shock tube test times. To address this issue, recent advances in shock tube techniques for achieving long uniform test times have been applied in order to measure low-pressure first-stage ignition times of normal heptane together with normal heptane fuel time-history records at times up to about 30 ms. These measurements were performed in the Negative Temperature Coefficient (NTC) region (T=664−792 K) in lean mixtures (21%O2/Ar, equivalence ratio ϕ=0.5) at pressures of roughly P=3.5 atm using both the conventional and Constrained Reaction Volume (CRV) shock tube filling strategies. The data have been used to evaluate the performance of several combustion models, have been compared with other higher-pressure shock tube first-stage ignition times in n-heptane/20–21%O2 mixtures found in the literature, and have been fit using a two-zone Arrhenius model for first-stage ignition delay times. The results showed that some combustion models yield ignition delay time predictions that differ from the experimental results by as much as an order of magnitude, that under these conditions n-heptane first-stage ignition times scale by roughly P−0.71 but are insensitive to ϕ, and that the fraction of fuel remaining after first-stage ignition increases with increasing initial experimental temperature.

Rate-Controlled Constrained-Equilibrium Application in Shock Tube Ignition Delay Time Simulation

The rate-controlled constrained-equilibrium (RCCE), a model order reduction method, assumes that the nonequilibrium states of a system can be described by a sequence of constrained-equilibrium kinetically controlled by relatively a small number of constraints within acceptable accuracies. The full chemical composition at each constrained-equilibrium state is obtained by maximizing (or minimizing) the appropriate thermodynamic quantities, e.g., entropy (or Gibbs functions) subject to the instantaneous values of the constraints. Regardless of the nature of the kinetic constraints, RCCE always guarantees correct final equilibrium state. Ignition delay times measured in shock tube experiments with low initial temperatures are significantly shorter than the values obtained by constant volume models. Low initial temperatures and thus longer shock tube test times cause nonideal heat transfer and fluid flow effects such as boundary layer growth and shock wave attenuation to gradually increase the pressure (and simultaneously increase the temperature) before ignition. To account for these effects, in this paper, the RCCE prescribed enthalpy and pressure (prescribed h/p) model has been further developed and has been applied to methane shock tube ignition delay time simulation using GRI-Mech 3.0. Excellent agreement between RCCE predictions and shock tube experimental data was achieved.

Ignition delay times of methane and hydrogen highly diluted in carbon dioxide at high pressures up to 300 atm

The need for more efficient power cycles has attracted interest in super-critical CO2 (sCO2) cycles. However, the effects of high CO2 dilution on auto-ignition at extremely high pressures has not been studied in depth. As part of the effort to understand oxy-fuel combustion with massive CO2 dilution, we have measured shock tube ignition delay times (IDT) for methane/O2/CO2 mixtures and hydrogen/O2/CO2 mixtures using sidewall pressure and OH* emission near 306 nm. Ignition delay time was measured in two different facilities behind reflected shock waves over a range of temperatures, 1045–1578 K, in different pressures and mixture regimes, i.e., CH4/O2/CO2 mixtures at 27–286 atm and H2/O2/CO2 mixtures at 37–311 atm. The measured data were compared with the predictions of two recent kinetics models. Fair agreement was found between model and experiment over most of the operating conditions studied. For those conditions where kinetic models fail, the current ignition delay time measurements provide useful target data for development and validation of the mechanisms.

Evaluating mixture rules and combustion implications for multi-component pressure dependence of allyl + HO2 reactions

Many of the reactions important to combustion proceed through rovibrationally excited complexes, whose evolution depends on energy-transferring collisions with molecules in the surrounding mixture—yielding a strong dependence of rate constants on pressure and collisional energy transfer characteristics of the mixture. In nearly all realistic combustion environments, a substantial fraction of multiple species of distinct collisional energy transfer characteristics are present—thus requiring a “mixture rule” to interpolate kinetic data from individual single-component bath gases. While mixture effects for reactions proceeding through a single potential well to a single channel have been extensively investigated, mixture effects and mixture rules for multi-well and/or multi-channel reactions are significantly less characterized. The present study presents an investigation of the mixture effects, suitability of mixture rules for representing them, and their impacts on combustion predictions for the allyl + HO2 reaction system. The performance of different mixture rules for representing multi-component pressure dependence of rate constants for allyl + HO2 was evaluated through comparisons against master equation calculations for the mixture. The comparisons revealed that the classic linear mixture rule (LMR,P) yields deviations reaching a factor of two for typical combustion mixtures, whereas newly proposed mixture rules based on reduced pressure exhibit lower deviations of generally less ∼ 3%. Simulations of shock tube ignition delay times and jet-stirred reactor species profiles for C3H6 combustion suggest that mixture effects for allyl + HO2 on combustion predictions can be significant and may serve as a plausible explanation of the rate parameter adjustments employed in modeling studies to reproduce combustion measurements. Newly proposed mixture rules based on the reduced pressure are shown to accurately capture these mixture effects within combustion simulations.

Ignition delay times, laminar flame speeds, and species time-histories in the H2S/CH4 system at atmospheric pressure

Hydrogen sulfide (H2S) composes up to 30% of certain natural-gas resources (“sour gas”) and can considerably alter combustion properties of methane (CH4), but few data on H2S/CH4 are available in the literature. In this work, new shock-tube and laminar flame speed data were obtained to facilitate future model validation. For the shock-tube experiments, a fuel-lean (φ = 0.5) 30/70 H2S/CH4 blend in 99% argon by volume was shock-heated to temperatures between 1538 and 2144 K and pressures near 1 atm. Laser absorption diagnostics at 4.5 and 1.4 µm were employed to measure CO and H2O time-histories, respectively. OH* chemiluminescence profiles were measured using an emission diagnostic at 307 nm. For the laminar flame speed experiments, measurements were carried out in a constant-volume vessel at 295 K and 1 atm for CH4/argon and H2S/CH4/argon (8.25% H2S) mixtures from φ = 0.7 to φ = 1.4. The predictions of several recent chemical kinetics mechanisms were compared to the data, leading to the conclusion that species containing both carbon and sulfur are unimportant for shock-tube conditions but can be quite influential for laminar flames. By combining the modeling efforts of two recent works, a tentative new model is proposed that shows marked improvement over the older models in terms of shock-tube ignition delay times. Flame speed predictions show a discrepancy with the new model but follow general experimental trends. To the best of the authors’ knowledge, this study provides the first shock-tube data and laminar flame speeds measured in the H2S/CH4 system.

Concerning shock-tube ignition delay times: An experimental investigation of impurities in the H2/O2 system and beyond

Shock-tube ignition delay time data have recently been called into question on the basis of possible hydrocarbon impurities that may serve to artificially accelerate ignition delay times. To assess potential sources of impurities, systematic tests were performed in highly dilute H2/O2/Ar and CH4/O2/Ar mixtures using H2O laser absorption at 1.388 µm and OH* emission at 307 nm near 1 atm and between 1257 and 2108 K. Factors investigated included common sources of impurity, namely shock-tube cleanliness, Ar purity, diaphragm fragments, leftover cleaning substances, turbopumping duration, and mixing tank cleanliness, but only mixing tank cleanliness was found to have any quantifiable effect on measured ignition delay times. Rate measurements of H + O2⇆OH + O using H2O time-histories were found to be unperturbed by impurity effects and were in excellent agreement with the Hong et al. measurement. Mixing tank impurities were found to perturb ignition delay times of a stoichiometric H2/O2 mixture in 98% Ar but had no effect on a stoichiometric CH4/O2 mixture in 99% Ar. Even with clean mixing tank conditions, H2/O2 ignition delay times in 98% and 99% Ar displayed a 25–30% discrepancy with predicted values but showed remarkable agreement with two sets of historical data. This discrepancy was modeled by including trace hydrocarbons (within certified purity levels) in simulations. For less-dilute (≤ 94% Ar) H2/O2 mixtures, literature data seem to agree rather well and were insensitive to trace hydrocarbons according to simulations. Furthermore, experimental and modeling evidence strongly suggests that almost every hydrocarbon C1 and higher is insensitive to impurities with respect to ignition delay times. It is thus concluded that the only data for which an impurity effect seems to exist is for highly dilute (98–99% Ar) H2/O2 data, while H2/O2 data below 94% dilution and almost every hydrocarbon C1 and greater are insensitive to common impurities.

High Pressure Shock Tube Ignition Delay Time Measurements During Oxy-Methane Combustion With High Levels of CO2 Dilution

Ignition delay times and methane species time-histories were measured for methane/O2 mixtures in a high CO2 diluted environment using shock tube and laser absorption spectroscopy. The experiments were performed between 1300 K and 2000 K at pressures between 6 and 31 atm. The test mixtures were at an equivalence ratio of 1 with CH4 mole fractions ranging from 3.5% to 5% and up to 85% CO2 with a bath of argon gas as necessary. The ignition delay times and methane time histories were measured using pressure, emission, and laser diagnostics. Predictive ability of two literature kinetic mechanisms (gri 3.0 and aramco mech 1.3) was tested against current data. In general, both mechanisms performed reasonably well against measured ignition delay time data. The methane time-histories showed good agreement with the mechanisms for most of the conditions measured. A correlation for ignition delay time was created taking into account the different parameters showing the ignition activation energy for the fuel to be 49.64 kcal/mol. Through a sensitivity analysis, CO2 is shown to slow the overall reaction rate and increase the ignition delay time. To the best of our knowledge, we present the first shock tube data during ignition of methane/CO2/O2 under these conditions. Current data provides crucial validation data needed for the development of future kinetic mechanisms.

Measurements and interpretation of shock tube ignition delay times in highly CO2 diluted mixtures using multiple diagnostics

Common definitions for ignition delay time are often hard to determine due to the issue of bifurcation and other non-idealities that result from high levels of CO2 addition. Using high-speed camera imagery in comparison with more standard methods (e.g., pressure, emission, and laser absorption spectroscopy) to measure the ignition delay time, the effect of bifurcation has been examined in this study. Experiments were performed at pressures between 0.6 and 1.2atm for temperatures between 1650 and 2040K. The equivalence ratio for all experiments was kept at a constant value of 1 with methane as the fuel. The CO2 mole fraction was varied between a value of XCO2=0.00 to 0.895. The ignition delay time was determined from three different measurements at the sidewall: broadband chemiluminescent emission captured via a photodetector, CH4 concentrations determined using a distributed feedback interband cascade laser centered at 3403.4nm, and pressure recorded via a dynamic Kistler type transducer. All methods for the ignition delay time were compared to high-speed camera images taken of the axial cross-section during combustion. Methane time-histories and the methane decay times were also measured using the laser. It was determined that the flame could be correlated to the ignition delay time measured at the side wall but that the flame as captured by the camera was not homogeneous as assumed in typical shock tube experiments. The bifurcation of the shock wave resulted in smaller flames with large boundary layers and that the flame could be as small as 30% of the cross-sectional area of the shock tube at the highest levels of CO2 dilution. Comparisons between the camera images and the different ignition delay time methods show that care must be taken in interpreting traditional ignition delay data for experiments with large bifurcation effects as different methods in measuring the ignition delay time could result in different interpretations of kinetic mechanisms and impede the development of future mechanisms.

Development of an Uncertainty Quantification Predictive Chemical Reaction Model for Syngas Combustion

An automated data-centric infrastructure, Process Informatics Model (PrIMe), was applied to validation and optimization of a syngas combustion model. The Bound-to-Bound Data Collaboration (B2BDC) module of PrIMe was employed to discover the limits of parameter modifications based on uncertainty quantification (UQ) and consistency analysis of the model–data system and experimental data, including shock-tube ignition delay times and laminar flame speeds. Existing syngas reaction models are reviewed, and the selected kinetic data are described in detail. Empirical rules were developed and applied to evaluate the uncertainty bounds of the literature experimental data. The initial H2/CO reaction model, assembled from 73 reactions and 17 species, was subjected to a B2BDC analysis. For this purpose, a dataset was constructed that included a total of 167 experimental targets and 55 active model parameters. Consistency analysis of the composed dataset revealed disagreement between models and data. Further analysis...

A comprehensive iso-octane combustion model with improved thermochemistry and chemical kinetics

Iso-Octane (2,2,4-trimethylpentane) is a primary reference fuel and an important component of gasoline fuels. Moreover, it is a key component used in surrogates to study the ignition and burning characteristics of gasoline fuels. This paper presents an updated chemical kinetic model for iso-octane combustion. Specifically, the thermodynamic data and reaction kinetics of iso-octane have been re-assessed based on new thermodynamic group values and recently evaluated rate coefficients from the literature. The adopted rate coefficients were either experimentally measured or determined by analogy to theoretically calculated values. Furthermore, new alternative isomerization pathways for peroxy-alkyl hydroperoxide (ȮOQOOH) radicals were added to the reaction mechanism. The updated kinetic model was compared against new ignition delay data measured in rapid compression machines (RCM) and a high-pressure shock tube. These experiments were conducted at pressures of 20 and 40 atm, at equivalence ratios of 0.4 and 1.0, and at temperatures in the range of 632–1060K. The updated model was further compared against shock tube ignition delay times, jet-stirred reactor oxidation speciation data, premixed laminar flame speeds, counterflow diffusion flame ignition, and shock tube pyrolysis speciation data available in the literature. Finally, the updated model was used to investigate the importance of alternative isomerization pathways in the low temperature oxidation of highly branched alkanes. When compared to available models in the literature, the present model represents the current state-of-the-art in fundamental thermochemistry and reaction kinetics of iso-octane; and thus provides the best prediction of wide ranging experimental data and fundamental insights into iso-octane combustion chemistry.

Development of a reduced n-butanol mechanism with combined reduction methods

N-butanol is a very competitive substitute fuel of gasoline due to its many advantages. In this study, a new reduced mechanism for n-butanol combustion simulation under engine conditions was developed based on the direct relationship graph with error propagation (DRGEP), sensitivity analysis and entropy production analysis methods. In the first stage, the DRGEP method and reaction pathway analysis (RPA) were used to remove unimportant species. In the second stage, the entropy production analysis method was utilized to eliminate insignificant reactions. In the last stage, for optimizing key reaction’s rate constant and accurately reproducing the auto-ignition behavior, sensitivity analysis method was used to identify the important reactions. The proposed combined reduction approach successfully reduced the detailed n-butanol mechanism (326 species and 1884 reactions) to a reduced mechanism containing 75 species and 285 reactions. Extensive validations were performed by comparing experimental data of shock tube ignition delay times, jet-stirred reactor (JSR) species profiles, laminar flame speed and premixed laminar flame species profiles with modeled results. The reduced mechanism was also validated by comparing with the detailed mechanism in a single-zone engine model. High accuracy is demonstrated in wide ranges of temperature (T=700–1500K), pressure (p=1–80bar), and equivalence ratio (Φ=0.5–2.0), which indicated that the present n-butanol reduced mechanism can well predict the experimental data.