Experimental Ignition Delay Times Research Articles

Relative reactivity of methyl anisole isomers: An experimental and kinetic modelling study

Due to the current interest in biomass derived carbon neutral fuels and fuel additives for gasoline engines and to the growing need of understanding the fundamentals of gas phase combustion in the context of wildfires, this study investigates the combustion behavior of key components of biomass pyrolysis oils, namely the three methylanisole isomers (ortho-, meta- and para-methylanisole). Specifically, this study presents the first experimental ignition delay time measurements for such fuels using a shock tube and a rapid compression machine. Ignition experiments were carried out for stoichiometric fuel/air mixtures (φ = 1) at compressed pressure pc = 10 and 20 bar, covering a temperature range Tc = 880–1220 K. A kinetic model based on previous efforts in the area of oxygenated aromatic hydrocarbon fuels is proposed to reproduce and interpret the experimental findings, specifically focusing on capturing the observed different reactivity of the three isomers. To this aim, thermodynamic properties of primary intermediates and bond dissociation energies were calculated highlighting relevant differences originated from the relative position of the O–CH3 and –CH3 substituents. In addition, an isomerization pathway specific to the ortho isomer was theoretically investigated and found to motivate the observed higher reactivity with respect to the meta and para isomers

Chemical Kinetic Analysis of High-Pressure Hydrogen Ignition and Combustion toward Green Aviation

In the framework of the “Multidisciplinary Optimization and Regulations for Low-boom and Environmentally Sustainable Supersonic aviation” project, pursued by a consortium of European government and academic institutions, coordinated by Politecnico di Torino under the European Commission Horizon 2020 financial support, the Italian Aerospace Research Centre is computationally investigating the high-pressure hydrogen/air kinetic combustion in the operative conditions typically encountered in supersonic aeronautic ramjet engines. This task is being carried out starting from the zero-dimensional and one-dimensional chemical kinetic assessment of the complex and strongly pressure-sensitive ignition behavior and flame propagation characteristics of hydrogen combustion through the validation against experimental shock tube and laminar flame speed measurements. The 0D results indicate that the kinetic mechanism by Politecnico di Milano and the scheme formulated by Kéromnès et al. provide the best matching with the experimental ignition delay time measurements carried out in high-pressure shock tube strongly argon-diluted reaction conditions. Otherwise, the best behavior in terms of laminar flame propagation is achieved by the Mueller scheme, while the other investigated kinetic mechanisms fail to predict the flame speeds at elevated pressures. This confirms the non-linear and intensive pressure-sensitive behavior of hydrogen combustion especially in the critical high-pressure and low-temperature region which is hard to be described by a single all-encompassing chemical model.

Probing the effects of NO2 and N2O additions on the auto-ignition behaviors of gasoline at engine relevant conditions

It is important to investigate the effects of NO2 and N2O additions on the auto-ignition behaviors of gasoline, as these species can affect the combustion of gasoline engines via exhaust gas recirculation (EGR). In this study, both a rapid compression machine and a high-pressure shock tube were used to measure the ignition delay times (IDTs) of China Stage-VI gasoline (Research Octane Number 95) in ‘air’ mixtures with NO2 and N2O additions, at equivalence ratios of 0.5, 1.0 and 2.0, at 10 bar, and over a temperature range of 600 ‒ 1400 K. The effects of NO2/N2O additions up to 1000 ppm were studied. The results show that the NO2 additions can obviously promote the oxidation of gasoline and decrease the IDTs at the studied conditions, and the promoting effects increase with NO2 concentrations. Meanwhile, NO2 additions show larger effects on IDTs at low temperatures than that at high temperatures. However, the N2O additions show little effects on the IDTs of gasoline. To further understand the chemistry, a multi-component gasoline surrogate model (n-heptane/isooctane/toluene) including the interaction chemistry between NOx and surrogates was proposed and employed to simulate these experimental IDTs. In general, the proposed kinetic model can simulate well these new IDTs over a wide temperature range. The flux analysis results show that the interaction chemistry between NO2 and toluene presents the largest effects on IDTs among the surrogates.

Development of a diesel surrogate for improved autoignition prediction: Methodology and detailed chemical kinetic modeling

While the surrogate fuel approach has been successfully applied to the simulation of the combustion behaviors of complex gasoline and jet fuels, its application to diesel fuels has been challenging. One of the main challenges derives from the large molecular size of the representative surrogate components necessary to simulate diesel blends, as the development of detailed chemical kinetic models and their validation becomes more complex. In this study, a new surrogate mixture that emulates the chemical and physical properties of a well-characterized diesel fuel is proposed. An optimization procedure was used to select surrogate components that can match both the physical and chemical properties of the target diesel fuel comprehensively. The surrogate fuel mixture composition was designed to have fuel properties (e.g., boiling point, cloud point, etc.) that enable its use in future diesel engine experiments. A detailed kinetic model for the surrogate fuel mixture was developed by combining well-validated sub-mechanisms of each surrogate component from Lawrence Livermore National Laboratory. The ability of the surrogate mixture and kinetic model to emulate ignition delay times was assessed by comparing the simulated results with measurements for the target diesel fuel. Comparison of the experimental and simulated ignition delay times shows that the current surrogate mixture and kinetic model well capture the autoignition response of the target diesel fuel at varying conditions of pressure, temperature, oxygen concentration, and fuel concentration. The current study is one of the first to demonstrate the efficacy of detailed chemical kinetics for diesel range fuels by assembling validated sub-mechanisms for palette compounds and successfully simulating the autoignition characteristics of a target diesel fuel. The experimental ignition delay times of diesel measured with a rapid compression machine, the surrogate mixture, and the kinetic model developed shall aid in progress of understanding diesel ignition under engine relevant conditions.

Development and Validation of Small-Size Mechanism for RP-3 Aviation Fuel with High Precision.

In this study, a kerosene surrogate model fuel containing 73% n-dodecane, 14.7% 1,3,5-trimethylcyclohexane, and 12.3% n-propylbenzene (percentage in mass) is developed by considering both the physical and chemical characteristics of practical aviation kerosene. By combining the small-size C0-C4 (carbon number) core mechanism and the large hydrocarbon submechanisms, a low- and high-temperature chemical kinetic mechanism including 43 species and 136 reactions is constructed for the kerosene surrogate model fuel. The performance of the 43-species mechanism is validated by examining various experimental ignition delay times and laminar flame speeds of single component of n-dodecane and practical kerosene. The predicted main species concentrations during the oxidation process in the jet-stirred reactor by this small-size mechanism exhibit generally acceptable performance with the corresponding experimental data of RP-3 kerosene. The results of brute force sensitivity analysis indicate that the mechanism retains key reaction paths. This relatively small size can be applied to the simulation of computational fluid dynamics to further explore the practical problems of aviation fuel application in engine.

Ignition delay times of C1–C7 natural gas blends in the intermediate and high temperature regimes: Experiment and correlation

This study presents new ignition delay time data for two multi-component natural gas (NG) blends composed of C1–C7n-alkanes with methane as the major component. New experimental data were recorded using a high-pressure shock tube (HPST) at reflected shock pressures (p5) of 10–30 bar, at temperatures (T5) in the range 770–1480 K, and at equivalence ratios (φ) of 0.5–1.5 in ‘air’. The current results together with published rapid compression machine (RCM) measurements show that higher concentrations of larger molecular weight n-alkanes in the NG blends increase fuel reactivity by more than an order of magnitude with mixtures that have ∼18.75% of C3–C5 components compared to mixtures that have ∼44.4% of C3–C7 components at temperatures below 1000 K. On the contrary, at higher temperatures the effect of increasing reactivity is reduced. NUIGMech1.2 is used to simulate the conditions studied and shows good agreement with the experimental ignition delay time data. A correlation equation is developed through regression analyses using NUIGMech1.2 to predict ignition delay times for a wide range of C1–C7 NG mixtures, in the pressure range 10–50 bar, at temperatures in the range 950–2000 K, and at φ = 0.3–3.0 in air. The proposed correlation expression that employs a traditional Arrhenius form is successfully validated against the new experimental data as well as previously published HPST experimental data.

Ignition delay times of C2H4, C2H4/n-C4H10, and n-C4H10/i-C4H10 under O2/CO2 atmospheres: Shock tube experiments and kinetic model

Oxy-fuel combustion of fossil fuels based on the Allam cycle has garnered increasing attention owing to its high-power cycle efficiency in carbon capture. The submechanism of ethylene, an important intermediate product in the oxidation of long-chain alkanes, is vital for establishing chemical kinetic models of alkanes for oxy-fuel combustion. In this study, the ignition delay times of C2H4 diluted in CO2 are measured at pressures of 1 and 10 atm under O2/CO2 atmospheres at three equivalence ratios (0.5, 1.0, and 2.0) in a shock tube. Meanwhile, the ignition delay times of C2H4/n-C4H10 mixtures and n-C4H10/i-C4H10 mixtures diluted in CO2 are measured at pressures of 1 and 10 atm, equivalence ratios of 1.0, and three mixing ratios (1:2, 1:1, and 2:1). The C2H4 submechanism in the Oxymech2.0 Plus model proposed in our previous study is modified by updating some C2H4-related reactions. The modified Oxymech model, NUIGMECH1.1, and Wang2021 are evaluated based on the present experimental ignition delay times and other experimental results from the literature, including the ignition delay times of C2H4 diluted in both CO2 and conventional atmospheres, as well as the laminar flame speeds of C2H4 and C2H4 profiles in pyrolysis. The results show that the modified Oxymech model accurately predicts the experimental results. The modified Oxymech model is compared with NUIGMECH1.1 to predict the ignition delay times in CO2 atmospheres and laminar flame speeds more accurately. The effects of CO2 on the ignition delay of C2H4 are discussed comprehensively.

Numerical and Experimental Investigations of CH4/H2 Mixtures: Ignition Delay Times, Laminar Burning Velocity and Extinction Limits

In this work, the influence of H2 addition on the auto-ignition and combustion properties of CH4 is investigated experimentally and numerically. Experimental ignition delay times (IDT) are compared with simulations and laminar burning velocities (LBVs), and extinction limits/extinction strain rates (ESRs) are compared with data from the literature. A wide variety of literature data are collected and reviewed, and experimental data points are extracted for IDT, LBV and ESR. The results are used for the validation of existing reaction mechanisms. The reaction mechanisms and models used are able to reproduce the influence of H2 addition to CH4 (e.g., shortening IDTs, increasing ESRs and increasing LBVs). IDTs are investigated in a range from 6 to 15 bar and temperatures from 929 to 1165 K with H2 addition from 10 to 100 mol%. We show that LBV and ESR are predicted in a wide range by the numerical simulations. Moreover, the numerical simulations using detailed Aramco Mech 3.0 (581 species) are compared with the derived reduced reaction mechanism UCB Chen (49 species). The results show that the reduced chemistry obtained by considering only the IDT is also valid for LBV and ESR.

Experimental and modelling study of syngas combustion in CO2 bath gas

Syngas produced from coal and biomass gasification has been proposed as a potential fuel for direct-fired supercritical power cycles. For instance, the Allam-Fetvedt cycle can offer price-competitive electricity production with 100 % inherent carbon capture while utilizing CO2 dilution of about 96 %. In this work, ignition delay times (IDTs) of syngas have been measured in CO2 diluted conditions using a high-pressure shock tube at two pressures (20 and 40 bar) over a temperature range of 1100 – 1300 K. Syngas mixtures in this study were varied in equivalence ratio and H2:CO ratios. The datasets were compared against the predictions of AramcoMech 2.0 and the University of Sheffield supercritical CO2 2.0 (UoS sCO2 2.0) kinetic models. Quantitative comparative analysis showed that the UoS sCO2 2.0 was superior in its ability to predict the experimental IDTs of syngas combustion. We found that the reaction of CO2 and H to form CO and OH caused the separation of H2 and CO ignition in two events, which increased the complexity of determining the IDTs. We investigated this phenomenon and proposed a method to determine simulated IDTs for an effective comparison against the experimental IDTs. The chemical kinetics of syngas combustion in a CO2 and N2 bath gas are contrasted by sensitivity and rate-of-production analyses. By altering the ratio of H2 and CO as well as mixture equivalence ratio, this work provides vital IDT data in CO2 bath gas for further development and validation of relevant kinetics mechanisms.

Development of a reduced chemical mechanism for ammonia/n-heptane blends

A reduced chemical mechanism for ammonia/n-heptane blends, which consists of 495 reactions and 74 species, is proposed in this study. This reduced mechanism is developed using the direct relation graph with error propagation (DRGEP) method based on the latest detailed kinetic mechanism of ammonia/n-heptane blends, which includes the important interactive reactions between n-heptane and ammonia (n-C7H16 + NH2 <=> C7H15 + NH3). The reduced mechanism was first validated against experimental ignition delay times (IDTs) and laminar burning velocities (LBVs) for different ammonia/n-heptane blends and can capture well these data. Moreover, both the detailed and reduced mechanisms were used to simulate the combustion and emission characteristics of a homogeneous charge compression ignition (HCCI) engine fueled by different ammonia/n-heptane blends. The simulation results show that the predicted auto-ignition timings and peak pressure values of the reduced mechanism match reasonably well with those of the detailed mechanism for different ammonia/n-heptane blends. Also, the predicted time-history profiles of the important pollutants, including NO, NO2, N2O and HCN, show reasonably good agreement between the detailed and reduced mechanisms, and the maximum discrepancy of the peak concentration is within a factor of two. The important reaction pathways of pollutant formations at HCCI engine conditions are discussed in detail. Furthermore, the current reduced mechanism is also used to simulate the combustion process of an ammonia/diesel dual-fuel engine. Overall, the engine combustion phasing, peak pressure, and heat release can be well simulated using the current mechanism, and the computational efficiency is acceptable.

An experimental and modeling study on autoignition of 2-phenylethanol and its blends with n-heptane

2-Phenylethanol (2-PE) is an aromatic alcohol with high research octane number, high octane sensitivity, and a potential to be produced using biomass. Considering that 2-PE can be used as a fuel additive for boosting the anti-knocking quality of gasoline in spark-ignition engines and as the low reactivity fuel or fuel component in dual-fuel reactivity controlled compression ignition (RCCI) engines, it is of fundamental and practical interest to understand the autoignition chemistry of 2-PE, especially at low-to-intermediate temperatures (<1000 K). Based upon the experimental ignition delay time (IDT) results of neat 2-PE obtained from our previous rapid compression machine (RCM) investigation and the literature shock tube study, a detailed chemical kinetic model of 2-PE is developed herein, covering low-to-high temperature regimes. Besides, RCM experiments using binary fuel blends of 2-PE and n-heptane (nC7) are conducted in this work to investigate the nC7/2-PE blending effects, as they represent a dual-fuel system for RCCI operations. Furthermore, the newly developed 2-PE model is merged with a well-validated nC7 kinetic model to generate the current nC7/2-PE binary blend model. Overall, the consolidated model reasonably predicts the experimental IDT data of neat 2-PE and nC7/2-PE blends, as well as captures the experimental effects of pressure, equivalence ratio, and blending ratio on autoignition. Finally, model-based chemical kinetic analyses are carried out to understand and identify the controlling chemistry accounting for the observed blending effects in RCM experiments. The analyses reveal that nC7 enhances 2-PE autoignition via providing extra ȮH radicals to the shared radical pool, while the diminished nC7 promoting effect on 2-PE autoignition with increasing temperature is due to the negative temperature coefficient characteristics of nC7.

Effects of nozzle diameter on the characteristic time scales of diesel spray two-stage ignition under cold-start conditions

Diesel engine cold-start performances are closely related to ignition delay time which in turn is strongly influenced by the nozzle diameter (Dnoz). In this study, ignition delay time and ignition location of No. −50 diesel spray at different Dnoz are investigated by high-speed direct photography in a constant volume combustion chamber (CVCC) under cold-start conditions. The experimental results clarify the non-monotonic variation of the high-temperature ignition delay time (τHTI) with decreasing nozzle diameter. The minimum experimental high-temperature ignition delay time of about 1.85 ms is obtained when Dnoz = 160 μm. To investigate the mechanism that the influence of nozzle diameter on high-temperature ignition delay time, numerical simulations of the n-heptane spray ignition process are performed under the same conditions as optical diagnostics. Five characteristic time scales for the two-stage ignition process are defined as: physical preparation time (τphy), low-temperature pre-reaction time (Δτ1), low-temperature ignition delay time (τLTI), high-temperature pre-reaction time (Δτ2), and high-temperature ignition delay time. The physical preparation time decreases linearly with nozzle diameter caused by better spray atomization, evaporation, and air-fuel mixing. Changes in nozzle diameter barely affect low-temperature pre-reaction time, indicating that the linear shortening of low-temperature ignition delay time is attributed to the acceleration of the physical preparation process. The high-temperature pre-reaction time initially decreases and then increases with decreasing Dnoz, which is the result of a smaller scalar dissipation rate promoting the decomposition of H2O2 and inhibiting flame propagation. The combined effect of decreasing Dnoz at each stage leads to a non-monotonic trend in high-temperature ignition delay time.

Reaction kinetics for high pressure hydrogen oxy-combustion in the presence of high levels of H2O and CO2

Novel shock tube experimental ignition delay time (IDT) data are provided to delineate the impact of high concentrations (45% by mole) of H2O and CO2 on IDTs of stoichiometric 4% H2. Ignition delay experiments were conducted using a high- and a low-pressure shock tube facility. High-pressure experiments were performed at pressures of 37-43.8 bar and temperatures of 1084-1242 K in four different bath gases, namely: Ar, 45%H2O/Ar, 30%H2O/15%CO2/Ar, and 45%CO2/Ar. Low-pressure experiments were conducted at 2.1-2.7 bar and 926-1198 K in Ar and 45%CO2/Ar bath gases. Impacts of non-ideal ignition phenomena that may occur in the presence of large amounts of H2O and CO2 were also analyzed. A minimally-tuned H2/CO reaction mechanism, CanMECH 1.0, targeting high-pressure combustion in the presence of large concentrations of H2O and CO2 is presented. The mechanism is constructed from unadjusted kinetic rate parameters from theoretical / experimental elementary reaction rate determinations. The only adjusted rate in CanMECH 1.0 is the much disputed Ar-specific low-pressure-limit pre-exponential factor of H + O2 (+Ar) = HO2 (+Ar) within the uncertainty bounds of the source rate. Validity of CanMECH 1.0 is confirmed against shock tube IDT data of this work, as well as selected H2 and H2/CO shock tube IDT datasets from literature. The performance of the model is compared to Keromnes et al. model (Combust. Flame 160 (2013) 995-1011). CanMECH 1.0 outperformed Keromnes et al. for 16 datasets out of 26 and exhibited a similar performance for another two. In particular, CanMECH 1.0 outperformed Keromnes et al. in predicting shock tube IDTs for H2O- and CO2-laden reactive mixtures, as well as all IDT data at pressures of 17-43.8 bar, which are of distinct value to pressurized oxy-fuel combustion applications relevant to this work.

An experimental auto-ignition and kinetic modelling study of binary and ternary cyclopentane/toluene/diisobutylene/iso-octane mixtures

Ignition delay times are measured for binary mixtures of cyclopentane/toluene, cyclopentane/2,4,4-trimethylpent-1-ene and ternary mixtures of cyclopentane/iso-octane/toluene, cyclopentane/iso-octane/2,4,4-trimethylpent-1-ene. The experiments are performed using a high-pressure shock tube and a rapid compression machine for stoichiometric mixtures in ‘air’, in the temperature range of 650 – 1450 K and at pressures of 25 and 40 bar. A detailed chemical kinetic model is used to simulate the experimental ignition delay times. It has been formulated using NUIGMech1.2 as the base chemistry to which has been added cyclopentane, iso-octane, 2,4,4-trimethylpent-1-ene and toluene sub-mechanisms from the literature. This work provides valuable insights into the reactivity of the blending behaviour of toluene, iso-octane and 2,4,4-trimethylpent-1-ene with cyclopentane. The model is used to understand the chemical influence of these fuels on the reactivity of cyclopentane. Reaction path and sensitivity analyses of each of the cyclopentane blends using the combined mechanism are performed and these are discussed. Our analysis furthers the understanding of binary (naphthene/aromatic, naphthene/olefin) and ternary (naphthene/iso-alkane/olefin and naphthene/iso-alkane/aromatic) mixtures and helps improve the development of predictive models for multi-component gasoline surrogate mixture blends.

Experimental and modelling study of hydrogen ignition in CO2 bath gas

Direct-fired supercritical CO2 power cycles, operating on natural gas or syngas, have been proposed as future energy technologies with 100 % carbon capture at a price competitive with existing fossil fuel technologies. Likewise, blue or green hydrogen may be used for power generation to counter the intermittency of renewable power technologies. In this work, ignition delay times (IDTs) of hydrogen were measured in a high concentration of CO2 bath gas over 1050 – 1300 K and pressures between 20 and 40 bar. Measured datasets were compared with chemical kinetic simulations using AramcoMech 2.0 and the University of Sheffield supercritical CO2 (UoS sCO2 2.0) chemical kinetic mechanisms. The UoS sCO2 2.0 mechanism was recently developed to model IDTs of methane, hydrogen, and syngas in CO2 bath gas. Sensitivity analyses were used to identify important reactions and to illustrate the trends observed among various datasets. The performance of both mechanisms was evaluated quantitatively by comparing the average absolute error between the predicted and experimental IDTs, which showed UoS sCO2 2.0 as the superior mechanism for modelling hydrogen IDTs in CO2 bath gas. The importance of OH time-histories is identified as the most appropriate next step in further validation of the kinetic mechanism.

When hydrogen is slower than methane to ignite

Hydrogen (H2) is known to be the fastest fuel to ignite among all practical combustion fuels. In this study, for the first time, longer ignition delay times (IDTs) for the H2 and H2 blended CH4 mixtures were measured compared to those for pure CH4. This work investigates the ignition characteristics of H2, CH4, and 50% CH4/50% H2 mixtures using a rapid compression machine at pressures ranging from 20 to 50 bar and at equivalence ratios (φ) from 0.5 to 2.0 in air in the temperature range 858–1080 K. The experimental IDTs are simulated using a newly updated kinetic mechanism, NUIGMech1.3, and good agreement is observed. At lower temperatures the IDTs of H2, CH4, and the 50% CH4/50% H2 mixtures are similar to one another, and the IDTs of the 50% CH4/50% H2 mixtures are longer than those for pure CH4 at temperatures below 930 K. At temperatures below 890–925 K, depending on the operating pressure and equivalence ratio, the hydrogen mixtures are the slowest to ignite, with IDTs being 2.5 times longer than those recorded for CH4 at a pressure of 40 bar at 890 K for φ = 1.0, and at 875 K for φ = 2.0. At low temperatures alkyl (Ṙ = ĊH3 and Ḣ) radicals add to O2 producing RȮ2 radicals, which then react with HȮ2 radicals forming ROOH (H2O2 and CH3OOH) and O2. For H2, the self-recombination of HȮ2 radicals leads to chain propagation which inhibits reactivity, whereas for CH4, the reaction between RȮ2 (CH3OȮ) and HȮ2 leads to chain branching, increasing reactivity. Furthermore, CH3OOH decomposes more easily to produce CH3Ȯ and ȮH radicals than does H2O2 to produce two ȮH radicals. Thus, mixtures containing higher H2 concentrations are slower to ignite compared to those with higher CH4 concentrations at low temperatures.

Motion and swelling of single coal particles during volatile combustion in a laminar flow reactor

Motion and swelling behavior of single bituminous coal particles during volatile combustion are investigated in a laminar flow reactor using a joint experimental and numerical approach. Three different particle samples with mean diameters of 90, 120, and 160 µm are studied in a conventional N2/O2 atmosphere with 20 vol% O2 mole fraction. Diffuse backlight-illumination (DBI) measurements with high temporal (10 kHz) and spatial (> 19 lp/mm) resolutions, combined with detailed parameter evaluation methods, provide fundamental insights into interactions of particle with flow and flame. The acceleration behavior of different particles is assessed based on the response time following the viscosity drag law. Rotation speed is determined by temporally tracking the orientation angle and shown to strongly correlate with the particle size and the devolatilization process. Simultaneously measured slip velocity and particle diameter enable evaluating time-dependent particle Reynolds numbers Rep. The swelling behavior is temporally synchronized with the devolatilization process and reveals a strong dependency on particle diameters. To better understand experimental observations, detailed simulations are first quantitatively validated against experimental ignition delay times and then applied to predict particle temperature histories. Further, the reduction of particle heating rates with increasing diameters is numerically quantified. The maximum swelling ratio decreases from 1.22 to 1.07 as the heating rate increases from approximately 3 × 104 to 8 × 104 K/s.

A shock tube experiment and an improved high-temperature diisopropyl ketone model by Bayesian optimization

Ignition delay times (IDTs) for di-isopropyl ketone (DIPK) were measured at pressures of 6 and 10 atm, equivalence ratios (ϕ) of 0.5, 1.0, and 2, and over a temperature range of 1100 - 1500 K. In the DIPK sub-model proposed by Barari et al., DIPK⇔IC3H7+IC3H7CO (R2) was erroneously written as DIPK⇔IC3H7+ IC4H7O and the consumption paths of dimethyl ketene (DMK) are missing. A DIPK sub-model was proposed on the basis of Barari model by correcting these errors, reassigning the rate coefficients of the DIPK unimolecular decomposition reactions (R1, R2), H-abstraction reactions by O˙H (R3, R4), fuel radical isomerization reaction (R5), and β-scission decomposition reactions (R5-R10) using the analogy method, and adopting the theoretical calculations of the H-abstraction reactions by H˙, C˙H3, and HO˙2 (R11-R16) provided by Allen et al.. The rate coefficients of R1-R10 were globally optimized by the Bayesian optimization algorithm using the experimental IDTs and profiles of DIPK and DMK. The optimized model well predicts not only the IDTs and profiles of DIPK and DMK, but also the laminar flame speeds and the profiles of many intermediates (propyne, propylene, and methyl ketene) and small species (H2, CO, CH4) during the pyrolysis of DIPK. The calculation using the method of Chen et al. shows that no optimized reaction violates the collision limit. The sensitivity and rate of production analysis were used to discuss the DIPK unimolecular decompositions, DIPK H-abstraction reactions, DIPK radicals isomerization and β-scission reactions, and the optimization of the DIPK sub-model.

Theoretical studies of real-fluid oxidation of hydrogen under supercritical conditions by using the virial equation of state

The Virial equation of state (EoS) was employed to describe the real-fluid impact on H2 oxidation simulation. Different from conventional empirical EoS methods fitted by using critical pressures and temperatures, such as Redlich-Kwong (RK) EoS, the Virial method was constructed with including intermolecular interactions, potentials, and molecular polarizations coupled with real-fluid partition function theory in this work. The Virial method was for the first time incorporated into Cantera software to realize real-fluid simulations with deeper-level physical insights. A series adiabatic/isothermal flow reactor and ignition delay simulations in H2O, N2, and CO2 diluents were performed. The calculations of species compressibility factors and thermodynamic properties by using the Virial method showed a better agreement with experimental data in literature than using the RK method. The Virial method also possessed a better performance in predicting experimental H2 ignition delay times and H2 speciation profiles from literatures. The effects of real fluid through corrections of compressibility factors, thermodynamic properties, and chemical potentials on supercritical H2 oxidation simulations in H2O diluents have been rigorously analyzed, respectively. Moreover, the Virial method performed well at adiabatic conditions in the bath gas of polar molecules like H2O, due to its consideration of molecular polarizations and higher accuracy of thermodynamic property calculations. We believed the Virial method could not only provide accurate estimations of real-fluid behavior, but also provided physical insights in the intermolecular interactions under supercritical conditions.

A comprehensive experimental and modeling study of the ignition delay time characteristics of ternary and quaternary blends of methane, ethane, ethylene, and propane over a wide range of temperature, pressure, equivalence ratio, and dilution

The ignition delay time (IDT) characteristics of new ternary and quaternary blended C1 – C3 gaseous hydrocarbons, including methane/ethane/ethylene and methane/ethane/ethylene/propane, are studied over a wide range of mixture composition, temperature (∼800 – 2000 K), pressure (∼1 – 135 bar), equivalence ratio (∼0.5 – 2.0), and dilution (∼75 – 90%) using both experimental data and kinetic modeling tools. In this regard, all of the experimental tests were designed using the Taguchi approach (L9) to fulfill the experimental matrix required to generate a comprehensive set necessary to validate a detailed chemical kinetic model. High- and low-temperature IDTs were recorded using low/high-pressure shock tubes (L/HPST) and rapid compression machines (RCM), respectively. The model predictions using NUIGMech1.2 are evaluated versus all of the newly recorded experimental data. Moreover, the individual effects on IDT predictions of the parameters studied, including mixture composition and pressure, are investigated over the temperature range. The results show that NUIGMech1.2 can reasonably reproduce the experimental IDTs over the wide range of the conditions studied. The constant-volume simulations using the chemical kinetic mechanism reveal the synergistic/antagonistic effect of blending on IDTs over the studied temperature range so that IDTs in certain temperature ranges are very sensitive to even small changes in mixture composition.