Ignition Delay Time Data Research Articles

Development of a Compact Skeletal Mechanism for Thermally Cracked Hydrocarbon Fuels

ABSTRACT The construction of a compact kinetic mechanism for thermally cracked hydrocarbon fuels holds great importance for developing advanced engine with regenerative cooling. In the present study, a skeletal oxidation mechanism consisting of 104 species and 373 reactions, which can be divided into two parts, i.e. a core mechanism for pyrolysis gas, and a skeletal sub-mechanism for pyrolysis liquid, was developed by a decoupling method. The combustion process of pyrolysis gas and its pure constituents was depicted by incorporating detailed reaction pathways of H2/CO/C1-C4 molecules, and variations of hydrocarbon classes, as well as carbon number distributions were also considered by introducing eight skeletal sub-mechanisms for heavy hydrocarbons which covered chain alkanes, cycloalkanes, and monocyclic aromatics in pyrolysis liquid, thus the newly developed mechanism was suitable for thermally cracked hydrocarbon fuels over a wide range of pyrolysis temperatures. The skeletal mechanism was extensively validated by experimental data of laminar burning velocities and ignition delay times, confirming its applicability for reproducing the flame characteristics of pyrolysis gases, pyrolysis liquids, and thermally cracked fuels. Sensitivity analyses showed that laminar burning velocities of thermally cracked fuels were only affected by reactions involving small radicals when the pyrolysis temperature was lower than 600°C, while reactions involving benzene rings started to dominate the flame propagation of thermally cracked fuel at 650°C under fuel-lean, stoichiometric, and fuel-rich conditions. Reactions of small radicals and heavy hydrocarbons both affected the ignition process of thermally cracked fuels.

High-pressure ignition behaviors of methane/ethane/propane-n-heptane mixtures representing natural gas-diesel dual fuel

Considering that natural gas is the key transition fuel towards the carbon-neutral future, the objective of the present study is to gain insight into evolution features of natural gas-diesel dual fuel during ignition process. Firstly, new experimental data of ignition delay times for the stoichiometric methane/ethane/propane-n-heptane mixtures, which is of significance for validating and optimizing chemical kinetic models of the dual fuel at engine-relevant conditions, were acquired through a shock-tube facility at pressure of 40 atmopshere within temperature range of 1200-1600 K, and quantitative influences of components of the fuel mixtures on ignition were determined. Then importance of species including typical radicals and alkenes during ignition processes were identified. Besides, stage characteristics of the fuel mixtures during ignition processes were analyzed. The result shows that the ignition of real natural gas which contains some ethane and propane is greatly different from that of methane. It can be seen that the C2 substances are significant to control ignition of the mixtures. For methane-n-heptane and methane/ethane-n-heptane mixtures, the whole ignition process can be divided into decomposition and oxidation stages. While for the fuel mixtures containing propane and n-heptane, it seems to be more reasonable to divide the whole ignition process into three-stages, i.e., decomposition, mixed and oxidation stages.

Auto-ignition characteristics of oxygenated aromatic compounds: Benzyl alcohol, benzaldehyde, and phenol

The growing attention towards biomass and biomass-derived fuels for transportation, energy production, forest fire prevention, and safety applications stimulated significant research activities focused on understanding the chemical kinetics of oxygenated aromatic hydrocarbons, which represent important reference components for biofuels. This motivates the current investigation of the ignition behavior of benzyl alcohol, benzaldehyde, and phenol, which exhibit intertwined reaction pathways. Novel ignition delay time data were measured in a shock tube and a rapid compression machine covering pressures from 10 to 40 bar and temperatures from 860–1250 K. In addition, a kinetic model was developed and used to analyze critical reaction pathways in comparison to models from literature. It is shown that the phenyl-phenoxy-phenyl peroxy subsystem plays a key role not only in the oxidation of benzaldehyde, but also in the oxidation of benzene and phenol. Comparison of the current model with a recent version of the CRECK mechanism reveals that differences are present with respect to the dominant reaction pathways that have a strong influence on the formation of Ö radicals. Further investigations are required to resolve this discrepancy reconciling theory, modelling, and the unfortunately limited number of systematic experimental data to which this work significantly contributes.

Effect of NO2 addition on the oxidation kinetics of n-pentane and natural gas blends with C1–C5n-alkanes

This study presents new ignition delay time (IDT) data for NO2 blended multi-component natural gas mixtures with methane as the major component including C2–C5 primary n-alkanes in different concentrations. The data are measured using a high-pressure shock tube and a rapid compression machine at compression pressures of 20 and 30 bar in the temperature range of 715–1480 K for stoichiometric mixtures. This paper builds upon our previous work on the development of a C1–C3/NOx kinetic mechanism and focusses on understanding the interaction chemistry of heavier n-alkane/NOx mixtures by determining the IDT characteristics of various NO2 blended n-C5H12/air combinations with 200, 400 and 1000 ppm concentrations of NO2 present at compression pressures of 10, 20 and 30 bar. The newly measured IDT results and existing experimental data from the literature are used to improve model predictions for the oxidation of n-C4H10/NOx, n-C5H12/NOx, NG2/NOx, and NG3/NOx by updating NUIGMech1.2. The improvement in the performance of GalwayMech1.0 is attributed to updated rate constants for the n-C5H12 + NO2 ↔ Ċ5H11–1, -2, -3 + HONO reactions as well as the inclusion of Ṙ + NO2 ↔ RONO reaction channels. The IDT decreased slightly when 1000 ppm of NO2 is added to n-pentane, which contrasts with propane (C3H8), where there was a significant decrease in the mixture reactivity as reported previously (A. A. E. S. Mohamed, A. B. Sahu, S. Panigrahy, M. Baigmohammadi, G. Bourque, H.J. Curran, The effect of the addition of nitrogen oxides on the oxidation of propane: An experimental and modeling study. Combust. Flame 245 (2022) 112306). This is because the competition between NO2 and O2 for 1-propyl radicals formed by H-atom abstraction from C3H8 decreases mixture reactivity, while in the case of n-C5H12, although NO2 and O2 compete for 1-pentyl radicals, the increase in reactivity is facilitated by the availability of 2-pentyl radicals in the system which also leads to chain branching when they add to O2, unlike 2-propyl (iso-propyl) radicals which ultimately do not lead to chain branching in the oxidation of propane.

Development and validation of a comprehensive combustion kinetic model for the oxidation of 3-hexene

Alkene with a double bond present in the middle of the molecule is attractive as a representative of the olefinic group present in the long-chain unsaturated methyl esters, which make up biodiesel typically. Also the study of alkenes has long gained importance owing to its potential as a gasoline surrogate and its formation in significant amounts during alkane oxidation. These considerations motivate the study of kinetics of trans-3-hexene. In the current work, 3-hexene mechanism is developed upon the base chemistry of comprehensive NUIG1.3 mechanism. Several important reactions are revisited and are assigned rate constants from the literature, including theoretical studies. The revised kinetic mechanism has been validated at various temperatures, pressures, and equivalence ratios for different experimental configurations. The mechanism shows good agreement with ignition delay time data in the low to high temperature range. The mechanism also predicts the well known lack of negative temperature coefficient (NTC) behaviour in the case of alkenes with the central double bond. With the proposed kinetic model, it is possible to predict intermediate species profiles produced during 3-hexene oxidation satisfactorily. Path flux analysis and sensitivity analysis are performed in order to understand important reaction pathways and their effect on the combustion characteristics.

Development of a new reduced n-pentane mechanism with NOX for combustion simulation at atmospheric pressure

Air-light hydrocarbon mixing gas is a new type of fuel with great potential, and its key component is n-pentane. In this study, a reduced n-pentane mechanism with NOX is developed for combustion simulation at atmospheric pressure, including 81 species and 348 elementary reactions. First, the dominant species and reactions for n-pentane combustion are recognized by the reaction pathway analysis based on the detailed mechanism. Then, a novel multistep reduction strategy with intersection thought is proposed to reduce the detailed mechanism of n-pentane. Ignition delay time and laminar flame speed are separately selected as the target parameters for validation. The two reduced mechanisms are developed by using the directed relation graphs (DRG) method, the directed relation graphs with error propagation (DRGEP) method, and the full species sensitivity analysis (FSSA) method. Species in the intersection mechanism of the above two mechanisms obtained with multistep reduction are treated as important species. Sensitivity analysis (SA) is further performed to remove the uncertain species, combined with the result of reaction pathway analysis, a reduced n-pentane mechanism including 77 species and 336 reactions is obtained. On this basis, a reduced NOX mechanism (4 species and 12 reactions) is incorporated. Then, the key kinetic parameters of the mechanism are optimized and adjusted according to the results of brute force sensitivity analysis. Finally, the proposed mechanism is validated by using the related data of ignition delay times, laminar flame speeds, premixed flame species profiles, and jet-stirred reactor (JSR) species profiles. The results indicate that the proposed mechanism shows good model performance. This study can provide significant assistance for subsequent research on air-light hydrocarbon mixing gas.

Role of O2 on nitrous oxide fuel blend ethylene auto-ignition sensitivity

An experimental and kinetic modeling study of the influence of O2 addition on the ignition behaviors of Ar diluted nitrous oxide/ethylene blend was performed. Ignition delay time (IDT) measurements were taken by detecting pressure evolution profiles in a heated rapid compression machine (RCM) at temperatures and pressures ranging from 910–1500 K to 35–45 bar. Results show that the addition of O2 significantly promotes the ignition of N2O/C2H4/Ar blend. With the O2 blending ratio increase from 10 % to 100 %, the IDT first decreases and then increases. Subsequently, the IDT data were employed to validate several kinetic models in literature. A reaction model developed in our previous work [Combustion and Flam, 2020, 221: 64-73] can reproduce the measured data and predict the promotion effect of O2 on N2O/C2H4/Ar blend well. Reaction pathway and rate of production (ROP) analyses show that high decomposition temperature of N2O inhibits the ignition of N2O/C2H4/Ar blend, whose ignition is mainly driven by reactions of N2O (+M) = N2 + O (+M) and N2O + H = N2 + OH. The dominant pathway is chain branching process: C2H4 + H/OH→C2H3 (+M)→C2H2 + O→HCCO + H2→CH2CO + H→CH3CO (+M)→CO + CH3. However, the presence of O2 in mixture induces reaction flux changes and decreases the global activation energy for chemical reaction. C2H3 radical is the main intermediate product during the ignition initial stage of C2H4 blends, whose ROP can reflect ignition process of C2H4 blends. For N2O/C2H4/O2/Ar and C2H4/O2/Ar ignition, C2H3 radical pool can be rapidly established via H-abstraction reaction of C2H4. However, the reaction of N2O + H = N2 + OH enhances the ROP of OH radical in N2O/C2H4/O2/Ar blends, resulting in C2H3 radical pool rapid establishment and a faster ignition.

Understanding fundamental effects of biofuel structure on ignition and physical fuel properties

The development of biochemicals and biofuels with advantaged properties of higher combustion efficiency at lower emission levels can be aided by a priori prediction of critical global characteristics based on molecular structure. Existing predictive techniques are incomplete for comprehensive ignition and physical fuel property prediction. This investigation focuses on filling this knowledge gap by developing a priori prediction techniques for predicting important ignition as well as physical fuel properties of promising advantaged biofuels. We demonstrate the utility of this a priori prediction technique on saturated (≤C5) alcohols, both linear and branched in nature by correlating their molecular structure with both important ignition and physical properties of the fuels. This work utilizes a semi-automated method of predicting ignition characteristics via accurate fundamental-based calculations of the fuel radical cascades that govern low-temperature combustion. This numerical methodology is supported by the relevant fuel ignition delay time (IDT) and research octane number (RON) data experimentally measured by the Advanced Fuel Ignition Delay Analyzer (AFIDA). Phyiscal fuel property trends are also analyzed by correlating the molecular structure of the fuel to its physical characteristics. Based on their molecular structure, recommendations are made on an a priori method for selecting biofuels that have advantaged properties for the selected fuel application. Lastly, the utility of this investigation is extended beyond the subset of alcohols considered by analyzing the IDT trends of i-Pentanol and n-Hexanol, revealing the fidelity of the novel kinetic modelling approach and the accuracy of the predicted trends in this study.

Experimental and kinetic modeling studies on oxidation of methanol and di-tert-butyl peroxide in a jet-stirred reactor

Methanol is a potential carbon neutral fuel, but it faces the challenge of ignition difficulties in engines. Di-tert-butyl peroxide (DTBP), a cetane number improver, is considered to be added into methanol to improve ignition. However, there is currently no research of experiments and simulations on the kinetic model of methanol and DTBP. Therefore, the oxidation characteristics of methanol and DTBP were experimentally studied in a jet-stirred reactor under temperatures of 400–1000 K and pressures of 1.03 atm, with equivalence ratios of 0.5, 1 and 2 for pure methanol, pure DTBP, blended fuel (97 % CH3OH + 3 % DTBP; 94 % CH3OH + 6 % DTBP). Then, a new reduced CH3OH-DTBP mechanism containing 75 species and 443 reactions was proposed, which was widely validated by experimental data of ignition delay times, laminar flame speeds and species concentration profiles. Next, the simulations were conducted with CHEMKIN PRO to analyze the effect of DTBP on methanol. The major findings are as follows. After adding DTBP, methanol can react at a lower temperature (550 K), which is due to CH3 generated by decomposition of DTBP promotes the enrichment of OH, thereby promoting the oxidation of methanol. The source of OH at 650 K is CH3 → CH3O2 → CH3O2H → CH3O. At 850 K, the early OH comes from R27 (CH3 + HO2=CH3O + OH), while the later OH comes from the decomposition of H2O2. When the temperature is above 850 K, the addition of DTBP has only a slight effect on the oxidation of methanol. As the temperature increases, the effect of DTBP on shortening the ignition delay time weakens. As DTBP concentration increases, the effect also shows a weakening trend. At low temperatures, the contribution of thermal effects caused by DTBP is greater. As DTBP concentration increases, the proportion of thermal effect contribution increases.

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.

Role of methyldioxy radical chemistry in high‐pressure methane combustion in CO2

AbstractThe combustion chambers of direct‐fired supercritical CO2 power plants operate at pressures of approximately 300 bar and CO2 dilutions of up to 96%. The rate coefficients used in existing chemical kinetic mechanisms are validated for much lower pressures and much smaller concentrations of CO2. Recently, the UoS sCO2 1.0 and UoS sCO2 2.0 mechanisms have been developed to better predict ignition delay time (IDT) data from shock tube studies at pressures from 1 to 260 bar in various CO2‐containing bath gas compositions. The chemistry of the methyldioxy radical (CH3O2) has been identified as an essential combustion intermediate for methane combustion above 100 bar, where mechanisms missing this species begin to vastly overpredict the IDT. The current literature available on CH3O2 is very limited and often concerned with atmospheric chemistry and low‐pressure, low‐temperature combustion. This means that the rate coefficients used in UoS sCO2 2.0 are commonly determined at sub‐atmospheric pressures and temperatures below 1000 K with some rate coefficients being over 30 years old. In this work, the rate coefficients of new potential CH3O2 reactions are added to the current mechanism to create UoS sCO2 2.1 It is shown that the influence of CH3O2 on the IDT is greatest at high pressures and low temperatures. It has also been demonstrated that CO2 has very little effect on the chemistry of CH3O2 at 300 bar meaning that CH3O2 rate coefficients can be determined in other bath gases, reducing the impact of non‐ideal effects such as bifurcation when studying in a CO2 bath gas. The updated UoS sCO2 2.1 mechanism is then compared to high‐pressure IDT data and the most important reactions which require reinvestigation have been identified as the essential next steps in understanding high‐pressure methane combustion.

Development of comprehensive kinetic models of ammonia/methanol ignition using Reaction Mechanism Generator (RMG)

Mechanism development is a complex process by itself, and it is usually combined with experimental studies to understand and evaluate the ignition chemistry of the investigated fuel mixtures. In this work, the software Reaction Mechanism Generator (RMG) is used to automatically generate a kinetic mechanism for the auto-ignition of ammonia/methanol blend mixtures. The ignition delay time (IDT) of ammonia/methanol mixtures was measured in a high-pressure shock tube (HPST) at the pressure of 10 bar, equivalence ratios of 0.5, 1.0 and 2.0, and temperatures between 1050 and 1550 K for methanol content of 0–20%. These IDT data were combined with prior literature data for laminar burning velocity (LBV) at 1 atm and 298–448 K and the ignition delay time (IDT) at 20 and 40 bar and 845 – 1100 K. The detailed mechanism, containing 68 species and 467 elementary reactions, satisfactorily predicted the combined set of IDTs and LBVs of ammonia/methanol blends.

A shock-tube and modeling study of syngas ignition delay times in rich CO2 environment at elevated pressures

Oxy-fuel combustion techniques are crucial for fossil fuel power plants with substantially reduced greenhouse gas emission. The chemical effect of CO2 as a reactant/product or a third-body is important in modeling oxy-fuel combustion. Many previous kinetic studies focused on evaluating the rate parameters in relevant reactions, but less attention was paid to CO2 collision efficiencies. In this study, ignition delay time data were measured in a shock-tube facility for stoichiometric syngas mixtures with 50%, 70% and 90% CO in fuel at 1 and 5 atm, diluted in 85% CO2 bath gas. Among the chemical effect of CO2, the CO2 collision efficiency in reaction H+O2(+M) = HO2(+M) shows a dominantly high sensitivity value to model prediction over the present experimental conditions. Therefore, the CO2 collision efficiency was evaluated using the current data and a recommended value was reported with uncertainty analysis. Moreover, the CO2 collision efficiencies in reaction H2O2(+M) = OH+OH(+M), HCO(+M) = H+CO(+M), H2+M = H+H+M and O+O+M = O2+M were evaluated using ignition delay time data at higher pressures (40 atm to 200 atm) and recommended CO2 collision efficiency values were reported with uncertainty analysis. The model with recommended CO2 collision efficiency values significantly improved prediction performance at 5 atm and was validated over a comprehensive set of syngas ignition delay time and laminar flame speed data.

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.

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.

A Reduced Reaction Mechanism for Diesel/2-Methyltetrahydrofuran Dual-Fuel Engine Application

2-methyltetrahydrofuran (MTHF2) has been recently regarded as a promising alternative engine fuel. However, the chemical reaction mechanism for MTHF2 combustion in the engine has not been reported to date. In this study, a reduced diesel/MTHF2 reaction mechanism with only 78 species among 233 reactions was constructed for diesel/MTHF2 dual-fuel engine simulations. Firstly, a diesel surrogate mechanism involving the sub-mechanisms of n-decane, iso-octane, methylcyclohexane (MCH), toluene, a reduced mechanism of C2-C3 species and a detailed mechanism of H2/CO/C1 was selected. Secondly, a skeletal MTHF2 mechanism containing 54 species and 294 reactions was formulated under engine-relevant conditions using combined mechanism reduction methods. Thirdly, a reduced sub-mechanism of MTHF2 oxidation with 11 species and 13 reactions was extracted and combined with the four-component diesel surrogate fuel mechanism. Subsequently, the reduced diesel/MTHF2 mechanism was obtained by improving the combined five-component mechanism based on sensitivity analysis. Finally, the proposed mechanism was validated with selected experimental data of ignition delay times, flame species concentrations and laminar flame speeds. In addition, the new measurements from diesel/MTHF2 dual-fuel engine combustion were obtained and then utilized to further assess the developed mechanism. Overall, the developed diesel/MTHF2 mechanism can be used for diesel and MTHF2 dual-fuel engine combustion simulation.

A data-driven framework to predict ignition delays of straight-chain alkanes

Ignition delay time (IDT) is an important global combustion property that affects the thermal efficiency of the engine and emissions (particularly NO). IDT is generally measured by performing experiments using Shock-tube and Rapid Compression Machine (RCM). The numerical calculation of IDT is a computationally expensive and time-consuming process. Arrhenius type empirical correlations offer an inexpensive alternative to obtain IDT. However, such correlations have limitations as these typically involve ad-hoc parameters and are valid only for a specific fuel and particular range of temperature/pressure conditions. This study aims to formulate a data-driven scientific way to obtain IDT for new fuels without performing detailed numerical calculations or relying on ad-hoc empirical correlations. We propose a rigorous, well-validated data-driven study to obtain IDT for new fuels using a regression-based clustering algorithm. In our model, we bring in the fuel structure through the overall activation energy () by expressing it in terms of the different bonds present in the molecule. Gaussian Mixture Model (GMM) is used for the formation of clusters, and regression is applied over each cluster to generate models. The proposed algorithm is used on the ignition delay dataset of straight-chain alkanes (C H for n = 4 to 16). The high level of accuracy achieved demonstrates the applicability of the proposed algorithm over IDT data. The algorithm and framework discussed in this article are implemented in python and made available at https://doi.org/10.5281/zenodo.5774617.

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.

A detailed chemical insights into the kinetics of diethyl ether enhancing ammonia combustion and the importance of NOx recycling mechanism

In this work, we investigated the combustion characteristics of ammonia (NH3) by blending it with various proportions of diethyl ether (DEE). We measured laminar flame speed of various NH3/DEE blends (DEE, 10–40% by mole) using a constant volume spherical vessel at Ti = 298 K and Pi = 3 and 5 bar and Φ = 0.8–1.3. We developed a detailed kinetic model to describe the trends of the current and previously published experimental data. For the robustness of the model, we first developed a comprehensive diethyl ether kinetic mechanism to accurately characterize neat DEE oxidation behavior. We validated the kinetic model using a large pool of experimental data comprising shock tube, rapid compression machine, jet-stirred and flow reactors, freely propagating, and burner-stabilized premixed flames. The developed kinetic model performs remarkably in capturing the combustion behavior of pure DEE and NH3. Importantly, our model captures the experimental data of laminar flame speed and ignition delay times of various NH3/DEE blends over a wide range of conditions. We found that DEE is a promising candidate to promote the combustion characteristics of NH3. A small portion of DEE (10%) enhances the laminar flame speed of NH3 by a factor of 2 at Pi = 1 bar, Ti = 298 K, and Φ = 1.0. A further doubling of the DEE mole fraction to 20% did not enhance the laminar flame speed of NH3 with the same propensity. At low temperatures, adding 5% DEE in NH3 blend has significantly expedited the system reactivity by lowering the autoignition temperature. A further 5% increment of DEE (i.e., 10% DEE in NH3) lowers the autoignition temperature by ∼120 K to achieve the same ignition delay time. The “NONO2” looping mechanism predominantly drives such reactivity accelerating effect. Here, the reactions, NO + HO2 = NO2 + OH and NO2 + H = NO + OH, appear to enhance the reactive radical pool by generating OH radicals. We observed that the HNO path is favored more with increasing DEE content which eventually liberates NO. Other key reactions in “NONO2” looping mechanism are: CH3 + NO2 = CH3O + NO, CH3O2 + NO = CH3O + NO2, C2H5 + NO2 = C2H5O + NO, C2H5O2 + NO = C2H5O + NO2. In addition, CH3 + NH2(+M) = CH3NH2(+M) reaction is also one of the important cross-reactions which leads to the formation of HCN. Therefore, cross-reactions between the nitrogen and carbon family are crucial in accurately predicting autoignition timing. This work provides a detailed chemical insight into the NH3 and DEE interaction, which could be applied to other fuel blends of NH3. The kinetic model is also validated for several C1C3 fuels including their interaction with NOx.

Development and verification of a physical–chemical surrogate model of RP-3 kerosene with skeletal mechanism for aircraft SI engine

A three-component surrogate model was developed to emulate both the physical and chemical characteristics of RP-3 kerosene for aircraft spark ignition (SI) engine. Due to the similar carbon number and physical–chemical properties, n-decane (C10H22), 1,3,5-trimethylbenzene (C9H12) and iso-dodecane (IC12H26) were selected as surrogate fuel (46.31%/25%/28.69% by mole, respectively). The surrogate well captured the target properties of RP-3, including RON, CN, H/C ratio, MW, LHV, liquid density, kinematic viscosity and surface tension. Furthermore, a skeletal mechanism of RP-3 was developed based on the provided surrogate model. The RP-3 sub-mechanism only included 65 species and 200 reactions which was efficient and compact for complex CFD simulation. The mechanism of each pure component and RP-3 surrogate had been verified with the ignition delay time data under various conditions. The measured species concentrations and laminar flame speed data were also used to test the kinetic mechanism. Surrogate model and skeletal sub-mechanism were implemented under evaporating spray environment, Bunsen burner and SI engine mode by CFD method. The acceptable agreement between experiments and simulations proved the reliability of the skeletal mechanism of RP-3 surrogate in SI engine simulation.