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Article

An n-Heptane Oxidation Mechanism Suitable for Low- to High-Temperature Combustion

by
Junfa Duan
*,
Aoqing Yang
,
Wei Wei
and
Gaolin Qin
School of Mechanical Engineering, North China University of Water Resources and Electric Power, Zhengzhou 450045, China
*
Author to whom correspondence should be addressed.
Energies 2025, 18(5), 1305; https://doi.org/10.3390/en18051305
Submission received: 15 January 2025 / Revised: 3 February 2025 / Accepted: 4 March 2025 / Published: 6 March 2025
(This article belongs to the Section I1: Fuel)
Browse Figures
Versions Notes

Abstract

:
The detailed n-heptane mechanism, which is widely used today, is suitable for a wide range of operating conditions. However, due to the large model involved, it is difficult to use this mechanism for computational fluid dynamics (CFD) simulation. In addition, the prediction accuracy of the existing simplified mechanism cannot meet simulation requirements with respect to low-temperature combustion and the negative temperature coefficient region. In this study, we sought to solve these problems by constructing a new simplified mechanism (NC2024) of the n-heptane chemical reaction based on the mechanism of Kuiwen Zhang using path analysis and sensitivity analysis. The mechanism involves 72 substances and 126 reactions. A comparison with the commonly used mechanism and an analysis of experimental data revealed that the NC2024 mechanism delivers high accuracy in predicting the ignition delay period under the low- to high-temperature conditions of 600–1100 K and a large pressure range of 13.5–42 bar and thus meets the accuracy requirements for CFD simulation of diesel low-temperature combustion.

1. Introduction

Diesel is a complex, multi-component fuel, and it is difficult to explore its combustion process in detail. The development of a detailed chemical-reaction mechanism for diesel oil can therefore be seen as a great challenge [1]. Because the cetane number of n-heptane is similar to that of diesel oil, and because its combustion characteristics are similar to those of diesel oil, most researchers have chosen n-heptane as the characterization fuel for diesel [2].
Since the 1970s, to further explore the n-heptane oxidation process, efforts have been made to establish kinetic models of n-heptane oxidation at low and high temperatures [3]. In the late 1970s and 1980s, Coats [3] and Westbrook [4,5] first modeled the oxidation of n-heptane (only high-temperature oxidation) [5].
In the past four decades, n-heptane has been studied in detail at institutions worldwide; these include the Lawrence Livermore National Laboratory [6,7,8,9], the Sandia National Laboratory [10], the University of California [11,12,13], the University of Aachen [14], the University of Michigan, the Polytechnic University of Milan [15,16,17,18], the University of Lorraine [19,20], the University of Heidelberg [21], Lund University [22], and the University of Connecticut [23].
In order to explore the combustion process of diesel oil in detail, researchers have successively established a number of detailed kinetic models of n-heptane chemical reaction.
With the deepening of research into alternative fuels for internal-combustion engines, the use of the computational fluid dynamics (CFD) method to simulate turbulence, spray, mixing, heat and mass transfer, and combustion and emission stages in internal-combustion engines has become an important technical means to study ignition and combustion phenomena in fuel cylinders [24,25].
However, the actual combustion process often involves multiple primitive reactions, forming a complex network of chemical reactions [26]. Even the combustion process of a single fuel is still very complex, involving thousands of elemental reactions and hundreds or thousands of intermediates. Accurate simulation of the occurrence and development of these chemical reactions and changes in concentrations of each substance is extremely difficult, so CFD calculations often involve small time steps to ensure the stability and accuracy of results [27]. Due to the very fast speeds of chemical reactions and fluid flows involved in the combustion process, especially in rapid combustion phenomena such as deflagration, it is necessary to use very small time steps to capture the dynamic process accurately [28]. Smaller time steps mean that a great deal of time iteration is required throughout the simulation, increasing the amount of computation [29]. As a result, CFD simulations using detailed mechanisms require significant amounts of computational resources and time.
Because of this, a number of institutions have proposed simplification mechanisms. These institutions include the Engine Research Center at Sloan Automotive Laboratory [30], the Combustion Research Group at UC San Diego (Chemical Mechanism: Combustion Research Group at UC San Diego), the University of Wisconsin–Madison [31,32], the German Institute of Physical Chemistry [33], the University of the Chinese Academy of Sciences [34], and Sichuan University (https://cds.scu.edu.cn/, accessed on 1 May 2024).
However, the simplified mechanisms proposed have delivered low rates of accuracy in predicting the flame retardation period under a wide range of operating conditions, and some mechanisms cannot even predict two-stage ignition [35].
In light of the above, we sought in the present study to construct a simplified mechanism of n-heptane suitable for medium- and low-temperature combustion and to verify its prediction accuracy.

2. Selection of Comparison Mechanism

A detailed mechanism can accurately predict the retardation period of diesel combustion over a large range of initial temperatures and pressures.
In order to explore the prediction accuracy of the existing detailed chemical-reaction kinetic models of n-heptane with respect to the flame retardation period, we reviewed chemical-reaction kinetic models of n-heptane with two-stage ignition behavior and more than 600 steps; these are summarized in Table 1.
As a component of primary reference fuel, n-heptane has been extensively studied in shock tubes [39,40,41,42,43,44], jet-stirred reactors and rapid compression machines (RCMs) [45,46,47,48], premixed laminar flames [49,50,51,52], and flow reactors [53].
Due to the need to construct a mechanism with a wide range of applicability in this study, the selected experimental parameters need to be comprehensive and detailed to facilitate subsequent comprehensive comparisons. The experimental parameters of the n-heptane ignition delay period selected for the present study are shown in Table 2.
The simulation was based on the experimental environment of adiabatic premixing and had previously been performed using the homogeneous closed reactor module within CHEMKIN, employing the transient solver, with an end time of 20 s [37]. The initial temperature was 600~1500 K (a group at 50 K intervals), the pressure was 13.5 bar/42 bar, and the equivalence ratio was 0.5/1.0/2.0 [54]. From a comparison with the experimental data measured by Ciezki [54] under the same working conditions, the accuracy of the reaction mechanism was evaluated and analyzed.
In this study, as stated above, eight extensively verified detailed chemical-reaction kinetic models of n-heptane were selected, as summarized in Table 1. The prediction accuracy of these models with respect to the flame retardation period was then compared under different working conditions.
For all eight models, variations in the ignition delay period with the increase in temperature under conditions of 13.5 bar, Ø = 0.5/1.0/2.0, and 42 bar, Ø = 0.5/1.0/2.0 are shown in Figure 1 and Figure 2, respectively.
It can be seen from Figure 1 that, under the condition of 13.5 bar, Ø = 0.5/1.0/2.0, the average deviation of the Yanzhao An mechanism for the lag period is larger, and its maximum deviation can reach three times that of the other mechanisms. It can also be observed that the prediction deviation of the Liming Cai mechanism and the Junjiang Guo mechanism between 600 K and 800 K is large and that the prediction accuracy of the Curran mechanism is comparable with that of the Chun Sang Yoo mechanism. However, the prediction bias of the Curran mechanism is slightly greater than those of Mehl and Kuiwen Zhang. In contrast, the Kuiwen Zhang mechanism has the smallest prediction bias and the highest prediction accuracy.
It can be seen from Figure 2 that, under the condition of 42 bar, Ø = 0.5/1.0/2.0, the overall prediction deviation of the Yanzhao An mechanism for the lag period is large, and its maximum deviation can reach four times that of the other mechanisms. It can also be observed that the prediction deviation of the Yulin Chen mechanism for the lag period between 800 K and 1000 K is larger, and that the prediction deviations of the Curran and Chun Sang Yoo mechanisms for the lag period are larger still, with a maximum deviation which can be twice that of the other mechanisms. The prediction biases of the Mehl and Liming Cai mechanisms are slightly larger when the mechanism is 900–1100 K. In contrast, the Kuiwen Zhang mechanism has the smallest prediction bias and the highest prediction accuracy.
To sum up, our comprehensive comparison of eight chemical-reaction kinetic models revealed that, within the working condition ranges of 13.5–42 bar pressure and an equivalence ratio of 0.5–2.0, the Kuiwen Zhang mechanism had the best prediction ability, as shown in Figure 1 and Figure 2. We therefore constructed a simplified mechanism based on the Kuiwen Zhang mechanism using the CHEMKIN 19.2 simulation platform.

3. Construction of a Simplified Mechanism of n-Heptane

3.1. Construction of Skeleton Mechanism

Combustion is a complex oxidation process involving a variety of chemical reactions, all of which must follow the law of conservation of mass, momentum, and energy. The formation rules for the main mechanisms [55,56] are shown in Figure 3.
As can be seen from Figure 3, in order to ensure the comprehensiveness of the generation mechanism, the generation laws for the main mechanisms include various reaction rules, including those for the isomerization of single molecules and those for bimolecular reactions.
However, the low-temperature oxidation process for macromolecular hydrocarbon compounds is very complicated. The detailed reaction mechanism generally includes hundreds of components and reaction numbers. Curran, of the LLNL National Laboratory in the United States, has divided all possible elementary reactions in the n-heptane oxidation process into twenty-five types, including nine high-temperature oxidation types and sixteen low-temperature oxidation types. These can be roughly summarized as covering the thermal decomposition of alkanes, the formation of alkyl radicals by alkyl dehydrogenation, the isomerization of alkyl radicals, the β-cracking of alkyl radicals, and a small number of additional low-temperature reactions. Curran’s research has shown that single-molecule isomerization is obviously much faster than the bimolecular reaction rate [6].
For this reason, the bimolecular reaction type was ignored in the construction of the simplified reaction mechanism model proposed in the present work.
The total process of n-heptane oxidation is shown in Figure 4. The diagram summarizes the main oxidation reaction types of n-heptane at different temperature ranges from low temperature to high temperature, in which each reaction type contains elementary reactions with similar characteristics.
At any ambient temperature, the initial reaction of n-heptane is always a reaction with oxygen [57]. After the oxidation process begins, the fuel can also react with active radicals with a stronger dehydrogenation ability to generate heptane. The active radicals HO2• and C7H15• are generated by the (R1) reaction. HO2• has a stronger dehydrogenation effect than O2. Therefore, as the reaction proceeds, the reaction of small-molecular radicals with NC7H16 becomes dominant. Such free radicals also include H•, OH•, and CH3; among these, OH• has the strongest dehydrogenation effect. In addition, the reaction between NC7H16 and OH dominates in the low-temperature oxidation stage. The specific reaction types are as follows:
NC7H16 + O2 => C7H15• + HO2•
NC7H16 + H => C7H15• + H2
NC7H16 + OH => C7H15• + H2O
NC7H16 + HO2 => C7H15• + H2O2
The emergence of C7H15• produces different reaction paths according to different ambient temperatures, and its related reactions are an important part of low-temperature oxidation. Figure 5 and Figure 6 show n-heptane oxidation paths at an initial pressure of 13.5 bar, an initial temperature of 600 K, and system temperatures of 850 K and 950 K, respectively.
It can be seen from Figure 5 that when the system temperature reaches 850 K, the main oxidation path of C7H15 is the chain branching reaction of primary oxygenation → isomerization → secondary oxygenation → isomerization → hydroxanthone. In this oxidation process, the alkyl radical β-cracking reaction and its own dehydrogenation reaction are both still very slow. At this time, the continuation of the low-temperature reaction chain of heptane is an important path for low-temperature oxidation.
It can be observed from Figure 6 that when the temperature rises to 950 K, the C7H15-related β-cracking reaction becomes the main path, and the low-temperature chain reaction is replaced during this process.
In light of this brief analysis of the n-heptane oxidation process, to construct a n-heptane oxidation mechanism suitable for a low-temperature environment, we specified a low-temperature oxidation reaction which was as detailed as possible and then selected the necessary high-temperature oxidation reaction to construct the skeleton mechanism.
The final n-heptane oxidation skeleton mechanism is as follows:
NC7H16 + O2 => C7H15• + HO2•
NC7H16 + H => C7H15• + H2
NC7H16 + OH => C7H15• + H2O
NC7H16 + HO2 => C7H15• + H2O2
C7H15• + O2 <=> C7H15OO•
C7H15OO• <=> C7H14OOH
C7H14OOH + O2 <=> •OOC7H14OOH
•OOC7H14OOH => C7KET + OH
C7KET <=> CH3CHO + NC3H7COCH2 + OH
C7H14OOH = C7H14• + HO2
C7H14OOH = C7H14O• + OH
C7H14OOH = C3H7CHO + C3H6 + OH
C7H15• = C5H11 + C2H4
C7H15·= C4H9 + C3H6
C7H15• + O2 = C7H14• + HO2
H2O2 = OH + OH
H + O2 = O + OH
CO + OH = CO2 + H
In addition to the above eighteen types of elementary reactions, the transition reaction of cracking stable olefins into small molecules in the decomposition products of C7H14OOH is also an important process in low- to high-temperature oxidation. In this study, the specific elementary reactions referred to by the skeleton mechanism were determined as set out in the following sections, which also describe how the subsequent oxidation paths were further supplemented and adjusted.

3.2. Important Primitives and Path Supplements in the Initial Stage of the Low-Temperature Reaction

The transition reaction of cracking stable olefins into small molecules in the decomposition products of C7H14OOH is also an important process in low- to high-temperature oxidation. From our analysis of previously reported results for the application range and accuracy of different reaction mechanisms, we determined that the Kuiwen Zhang mechanisms delivered high prediction accuracy with respect to the ignition delay period, and this was verified by a variety of experiments. Therefore, this mechanism was selected for further analysis.
It was found that the main macromolecular (C7) isomers directly related to the low-temperature oxidation process accounted for 58 species. By analyzing the yields of important intermediate substances in the low-temperature oxidation chain, those elements with a relatively large influence on the oxidation process were retained, the number of isomers was reduced, and the main reactions were selected to initially fill the skeleton mechanism. In addition, in the process of yield analysis, any reaction that had little effect on the target material was directly omitted, and no analysis or comparison was carried out. If there were multi-step elementary reactions in the same substance or in the same class of substances, the three or four elementary reactions with the most significant formation rate were retained in the figure.
After analysis, the skeleton mechanism related to low-temperature oxidation was determined, including the following 24 elementary reactions:
NC7H16 + O2 => C7H15-2 + HO2C7H14OOH2-4O2 => NC7KET35 + OH
NC7H16 + H => C7H15-2 + H2C7H14OOH2-4O2 => NC7KET42 + OH
NC7H16 + OH => C7H15-2 + H2OC7H14OOH2-4 + O2 => C7H14OOH2-4O2
NC7H16 + HO2 => C7H15-2 + H2O2NC7KET42 => CH3CHO + NC3H7COCH2 + OH
C7H15-2 + O2 => C7H15O2-2NC7KET24 => NC3H7CHO + CH3COCH2 + OH
C7H15O2-2 => C7H14OOH2-4NC7KET35 => C2H5CHO + C2H5COCH2 + OH
C7H15-2 => C7H14-1 + HC7H14OOH2-4 => C7H14-3 + HO2
C7H15-2 => pC4H9 + C3H6C7H14OOH2-4 => C7H14O2-4 + OH
C7H15-2 => C4H8-1 + NC3H7C7H14OOH2-4 => OH + C2H5CHO + C4H8-1
C7H15-2 => C2H5 + C5H10-1C7H14OOH2-4 => OH + CH3CHO + C5H10-1
C7H14OOH2-4O2 => NC7KET24 + OHC7H14OOH2-4 => OH + NC3H7CHO + C3H6

3.3. Study on Reaction Intermediates and Paths

A mature and widely applicable skeletal mechanism plays an important role in constructing the combustion mechanism of large hydrocarbon fuels. In order to accurately simulate the effect of fuel on engine performance, chemical models must predict ignition behavior (including low-temperature heat release, the negative temperature coefficient (NTC) region, two-stage ignition, and cold flame phenomena), as well as flame propagation and pollutant formation [58].
Based on the mechanism, the main oxidation paths of important intermediate substances were studied. The main oxidation paths of important intermediate substances are shown in Figure 7, where red refers to the high-temperature oxidation path and blue refers to the low-temperature oxidation path.
C5H10-1 is mainly obtained from the decomposition of C7H14O, and its consumption path is divided into two parts: a low-temperature oxidation stage, in which it is mainly decomposed into macromolecule C5H9, and a high-temperature oxidation stage, in which it is directly decomposed into small molecules of C2 and C3. C7H14-1 is decomposed into C3H5-a and pC4H9 during the low-temperature oxidation process, and the main path of the latter is also the source of C4H8-1. During the high-temperature oxidation process, C7H14-1 is mainly decomposed into C4H7 and NC3H7. C4H8-1 does not have a high-temperature oxidation source; mainly due to the high-temperature oxidation stage, the macromolecules are directly decomposed into C2 and C3 small molecules, such as pC4H9 cleavage into C2H4 and C2H5. The main decomposition product of C4H8-1 in the low-temperature oxidation stage is C4H7, and the main decomposition product in the high-temperature oxidation stage is C3H5-a. During the low-temperature oxidation process, C3H6 is mainly derived from the decomposition product C5H11 of C7H15O2, which is subsequently dissociated into C3H4, C3H3, C3H2, and HCCO. In the process of high-temperature oxidation, C3H6 is obtained by dissociation of NC3H7 and then rapidly decomposed into small molecular units.
By combining the oxidation pathways of important intermediates, the elementary reactions involved in the main pathways were obtained, as summarized in Table 3.
We used temperature as a reference variable, and sensitivity analysis was carried out according to the sensitivity equation of the lag period.
The sensitivity equation for the lag period is as follows:
S i g T = 100 × ( l o g 10 t k 1 − l o g 10 t k 2 ) l o g 10 t b a s e l o g 10 ( k 1 / k 2 )
where S i g T is the ignition delay sensitivity coefficient in the percentile at initial temperature T; t k 1 and t k 2 are the ignition delay times obtained using the factors of k1 and k2, respectively; and t b a s e is the ignition delay time of the baseline case corresponding to initial temperature T. If S i g is positive, the ignition delay time shortens as the reaction rate (pre-exponential factor) increases.
In the sensitivity analysis, the temperature is divided into four groups: 600 K, 700 K, 900 K, and 1100 K. The homogeneous reaction model was selected to simulate the retardant period under the working conditions of 13.5 bar and 42 bar. The influence of each reaction on the retardant period in the constructed chemical-reaction kinetic model was then explored, and the 15 groups of reactions that were most sensitive to the retardation period were selected [59]. The sensitivity analysis results are shown in Figure 8.
When using sensitivity analysis, it is difficult to improve the prediction accuracy of the global lag period by adjusting a single reaction. In this study, therefore, we adjusted the local prediction accuracy under the condition of ensuring the global prediction accuracy. The adjustment for the pre-index factor was as follows:
1.NC7H16 + O2 <=> C7H15-2 + HO24.000 × 1013 => 4.1 × 1013
2.NC7H16 + OH <=> C7H15-2 + H2O9.400 × 107 => 1.6 × 108
3.NC7H16 + HO2 <=> C7H15-2 + H2O23 × 1013 => 2.1 × 1013
4.C7H15O2-2 <=> C7H14OOH2-42.500 × 1010 => 3 × 1010
5.C7H14OOH2-4O2 <=> NC7KET42 + OH1.250 × 1010 => 1.5 × 1010
6.C7H14OOH2-4 <=> C7H14-1 + HO26.615 × 1018 => 1.3 × 1019
7.HO2 + H = H2 + O21.660 × 1013 => 0.7 × 1013
After the supplementation of the free-radical oxidation reaction, intermediate oxidation reaction, and high-temperature oxidation reaction; the adjustment of the relevant parameters of the constructed mechanism; and the sensitivity analysis, the NC2024 mechanism with 72 elementary substances and 126 elementary reactions was finally constructed.

4. Ignition Delay Period Verification

The NC2024 mechanism was verified by comparison with Kuiwen Zhang’s detailed mechanism and with simplified mechanisms involving similar species and reaction numbers proposed by other institutions.
The selected simplified mechanisms are shown in Table 4.
The results are shown in Figure 9. Based on the zero-dimensional homogeneous reaction model (conditions: homogeneous, adiabatic) of CHEMKIN 19.2, the simulation results of NC2024 for the ignition delay period under different working conditions were compared with the selected comparison mechanism simulation results and shock tube experimental results, as shown in Figure 9.
It can be seen from Figure 9a that the San Diego mechanism has the largest prediction deviation for the flame retardation period, and its maximum deviation can be more than twice that of the other mechanisms. In addition, the Yachao Chang mechanism has a large prediction deviation for the flame retardation period at 700–850 K, and the NC2024 mechanism has the same prediction bias as the Kuiwen Zhang mechanism, with a prediction accuracy of more than 90%. It can be seen from Figure 9b that the San Diego mechanism has the largest prediction bias for the lag period, and the Yachao Chang mechanism has the smallest prediction bias, followed by the Kuiwen Zhang mechanism. In addition, the NC2024 mechanism has a slightly inferior prediction accuracy compared with the Kuiwen Zhang mechanism. It can be seen from Figure 9c that the prediction deviation of the San Diego mechanism is large, with a maximum deviation which can be more than twice that of the other mechanisms. In addition, the prediction deviation of the Yachao Chang mechanism between 750 K and 900 K is large, with a maximum deviation which can reach 1.5 times that of the other mechanisms. It can also be seen that the prediction bias of the NC2024 mechanism is second only to that of the Kuiwen Zhang mechanism, and its prediction accuracy can reach 80%. It can be seen from Figure 9d that the prediction deviation of the Yachao Chang mechanism is the smallest, between 800 K and 900 K, while the prediction deviations of the San Diego mechanism and the Kuiwen Zhang mechanism are larger, with maximum deviations about twice that of the NC2024 mechanism. In addition, the prediction deviation of the lag period of the NC2024 mechanism between 900 K and 1000 K is slightly larger. It can be seen from Figure 9e that, between 700 K and 850 K, the prediction bias of the NC2024 mechanism is slightly better than that of the San Diego mechanism and the Yachao Chang mechanism. Between 900 K and 1000 K, the San Diego mechanism has the smallest prediction bias, followed by the NC2024 mechanism, and its prediction accuracy is better than that of the Kuiwen Zhang mechanism and the Yachao Chang mechanism. In addition, the prediction bias of the Chang mechanism is the largest, the overall prediction bias of the Kuiwen Zhang mechanism is the smallest, and the prediction accuracy of the NC2024 mechanism is second only to the Kuiwen Zhang mechanism. It can be seen from Figure 9f that the prediction accuracy of the Kuiwen Zhang mechanism and the Yachao Chang mechanism is comparable; in addition, their overall prediction biases are small. The prediction accuracy of the NC2024 mechanism is comparable to that of the Yachao Chang mechanism, except for a large prediction deviation between 900 K and 1050 K. And the San Diego mechanism has a better prediction bias than the other mechanisms between 850 K and 950 K, and lower prediction deviation between 950 K and 1150 K.
In summary, the comparison with simulation results under various working conditions shown in Figure 9 revealed that the NC2024 mechanism constructed at 13.5 bar, Ø = 0.5/1.0/2.0, exhibited a consistent prediction trend when compared with experimental data and with other simplified mechanisms; its prediction deviation for the lag period was small, and its prediction accuracy was second only to the Kuiwen Zhang mechanism, with a deviation in accuracy of no more than an order of magnitude. At 42 bar, Ø = 0.5/1.0/2.0, the prediction accuracy of the NC2024 mechanism was slightly inferior to that of the Yachao Chang mechanism, but the deviation in its prediction accuracy was not more than half an order of magnitude.
On the whole, the NC2024 mechanism was found to be applicable to a wider range of working conditions and to deliver smaller prediction bias and better prediction accuracy than other mechanisms with similar species and response numbers.

5. Conclusions

In this study, we used CHEMKIN software to verify the simulation accuracy of multiple mechanism models with respect to the ignition delay period under different working conditions and compared their results with experimental data. The results showed that the Kuiwen Zhang mechanism had a wide range of application and delivered high prediction accuracy with respect to the ignition delay period.
In light of this, we conducted a reaction path analysis of the Kuiwen Zhang mechanism at different temperatures according to the formation rules of the main mechanisms and carried out a path supplementation under low-temperature conditions to construct a skeleton mechanism including 24 steps. Next, by combining the oxidation paths of important intermediate substances, we summarized the main elementary reactions involved in the path. Finally, an NC2024 mechanism with 72 elementary substances and 126 elementary reactions was constructed with only a small amount of calculation and consequent savings in calculation time.
Based on experimental data, the simulation accuracy of the NC2024 mechanism with respect to the ignition delay period was explored, along with the accuracy of commonly used detailed and simplified mechanisms, under different working conditions. The results showed that the NC2024 mechanism delivers high prediction accuracy for low- to high-temperature combustion processes. Under the conditions of temperature 600–1100 K, 13.5–42 bar pressure and an equivalence ratio of 0.5–2.0, the combustion process of n-heptane can be accurately predicted.

Author Contributions

Conceptualization, J.D.; Methodology, G.Q.; Validation, A.Y.; Formal analysis, W.W. and G.Q.; Investigation, A.Y.; Writing—original draft, A.Y.; Supervision, J.D. and W.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by [the National Natural Science Foundation of China] grant number [No. 51976012] and [Henan Province key research and development and promotion special project (science and technology research)] grant number [252102220117] and the APC was funded by [Laser echo simulator development].

Data Availability Statement

Data are provided within the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Energies 18 01305 g001
Figure 1. Variations in ignition delay periods of different models with temperature under different working conditions of 13.5 bar, Ø = 0.5/1.0/2.0.
Figure 1. Variations in ignition delay periods of different models with temperature under different working conditions of 13.5 bar, Ø = 0.5/1.0/2.0.
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Figure 2. Variations in ignition delay periods of different models with temperature under different working conditions of 42 bar, Ø = 0.5/1.0/2.0.
Figure 2. Variations in ignition delay periods of different models with temperature under different working conditions of 42 bar, Ø = 0.5/1.0/2.0.
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Figure 3. Generation laws for main mechanisms [55,56].
Figure 3. Generation laws for main mechanisms [55,56].
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Figure 4. General flowchart of n-heptane oxidation process [6,15,18].
Figure 4. General flowchart of n-heptane oxidation process [6,15,18].
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Figure 5. The reaction path at 850 K system temperature with an initial temperature of 600 K.
Figure 5. The reaction path at 850 K system temperature with an initial temperature of 600 K.
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Figure 6. The reaction path at 950 K system temperature with an initial temperature of 600 K.
Figure 6. The reaction path at 950 K system temperature with an initial temperature of 600 K.
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Figure 7. The main oxidation pathways of important intermediates [37].
Figure 7. The main oxidation pathways of important intermediates [37].
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Figure 8. Sensitivity analyses at 13.5 bar and 42 bar.
Figure 8. Sensitivity analyses at 13.5 bar and 42 bar.
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Figure 9. Variations in ignition delay period of NC2024 mechanism and different mechanisms with temperature under different working conditions of 13.5 bar/42 bar Ø = 0.5/1.0/2.0.
Figure 9. Variations in ignition delay period of NC2024 mechanism and different mechanisms with temperature under different working conditions of 13.5 bar/42 bar Ø = 0.5/1.0/2.0.
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Table 1. Summary of detailed mechanism investigations carried out in recent years.
Table 1. Summary of detailed mechanism investigations carried out in recent years.
NumberAuthorYearSpeciesReactionTemperature
(K)		Pressure
(Bar)		Equivalence
Ratio
1Curran et al. [6] 19985602539550–17001–420.3–1.5
2Mehl [7,8] 201115506000650–12003–500.3–1.0
3Chun Sang Yoo [23] 2011188939///
4Yanzhao An [36] 20152191229750–180 K15–600.5–2.0
5Liming Cai [14] 2016169211,015//0.5–2
6Kuiwen Zhang [37] 201612685336726–141215–380.25–4.0
7Chen Yulin [13] 2018126680750–110020–400.5–2.0
8Junjiang Guo [38] 20197323022///
Table 2. Experimental comparison parameters selected for this study.
Table 2. Experimental comparison parameters selected for this study.
AuthorTemperature
(K)		Pressure
(Bar)		Equivalent RatioTest EquipmentYear
Ciezki [54]660–13503.2–420.5–3.0shock tube1993
Table 3. Summary of elementary reactions of main olefin oxidation pathways.
Table 3. Summary of elementary reactions of main olefin oxidation pathways.
Equation
C7H14O2-4 + OH => CH3CO + C5H10-1 + H2OC3H5-a <=> C3H4-a + H
C7H14-1 <=> pC4H9 + C3H5-aC3H4-a + OH => C3H3 + H2O
C7H14-2 => C4H7 + NC3H7C3H3 + H => C3H2 + H2
C5H10-1 => C2H5 + C3H5-aC3H2 + O2 => HCCO + CO + H
C5H10-1 + OH => C5H9 + H2OC2H3CHO + H => C2H3CO + H2
C5H9 => C2H3 + C3H6C2H3CO => C2H3 + CO
NC4H9O2 => pC4H9 + O2C2H5(+M) <=> H + C2H4(+M)
pC4H9 => C2H5 + C2H4C2H4 + H => C2H3 + H2
C4H8-1 <=> C3H5-a + CH3C2H4 + O => CH3 + HCO
C4H8OOH1-2 => C4H8-1 + HO2C2H3 + O2 => C2H2 + HO2
C4H8-1 + OH => C4H7 + H2OC2H3 + O2 => CH2O + HCO
C4H7 + HO2 => C4H7O + OHC2H2 + O => HCCO + H
C4H7O => CH3CHO + C2H3CH3CHO + H => CH3CO + H2
C4H7O => C2H3CHO + CH3CH3CO(+M) => CH3 + CO(+M)
C4H7 + NC3H7 => C7H14-2HCCO + H => CH2(s) + CO
C4H7 => C4H6 + HCH4 + O => CH3 + OH
C4H6 + H => C2H3 + C2H4CH3 + O2 => CH2O + OH
NC3H7 <=> CH3 + C2H4CH3 + O => CH2O + H
NC3H7 <=> H + C3H6CH2O + OH => HCO + H2O
C3H6 + OH => C3H5-a + H2OCH2O + H => HCO + H2
C3H6 + H => C3H5-a + H2CH2(s) + CO => HCCO + H
C3H6 + CH3 => C3H5-a + CH4CH2(s) + M <=> CH2 + M
C2H4 + OH => C2H3 + H2OCH2(s) + CH4 => 2CH3
C2H3 + CH3 => C3H6CH2(s) + O2 => CO + OH + H
C3H5-a + HO2 => C3H5O + OHHCO + M => H + CO + M
C3H5O => C2H3CHO + HCO + OH => CO2 + H
C3H5O => C2H3 + CH2OOH + H2 => H + H2O
Table 4. Simplified mechanisms with species and reaction numbers similar to NC2024.
Table 4. Simplified mechanisms with species and reaction numbers similar to NC2024.
NumberAuthorYearSpeciesReactionTemperature
(K)		Pressure
(Bar)		Equivalence
Ratio
1San Diego201664301600–15001–400.5–2.0
2Yachao Chang [60]202056131///
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Duan, J.; Yang, A.; Wei, W.; Qin, G. An n-Heptane Oxidation Mechanism Suitable for Low- to High-Temperature Combustion. Energies 2025, 18, 1305. https://doi.org/10.3390/en18051305

AMA Style

Duan J, Yang A, Wei W, Qin G. An n-Heptane Oxidation Mechanism Suitable for Low- to High-Temperature Combustion. Energies. 2025; 18(5):1305. https://doi.org/10.3390/en18051305

Chicago/Turabian Style

Duan, Junfa, Aoqing Yang, Wei Wei, and Gaolin Qin. 2025. "An n-Heptane Oxidation Mechanism Suitable for Low- to High-Temperature Combustion" Energies 18, no. 5: 1305. https://doi.org/10.3390/en18051305

APA Style

Duan, J., Yang, A., Wei, W., & Qin, G. (2025). An n-Heptane Oxidation Mechanism Suitable for Low- to High-Temperature Combustion. Energies, 18(5), 1305. https://doi.org/10.3390/en18051305

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