Influence of the functional group of fuels on the construction of skeletal chemical mechanisms: A case study of 1-hexane, 1-hexene, and 1-hexanol
Introduction
To cope with the energy crisis and environmental issues, higher thermal efficiency and lower pollution emissions are required for internal combustion engines. Advanced low-temperature combustion (LTC) strategies are effective ways to achieve this goal [1], [2], [3]–4]. To investigate the combustion and emission behaviors of engines with different combustion strategies, an accurate chemical kinetic mechanism is crucial [5].
As the compositions of commercial fuels are extremely complex, it is difficult to develop their detailed oxidation mechanisms [6]. The surrogate fuel containing a limited number of components is usually employed to describe specific chemical or physical behaviors of commercial fuel. The detailed mechanism of fuels can successfully reproduce the oxidation and combustion characteristics. However, the scale of the detailed mechanism of large hydrocarbons is usually extremely huge, e.g., the detailed mechanism of the gasoline surrogate composes 2315 species and 10,079 reactions [7]. It needs insufferable computational time to couple the detailed mechanism into computational fluid dynamic (CFD) simulations [8]. Reduced mechanisms are capable of significantly shortening the computational time and maintaining the specific prediction performance of detailed mechanisms, thus attracting increasing attention recently. Thus, the reduced mechanism of a surrogate fuel is generally developed by coupling the reduced mechanisms of the pure components in the surrogate fuel [9]. It should be noted that petrol-derived fuels contain various types of components, and their reactivities vary greatly [6]. The reactivity of fuel is mainly influenced by the functional group of the components in it. For example, Heufer et al. [10] indicated that the hydroxy group in alcohol fuels led to a significantly shorter ignition delay time at high temperatures and lower reactivity at low temperatures compared with alkanes with the same carbon number. Consistently, Minetti et al. [11] found that the addition of radicals to the double bond of alkenes resulted in its lower reactivity compared with alkanes with the identical carbon number. Therefore, the recognition of the influence of the functional group of different components is very crucial to build a reliable reduced mechanism for the surrogate fuel.
The components of 1-hexane, 1-hexene, and 1-hexanol all have a linear structure with six carbon atoms but own different functional groups. Thus, it is effective to explore the influence of the functional group on the construction of the reduced mechanism for different components. Furthermore, 1-hexane and 1-hexene were adopted as the representative components for gasoline and diesel surrogate fuels [6,12]. Moreover, 1-hexanol has attracted increasing attention because of higher cetane number and energy density [13] and smaller energy for production [14] compared with other alcohols with lower carbon numbers. The better stability of 1-hexanol also makes it easy to mix with petrol-derived fuels [15]. To deeply understand the kinetic behavior of 1-hexane, 1-hexene, and 1-hexanol, several detailed mechanisms have been built. Touchard et al. [16] established a detailed oxidation mechanism for 1-hexene containing 1250 species and 4526 reactions using EXGAS. By adding the large-molecule reactions into a C0–C4 sub-mechanism, a detailed 1-hexene mechanism consisting of 350 species and 8000 reactions was proposed by Mehl et al. [17]. Fan et al. [18] studied the pyrolysis process of 1-hexene in a flow reactor, and a detailed mechanism including 122 species and 919 reactions was developed to reproduce the experimental data. Recently, Meng et al. [19] conducted a detailed mechanism containing 1113 species and 4794 reactions for 1-hexene by updating the Bounaceur et al. model [20] based on the latest experimental and theoretical work. On the contrary, for 1-hexane and 1-hexanol, much fewer mechanisms exist. By adding the 1-hexanol sub-mechanism into the 1-pentanol mechanism, Togbé et al. [15] proposed a detailed 1-hexanol oxidation mechanism, which consists of 600 species and 2977 reactions. For 1-hexane, only Zhang et al. [21] established a detailed oxidation mechanism composing 1118 species and 4808 reactions by adding several new reactions about O2QOOH racial consumption into the previous n-heptane and isooctane oxidation models [22,23]. The scale of the detailed mechanisms above is large, which prevents their applications in computational fluid dynamics simulations. At present, very few reduced mechanisms of 1-hexane, 1-hexene, and 1-hexanol exist. To our best knowledge, only Luo et al. [12] built a 1-hexene reduced mechanism included in a multi-component chemical mechanism of diesel surrogate fuel. By comparing the predicted and experimental value of ignition delay times of 1-hexene diluted by argon, the reliability of the reduced mechanism of 1-hexene was verified.
To obtain a reliable skeletal/reduced mechanism, the reactions and species dominating the reduction targets should be diagnosed correctly. Many methods have been proposed to cope with this task effectively. These methods can be classified as direct reduction method, time scale method, and calculation error based method. In the direct reduction method, a reduced mechanism is obtained by removing unimportant species and reactions from the detailed mechanism. The representative methods include the directed relationship graph (DRG) series method [24], [25]–26], sensitivity analysis [27], principal component analysis [28], path flux analysis [29], and so on. The time scale method is derived from the partial equilibrium or quasi-steady hypothesis, in which the time evolutions of the fast species and reactions are described by algebraic equations [30]. The typical methods contain computational singular perturbation [31] and intrinsic low-dimensional manifolds method [32]. The calculation error based method includes genetic algorithms [33], simulation error minimization [34], linearized error propagation [35], automated simulation error based reduction method [36], error-controlled kinetics reduction method [37], etc. Although many methods existing, it was found that the size of the final reduced mechanism is still large, especially for the conditions with the low-temperature chemistry being included [38]. In order to obtain a reduced mechanism with a very small size, a complicated combination of the above methods through multiple iterations is usually required [39]. The decoupling method is an effective method to build the skeletal mechanism of various fuels by integrating the skeletal fuel-specific sub-mechanism and the detailed small-molecule sub-mechanism [40]. The comparison between the predicted and measured data indicates that the skeletal mechanism built by decoupling methodology can well reproduce the ignition, oxidation, and flame characteristics of fuels over wide conditions [41,42]. In the previous applications of the decoupling methodology, the skeletal large-molecule sub-mechanism is usually constructed using the local sensitivity and path sensitivity analyses on every single reaction, which cannot capture the non-linear behavior between the reaction rate coefficients and the mechanism predictions, as well as the coupling relationship of different reactions.
Xin et al. [43] indicated that a reliable reduced mechanism should retain the dominant reactions and the reactions strongly coupled with these important reactions. At present, it is still a challenge to capture the coupled relationship between different reactions. Recently, Chang et al. [44] proposed a reaction class-based global sensitivity analysis (RC-GSA) method to identify the crucial reactions and the coupled relationship of different reactions for the establishment of skeletal chemical mechanisms.
In this work, skeletal mechanisms of 1-hexane, 1-hexene, and 1-hexanol were constructed by integrating the decoupling methodology and the RC-GSA method. In the mechanism construction process, the functional group-relevant reactions were emphasized for the three fuels. The paper consists of these sections. First, the construction processes of the skeletal mechanism are introduced in detail in Section 2. Then, the structure of the skeletal mechanism of the three fuels is analyzed to illustrate the role of the distinctive functional group in fuels on the development of skeletal mechanisms in Section 3. Third, the skeletal mechanism is evaluated by comparing the predicted results and the measured data of laminar flame speed, ignition delay times, and species concentrations in Section 4. The major conclusion is summarized in the last section.
Section snippets
Construction of skeletal mechanism
In this study, the skeletal chemical mechanisms of 1-hexane, 1-hexene, and 1-hexanol were formed using the decoupling methodology [40], the reaction class-based global sensitivity (RC-GSA) method, the path sensitivity analysis method [44], the rate of production (ROP) analysis, and the genetic algorithm [45]. Figure 1 displays the procedure of the skeletal mechanism construction. First, the global sensitivity and path sensitivity analyses are adopted to evaluate the importance of the reaction
Comparison of reaction paths for 1-hexane, 1-hexene, and 1-hexanol
The overall reaction paths of 1-hexane, 1-hexene, and 1-hexanol are compared in Fig. 12. For 1-hexane, the fuel molecule (C6H14) is first reacted by the H-atom abstraction reactions to produce fuel radical (C6H13). Then, C6H13 is oxidized following the classic low-temperature reaction path of n-alkane: R (C6H13)→RO2 (C6H13O2)→QOOH (C6H12OOH)→O2QOOH (O2C6H12OOH)→KET (C6KET)→small species. Moreover, the reaction C6H13O2C6H12+HO2 (RO2=E+HO2) is also involved in the present skeletal mechanism as it
Validation of skeletal chemical mechanism
To verify the accuracy of the skeletal chemical mechanism developed above, the measurements of laminar flame speed, ignition delay time, and important species concentrations are introduced to evaluate the mechanism under extended conditions in this section.
Conclusions
In the present work, a skeletal chemical mechanism for 1-hexane, 1-hexene, and 1-hexanol is built by a combined method of the decoupling methodology and the global sensitivity analysis. The reaction classes dominating the oxidation characteristics of fuels are first identified by the reaction class-based global sensitivity and path sensitivity analyses. A representative reaction is chosen from each dominant reaction class by the ROP analysis, and the skeletal fuel-specific sub-mechanism is
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The present study is supported by the National Nature Science Foundation of China (Grant Nos. 51706033 and 51961135105).
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