Elsevier

Combustion and Flame

Volume 161, Issue 4, April 2014, Pages 866-884
Combustion and Flame

A chemical mechanism for low to high temperature oxidation of n-dodecane as a component of transportation fuel surrogates

https://doi.org/10.1016/j.combustflame.2013.10.012Get rights and content

Abstract

Using surrogate fuels in lieu of real fuels is an appealing concept for combustion studies. A major limitation however, is the capability to design compact and reliable kinetic models that capture all the specificities of the simpler, but still multi-component surrogates. This task is further complicated by the fairly large nature of the hydrocarbons commonly considered as potential surrogate components, since they typically result in large detailed reaction schemes. Towards addressing this challenge, the present work proposes a single, compact, and reliable chemical mechanism, that can accurately describe the oxidation of a wide range of fuels, which are important components of surrogate fuels. A well-characterized mechanism appropriate for the oxidation of smaller hydrocarbon species [G. Blanquart, P. Pepiot-Desjardins, H. Pitsch, Chemical mechanism for high temperature combustion of engine relevant fuels with emphasis on soot precursors, Combust. Flame 156 (2009) 588–607], and several substituted aromatic species [K. Narayanaswamy, G. Blanquart, H. Pitsch, A consistent chemical mechanism for the oxidation of substituted aromatic species, Combust. Flame 157 (10) (2010) 1879–1898], ideally suited as a base to model surrogates, has now been extended to describe the oxidation of n-dodecane, a representative of the paraffin class, which is often used in diesel and jet fuel surrogates. To ensure compactness of the kinetic scheme, a short mechanism for the low to high temperature oxidation of n-dodecane is extracted from the detailed scheme of Sarathy et al. [S. M. Sarathy, C. K.Westbrook, M. Mehl, W. J. Pitz, C. Togbe, P. Dagaut, H. Wang, M. A. Oehlschlaeger, U. Niemann, K. Seshadri, Comprehensive chemical kinetic modeling of the oxidation of 2-methylalkanes from C7 to C20, Combust. Flame 158 (12) (2011) 2338–2357] and integrated in a systematic way into the base model. Rate changes based on recent rate recommendations from literature are introduced to the resulting chemical mechanism in a consistent manner, which improve the model predictions. Extensive validation of the revised kinetic model is performed using a wide range of experimental conditions and data sets.

Introduction

Computational combustion studies in engines typically use surrogates to model real fuels. However, it is challenging to develop kinetic models that describe the oxidation of all individual components in multi-component surrogates accurately. Further, the nature of the hydrocarbons commonly considered as surrogate components often leads to extremely large reaction schemes for surrogate mixtures, owing to the large detailed reaction schemes for the individual component description. As a result, designing compact kinetic models is yet another formidable task. It is our objective to meet these challenges, by developing a single, consistent, reliable, and compact chemical mechanism, that can describe the oxidation of essential components of transportation fuel surrogates, and the present work makes a contribution towards achieving this goal.

In a recent work, a single chemical mechanism describing the oxidation of a wide range of hydrocarbon species, from C1 to C8 species was proposed and validated extensively against experimental data for the oxidation of several compounds [1], with emphasis on detailed soot modeling and surrogate fuel formulations. In addition to smaller hydrocarbons, which are well considered in that model, jet fuels consist of up to 16–18% of aromatic compounds [2], [3], and these play a crucial role in soot formation. Accordingly, the mechanism was extended in a consistent manner to describe the moderate to high temperature oxidation of several aromatics, viz. toluene, ethylbenzene, styrene, m-xylene, and α-methylnaphthalene [4]. The resulting scheme was validated thoroughly against available experimental data for the substituted aromatics under consideration. The purpose of this work is to extend this mechanism, ideally suited as a base to model surrogate blends, to include the low to high temperature oxidation pathways of a representative of the paraffin class, important in engine fuels.

Longer chain alkanes, such as n-decane, n-dodecane, and n-tetradecane, are potential candidates to represent the paraffin class in transportation fuel surrogates. Out of these normal alkanes, n-dodecane could be interesting as a surrogate component [5], [6], and has been identified as a good compromise between a longer straight chain alkane, typical for transportation fuels, and a reasonable size of the molecule [7]. Following this, n-dodecane is chosen as the paraffin representative for this work.

Few detailed mechanisms for n-dodecane exist in the literature. A detailed reaction scheme to describe the pyrolysis of n-dodecane was developed by Dahm et al. [8], and later improved and extended by Herbinet et al. [9]. Their improved model was used to predict the results of pyrolysis experiments and thermal decomposition in a jet-stirred reactor at low to moderate temperatures (773–1073 K). Biet et al. [10] suggested improvements to the rate rules used in the EXGAS software for better modeling of low temperature oxidation of large linear alkanes (>C10). They proposed a semi-detailed kinetic model applicable at low through intermediate temperatures (550–1100 K), and modeled pressurized flow reactor data for n-dodecane using their kinetic model. Ranzi et al. [11] proposed a lumped mechanism for n-alkanes including n-dodecane for low to high temperature oxidation and validated their model against pressurized flow reactor data and in counter flow flame configurations. Recently, a detailed kinetic scheme to describe the low to high temperature kinetics of n-undecane and n-dodecane was developed by Mzé-Ahmed et al. [12]. The proposed scheme was validated against their jet stirred reactor data for n-dodecane, and their model predictions have been compared against ignition delays and species profile measurements for n-dodecane.

Of particular interest to this work are the chemical models for alkane oxidation developed by the kinetics group at Lawrence Livermore National Laboratories (LLNL). Westbrook et al. [13] proposed a detailed kinetic scheme to describe the pyrolysis and oxidation of several n-alkanes up to n-hexadecane. This reaction mechanism includes high and low temperature oxidation pathways. Sarathy et al. [14] improved this detailed model for normal alkanes, further extending it to methyl alkanes from C8 to C20. Specifically, the C0–C5 sub-mechanism was updated based on Healy et al. [15], and new reaction pathways such as the concerted elimination pathway to form alkene and HO2 from the alkyl peroxy radical were introduced to the normal alkane chemistry. The reaction rates for H-abstraction from alkenes were updated, and the activation energies for alkenyl decompositions were modified. These reaction schemes are built in a modular approach, based on well-established reaction classes, and associated rate rules originally developed for n-heptane by Curran et al. [16], [17], and further updated as per the Mehl et al. mechanism [18] for gasoline surrogate.

Considering moderate and high temperature oxidation, You et al. [19] proposed a kinetic model for normal alkanes up to n-dodecane applicable above 850 K. The reaction mechanism includes the high temperature pyrolysis and oxidation of normal alkanes (C5–C12), and a global 4-species, 12-step reaction set appended to this captures some of the intermediate temperature chemistry. They validated their kinetic model against fuel pyrolysis in plug flow and jet-stirred reactors, laminar flame speeds, and ignition delays behind reflected shock waves, with n-dodecane being the emphasis. This reaction scheme forms the basis for the n-dodecane sub-mechanism in JetSurF [20], which is an ongoing effort towards a jet fuel surrogate mechanism. The JetSurF mechanism includes some revisions to the previous You et al. model, and has been tested more widely against experimental data at moderate to high temperatures.

In summary, among the several reaction mechanisms that have been proposed for n-dodecane oxidation, these models [10], [11], [12], [13], [14] have the capability to describe low through high temperature chemistry, which is of interest to this work. It should also be noted that the relevance of n-dodecane as a component of jet fuel surrogates has attracted a number of experimental studies in the last couple of years, thus widening the experimental database on n-dodecane oxidation [12], [21], [22], [23], [24], [25], [26], [27], [28], [29]. A vast majority of these experimental data were obtained after the development of the above mentioned kinetic models, and in some cases, those reaction mechanisms were not validated against all existing data, for example, species profile measurements. There is therefore a rich experimental database that is yet to be fully utilized for model evaluation and improvement.

The prime objective of the present work is to (i) leverage this recent experimental knowledge to develop and extensively validate a model for low through high temperature oxidation of n-dodecane, (ii) ensure that the proposed reaction scheme retains a compact size, which is amenable to comprehensive kinetic analysis, and (iii) progress towards a single chemical mechanism that can accurately describe the oxidation of a wide range of fuels, which are important surrogate components. The present model is built as an additional module on an existing well-validated model, whose base chemistry has been treated consistently [1], [4], and thereby ensures kinetic compatibility between the various individual components included in the multi-component scheme by construction. Also, great care is taken to ensure that the oxidation of n-heptane, iso-octane, and aromatics, well described in this base model, is little impacted by the introduction of the n-dodecane model.

In extending the base model of Ref. [4] to include the low through high temperature oxidation pathways of n-dodecane, it is desired to introduce only the necessary kinetics to ensure the compactness of the model. Therefore, in the present approach, mechanism reduction techniques developed previously by Pepiot and Pitsch [30], [31] are employed to first obtain a reduced reaction scheme applicable to low through high temperature oxidation of n-dodecane from a reference mechanism, which is then incorporated into the base model. In this reduction technique, each reduction step, i.e. elimination of species, elimination of additional reactions, and lumping, is performed in one sweep with a single evaluation of source terms at the considered conditions. This technique therefore avoids reduction by cancelation of errors by only neglecting species and reactions that have truly a small influence on the reaction fluxes. Since the chemical mechanisms being combined are small in size, the risk of introducing truncated paths or involuntarily duplicating reaction pathways in the combined mechanism is best circumvented by this approach.

Considering the choice of the reference mechanism, while much effort has gone into developing the JetSurF [20] model that describes the oxidation of n-dodecane, since low temperature chemistry is also of interest here, for consistency, it is found best to start with a reaction mechanism that already includes these pathways. The recent detailed reaction scheme proposed by Sarathy et al. [14] is chosen as the reference mechanism for the present work. This kinetic scheme describes the low through high temperature chemistry of normal alkanes (including n-dodecane) and is constructed from elementary reactions, which is more consistent with our base model and the aforementioned mechanism reduction approach than the Biet et al. [10] and Ranzi et al. [11] models, which are semi-lumped in nature. The reaction mechanism of Sarathy et al. [14] also takes into account the recent knowledge on newer reaction pathways, for instance, the concerted elimination pathway, as well as a better description of auto-ignition, compared to the Mzé-Ahmed et al. [12] model.

Note that the reduction techniques referred above involve chemical lumping of isomer species, which contributes significantly to the reduction in model size. Therefore, the incremental n-dodecane module added to the base model involves reactions among some lumped species as well. This imposes certain restrictions on the modifications of reaction rate data that could be introduced in this combined reaction mechanism. For instance, it is not possible as such to introduce rate changes for reactions involving single isomer species that exist as lumped species in the combined model. However, owing to the large size of the reference mechanism, the alternative approach of combining the reference n-dodecane mechanism with the base model, and then introducing rate changes and validating the combined model, followed by model reduction is nearly impossible. The huge size of the combined mechanism prior to model reduction discourages reaction flux analysis and sensitivity studies, which are often used to identify deficiencies in the reaction mechanism, such as missing pathways and incorrect reaction rates, thereby pointing to changes required to improve the description of the underlying kinetics.

The approach adopted in this work wins on this front, and has the advantages of simplicity, by minimizing the kinetic incompatibilities required to be dealt with when merging kinetic modules from different sources, and assures a quick turn around time to arrive at a reliable chemical model. In the event that an elementary reaction rate needs to be updated for a single isomer species that exists as a lumped species in the combined model, the rate change could be introduced in the reference mechanism, the automatic reduction procedure could be repeated easily, and the revised reduced mechanism combined with the base model to give the final updated model in a short time.

The methodology involved in arriving at a reduced skeletal level model from the reference n-dodecane mechanism is described in Section 2.1. This is followed by a discussion on incorporating the reaction pathways in the condensed scheme into the base model in Section 2.2. A few reaction rate changes are introduced to the combined model based on recent theoretical and experimental studies, and these help achieve improved model predictions. These changes, described in Section 2.3, have been consistently incorporated by treating all C4–C12 alkane derivatives in a similar manner. A demonstration of the performance of the revised reaction model for different targets follows in Section 3. The article is then concluded by summarizing the capabilities of this model in describing low through high temperature oxidation of n-dodecane.

Section snippets

Skeletal mechanism for n-dodecane

The detailed mechanism for n-dodecane oxidation from Sarathy et al. [14], chosen here as the reference model, and referred to below as the LLNL mechanism, has tens of thousands of reactions among ∼1480 species, not counting the sub-mechanisms for the larger alkanes (C13–C16) and the 2-methyl alkanes (C8–C20), which are also described in the reference model. First, this extensive mechanism is reduced to a skeletal level using a multi-stage reduction strategy put forth by Pepiot and Pitsch,

Validation tests

Having discussed the changes introduced to the reaction model, this section evaluates the ability of this revised mechanism to predict targets for different idealized configurations of interest, by comparing the simulated results against several experimental data sets. The validation cases focus on oxidation environments, and include (i) ignition delays spanning wide ranges of temperatures and pressures, (ii) species time histories measured in shock tubes, (iii) concentration profiles of fuel,

Conclusions

With the long term objective of a kinetic model for jet fuel surrogates, a reaction mechanism has been developed to describe the oxidation of a representative paraffin molecule, n-dodecane. This has been accomplished by starting with a chemical mechanism proposed earlier for smaller hydrocarbons along with a few substituted aromatics [1], [4] as base model, and extending this model to include the reaction pathways of n-dodecane oxidation. Starting with the LLNL detailed mechanism for methyl

Acknowledgments

The first and the third author gratefully acknowledge funding by the Air Force Office of Scientific Research and NASA. The second author acknowledges support from the Department of Energy under Grant DE-FG02-90ER14128. The third author also acknowledges support from the German Research Foundation (DFG) within the Collaborative Research Centre SFB 686 – Model-Based Control of Homogenized Low-Temperature Combustion at RWTH Aachen University, Germany, and Bielefeld University, Germany. The first

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