Article pubs.acs.org/EF
Experiments and Computational Fluid Dynamics Modeling Analysis of Large n‑Alkane Ignition Kinetics in the Ignition Quality Tester Gregory E. Bogin, Jr.,*,† Eric Osecky,# J. Y. Chen,‡ Matthew A. Ratcliff,§ Jon Luecke,§ Bradley T. Zigler,§ and Anthony M. Dean# †
Department of Mechanical Engineering, and #Department of Chemical and Biological Engineering, Colorado School of Mines, Golden, Colorado 80401, United States ‡ Department of Mechanical Engineering, University of California, Berkeley, Berkeley, California 94720, United States § National Renewable Energy Laboratory, Golden, Colorado 80401, United States S Supporting Information *
ABSTRACT: This paper presents experimental measurements of ignition delays from low- to high-volatility n-alkanes representative of diesel and jet fuel compounds that are supplemented with a computational fluid dynamics (CFD) analysis. The ignition quality tester (IQT) is shown to be effective for studying ignition of low-volatility fuels, such as n-hexadecane, which are typically difficult to measure. Ignition delays, both experimental and modeled, are presented using an eight-point experimental design matrix (1.5 and 3.0 MPa, 823 and 723 K, and 15 and 21% O2). A detailed n-alkane mechanism (C8−C16 with a total of 2115 species) was reduced to a skeletal 237 species n-hexadecane mechanism using a targeted search algorithm. A CFD model of the IQT (developed using KIVA-3V) coupled with skeletal mechanisms predicted ignition delays of n-heptane and n-hexadecane with reasonable accuracy over the eight-point matrix, with the exception of the highest temperature, lowest pressure, and oxygen concentration conditions. Temperature sweeps across a range of pressures (0.1−1.0 MPa) and temperatures (673−973 K) were performed for n-heptane, n-decane, n-dodecane, and n-hexadecane. The negative temperature coefficient (NTC) region was observed experimentally for the first time for n-hexadecane. The NTC region for n-dodecane and n-decane has previously been observed in shock tubes and rapid compression machines and is reported here for the first time in the IQT. The IQT is thus capable of capturing NTC behavior for large alkanes and can serve as an additional experimental validation tool for chemical kinetic mechanisms of low-volatility surrogates for diesel and jet fuels.
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INTRODUCTION As a result of an increase of fossil fuel use and related emissions, there is growing motivation to develop more efficient internal combustion engines (ICEs) as well as suitable alternative fuels. Such efforts will be facilitated by accurate modeling of the combustion kinetics of the candidate fuels within the ICE. Modeling combustion in ICEs commonly uses computational fluid dynamic (CFD) models, coupled with chemical kinetic mechanisms. Along with accurately capturing the spray physics and fluid dynamics, it is equally important for these combustion models to include validated chemical mechanisms. Such combinations allow for more efficient experimentation by identifying conditions most likely to lead to improved engine efficiency and reduced emissions. Hydrocarbon fuels used in transportation are complex mixtures of several hundred to several thousand hydrocarbon species. This complexity makes mechanism development and modeling of these fuels difficult, and thus, surrogate fuels, which are much simpler but have chemical and physical properties similar to the real fuels, are preferred for experiments and numerical modeling. Diesel and jet fuel compositions can be classified into four chemical classes: n-alkanes, isoalkanes, cycloalkanes, and aromatics.1 This paper focuses on n-alkanes because they are important for low-temperature combustion modes that rely heavily on ignition kinetics, because they have shorter ignition delays than similar molecular weight isoalkanes, © 2014 American Chemical Society
cycloalkanes, and aromatics. Both n-heptane and n-hexadecane have significant low-temperature oxidation chemistry involving peroxy radicals and hydroperoxyalkyl radicals, which play a crucial role in their early ignition kinetics and produce the characteristic negative temperature coefficient (NTC) region and low-temperature heat release.2−6 Accurate characterization of the ignition kinetics of fuels is even more important for lowtemperature combustion engines, such as homogeneous charge compression ignition, premixed charge compression ignition, and other variations of the low-temperature combustion regimes. Large n-alkanes are significant components in jet (≤C16) and diesel fuels (≤C22); therefore, an improved understanding of their ignition kinetics is necessary to accurately model the combustion behavior of more complex surrogate and real fuels. Comprehensive mechanisms have been developed for many of the smaller hydrocarbon fuels (C1−C8)7−22 and have been wellvalidated as a result of available experimental data for these fuels. Unfortunately, for the large n-alkanes of relevance to diesel and jet fuels, there are fewer such experimental studies that provide good validation benchmarks.23−33 A large n-alkane mechanism (C8−C16) has been developed by Westbrook et Received: April 7, 2014 Revised: June 13, 2014 Published: June 16, 2014 4781
dx.doi.org/10.1021/ef500769j | Energy Fuels 2014, 28, 4781−4794
Energy & Fuels
Article
Figure 1. Schematic of the IQT combustion chamber.
al.34 and validated primarily with data obtained from n-decane experiments in shock tubes, rapid compression machines, and jet-stirred reactors. Less data were obtained from the larger nalkanes primarily because of the difficulty with studying these low-volatility fuels in traditional experimental systems. Thus, the main objective of this paper is to present new experimental results for n-hexadecane, n-decane, and n-dodecane that could be used for mechanism validation of large n-alkanes.
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front and rear of the chamber near the injector nozzle and pressure transducer, respectively. This drop in temperature is due to heat transfer from the coolant flow, which is used to maintain a constant temperature at the injector nozzle (323 ± 4 K) and pressure transducer (403 ± 20 K), as specified by the ASTM D6890 standard. For the experiments in this paper, the charge-air temperature is allowed to reach steady-state conditions by allowing ∼10 s to elapse between charging the chamber and injecting the fuel. The charge-air temperature is within ±1K of the set point. A pressure transducer (Kistler 601B1, with an insert and coolant jacket installed by the manufacturer to protect the pressure transducer against thermal shock and overheating) installed in the rear of the combustion chamber measures the pressure rise during the combustion event; the charge air pressure is ±7 kPa of the set point. The ID is defined as the time interval between the start of injection and the rise in combustion pressure to the “pressure recovery point” (normally 0.138 MPa above the initial chamber pressure prior to injection).35,36 For this study, the default 0.138 MPa was used for the short ID studies in the highpressure regime (≥1.5 MPa); for the longer ID studies in the lowpressure regime, a 0.276 MPa pressure rise was used to compensate for low-temperature heat-release-induced pressure increases to more accurately determine the start of ignition. Studies were also performed with n-heptane and n-hexadecane using the maximum rate of change in pressure with respect to time (dP/dt = maximum) as the definition for ignition to compare to the fixed pressure recovery point methods. The ID (averaged over 32 injections) is converted to a derived cetane number (DCN) by an algorithm described in ASTM method D6890.37 The IQT apparatus and setup were described in more detail previously.38,39 For the low-pressure (40 ms for low-volatility fuels, such as n-hexadecane), such that the fuel−air mixture in the IQT becomes pseudo-homogenous. NTC was also observed for n-dodecane and n-decane, which was also previously measured in shock tubes and RCMs. Twostage ignition is observed for all three large n-alkanes, as expected. This work further demonstrates that the IQT can complement shock tubes and RCMs to validate chemical kinetic mechanisms, provided that the test conditions are adjusted to yield longer IDs. The modification of the IQT to have extended capabilities that allow for the ability to rapidly measure IDs for liquid fuels across a wide range of pressures and temperatures is unique among these devices and offers the possibility of faster validation feedback. This is especially true for low-volatility fuels, which can be difficult to study using shock tubes and RCMs. Some key next steps are to develop skeletal mechanisms for n-decane and n-dodecane.
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ASSOCIATED CONTENT
S Supporting Information *
The 237 n-hexadecane skeletal mechanism. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the U.S. Department of Energy Vehicle Technologies Office and Fuel Technologies Program Manager Kevin Stork for their support of this fuel research. This work was supported by the U.S. Department of Energy under Contract DE-AC36-08-GO28308 with the NREL. A portion of the research was performed using computational resources sponsored by the Department of Energy’s Office of Energy Efficiency and Renewable Energy and located at the NREL.
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