Experimental and Kinetic Modeling Study on Self-ignition of Î±

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Experimental and Kinetic Modeling Study on Self-ignition of #Methylnaphthalene in a Heated Rapid Compression Machine Shuzhou Sun, Liang Yu, Sixu Wang, Yebing Mao, and Xingcai Lu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00987 • Publication Date (Web): 30 Aug 2017 Downloaded from http://pubs.acs.org on August 31, 2017

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Experimental and Kinetic Modeling Study on Self-ignition of α-Methylnaphthalene in a

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Heated Rapid Compression Machine

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Shuzhou Sun, Liang Yu, Sixu Wang, Yebing Mao, Xingcai Lu*

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Key Lab. for Power Machinery and Engineering of M. O. E, Shanghai Jiao Tong University, 200240, Shanghai, P. R. China

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Abstract: As an important component of diesel and kerosene surrogate, the experimental study and chemical kinetic

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modeling of α-methylnaphalene (AMN) are still very insufficient. The ignition delay of AMN/O2/Ar mixture in a heated

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rapid compression machine (RCM) was measured in this study. The data was obtained for equivalence ratios of 0.7, 1, 1.2,

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at pressures of 12bar, 15bar, 20bar, over the temperature range of 860K~1040K. A semi-detailed kinetic mechanism for the

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oxidation of AMN was established, which consists of 196 species and 1330 reactions. Compared with the different previous

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mechanism, the new mechanism can more accurately predict the ignition delay of AMN in RCM and shock tube

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experiments. It can also accurately predict the experimental data obtained in a jet stirred reactor (JSR) from the literature.

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The ignition delay using adiabatic constant volume simulation (CV simulation) and RCM simulation were compared, which

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indicated that RCM simulation could better predict the experimental data. Sensitivity and reaction path analysis were also

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carried out to explore the effect of key reactions and paths on AMN ignition.

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Keywords: α-methylnaphalene, RCM, ignition delay, kinetic modeling

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1. Introduction

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The more and more severe energy crisis and environmental problems make a higher request for the design of internal

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combustion engines. In order to achieve a precise control of the combustion and emissions in engines, it’s necessary to

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study the combustion chemical mechanism of the actual fuels. However, the composition of the actual fuels is extremely

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complex. There are thousands of components in gasoline and diesel. The mechanisms of many components are not clear.

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Corresponding author: E-mail address: [email protected]. Tel.: +86-21-34206039; Fax: +86-21-34206139. 1

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The interaction between the various components is more difficult to study. At present, the common method is to construct

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the surrogate fuel which can characterize the physical and chemical properties of the actual fuel. The research on the

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mechanism of gasoline surrogate represented by toluene reference fuel (TRF) has been extensively studied in the past few

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years [1-4]. Other mixtures related to gasoline like PRF with ethanol [5] and n-heptane/toluene [6] are also studied in

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mechanism and experiment. The main components of diesel include alkanes, olefins, cycloalkanes, aromatic hydrocarbons,

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and a small amount of other substances. Compared with gasoline, the diesel consists of larger molecules with more complex

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structures and lower vapor pressure at room temperature. Higher temperature is needed to obtain the homogeneous mixture.

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Therefore, the experimental and kinetic modeling of diesel surrogate are very lacking.

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With the development of chemical kinetics, the mechanisms of ordinary alkanes (n-heptane [7], iso-octane [8] et al. ),

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cycloalkanes (cyclohexane [9], methyl-cyclohexane [10] et al. ), aromatic hydrocarbons (toluene [11,12] et al. ) have been

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extensively proposed and validated in different reactors, which makes it possible to construct a more complex diesel

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surrogate mechanism. As one of the fuels that characterize the cetane number of diesel, α-methylnaphalene is considered to

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be one of the representatives of polycyclic aromatic hydrocarbons in diesel. Suzuki et al. [13] investigated the ignition and

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combustion characteristics of the diesel surrogate in a micro flow reactor, which includes n-hexadecane, n-decane,

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n-heptane, iso-hexadecane, and α-methylnaphthalene. Ramirez et al. [14] used n-decane and α-methylnaphthalene as the

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diesel surrogate and mixed it with biodiesel as the surrogate fuel of B30, constructing a chemical mechanism involving

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1964 species and 7748 reactions. Lemaire et al. [15] compared the soot formation of commercial diesel and diesel surrogate

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components n-decane and α-methylnaphthalene based on the turbulent diffusion flame, and found that the proportion of

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α-methylnaphthalene has an important effect on soot formation. Barths et al. [16] used 70% n-decane and 30%

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α-methylnaphthalene as a diesel surrogate.

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In addition, α-methylnaphthalene is also considered to be one of the components of the kerosene surrogate. Schulz et al.

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[17] proposed a twelve-component surrogate fuel for kerosene, in which α-methylnaphthalene was a major component.

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Agosta et al. [18] used n-dodecane and iso-octane to represent the normal and isomeric components of alkanes,

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methyl-cyclohexane and decalin for cycloalkanes, and α-methylnaphthalene for aromatics to build a five-component

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kerosene surrogate fuel. α-methylnaphthalene is also an important component in the 5-7 component surrogate fuel proposed

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by Violi et al. [19]. Slavinskaya et al. [20] used 10% n-propylcyclohexane, 13% iso-octane, 20% n-dodecane, 23%

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α-methylnaphthalene, 32% n-hexadecane as the surrogate fuel for kerosene. Mensch et al. [21] also considered that

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α-methylnaphthalene is an important component of the surrogate fuel for JP8 and Jet-A (aviation fuels).

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The experimental and kinetic modeling study for α-methylnaphthalene is still relatively lacking. Pfahl et al. [22]

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measured the ignition delay of α-methylnaphthalene in a shock tube and observed two kind of combustion characteristics

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which were described as mild ignition and strong ignition. The detailed mechanism of α-methylnaphthalene constructed by

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Pitsch [23] was also verified. Mati et al. [24] conducted a JSR experiment for the oxidation of α-methylnaphthalene at 1-13

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atm. The concentration profiles of reactants, stabilized intermediates and final products were measured using sonic probe

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sampling and GC-MS analysis. A detailed kinetic mechanism for the oxidation of α-methylnaphthalene with 146 species

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and 1041 reactions was constructed. And then a detailed mechanism for diesel surrogate consist of five components

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including α-methylnaphthalene was proposed [25]. Wang et al. [26] have also measured the ignition delay of

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α-methylnaphthalene in the shock tube. Based on the mechanism of Mati [24] and Bounaceur [27], a detailed mechanism of

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α-methylnaphthalene was constructed and verified with various experimental data, showing a good agreement. More

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recently, Kukkadapu et al. [28] measured the ignition delay of AMN/air mixtures in a RCM at pressure of 15-40 bar,

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temperature of 837-980 and equivalence ratio of 0.5-1.5. This is the only α-methylnaphthalene experiments in the RCM as

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is known by the authors. The mechanisms of Wang et al. [26] and Narayanaswamy et al. [29] were used to simulate the

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experiments. The results show that the mechanism of Wang et al. has an obvious defect in predicting the ignition delay of

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AMN in the low temperature.

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Somers et al. [30] measured the pyrolysis products of α-methylnaphthalene in 585 oC, 110 atm, 140s with

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high-pressure liquid chromatography (HPLC) with ultraviolet-visible (UV) diode-array detection in series with a mass

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spectrometer. A total of 37 PAHs with 2-7 rings were observed, many of which were not previously observed. The

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consumption path studies of α-methylnaphthalene by Shaddix et al. [31] show that, in the flow reactor, abstraction of a

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''benzylic'' hydrogen from the methyl side chain dominates the consumption of α-methylnaphthalene, although O atom

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addition to the aromatic ring, displacement of the side chain by H atom, and homolytic decay also contribute significantly.

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Changes in the α-methylnaphthalene decay mechanism under higher temperature combustion environments and the

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oxidation mechanisms of even larger polycyclic aromatic hydrocarbons are postulated. The study of Yang et al. [32]

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indicated that in the combustion process, resonance stabilize radicals can result in more soot and PAC (polycyclic aromatic

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compounds) formation in combustion processes. As the largest single polycyclic aromatic constituent in petroleum fuels, 1-

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and 2-methylnaphthalenes can readily form resonance-stabilized radicals in combustion, and yet the reaction mechanisms

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are not well understood. The authors investigated the pyrolysis products of 1- and 2-methylnaphthalene between

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650-800 °C and the oxidation products of 2-methylnaphthalene between 650-950 °C in a flow reactor. Naphthalene is the

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most abundant product and is formed from methyl loss by H displacement. Isomerization in pyrolytic conditions results in

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the second most abundant product. The methyl radicals recombine with the reactants to form ethylnaphthalenes and

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dimethylnaphthalenes. Experimental results show that the lack of carbon dimerization products in significant quantities may

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be due to their further growth into soot.

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In general, the experimental studies of α-methylnaphthalene are still relatively lacking, especially for the study of

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ignition characteristics in low and moderate temperature. In this study, the ignition delay measurments of

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α-methylnaphthalene in a heated rapid compression machine was carried out. The data was obtained for equivalence ratios

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of 0.7, 1, 1.2, at pressures of 12bar, 15bar, 20bar, over the temperature range of 860K~1040K. A semi-detailed kinetic

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mechanism containing 196 species and 1330 reactions was established and compared with the mechanism of Wang et al. [26]

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and Narayanaswamy et al. [29]. The mechanism of this study shows a better agreement with the experimental results.

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2. Experimental

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2.1 Heated rapid compression machine

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The experiment in this study is carried out in the heated rapid compression machine of Shanghai Jiao Tong University,

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which is modified based on the previous one [33,34]. Some major parameters like the diameter of the combustion chamber

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and the compression ratio have been changed. So the experiment system is been introduced here. The system consists of

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driving system, braking system, fuel pre-mixing system, heating and temperature control system, combustion test system

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and data acquisition system, as shown in Fig.1. The pneumatically driven impact piston diameter is 125mm and the

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combustion chamber diameter is 35mm. Creviced piston is adopted due to its ability to restrain

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vortex forming at the boundary layer and to ensure the consistency of the temperature during compression and ignition

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process. The creviced piston design has referred to that of Case Western Reserve University [35] and optimized by

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FLUENT software subsequently. The piston stroke in the combustion chamber is fixed at 215mm. The compression process

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of RCM typically takes 30-40ms and 50% pressure rise during the compression occurred in the last 5ms. The length of the

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connecting rod between the two pistons can be adjusted to change the initial and the end volume so as to alter the

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compression ratio. The compression ratio can change from 4 to 20. Pressure measurements are taken using a pressure sensor

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(Kistler 6125B) on the end wall and processed by a charge amplifier (Kistler 5015) and saved to a computer using a data

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acquisition card. All data acquisition and system operation are controlled by the Labview program.

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A significant feature of the second-generation RCM is the adding of a high-temperature heating system. The mixing

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tanks for fuel and inert gas, the intake pipe and the combustion chamber can be heated precisely and controllable. The

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maximum operating temperature can reach more than 200 °C with a temperature accuracy of ±1 °C. So the large-molecular

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fuels with high boiling point and low saturation pressure can form homogenous gas mixture. The heating and temperature

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control system is self-designed, including PT100 thermistor, thermal silicone oil, flexible heating tape and PID temperature

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controller. In order to avoid the fuel mixture reacting in the preparation and mixing process, a lower temperature is adopted

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in the prerequisite to ensure that the fuel pressure is much lower than the saturated vapor pressure under that temperature.

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The compressed temperature Tc is calculated using the adiabatic core assumption:

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∫

Tc

T0

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P  dT = ln  c  γ −1 T  P0 

γ

(1)

Where Pc is the compressed pressure; γ is the specific heat ratio, as a function of temperature. The compressed 5


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temperature is obtained by solving the integral. Pc and Tc can be controlled by changing the initial fuel temperature (T0), the

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initial pressure (P0) and the compression ratio.

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2.2 Mixture preparation

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The homogeneous mixture of fuel and oxidant was prepared in a 27.68L stainless steel tank which is heated by

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circulating silicone oil to ensure the uniformity of temperature. Digital pressure gauges (OMEGA DPG4000) and

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PT100-type thermistor are connected to the top of the fuel tank to real-time monitor the pressure and temperature of the

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mixture. The fuel tank was maintained at 130 °C and the pressure of AMN is less than 2.5 kPa, which is below the

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saturation pressure (about 3.37 kPa) of AMN at 130°C. A fuel vaporization experiment was conducted before the formal

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experiment. Fuel can be vaporized within a few minutes, and vaporization rate is 98% or more. There was no significant

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change in the pressure of the fuel tank after several hours, demonstrating that the mixture was reliable. The purity of O2, Ar

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and α-methylnaphthalene in this study was 99.999%, 99.999% and 98%, respectively. The fuel tank is first heated to the

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specified temperature, and then fuel, Ar, and O2 were injected successively after vacuuming. The mixture preparation

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process is similar with the literature [36]. Each component was kept for a certain period of time after injection to stabilize

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the temperature and then inject other components. The mixture is maintained for 2-3 hours after completion of the

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preparation to ensure adequate gas mixing.

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2.3 Definition of ignition delay

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The typical pressure trace and the derivative of pressure obtained in the experiment are shown in Fig.2. It can be seen

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that the ignition process of α-methylnaphthalene is very intense, and the rate of pressure rise is high. The time of the end of

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compression with the maximum pressure in the compression process is set to 0. The time when the rate of pressure rise is

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the highest during the combustion is taken as the ignition time. And the time interval between the end of compression and

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the ignition time is defined as the ignition delay (τ).

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2.4 Experimental reproducibility

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The reproducibility experiments are shown in Fig.3 when the equivalence ratio is 1 and Pc = 15 bar. The slight error of

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the initial pressure will result in a slight difference in the compressed temperature and pressure. The error of compressed

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pressure in this experiment is within ± 0.5bar, and the overall reproducibility is good. O2 in the mixture was replaced by N2

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in the inert run. According to the simulation, such a method will result in a slight increase in the compressed temperature in

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the inert run under the same initial conditions due to the small difference in heat capacity between O2 and N2. The

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calculation shows that the inert-run compressed temperature is about 13 °C higher than that of the experiment in this study.

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Comparing the inert-run heat loss curves under different experimental conditions, it can be found that the heat loss before

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ignition is more affected by the volume of the combustion chamber (surface-to-volume ratio) rather than the temperature.

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Therefore, this inert-run experiments are reasonable.

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2.5 Experimental conditions

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Pitz et al. [37] noted that α-methylnaphthalene could not ignite at time within the diesel engine's time scale below 860

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K. In order to reach a higher temperature, Ar was chosen as the dilution gas for all experiments in this study. The molar ratio

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of O2:Ar was 0.21: 0.79, and the temperature was 860K~1040K. A total of 7 different ignition delay curves of

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α-methylnaphthalene were measured under different conditions. The specific conditions are shown in Table 1.

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3. Kinetic Modeling

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In this paper, a semi-detailed chemical kinetic mechanism of α-methylnaphthalene with 196 species and 1330 reactions

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was established. The mechanism mainly consists of the toluene sub-mechanism from the literature [12] and the

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α-methylnaphthalene sub-mechanism from the literature [24]. The mechanism of PAHs formation in these two mechanisms

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is not considered in this study. The mechanism can be found in Supplementary Materials.

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The sub-mechanism of toluene and styrene in this study is from the literature [12]. This mechanism consists of the

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mechanism of C0-C4, benzene, toluene, and larger molecular aromatic hydrocarbons. The mechanism provides different

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Arrhenius coefficients of a number of key reactions at different pressures with PLOG (Pressure Dependence through

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Logarithmic Interpolation), so that it can be applied to a larger pressure range.

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The α-methylnaphthalene sub-mechanism is an skeleton mechanism containing 60 reactions, which is from literature

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[24]. The reactions of α-methylnaphthalene sub-mechanism are mainly concentrated in the methyl, while the reactions on

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the naphthalene ring are not considered. The main reactions include H-atom abstraction and oxidation at methyl, after that

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the naphthalene ring opens to form indene and continues to produce styryl. The main reactions classes of

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α-methylnaphthalene include unimolecular decomposition of methylnaphthalene and H-atom abstraction reactions at methyl

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radicals by radicals, atoms and molecules.

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The chain initiation reactions of α-methylnaphthalene include unimolecular decomposition reactions and H-atom abstraction reactions. Unimolecular decomposition occurs through the following two reactions:

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C10H7CH3=C10H7CH2+H

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and

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C10H7CH3=C10H7+CH3

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H-atom abstraction reactions include reactions with O2, HO2, O, OH, H, CH3, C10H7 and C10H7CO. All of the rate

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constants are also taken from [24].

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4. Experiment result and mechanism validation

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The ignition delay under different temperatures in seven different conditions were measured. Parts of pressure curves are shown in Fig.4. The detailed ignition delay data can be found in Supplementary Materials.

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The compressed pressures were controlled within ±0.05bar in the experiments. As we can see from Fig.4, the pressure

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curve during ignition process does not exhibit the characteristics of two-stage ignition. Single stage ignition is observed and

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the rate of pressure rise is very high. There is no significant difference in the maximum combustion pressure at different

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temperatures. The experimental compressed temperature is controlled in 860K-1040K. The minimum measured ignition

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delay is about 2ms. There is no obvious reaction during compression for the weak low-temperature chemistry of AMN.

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Fig.5 shows the comparison of the ignition delay obtained in this experiment with the RCM experiments of Kukkadapu

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et al. [28], the shock tube experiments of Wang et al. [26] and Pfahl et al. [22] in the similar conditions. It can be seen that

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the ignition delay of AMN of this study is in good agreement with the data of Kukkadapu et al. It should be noted that the

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facility has a significant effect on the RCM ignition delay [38]. By comparison, the time interval of 50% pressure to the end

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of compression is 3.8ms [28] and 4.5ms (this work) respectively, which is very close. The heat loss after the end of

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compression is also similar. So the comparison is reasonable. Furthermore, compared with the data of Kukkadapu et al., The

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temperature range of this study is wider and the maximum temperature is above 1020K, which can basically connect with

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the shock tube data. Compared with the shock tube data, the ignition delay obtained under similar pressure in this

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experiment is slightly larger, which is due to the heat loss after the end of compression in RCM experiment. It can also be

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seen in Fig.5 (b) that the logarithm of the ignition delay of AMN is linearly related to 1000 / T over a very wide temperature

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range, with no obvious NTC phenomenon.

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The ignition delay curves under different conditions are shown in Fig.6. The ignition delay of AMN decreases with the increase of temperature, pressure and equivalence ratio.

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The Wang’s mechanism [26] and the mechanism of this work were used to perform the simulation with CHEMKIN.

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The 0-D adiabatic constant volume (CV) simulation was adopted in the calculation first. CV simulation is the ideal

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condition of the RCM experiment and it has the advantage of saving time and computing resource. In order to get closer to

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the actual experimental condition, full RCM simulation is also used to take the compression and post-compression process

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into consideration for the new established mechanism. Only CV simulations were carried out for Wang’s mechanism

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because there are issues with convergence when RCM simulation are used with the this mechanism. It can be seen from

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Fig.6, the ignition delay curve using Wang's mechanism has obvious difference with the experimental data, specifically as

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follows:

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1. The mechanism does not correctly predict the effect of the equivalence ratio on the ignition delay. At the same

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temperature and pressure in the experiment, the ignition delays decrease with the increase of the equivalence ratio.

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While the simulation result of Wang’s mechanism is the opposite. Moreover, the ignition delay curves of different

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equivalence ratios are crossed.

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2. The activation energy (slope) of the experimental curve and the simulation curve are significantly different. The

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slope of the simulation curve at the lower temperature is obviously decreased which has not been observed in the

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experiment.

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Since the effects of compression and heat loss on the ignition delay are not taken into account, the ignition delay

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calculated by this mechanism is generally less than the experimental value. And with the increase of ignition delay, the

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effect of heat loss on the experiment is more and more obvious, leading the error of the experimental ignition delay and the

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simulation with CV simulation obviously increasing. The effect of the equivalence ratio on the ignition delay can be

213

predicted correctly using the new established mechanism of this work and the calculated activation energy is closer to the

214

experimental result. However, it can be found that the effect of the equivalence ratio on the ignition delay is much greater

215

than that of the simulation.

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Afterwards, the RCM simulation in [39] is adopted in this work. According to the adiabatic core assumption, the heat

217

loss before ignition is converted to volume expansion, and the compression process is also considered. The changes of the

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inert-run pressure at Φ = 1 are transformed into the volume changes by formula (2). The volume history can be found in

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Supplementary Materials. The compressed temperature and pressure calculated by CHEMKIN when introduced the

220

volume-time-dependent relationship is slightly different from the experiment. While compared with the adiabatic constant

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volume method, this method can significantly reduce the simulation error. Fig.7 shows the comparison of simulated

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pressure trace obtained by this method with the experimental one. It can be seen that there is a significant exothermic

223

phenomenon during the induction period in the simulation. The pressure before ignition shows a significantly increase

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compared with the inert run, which is not observed in the experiment. In the experiment, the experimental pressure and the

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inert run pressure are consistent between the end of compression and ignition without exothermic phenomenon, which is

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consistent with the experimental phenomenon of Kukkadapu et al. [28] That means the current mechanism is still somehow

227

insufficient. 1

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 P (t ) γ V (t ) = V0    P0 

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Fig.8 compares the ignition delay using the CV simulation and the RCM simulation with the mechanism of this work.

(2)

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The simulated ignition delay is the same definition as the experiment. The compression process is more important than the

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heat loss process when the temperature is high and the ignition delay is small. The low-temperature reaction during the

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compression process results in the ignition delay of the RCM simulation being less than that of the CV simulation. Case is

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the opposite while the temperature is low and the ignition delay is long. Obviously, the ignition delay with RCM simulation

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is closer to the experimental result. Especially in the case of low temperature and long ignition delay, the RCM simulation

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considers the effect of heat loss on the ignition delay. The simulated ignition delay is very close to the experimental result.

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At higher temperatures, the simulated ignition delay is less than the experimental value. The maximum error is about -60%.

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Since there were convergence problems in RCM simulations using CHEMKIN with the mechanism of Wang et al. [26],

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the mechanism of Narayanaswamy et al. [29] was adopted in RCM simulations, as is shown in Fig.9. The results of the two

239

are similar in about 1000K, but Narayanaswamy's mechanism underestimate the ignition delay of AMN near the low and

240

moderate temperature.

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In addition, the simulation results of current mechanism are also compared with the measures of shock tube of Wang et

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al. [26] and JSR of Mati et al. [24]. The ignition delay of α-methylnaphthalene/air mixtures at equivalence ratios of 0.5, 1,

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1.5, pressure of 10 bar, 40 bar, and temperature of 1032K~1445K were measured by Wang et al. The experimental data and

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simulation results with different mechanisms are shown in Fig.10. In the experiments, the ignition point was defined by

245

extrapolating the peak in the slope of the OH signal to the baseline value. The time interval between the ignition point and

246

the arrival of reflected shock at the end wall is defined as the ignition delay. The same definition is adopted in the simulation.

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The measured ignition delay times have been scaled using τ ∝ P −0.90 . It can be seen from the figure that the simulated

248

ignition delay is slightly longer than the experimental value in rich mixture. But in general, the simulation results are in

249

good agreement with the shock tube experiment. The mechanism of Wang erroneously predicts the effect of equivalence on

250

ignition delay. The mechanism of Narayanaswamy et al. overestimates the ignition delay of AMN, especially at high

251

pressure. And at low pressure, the ignition delay curves of different equivalence ratios are crossed.

252

The species concentration profiles of the reactants, products, and various stable intermediates of α-methylnaphthalene

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253

oxidation in JSR were measured by Mati et al. The simulated and experimental results of α-methylnaphthalene, O2, CO,

254

CO2, CH4, C2H2, C2H4, C6H6 in 10atm, Φ = 1 were compared in this study, as shown in Fig.11. The results show that the

255

prediction of a variety of reactants, products and intermediates are in good agreement with the experimental data. The

256

predictions of C2H2, C2H4, and C6H6 are slightly error compared with the experiment, but the general trend is consistent.

257

5. Kinetic Analysis

258

The main reaction path of α-methylnaphthalene under the conditions of initial temperature of 900K, pressure of 15bar

259

and Φ = 1 is shown in Fig.12. The reaction path analysis was carried out at 29.2 ms, when the consumption of

260

α-methylnaphthalene was 19.78%, and the temperature was 968 K. The main reaction path is highlighted in bold arrow. It

261

can be seen from the figure that the initial reaction of α-methylnaphthalene includes the H-atom abstraction reaction of

262

methyl yielding naphthylmethyl radical and the C-C bond cleavage yielding naphthalene, in which the path yielding

263

naphthalenemethyl

264

α-methylnaphthalene and the rest yields naphthaldehyde and naphthyl radical. The naphthyl radical is oxidized to the

265

naphthoxy radical and then undergoes a decomposition reaction to produce the indenyl. The indenyl ring is then opened to

266

form C6H5CCH2, which will be oxidized to benzaldehyde by O2. The benzaldehyde breaks down into phenyl and CO. The

267

phenyl can be oxidized in a variety of different ways, but mainly yields phenoxy. The phenoxy continues to be oxidized by

268

the O atom, producing various small molecules

radical

occupies

an

absolute

advantage.

35.6%

naphthalenemethyl

radical

re-generate

269

Fig.13 shows the sensitivity analysis of OH at P = 15 bar, Φ = 1 and initial temperatures of 900 K and 1300 K,

270

respectively. The negative sensitivity coefficient indicates that the reaction can promote the formation of OH and reduce the

271

ignition delay.

272

The sensitivity analysis shows that there are no significant differences in the main reactions affecting the ignition of

273

α-methylnaphthalene at 900K and 1300K. The H-atom abstraction reaction of α-methylnaphthalene with O2 have the

274

greatest enhancement on the ignition. In the reaction path analysis, 62.1% of the naphthoxy radical yields the stable

275

naphthol via the chain termination reaction R44.C10H7O + H = C10H7OH, which in turn reacts with OH and O to form

12


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276

naphthyloxy. Therefore, R44 has the greatest inhibition on ignition. R43 produces indenyl and CO in the opposite direction.

277

This reaction also has a significant promotion on ignition by producing a relatively active indenyl and competing with R44.

278

R15.C10H7CH3+C10H7=C10H7CH2+C10H8 consumes the active naphthalene and produces a relatively stable naphthalene. So

279

it also has a greater inhibition on the ignition. In the reaction of small molecules, R1308. H+2O2=HO2+O2 and R1321.

280

H+O2=O+OH have obvious promoting effect on ignition at low temperature and high temperature, respectively. In general,

281

the initial reaction of α-methylnaphthalene, the reactions in the path of naphthyl radical → naphthoxy radical → indenyl

282

play the most important role in the oxidation of α-methylnaphthalene.

283

6. Summary

284

In this study, the ignition delay of α-methylnaphthalene on the rapid compression machine was measured. Inert run

285

experiments were carried out on three different pressures with equivalence ratio of 1. A new semi-detailed chemical kinetic

286

mechanism of α-methylnaphthalene was established. The calculation was carried out using the current mechanism, Wang’s

287

mechanism and Narayanaswamy's mechanism with CV simulation and RCM simulation. Comparing the different

288

mechanisms and different simulation methods, the mechanism of this study with the RCM simulation can predict the

289

experimental data well. In this work, the data of shock tube and JSR from literature is also compared with the simulation,

290

which indicates that this mechanism has wide applicability in different reactors. According to the reaction path analysis and

291

sensitivity analysis, α-methylnaphthalene → naphthyl radical → naphthoxy radical → indenyl is the main path of the

292

consumption of α-methylnaphthalene, of which the main reactions play a great role in promoting the ignition of

293

α-methylnaphthalene.

294

Acknowledgements

295

This work was supported by the Nature Science Foundation of China (Grant No. 91641202, 51425602).

296

References

297 298 299 300 301

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mixtures including cross reactions between aromatics and aliphatics. Proceedings of the Combustion Institute. 2009;32:411-8. [4] Li H, Yu L, Lu X, Ouyang L, Sun S, Huang Z. Autoignition of ternary blends for gasoline surrogate at wide temperature ranges and at elevated pressure: Shock tube measurements and detailed kinetic modeling. Fuel. 2016;181:916-25. [5] Zhong B J, Zheng D. Chemical kinetic mechanism of a three-component fuel composed of iso-octane/n-heptane/ethanol. Combustion Science and Technology, 2013, 185(4): 627-644. [6] Di Sante R. Measurements of the auto-ignition of n-heptane/toluene mixtures using a rapid compression machine. Combustion and flame, 2012, 159(1): 55-63. [7] Curran HJ, Gaffuri P, Pitz WJ, Westbrook CK. A Comprehensive Modeling Study of n-Heptane Oxidation. Combustion and Flame. 1998;114:149-77. [8] Curran HJ, Gaffuri P, Pitz WJ, Westbrook CK. A comprehensive modeling study of iso-octane oxidation. Combustion and Flame. 2002;129:253-80. [9] Silke EJ, Pitz WJ, Westbrook CK, Ribaucour M. Detailed Chemical Kinetic Modeling of Cyclohexane Oxidation†. The Journal of Physical Chemistry A. 2007;111:3761-75. [10] Pitz WJ, Naik CV, Mhaoldúin TN, Westbrook CK, Curran HJ, Orme JP, et al. Modeling and experimental investigation of methylcyclohexane ignition in a rapid compression machine. Proceedings of the Combustion Institute. 2007;31:267-75. [11] Dagaut P, Pengloan G, Ristori A. Oxidation, ignition and combustion of toluene: Experimental and detailed chemical kinetic modeling. Physical Chemistry Chemical Physics. 2002;4:1846-54. [12] Yuan W, Li Y, Dagaut P, Yang J, Qi F. Investigation on the pyrolysis and oxidation of toluene over a wide range conditions. I. Flow reactor pyrolysis and jet stirred reactor oxidation. Combustion and Flame. 2015;162:3-21. [13] Suzuki S, Hori M, Nakamura H, Tezuka T, Hasegawa S, Maruta K. Study on cetane number dependence of diesel surrogates/air weak flames in a micro flow reactor with a controlled temperature profile. Proceedings of the Combustion Institute. 2013;34:3411-7. [14] Ramirez HP, Hadj-Ali K, Diévart P, Dayma G, Togbé C, Moréac G, et al. Oxidation of commercial and surrogate bio-Diesel fuels (B30) in a jet-stirred reactor at elevated pressure: Experimental and modeling kinetic study. Proceedings of the Combustion Institute. 2011;33:375-82. [15] Lemaire R, Faccinetto A, Therssen E, Ziskind M, Focsa C, Desgroux P. Experimental comparison of soot formation in turbulent flames of Diesel and surrogate Diesel fuels. Proceedings of the Combustion Institute. 2009;32:737-44. [16] Barths H, Hasse C, Bikas G, Peters N. Simulation of combustion in direct injection diesel engines using a eulerian particle flamelet model. Proceedings of the Combustion Institute. 2000;28:1161-8. [17] Schulz W D. Oxidation products of a surrogate JP-8 fuel. Preprints - American Chemical Society. Division of Petroleum Chemistry, 1992.. [18] Agosta A. Development of a chemical surrogate for JP-8 aviation fuel using a pressurized flow reactor. Drexel University. 2002. [19] Violi A, Yan S, Eddings EG, Sarofim AF, Granata S, Faravelli T, et al. Experimental formulation and kinetic model for JP-8 surrogate mixtures. Combustion Science and Technology. 2002;174:399-417. [20] Slavinskaya N A, Zizin A, Riedel U. Towards surrogate reaction model development. ASME Turbo Expo 2011, Power for Land, Sea and Air. DLR, 2011:203-214. [21] Mensch A, Santoro RJ, Litzinger TA, Lee SY. Sooting characteristics of surrogates for jet fuels. Combustion and Flame. 2010;157:1097-105. [22] Pfahl U, Fieweger K, Adomeit G. Self-ignition of diesel-relevant hydrocarbon-air mixtures under engine conditions. Symposium (International) on Combustion. 1996;26:781-9. [23] Pitsch H. Detailed kinetic reaction mechanism for ignition and oxidation of α-methylnaphthalene. Symposium (International) on Combustion. 1996;26:721-8. [24] Mati K, Ristori A, Pengloan G, Dagaut P. Oxidation of 1-methylnaphthalene at 1-13 atm: experimental study in a JSR and detailed chemical kinetic modeling. Combustion Science and Technology. 2007;179:1261-85. [25] Mati K, Ristori A, Gaïl S, Pengloan G, Dagaut P. The oxidation of a diesel fuel at 1–10 atm: Experimental study in a 14


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JSR and detailed chemical kinetic modeling. Proceedings of the Combustion Institute. 2007;31:2939-46. [26] Wang H, Warner SJ, Oehlschlaeger MA, Bounaceur R, Biet J, Glaude P-A, et al. An experimental and kinetic modeling study of the autoignition of α-methylnaphthalene/air and α-methylnaphthalene/n-decane/air mixtures at elevated pressures. Combustion and Flame. 2010;157:1976-88. [27] Bounaceur R, Glaude PA, Fournet R, Battinleclerc F, Jay S, Cruz APD. Kinetic modelling of a surrogate diesel fuel applied to 3D auto-ignition in HCCI engines. International Journal of Vehicle Design. 2007;44:124-42. [28] Kukkadapu G, Sung C-J. Autoignition Study of 1-Methylnaphthalene in a Rapid Compression Machine. Energy & Fuels. 2017;31:854-66. [29] Narayanaswamy K, Blanquart G, Pitsch H. A consistent chemical mechanism for oxidation of substituted aromatic species. Combustion and Flame. 2010;157:1879-98. [30] Somers ML, McClaine JW, Wornat MJ. The formation of polycyclic aromatic hydrocarbons from the supercritical pyrolysis of 1-methylnaphthalene. Proceedings of the Combustion Institute. 2007;31:501-9. [31] Shaddix CR, Brezinsky K, Glassman I. Analysis of fuel decay routes in the high-temperature oxidation of 1-methylnaphthalene. Combustion and Flame. 1997;108:139-57. [32] Yang J, Lu M. Thermal growth and decomposition of methylnaphthalenes. Environmental Science & Technology. 2005;39:3077-82. [33] Guang H, Yang Z, Huang Z, Lu X. Experimental study of n-heptane ignition delay with carbon dioxide addition in a rapid compression machine under low-temperature conditions. Chinese Science Bulletin. 2012;57:3953-60. [34] Yang Z, Wang Y, Yang X, Qian Y, Lu X, Huang Z. Autoignition of butanol isomers/n-heptane blend fuels on a rapid compression machine in N2/O2/Ar mixtures. Science China Technological Sciences. 2014;57:461-70. [35] Mittal G, Sung* C-J. A rapid compression machine for chemical kinetics studies at elevated pressures and temperatures. Combustion Science and Technology. 2007;179:497-530. [36] Kukkadapu G, Kumar K, Sung C J, et al. Experimental and surrogate modeling study of gasoline ignition in a rapid compression machine. Combustion and Flame, 2012, 159(10): 3066-3078. [37] Pitz WJ, Mueller CJ. Recent progress in the development of diesel surrogate fuels. Progress in Energy and Combustion Science. 2011;37:330-50. [38] Darcy D, Nakamura H, Tobin C J, et al. An experimental and modeling study of surrogate mixtures of n-propyl-and n-butylbenzene in n-heptane to simulate n-decylbenzene ignition. Combustion and Flame, 2014, 161(6): 1460-1473. [39] Sung C-J, Curran HJ. Using rapid compression machines for chemical kinetics studies. Progress in Energy and Combustion Science. 2014;44:1-18.

15


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Table 1 The experimental conditions and mixture composition in this study (mol%). No.

Pc (Bar)

Φ

Fuel

O2

Ar

1

12

1

1.53%

20.68%

77.79%

2

12

1.2

1.83%

20.62%

77.55%

3

15

0.7

1.08%

20.77%

78.15%

4

15

1

1.53%

20.68%

77.79%

5

15

1.2

1.83%

20.62%

77.55%

6

20

0.7

1.08%

20.77%

78.15%

7

20

1

1.53%

20.68%

77.79%

16


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Energy & Fuels

Fig.1. Heated rapid compression machine

17


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Fig.2. Definition of ignition delay

18


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Energy & Fuels

Fig.3. Experiment reproducibility

19


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(a) Pc=15bar Φ=1 (b) Pc=15bar Φ=0.7 Fig.4. Typical pressure trace at various temperatures

20


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(a) RCM data comparison Φ=1 (b) Shock tube data comparison Φ=1 Fig.5. Comparison of ignition delays of AMN with the experimental data obtained by Kukkadapu et al. [28], Wang et al. [26] and Pfahl et al. [22]

21


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(a) Pc=12bar Φ=1, 1.2

Page 22 of 29

(b) Pc=15bar Φ=0.7, 1, 1.2

(c) Pc=20bar Φ=0.7,1 (d) Φ=1, Pc=12bar, 15bar, 20bar Fig.6. Comparisons of experimental and simulated ignition delay with adiabatic constant-volume model; symbols: experiment data; dash lines: Wang’s model [26]; solid lines: model of this study

22


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Energy & Fuels

Fig.7. Comparison of experimental and RCM simulated pressure trace, Pc=15bar, Tc=905K, Φ=1

23


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Fig.8. Comparison of experimental and simulated ignition delay at different pressures; symbols: experiment data; dash lines: constant volume simulation; solid lines: RCM simulation

24


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Energy & Fuels

Fig.9. Comparison of experimental and RCM simulated ignition delay at different pressures; symbols: experiment data; dash lines: Narayanaswamy's model; solid lines: current model

25


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(a) Comparison with Wang’s model, P=10bar

Page 26 of 29

(b) Comparison with Wang’s model, P=40bar

(c) Comparison with Narayanaswamy's model, P=10bar (d) Comparison with Narayanaswamy's model, P=40bar Fig.10. Comparison of simulated ignition delay with experiment data of Wang et al. [26] in a shock tube. P=10bar, 40bar, Φ=0.5,1,1.5 Model of Wang et al. [26] and Narayanaswamy et al. [29] were compared with the current model.

26


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Energy & Fuels

Fig.11. Comparison of simulations with measurements of Mati et al. [24] in a JSR. P=10atm Φ=1; symbols: experiment data; lines: simulation

27


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Fig.12. Reaction pathways of AMN with initial temperature of 900K, Φ=1 P=15bar, 20% AMN consumption

28


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Energy & Fuels 1300K 900K

R44.C10H7O+H=C10H7OH R15.C10H7CH3+C10H7=C10H7CH2+C10H8 R45.C10H7OH+OH=C10H7O+H2O R16.C10H7CH2+O=C10H7HCO+H R12.C10H7CH3+H=C10H7CH2+H2 R11.C10H7CH3+OH=C10H7CH2+H2O R1321.H+O2=O+OH R20.C10H7CH2+HO2=C10H7+CH2O+OH R59.Indenoxy=C6H5CCH2+CO R1248.CH2O+OH=HCO+H2O R35.C10H7HCO+C10H7CH2=C10H7CO+C10H7CH3 R50.Indenyl+O=C6H5CCH2+CO R1308.H+2O2=HO2+O2 R42.C10H7+O2=C10H7O+O R43.Indenyl+CO=C10H7O R1312.H+O2+AR=HO2+AR R5.C10H7CH3+O2=C10H7CH2+HO2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

Normalized Sensitivity Coefficient

Fig.13. Normalized sensitivity coefficients of OH radical. P=15bar, Φ=1, T=900K,1300K

29


Experimental and Kinetic Modeling Study on Self-ignition of Î±

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