Polycyclic

Article pubs.acs.org/EF

Development of a Reduced n‑Decane/α-Methylnaphthalene/ Polycyclic Aromatic Hydrocarbon Mechanism and Its Application for Combustion and Soot Prediction Liang Qiu, Xiaobei Cheng,* Xin Wang, Zhongqiu Li, Ying Li, Zhaowen Wang, and Hui Wu* State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China S Supporting Information *

ABSTRACT: In this work, a reduced n-decane/α-methylnaphthalene/polycyclic aromatic hydrocarbon (PAH) kinetic mechanism for calculation of combustion and soot behavior of diesel and its surrogate fuel was developed. This mechanism consists of the pyrolysis of n-decane (C10H22) and α-methylnaphthalene (C10H7CH3), C0−C3 core species reaction, and the formation of initial benzene and PAHs, including 77 species and 287 reactions. The pyrolysis reaction of fuel was obtained by pathway analysis from a detailed mechanism, while the reactions of core species and PAHs were reduced by a directed relation graph with error propagation, computational singular perturbation method, and direct sensitivity analysis, sequentially. The mechanism was validated by the mole fraction of main species and key PAH species in the ethylene premixed flame, ignition delay times of pure and mixed fuel in shock tubes, and the concentrations of major species in jet-stirred reactors. Finally, the new developed mechanism was coupled with a soot phenomenological model, where A4 was employed as the precursor in soot inception, and a multi-dimensional turbulent model, to calculate the combustion and soot emission processes in a diesel engine. The pressure in the cylinder, apparent heat release rate, and normalized soot fraction were obtained in this calculation. Both the fundamental modeling and engine modeling agree well with the data from the literature.

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commonly used mixture includes n-heptane/toluene,10−12 nheptane/isooctane,13,14 n-decane/isooctane/methylcyclohexane/toluene,15 n-hexadecane/cyclohexane/isocetane/toluene,16 etc. The literature16 sorted out most of the current multicomponent surrogate fuel for commercial diesel. Constructing an accurate reduced mechanism for the diesel surrogate is very important, because it is the basis for further development of fuel mixture mechanisms, such as diesel/alcohols17,18 and diesel/biodiesel mixtures.19−22 In the integrated development on engine action, a diesel surrogate fuel named IDEA was proposed in Europe, which consists of 70% n-decane and 30% α-methylnaphthalene by mole fraction.23 The chemical and physical characteristics of the mixed fuel are close to that of the conventional diesel fuel, whose density in 293 K, cetane number, and C/H ratio are 0.798 g/L, 53, and 1.8, respectively.24 The kinetic mechanism used in the engine combustion and emission calculation is generally reduced from the detailed mechanism with thousands of species and reactions. Lebedev et al.25 reviewed 10 kinds of commonly used mechanism reduction methods, including principal component analysis (PCA), directed relation graph with error propagation (DRGEP), computational singular perturbation (CSP) method, direct sensitivity analysis (DSA), and two kinds of mechanism analysis methods, including quasi-steady-state index (QSSA) and rate of production (ROP) analysis. In comparison to the detailed mechanism, some reaction information on the

INTRODUCTION Diesel fuel is a mixture of hundreds of hydrocarbons. Subject to the limitations of computational cost, developing a dynamic model that contains all of the components is not realistic. In addition, a detailed reaction mechanism of pure large hydrocarbon fuel typically contains hundreds of species and reactions. In the multi-dimensional in-cylinder turbulent combustion calculation, when coupled with computational fluid dynamics (CFD), a detailed chemical reaction mechanism, and a soot model, the calculation expense is still unacceptable. Therefore, a single component or multi-component mixture was commonly used as the surrogate fuel in the combustion simulation of real diesel fuel. Surrogate fuel is the fuel that consists of a few pure components, which has some similar characteristics as a real fuel composed of many compounds, such as a commercial diesel fuel. Both the physical and chemical behaviors of the target fuel need to be reproduced by the specified surrogate fuel; thus, the surrogate can properly represent the combustion processes of the target fuel as well as the injection, atomization, evaporation, and gas mixing characteristics before ignition in combustion devices.1,2 Commercial diesel fuel mainly consists of n-alkanes, isoalkanes, aromatics, and cycloalkanes.3 Among which, n-alkanes were often used as single-component surrogate fuel, including n-heptane,4−6 n-decane,7 n-dodecane,8 etc. However, on the basis of the experiments conducted by Weber et al.,9 n-decane ignites earlier than diesel fuel. In addition, studies indicate that real diesel fuel contains nearly 30% aromatic hydrocarbons, and its oxidation and emission characteristics cannot be completely represented by pure nalkane; thus, a multi-component mixture is employed. The © XXXX American Chemical Society

Received: August 30, 2016 Revised: October 19, 2016 Published: October 21, 2016 A

DOI: 10.1021/acs.energyfuels.6b02186 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

decane and α-methylnaphthalene and the formation of A1 are listed in Table 1. Pyrolysis of n-decane can be divided into a medium-to-high temperature pathway and a low-temperature pathway. However, the first step is hydrogen atom abstraction of C10H22 by O2/OH/H/HO2 to form C10H21 in both pathways (during the initiation period, by O2, and then mainly by OH groups). The related reactions are reactions R1−R4. Then, in medium and high temperatures (T > 900 K), C10H21 decomposes to C2−C3 species by β scission. According to the investigation of Chang et al.,7 this part is merged into reaction R5. In the low temperature below 900 K, the further oxidation and decomposition of C10H21 is mainly carried out by reactions R6−R11. First, O2 is added to C10H21 to yield C10H21OO (reaction R6). The C10H21OO radical first isomerizes to give C10H20OOH (reaction R7). Then, the rapid decomposition of the C10H20OOH group results in three groups (two OH and C5H11CO), which is a chain-branching process (reactions R9 and R10). In this reaction step, a medium product named alkylhydroperoxide is generated. Alkylhydroperoxide includes a carbonyl group, primarily ketohydroperoxides (C10ket), which have been observed in experiments recently under conditions detected close to autoignition.30 In this pathway, the HO2 or OH group reacting by hydrogen abstraction with C10H22 leads to the formation of three free groups in a chain-branching reaction. Therefore, the oxidation of C10H22 is strongly promoted by this autoacceleration process below 700 K. The oxidation reactions of α-methylnaphthalene (C10H7CH3) were taken from a detailed mechanism proposed by Mati et al.31 through reaction pathway analysis. Under stoichiometric conditions, at atmospheric pressure and 1300 K, C10H7CH3 mainly reacts through hydrogen atom abstraction by O2, HO2, H, and OH to form the naphthylmethyl radical (C10H7CH2) through reactions R12, R13, R15, and R16, respectively. The consumption of C10H7CH2 yields naphthaldehyde (C10H7HCO) by reactions R17 and R18. Naphthaldehyde is consumed via reactions R19−R21, which essentially leads to the formation of the naphthoyl radical (C10H7CO). C10H7CO is consumed through reaction R22 to form naphthyl (A2−). A2− reacts with O2 to form C10H7O, and then C10H7O, in turn, decomposes to generate indenyl. Indenyl then further decomposes into C2−C3 species. The specific reaction path is shown in Figure 1. In the ABF mechanism,32 only the high-temperature reaction R28, low-temperature reaction R25, and the combination of propargyl (reaction R24) were included in the formation of initial benzene (A1). In the present work, the reduced mechanism of the A1 formation was taken from the investigation of Chernov et al.,33 where a combination of propargyl and propyne (C3H4), cyclopentadienyl (C5H5), and methyl were also taken into consideration. With regard to other parts, the C2−C3 and H2/CO/C1 reactions were reduced from the detailed San Diego mechanism.34 PAH reactions were reduced from the PAH mechanism up to A4 formation (Chernov et al.33), where C2H2, C3H3, C4H2, C4H3, C4H4, and C5H5 were considered as the precursors of benzene and PAH. The NOx mechanism was taken from GRI 3.0.35 The reaction rates of oxidation of n-decane and α-methylnaphthalene were calculated by the Arrhenius equation, where the pre-exponential factor, temperature index, and activation energy were first taken from the literature7,31 and then adjusted to match the ignition delay time from experimental data in the literature. Mechanism Reduction Methodology. Figure 2 shows the main procedure of this investigation. The mechanism development was mainly carried out according to the four following steps: (1) Add the reactions of A1/PAH and their precursors to the San Diego C0−C3 mechanism to yield a core reaction mechanism, which has 108 species/ 846 reactions, including the formation of A1 and PAH. (2) In a zerodimensional homogeneous reactor, reduce the mechanism obtained from step 1 through DRGEP,36 CSP method,37 and DSA,25 in sequence. Then, the mechanism was degenerated into a smaller size that contains only 56 species/236 reactions. (3) Analyze the reaction pathway of n-decane and α-methylnaphthalene from their detailed mechanisms and add the skeletal oxidation reaction of this two-fuel and NOx mechanism to the base mechanism obtained from step 2.

intermediate component is ignored in the reduced mechanism. Thus, the computational time decreases significantly. However, the reduced mechanism still maintains a certain accuracy, so that the simulation result of the ignition delay time, mole fraction of key species in the premixed flame, and jet-stirred reactor (JSR) were kept in an acceptable range. Peters et al.26,27 proposed a reduced n-decane/α-methylnaphthalene kinetic mechanism that contains 118 species/553 reactions to model combustion and emission processes of this binary fuel. Unfortunately, the formation reactions of PAHs were not included in this mechanism, and the mechanism was only validated by the in-cylinder pressure of the diesel engine, soot, and NOx emission. There was no validation of the ignition delay time or species concentration in fundamental reactors. In this work, a reduced chemical mechanism for n-decane and α-methylnaphthalene binary fuel that contains the formation of PAH up to A4 was first developed by reaction pathway analysis, DRGEP, CSP method, and DSA, sequentially. Then, the newly developed mechanism was validated by the mole fraction of main species and key PAH species in the ethylene premixed flame, the ignition delay times of n-decane, α-methylnaphthalene, and the n-decane/α-methylnaphthalene mixture in shock tubes at different equivalence ratios, initial temperatures, and pressures, and concentrations of major species in JSRs for n-decane/O2/N2 and α-methylnaphthalene/ O2/N2 mixtures. Finally, this mechanism was coupled to KIVA3V,28 which includes a phenomenological soot model, to simulate the combustion, heat release, and soot emission.

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MODELING METHODOLOGY

Mechanism Development. Figure 1 shows the major reaction pathway for the proposed diesel surrogate fuel, which includes the

Figure 1. Major reaction pathway of the reduced mechanism for proposed diesel surrogate fuel. pyrolysis and oxidation reactions of n-decane and α-methylnaphthalene, reduced C2−C3 reactions, H2/CO/C1 mechanism, and A1 and PAH formation reactions. The reaction pyrolysis and oxidation pathway of n-decane was taken from the previous work of Chang et al.,7 with the elimination of C10H20 to further simplify the submechanism, while the main oxidation pathway of α-methylnaphthalene was obtained from rate analysis in CBR3 (shown in Table 4) using Chemical Workbench.29 Key reactions of the pyrolysis of nB

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Energy & Fuels Table 1. Key Reactions in the Pyrolysis of Fuel and A1 Formation number

reaction

number

reaction

R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15

C10H22 + O2 ⇄ C10H21 + HO2 C10H22 + OH ⇄ C10H21 + H2O C10H22 + H → C10H21 + H2 C10H22 + HO2 ⇄ C10H21 + H2O2 C10H21 → 2C3H6 + C2H5 + C2H4 C10H21 + O2 ⇄ C10H21OO C10H21OO ⇄ C10H20OOH C10H20OOH + O2 ⇄ OOC10H20OOH OOC10H20OOH ⇄ C10ket + OH C10ket = >CH2O + C5H11CO + OH + C3H6 C5H11CO + O2 → C3H7 + C2H3 + CO + HO2 C10H7CH3 + O2 ⇄ C10H7CH2 + HO2 C10H7CH3 + H ⇄ C10H7CH2 + H2 C10H7CH3 + H ⇄ A2 + CH3 C10H7CH3 + OH ⇄ C10H7CH2 + H2O

R16 R17 R18 R19 R20 R21 R22 R23 R24 R25 R26 R27 R28 R29

C10H7CH3 + HO2 ⇄ C10H7CH2 + H2O2 C10H7CH2 + O ⇄ C10H7HCO + H C10H7CH2 + HO2 ⇄ C10H7HCO + OH + H C10H7HCO ⇄ C10H7CO + H C10H7HCO + H ⇄ C10H7CO + H2 C10H7HCO + OH ⇄ C10H7CO + H2O C10H7CO ⇄ CO + A2− A2− + O2 ⇄ C10H7O + O 2C3H3 ⇄ A1 C4H3 + C2H3 ⇄ A1 C3H3 + C3H4 ⇄ A1 + H C2H3 + C4H4 ⇄ A1 + H C4H5 + C2H2 ⇄ A1 + H C5H5 + CH3 ⇄ A1 + 2H

The main ideas of DRGEP, CSP method, and DSA adopted in step 2 are briefly described below. The DRGEP method is derived from the directed relation graph (DRG) method. In the DRG method, some important species that need to be accurately reproduced by the reduced mechanism are specified as target species. Direct species coupling can be defined by the immediate error to the production rate of a target species, introduced by the removal of another non-target species from the mechanism with a controlling threshold. As an extension of DRG, DRGEP takes into account the indirect relations between species to further reduce the mechanism. The CSP method is an automated reduction method that is based on the separation of time scales between chemical species. In this method, the Jacobian matrix is used to separate reactions to slow and fast subdomains, and the most important reactions in each subdomain form the reduced mechanism. In the DSA method, the maximum value of the concentration sensitivity to each reaction is calculated and compared to a threshold value, and the reactions with all sensitivity coefficients smaller than the threshold value are excluded from the mechanism. The detailed algorithms of these mechanism reduction methods were described in the literature.25 In step 4, the original pre-exponential factors of the n-decane and PAH submechanism were taken from the work of Chang et al.7 and Chernov et al.,33 respectively, and some of them were adjusted to match the ignition delay time of n-decane and the mole fraction of PAH in the ethane/air premixed flame, as show in Table 2. Sensitivity analysis of the temperature was conducted in CBR1 (shown in Table 4) to determine which reaction has the greatest impact on ignition at different initial temperatures. Then, the pre-exponential factors of these chosen reactions were adjusted to match the ignition delay time with the measured data in the literature. Similarly, sensitivity analysis of the mole fraction of PAHs was also conducted in PF1 (shown in Table 4) to assign the importance of each reaction in the PAH submechanism, and the pre-exponential factors of the chosen reactions were adjusted to match the mole fraction of PAHs with the measured data in the literature. It should be pointed out that the pre-exponential

Figure 2. Procedure of this work.

However, the reactions from the oxidation of α-methylnaphthalene that are redundant with PAH reactions were removed. The final mechanism contains 77 species and 287 reactions. (4) Adjust the preexponential factor of the reactions relating to the formation of A1 and PAH to match the mole fraction of A1/PAH in the ethylene premixed flame with experimental data in the literature.38 In addition, adjust the pre-exponential factors of the skeletal oxidation reactions of n-decane to match the ignition delay time with experimental data in the literature.39

Table 2. Adjustment of Pre-exponential Factors original A7,33

reaction C10H22 + HO2 ⇄ C10H21 + H2O2 C10H21 + O2 ⇄ C10H21OO C10H20OOH + O2 ⇄ OOC10H20OOH C10H21 → 2C3H6 + C2H5 + C2H4 C4H5 + A1 → A2 + H2 + H A2R5 + C2H2 → A3 C4H2 + A2R5 → A4 A1C2H + A1C2H− ⇄ A4 + H

2.120 2.000 2.000 4.000 1.000 2.765 5.000 1.100 C

× × × × × × × ×

1014 1011 1011 1013 1012 104 102 1024

revised A 1.217 1.097 1.217 5.000 1.000 2.765 5.000 1.100

× × × × × × × ×

1014 1011 1011 1013 1011 102 10−2 1023

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Energy & Fuels factors of reactions in the α-methylnaphthalene submechanism were kept the same as the detailed mechanism, after a comparison between the calculated ignition delay time and measured data in the literature.40 Soot Model. In this work, a soot phenomenological model proposed by Vishwanathan et al.41 was employed to calculate the soot formation in a diesel engine. Soot inception by A4, surface growth by acetylene, particle coagulation, and particle oxidation by O2 and OH were included in this model. The specific reaction steps are listed in Table 3. Calculations of the combustion and emission of the diesel engine were carried on KIVA-3V.28

Validation in the Ethylene Premixed Flame. To validate the reduced PAH mechanism, the mole fraction of the key PAH species from C2H4 premixed flames that predicted by the current model was compared to the measured results from the literature. Castaldi et al.38 investigated the reaction pathway of PAHs in a C2H4/O2/Ar premixed flame. The laminar, premixed, flat flame with 21.3% C2H4/20.9% O2/57.8% Ar by mole fraction (Φ = 3.06, and gas volume flow rate is 7.56 L/ min) at 1 bar was adjusted to stabilize over a copper burner. The detailed experimental system was described in the literature.42 Figure 3 shows the mole fraction of major species, including C2H4, O2, CO, CO2, H2O, and an extremely important species in surface growth of PAH and soot particles, C2H2. Figure 4 shows the key PAH species at different special distances, including A1, A2, A3, and A4. In these two figures, the red symbols were measured data reported by Castaldi et al.,38 while lines were the calculated results by the ABF mechanism32 and the newly developed mechanism in this work. The comparison of these two simulated results shows the similar precision in the prediction of major species. However, the newly developed mechanism in this investigation is much more accurate in the prediction of A3 and A4. Shock-Tube Ignition Delay Time Validation. The ignition behaviors of the fuel/air mixture are much too important for advanced compression ignition combustion strategies, such as homogeneous charge compression ignition (HCCI), premixed charge compression ignition (PCCI), and reactivity controlled compression ignition (RCCI) combustion, in an internal combustion engine. The ignition delay time determines both the combustion and emission processes. Thus, the newly developed mechanism needs to be validated by the measured results in shock tubes. According to the literature,7 the ignition delay time refers to the time where the maximum

Table 3. Soot Phenomenological Model Proposed by Vishwanathan et al.41 soot process

global reaction step

inception C2H2 surface growth soot coagulation O2 oxidation OH oxidation

A4 → 16C(s) + 5H2 C(s) + C2H2 → 3C(s) + H2 nC(s) → C(s)n 2C(s) + O2 → 2CO 2C(s) + 2OH → 2CO + H2

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VALIDATION IN FUNDAMENTAL REACTORS The newly developed mechanism was validated by experimental data in fundamental reactors, including mole fraction of key species in the ethylene premixed flame and JSR and ignition delay time of pure and mixed fuel. All of the fundamental calculations were carried on Chemical Workbench software.29 The premixed flame (burner-stabilized), calorimetric bomb reactor (CBR), and well-stirred reactor (WSR) were used. The in-cylinder pressure at engine conditions is typically 50−100 bar; however, because there was no experimental data for these two fuels at those elevated pressures available in the literature, the measured results at lower pressures were employed in this validation for fundamental reactors. The initial conditions of each calculation case are listed in Table 4.

Table 4. Initial Conditions in the Calculation of Fundamental Reactors

PF1

a

1.0

name

P (bar)

CBR1 CBR2

13.5 50

CBR3 CBR4 CBR5

10/40 10/40 10/40

CBR6

10/40

CBR7 CBR8 CBR9

10 10 20

WSR1 WSR2

10 1.0

WSR3 WSR4

10 10

Φ

VFa

P (bar)

name

7.56 T (K)

XAr

XO2

C2H4/O2/AR Premixed Flame 3.06 0.578 Φ

XC10H22

0.209

XC10H7CH3

C10H22 Ignition Delay 0.5/1.0/2.0 0.0 0.5/1.0/2.0 0.0 C10H7CH3 Ignition Delay 1000−1550 0.5 0.0 0.5 1000−1550 1.0 0.0 1.0 1000−1550 1.5 0.0 1.5 70% C10H22/30% C10H7CH3 Ignition Delay 750−1450 1.0 0.7 0.3 Diesel Ignition Delay (70% C10H22/30% C10H7CH3 for Surrogate 625−1250 0.5 0.35 0.15 625−1250 1.0 0.7 0.3 625−1250 1.0 0.7 0.3 C10H22/N2/O2 JSR 550−1150 1.0 0.001 0.0 900−1300 2.0 0.0007 0.0 C10H7CH3/N2/O2 JSR 800−1200 1.0 0.0 0.0005 800−1200 1.5 0.0 0.0005 630−1430 630−1430

0.5/1.0/2.0 0.5/1.0/2.0

X C2 H 4

reference

0.213 X O2

38 X N2

reference

15.5 15.5

58.310 58.310

39 39

13.5 13.5 13.5

50.786 50.786 50.786

40 40 40

14.9 Fuel) 14.9 14.9 14.9

56.052

40

56.052 56.052 56.052

44 44 44

0.0155 0.005425

0.9835 0.993875

45 46

0.00675 0.0045

0.99275 0.995

31 31

VF = volume flow rate (L/min). D

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Figure 3. Comparisons between calculated and measured concentrations of the main species in the C2H4 premixed flame38 (1 atm and C2H4/O2/Ar = 21.3/20.9/57.8%).

Figure 4. Comparisons between calculated and measured concentrations of initial benzene and key PAHs in the C2H4 premixed flame38 (1 atm and C2H4/O2/Ar = 21.3/20.9/57.8%).

Pfahl et al.39 investigated the ignition delay time of the C10H22/air mixture at a pressure of 13 ± 1.5 bar for equivalence

temperature gradient is reached in zero-dimensional, homogeneous calculations. E

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Figure 5. Comparisons between calculated and measured ignition delay times of C10H22/air mixtures: (a) P = 13 ± 1.5 bar and Φ = 0.5, 1.0, and 2.0 and (b) P = 50 ± 3 bar and Φ = 0.67, 1.0, and 2.0. Symbols were measured results reported by Pfahl et al.,39 while lines were calculated results for C10H22/air mixtures.

±20%,40 and for most of the conditions, the calculated results fall within ±20%. Wang et al.40 also investigated the ignition delay time of 70% C10H22 and 30% C10H7CH3 by mole fraction at 10 and 40 bar for an equivalence ratio of 1.0, within the temperature range of 848−1349 K. Figure 8 displays the comparisons between the measured data and model computations. It shows that the simulated ignition delay times by the model at a high temperature (T > 1000 K) agree well with experimental data. However, in comparison to the measured results of Wang et al.,40 the NTC regime of the calculated results displays an earlier beginning at 40 bar and the experimental ignition delay time is almost 2 times longer than that of the calculated result. This is mainly due to the following two reasons. First, the uncertainty of pressure is ±2 bar. Second, the shorter ignition delay time is mainly caused by the underestimation of the ignition delay time for α-methylnaphthalene at low to medium temperature. Gowdagiri et al.44 measured the ignition delay time of two different diesel fuels, F-76 (cetane number of 48.8), a militarygrade petroleum-derived diesel fuel, and HRD-76 (cetane number of 78.5), an alternative diesel fuel from hydroprocessed algal oil, at 10 and 20 bar for equivalence ratios of 0.5 and 1.0, within the temperature range of 671−1266 K. In this work, 70% C10H22 and 30% C10H7CH3 by mole fraction were prescribed to represent F-76 diesel fuel to calculate its ignition delay time. On the one hand, as described in the Introduction, recent studies indicate that real diesel fuel contains nearly 30% aromatic hydrocarbon in addition to n-alkanes. On the other hand, in the integrated development on engine action, a diesel surrogate fuel named IDEA was proposed in Europe, which consists of 70% n-decane and 30% α-methylnaphthalene by mole fraction.23 Figure 9 shows the comparison between the model computations of surrogate fuel and experimental data of F-76 diesel fuel from Gowdagiri et al.44 As the situation above, the ignition delay time at a high temperature (T > 1000 K) at various pressures and equivalence ratios is accurately reproduced by the mechanism. However, the model prediction is slightly lower than the measured results for an equivalence ratio of 1.0 at 20 bar. The main reason is the same as the case of 70% n-decane and 30% α-methylnaphthalene described above. Validation in a JSR. Dagaut et al.45,46 measured the mole fraction of different species for n-decane diluted within nitrogen (N2) in a JSR at different pressures, initial temperatures, equivalence ratios, and residence times. Figure 10 displays the comparison of measured data and simulation results at the same condition. Figure 10a represents the results of 0.1% n-decane in mole fraction diluted in N2 at a pressure of 10 bar, a residence

ratios of 0.5, 1.0, and 2.0 and at a pressure of 50 ± 3 bar for equivalence ratios of 0.67, 1.0, and 2.0, within the temperature range of 650−1300 K. At different initial temperatures, different ignition modes were observed in the experiment. Figure 5 displays the comparisons of the ignition delay time calculated by the model developed in this work to the experimental data from the literature.39 It can be seen from the figure that the simulated results fit well with experimental data. The negative temperature coefficient (NTC) phenomenon is accurately reproduced by this model at 13 bar. Figure 6 displays the normalized sensitivity analysis for the ndecane/air mixture of 800, 900, and 1000 K at 40 bar, for

Figure 6. Sensitivity analysis for ignition delay times of C10H22/air mixtures under different initial temperatures. Φ = 1.0, P = 40 bar, and T = 800, 900, and 1000 K.

equivalence ratio of 1.0. It shows that the dominant reaction at 800, 900, and 1000 K is reactions R8, R10, and R4, respectively. That indicates that the key reaction at a low temperature (T < 900 K) is the oxygen addition of hydroperoxydecyl (C10H20OOH), which agrees well with the theory of lowtemperature combustion proposed by Sarathy et al.43 Wang et al.40 investigated the ignition delay time of the C10H7CH3/air mixture at 10 and 40 bar for equivalence ratios of 0.5, 1.0 and 1.5, within the temperature range of 1032−1445 K. Figure 7 shows the comparison of predicted ignition delay times by the current model to experimental data. The results fit well at 10 bar, while the ignition delay was underestimated by almost a factor of 2 when the equivalence ratio is 1.0 and 1.5 at 40 bar. However, the uncertainty of this measurements is F

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Figure 7. Comparisons between calculated and measured ignition delay times of C10H7CH3/air mixtures: (a) P = 10 and 40 bar and Φ = 0.5, (b) P = 10 and 40 bar and Φ = 1.0, and (c) P = 10 and 40 bar and Φ = 1.5. Symbols were measured results reported by Wang et al.,40 while lines were calculated results for C10H7CH3/air mixtures.

Figure 9. Comparisons between calculated and experimental ignition delay times of diesel and its surrogate fuel. Symbols were measured results reported by Gowdagiri et al.,44 while lines were calculated results for 70% C10H22/30% C10H7CH3/air mixtures.

C10H22 is lower than the model prediction. That is mainly due to the lower oxidation rate of C10H22 in the model above 800 K compared to the experimental data. Figure 10b represents the result of 700 ppm of n-decane in mole fraction at atmospheric pressure, for a residence time of 0.07 s and an equivalence ratio of 2.0 in the temperature range of 900−1300 K.46 Comparisons of the major species concentrations were carried on C10H22, CO, CO2, H2, and C2H4 between the simulated and experimental data. Similar to Figure 10a, the calculated result of the C10H22 concentration is higher than experimental data, while the predicted concentration of CO is lower than experimental data. In addition, the calculated mole fraction of CO2 is slightly lower than the measured result in the whole temperature range, as suggested by Mehl et al.;47 one of the possible reasons is that the kinetic mechanism cannot fully describe the partially oxidized products. Mati et al.31 investigated the chemical species concentration in a JSR of 0.05% α-methylnaphthalene diluted in N2 at 10 bar, for a residence time of 0.5 s and equivalence ratios of 1.0 and 1.5 in the temperature range of 800−1200 K. Figure 11 shows the comparison of mole fractions of C10H7CH3, O2, CO, and CO2 between the simulated and experimental data under different initial temperatures. It shows that the simulated concentrations of C10H7CH3 and O2 agree well with the measured results, while the predicted concentrations of CO and CO2 are lower than the measured data, which also occurs in the work of Mati et al.31 by the detailed α-methylnaphthalene oxidation mechanism. That indicates that the kinetic model of α-methylnaphthalene still needs to be improved.

time of 1.0 s, and an equivalence ratio of 1.0 within the initial temperature range of 550−1150 K.45 It shows that the mole fraction of major species, C10H22, CO, CO2, H2, and C2H2, match well with the measured result in magnitude. Above 800 K, the measured concentration of CO is almost 2 times higher than the simulated result, while the measured concentration of

VALIDATION IN AN OPTICAL DIESEL ENGINE At last, the reduced n-decane/α-methylnaphthalene/PAH mechanism was coupled with a three-dimensional turbulent model to calculate the combustion and soot processes in a diesel engine. The chemical chemistry and flow characteristics

Figure 8. Comparisons between calculated and measured ignition delay times of 70% C10H22/30% C10H7CH3/air mixtures, with Φ = 1.0 and P = 10 and 40 bar. Symbols were measured results reported by Wang et al.,40 while lines were calculated results.

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Figure 10. n-Decane oxidation in a JSR. Initial conditions: (a) P = 10 bar, Φ = 1.0, 0.1% n-decane, 1.55% O2, 98.35% N2, and τ = 1.0 s. Symbols were the measured data reported by Dagaut et al.,45 while lines were the calculated results. (b) P = 1 bar, Φ = 2.0, 0.07% n-decane, 1.085% O2, 98.845% N2, and τ = 0.07 s. Symbols were the measured results reported by Dagaut et al.,46 while lines were the calculated results.

Figure 11. α-Methylnaphthalene oxidation in a JSR. Initial conditions: (a) P = 10 bar, Φ = 1.0, 0.05% α-methylnaphthalene, 0.675% O2, 99.275% N2, and τ = 0.5 s and (b) P = 10 bar, Φ = 1.5, 0.05% α-methylnaphthalene, 0.45% O2, 99.50% N2, and τ = 0.5 s. Symbols were the measured results reported by Mati et al.,31 while lines were the calculated results.

Table 5. Sandia/Cummins Engine Specifications53 parameter

value

bore × stroke (mm) connecting rod length (mm) displacement (L) simulated compression ratio nozzle diameter of injector (mm) number of holes fuel type

139.7 × 152.4 304.8 2.34 16:1 0.196 8 number 2 diesel

Table 6. Operating Conditions of the Sandia/Cummins Engine53 parameter engine speed (rpm) rail pressure (MPa) injection timing (deg CA ATDC) injection duration (deg CA) temperature of intake air (K) pressure of intake air (MPa) fuel injection mass (mg/cycle)

H-T diffusion combustion

H-T premixed combustion

1200 120 −7

1200 120 −5

10 384 0.233 61

10 320 0.192 61

Figure 12. Computational grids at TDC (with 13 527 cells) employed in the calculation of the optical diesel engine.53

a phenomenological soot model developed by Vishwanathan et al.41 Singh et al.51 investigated the ignition, combustion, and emission characteristics of a single-cylinder, direct-injection, four-stroke optical diesel engine based on a Cummins N-14 production engine. Detailed parameters of this diesel engine and operating condition were specified in Tables 5 and 6, respectively. Two different conditions were listed as the computational cases in this work. The first condition is typical high-temperature combustion in a diesel engine that has a shorter ignition delay time, where diffusion combustion dominates the whole process. While in the other condition,

was calculated by KIVA-3V,28 coupled with the kinetics code CHEMKIN-III.48 Because the main work of this study is the development of the gas chemical reaction mechanism of surrogate fuel with PAH formation, other models used in this calculation were kept the same as the original KIVA-3V code,28 such as the RNG k−ε turbulence model49 and KH-RT breakup model.50 The in-cylinder soot volume fraction was calculated by H

DOI: 10.1021/acs.energyfuels.6b02186 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 13. Comparisons between calculated and measured pressure in cylinder and apparent HRR. Measured results were taken from Singh et al.53

Figure 14. Comparisons between calculated and measured normalized soot fractions. Measured results were taken from Singh et al.53

which will further affect the calculated atomization and evaporation processes after fuel injection. Second, in this investigation, the measured data of Singh et al.51 was used for the mechanism validation; nevertheless, only the engine specifications and initial conditions were provided in the literature, while the fuel injection rate and detailed information on the spray are not available for the spray model calibration. Thus, some parameters in the physical process of the injected fuel were estimated by experience. These uncertainties eventually lead to faster atomization and evaporation processes, and more premixed gas mixture was formed before ignition. Therefore, the premixed combustion dominates the whole combustion process in the calculation, while diffusion combustion is not significant. Third, as stated above, the mechanism developed in this work needs to be further improved at high pressure mainly as a result of the lack of kinetic information available in the literature for α-methylnaphthalene oxidation and pyrolysis.40 The comparison of experimental and simulated normalized soot fractions is displayed in Figure 14. It shows that the simulated soot formation and oxidation follow the same trend as that of the experimental data. However, a second peak occurs near 11° CA ATDC at the measured soot volume concentration in the high-temperature (H-T) diffusion combustion conditions. This mainly due to the temperature changes of soot particles when those particles collide with the surface of the piston; besides, this may also result from the convection of particles from squish to piston bowl.52,53 Apart from this difference, the present model can well-predict the soot trend, which also indicates that the prediction of soot precursors, surface growth species, oxidant (O2 and OH), and the in-cylinder temperature was accurately reproduced by the model.

the intake temperature was reduced to yield a longer ignition delay. This condition has a large premixed burn, with relatively little diffusion combustion. Commercial number 2 diesel fuel (cetane number of 46) was used in this investigation. nTetradecane was employed to calculate the physical process of number 2 diesel, such as atomization and evaporation.41 A total of 70% n-decane and 30% α-methylnaphthalene by mole fraction were employed to calculate the kinetic reactions in the combustion process. Because there are eight injection holes distributed evenly in the injector, a 45° sector mesh (as shown in Figure 12) with assumed periodic boundary conditions was used in the simulation. The mesh contains 13 527 cells at the top dead center (TDC) with 2.0 × 2.0 × 2.0 mm cell size. According to the literature,51 the computations were started from intake valve closure [IVC = −165° after top dead center (ATDC)] with an assumed uniform mixture distribution in the cylinder and ended at 125° ATDC. Initial pressure, initial temperature, and injection rate shape were taken from experimental data in the literature.51 A parallel KIVA-3V code was used in the calculation, and it takes 6 h to finish one case on 12 central processing units (CPUs). Figure 13 displays the comparisons for in-cylinder pressure and the apparent heat release rate (HRR) profile between the calculated results and measured data. It shows that the simulated pressure and HRR profile agree well with the measured data. In the high-temperature diffusion combustion case, pressure peak is slightly lower than the experimental data; however, the difference is within 5%. The crank angle (CA) phase for the measured peak of pressure and HRR profile is close to that of the simulation result. As shown in Figure 13, the two-staged combustion process of the case with start of injection (SOI) = −7° CA ATDC was not well captured by the present model. This discrepancy mainly results from the following three causes. First, referred to the literature,41 ntetradecane was employed to calculate the physical process of number 2 diesel. However, the physical parameter of ntetradecane is slightly different from that of number 2 diesel,

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CONCLUSION In this work, a reduced chemical kinetic mechanism with 77 species and 287 reactions for a diesel surrogate that contains nI

DOI: 10.1021/acs.energyfuels.6b02186 Energy Fuels XXXX, XXX, XXX−XXX

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decane and α-methylnaphthalene binary fuel and the formation of PAHs was developed by reaction pathway analysis, DRGEP, CSP method, and DSA, sequentially. This newly developed mechanism was validated by both fundamental reactors and a diesel engine. The fundamental experimental data employed to validate the mechanism includes the mole fraction of major species and key PAH species in the ethylene premixed flame, ignition delay times of pure and mixed fuel in shock tubes, and concentrations of major species in a JSR. Both the fundamental modeling and engine modeling agree well with the data from the literature. Finally, the reduced n-decane/α-methylnaphthalene/PAH mechanism was used to calculate the ignition, combustion, and emission characteristics of the diesel engine. The predicted pressure, apparent HRR, and soot trend agree well with the experimental results, which indicates that the newly develop mechanism is accurate in the prediction of combustion and soot emission in engines.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b02186. Mechanism (TXT)

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AUTHOR INFORMATION

Corresponding Authors

*Telephone: +86-27-87543458. Fax: +86-27-87540724. E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support of this research provided by the National Natural Science Foundation of China (NSFC) through its Project 51576083.

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NOMENCLATURE ATDC = after top dead center CA = crank angle CBR = calorimetric bomb reactor CFD = computational fluid dynamics CSP = computational singular perturbation DRG = directed relation graph DRGEP = directed relation graph with error propagation DSA = direct sensitivity analysis HCCI = homogeneous charge compression ignition HRR = heat release rate H-T = high temperature IDEA = integrated development on engine action JSR = jet-stirred reactor NTC = negative temperature coefficient PAH = polycyclic aromatic hydrocarbon PCA = principal component analysis PCCI = premixed charge compression ignition QSSA = quasi-steady-state index RCCI = reactivity controlled compression ignition ROP = rate of production rpm = revolutions per minute WSR = well-stirred reactor J

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K

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