The effects of naphthalene-addition to alkylbenzenes on soot formation

Combustion and Flame 215 (2020) 169–183

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The effects of naphthalene-addition to alkylbenzenes on soot formation Carson Chu, Murray J. Thomson∗ Department of Mechanical and Industrial Engineering, University of Toronto, 5 Kings College Road Toronto, ON M5S 3G8, Canada

a r t i c l e

i n f o

Article history: Received 29 July 2019 Revised 21 August 2019 Accepted 21 January 2020

Keywords: Soot formation Naphthalene Alkylbenzenes Aromatics Polycyclic aromatic hydrocarbons PAH formation

a b s t r a c t Naphthalene and alkylbenzenes are present in practical transportation fuels. This study investigates the impact of naphthalene addition to alkylbenzenes on soot formation. Naphthalene was added to two kinds of alkylbenzenes, namely, 1,2,4-trimethylbenzene and n-propylbenzene. Because they are isomers, the effect of molecular structure is isolated. The sooting characteristics of naphthalene-added alkylbenzenes are compared to pure alkylbenzenes in laminar coflow flames. The fuel and carbon mass flow rates were kept constant for all cases. The soot volume fraction measurements show that n-propylbenzene is sensitive to naphthalene addition. In contrast, no significant changes in soot volume fraction were observed for the 1,2,4-trimethylbenzene flames. A slight increase in primary particle diameter was observed for both naphthalene-added n-propylbenzene and 1,2,4-trimethylbenzene, suggesting that naphthalene promotes soot surface growth. The calculated number densities show that naphthalene addition promotes soot nucleation for n-propylbenzene but not for 1,2,4-trimethylbenzene. The flames were simulated with the CoFlame code with the CRECK mechanism. The model partially agrees with the experimental results, as the model agrees with the case of 1,2,4-trimethylbenzene but underestimates the effect of naphthalene addition to n-propylbenzene. More understanding of the PAH formation beyond naphthalene is required. In conclusion, the study suggests that the effect of naphthalene addition on soot formation is fuel-type dependent. © 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction Soot is a by-product of incomplete combustion and has a negative impact on the environment and health. Soot is considered the second largest contributor to climate change [1,2] because it is a strong absorbent of solar radiation. In addition, as a carcinogen to humans, soot is associated with over 4.2 million premature deaths worldwide [3]. Approximately 16% of global anthropogenic soot emissions are estimated to originate from the transportation sector [4]. In the aviation sector, the use of non-premixed combustion combined with aromatics-containing jet fuels generates soot. The International Civil Aviation Organization (ICAO) is set to adopt a new emission scheme after 2020 [5]. To facilitate future engine development, a better understanding of how soot forms in jet fuel is necessary. Jet fuel, e.g. Jet A, is a mixture of various hydrocarbons with different chemical and physical properties. The complexity of practical fuels makes studying soot formation, which is sensitive to

∗

Corresponding author. E-mail address: [email protected] (M.J. Thomson).

aromatic contents[6], difficult to control. How the interactions between the aromatic hydrocarbons affect soot formation are unclear. Thus, to better understand the interactions, it is necessary to isolate individual fuel species. In the present study, naphthalene, 1,2,4-trimethylbenzene, and n-propylbenzene were selected as fuels. Naphthalene is a 2ring aromatic hydrocarbon and forms 3% of Jet A [7], whereas 1,2,4-trimethylbenzene and n-propylbenzene are constituents of a jet fuel surrogate [8]. Also, 1,2,4-trimethylbenzene and npropylbenzene are both alkylbenzene isomers with different chemical and physical properties. Naphthalene was doped into both alkylbenzenes in laminar coflow diffusion flames and soot characteristics were measured. The goal of the present study is to investigate how the interactions between naphthalene and alkylbenzenes impact soot formation. To assess how chemical properties affect soot formation, fuel mass flow rates were kept constant for all cases. Naphthalene is the smallest member of the polycyclic aromatic hydrocarbon (PAH) family, which is considered as the precursor of soot. Because of its direct relationship with soot formation, the presence of naphthalene is a good indicator for soot formation in flames [9]. By bypassing the formation of the first ring, the

https://doi.org/10.1016/j.combustflame.2020.01.024 0010-2180/© 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

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C. Chu and M.J. Thomson / Combustion and Flame 215 (2020) 169–183 Table 1 Flow conditions. Fuel

PBZ/124TMB (%-by-weight)

A2 (%-byweight)

Liquid (g/h)

CH4 (l/min)

Air (l/min)

124TMB 124TMB+A2 PBZ PBZ+A2

100 90 100 90

– 10 – 10

1.1 1.1 1.1 1.1

0.33 0.33 0.33 0.33

60 60 60 60

formation of naphthalene, i.e. the second ring, is the critical step for soot formation from monoaromatics [10]. This theory was further emphasized in a recent review paper [11]. To strengthen the importance of naphthalene in soot and higher PAH formation, naphthalene was experimentally found to be the most abundant low molecular weight PAH species in diffusion laminar flames[12,13]. In some modeling studies [14,15], naphthalene was chosen as the soot nucleating species to predict soot formation. Therefore, it is expected that the presence of naphthalene in fuels, with the formation of the second ring being circumvented, would promote soot formation. Because of the importance of naphthalene in soot formation, the formation of naphthalene from alkylbenzenes in flames has been previously investigated. McEnally and Pfefferle [16] found that the sooting tendencies of alkylbenzenes depend on the structure of alkylbenzene’s side chains, which determines the effectiveness of forming naphthalene. Primary alkylbenzenes with an alkane chain, such as n-propylbenzene, dissociate to substituted benzyl radicals and grow to naphthalene by propargyl addition. This sequential HACA (Hydrogen Abstraction and C2 H2 Addition) like reaction was consistent with [17–19]. Methyl-substituted alkylbenzenes, such as trimethylbenzenes, were also assessed [20]. Similar to primary alkylbenzenes, methyl-substituted alkylbenzenes form naphthalene by the recombination of benzyl and propargyl. The additional methyl groups increase soot formation as they provide more sites for H-abstraction for the benzyl formation. Therefore, for the same molecular weight, methyl-substituted alkylbenzenes have higher tendencies than primary alkylbenzenes to form naphthalene, leading to more soot formation. Another possible explanation for methyl-substituted alkylbenzenes to form more soot than primary alkylbenzenes is the differences in the reaction pathways to form higher PAHs. As suggested by [19,21], methylsubstituted alkylbenzenes may form higher PAH by faster radical recombination [22], making naphthalene formation less important. On the other hand, primary alkylbenzenes form higher PAHs via the slower sequential HACA-like mechanism [19]. As a result, methyl-substituted alkylbenzenes have higher sooting tendencies than primary alkylbenzenes. How naphthalene addition affects soot and PAH formation has been previously studied in various flame configurations but was limited to non-aromatic fuels. Naphthalene was added to an ethylene stream in a jet-stirred/plug-flow reactor system (JSR/PFR) and the soot and PAH productions were compared to pure ethylene [23]. The study shows that naphthalene addition produces fewer heavy PAHs and has no impact on the rate of soot mass growth. In addition, another study shows that naphthalene addition to ethylene promotes particle inception in a well-stirred/plug-flow reactor, as compared to benzene addition and pure ethylene [24]. For a more practical fuel, a group from the Meiji University conducted a series of studies on the effect of naphthalene addition on Fischer-Tropsch Diesel (FTD) spray flames [25–27]. Consistent with previous findings [23,24], naphthalene addition promotes soot nucleation but has no significant impact on soot growth. While naphthalene addition promotes soot aggregation by an increased number of soot particles, it also promotes soot oxidation as the change of lattice fringe separation of primary particles from up-

Carbon (mol-C/h) 0.964 0.964 0.964 0.964

stream to downstream becomes larger and faster [25,26]. For soot nanostructures, naphthalene addition produces a more tightened and homogeneous layer structure in soot primary particles [25,27]. The objective of this work is to examine how the interactions between naphthalene and alkylbenzenes affect soot formation in methane laminar diffusion flames. The soot measurements of pure alkylbenzenes were previously reported [19]. In the present study, naphthalene was separately mixed with 1,2,4-trimethylbenzene and n-propylbenzene. The sooting characteristics of the mixtures were measured in co-flow diffusion flames and were compared to their undoped counterparts. Moreover, the flames were modeled to investigate if the phenomenon could be sufficiently predicted. 2. Methodology A co-annular burner described in [28] was used to generate laminar co-flow diffusion flames. The alkylbenzenes, which are liquid fuels, were vaporized by the Bronkhorst CEM Liquid Delivery System. Methane was used as a carrier gas to carry the vaporized fuels. Four cases, i.e. pure 1,2,4-trimethylbenzene (124TMB), pure n-propylbenzene (PBZ), naphthalene-doped 1,2,4-trimethylbenzene (124TMB+A2), and naphthalene-doped n-propylbenzene (PBZ+A2). The flow conditions are summarised in Table 1. The resultant flame heights are approximately 85 mm for 124TMB and 124TMB+A2, 77 mm for PBZ+A2 and 70 mm for PBZ, as shown in Fig. 1. It should be noted that a noticeable height increase was observed when naphthalene was doped into n-propylbenzene. The modified Artium LII-200 time-resolved Laser-induced Incandescence (TiRe-LII) system with an Nd-YAG laser centred at 1064 nm was used to non-intrusively measure in situ soot volume fractions and primary particle diameters in laminar coflow flames. The system, which was originally designed for engine

Fig. 1. The appearance and the flame heights of the flames in this study.

C. Chu and M.J. Thomson / Combustion and Flame 215 (2020) 169–183

exhaust measurement, was modified to include an additional neutral-density (ND) filter and a smaller aperture. The purpose of the modification is to increase the detection range and spatial resolution. The resultant laser sheet is approximately 0.5 mm by 1.5 mm, with the viewing field of 0.5 mm-dia. A laser fluence of 2.25 mJ/mm2 was used to heat up the soot particles. The measured values are the mean values over 500 counts. The system is controlled by the Artium AIMS software. Due to the limited range of the stage that supports the burner, the maximum measurable height for LII is 70 mm HAB. In the LII technique, soot particles are heated up by laser radiation. The soot volume fraction, fv , is determined by the peak soot temperature and is expressed as [29]:

fv =

hc kλ1 T

− 1) VEXP (λ ) λ (e η (λ )wb 12π c2 hE (m ) 6 1

(1)

where wb is the equivalent width of the laser sheet, c is the speed of light, VEXP is the observed signal, η is the calibration factor, λ is the detection wavelength, h is the Planck constant, k is the Boltzmann constant, and E(m) is the complex index of refraction function of soot, which is assumed to be a constant value of 0.4. T is the soot particle surface temperature, which is obtained by twocolour pyrometry between 400 nm and 780 nm and is the function E ( mλ )

of relative E(m), E (m 1 ) . The system assumes the relative E(m) to be λ2 unity. The primary particle diameter, dp , is evaluated based on the temporal decay of the particle temperature. Assuming monodispersed primary particles, dp is expressed as [30]:

dp =

12kg τ GλMEP cp ρp

(2)

where kg is the thermal conductivity of the ambient gas, α T is the thermal accommodation coefficient, λMEP is the mean free path in the ambient gas, cp is the specific heat of soot, and ρ p is the density of soot. The time constant, τ , is the function of the difference between the peak soot and the ambient flame temperatures, T, t defined as T = Ae− τ , where A is a constant fit to the temperature data. The geometry-dependent heat transfer coefficient is defined as [31,32]:

G=

8f

αT (γ + 1 )

(3)

where f is Eucken factor for monoatomic species equal to 5/2 [33,34], γ is the ratio of specific heat capacity, which is 1.4 for air. The expression of G is for the heat conduction from a single sphere, i.e. an aerosol particle [32]. In the present experiment, dp was obtained from the Artium AIMS software, which employs the above equations (Eq. (2) and (3)), with the input of the a priori flame temperatures. The soot temperatures were obtained by the Spectral Soot Emission (SSE) technique [35], which derives soot temperatures from soot particle radiation. SSE measurements agree well with gas temperature measurements obtained from Coherent Anti-Stokes Raman (CARS) [35,36], which in turn also agree well with the Rapid Thermocouple Insertion (RTI) in the high soot containing regions, e.g. annular [28,37]. However, SSE becomes unreliable in the low soot containing regions [28,35], where immature soot is present [38], e.g. the center regions at lower HABs. Ref. [28] suggested that SSE provides reliable measurements in the regions where fv is greater than 0.5 ppm. The line-of-sight spectral radiance was captured by a Princeton Instrument SP2105i spectrometer. The size of the entrance slit of the spectrometer is 100 μm. The spectrometer is attached to a PIXIS100 digital camera, which has 1340 × 100 imaging arrays with 20 μm × 20 μm pixel size.

171

The setting allows 100 plots of spectral radiance to be taken in every snapshot. An optical assembly, consisting of an achromatic lens with a 100 mm focal length and an iris with a 2 mm aperture, is placed between the burner and the spectrometer. The resulting equivalent focal number is f/42. An average of intensity is taken over 10 to 20 frames, subject to signal stability. The exposure time for each frame is 100 ms. Both LII and SSE share the same burner. The detailed setup and evaluation methods of the LII and SSE were described in [19,28]. Assuming the flame is optically thin, the flame temperature, TF , can be derived by the following equation:

ln

 E (m )  hc = + c1 λkTF (r ) Gλ (r j )λ6

(4)

where Gλ (rj ) is the local spectral emission property field, recovered by the Abel Inversion and c1 consists of all variables that are constant with λ. The Abel Inversion is computed using the Nestor–Olsen method [39]. Eq. (4) has a form of the linear equation y = ax + b, where a, the slope of the function, is represented by kThc(r ) , y is represented by the function on the left-hand side of F

Eq. (4), and x is represented by 1/λ. Rearranging the expression of a, TF can be expressed as TF (r ) = a(hc , where a is determined by r )k curve fitting the function on the left-hand side of Eq. (4) plotted against 1/λ. Both the LII and SSE require E(m) for the computation. The values were set to be constant and were valued at 0.4 for both techniques. The choice of E(m) is debatable in the combustion community as it is influenced by flame location, flame configuration, particle size, soot maturity and detection wavelength [40–42]. The justification and the uncertainties of the choice of E(m) were discussed in [19]. Another uncertainty is α T , which is used to evaluate primary particle diameters. This parameter specifies the energy transferred between a particle surface and a colliding gas molecule. In our experiment, α T is treated as a constant and has a value of 0.26 [43]. However, this assumption may not be sufficient as the value varies with a number of factors like soot maturity, particle composition, flame configuration, and soot aggregation [40]. The impact of α T on soot particle sizing is to be discussed in Section 3.5. The CoFlame code [44] with updates made in a recent numerical soot study [17] was used to simulate the cases. The employed chemical kinetic mechanism was the CRECK mechanism, which was developed by Ranzi et al [45]. The full CRECK mechanism, which has 200 species and 6907 reactions, was reduced to 115 species and 2456 reactions using the DRGESPA method with Reaction Workbench’s Closed Homogeneous Reactor model. The validation of the mechanism was described in [19]. To model soot formation, several assumptions are made in the CoFlame code. First, the soot aerosol dynamics is described by a fixed sectional method with 35 discrete sections for each soot aggregate number density and primary particle number density. Second, soot nucleation is modeled based on the collision and subsequent dimerization of two gaseous soot precursors in the freemolecular regime. Third, soot surface growth is modeled by the HACA mechanism and PAH addition. A detailed description of the governing equations, sectional soot model, boundary conditions, and solution methodology can be found in [44]. For radiation heat transfer, the code uses the discrete-ordinate method (DOM) and the statistical narrow-band correlated-k (SNBCK) method to solve the radiation transfer equation (RTE). The code considers CO, CO2 , H2 O and soot as radiation sources. Consistent with previous work [17], pyrene (C16 H10 , A4), BIN1A (C20 H16 ), and BIN1B (C20 H10 ) were considered as the gaseous soot precursors. The computational domain is 12.29 cm in the axial direction by 4.75 cm in the radial direction, and is divided into

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192(z) × 88(r) non-uniform control volumes. The uncertainties of the applied models are addressed in Section 4.7. 3. Experimental results 3.1. Flame length and soot propensity Figure 1 compares the visible flame lengths, Lf , of the baseline (CH4 ), 124TMB, 124TMB+A2, PBZ, and PBZ+A2 flames. The baseline flame (pure CH4 ) is around 60 mm high. After adding liquid fuels (124TMB, 124TMB+A2, PBZ, and PBZ+A2), the flame lengths increase accordingly. For the pure alkylbenzene cases, 124TMB exhibits a longer flame length than PBZ (~85 mm vs ~70 mm). Apparently, naphthalene addition has a more pronounced impact on PBZ than 124TMB, as the PBZ+A2 flame (~77 mm) is noticeably longer than the PBZ flame (~70 mm) and the 124TMB and 124TMB+A2 flames have similar heights. The flame length measurement can serve as an indicator of overall soot propensity [46–48]. Thus, the descending order of soot propensity is 124TMB ≈ 124TMB+A2 > PBZ+A2 > PBZ. The order is consistent with the centerline soot volume fraction measurement as shown in Fig. 2. Furthermore, 124TMB exhibits a longer flame height than PBZ. This is consistent with ref. [48], which compared 135TMB with PBZ in a co-flow burner. The centerline soot volume fraction measurements, shown in Fig. 2, provide a general view of soot production of all four cases. Trendlines were added to help visualize the trends of soot volume fractions. All four cases exhibit a dome shape, with soot volume fractions rising from 30 mm HAB (height above burner), peaking at between 50 mm HAB and 60 mm HAB, and decreasing after the peak. The differences among all four cases are minimal at lower HABs but become wider at upper HABs. Both 124TMB and 124TMB+A2 have the highest and similar peak soot. PBZ+A2 has lower peak soot and PBZ has the lowest peak soot. Apparently, naphthalene addition has a more pronounced effect on PBZ, than on 124TMB, as the peak soot volume fractions increase by ~ 25% for PBZ+A2 as compared to PBZ. One should be aware that LII only provides the measurement of mature soot and cannot measure

Fig. 2. Centerline soot volume fractions (fv ) for naphthalene-added 1,2,4trimethylbenzene (124TMB+A2), naphthalene-added n-propylbenzene (PBZ+A2), 1,2,4-trimethylbenzene (124TMB), and n-propylbenzene (PBZ).

nascent soot [49]. Therefore, at 30 mm HAB, where nascent soot is likely to be dominant [13], minimal soot was reported. The same measurements were plotted against the normalized flame heights, which are defined as Z = HAB/Lf . The plot is shown in the Supplementary Materials. The effect of naphthalene addition is more obvious as PBZ+A2 shows a sign of early soot nucleation as compared to PBZ. 3.2. Soot temperature Figure 3 shows the radial soot temperature profiles at 40, 50, 60, and 70 mm HABs. At 40 mm HAB, all cases exhibit higher temperatures in the wing than on the centerline. The differences are approximately 200 K. The temperature differences between the wing and the centerline reduce at higher HABs. A similar observation was previously reported in [50]. The soot temperatures of all four cases overlap at lower heights but gradually depart at upper heights. This can be explained by the difference in soot production, which is a source of radiation and consequently lower soot temperatures. The PBZ+A2 case has lower soot temperatures than PBZ because the former case has higher soot concentrations than the latter case. The soot temperature profiles are similar for both 124TMB+A2 and 124TMB because both cases have similar soot production. 3.3. Soot volume fraction and primary particle diameter Figure 4 shows the radial soot volume fraction profiles at 40, 50, 60, and 70 mm HABs. For all cases, peak soot appears in the wing regions at 40 mm HAB and gradually moves toward the center at higher HABs. The phenomenon can be explained by the higher flame temperatures (Fig. 3) and longer residence time at the wing [51] as compared to the centerline. Soot formation is highly associated with temperature [52]. While the higher flame temperatures in the wing correlate with early soot nucleation and carbonization [50], a longer residence time along the wing provides more time for soot surface growth [51], resulting in annular profiles at 40 mm HAB. As the centerline flame temperatures gradually increase with the HAB (Fig. 3), the peak soot moves toward the centerline. The 124TMB produces more soot than PBZ, possibly due to different tendencies to form the soot nucleating species. The reaction pathway analysis performed in [19] suggests that 124TMB forms the soot nucleating species, i.e. pyrene, via efficient radical recombination [22]. In contrast, PBZ forms pyrene through the conventional, but less efficient, HACA mechanism. As a result, more soot nucleates in the 124TMB flame. When comparing the pure alkylbenzene cases (124TMB, PBZ) to the naphthalene addition cases (124TMB+A2, PBZ+A2), PBZ+A2 has higher soot volume fractions than PBZ by approximately 25%, whereas 124TMB+A2 exhibits slightly lower soot volume fractions as compared to 124TMB. This observation implies that naphthalene addition has a more significant effect on PBZ than on 124TMB. Figure 5 shows the radial primary particle diameter profiles at all HABs. The profiles are considered relatively flat, with no pronounced difference between the wing and the core regions. At 40 mm HAB, the primary particle sizes in the wing are slightly larger than those on the centerline. At 50 mm HAB, the primary particle sizes on both centerline and wing are almost the same. At higher HABs, the primary particle sizes are larger on the centerline than those in the wing. This is consistent with [53], which explains that the larger primary particles sizes in the wing at a lower HAB is possibly due to higher flame temperatures in the wing, which promotes fuel pyrolysis [54]. As the soot particles move up, the primary particle size decreases due to oxidation [55].

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Fig. 3. Radial soot temperatures for naphthalene-added 1,2,4-trimethylbenzene (124TMB+A2) and n-propylbenzene (PBZ+A2) and pure 1,2,4-trimethylbenzene (124TMB) and n-propylbenzene (PBZ) [19]. The error bars represent the root-mean-square-error of curve-fitting on the raw measurements and are only shown on the 124TMB+A2 profiles for better readability. The raw profiles for 124TMB+A2 are presented in the Supplementary Materials.

At lower HABs, the naphthalene addition cases (124TMB+A2 and PBZ+A2) have slightly higher values than the pure alkylbenzene cases (124TMB and PBZ). This suggests that, at first glance, naphthalene addition has a minor impact on soot surface growth. This coincides with previous studies [25–27], which assessed the impact of naphthalene addition on a non-aromatic fuel. The larger primary particles for the naphthalene addition cases suggest naphthalene addition promotes soot surface growth [25]. However, such growth is cancelled out by tightening nanostructures, i.e. carbonization, resulting a minor change in primary particle size [25]. The enhancement of soot surface growth could be due to the promotion of PAH-soot surface addition, as proposed by Frenklach and Wang [56]. The involvement of smaller PAH, e.g. A2, in this process should be investigated in future studies. As we move to higher HABs, the differences between PBZ and PBZ+A2 become more distinguishable, suggesting these two flames exhibited different oxidation rates. It is possible that PBZ and PBZ+A2 have different soot nanostructures, with PBZ+A2 having more carbonized nanostructures. Improved soot carbonization also retards the burnout rate of the particle [57], leading to wider gaps

between PBZ and PBZ+A2 at higher HABs. To confirm this speculation, TEM imaging of soot primary particles should be carried out in future work. 3.4. The impact of naphthalene addition The above sections demonstrate how naphthalene addition affects the soot formation of pure alkylbenzenes, in terms of flame length, soot temperature, soot volume fraction, and primary particle diameter. The results suggest that the effect of naphthalene addition is species-dependent, as, clearly, naphthalene addition affects PBZ considerably more than 124TMB. PBZ+A2 has obviously higher soot volume fractions than PBZ whereas 124TMB and 124TMB+A2 have similar soot volume fraction profiles. Meanwhile, naphthalene addition only slightly increases primary particle diameters. The next question would be what contributes to the changes in soot volume fraction. In our experiment, naphthalene addition to PBZ causes ~ 6 % increase in average primary particle diameter, dp, avg (the average of all measured primary particle diameters), and ~ 25 % increase

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Fig. 4. Radial soot volume fractions (fv ) for naphthalene-added 1,2,4-trimethylbenzene (124TMB+A2), naphthalene-added n-propylbenzene (PBZ+A2), 1,2,4-trimethylbenzene (124TMB), and n-propylbenzene (PBZ) [19]. The error bars represent one standard deviation over ~ 500 counts and are only shown on the 124TMB+A2 profiles for better readability. Table 2 The summary of the effect of naphthalene (A2) addition to 124TMB and PBZ. Fuel

Change in dp, avg (Experimental)

Change in fv, avg (Experimental)

124TMB+A2 PBZ+A2

~ +4% ~ +6%

~ -6% ~ +25%

a

Change in fv, avg (Calculated)a ~ +12% ~ +19%

Calculated using Eq. (5), assuming constant np.

in average soot volume fraction, fv, avg (the average of all measured soot volume fractions), whereas naphthalene addition to 124TMB causes ~ 4 % increase in average primary particle diameter but ~ 6 % decrease in average soot volume fraction. The changes are summarized in Table 2. An analysis using the following equation can provide some insight in how soot surface growth contributes to soot formation:

fv =

np dp3 π 6

(5)

where np is number density, which can serve as an indicator of soot nucleation. Equation (5) relates soot volume fraction with primary particle diameter and number density. The equation assumes monodispersed spherical soot primary particles with no aggregation. The justification of the monodispersity assumption is explained in the Supplementary Materials. This approach, with the use of LII-yield primary particle diameters, was first demonstrated in [58]. From Eq. 5, fv ∝ dp3 . Thus, it is expected that a slight fluctuation in primary particle diameter will amplify in the derivation of soot volume fraction. The following analysis assumes a constant number density. For the PBZ case, a ~ 6 % increase in average primary particle diameter yields a ~ 19 % increase in average soot volume fraction, which is somewhat close to the experimental result of a ~ 25 % increase in average soot volume fraction. Although this is a first estimation relating primary particle diameter to soot volume fraction, the calculation shows that the impact of soot surface growth on soot formation cannot be neglected. On the other hand, by

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175

Fig. 5. Radial soot primary particle diameters (dp ) for naphthalene-added 1,2,4-trimethylbenzene (124TMB+A2), naphthalene-added n-propylbenzene (PBZ+A2), 1,2,4trimethylbenzene (124TMB), and n-propylbenzene (PBZ) [19]. The error bars represent one standard deviation over ~ 500 counts and are only shown on the 124TMB+A2 profiles for better readability.

Table 3 Reactions and corresponding rates of production of the reaction pathways shown in Fig. 9. Reaction # R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 Overall ROPs

Reactions PBZ → CH3 + 0.67H + 0.67C6 H5 C2 H3 + 0.33C2 H4 + 0.33C6 H5 H + C6 H5 C2 H3 → C6 H6 + C2 H3 C6 H6 + C H3 → C H4 + C6 H5 C2 H2 + C6 H5 → C6 H5 C2 H + H CH3 + H5 C2 H → C9 H8 (Indene ) + H C9 H7 (Indenyl ) + C3 H3 → C12 H10 (Biphenyl ) C12 H9 (RBiphenyl ) + C2 H2 → C14 H10 C6 H5 + H5 C2 H → C14 H10 + H C6 H4 C2 H + C2 H2 → C10 H7 C10 H7 + C2 H2 → C12 H8 + H C12 H7 + C6 H6 → CH3 + 0.75 C16 H10 + 0.25BIN1B C10 H7 + C6 H6 → C16 H10 + H + H2 C14 H9 + C2 H2 → C16 H10 + H 2C9 H7 (Indenyl ) → 0.75C16 H10 + 0.25BIN1B + CH3 + H for pyrene

Rates of Production (PBZ) (mol/(cm3 s )) −10

Rates of Production (PBZ+A2) (mol/(cm3 s ))

1.12 × 10 3.95 × 10−7 1.84 × 10−6 7.09 × 10−7 1.41 × 10−7 6.18 × 10−8 1.4 × 10−8 1.24 × 10−8 3.77 × 10−7 1.42 × 10−7

1.17 × 10−10 3.73 × 10−7 1.73 × 10−6 6.41 × 10−7 1.29 × 10−7 7.8 × 10−8 1.38 × 10−8 1.06 × 10−8 3.24 × 10−7 1.79 × 10−7

1.22 × 10−8 1.07 × 10−8 9.17 × 10−8 4.4 × 10−9 1.08 × 10−7

1.53 × 10−8 1.32 × 10−8 7.75 × 10−8 6.27 × 10−9 1.12 × 10−7

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Table 4 Reactions and corresponding rates of production of the reaction pathways shown in Fig. 10. Rates of Production (124TMB+A2) (mol/(cm3 s ))

Reaction #

Reactions

Rates of Production (124TMB) (mol/(cm3 s ))

R1 R2 R3 R4 R5

124TMB + CH3 → CH4 + RC9 H11 124TMB + H → H2 + RC9 H11 124TMB → CH3 + C8 H9 (RXylene ) 124TMB + H → C8 H10 (Xylene ) + CH3 CH3 + C8 H10 (Xylene ) → CH4 + C8 H9 (RXylene )

1.86 × 10−6 1.35 × 10−6 3.32 × 10−7 8.32 × 10−7 1.24 × 10−7

1.72 × 10−6 1.23 × 10−6 3.11 × 10−7 7.57 × 10−7 1.17 × 10−7

2.97 × 10−9 1.9 × 10−7 1.16 × 10−6 1.35 × 10−6

2.59 × 10−9 1.7 × 10−7 1.03 × 10−6 1.2 × 10−6

R6 RC9 H11 + C8 H10 (Xylene ) → 0.5C14 H10 + 0.5C16 H10 + CH3 + 3H2 + 0.5C2 H4 R7 C6 H4 C2 H + C8 H9 (RXylene ) → 0.5C16 H10 + 0.5C14 H10 + CH3 + H R8 2RC9 H11 → C16 H10 + 3 H2 + 2H + C2 H4 Overall ROPs for pyrene

[13,17,59] shows that PAH production is positively correlated with number density. A detailed analysis is presented in the Supplementary Materials. As mentioned previously, both cases recorded slight increases in primary particle diameter. Therefore, it can be said that, for PBZ, there is a positive effect on both soot surface growth and soot nucleation, and, for 124TMB, although there is a positive effect on soot surface growth, it is outweighed by the negative effect on soot nucleation. Our findings partially agree with ref. [25–27], which assessed the impact of naphthalene addition on a non-aromatic fuel using TEM imaging. While observing a minor increase in primary particle diameter, noticeable increases in the number and the size of soot aggregate lead to a conclusion that naphthalene addition promotes soot formation by predominantly soot nucleation. However, our analysis suggests that although the increase in primary particle diameter is minor, it could have a significant contribution to the soot formation. Naphthalene addition does not necessarily promote soot nucleation. Its effect on soot nucleation is likely to be fuel-dependent, which also suggests that naphthalene addition may affect the formation of soot nucleating species, i.e. PAH. The discussion on PAH formation is presented in Section 4.5. Fig. 6. Radial number densities (np ) for naphthalene-added 1,2,4-trimethylbenzene (124TMB+A2), naphthalene-added n-propylbenzene (PBZ+A2), 1,2,4trimethylbenzene (124TMB), and n-propylbenzene (PBZ) [19] at 40 mm HAB. The error bars account for one standard deviation over ~ 500 counts of both dp and fv and are only shown on the 124TMB+A2 profiles for better readability.

calculation, a ~ 4 % increase in average primary particle diameter for the 124TMB case yields a ~ 12 % increase in average soot volume fraction. This does not agree with the experimental results of a ~ 6 % decrease in average soot volume fraction, suggesting that there is a reduction in primary particle number density. It is believed that the soot surface growth promoted by naphthalene addition is eventually outweighed by the reduction in soot nucleation. The comparisons are summarized in Table 2 and the statistics are summarized in the Supplementary Materials. To assess the impact of soot nucleation, the number density calculation at the soot nucleation region, i.e. 40 mm HAB, using Eq. 5 was performed and is shown in Fig 6. Naphthalene addition increases the averaged number density by ~ 11% for the PBZ case but decreases the averaged number density by ~ 13% for 124TMB. These correspond to a ~ 25 % increase and a ~ 6 % decrease in soot volume fraction for PBZ and 124TMB respectively. The change in calculated number density can be explained by the difference in PAH production in flames. An interpretation of Refs.

3.5. Uncertainties in the experiments The values reported in Figs. 4 and 5 are the mean values over 500 counts and the error bars represent one standard deviation. The repeatability of soot volume fraction and primary particle diameter measurements are within 0.03 ppm and 0.28 nm respectively. The values were evaluated by computing the 95 % confidence interval for the difference from the mean over multiple pairs using a paired t-test. As these paired measurements were taken at the same locations on different days, the values account for the variation in position, flow, and measurements. The absolute uncertainty of LII is primarily attributed to the value of E(m) for soot, which varies with multiple factors like soot maturity, wavelength, location within a flame, and flame configuration [40]. The correspondence between absolute E(m) and soot volume fraction is 1:1 while the correspondence between relative E(m) and soot volume fraction is 1:1.5. These values were verified by the sensitivity analysis in [19]. The impacts of absolute and relative E(m) were addressed in detail in the previous study [19] and will not be repeated here. The evaluation of the primary particle diameter (Eq. (2)) involves several assumptions, such as monodispersed primary particles without aggregation and constant values for parameters, including α T , f, γ , and ρ p . However, the assumptions introduce uncertainties to the evaluation. The following paragraphs address the issues of the mentioned assumptions.

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As opposed to the first assumption, i.e. monodispersed primary particles without aggregation, actual soot particles are aggregates, with size distributions for primary particles. The LII-derived primary particle diameters are regarded as “apparent” or “effective” size because it is evaluated by determining the conductive cooling rate to the surrounding gas of a particle. The primary particle diameter is inversely proportional to the specific surface area available for conductive cooling [60]. In other words, smaller particles would cool down faster and vice versa. The cooling curve is thus highly sensitive to the primary particle diameter, as shown in [61]. Assuming monodispersed primary particles with just point contact would definitely introduce uncertainties. Studies [62,63] show that the monodispersity of primary particles would overpredict primary particle diameters. A further explanation for the monodispersity assumption can be seen in the Supplementary Materials. The shielding and bridging effects, which refer to primary particles hidden from the exterior of an aggregate and the merging area among primary particles respectively, reduce the specific surface area available for heat conduction and consequently the overall cooling rate [62,64]. As a result, the LII-derived primary particle diameters are likely to be overpredicted [62,64]. However, ref. [28] suggested that at low soot concentration the shielding and bridging effects are not significant, bringing the LII-derived primary particle diameters closer to the determined to those TEM images. Furthermore, aggregates can come in different sizes, and the effect of aggregate size polydispersity can become important in the near transition regime (Knudsen number, Kn ~ 1) but unimportant in the free-molecular regime (Kn  1) [62]. In the flame environment, Kn is likely to be  1, as flame temperatures can exceed 1500 K [62]. Therefore, the effect of aggregate size polydispersity is likely to be unimportant. The temporal decay of the particle temperature relies on the conductive cooling of a soot particle, which is highly influenced by α T . The coefficient specifies the energy transferred between a particle surface and a colliding gas molecule. As mentioned previously, for the LII200 system, α T is assumed to be 0.26, which was first reported by Leory et al. [43]. The value for α T is debatable in the combustion community, as the coefficient is a function of various factors, such as soot maturity, surface property, and bath gas conditions, etc. and there is no direct measurement for soot. Representative values for α T range from 0.2 to 0.44 [40,42,65,66]. The value 0.26, which is used in the present study, was obtained for room-temperature N2 interacting with graphite at 1200 K [43]. Thus, this value is not able to reflect the actual environment in combustion. Since the value for α T varies widely and is present in the expression of G (3), the variation could affect the accuracy of primary particle sizing. From Eq. 2 and 3, the relationship between the primary particle diameter and the thermal accommodation coefficient is dp ∝α T [30]. Because young soot tends to have a higher α T [41], primary particle diameters for young soot (soot in the growth region) derived by the system are likely to be underestimated [49]. The Eucken factor, f, in Eq. (3) is a molecular model-dependent numerical factor that relates the thermal conductivity to the specific heat capacity of a gas and is assumed to be 5/2. For monatomic gases, e.g. argon, f is 5/2 [33]. For polyatomic gases, which are predominant in air, f is less than 5/2 [34]. In other LII 9γ −5 literature, an expression f = 4 was used [33,60], where γ is the specific heat ratio, and, for γ equal 1.4 (air at 300 K), f becomes 1.9. On the other hand, Ref. [30] reported that f has a value of 1.656. Both values, i.e. 1.9 and 1.656, are ~ 24% and ~ 34% lower than the assumed value, respectively. As f is inversely proportional to dp , using the value of 1.656 for f yields a ~ 50% increase in dp . The geometry-dependent heat transfer factor, G, is also a function of γ (Eq. (3)), which has an assumed value of 1.4 for air at 300 K. However, typical flame temperatures could reach > 1500 K,

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which causes γ to reduce to 1.3. As shown in Eqs. (2) and (3), dp is proportional to γ + 1. Thus, reducing the γ from 1.4 to 1.3 ( 7% of reduction) would lead to a 4% decrease in dp . Fortunately, this error is relatively insignificant as compared to the uncertainty in α T and f. Eq. (2) is also a function of the temperature-dependent specific heat of soot, cp . The LII system looks up the value from the material property table for graphite [67] according to the temperature of heated soot. The value ranges from 4.94 J/(Kmol) at 200 K to 37.0 J/(Kmol) at 50 0 0 K [67]. However, compared to the NISTJANAF databases [68], which are more up-to-date and were used in other literature [69], the database [67] has higher values at higher temperatures. At a typical LII temperature of around 40 0 0 K, the value in [67] is 3% greater than the value in [68]. As primary particle diameter is inversely proportional to the specific heat of soot (Eq. (3)), using the values from [67] would yield smaller primary particle sizes. The density of soot, ρ p , is assumed to be 1900 kg/m3 [60]. However, various values, ranging from 1800 to 2260 kg/m3 , were reported in other literature [69–72]. The value of 1900 kg/m3 can be said to be a good average of this range [31]. Furthermore, soot density varies with temperatures, as the quantity can range from 2300 kg/m3 below 1000 K to 1900 kg/m3 at around 5000 K [73]. As LII can heat soot particles to around 40 0 0K, 190 0 kg/m3 is a good estimate of the quantity. After considering the above-mentioned uncertainties, the possible resultant propagation of the uncertainties for LII-derived primary particle diameter evaluation is around 85%. The uncertainties of both primary particle diameters and soot volume fractions are carried forward to the calculation of number density in Section 3.4. Because np ∝ 13 , the possible resultant propagation of the uncerdp

tainties for number density calculation would exceed 200%. However, since all measurements were performed under the same assumptions, the propagation of the uncertainties will not affect our conclusions about the relative differences between fuels. Readers should be reminded that the present study focuses on the relative differences induced by naphthalene addition as opposed to the absolute values of the measurements. Figure 3 shows the radial soot temperatures measured by SSE at different HABs (Height Above Burner). The technique measures line-of-sight emission from soot and requires a mathematical deconvolution technique, e.g. the Abel inversion, to recover the temperature field. For a highly annular radial soot profile, e.g. 40 mm HAB, the uncertainty in the core regions is high. This is potentially caused by background radiation, which becomes significant when soot concentration, soot maturity, and soot temperatures are relatively low [28]. In a laminar diffusion flame, the emission from the core is relatively weak, as compared to the wing. Thomson [74] commented that the emission measurement in the core of the flame is dominated by the emission from the wing, where soot concentration and temperature are high along the measurement chord; the emission from the core has a little contribution. As a result, in the present study, the soot temperatures at 40 mm HAB near the centerline are likely to be overpredicted. The Abel inversion, which is a mathematical deconvolution technique, transforms a lateral spatially resolved property field into a radially resolved equivalent for a symmetrically cylindrical property field [75]. The inversion algorithm is well-known for being very sensitive to noise, the effect of which is accumulated in the radial centre of the emission source [74,75]. As a result, the output profiles tend to have oscillation, which magnifies toward the centerline. In the present study, curve-fitting was applied to smooth out the measured temperature profiles. In Fig. 3, error bars represent the root-mean-square-error of curve-fitting and are only shown on the 124TMB+A2 profiles for better readability. It can be seen that the error bars are wider at 40 and 50 mm HABs,

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but narrower on at 60 and 70 mm HABs. These are caused by more data points recorded at lower HABs, i.e. more error accumulation. To better present the oscillation, the raw and the curve fitted profiles for 124TMB+A2 are presented in the Supplementary Materials. 4. Comparison of experimental and numerical results The CoFlame code [44] was used to model the flames and the calculated results were compared to the experimental results in order to provide information, such as soot nucleation, that is difficult to obtain through experiments. The calculations for pure 124TMB and PBZ were previously reported in [19]. While radial temperature (Fig. 3), soot volume fraction (Fig. 4), and primary particle diameter (Fig. 5) profiles are provided, two-dimension (2D) contour plots of different flame quantities, such as temperatures and species concentrations (Figs. 7 and 8) are also provided to give information about the structures of the flames. Pure alkybenzenes (124TMB and PBZ) and naphthalene-added alkylbenzenes (124TMB+A2 and PBZ+A2) are shown respectively on the left and right side of each image. The uncertainties of the numerical results are also discussed in Section 4.7. 4.1. Modeled flame temperature Figure 3 shows the calculated flame temperatures and they agree with the experimental soot temperature measurements. The model successfully predicts lower flame temperatures in the core but higher flame temperatures on the wing at lower HABs. As we move to higher HABs, the differences between the wing and the core reduce and eventually even out at the tip of the flames. 4.2. Modeled soot volume fraction In general, the model underpredicts soot volume fractions by a factor of 2 for both 124TMB and PBZ cases (Fig. 4). In particular, the soot volume fractions on the centerline are lower than in the wing. There are minor differences between the naphthaleneadded cases and the pure alkylbenzene cases. For the PBZ case, PBZ and PBZ+A2 are very close, with PBZ+A2 exhibiting slightly higher soot volume fractions. The noticeable increase shown in the corresponding experimental results was not reflected in the modeling results. For the 124TMB case, 124TMB+A2 shows a slight decrease in soot volume fraction compared to 124TMB. This trend is consistent with the corresponding experimental results. The results indicate that the model cannot completely predict the effects of naphthalene addition to alkylbenzenes. The underprediction of the model is possibly due to (1) inaccuracy of PAH mechanism, which may cause lower prediction in the core [17,19], (2) the uncertainties of the current nucleation and PAH addition models and (3) the underprediction of soot surface growth. The above uncertainties are discussed in Section 4.7 and have been discussed in [19]. Moreover, readers should also be reminded that the absolute uncertainty in LII-derived soot volume fractions is large and therefore the comparison between the experimental and numerical results should be taken with caution. The comparison focuses on the relative changes in soot volume fraction after naphthalene addition, as opposed to predicting the absolute values of soot volume fraction. 4.3. Modeled primary particle diameter Figure 5 shows the calculated primary particle diameters. As opposed to the experimental results, the modeled results exhibit upward trends from the centerline toward the wing. This is possibly due to the uncertainty in the value for the empirical surface reactivity parameter α , which correlates to the HACA surface growth

Fig. 7. 2D contour plots for calculated flame temperatures (K), naphthalene (A2, mol/mol), pyrene (A4, mol/mol), hydroxyl radical (OH, mol/mol), soot volume fraction (fv , ppm), inception rate (Incept. R., g/(cm3 s)), surface HACA growth rate (HACA G. R., g/(cm3 s)), and OH oxidation rate (OH Ox. R., g/(cm3 s)) for n-propylbenzene (PBZ, left in each sub-figure) and naphthalene-added n-propylbenzene (PBZ+A2, right in each sub-figure).

rate and is set to a constant. In the present study, because soot surface growth accounts for 90% of soot mass addition in the wing, the primary particle size is highly influenced by the choice of α [76,77], which was tuned to match as much as possible the soot volume fractions at 40 mm HAB. The effects of α are discussed in Section 4.7. As mentioned in Section 3.5, since the LII-derived primary particle diameters are not actual but “apparent” or “effective” diameters and have a large propagation of uncertainties, the comparison between the experimental and modeling results should be taken with caution. The authors would like to emphasis that the comparison focuses on the relative changes in primary particle diameter after naphthalene addition, as opposed to predicting the absolute values of primary particle diameter.

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Fig. 9. Reaction pathways for n-propylbenzene (PBZ) to form pyrene. The thickness of the arrows refers to the degree of the production rate, i.e. a thicker arrow for a faster production rate. Refer to Table 3 for corresponding reactions and rates of production (ROP).

Fig. 8. 2D contour plots for calculated flame temperatures (K), naphthalene (A2, mol/mol), pyrene (A4, mol/mol), hydroxyl radical (OH, mol/mol), soot volume fraction (fv , ppm), inception rate (Incept. R., g/(cm3 s)), surface HACA growth rate (HACA G. R., g/(cm3 s)), and OH oxidation rate (OH Ox. R., g/(cm3 s)) for pure 1,2,4trimethylbenzene (124TMB, left in each sub-figure) and naphthalene-added 1,2,4trimethylbenzene (124TMB+A2, right in each sub-figure).

In general, the model can predict the correct magnitude reasonably well for both cases, except for the PBZ case at 40 mm HAB, which is moderately underpredicted. From the perspective of the effects of naphthalene addition, the model partially agrees with the experimental results. There is a small increase in primary particle diameter for the PBZ+A2. However, for the 124TMB case, the change is insignificant. 4.4. Modeled PAH species formation Because there are discrepancies between the numerical and experimental results, especially for the PBZ case, it is worth to explore the PAH formation of the PBZ case. Figure 7 shows the 2D

contour plots for naphthalene and pyrene concentrations of both PBZ and PBZ+A2. Readers should be aware that the distribution of A4 is influenced by the soot nucleation and PAH addition processes. It is not surprising to observe that PBZ+A2 has a higher naphthalene concentration than PBZ, as naphthalene is initially present in the fuel stream. However, in spite of a higher naphthalene concentration for the PBZ+A2 case, both PBZ and PBZ+A2 show roughly the same pyrene concentration. This result suggests that there are missing links between naphthalene and pyrene in the mechanism, causing the underprediction of pyrene formation and subsequently soot production for PBZ+A2, which is reflected in the 2D contours plots (Fig. 4) for soot volume fraction (fv ) and the inception rate (Incept. R.). More work on PAH mechanisms is required. Figure 7 also shows the 2D contour plots for OH concentration, the surface HACA growth rate and the OH oxidation rate. Naphthalene addition does not significantly alter these quantities. Similar results are also shown for the 124TMB case (Fig. 8). 4.5. Reaction Pathway Analysis To shed light on how naphthalene addition affects pyrene formation in the model, a reaction pathway analysis was conducted. The reaction pathways, which are derived from the CRECK mechanism, for PBZ and 124TMB to form pyrene are shown in Figs. 9 and 10 respectively. The reactions and their rates of production (ROP) are shown in Tables 3 and 4. Only representative reactions are presented in Figs. 9 and 10 and Tables 3 and 4 to facilitate the following discussion. Readers are reminded that the reaction pathway

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Fig. 10. Reaction pathways for 1,2,4-trimethylbenzene (124TMB) to form pyrene. The thickness of the arrows refers to the degree of the production rate, i.e. a thicker arrow for a faster production rate. Refer to Table 4 for corresponding reactions and rates of production (ROP).

analysis aims to compare reactions within the model but not to represent the actual reactions in the experiments. The presented reaction pathways are not validated with experimental data and therefore this section should be read with caution. In general, PBZ first dissociates into phenyl (C6 H5 ) and styrene (C6 H5 C2 H3 )(R1). Styrene further dissociates and forms phenyl through H-abstraction (R2 and R3). Acetylene (C2 H2 ) is added to phenyl to form C6 H5 C2 H. There are three alternatives pathways for C6 H5 C2 H: (1) methyl addition to form indene, (2) phenyl recombination to form C14 H10 , and (3) acetylene addition to form naphthalen-1-ide (C10 H7 ), or naphthalene radical. For the first route, indene (C9 H8 ) reacts with propargyl (C3 H3 ) to form biphenyl (C6 H5 C6 H5 ) (R6), which later forms C14 H10 via acetylene addition (R7). On the other hand, indene can also form pyrene via selfrecombination (R14). For the second route, pyrene is formed via acetylene addition to C14 H10 (R13). For the third route, naphthalen1-ide either reacts with phenyl to form pyrene (R12) or with acetylene to form acenaphthylene (R10), which further forms pyrene via acetylene addition. Naphthalene addition does not alter the reaction pathways for pyrene formation from PBZ but change the production rates of certain species in the course of reactions. To demonstrate the changes, Table 3 compares the rates of production (ROPs) of PBZ and PBZ+A2. It is expected that reactions that involve 2-ring species such as naphthalen-1-ide and indene, would be positively affected by naphthalene addition. As seen in Table 3, we observe some significant increase for PBZ+A2 in R10, R12, and R14 (by ~ 30% to 40%), where naphthalen-1-ide and indene are directly involved as reactants. The ROP of R11, which produces pyrene from the reaction between acenaphthylene and benzene, also increases by around 30%, for acenaphthylene being a major product of the PAH growth from naphthalene [78,79]. However, R13, which is the main

route for pyrene production, decreases by around 15%. Thus, the reduction in the ROP of R13 balances out the promotion in the ROPs of R10, R11, R12, and R14, making the overall ROP of pyrene production relatively unchanged as shown in Table 3. The calculations support the previous claim that there are missing links for pyrene production. The reaction pathway of pyrene formation from PBZ as shown in Fig. 9 is consistent with [17,18], which also showed PBZ exhibiting a sequential HACA-like mechanism to form pyrene. Experimentally, Liu and Tian [80] observed an abundance of naphthalene in a study of the oxidation of PBZ in a JSR. It is suggested that styrene is the key precursor of naphthalene as styrene already has C = C bond and C2 branch chain, which allow styrene to easily form naphthalene [80]. The reaction for styrene to form naphthalene is not shown in the reaction pathways, suggesting there are uncertainties in the PAH mechanism. As opposed to PBZ, as shown in Fig. 9, 124TMB (Fig. 10) forms pyrene via radical recombination [21,81]. The majority of 124TMB forms 124TMB radical RC9 H11 through H-abstraction (R1 and R2). 124TMB radical self-recombines to pyrene (R8). Alternatively, 124TMB dissociates into xylene (C8 H10 )(R4) and xylene radical (C8 H9 )(R3). Xylene either forms xylene radical through Habstraction (R5) or forms pyrene by combining with 124TMB radical (R6). Xylene radical reacts with C6 H4 C2 H to form pyrene. Since the reactions do not involve the formation of any 2-ring species, naphthalene addition does not enhance any production rate of these reactions. In contrast, by adding naphthalene, some of the fast-reacting 124TMB is replaced by naphthalene. As seen in Table 4, all reactions show reduced ROPs for 124TMB+A2, resulting in the decreased overall pyrene ROP. However, one should not underestimate the role of naphthalene as ref [21] experimentally shows trimethylbenzene forms a noticeable amount of naphthalene, A3, and other higher PAHs, and models generally underpredict naphthalene formation from trimethylbenzene pyrolysis [21,81]. However, in a recent study, Liu and Tian [80] reported that naphthalene was rarely found in 124TMB pyrolysis. Future work should involve PAH species measurement for both PBZ and 124TMB. 4.6. Findings from the modeling results The results show that not only the model underpredicts the soot quantities but also fails to fully predict how naphthalene addition affects soot formation. For the 124TMB case, the model correctly predicts the decrease in soot volume fraction but fails to predict the increase in primary particle diameter. On the other hand, for the PBZ case, while the model correctly predicts an increase in primary particle diameter, it only predicts a slight increase in soot volume fraction, as opposed to a noticeable change as observed in the experiment. The reaction pathway analysis indicates that naphthalene addition alters the ROPs of relevant reactions. However, the net change in the ROP of pyrene is insignificant. The inconsistency could be potentially due to our poor understanding of soot surface growth, soot nucleation, and PAH formation mechanisms. The next section highlights the uncertainties in the model. 4.7. Uncertainties in the model Although the CRECK mechanism was designed to model real fuels and covers several higher aromatics such as PBZ, 124TMB, and pyrene, the PAH growth mechanisms are very limited. Indeed, how PAH grows beyond the 2nd ring is still unclear to the community. The conventional HACA mechanism is thought to be not enough to explain the PAH growth beyond the 2nd ring [78,82,83] and therefore other growth schemes were proposed [82–88]. It is clear that the CRECK mechanism does not cover every proposed PAH growth

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mechanism, leading to uncertainties in intermediate production. In conclusion, the uncertainties in PAH growth mechanisms probably cause the underestimation of soot production. Other than the uncertainties in the PAH growth mechanisms, the underprediction of soot production could be caused by the assumptions in the soot nucleation models, which define the soot nucleating species and the soot nucleation mechanism. The model assumes pyrene as the soot nucleating species, which physically dimerizes to form soot nuclei. However, studies have shown that species other than pyrene can nucleate [82,86,89]. To examine the possibility of naphthalene dimerization, flames were simulated with naphthalene as the soot nucleating species and are presented in the Supplementary Materials. Apart from physical dimerization, other mechanisms that involve chemical bonding have also been suggested [90–92]. The model prediction of primary particle diameters depends on the empirical parameter α . The value of α accounts for the uncertainties in the assumptions applied to the HACA growth model, which are (1) constant number of surface reaction sites; (2) the adoption of benzene kinetics for surface reactivity; and (3) a HACAonly reaction. However, studies [57,76,93] have shown that the above assumptions are not sufficient to describe the soot surface growth process, as these assumptions vary with the soot growth process. Thus, assuming α constant across all locations and conditions would be error-prone. How α affects primary particle size profiles was explored in [19]. The assessment shows that primary particle sizes in the wing increase with α . However, primary particle sizes in the core region are insensitive to the change of α .

5. Conclusions The impact of naphthalene addition to alkylbenzenes on soot formation was investigated. The carbon flow rates were kept constant for all cases to isolate the impact of fuel structures. The soot volume fractions, soot primary particle sizes, number densities, and soot temperatures at different heights above the burner for naphthalene-added alkylbenzenes were measured and compared with pure alkylbenzenes. The conclusions are as follows: 1. Naphthalene addition has a positive impact on soot formation for PBZ, as PBZ+A2 has noticeably higher soot volume fractions than PBZ. The effect of naphthalene addition to 124TMB on soot formation is minimal as 124TMB and 124TMB+A2 have similar soot volume fractions. The effect of naphthalene addition on soot formation is fuel-dependent. Both PBZ+A2 and 124TMB+A2 have slightly larger primary particle diameters than their pure counterparts, suggesting that naphthalene promotes soot surface growth. 2. An analysis using the fv − dp − np relationship shows that naphthalene addition positively affects both soot surface growth and soot nucleation for PBZ. In contrast, although naphthalene addition promotes soot surface growth for 124TMB, it is likely that the decrease in soot nucleation counteracts the positive effect on soot surface growth, leading to an unnoticeable impact on soot formation. 3. The simulated soot quantities using CoFlame with a published chemistry mechanism do not fully agree with the experimental results. The prediction of the effect of naphthalene addition on primary particle diameter of both alkylbenzenes on primary particle diameter is acceptable. The predicted effect on soot formation of 124TMB agrees reasonably well with the experimental result. However, the model underestimates the effect of naphthalene addition on soot formation of PBZ. An assessment of calculated PAH species suggests that there could be missing pathways in the PAH growth for PBZ. Assumptions, such as soot

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nucleating species and soot nucleation mechanisms, made in the CoFlame code may also contribute to the underestimation. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment The authors acknowledge the support of the Natural Sciences and Engineering Research Council of Canada (NSERC), PGSD2534476-2019 Computations were performed on the GPC supercomputer at the SciNet HPC Consortium. SciNet is funded by: the Canada Foundation for Innovation under the auspices of Compute Canada; the Government of Ontario; Ontario Research Fund Research Excellence; and the University of Toronto. The authors would like to express their gratitude to Dr. Gregory Smallwood and Dr. Fengshan Liu of National Research Council Canada and the staff of Artium Technologies, Inc. for the discussion on the evaluation of LII primary particle diameters. The authors would also like to thank Tirthankar Mitra of the Combustion Research Lab of the University of Toronto for advice on PAH production, and for comments that greatly improved the manuscript. Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.combustflame.2020.01. 024 References [1] T.C. Bond, S.J. Doherty, D. Fahey, P. Forster, T. Berntsen, B. DeAngelo, M. Flanner, S. Ghan, B. Kärcher, D. Koch, et al., Bounding the role of black carbon in the climate system: A scientific assessment, J. Geophys. Res.: Atmosp. 118 (11) (2013) 5380–5552. [2] R.F. Service, Climate change. study fingers soot as a major player in global warming., Science 319 (5871) (2008) 1745. [3] World Health Organization, World Health Statistics 2018: Monitoring Health for the SDGs Sustain-able Development Goals, World Health Organization, 2018. [4] D. Koch, T.C. Bond, D. Streets, N. Unger, G.R. Van der Werf, Global impacts of aerosols from particular source regions and sectors, J. Geophys. Res.: Atmosp. 112 (D2) (2007). [5] International Civil Aviation Organization, ICAO Environmental Report 2016: On Board a Sustainable Future,International Civil Aviation Organization Montréal, 2016. [6] C. Saggese, A.V. Singh, X. Xue, C. Chu, M.R. Kholghy, T. Zhang, J. Camacho, J. Giaccai, J.H. Miller, M.J. Thomson, et al., The distillation curve and sooting propensity of a typical jet fuel, Fuel 235 (2019) 350–362. [7] T.M. Lovestead, J.L. Burger, N. Schneider, T.J. Bruno, Comprehensive assessment of composition and thermochemical variability by high resolution GC/QTOF-MS and the advanced distillation-curve method as a basis of comparison for reference fuel development, Energy Fuels 30 (12) (2016) 10029–10044. [8] S. Dooley, S.H. Won, J. Heyne, T.I. Farouk, Y. Ju, F.L. Dryer, K. Kumar, X. Hui, C.-J. Sung, H. Wang, et al., The experimental evaluation of a methodology for surrogate fuel formulation to emulate gas phase combustion kinetic phenomena, Combust. Flame 159 (4) (2012) 1444–1466. [9] H. Anderson, C.S. McEnally, L.D. Pfefferle, Experimental study of naphthalene formation pathways in non-premixed methane flames doped with alkylbenzenes, Proc. Combust. Inst. 28 (2) (20 0 0) 2577–2583. [10] C.S. McEnally, L.D. Pfefferle, Experimental assessment of naphthalene formation mechanisms in non-premixed flames, Combust. Sci. Technol. 128 (1–6) (1997) 257–278. [11] C.S. McEnally, L.D. Pfefferle, B. Atakan, K. Kohse-Höinghaus, Studies of aromatic hydrocarbon formation mechanisms in flames: Progress towards closing the fuel gap, Prog. Energy Combust. Sci. 32 (3) (2006) 247–294. [12] G.D.G. Peña, V. Pillai, A. Raj, J.L. Brito, Effects of fuel-bound methyl groups and fuel flow rate in the diffusion flames of aromatic fuels on the formation of volatile pahs, Combust. Flame 198 (2018) 412–427. [13] T. Mitra, T. Zhang, A.D. Sediako, M.J. Thomson, Understanding the formation and growth of polycyclic aromatic hydrocarbons (PAHS) and young soot from n-dodecane in a sooting laminar coflow diffusion flame, Combust. Flame 202 (2019) 33–42.

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