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Effect of toluene addition on the PAH formation in laminar coflow diffusion flames of n-heptane and isooctane Fushui Liu, Yang Hua, Han Wu, and Chia-fon F. Lee Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00745 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

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Title Page Title: Effect of toluene addition on the PAH formation in laminar coflow diffusion flames of n-heptane and isooctane

Authors: Fushui Liua,b, Yang Huaa, Han Wua,*, Chia-fon Leea,c

Author affiliations: a. School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China b. Beijing electric vehicle Collaborative Innovation Center, Beijing 100081, China c. Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, IL 61801, USA *

Corresponding authors:

Han Wu, Email: [email protected]; Tel: +86-10-68918581 School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China ORCID Han Wu: 0000-0002-9113-9774

ACS Paragon Plus Environment

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Effect of toluene addition on the PAH formation in laminar coflow diffusion flames of n-heptane and isooctane Fushui Liua,b, Yang Huaa, Han Wua,*, Chia-fon Leea,c a. School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China b. Beijing electric vehicle Collaborative Innovation Center, Beijing 100081, China

c. Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, IL 61801, USA (*Corresponding authors: H. Wu, E-mail: [email protected]; Tel: +86-10-68918581)

Abstract: Characterization of soot formation is becoming more important for gasoline surrogate fuels, requiring a deeper understanding of the effect of toluene ratio on PAHs formation. In this work, the different size PAHs distribution in laminar diffusion flames was measured by PLIF technique, and the chemiluminescences of OH and CH radicals were recorded by ICCD coupled with band-pass filters. The effect of toluene addition to n-heptane and isooctane on the flame and PAHs formation was comparatively studied. Then, the chemical kinetic of PAHs was analyzed using the CHEMKIN Diffusion Flame model. The experimental results showed that the cool flame area ranks as n-heptane>isooctane>toluene may due to the NTC action. The OH intensity and flame lift-off height show a monotonous increasing trend with the toluene ratio. At the same toluene ratio, the peak OH intensity and flame lift-off height of n-heptane/toluene is lower than that of isooctane/toluene. The CH intensity in n-heptane, isooctane and toluene flames shows completely different distribution characteristics. As the toluene ratio increases, the 320nm-LIF in n-heptane and isooctane flames shows a weakening monotonous increasing trend with different inflection points. The 360/400/450nm-LIF in n-heptane and isooctane flames show a non-monotonic trend with different peak points. The peak point is 50% toluene ratio in n-heptane/toluene flames, 40% in isooctane/toluene flames,


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and it corresponds exactly to the smoke point of the flame. The kinetic analysis showed that the benzyl has a significant effect on the formation of A2-A4 through the generation of C10H9, C9H7, and C14H12. A1 trend is dominated by C6H5CH3+H=A1+CH3 and A1-+CH4=>A1+CH3. The trends of A2, A3 and A4 are mainly dominated by the reactions between their own dehydrogenation groups with H2. Key words: toluene; n-heptane; isooctane; polycyclic aromatic hydrocarbons; laser-induced fluorescence 1. Introduction Currently, gasoline is the most widely used fuel for transportation. In the current situation of environmental pollution and energy crisis, gasoline engines need to further reduce emissions and improve thermal efficiency 1. In order to optimize combustion process and improve sustainability, the specific physical and chemical kinetic properties of gasoline need to be in-depth understood. However, gasoline is a complex mixture of alkanes, alkenes and aromatics, its true chemical kinetic model including all components is not available for combustion simulation

2, 3

. Therefore, the surrogate fuel is formulated to

characterize the main physicochemical properties of real gasoline 4, 5. Previously, a single component

6

or the binary mixture of n-heptane and isooctane

7

was used as

gasoline surrogate fuel. Subsequently, toluene reference fuel (TRF) primarily containing n-heptane, isooctane and toluene are widely accepted as the gasoline surrogate fuel. TRF has been formulated for characterizing the ignition delay, the laminar burning speed, and the combustion performance of gasoline with various component proportions 8-12. Recently, the ability of TRF to characterize the soot formation characteristics of gasoline has drawn more attention since that gasoline direct injection (GDI) engines are facing the soot emissions due to non-homogeneous air-fuel mixing resulted by in-cylinder mixture preparation

13-16

and emissions

regulations are becoming more stringent 17. Toluene is the representative of aromatics in gasoline, which is


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crucial for the prediction of soot formation. In gas phase kinetics, polycyclic aromatic hydrocarbons (PAHs) are considered as soot precursors 1, whose formation and growth originate in chemical reactions, starting with the fuel pyrolysis result in the formation of incipient aromatic ring, and then forming the soot inceptions via coagulation and surface growth 18, 19. In order to understand the effect of fuel chemistry on soot formation characteristics, it is necessary to study the effect of toluene on PAHs forming process in a well-controlled combustion system. Li et al. 20, 21 studied the PAHs formation in premixed flame of toluene using VUV photoionization mass spectrometry. They observed that the main degradation products of toluene are benzene (C6H6) and benzyl (C6H5CH2), pointed out that in rich toluene flames, benzene is mainly produced by the dealkylation reaction induced by H atom attack rather than free radicals recombination, and benzyl plays a significant role in the formation and growth process of PAHs. El Bakali et al.

22

investigated the pyrolysis of toluene in tolunene/methane

premixed flames, and reported that the reaction pathways of toluene degradation in tolunene/methane flames is mainly controlled by C6H5CH3 + H = C6H6 + CH3 and C6H5CH3 + H = C6H5CH2 + H2. Harris et al. 23 investigated the soot growth characteristics in toluene/ethylene premixed flames. They reported that at the same C/O ratio, the amount of soot formed at the initial stage of the particles in the toluene/ethylene mixture flame is much higher than in the ethylene flame, but the surface growth rate in the ethylene flame is higher than in the toluene/ethylene mixture flame due to the higher acetylene (C2H2) concentration. Subsequently, Choi S K et al.

24

compared the formation characteristics of PAHs and soot in 1-D

counterflow diffusion flames of ethylene mixed with toluene, n-heptane, or benzene. They observed that the PAHs and soot formed more in ethylene/toluene flame than the other mixtures, indicating that the addition of a small amount of toluene to ethylene can effectively promote the formation of PAHs and soot. They pointed out that the toluene pathway contributes to the pyrene formation that promotes the formation


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of large size PAHs. The above studies on toluene and ethylene/toluene have illustrated the major pyrolysis pathways of toluene and its important impact on PAH and soot formation. Of course, some studies have been carried out in n-heptane/isooctane/toluene mixture. Alexiou et al. 25 investigated the soot formation of toluene mixed with n-heptane or isooctane in shock tube pyrolysis. The results showed that as the n-heptane or isooctane mixing ratio increases, the peak soot concentration decreases due to the fact that the rapid decomposition of n-heptane or isooctane shifts the soot formation process from the more efficient pyrolysis of toluene to a slower path. Choi B C et al.26 investigated the soot and PAHs formation characteristics in 1-D counterflow diffusion flames of gasoline surrogate fuels. They reported that as the toluene content increases, the soot formation increases monotonically, while PAHs shows a synergistic effect. Consalvi et al.

27

studied numerically the effect of adding n-heptane/toluene or

isooctane/toluene binary mixtures to methane on the formation of PAHs and soot in laminar diffusion flames. They found as the toluene content in binary mixtures increases, soot formation increases monotonically, while the benzene and pyrene formation shows a non-monotonic trend in methane diffusion flames. Park et al.

28

investigated the compositional effect of gasoline surrogate fuels on the formation of

PAHs and soot. They reported that the composition of gasoline surrogates has a strong effect on the formation of PAHs and soot, and the reactions involving benzyl groups play an important role in the PAHs formation of n-heptane/isooctane/toluene mixtures. The aforementioned studies have shown that the composition of TRF (especially toluene) has a strong effect on the formation of PAHs and soot

28

, and PAHs shows a synergistic effect with the increasing

toluene ratio 26, 27. However, the difference of PAHs trend and flame structure between n-heptane and isooctane for the toluene addition needs to be further studied. Furthermore, the 1-D distribution features in counterflow diffusion flames has been reported, while the distribution characteristics of different size PAHs


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in 2-D flames commonly used in the study of soot is unknown. Thus, in this work, the 2-D relative concentration distribution of different size PAHs in laminar diffusion flames of n-heptane/toluene and isooctane/toluene mixtures with the toluene-mixing ratio from 0% to 100% were measured through the planar laser-induced fluorescence (PLIF) technique. Furthermore, the chemiluminescences of OH and CH radicals were captured by the ICCD with the band pass filters. Then, the chemical kinetic analysis of PAHs formation and growth has been performed using the CHEMKIN Diffusion Opposed-flow Flame model. This work aimed to understand the formation and growth characteristics of PAHs in the n-heptane/toluene and isooctane/toluene laminar diffusion flames and the differences between them, and to provide reference data for the development of soot prediction models for gasoline. 2. Experimental method and setup 2.1 Fuels This study investigated the n-heptane, isooctane, toluene, and the mixtures of n-heptane/toluene or isooctane/toluene. In the mixtures, the volume fraction of toluene increases from 0% to 100%, denoted as T0-100. The physical and chemical properties of n-heptane, isooctane, toluene, and gasoline are displayed in Table 1. In the test, the fuel mass flow rate was maintained at 7 g/h (with an uncertainty of about 0.05 g/h). 2.2 Test system The experimental laminar diffusion flames with a height fluctuation of ~1 mm was obtained based on the liquid burner system previously used and described in detail

29, 30

, and is only briefly described here.

The burner system shown in Fig. 1 includes the accumulator, the controlled evaporator mixer (CEM), the modified Gülder burner

31

, mass flow controllers (MFC), pressure stabilizing cavity, air compressor, and

heating bands. The liquid fuel flow rate was controlled by the Coriolis high-precision mass flow meter


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(Mini CORI-FLOW, Bronkhorst). The nitrogen flow rate was regulated by a thermal flow meter (EL-FLOW, Bronkhorst) with an uncertainty of about 0.5%. The fuel stream was first diluted with nitrogen and then evaporated in the CEM device. The nitrogen and air flow rate was respectively controlled at about 0.3 and 200 L/min (1 atm, 273 K), and the temperatures of CEM and heating bands were set at 423 K. The two-dimensional distribution of PAHs were measured by the planar laser-induced fluorescence (PLIF) technique. The test system was shown in Fig. 1. In the test, a sheet laser with a thickness of 0.8 mm forming based on the fourth harmonic of the pulsed Nd:YAG laser with a 80 mJ/pulse energy passed through the flame center. Then the LIF signals were collected by the ICCD camera with a gate width of 30 ns. In order to differentiate relative size of PAHs, four band pass filters of 320, 360, 400, and 450 nm (FWHM 10 nm) were selected. Previous research have shown that as the detection wavelength increases, the LIF signal represents the larger aromatic rings 19, 32. In addition, the chemiluminescences of OH and CH radicals were collected by the ICCD with the band pass filters of 310 and 430 nm (FWHM 10nm) respectively. 3. Results and discussion 3.1 Experimental results 3.1.1 Natural luminosity flames and flame structure Fig. 2 shows the natural luminosity flames and the OH and CH chemiluminescences of n-heptane (n-C7H16), isooctane (i-C8H18), and toluene (C6H5CH3). From Fig. 2a, it can be seen that n-heptane and isooctane flames closed that there is a well-defined sharp boundary at the flame top without emitting visible smoke, while the toluene flame reveal a markedly different appearance. It is open and emits visible smoke. Furthermore, around the flame root, the blue area in the n-heptane flame is the largest, followed by isooctane, that in toluene is the smallest, which indicates that the cool flame area shown in Fig. 2a is


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n-heptane>isooctane>toluene. The n-heptane has a very large cool flame area. This may be due to the action of negative temperature coefficient (NTC). Law et al.

33

and Peng et al. 34 determined the existence

of special NTC-influenced chemical reactivity and related weak combustion flames in the counterflow flame system computationally. Deng et al. 35 and Peng et al. 36 reported that the phenomenon of NTC is an essential feature of the oxidation kinetics of large hydrocarbons, and is closely related to the cool flames. Ji et al.

37

pointed out that the NTC behavior is closely related to the competition between the

low-temperature chain branch reactions and the decomposition of intermediate species. Therefore, as the temperature increases with the height above burner, the chain branching reactions in the low temperature zone of n-heptane diffusion flames are replaced by the chain transfer reactions in the mid-temperature zone, resulting in the phenomenon of NTC. However, toluene ignition temperature is higher, whose reaction process does not appear NTC phenomenon, so its cool flame zone is very small, almost negligible. It is known that the OH and CH radicals can produce the strongest chemiluminescent intensity in hydrocarbon flames

38

. From the OH chemiluminescene (see Fig. 2b), the flame lift-off height of toluene

diffusion flame is the highest, followed by isooctane, n-heptane is the minimum. The flame lift-off height is defined as the distance from the nozzle exit to the OH peak position. From the laminar flame speed, n-heptane is the largest, and the difference between isooctane and toluene is small. However, the ignition temperature of toluene is higher 39. Therefore, the flame lift-off height of the three shows such a result. It can be inferred that after the fuel stream leaves the nozzle, toluene has the longest ignition delay. This in turn increases the mutual diffusion of fuel and air and increases the entrainment of air around the flame root, resulting in an increase in the intensity of reaction, which can be evidenced by the OH intensity at the flame lift-off height in Fig. 2b. CH radical as the simplest hydrocarbon group can be a marker of the completion of macromolecular


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degradation, the distribution of which indicates the different reaction zones of the flame. The CH luminescence intensity distribution in n-heptane, isooctane, and toluene flames is compared in Fig. 2c. It is observed that the distribution characteristics of the three flames are distinctly different. In the n-heptane flame, CH radicals mainly distributes at the top of the flame, and in the isooctane flame, the CH radicals mainly distributes in the interior of the middle and upper flame (HAB 25~40 mm). In the toluene flame, CH groups mainly distributes in the periphery of the flame, and it peaks at lower height than n-heptane and isooctane. In addition, it can be seen that the CH luminescence intensity for isooctane flame is the highest, and that for the toluene flame is the lowest. Toluene is an aromatic hydrocarbon and its reaction path is different from that of n-heptane and iso-octane, which may leads to differences in flame structure. Fig. 3 shows the radial OH maximum intensity and the flame lift-off height of n-heptane and isooctane flames mixed with different toluene ratio. The flame lift-off height is defined as the distance from the nozzle exit to the OH peak position. In the n-heptane and isooctane flames, the peak value of OH chemiluminescence intensity gradually increase with the increasing toluene ratio. Specifically, in n-heptane/toluene flames (see Fig. 3a), compared with toluene (T100), the OH peak intensity in the T0, T20, T40, and T60 flames decrease by 39.11%, 28.68%, 25.83%, and 11.84% respectively. In isooctane/toluene flames (see Fig. 3b), compared with toluene (T100), the OH peak intensity in the T0, T20, T40, and T60 flames decrease by 21.76%, 15.00%, 10.93%, and 4.08% respectively. It can be seen that at the same toluene mixing ratio, the peak of OH intensity in n-heptane/toluene flame is lower than that of isooctane/toluene. In the n-heptane and isooctane flames, the flame lift-off height all show a monotonous increasing trend with the increasing toluene-mixing ratio. However, the upward trend in n-heptane/toluene flames is more pronounced than isooctane/toluene flames. In n-heptane/toluene flames (see Fig. 3a), the flame lift-off lengths are 2.77, 3.23, 3.74, 5.23, and 9.08 mm for the T0, T20, T40, T60, and T100 flames.


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In isooctane /toluene flames (see Fig. 3b), the flame lift-off lengths are 6.15, 6.40, 6.81, 8.46, and 9.08 mm for the T0, T20, T40, T60, and T100 flames. Thus, at the same toluene-mixing ratio, the flame lift-off height in isooctane /toluene flame is higher than that of n-heptane/toluene, inferring that n-heptane has higher reactivity than isooctane. 3.1.2 PAHs distribution in n-heptane/toluene flames The normalized maximum PAHs-LIF signals detected at 320, 360, 400, and 450 nm as a function of toluene mixing ratio in n-heptane coflow diffusion flame are shown in Fig. 4. It has been accepted that 320 nm represents one ring aromatics (A1) 18, 360 nm indicates 2/3-ring aromatics (A2-A3) 19, 400 represents 3/4-ring aromatics (A3-A4), and 450 nm indicates 4/5-ring aromatics (A4-A5) 32, 40. There is only a few PAHs formation in the n-heptane flame compared to that in toluene flame, which is mainly due to the ring structure in toluene. The ring structure is more conducive to the formation of polycyclic aromatic hydrocarbons than the chain structure. Besides, the cool flame may help reduce PAHs growth component formation. As shown in Fig. 2a, the cool flame area in the n-heptane flame occupies almost a half of the total flame area with a typical two-stage combustion phenomenon, whereas the cool flame of toluene is almost negligible. The cool flame increases the area of the low temperature reaction zone, promotes the production of a large number of free radicals, and inhibits the production of olefins in high temperature regions 37. It can be seen that n-heptane flame does not contain toluene, but there is also one ring aromatics formation. Đnal et al. 41 investigated the PAHs formation in n-heptane laminar premixed flames and pointed out that benzene is the most abundant aromatics. The 320 nm-LIF signal shows a monotonically increasing trend with the toluene-mixing ratio. However, the increasing effect is weakened with the increasing toluene ratio. As the proportion of toluene increases from 0% to 50%, the peak of the 320 nm-LIF signal rapidly


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increases. When the proportion of toluene reaches 50%, the tendency of increase slowed down significantly. The tendency of 320 nm-LIF attributes to the increase of toluene content, since itself belongs to one-ring aromatics. Unlike the 320nm-LIF trend, the 360/400/450 nm-LIF signals exhibit a non-monotonous trend consistently. As the toluene ratio increases from 0% to 50%, the PAH-LIF signals gradually increase, and as the toluene ratio increases from 50% to 100%, the PAH-LIF signals decrease gradually. They all peak at 50% toluene mixing ratio, exactly at the point where the increasing trend of 320 nm-LIF signal is weakened. The tendency of 360/400/450 nm-LIF signals may attributes to the synergistic effect of toluene. As the toluene content increases, on the one hand, the content of one ring aromatics increase, which diverts the PAH formation process from the slow route of alkanes to the more effective path of toluene pyrolysis

25

.

However, on the other hand, some certain free radicals like H, C2H2 decrease, which inhibits the growth of PAHs 42, 43. This requires further chemical kinetic analysis. Besides, it can be seen that adding less than 10% toluene to n-heptane has little effect on the 450 nm-LIF signal, indicating there is a tolerance in terms of toluene mixing ratio, below which the effect of toluene addition on the formation of large size aromatics can be neglected. Since large size aromatics are the direct precursors of soot nucleation, the suitable toluene ratio is between 10% and 50% when determining the components fraction of gasoline surrogate fuels that does not contain isooctane to predict the soot and PAHs formation characteristics. This situation is different after adding isooctane. In addition, from the flame picture marked in the Fig. 4, the peak point corresponds exactly to the smoking point of the flame, before which the flame top is closed and non-smoking, after which the flame top is open and smoking. Thus, it can be concluded that as the toluene content increases, the formation of PAHs begins to decrease after the n-heptane / toluene flame reaches the point of smoking.


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In order to better understand the effect of toluene addition to n-heptane on the PAH growth process, the distributions of PAHs-LIF detected at 320 and 450 nm in the T0-100 flames are displayed in Fig. 5. It can be seen that the 320 nm-LIF signal appears near the nozzle outlet (HAB=0~10 mm), and the 450 nm-LIF signal appears at HAB > 5 mm. Choi et al. 26 also found the order in which different size aromatics appear in the n-heptane/toluene 1-D counterflow diffusion flames. However, it is different from the one-dimensional distribution features in only one direction. In 2-D coflow diffusion flames, the high concentration of one-ring aromatics locate at the flame center, while the high concentration of large size aromatics locate in the two wings of the flame, which may finally results in the two wings distribution characteristic of soot in laminar diffusion flames (as shown in our previous studies

44, 45

). Moreover, it is

found that toluene addition to n-heptane has a different effect on the formation and growth of PAHs. With the increase of the toluene proportion, the distribution area and strength of 320nm-LIF signal that can characterize PAHs initially formation gradually increases, while the distribution area and strength of 450nm-LIF signal that can characterize PAHs finally growth 40 first increases and then gradually decreases. Its distribution area reaches its maximum at a 40% toluene-mixing ratio. Fig. 6 shows the profiles of normalized PAHs-LIF signals detected at 320 and 450 nm on the flame centerline of T0-100. It is found that the 320nm-LIF signal rapidly increases after the fuel stream leaves the nozzle exit. In the T0 and T20 flames, the 320-LIF signals show a steady trend after the rapidly increasing, and then gradually decreases. In the T40-100 flames, the 320-LIF signals appear a second peak after the first rapidly increase, and then decreases gradually, which shows that adding more than 20% toluene helps to promote the formation of one-ring aromatic hydrocarbons. The decreasing range (around HAB = 10 mm) of 320nm-LIF signal corresponds to the increasing range of 450 nm-LIF signal, indicating that one-ring aromatics are largely converted to large size aromatics before oxidized. Overall, the centerline profiles of


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normalized 450 nm-LIF signals shows a trend of first increase and then decrease. The 450-LIF signal in the n-heptane flame increase more slowly, and peaks at HAB > 30 mm. After adding 20% - 60% toluene, the growth rate of large size aromatic hydrocarbons obviously accelerates, and the 450 nm-LIF signals peak around HAB = 13 mm. However, after adding 80% - 100% toluene, the growth rate of large size aromatics is weakened, with the 450 nm-LIF signals peak around HAB = 23 mm. 3.1.3 PAHs distribution in isooctane/toluene flames The normalized maximum PAHs-LIF signals detected at 320, 360, 400, and 450 nm as a function of toluene mixing ratio in isooctane coflow diffusion flames are shown in Fig. 7. It is found that the 320 nm-LIF signal shows a monotonically increasing trend with the toluene ratio. However, the increasing effect is first enhanced and then weakened with the increasing toluene ratio. As the toluene proportion increases from 0% to 40%, the peak of the 320 nm-LIF signal rapidly increases, then the tendency of increase slowed down significantly when the toluene proportion continues to increase from 40%. This turning point is 50% in the n-heptane/toluene flames. In addition, unlike the n-heptane/toluene flames, the 360/400/450 nm-LIF signals in the isooctane/toluene flames shows a three-stage trend. As the toluene ratio increases from 0% to 40%, the three LIF signals rapidly increase, as the toluene ratio increases from 40% to 80%, the LIF signals decrease gradually, then increase slightly with the toluene ratio increases from 80% to 100%. Moreover, it is noted that when the toluene proportion increases from 40% to 80%, the decrease tendency is gradually weakened. The 360/400/450 nm-LIF signals all peak at 40% toluene mixing ratio, which is 50% in n-heptane/toluene flames (see Fig. 4). Thus, as the toluene content increases, the decreasing trend of PAHs in the isooctane flame will appear earlier than that in the n-heptane flame. In addition, it is observed that adding less than 20% toluene to isooctane has little effect on the 450 nm-LIF signal, indicating there exists a tolerance for toluene mixing ratio, below which the effect of toluene


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addition on the large size PAHs formation can be neglected. Thus, the suitable toluene ratio is between 20% and 40% for gasoline surrogate fuels to predict the soot and PAHs formation characteristics. Furthermore, from the flame picture marked in the Fig. 7, the peak point also corresponds to the flame of the smoking point, which is consistent with the finding in n-heptane/toluene flames (see Fig. 4). Thus, as the toluene content increases, the formation of PAHs begins to decrease after the isooctane toluene flame reaches the point of smoking. In addition, from Fig. 4 and 7, it is observed that as the toluene content in the flame increases, the flame height also increases. Fig. 8 shows the flame height and normalized PAH residence time as a function of toluene mixing ratio. The PAHs residence time is qualitatively evaluated based on the fuel exit velocity and flame height. Since the proportion of toluene reaches 40%, the iso-octane-toluene flame begins to smoke, Fig. 8 shows only the 0-30% toluene ratio. It can be seen that the residence time of PAHs in the flame keeps a monotonically increasing trend with the proportion of toluene. At the same toluene blending ratio, the flame height of isooctane-toluene and PAHs residence time are larger than that of n-heptane-toluene flame. The distributions of PAHs-LIF detected at 320 and 450 nm in the isooctane/toluene flames are displayed in Fig. 9. It is found that the 320 nm-LIF signal is similar to that in the n-heptane/toluene flames, while the 450 nm differs from n-heptane/toluene (see Fig. 5). In the isooctane/toluene flames, the high concentration region of the 450 nm-LIF signal locates in the HAB = 10~30mm two wings of the flame, while which mainly locates in HAB=10~15 mm in n-heptane/toluene flames. This indicates that the growth rate of large size PAHs in isooctane/toluene flame is slower than that in n-heptane/toluene flames. In addition, As the toluene ratio increases, the distribution area and strength of 450nm-LIF signal first increases, then gradually decreases, and finally increases. The distribution area and strength reach the maximum at a 40% toluene-mixing ratio.


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Energy & Fuels

3.2 Numerical results In order to better understand the non-monotonous behavior of PAHs, chemical kinetic analysis has been performed using the CHEMKIN Diffusion Opposed-flow Flame model with the two reaction mechanisms of An et al.

46

(An mechanism) and Wang et al.

42

(Wang mechanism). In the calculation, the

number of uniform grid points is set to 11, and the absolute tolerance is set to 1.0E-9, the inlet temperature is 423K and the pressure is 1atm. The two mechanisms have been developed to simulate the formation of soot precursors and soot for gasoline surrogate fuels, and contain up to four ring aromatics. An mechanism contains 219 species and 1229 reactions, and Wang mechanism contains 109 species and 543 reactions. They also have been fully validated for laminar flame speeds and ignition delays over a wide range of temperatures and pressures, and also performs well in predicting the PAHs in premixed or counter flow diffusion flames. 3.2.1 Mole fraction of PAHs Fig. 10 shows the normalized maximum mole fraction of benzene (A1), naphthalene (A2), phenanthrene (A3), and pyrene (A4) for n-heptane/toluene mixture and isoocatane/toluene mixture calculated by using the An and Wang mechanisms. For n-heptane/toluene mixture, both mechanisms can predict the non-monotonic behavior of A2, but can not predict A3 and A4. A2 both peaks at 80% toluene ratio instead of 50% of the experiment (Fig. 4). For isooctane/toluene mixture, the An mechanism predict the non-monotonic behavior of A1-A4. One-ring aromatic hydrocarbons showed a monotonic increasing trend in the experiment because of the LIF signal of toluene, whereas the calculated A1 only considered the content of benzene. The Wang mechanism only predicts the non-monotonic behavior of A2, but fails to predict A3 and A4. Thus, the An mechanism shows a good prediction ability for the formation and growth characteristics of PAH for the isooctane/toluene mixture, in which the reaction pathways and rate of


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production (ROP) will be further analyzed below. 3.2.2 Reaction pathways and ROP Fig. 11 shows the major reaction pathways of the formation of A1, A2, A3, and A4 for isooctane and isooctane/toluene mixture calculated by the An mechanisms. It is found that after adding toluene, the formation pathways of A1, A2, A3 and A4 are changed. The reaction pathway of toluene degradation in isooctane/toluene flames is mainly controlled by C6H5CH3 + H = A1 + CH3 and C6H5CH3 + H = C6H5CH2 + H2, which is consistent with the path reported by Li et al. 21 in the toluene premixed flame and El Bakali et al.

22

in the toluene/methane premixed flame. A1 is mainly produced via AC3H4+C3H3=A1+H for

isooctane, and C6H5CH3+H=A1+CH3 for isooctane/toluene mixture. A2 is mainly produced via A1-+C4H4=>A2+H and n-A1C2H2+C2H2=>A2+H for isooctane. After adding toluene, the reaction C10H9+H=A2+H2 also becomes the main path for the A2 formation, where C10H9 mainly from benzyl groups (C6H5CH2). A3 is mainly produced via P2-+C2H2=>A3+H and A2CH2+C3H3=>A3+2H for isooctane. After adding toluene, the reaction C14H12=A3+H2 and C9H7+c-C5H5=>A3+2H also become the main path for the A3 formation, where C14H12 and C9H7 are also mainly produced from benzyl groups (C6H5CH2).

After

adding

toluene,

A4

is

mainly

produced

via

C6H5CH2+C9H7=>A4+2H2,

2C9H7=>A4+C2H2+H2, and A3-4+C2H2=>A4+H, where benzyl groups (C6H5CH2) also play an important role. Park et al. 28 pointed out that the reactions involving benzyl groups play an important role in the PAHs formation of n-heptane/isooctane/toluene mixtures. From Fig. 11b, it can be seen that the benzyl (C6H5CH2) has a significant effect on the formation of A2-A4 through the generation of C10H9, C9H7, and C14H12 in isooctane/toluene diffusion flames. Fig. 12 shows the major reaction pathways of the formation of A1, A2, A3, and A4 for isooctane/toluene mixture calculated by the Wang mechanism. It can be seen that the reaction path of


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Energy & Fuels

benzyl (C6H5CH2) is significantly different from that in the An mechanism. In Wang mechanism, the main pyrolysis products of benzyl are C4H3 and C3H4. For isooctane/toluene mixture, A1 is mainly produced via C6H5CH3+H=A1+CH3 and C6H5OH+H=A1+OH. A2 is mainly produced via 2C5H5=A2+2H, A1+iC4H5=A2+2H+H, and A1-+C4H3=A2. A3 is mainly produced via A1C2H+A1-=A3+H, A2-+C4H4=A3+H. A4 is mainly produced via A3-+C2H2=A4+H. These differences in reaction pathways may lead to the different predictions as compared to the experimental results above 40% of toluene. In order to understand the reason for the non-monotonic trend caused by the toluene addition, the rate of production (ROP) of the main reactions was analyzed. Fig. 13 shows the maximum ROP of PAHs formation reactions related to the non-monotonic behavior for isooctnae/toluene mixture calculated by the An mechanism. It can be seen that the non-monotonic behavior of A1 is dominated by the reactions C6H5CH3+H=A1+CH3 and A1-+CH4=>A1+CH3. The relationship between the mole fraction of H, CH4 and H2 as a function of toluene ratio is shown in Fig.14. It can be seen that as the toluene ratio increases, the content of C6H5CH3 increases, but the mole fraction of H decreases monotonically, which offsets the effect of increasing toluene content. This finally leading to the synergistic effect of toluene on the reaction C6H5CH3+H=A1+CH3. As shown in Fig. 11b, except A1 dehydrogenation, A1- is mainly produced by the reaction of A1C2H5 with H radicals. Moreover, as the toluene ratio increases, the mole fraction of CH4 first increases and then decreases significantly (see Fig. 14). The combined effect of the two reasons leads to the synergistic effect of A1-+CH4=>A1+CH3. The main reactions that dominate the non-monotonic behavior of A2, A3 and A4 are very similar, all of which are reactions of their own dehydrogenation groups with H2. Specifically, the most important synergistic reactions are A2-1+H2=>A2+H for A2, what is A3-1+H2=>A3+H for A3, and A4-4+H2=>A4+H for A4. It can be seen from Fig. 11 that A2-1, A3-1, and A4-4 are formed by the dehydrogenation of A2, A3,


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Page 18 of 34

and A4, respectively. The ROP of these reactions may depend on the amount of H2 involved in these reactions. It can be seen from Fig. 14 that as the toluene ratio increases, the mole fraction of H2 shows a three-stage trend, first decreases, then increases, and finally decreases. In addition to the reactions with H2, C10H9+H=A2+H2 and A1-+C4H4=>A2+H also contribute to some degree to the non-monotonic behavior of A2. C14H12=A3+H2, C9H7+c-C5H5=>A3+2H, and A2CH2+C3H3=>A3+2H also contribute to some degree to the non-monotonic behavior of A3. C6H5CH2+C9H7=>A4+2H2 and 2C9H7=>A4+C2H2+H2 also contribute to the non-monotonic behavior of A4. In addition, it is also found that the PAH growth reactions based on the H-abstraction-C2H2-addition mechanism

47

, such as n-A1C2H2+C2H2=>A2+H and

A3-4+C2H2=>A4+H, have little effect on the non-monotonic behavior of PAHs. 4. Conclusions Based on laminar diffusion flames, this work measured the relative concentrations of different size PAHs by the PLIF technique, and recorded the chemiluminescences of OH and CH radicals with the band-pass filters. The effect of toluene addition to n-heptane and isooctane on the PAHs formation was comparatively studied. The chemical kinetic analysis of PAHs was performed using the CHEMKIN Diffusion Opposed-flow Flame model. As the toluene ratio increases, the 320nm-LIF in n-heptane and isooctane flames shows a weakening monotonous increasing trend with different inflection points. And the 360/400/450nm-LIF in n-heptane and isooctane flames show a non-monotonic trend with different peak points. The peak point is 50% toluene ratio in n-heptane/toluene flames, 40% in isooctane/toluene flames, and it corresponds exactly to the smoke point of the flame. The suitable toluene ratio is between 20% and 40% to predict the PAHs formation of gasoline. As the ring size of PAHs increases, the high-concentration region of aromatics evolves from the center to the two wings of the flame.


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The cool flame area ranks as n-heptane>isooctane>toluene, which is may be due to the NTC action. The cool flame area of the n-heptane occupies nearly half of the total flame, exhibiting a typical two-stage combustion phenomenon, while the cool flame area of toluene is almost negligible. The OH intensity and flame lift-off height show as n-heptaneA2+H also contribute to the trend of A2. C14H12=A3+H2 and C9H7+c-C5H5=>A3+2H also contribute to the trend of A3. C6H5CH2+C9H7=>A4+2H2 and 2C9H7=>A4+C2H2+H2 also contribute to the trend of A4. Acknowledgements

This study is based upon work supported by National Natural Science Foundation of China (No. 91741124). Any findings, opinions, and conclusions presented in this paper are the point of the author(s) and do not necessarily reflect the views of the funded organization. References 1.

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Liu, Y. D.; Jia, M.; Xie, M. Z.; et al. Development of a New Skeletal Chemical Kinetic Model of Toluene Reference Fuel with Application to Gasoline Surrogate Fuels for Computational Fluid Dynamics Engine Simulation. Energ. Fuel. 2013, 27(8), 4899-4909.


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Sarathy, S. M.; Farooq, A.; Kalghatgi, G. T. Recent progress in gasoline surrogate fuels. Prog. Energ. Combust. 2018, 65, 67-108.

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Zhen, X.; Wang, Y.; Liu, D. An overview of the chemical reaction mechanisms for gasoline surrogate fuels. Appl. Therm. Eng. 2017, 124, 1257-1268.

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Andrae, J.; Johansson, D.; Björnbom, P.; et al. Co-oxidation in the auto-ignition of primary reference fuels and n -heptane/toluene blends. Combust. Flame 2005, 140(4), 267-286.

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Gauthier, B. M.; Davidson, D. F.; Hanson, R. K. Shock tube determination of ignition delay times in full-blend and surrogate fuel mixtures. Combust. Flame 2004, 139(4), 300-311.

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Machrafi, H.; Cavadias, S. Three-stage autoignition of gasoline in an HCCI engine: An experimental and chemical kinetic modeling investigation. Combust. Flame 2008, 155(4), 557-570.

10. Sileghem, L.; Alekseev, V. A.; Vancoillie, J.; et al. Laminar burning velocity of gasoline and the gasoline surrogate components iso-octane, n-heptane and toluene. Fuel 2013, 112(3), 355-365. 11. Pera, C.; Knop, V. Methodology to define gasoline surrogates dedicated to auto-ignition in engines. Fuel 2012, 96(7), 59-69. 12. Knop, V.; Pera, C.; Duffour, F. Validation of a ternary gasoline surrogate in a CAI engine. Combust. Flame 2013, 160(10), 2067-2082. 13. Attar, M. A.; Xu, H. Correlations between particulate matter emissions and gasoline direct injection spray characteristics. J. Aerosol Sci. 2016, 102, 128-141.


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Energy & Fuels

14. Liu F.; Kang N.; Li Y.; Wu Q. Experimental investigation on the spray characteristics of a droplet under sinusoidal inertial force. Fuel 2018, 226, 156-62. 15. Cen C.; Wu H.; Lee C.; Liu F.; Li Y. Experimental investigation on the characteristic of jet break-up for butanol droplet impacting onto a heated surface in the film boiling regime. Int J Heat Mass Tran 2018, 123, 129-136. 16. Wang Z.; Dai X.; Li F.; Li Y.; Lee C.; Wu H.; Li Z. Nozzle internal flow and spray primary breakup with the application of closely coupled split injection strategy. Fuel 2018, 228(15), 187-196. 17. Zhang S.; Lee T.; Wu H.; Pei J.; Wu W.; Liu F. Experimental and kinetical study of component volumetric effects on laminar flame speed of Acetone–Butanol–Ethanol (ABE). Energy & Fuels 2018, DOI: 10.1021/acs.energyfuels.8b00003. 18. Hayashida, K.; Mogi, T.; Amagai, K.; et al. Growth characteristics of polycyclic aromatic hydrocarbons in dimethyl ether diffusion flame. Fuel 2011, 90(2), 493-498. 19. Lee, S. M.; Yoon, S. S.; Chung, S. H. Synergistic effect on soot formation in counterflow diffusion flames of ethylene–propane mixtures with benzene addition. Combust. Flame 2004, 136(4), 493-500. 20. Li, Y.; Zhang, L.; Tian, Z.; et al. Experimental study of a fuel-rich premixed toluene flame at low pressure. Energ. Fuel. 2009, 23(3), 1473-1485. 21. Li, Y.; Cai, J.; Zhang, L.; et al. Investigation on chemical structures of premixed toluene flames at low pressure. P. Combust. Inst. 2011, 33(1), 593-600. 22. Bakali, A. E.; Dupont, L.; Lefort, B.; et al. Experimental study and detailed modeling of toluene degradation in a low-pressure stoichiometric premixed CH4/O2/N2 flame. J. Phys. Chem. A 2007, 111(19), 3907-3921.. 23. Harris, S. J.; Weiner, A. M. Soot particle growth in premixed toluene/ethylene flames. Combust. Sci.


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Technol. 1984, 38(1-2), 75-87. 24. Choi, S. K.; Choi, B. C.; Lee, S. M.; et al. The effect of liquid fuel doping on PAH and soot formation in counterflow ethylene diffusion flames. Exp. Therm. Fluid Sci. 2015, 60, 123-131. 25. Alexiou, A.; Williams, A. Soot formation in shock-tube pyrolysis of toluene-n-heptane and toluene-iso-octane mixtures. Fuel 1995, 74(2), 153-158. 26. Choi, B. C.; Choi, S. K.; Chung, S. H. Soot formation characteristics of gasoline surrogate fuels in counterflow diffusion flames. P. Combust. Inst. 2011, 33(1), 609-616. 27. Consalvi, J. L.; Liu, F.; Kashif, M.; et al. Numerical study of soot formation in laminar coflow methane/air diffusion flames doped by n-heptane/toluene and iso-octane/toluene blends. Combust. Flame 2017, 180, 167-174. 28. Park, S.; Wang, Y.; Chung, S. H.; et al. Compositional effects on PAH and soot formation in counterflow diffusion flames of gasoline surrogate fuels. Combust. Flame, 2017, 178, 46-60. 29. Liu, F.; Hua, Y.; Wu, H.; et al. Experimental and kinetic investigation on soot formation of n-butanol-gasoline blends in laminar coflow diffusion flames. Fuel 2018, 213, 195-205. 30. Liu, F.; Hua, Y.; Wu, H.; et al. Effect of alcohol addition to gasoline on soot distribution characteristics in laminar diffusion flames. Chem. Eng. Technol. 2018, DOI: 10.1002/ceat.201700333. 31. Khosousi, A.; Liu, F.; Dworkin, S. B.; et al. Experimental and numerical study of soot formation in laminar coflow diffusion flames of gasoline/ethanol blends. Combust. Flame 2015, 162(10), 3925-3933. 32. Sun, R.; Zobel, N.; Neubauer, Y.; et al. Analysis of gas-phase polycyclic aromatic hydrocarbon mixtures by laser-induced fluorescence. Opt. Laser. Eng. 2010, 48(12), 1231-1237. 33. Law, C. K.; Zhao, P. NTC-affected ignition in nonpremixed counterflow. Combust. Flame 2012,


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159(3), 1044-1054. 34. Peng, Z.; Law, C. K. The role of global and detailed kinetics in the first-stage ignition delay in NTC-affected phenomena. Combust. Flame 2013, 160(11), 2352-2358. 35. Deng, S.; Zhao, P.; Zhu, D.; et al. NTC-affected ignition and low-temperature flames in nonpremixed DME/air counterflow. Combust. Flame 2014, 161(8), 1993-1997. 36. Peng, Z.; Liang, W.; Deng, S.; et al. Initiation and propagation of laminar premixed cool flames. Fuel 2016, 166, 477-487. 37. Ji, W.; Zhao, P.; He, T.; et al. On the controlling mechanism of the upper turnover states in the NTC regime. Combust. Flame 2016, 164, 294-302. 38. Leo, M. D.; Saveliev, A.; Kennedy, L. A.; et al. OH and CH luminescence in opposed flow methane oxy-flames. Combust. Flame 2007, 149(4), 435-447. 39. Sileghem, L.; Alekseev, V. A.; Vancoillie, J.; et al. Laminar burning velocity of gasoline and the gasoline surrogate components iso-octane, n-heptane and toluene. Fuel 2013, 112, 355-365.a 40. Kobayashi, Y.; Furuhata, T.; Amagai, K.; et al. Soot precursor measurements in benzene and hexane diffusion flames. Combust. Flame 2008, 154(3), 346-355. 41. Đnal, F.; Senkan, S. M. Effects of equivalence ratio on species and soot concentrations in premixed n-heptane flames. Combust. Flame 2002, 131(1-2), 16-28. 42. Wang, H.; Yao, M.; Yue, Z.; et al. A reduced toluene reference fuel chemical kinetic mechanism for combustion and polycyclic-aromatic hydrocarbon predictions. Combust. Flame 2015, 162(6), 2390-2404. 43. Jia, G.; Yao, M.; Liu, H.; et al. PAHs formation simulation in the premixed laminar flames of TRF with alcohol addition using a semi-detailed combustion mechanism. Fuel 2015, 155, 44-54.


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44. Liu, F.; Hua, Y.; Wu, H.; et al. An experimental study on soot distribution characteristics of ethanol-gasoline

blends

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J.

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https://doi.org/10.1016/j.joei.2017.07.008 45. Liu, F.; Hua, Y.; Wu, H.; et al. Experimental and kinetic studies of soot formation in methanol-gasoline coflow diffusion flames. J. Energy Inst. 2017, https://doi.org/10.1016/j.joei.2017.12.002. 46. An, Y.; Pei, Y.; Qin, J.; et al. Kinetic modeling of polycyclic aromatic hydrocarbons formation process for gasoline surrogate fuels. Energ. Convers. Manage. 2015, 100, 249-261. 47. Shukla, B.; Koshi, M. Comparative study on the growth mechanisms of PAHs. Combust. Flame 2011, 158(2), 369-375.


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Energy & Fuels

Table list with Caption Table 1 Physical and chemical properties of gasoline, n-heptane, isooctane, and toluene Fuel

Gasoline

N-heptane

Isooctane

Toluene

Carbon content (Weight %)

86.24

84.00

84.21

91.30

Hydrogen content (Weight %)

13.76

16.00

15.79

8.70

Density(20°C) (kg·m-3)

725

684

691.9

866

Low heating value (MJ·kg-1)

43.5

48.06

47.97

42.45

Boiling point (°C)

40-210

98.5

99.3

110.6

A/F stoichiometric

14.7

15.18

15.13

13.5


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure list with Caption Figure 1: Test system diagram Figure 2: The natural luminosity and the OH and CH chemiluminescences of n-heptane, isooctane, and toluene flames Figure 3: The radial OH maximum intensity and the flame lift-off height of n-heptane and isooctane flames mixed with different toluene ratio Figure 4: The normalized maximum PAHs-LIF signals detected at 320, 360, 400, and 450 nm as a function of toluene mixing ratio in n-heptane diffusion flame Figure 5: The distributions of PAHs-LIF detected at 320 and 450 nm in the n-heptane/toluene coflow diffusion flames Figure 6: The profiles of normalized PAHs-LIF signals of 320 and 450 nm along the axial centerline of the T0-100 flames Figure 7: The normalized maximum PAHs-LIF signals detected at 320, 360, 400, and 450 nm as a function of toluene mixing ratio in isooctane diffusion flame Figure 8: Flame height and normalized PAH residence time as a function of toluene mixing ratio Figure 9: The distributions of PAHs-LIF detected at 320 and 450 nm in the isooctane/toluene coflow diffusion flames Figure 10: Normalized maximum mole fraction of A1, A2, A3, and A4 for n-heptane/toluene mixture and isoocatane/toluene mixture calculated by the An 46 and Wang 42 mechanisms

Figure 11: Major reaction pathways of the formation of A1, A2, A3, and A4 for isooctane and isooctane/toluene mixture in An mechanism Figure 12: Major reaction pathways of the formation of A1, A2, A3, and A4 for isooctane/toluene mixture in Wang mechanism Figure 13: Maximum ROP of PAHs formation reactions related to the non-monotonic behavior for isooctnae/toluene mixture Figure 14: Calculated normalized maximum mole fractions of H, CH4, and H2


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Energy & Fuels

Fig. 1. Test system diagram

Fig. 2. The natural luminosity and the OH and CH chemiluminescences of n-heptane, isooctane, and toluene flames


Energy & Fuels

(b) i-C8H18/C6H5CH3

0.8 0.6

1.0

8 6 4 2 0

0

40 20 60 Toluene ratio (%)

0.4 0.2 0.0

0

10

20 30 Height above burner (mm)

100

T0 T20 T40 T60 T100 40

Flame lift-off height (mm)

1.0

Normalized max. OH intensity

Flame lift-off height (mm)

(a) n-C7H16/C6H5CH3 Normalized max. OH intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 34

0.8 0.6

8 6 4 2 0

0

40 20 60 Toluene ratio (%)

0.4 0.2 0.0

0

10

20 30 Height above burner (mm)

100

T0 T20 T40 T60 T100 40

Fig. 3. The radial OH maximum intensity and the flame lift-off height of n-heptane and isooctane flames mixed with different toluene ratio

Fig. 4. The normalized maximum PAHs-LIF signals detected at 320, 360, 400, and 450 nm as a function of toluene mixing ratio in n-heptane diffusion flame


Page 29 of 34

(a) 320 nm T0 T20 T40 50 50 T50 50 T6050 T8050 T100 50 50

(b) 450 nm 50 T6050 T80 50 T050 T2050 T40 50 T50

900

T100

40

40

40

40

40

Height above burner (mm)

700 600

30

30

30

30

30

30

30 500

20

20

20

20

20

20

400

20

300

10

10

10

10

10

10


40

40

200

10

40

40

40

40

40

40

30

30

30

30

30

30

1400 1200 1000 800

20

20

20

20

20

20 600

10

10

10

10

10

10

400 200

100

0 0 0 0 0 0 0 2 4 0 2 4 0 2 4 0 2 4 0 2 4 0 2 4 0 2 4 r (mm) r (mm) r (mm) rr (mm) (mm)r (mm) r (mm) r (mm)

1800 1600

800

0 0 0 0 0 0 2 4 0 2 4 0 2 4 0 2 4 0 2 4 0 2 4 0 2 4 r (mm) r (mm) r (mm) r (mm) (mm) r (mm) r (mm) r (mm)

0

0

Fig. 5. The distributions of PAHs-LIF detected at 320 and 450 nm in the n-heptane/toluene coflow diffusion flames

T0 T20 T40 T50 T60 T80 T100

0.8 0.6 0.4 0.2 0.0

1.2 Normalized PAHs-LIF signals

(a) 320 nm

1.0 Normalized PAHs-LIF signals

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1.0 0.8 0.6

10

20 30 40 Height above burner (mm)

50

T0 T20 T40 T50 T60 T80 T100

0.4 0.2 0.0

0

(b) 450 nm

0

10

20 30 40 Height above burner (mm)

50

Fig. 6. The profiles of normalized PAHs-LIF signals of 320 and 450 nm along the axial centerline of the T0-100 flames


Energy & Fuels

Fig. 7. The normalized maximum PAHs-LIF signals detected at 320, 360, 400, and 450 nm as a function of toluene mixing ratio in isooctane diffusion flame

52

n-heptane/toluene isooctane/toluene

(a)

Normalized PAHs residence time

54

Flame height (mm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 34

50 48 46 44 42 40

1.0

10 20 Toluene mixing ratio (%)

30

(b)

0.9

0.8

0.7

0.6

0

n-heptane/toluene isooctane/toluene

0

10 20 Toluene mixing ratio (%)

30

Figure 8. Flame height and normalized PAH residence time as a function of toluene mixing ratio


Page 31 of 34

(a) 320 nm 50 T100 50 T050 T2050 T40 T50 50 T60

(b) 450 nm 50 T5050 T6050 T100 50 T050 T2050 T40

1800

900

1600

800


40

40

40 700 600

30

30

30

30

30 500

20

20

20

20

400

20

300

10

10

10

10


40

40

200

10

40

40

40

40

40

40

30

30

30

30

30

30

1400 1200 1000

20

20

20

20

20

800

20

600

10

10

10

10

10

400

10

200

100

0 0 0 0 0 0 2 4 0 2 4 0 2 4 0 2 4 0 2 4 0 2 4 r (mm) r (mm) r (mm) r (mm) r (mm) r (mm) r (mm)

0 0 0 0 0 0 0 2 4 0 2 4 0 2 4 0 2 4 0 2 4 0 2 4 r (mm) r (mm) r (mm) r (mm) r (mm) r (mm) r (mm)

0

0

Fig. 9. The distributions of PAHs-LIF detected at 320 and 450 nm in the isooctane/toluene coflow diffusion

NC7H16/C6H5CH3

1.0 0.8 0.6 0.4

A1 A2 A3 A4

0.2 0.0

0

20

40 60 Toluene ratio (%)

80

100

IC8H18/C6H5CH3

(a) An mechanism 2.0

A1 A2 A3 A4

1.5

1.0

0.5

0.0

0

20


80

1.8 Normalized maximum mole fractions

(a) An mechanism

1.2

100

Normalized maximum mole fractions


flames


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(b) Wang mechanism

NC7H16/C6H5CH3

1.6 1.4 1.2 1.0 0.8 0.6

A1 A2 A3 A4

0.4 0.2 0.0

0

20


(b) Wang mechanism

3.2

80

100

IC8H18/C6H5CH3 A1 A2 A3 A4

2.8 2.4 2.0 1.6 1.2 0.8 0.4 0.0

0

20


80

100

Fig. 10. Normalized maximum mole fraction of A1, A2, A3, and A4 for n-heptane/toluene mixture and isoocatane/toluene mixture calculated by the An 46 and Wang 42 mechanisms


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 11. Major reaction pathways of the formation of A1, A2, A3, and A4 for isooctane and isooctane/toluene mixture in An mechanism


Page 32 of 34

Page 33 of 34

Fig. 12. Major reaction pathways of the formation of A1, A2, A3, and A4 for isooctane/toluene mixture in Wang mechanism (a) A1

-4

(b) A2 IC8H18/C6H5CH3

C6H5CH3+H=A1+CH3 A1-+CH4=>A1+CH3 -4

2.0x10

-4

1.5x10

-4

1.0x10

-5

5.0x10

0.0 0

20


80

100

(c) A3

-6

9.0x10

-6

6.0x10

-6

3.0x10

IC8H18/C6H5CH3

-6

6.0x10

-6

4.0x10

-6

2.0x10

0.0 0

20


80

100

(d) A4

-4

Maximum ROP (mole/cm3-sec)

-5

1.2x10

-6

8.0x10

A4-4+H2=>A4+H A4-1+H2=>A4+H A4-2+H2=>A4+H C6H5CH2+C9H7=>A4+2H2 2C9H7=>A4+C2H2+H2 A3-4+C2H2=>A4+H

IC8H18/C6H5CH3

A3-1+H2=>A3+H A3-1+CH4=>A3+CH3 C14H12=A3+H2 C9H7+c-C5H5=>A3+2H A2CH2+C3H3=>A3+2H P2-+C2H2=>A3+H

1.5x10

1.0x10

1.0x10

-5

1.8x10

-5

A2-1+H2=>A2+H A2-2+H2=>A2+H A2-2+CH4=>A2+CH3 C10H9+H=A2+H2 A1-+C4H4=>A2+H n-A1C2H2+C2H2=>A2+H

-5



2.5x10


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

-5

8.0x10

-5

6.0x10

IC8H18/C6H5CH3

-5

4.0x10

-5

2.0x10

0.0

0.0 0

20


80

100

0

20


80

100

Fig. 13. Maximum ROP of PAHs formation reactions related to the non-monotonic behavior for isooctnae/toluene mixture


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Energy & Fuels

Page 34 of 34

1.0 0.8 0.6 0.4 H CH4 H2

0.2 0.0

0

20


80

100

Fig. 14. Calculated normalized maximum mole fractions of H, CH4, and H2


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