Size Distribution of Soot Particles in Premixed n-Heptane and

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Size Distribution of Soot Particles in Premixed n-Heptane and Methylcyclohexane Flames Baiyang Lin, Hao Gu, Hong Ni, Bin Guan, Zhongzhao Li, Dong Han, Zhen Huang, and He Lin Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03295 • Publication Date (Web): 13 Feb 2018 Downloaded from http://pubs.acs.org on February 23, 2018

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Size Distribution of Soot Particles in Premixed nHeptane and Methylcyclohexane Flames Baiyang Lina, Hao Gua, Hong Nib, Bin Guana, Zhongzhao Lia,*, Dong Hana, Zhen Huanga, He Lina

a

Key Laboratory for Power Machinery and Engineering of M.O.E, Shanghai Jiao Tong

University, Shanghai 200240, PR China b

Chinese Research Academy of Environmental Sciences, Beijing 100012, PR China

Abstract The size distributions of soot particles from premixed n-heptane and methylcyclohexane flames were measured and compared at different particle residence times. The micro-orifice probe sampling technique and the scanning mobility particle sizer (SMPS) were used for the particle size measurement in the burner stabilized stagnation (BSS) flame configuration. For both flame series, the mixture C/O mole ratio was 0.6, and the maximum flame temperature was controlled around 1835 K to investigate the fuel structure effects on soot propensity. It was observed that with the increased particle residence time, the particle size distribution function (PSDF) of both flames evolved from the unimodal (only the nucleation mode) to the bimodal (both the nucleation and coagulation modes) distribution. The particle nucleation strength and the particle growth rate of the methylcyclohexane flame were quantitatively lower than those of the n-

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heptane flame. Considering the equivalence ratio difference between the two flames, the sooting propensity disparity between cycloalkanes and n-alkanes is not considerable under the test conditions.

Key words: Particle size distribution; Soot; n-Heptane; Methylcyclohexane; Premixed flame


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1. Introduction Understanding of the engine pollutants formation processes, e.g. soot formation, could contribute to the development of clean engine combustion strategies. The experimental sooting behavior of different fuel structures is required for the development of soot model. It is therefore of significance to study the sooting characteristics of different fuel molecules. Some researchers have investigated the effect of methane 1-3 on PAH and soot formation. Some other studies focused on the molecular structure influences, such as the branched chain 4, 5, the alkyl substitute on aromatic ring 6, 7, the C=C double bond 8-10 and the aromatic ring 11-17. These results mostly revealed that the fuel molecular structures have observable effects on both the global sooting tendency and the particle nucleation and growth process. The sooting characteristics of cycloalkanes (especially cyclohexane and alkylated cyclohexane) are drawing increased attention, since cycloalkanes account for a considerable proportion of the practical fuels. Zhou et al. found that cyclohexane produced more soot than nhexane in diffusion flames 18. Botero et al. observed a higher mean particle size in the methylcyclohexane diffusion flame than that in the n-heptane diffusion flame 19. Ciajolo et al. detected an earlier soot inception in the premixed cyclohexane flames than in the premixed nhexane flames, and attributed this to benzene formation by cyclohexane dehydrogenation 20. Hansen et al. 21, Law et al. 22 and Zhang et al. 23 found that cyclohexane dehydrogenation was the most important pathway toward benzene formation in premixed flames. Similar observations are obtained in engine combustion studies 24. Soot emissions of cyclohexane were found to be higher than those of hexane in a compression-ignition engine 25. Kaiser et al. observed substantial benzene emissions from cyclohexane and methylcyclohexane combustion in a spark-ignited engine 26. All these studies supported that the dehydrogenation of six-carbon aliphatic ring led to


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the formation of benzene ring, and as such cyclohexane and alkylated cyclohexane have stronger soot propensity than the straight-chain alkanes with the same carbon number. The objective of this study is to further investigate the soot propensity of cycloalkanes relative to straight-chain alkanes in premixed flames. The special feature of present work is that both the maximum flame temperature and temperature-time histories were well controlled to achieve a direct comparison of the sooting tendency from the aspect of fuel structure. In addition, the measurements were made in the burner stabilized stagnation (BSS) flames with well-defined boundary conditions to facilitate numerical simulations 27. This is the second objective of the present study as the experimental results are expected to benefit the soot model development. Methylcyclohexane and n-heptane were selected as the test fuels. They are both important surrogate components for the jet fuel 28. N-heptane is also a crucial gasoline and diesel surrogate 28

. To the best of our knowledge, the particle size distribution functions (PSDF) of these two

fuels have not been compared in premixed flame conditions and the detailed PSDF measurements carried out in the BSS flames here filled this gap.

2. Experimental and computational method The experimental and computational methods have been described in detail in the authors’ previous work 29-31. Figure 1 schematically shows the experimental setup. The flame burner consisted of a bronze porous plug and a concentric porous ring. The diameter of the plug is 5 cm. Atmospheric fuel / oxygen / nitrogen mixture flowed into the plug, and nitrogen was introduced into the ring isolating the surrounding air. A water-cooled aluminum alloy plate served as the flow stagnation surface, and the BSS flame formed between the burner and the stagnation plate. More details about the BSS flame configuration could be found in literature 27, 32. A syringe


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pump (Harvard PHD 2000 Series) was used to control the flow rate of the liquid fuel. The liquid fuel broke up into droplets through a nebulizer (Precision Glassblowing, Glass Concentric Nebs). The upstream gauge pressure for the carrier gas of the nebulizer was 199.26 kPa (28.9 PSI), corresponding to a nitrogen flow of 0.68 L/min (at 298 K and 1 atm) at the nebulizer’s outlet. The fuel droplets were mixed with the preheated oxygen / nitrogen flow and were vaporized in the heated gas tubes. Temperature of the reactant gas tubes and the burner was maintained at 440 K to prevent fuel vapor condensation. No residual fuel was observed inside the reactant gas tubes. Critical orifices were used to meter the volumetric flow rate of oxygen and nitrogen. An S-type thermocouple, whose measurement range was up to 2000 K, was used for measuring the axial temperature field of the flame. The fine wire diameter of the S-type thermocouple enabled accurate temperature measurements, minimizing the interference to the flame. The wire and bead diameter for the coated thermocouple was approximately 0.014 cm and 0.028 cm, respectively. The measured temperatures close to the burner surface were linearly extrapolated to obtain the burner surface temperature 32, 33. The flame temperature at the downstream boundary was held at around 400 K, and a K-type thermocouple was inserted into the bottom of the stagnation plate for the measurement of the stagnation surface temperature. Radiation correction of the measured temperature was made using the Shaddix procedure 34. When conducting the radiation correction, the local transport and flow property values were calculated iteratively by a modified OPPDIF code 35, 36. The error of the radiation-corrected temperature mainly originated from the uncertainty of the thermocouple radiation coefficient. The lower and upper emissivity limits of the coating in literature were 0.3 and 0.6, respectively 37. The temperatures corrected by the two emissivity limits were averaged as the final flame temperature value 27, 32.


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The gas physical properties were calculated for radiation correction. Flame temperatures and species concentrations were computed from the quasi one-dimensional OPPDIF code, in which the burner surface and stagnation plate temperatures were used as the boundary conditions 27, 32

. The reaction kinetic model used was JetSurF2.0 consisting of 348 species and 2163

reactions 38.

Figure 1. Schematic of the experimental setup. A stainless-steel tube whose outer diameter and wall thickness were 6.35 mm and 0.127 mm, respectively, was embedded in the stagnation plate. An orifice with a diameter of 0.13 mm was on the tube for sampling. The soot sample was drawn through the orifice into the tube and immediately diluted by the nitrogen flow (30 L/min at 298 K and 1 atm). A small fraction of the diluted sample flowed into the scanning mobility particle sizer (SMPS) system consisting of a 3080 Electrostatic Classifier (EC) and a 3776 ultrafine Condensation Particle Counter (CPC).


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The EC was composed of a 3085 nano-Differential Mobility Analyzer (DMA) and a Kr-85 neutralizer. The upward scan time was 50 seconds, during which the size distribution was obtained for particles from 2.5 nm to 66.1 nm. The determination of the dilution ratio (DR) was by calibrating the orifice flow rate variation with the pressure drop at the orifice (∆P), using the bubble flowmeter method 27, 32, 39, 40. In the upstream and downstream there were two manometers for ∆P measurement, and the flow of secondary air was adjusted to control ∆P. Within the appropriate DR range for each test condition, the dilution-corrected PSDF was insensitive to DR. The optimum dilution ratio ranged from about 600 to 1500 in this study. The studied flame conditions were listed in Table 1, which were typical lightly sooting flame conditions 29-31 with C/O mole ratio of 0.6, the maximum temperature around 1830 K and diluent gas proportion of 60%. The PSDF measurement of n-heptane and methylcyclohexane was carried out at different plate to burner separations (Hp, namely the flame height). For a given reactant gas mixture, each Hp yielded a specific flame temperature field, while the maximum flame temperature was similar 27. The “flame series” named in this study referred to the flames tested under a series of plate to burner separations burning a given fuel. Due to different C/H ratios of n-heptane and methylcyclohexane, the equivalence ratios of the two flames were not exactly the same when the carbon to oxygen ratio was controlled constant. Under the test conditions, the equivalence ratio of the heptane flame was higher than that of the methylcyclohexane flame by approximately 4.4%.


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Table 1. Flame conditions studied.

Φ

Veloc ity, b v0 (cm/s)

Maximum temperature , cTm (K)

Stagnation surface temperature , cTs (K)

Largest separation from plate to burner, Hp,max (cm)

Mole fraction Flame series, a

n-Heptane (HEP) Methylcycl ohexane (MCH) a

Fuel

O2

0.0585

0.3415

1.88

4.14

1837 ± 89

405 ± 10

1.8

0.0585

0.3415

1.8

3.55

1834 ± 81

403 ± 10

1.55

In both flame series, the C/O mole ratio is 0.6 and the mole fraction of the diluent gas

(nitrogen) is 0.6. b

At 298 K and 1 atm.

c

Measured at Hp,max. It was noted that soot particles evolved with residence time, so the PSDFs were compared

under similar particle residence times for the two flame series. The definition of the particle residence time or reaction time (t) was the time duration to traverse from the calculated peak flame temperature location to the stagnation surface for the particles (or precursors), as shown in Eq. (1)

t=

∫

Hp

xTm

dx vc ( x ) + vT ( x )

(1)

where xTm represents the distance between the burner surface and the peak flame temperature location, vc(x) is the convective velocity determined by the OPPDIF non-slip boundary solution 41, and vT ( x ) is the thermophoretic velocity calculated by Eq. (2)


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vT ( x) =

λ

dT 5(1 + πϕ / 8) NgkBT dx

(2)

where λ is the thermal conductivity calculated from the gas transport properties in the flame based on the multi-component formulation, Ng represents the number density of gas molecules, kB is the Boltzmann constant, ϕ is the momentum accommodation factor taken to be 0.9 42 and T is the flame temperature. Previous studies 32, 43 revealed that owing to the orifice flow, the sample taken with the current experimental technique represented an average of the gas volume immediately adjacent to the orifice, and that such a volume could be represented by shifting the spatial position a few millimeters upstream from the stagnation surface. Here we chose 1 mm 30, 31 as the approximate shift for particle residence time modification. Hence, a modified particle residence time is given by Eq. (3) t′ =

∫

xs

xTm

dx vc ( x ) + vT ( x )

(3) .

where xs = Hp – 0.1 cm. All of the particle residence times mentioned below refer to the modified particle residence times (t’). The flame heights studied and the corresponding particle residence times calculated are listed in Table 2. Table 2. Plate to burner separations studied and the corresponding particle residence times. HEP

MCH

Hp

t'

Hp

t'

(cm)

(ms)

(cm)

(ms)

0.91

35.7

0.8

35.4

1.13

50.8

0.98

50.3


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1.31

63.7

1.13

62.6

1.56

81.2

1.33

80.4

1.8

101.1

1.55

103.3

3. Results and discussion Figures. 2 and 3 show the radiation-corrected axial flame temperature profiles of the HEP and MCH flames measured at their largest plate to burner separation distances (Hp,max). The uncertainty of the thermocouple position was assumed to be the radius of the coated thermocouple bead and the temperature measurement uncertainty was considered to originate from the emissivity coefficient uncertainty (0.3~0.6) of the coated thermocouple. Generally, the agreement between the measurement and simulation results was satisfactory for both flames.

Figure 2. Measured (symbols) and simulated (line) axial temperature profiles of the HEP flame at the largest separation (Hp,max=1.8 cm).


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Figure 3. Measured (symbols) and simulated (line) axial temperature profiles of the MCH flame at the largest separation (Hp,max=1.55 cm). Figure 4 shows the calculated temperature-time histories of both flames at five particle residence times. At each particle residence time, the T-t’ profiles of the two fuels were similar. Since the temperature is one of the key factors influencing the nucleation and growth of soot particles, the consistency in temperature-time histories laid the foundation for soot formation comparison of the two flames.


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Figure 4. Temperature-time histories calculated at five particle residence times for the flames studied.


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Figure 5 shows the measured PSDFs of the HEP and MCH flames at different Hp. For the two flames, the size evolution was qualitatively similar. At the early stage, the particle size distribution only consisted of nucleated particles. The growth of these particles produced a shoulder in the size distribution, showing a log-normal distribution when the Hp was larger. At the same time, the persistent nucleation resulted in a strong tail in the PSDF throughout the period of particle size growth. In general, the PSDFs of the two flames both evolved from the unimodal (nucleation mode only) to the bimodal (nucleation and coagulation mode) distribution. The nucleation mode formed the tail branch of the bimodal PSDF, reflecting the number density of recently nucleated small-size particles that were typically in a few nanometers. The coagulation mode was the lognormal branch representing the larger-size particles that coagulated from their older brethren, as well as by surface growth. The coupling between the nucleation and coagulation mode could be represented by the trough in the bimodal distribution. The competition between persistent nucleation and size growth of soot particles led to the observed bimodality 44. This typical bimodal PSDF has been observed previously in premixed flames of ethylene 29, 39, 40, 44-46, propene 30, butane/butanol isomers 5, n-dodecane 47, heptane/toluene blends and practical gasoline 31.With increased Hp, the nucleation tail was attenuated in the MCH flame, while it remained relatively pronounced in the HEP flame. The nucleation tail strength is expected to largely influence the total particle number density, as to be discussed later.


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Figure 5. PSDF measurements of the two flame series. Figure 6 shows the PSDF comparison at different particle residence times. At the residence time of 35 ms, the two flames only generated nucleated particles, and the particle number density of the MCH flame was lower than that of the HEP flame. After 50 ms particle residence time, nucleated and coagulated particles coexisted and the number densities of the two particle classes were comparable. At the same particle residence time, particles in the n-heptane flame developed faster than those in the methylcyclohexane flame. At particle residence times of 80 ms and 102 ms, both the particle nucleation strength and the particle growth rate of the MCH flame were lower than those of the HEP flame. At the residence time of 102 ms, the PSDFs of the HEP and


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MCH flames presented similar shapes, while the PSDF of the HEP flame was shifted to the region of smaller particle sizes.

Figure 6. PSDF comparison between the HEP and MCH flames at different particle residence times.


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Figure 7 shows the total particle number density (N) versus t′ . The error bars were calculated based on the repeatability of multiple PSDF tests. The value of N is calculated according to the following formula

N=

∫

Dm

Dm ,min

dN d log Dm d log Dm

(4)

The criteria for the integral range is that the particles’ dN / dlog(Dm) should be higher than 106 (cm-3). For particles whose dN / dlog(Dm) was less than 106 (cm-3) or particles out of the measuring range (Dm > 66.1 nm), their contribution to the total number density was neglected. It was seen that the number density of the two flames were very close at the particle residence time of about 50 ms. However, with the increased particle residence time, the number density in the MCH flame declined faster than that in the HEP flame, which was consistent with the observation in Fig. 5. At particle residence times of 63 ms / 80 ms / 102 ms, the total number densities of the HEP flame were about 1.5 / 2.5 / 3.2 times as those of the MCH flame.

Figure 7. Particle number density versus particle residence time, lines are for guiding the eyes.


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The soot volume fraction ( Fv ) development with particle residence time was shown in Fig. 8, and the error bars were based on multiple PSDF tests. Here the calculated value of Fv was based on the spherical particle assumption, as shown in Eq. (5)

Fv ( Dm ) =

Dm

π

Dm ,min

6

∫

3 Dm

dN d log Dm d log Dm

(5)

Similarly, only the particles whose dN / dlog(Dm) was higher than 106 (cm-3) were considered in the calculation. For particles whose dN / dlog(Dm) was less than 106 (cm-3), their contribution to Fv was neglected. While for particles larger than 66.1 nm, their contribution to Fv was not negligible, and the value of dN / dlog(Dm) for these particles was taken from the fitting value of the experimental PSDF. The experimental PSDF could be fitted by the following bilognormal distribution function,

dN = d log Dm

∑

2 i =1

Ni 2π log σ g,i

(

 log D m − log Dm ,i exp  − 2 2 log σ g,i 

(

)

2

)

   

(6)

In Eq. (6), N represents the number density of soot particles, σg is the geometric standard deviation, Dm is the mobility diameter, and represents the median mobility diameter. For the nucleation mode, particles were labeled i = 1; for the large-size mode (coagulation mode), i = 2. Details about the bi-lognormal distribution fitting could be found in literature 30, 31. The trend of Fv versus particle residence time was consistent between the two flames. At the five residence times, Fv of the HEP flame was quantitatively higher than that of the MCH flame. Clearly the volume fraction results based on the spherical particle assumption was not sufficient


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to compare the particle growth rate between the two flames, and as such the response of the PSDF parameter, especially that of the large size mode is to be discussed.

Figure 8. Soot volume fraction development with particle residence time, lines are for guiding the eyes. Figure 9 shows the evolution of with t’, and the error bars were based on multiple PSDF tests. The value of was taken from the bi-lognormal distribution function fitted to the experimental PSDF. It is seen that the values of the two flames both increased with particle residence time as expected. Quantitatively, of the MCH flame remained smaller than that of the HEP flame, and the differences were 6.2 nm / 6.1 nm / 7.5 nm / 10 nm at four particle residence times, respectively. This demonstrated that the particle growth rate of the MCH flame was slower than that of the HEP flame.


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Figure 9. evolution with particle residence time, lines are for guiding the eyes. The experimental results above indicated that both the particle nucleation strength and the particle growth rate of the MCH flame were lower than those of the HEP flame. It should also be noticed that the equivalence ratio of the HEP flame was slightly higher than that of the methylcyclohexane flame by about 4.4%. Considering the difference in equivalence ratio, the disparity in soot propensity between the two fuels is not quite significant. Figure 10 shows the computed mole fraction profiles of benzene, acetylene, and methyl for the HEP and MCH flame at their Hp,max. These species concentrations were solved by the OPPDIF code 27, 32 with the reaction kinetic model of JetSurF2.0 38 and the boundary conditions of the inlet and stagnation surface. The computed results shown here were used to illustrate the qualitative consistency between the experimental results and the kinetic understanding. Since the particle residence time was the time duration to traverse from the calculated peak flame temperature location to the stagnation surface, the benzene mole fraction variation with the height above burner surface before the temperature peak was separately shown in Fig.10b. As seen, the benzene concentration of the MCH flame had an observable increase compared to the


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HEP flame at first, probably due to the cycloalkane dehydrogenation. However, this did not result in a faster particle nucleation of the MCH flame, as indicated in our experimental results. The possible reason was that in the MCH flame, the concentration of acetylene (acetylene and methyl are two species considered crucial for PAH formation and growth 48, 49) was always lower than that in the HEP flame, as shown in Fig.10c. Another possible reason was that the PAHs formed were consumed in the high temperature environment which would enhance the oxidation of soot precursors. The benzene concentration from methylcyclohexane kept lower than that from n-heptane after the temperature peak (the origin of particle residence time) till the sampling plate, as seen in Fig.10a. This was consistent with the final experimental results that soot particles from methylcyclohexane had somewhat weaker particle nucleation strength and lower particle number density. While it is beyond the scope of this study, a detailed modeling study on the test flames will definitely contribute to understanding the chemical kinetics during aromatics and soot formation.


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Figure 10. Calculated mole fraction profiles of benzene, acetylene, and methyl for the HEP and MCH flames at their largest plate to burner separation distances.

4. Conclusions The detailed particle size distributions from premixed n-heptane and methylcyclohexane flames were measured and compared in the burner stabilized stagnation flame at different particle residence times. It was observed that the size evolution was qualitatively similar between nheptane and methylcyclohexane flames, both from the unimodal (only nucleation mode) to the bimodal (both nucleation and coagulation mode) distribution. Under the test conditions, the particle nucleation strength and the particle growth rate of the methylcyclohexane flame were lower than those of the n-heptane flame. However, considering the difference in the equivalence


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ratios between the two flames, the molecular structure effects on soot propensity might not be significant for cycloalkanes and n-alkanes,.

AUTHOR INFORMATION Corresponding Author *Telephone: +86 21 34205949 (Zhongzhao Li). Fax: +86 21 34205949 (Zhongzhao Li). E-mail address: [email protected] (Zhongzhao Li). Notes The authors declare no competing financial interest.

Acknowledgement This work was supported by the National Natural Science Foundation of China (Nos. 51210010 and 91441129), and the National Key R&D Program of China (2016YFC0208000).

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