Nascent soot_tang

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Proceedings of the Combustion Institute 000 (2016) 1–8 www.elsevier.com/locate/proci

Nascent soot particle size distributions down to 1 nm

from a laminar premixed burner-stabilized stagnation

ethylene flame

Quanxi Tang

a , b , 1 , Runlong Cai c , d , 1 , Xiaoqing You

a , b , ∗, Jingkun Jiang

c , d , ∗∗

a Center for Combustion Energy, Tsinghua University, Beijing 100084, China b Key Laboratory for Thermal Science and Power Engineering of the Ministry of Education, Tsinghua University, Beijing

100084, China c State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua

University, Beijing 100084, China d State Environmental Protection Key Laboratory of Sources and Control of Air Pollution Complex, Beijing 100084, China

Received 4 December 2015; accepted 31 August 2016 Available online xxx

Abstract

In this study, spatially resolved measurement of soot particle size distribution functions (PSDFs) down to

∼1 nm from a laminar premixed burner-stabilized stagnation ethylene flame was made by paralleling a commercial 3936 Scanning Mobility Particle Spectrometer (3936 SMPS) and a Diethylene Glycol (DEG) SMPS. While the 3936 SMPS may detect particles with a mobility diameter of 3–150 nm, DEG SMPS can be used

to measure incipient soot particles of 1–10 nm. We found that the minimum diameter of the incipient soot particles appeared at ∼1.5 nm (though with some uncertainty caused by the classification device). A complete bimodality of the PSDFs was observed quantitatively when the burner-to-stagnation surface separation distance ( H p ) was greater than 0.6 cm. Characterized by a lognormal distribution, the first peak appears to be relatively stable at different H p , with the geometric standard deviation varying from 1.1 to 1.3 and the peak

diameter ranging from 1.9 to 2.9 nm. The absolute number density of particles no bigger than the first peak

diameter was found to be positively related to the first peak diameter and the geometric mean diameter of these particles. © 2016 by The Combustion Institute. Published by Elsevier Inc.

Keywords: Soot inception; Premixed flame; 1–3 nm particles; SMPS; Bimodality

∗ Corresponding author at: Center for Combustion En- ergy, Tsinghua University, Beijing 100084, China. ∗∗ Corresponding author at: State Key Joint Labora-

tory of Environment Simulation and Pollution Control,

http://dx.doi.org/10.1016/j.proci.2016.08.085 1540-7489 © 2016 by The Combustion Institute. Published by E

Please cite this article as: Q. Tang et al., Nascent sootlaminar premixed burner-stabilized stagnation ethylene(2016), http://dx.doi.org/10.1016/j.proci.2016.08.085

School of Environment, Tsinghua University, Beijing 100084, China.

E-mail addresses: [email protected] (X. You), [email protected] (J. Jiang).

1 Quanxi Tang and Runlong Cai contributed equally to this work and should be considered as co-first authors.

lsevier Inc.

particle size distributions down to 1 nm from a flame, Proceedings of the Combustion Institute

http://www.sciencedirect.com

http://dx.doi.org/10.1016/j.proci.2016.08.085

http://www.elsevier.com/locate/proci

mailto:[email protected]

mailto:[email protected]



2 Q. Tang et al. / Proceedings of the Combustion Institute 000 (2016) 1–8

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. Introduction

Soot generated from combustion is of consid-rable interest not only for its negative influence onnvironment and human health, but also for a num-er of applications, such as carbon black for au-omobile tires and pigment in laser-printer toners,ll of which require a deep understanding of theechanism of soot formation. One of the biggest

hallenges in this field of research is to understandhe process of nucleation and mass growth, espe-ially for nascent soot particles below 10 nm inames.

Recent developments in experimental tech-iques have paved way for a few studies on nascentoot. The on-line dilution probe in conjunctionith the scanning mobility particle sizer (SMPS), inarticular, can follow the evolution of particle sizeistribution functions (PSDFs) from nucleation toass growth for particles as small as 3 nm (if not

pecially mentioned, all diameters are referred tolectrical mobility diameters). Based on this tech-ique, previous studies [1–6] have indicated that theremixed, lightly sooting ethylene flames are of bi-odal characteristics at the region dominated by

oth soot nucleation and mass growth. However,he first peak was only partly observed because of he 3 nm cutoff [7] in commercial SMPS such asSI 3936 used in these studies.

The 3 nm detection limit of commercial SMPSas caused several problems. First, without quan-itative information of the sub-3 nm particles, theumerical simulations [1,2,6,8–10] with a detailedas-phase chemistry, nucleation and mass growthodel could not be completely validated. Secondly, previous study [10] indicated that the trough of SDFs was very sensitive to the size of the nuclei,owever a consensus about the size of the incipientoot particles has not been reached in variousoot models. Mckinnon and Howard [11] maden operational definition for the size of a sootucleus as 2000 amu and a projected area diameterf ∼1.5 nm based on off-line transmission electronicroscopy observations. The smallest particleith a volume diameter of 0.87 nm was used inef. [2] , while some nucleation models regarded theyrene–pyrene dimer as the minimum-sized parti-le with a volume diameter of ∼0.8 nm [1] . Singht al. [10] , however, employed 224 carbon atoms,orresponding to spherical particles of 1.68 nmn volume diameter, to model the PSDFs, which

atched well with experimental data. Thirdly,ome experimental phenomena cannot be ob-erved because of the limitation from experimentalnstruments. For example, the experimental resultsn Refs. [12,13] indicate that the PSDFs will evolvento an apparent unimodal distribution in particleize larger than 3 nm in a high temperature flame,ut it is unclear whether the observed unimodalistribution was affected by the instrument’s
etection limit.

While searching for solutions to the above-mentioned problems, the interests on studyingparticles smaller than 3 nm have continued overthe years. Sgro et al. [14] have attempted to fol-low the gas to particle conversion process usinga differential mobility analyzer (DMA) and afaraday cup electrometer (FCE). Based on theirobservations, there were three peaks in the PSDFsof the ethylene/air premixed flames at a samplingheight of 10 mm. Particle diameters at the firstand the second peaks of the PSDF were approx-imately 0.8 and 2 nm, respectively. According totheir analysis, the first peak was attributable tocarbon compounds or molecular clusters in theflame. Abid et al. [15] adopted a similar method toinvestigate the premixed flat ethylene flames andsucceeded in extending the range of measurementdown to ∼1.9 nm. Different from the results re-ported by Sgro et al. [14] , minor drop-offs wereobserved around 2.1 nm, but they were attributedto inefficient non-steady state bipolar charging of the instrument. For super 3 nm soot particles, themeasurement results obtained by FCE are com-parable with those obtained by UCPC [15] . Thecommon problem of these FCE measurements isthat the results are interfered by charged molecularclusters (ions) when measuring particles smallerthan 2.1 nm. In addition, FCE often suffers fromhigh background noises. Although nascent sootparticles smaller than 3 nm have been detectedwith FCE, it is difficult to obtain quantitativeinformation due to these problems, and it remainsunclear how small the particles could be in thefuel-rich premixed ethylene flames due to an am-biguous boundary between the nascent soot andthe molecular clusters.

The lower detection limit of commercial SMPS( ∼3 nm) mentioned above is due to one of its com-ponents, ultrafine condensation particle counter(UCPC, TSI 3025 or 3776), which uses butanol asthe working fluid [7] . It can hardly activate particlessmaller than 3 nm because of the significant Kelvineffect of these particles. Recent studies [16,17] sug-gested that the working fluids with high surfacetension and low vapor pressure may overcome thisproblem without causing homogenous nucleation.Diethylene glycol (DEG) meets with these criteria.UCPC using DEG as the working fluid has beenproved to be able to activate particles down to∼1 nm [16,18–20] . Compared with FCE, DEGUCPC has low background noise and can be tunedto become insensitive to selected charged molecu-lar clusters (ions) by adjusting the vapor saturationratio inside the activation/growth unit [18] . Basedon the new UCPC, DEG SMPS has been devel-oped as a size spectrometer for sub-3 nm particles,which has been successfully deployed in measuringnewly formed particles during atmospheric nu-cleation events [18,19,21] . This new spectrometerhas not been used in any previous soot formation
studies.


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Fig. 1. Schematic of experiment setup.

The purpose of this study is to expand thesoot particle detection limit down to 1 nm, obtaindetailed PSDFs in a burner-stabilized stagnationethylene flame using the burner-stabilized stagna-tion probe together with the prototype DEG SMPSand a commercial TSI 3936 SMPS, and analyze theevolution of PSDFs, especially for sub-3 nm sootparticles.

2. Experimental

The experimental setup is presented inFig. 1 . Similar to what was described in ourprevious work [22,23] , a laminar premixed flatethylene flame with an unburned composition of 14.4% (mol) ethylene, 21.6% (mol) oxygen and64% (mol) argon (equivalence ratio, ϕ = 2) wasgenerated by a commercial McKenna burner witha stainless outer layer and a 6 cm-diameter-bronzewater-cooled porous sintered plug. The cold gasvelocity was 7 cm/s (STP), which was controlled bya sonic nozzle and calibrated by a BUCK soap-filmflow meter (Model M-30). The flame was shroudedfrom the ambient air by a nitrogen flowing at40 cm/s through a concentric porous plug.

The burner-stabilized stagnation flame configu-ration was used to probe soot PSDFs. A samplingtube made of stainless steel with an outer diameterof 6.35 mm and wall thickness of 0.125 mm wasembedded in a 1.3 cm thick aluminum disc. Theflame gas containing soot particles was drawninto the sampling probe through a 160 μm orifice,which was cleaned by a fine stainless needle aftereach scan because it can be easily clogged by sootparticles after sampling for a long time. Watercooling copper coils were attached to the top of the disc to avoid an excessively high temperatureof the disc. The temperature of the orifice wasmeasured by a type-K thermocouple imbedded in-side of the aluminum disc. The orifice temperature


was about 436 ± 30 K when sampling. The burner- to-stagnation surface separation distance H p was determined by a Vernier height gauge with an

accuracy of ± 0.02 cm. The flame temperature was measured by a type-S thermocouple coated with

a Y/Be/O mixture to prevent surface catalytic re- actions. The diameter of the thermocouple before and after coating is 125 and 137 μm, respectively. Radiation corrections were made using the proce- dure of Shaddix [24] , with gas mixture properties calculated iteratively by a modified OPPDIF code [25] using a detailed reaction model of USC-Mech

II [26] . The upper and lower temperature limits were determined on the basis of the uncertainties of emissivity of the coated thermocouple that varies from 0.3 to 0.6 [27] . The radiation-corrected

temperature was assumed to be the average of the two limiting values. The standard test of total number density as a function of dilution ratio was performed with 3936 SMPS and DEG SMPS similarly as in our previous work [23] and the results were presented in Fig. S1 of the Supplemental material.

Two sets of SMPS were used to measure size distributions of the diluted soot particles in paral- lel. The first set is a TSI 3936 SMPS without any modification, which has been used in our previous research [22,23] . Another set of DEG SMPS

was applied to measure 1–10 nm soot particles. It consists of a neutralizer (TSI Model 3087), a nanoDMA (TSI Model 3085), a DEG UCPC

(modified from TSI Model 3776), and a “booster”CPC (TSI Model 3772). The sample flow rate of the nanoDMA was 2 L/min, and the sheath flow

rate was 20 L/min. The sheath-to-aerosol ratio

is the same with that of the 3936 SMPS. The temperatures of the saturator and the condenser were set at 63 and 20 °C, respectively. 2 L/min of classified monodisperse flow went into the DEG

UCPC, of which 0.25 L/min flowed through the saturator after the particles were filtered and water vapor was removed, and 0.05 L/min went directly




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nto the condenser as the capillary flow. Havingeen activated by DEG, particles with a flow ratef 0.3 L/min went into the CPC 3772 to furtherrow into micrometer size range and got detected. filtered makeup flow of 0.7 L/min was applied

o match with the required inlet flow rate of CPC772. Compared with the 50% cut-off size, D 50 , of .5 nm of the UCPC 3776 or 3025, DEG UCPCad a lower D 50 of 1.4 nm. The detection efficiencyf 1.1 nm particles is ∼10%, which has been cali-rated by negatively charged NaCl particles. Theaximum measurement range of DEG SMPS is

rom 1 to 10 nm, which is divided into 30 bins.ach scan cycle took 130 s, an up scan and a down

can of 60 s each; and a zeroing time of 10 s. Thealibrated transport time between the nanoDMAnd the DEG-UCPC was 2.4 s. Up and downcans agreed well at low soot concentrations. How-ver, DEG UCPC needs several seconds to zerofter it measures extremely high concentrationsf particles. Therefore, only up scan data weredopted to avoid the problem of zeroing instantlyfter measuring extremely high concentrationsf soot particles in the down scan and to keephe scan time consistent with that of 3936 SMPS.lectrical mobility diameter was used in this study,

hough the mobility-volume relationship has beeneported and can be readily applied [15,28] .

Independent procedures were used to calculatehe soot particle number density, n ( n = dN / d log D m

,here N is the number of particles and D m

is thelectrical mobility diameter) in the flames, whichas related to the number density measured by936 SMPS, n 3936 , and raw count data, n DEG , byEG SMPS. In regard to 3936 SMPS, the TSIIM software was used to obtain the inversedata of n 3936 , while n is related to n 3936 through the

ollowing equation,

= DR

n 3936

ηdiff 3936 (1)

here DR is the dilution ratio; ηdiff3936 is the particleiffusion loss factor in the sampling line betweenhe inlet of orifice and the impactor, which cane calculated by the empirical equations providedy Cheng et al. [29] . n DEG was recorded by a Lab-IEW program and inverted by a C ++ program

o calculate number density, which is related to nhrough a series of corrections including dilution,harging, classification, activation, and diffusionoss by Eq. (2) .

= DR

n DEG

ηch arg ing · ηDMA

· ηDEG UCPC · ηdiff , DEG

(2)

here ηchrging is the charging fraction of particles.ipolar charging fraction was estimated using theodified Fuchs theory [30] . Its uncertainty for sub- nm particles needs to be addressed experimentallyn future study. ηDMA

refers to the penetration of he nanoDMA calculated by diffusional loss of quivalent length method [31] . ηDEG UCPC is the


detection efficiency of DEG UCPC. ηdiff, DEG

is thediffusion loss in the sampling line from the inlet of the sampling orifice to DEG UCPC.

3. Results and discussion

We measured the temperature profiles at severalburner-to-stagnation surface separations and maderadiation corrections using the method describedabove. As shown in Fig. 2 , the vertical error barsrepresent the uncertainty of emissivity of thecoated thermocouple, which generates an approx-imate ± 90 K uncertainty in peak temperatures.The horizontal error bars are due to the positionuncertainty. The model predicted temperatureprofiles agree well with the radiation-correctedmeasured temperature profiles within experimentaluncertainties.

Figure 3 presents the PSDFs at several selectedrepresentative burner-to-stagnation surface separa-tion distances, H p = 0.45–1.2 cm. The parameters’effect of BSS flame technique on soot mobility havebeen introduced in previous studies [23] and theprobe effect on measuring nascent soot particleswas discussed in [32] . The PSDFs measurementswere repeated at least three times for each positionwith an average relative error of 14% and alldata from each measurement were included inFig. 3 . As illustrated here, PSDFs are reproducible.The DEG SMPS data agree well with the 3936SMPS data at lower H p ( ≤0.5 cm) for particlesbigger than 3 nm in diameter, but fewer particleswith a diameter of 6–10 nm at H p ≥ 0.6 cm weremeasured by the DEG SMPS than those by the3936 SMPS. One possible deviation is due to theuncertainty of charging fractions, since the 3936SMPS classifies positively charged particles, whilethe DEG SMPS classifies negatively charged onesfor higher detection efficiency [18] .

As shown in Fig. 3 , a distinctive peak at∼2.4 nm mobility diameter can be observed for thewhole range of separation distances from 0.45 to1.2 cm, which might be caused by the competitionof continuous nucleation and growth. A similarpeak has also been reported in Refs. [33,34] usingDMA (TapCon 3/150) equipped with a FCE if hy-pothetically excluding the interference of ions. Theobservation of this peak indicates that nucleationdoes exist in the post-flame region, and the uni-modality of PSDFs [12,13] of the high temperatureflames only represents part of the complete PSDFdue to the 3 nm cutoff limitation of the commercialSMPS. While the smallest soot particles detectedby DEG SMPS appear at ∼1.5 nm and remainsnearly the same at different H p , it does not meanthe smallest soot particles generated by nucleationin the flame is 1.5 nm, as will be discussed later. It isworthy to mention that the detection limit of DEGUCPC was calibrated by particles with differentchemical compositions [16,18] , and none of the




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Fig. 2. Radiation-corrected temperature profiles. Symbols and lines are measured and simulated data, respectively.

Fig. 3. Evolution of PSDFs at different burner-to-stagnation separation distances. The open symbols are measured by the DEG SMPS while the filled ones by the 3936 SMPS. Lines are fitting results. A lognormal distribution and a bi-lognormal distribution are used as fitting functions at H p = 0.45–0.55 cm, 0.6–1.2 cm, respectively. The dashed lines are drawn to guide the eye.

detection efficiency was zero for 1.5 nm particles.It is thus reasonable to believe that 1.5 nm is notthe detection limit of DEG UCPC. In addition,the PSDFs of a fuel-lean flame and a slightlyfuel-rich flame (equivalence ratio ≤1.5) have beenmeasured. No signal was observed for these flames.Considering that ions are ubiquitous in flames and


the detection efficiency of 1.47 nm tetra-heptyl ammonium ion was less than 1% at the operated

conditions of DEG UCPC, we conclude that these signals corresponding to ∼1.5 nm particles are not caused by ions from the flame. Future work will be performed to further explore the mechanism of soot nucleation.




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Fig. 4. A typical unimodal distribution of nascent soot at H p = 0.5 cm. Symbols are experimental data, and the solid line is fitted to the data using a lognormal function. The dash-dotted and dashed lines are theoretical transfer functions of nanoDMA multiplied by a factor (see text) to match with measured number density at 1.54 and 1.67 nm, respectively.

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Fig. 5. N T (absolute number density of particles no bigger than the first peak diameter), D p (peak diameter at the first peak), and D gm

(geometric mean diameter of particles no bigger than the first peak diameter) as a function of burner-to-stagnation separation distance. Symbols are the experimental data. Lines are drawn to guide the eye.

Figure 3 indicates that the evolution of PSDFss a function of H p experiences two different pro-esses: lognormal and bi-lognormal distributions.t H p = 0.45–0.5 cm, newly nucleated particles are

haracterized by the lognormal distribution. As fars the actual size of the smallest particles in flamess concerned, despite the fact that the smallest par-icles detected by the DEG SMPS were ∼1.5 nm iniameter, the real size might have been a little big-er than 1.5 nm due to the resolution limitation of he nanoDMA used in this study. Figure 4 showshe measured PSDF at H p = 0.5 cm, and the theo-etical transfer functions of nanoDMA [31] multi-lied by a factor to match with the measured num-er density at 1.54 and 1.67 nm. Transfer function

s defined as the probability that an aerosol particlehich enters the mobility analyzer via the aerosol

nlet will leave via the sampling flow. A multiplica-ive factor ( n / �∗, where �∗ is the peak value of theransfer function) was used in these transfer func-ions. Whether particles smaller than 1.54 nm ex-sted or not cannot be proved due to the diffusionroadening effect in the nanoDMA, while the mea-ured data points between 1.4 and 1.5 nm are signif-cantly higher than the inverted number density us-ng the transfer function of 1.67 nm, which suggestshat particles bigger than 1.67 nm surely existed inhe flame. Future work will be performed to reducehe diffusion broadening effect by using a DMA of igher resolution.

A sum of a power-law function and a lognor-al distribution function is usually used to fit theSDFs with mobility diameters larger than 3 nm

35] . With these new sub-3 nm PSDFs data fromhe DEG SMPS, the combined PSDFs have shown distinctively bi-lognor mal for m (the fitting equa-ions can be found in supplemental material) at theeights from 0.6 to 1.2 cm as shown in Fig. 3 , which


are the results of continuous nucleation, coagula-tion, coalescence, and surface growth. However, atH p = 1.2 cm, particle nucleation became less dom-inant and the number density at the first peak de-creased by an order of magnitude compared withthat at H p = 0.7 cm. At the same time, the secondpeak of PSDF is wider, indicating that soot dynam-ics has gradually changed due to a longer residencetime.

Figure 5 presents the absolute number densityof particles no bigger than the first peak diame-ter, N T (obtained from integration of their numberdensity), the diameter at the first peak, D p , and thegeometric mean diameter of these particles, D gm

.As H p gets higher, all three quantities N T , D p , andD gm

increase first and then decrease. The overalltrends of N T , D p , and D gm

look similar, suggestingD p and D gm

are positively related with N T , whichmakes sense because in theory a larger N T meansa higher collision frequency that usually leads tolarger particles at the first peak as well as larger ge-ometric mean diameter, and vice versa. In fact, D p

obtained at different H P in this study is similar tothat reported in Ref. [14] and the minor drop-off inthe number density in Ref. [15] using FCE is alsosimilar.

To understand the variation trend of peakheight and peak width of the measured PSDFs,we present in Fig. 6 the geometric standard devia-tions, σ , of the lognormal part of PSDFs as a func-tion of separation distance, H p . The values of σ atthe second peak increasing from 1.36 to 1.65 areconsistent with those in the study of Zhao et al.[35] . The continuous upswing of σ at the secondpeak at H p = 1.2 cm corresponds to a visualizedchange of size distributions, indicating a change inparticle dynamics, such as the formation of soot ag-gregates. As a result of the competition of small




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Fig. 6. Geometric standard deviations of the lognormal part of PSDFs as a function of burner-to-stagnation separation distance.

particles generated by nucleation and those con-sumed by surface growth and coagulation, the firstlognormal peak has smaller values of σ varyingfrom 1.14 to 1.32. As can be seen from Figs. 5 and6 , the trend of the diameter at the first peak of the lognormal distribution and that of σ is simi-lar, which is marginally different from those in Ref.[34] , keeping nearly constant at different separationdistances.

4. Conclusion

By expanding the lower detection limit down to∼1 nm using a DEG SMPS, we observed a com-plete bimodality in the size distribution of nascentsoot in a laminar burner-stabilized stagnationpremixed ethylene flame. The smallest particlesdetected by the DEG SMPS in flame at differentseparation distances appeared at ∼1.5 nm in di-ameter, although only particles with a diameterlarger than 1.67 nm were sure to exist in the flamewith the nanoDMA. The diameter at the first peakwas ∼2.4 nm, which was affected by the absolutenumber density of small particles. The increasedabsolute number density led to a bigger D p . Thegeometric mean diameter of particles no biggerthan the first peak diameter has similar varia-tions to those of the peak diameter. At a lowerburner-to-stagnation surface separation distance( H p = 0.45–0.55 cm), the flame was dominated bynucleation, resulting in a lognormal distribution.As H p was increased, the bi-lognormal distributionwas observed because of the enhanced particlecoagulation.

Acknowledgment

Financially support from the National Key Ba-sic Research and Development Program of China


( 2013CB228502 and 2013CB228505 ), the National Key Foundation for Exploring Scientific Instru- ment of China (20121318549) , the National Nat- ural Science Foundation of China ( 91541122 , 21221004 , 41227805 , and 21422703 ), and “Strate- gic Priority Research Program” of the Chinese Academy of Sciences ( XDB05000000 ) are ac- knowledged.

Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi: 10.1016/j.proci.2016.08.085 .

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