Soot measurements at the axis of an ethylene/air non-premixed turbulent jet flame

Bing Hu, Bo Yang, Umit O. Koylu*Department of Mechanical and Aerospace Engineering and Engineering Mechanics, University of Missouri-Rolla,

Rolla, MO 65409-0050

Received 12 August 2002; received in revised form 17 January 2003; accepted 10 March 2003

Abstract

The size, morphology, and volume fraction of soot particles within the fuel-rich regions of a non-premixedturbulent jet flame fueled by ethylene/air at atmospheric pressure were investigated. Experiments involvedthermophoretic sampling followed by transmission electron microscopy and laser extinction techniques todetermine mean soot properties of principal interest at various heights along the flame axis. Similar to numerouspast studies, soot in this turbulent flame consisted of nearly uniform spherical particles that collected intoaggregates of different shapes and sizes. Soot spherule (primary particle) diameters were 19 to 35 nm, inagreement with the narrow size range reported in many combustion environments. With fractal dimensions andprefactors in the range 1.74 � 0.11 and 2.2 � 0.4, respectively, aggregate morphology was also found to bealmost identical to soot formed in various laminar flames and emitted from turbulent flames. These universalparameters apparently resulted from the dominant cluster-cluster agglomeration mechanism, which was alsoresponsible for the monotonic increase in the average aggregate size with height above the burner. The mean sootvolume fraction reached a maximum of 1.1 ppm at a height lower than the peak axial location of the spherule size,while the particle number density varied in the range of 1010–1011 particles/cm3 with a similar trend. The peakvalue of the actual specific surface area was 2.5 cm2/cm3, which was about a factor of three larger than the valueestimated by a volume-equivalent spherical model. The results clearly identified various axial zones of prevailingsoot nucleation, surface growth, oxidation, and agglomeration processes in the non-premixed turbulent flameconsidered here. © 2003 The Combustion Institute. All rights reserved.

Keywords: Soot formation; Turbulent flames; Particle size/morphology; TEM

1. Introduction

Soot particles formed during combustion pro-cesses significantly affect the performance and dura-bility of many engineering systems such as gas tur-bines and diesel engines. Soot emissions fromvarious sources have also been reported to have se-rious adverse effects on human health because thesenanometer–size particles are believed to be responsi-

ble for increasing the risk of death by as much as15% in cities with heavy air pollution [1]. Moreover,it has recently been argued that flame-generated sootmight be a major contributor to global warming, onlysecond to CO2 [2]. These important technologicaland environmental implications motivate advancedresearch for a complete understanding of the factorsgoverning the complex soot processes in flames. Thedetailed study of soot phenomena is also a necessarystep toward the development of accurate numericalsimulations of practical combustion devices with acrucial progress from ad-hoc/empirical approaches to

* Corresponding author.E-mail address: koyluu@umr.edu (U.O. Koylu).

Combustion and Flame 134 (2003) 93–106

0010-2180/03/$ – see front matter © 2003 The Combustion Institute. All rights reserved.doi:10.1016/S0010-2180(03)00085-3

reliable computational models during the designphase of combustion devices.

Past studies of soot processes in flames have been

reviewed in [3–5]. Although turbulent flame condi-tions prevail in many large-scale combustion sys-tems, they are relatively difficult to probe due to theinherent fluctuations in the flow field. Consequently,crucial investigations of soot formation in non-pre-mixed flames have primarily been based on the dataobtained under laminar conditions (e.g., [6–9]),which are experimentally and computationally moretractable. The findings in laminar flames have beenwidely used to describe the processes in turbulentflames using the laminar flamelet concept. In general,there is relatively limited number of studies on soot-containing turbulent flames compared to the laminarones in the literature. Experimental investigations ofsoot-containing turbulent flames have usually re-ported the soot volume fraction, fv, from either clas-sical light extinction measurements [10–13] or therecently developed laser-induced incandescence (LII)technique [14]. Some of the investigations also mea-sured the volume-equivalent soot diameter, deq, fromthe analysis of scattering measurements based onunsuitable spherical models [15,16]. Although suchcharacterizations of the soot field were useful, thepioneering work of Magnussen [17] in an acetyleneturbulent flame demonstrated that the aggregationprocess must be accounted for during the scatteringanalysis for reliable results. This is due to the factthat, after the initial nucleation region, the coagula-tion of soot particles leads to chained-like aggregatesrather than coalesced spherical particles, as verifiedin a wide range of laminar flame environments[8,18,19]. Furthermore, it is difficult to decouple var-ious soot processes from each other using fv and deq,which are not fundamental quantities for describingand modeling the soot phenomena. For example, fv �(�dp

3/6)np alone, without supplemented by the sootsize, yields only an overall effect of growth (repre-sented by the spherule diameter, dp) and nucleation(represented by the particle number density, np) pro-cesses. A similar argument also holds for deq �N1/3dp, which is insufficient to separate soot aggre-gation, N, from surface growth, dp.

In addition to the above shortcomings, the mor-phology of soot and its variation within turbulentflames are not well established in the literature.Koylu and Faeth [20] extensively investigated thesize and morphology of soot emitted from variousgas- and liquid-fueled non-premixed buoyant turbu-lent flames. However, their experimental conditionswere limited to the overfire (fuel-lean) regions offlames where chemical reactions were diminisheddue to relatively cold temperatures. Recently, Fang etal. [21] reported measurements of particle and aggre-gate size statistics at five locations within a turbulentcombustor. Their experiments excluded soot concen-tration and involved a confined, liquid-fueled, and

Nomenclature

Aa projected area of an aggregatedp spherule (primary particle) diameterD burner exit diameterDf aggregate fractal dimensiondeq volume-equivalent diameter, N1/3dp

E refractive index functionfv mean soot volume fractiong gravitational accelerationHf visible flame heightI transmitted laser intensity through

the flameI0 incident laser intensitykf fractal prefactorkL correlation constant based on L, Eq.

4Kext extinction coefficientL maximum projected length of an ag-

gregatem mass flow rate of fuel at the burner

exitnp number of spherules per unit volumeN number of spherules in an aggregater radial location (flame axis at r � 0)Rg gyration radius of an aggregateRe Reynolds number at the burner exitRi Richardson number at the burner

exitSp soot surface area per unit volumeV average fuel velocity at the burner

exitz axial location (height above the

burner exit)

Greek Symbols

� wavelength of light�s optical path length (distance be-

tween the optical probes)� fuel viscosity� fuel density�p standard deviation of the mean

spherule diameter

� � average aggregate size

94 B. Hu et al. / Combustion and Flame 134 (2003) 93–106

re-circulating flame. Although such a complex con-figuration was relevant to practical gas turbine com-bustors, it was not ideally suited for convenient com-parisons to computational predictions, i.e., sootmodel validation. Both of these studies exclusivelyused thermophoretic sampling technique, which hasalso been successfully implemented in laminarflames in the past [22–24]. This experimental methodinvolves rapid insertion of a small probe into a de-sired flame location and subsequent analysis of par-ticles deposited onto a grid under a transmissionelectron microscope (TEM). It should be emphasizedthat a prior knowledge of particle bulk density andrefractive index is not required with this novel diag-nostic technique.

The main objective of the present study was toquantify the evolution of soot particles within thefuel-rich conditions of non-premixed turbulentflames. A simple burner and ethylene fuel were cho-sen to provide data in a well-defined experimentalconfiguration involving relatively light soot concen-trations. Measurements included thermophoreticsampling followed by TEM observations and laserextinction to determine time-averaged soot proper-ties, i.e., spherule (primary particle) diameter, aggre-gate size distribution, fractal morphology, and vol-ume fraction. Mean soot number density, np, andspecific surface area, Sp, were also determined. Ex-periments were conducted at various heights alongthe centerline of an axisymmetric turbulent jet flameburning ethylene in atmospheric-pressure air. Theresults are compared with and contrasted against pre-vious data in the fuel-rich conditions of laminarflames and in the fuel-lean (overfire) regions of tur-bulent flames. Accordingly, the experimental condi-tions considered here involved characteristic timesbetween these two flame conditions.

The present research couples the actual size andmorphology information with the soot volume frac-tion so that the parameters of principal interest, suchas dp, np and Sp, in soot formation studies can accu-rately be depicted. This key aspect is a step towardimproving our understanding of various soot pro-cesses within hydrocarbon-fueled turbulent flamesand therefore helpful in the development of reliablemodeling of practical combustion devices [25]. Sucha contribution is also valuable in the interpretation ofin situ optical measurements, which can allow explo-ration of not only the mean but also the fluctuatingsoot field during turbulent combustion for a widerrange of experimental conditions. Finally, the presentfindings are relevant to the predictions of radiationproperties of luminous turbulent flames with impli-cations in natural fires and industrial furnaces.

2. Experimental methods

2.1. Apparatus

A sketch of the general experimental arrangementis shown in Fig. 1. Combustion was in stagnant air atatmospheric pressure within a 2.4-m-high enclosure,which had a metal hood and an exhaust system at thetop to remove combustion products from the labora-tory. Plastic strips, which covered the sidewalls of theenclosure until near the floor, helped minimize po-tential flame disturbances from the surroundings. Awater-cooled stainless steel tube with an inside diam-eter of D � 4.56 mm was used as the burner toproduce non-premixed turbulent jet flames. Theburner was mounted at the center of the enclosure ona computer-controlled traversing system that offeredvertical and horizontal positioning within an accu-racy of 10 �m. This accommodated the fixed instru-mentation, and, with the coordinate system beingattached to the center of the burner exit, allowedmeasurements at various axial (z) and radial (r) lo-cations within this axisymmetric flame.

The present test conditions are given in Table 1.The ethylene gas (98% purity) stored in a high-pressure cylinder was vertically delivered to theburner using a two-stage pressure regulator, a flowmeter, and polyethylene tubing. The burner was longenough (150 mm) to result in fully developed flowconditions at its exit. The flow rate of ethylene mea-sured by the flow meter corresponded to a Reynoldsnumber of 13,500 at the burner exit using the averagefuel velocity and viscosity at room conditions. Al-though the source Richardson number was relativelysmall at the burner exit, the downstream flame loca-

Fig. 1. Sketch of the general experimental configuration.

95B. Hu et al. / Combustion and Flame 134 (2003) 93–106

tions were significantly influenced by buoyancy. Theflame was naturally attached at the burner rim andobserved to be yellow except near its base where ablue color indicated a soot-free region. However,continuum radiation due to the formation of soot afterthese lowest positions caused a primarily luminousflame. Visible flame height was measured to be0.84 m by averaging a number of images capturedwith a digital camera. This measured value was inexcellent agreement with the past observations; e.g.,the correlations given by Delichatsios [26]. Thepresent relatively simple and free-to-develop flameconfiguration was chosen to establish a well-definedand highly reproducible experimental environmentwithout the complications of any physical restric-tions.

2.2. Thermophoretic sampling experiments

Ex-situ thermophoretic sampling and transmis-sion electron microscope (TS/TEM) experimentswere conducted to obtain time-averaged soot proper-ties at various heights along the axis of the flame. TheTS technique employed here was an extension of theone originally developed by Dobbins and Megaridis[22] based on the theoretical treatment of Eisner andRosner [27] for determining the size and morphologyof flame-generated particles. It has been extensivelyemployed for soot formed in laminar flames (see,Megaridis and Dobbins [23], and references citedtherein) and soot emitted from turbulent flames [20].Recently, it was also demonstrated that the TS tech-nique could yield not only the soot size and morphol-ogy but also the volume fraction in laminar flames[24]. Although intrusive in nature, it has the keyadvantage over other sampling methods because ofthe one-step fast extraction of flame-generated sootwithout any further sample manipulations that maypossibly alter the original state of particles. Addition-ally, particle mass transport to a cold surface due tothermophoresis is relatively insensitive to the sizeand morphology for aerosols in the free molecularand transition regimes [27]. Therefore, extensivecomparisons to independent optical measurements[8,18,19] have established thermophoretic samplingas a reliable experimental technique to discover rep-resentative particle properties of their natural envi-

ronment. Characterization of particles with visualTEM inspections is also independent of soot refrac-tive index, which introduces additional experimentaluncertainties to the interpretation of laser-based par-ticulate diagnostics.

The present TS instrumentation was designed fol-lowing the past practice [20–24], while it involvedparticular improvements for sampling only at thedesired flame locations, precise sampling times, andaccurate correspondences between the flame and mi-croscope coordinates. As illustrated in Fig. 2, thesampling system consisted of a collection surface(TEM grid), a carrier probe stored in a protectivetube, a piston activated by compressed air/solenoidvalve, a sliding base, and an electronic controllerwith a digital timer. Special 3 mm-diameter coppermicroscope grids (Electron Microscopy Sciences CF-H4-Spec-Cu) served as cold collection surfaces.These carbon-supported grids had arrows pointingfrom the edges toward their marked centers forachieving orientation and alignment with respect tothe flame axis. For each sampling, a single grid wasattached to the tip of a stainless steel substrate (3.8mm width and 0.8 mm thickness) by aligning thearrow at the edge of a TEM grid in the verticaldirection parallel to the mean flow direction at theflame centerline. The substrate was stored in a sta-tionary metal tube of rectangular cross section (4.8mm by 2.4 mm). When activated by a 75-mm-strokedouble-acting pneumatic air cylinder, the probe car-rying a grid rapidly moved, extending only 4 mmbeyond the end of the protective tube. This exposedeach TEM grid only to the intended flame location,preventing soot deposition before and after sampling.Consequently, the protective cover ensured no parti-cle contamination from other radial flame locationsduring the transit time of the probe. When fullyinserted into flame, hot particles from the flame weredriven to the cold probe, which was aligned in thevertical direction approximately parallel to the meanflow. Controlled by an electronic circuit and a four-way valve, the probe stayed at the sampling locationfor a specified time before being extracted from theflame.

The positioning accuracy and sampling timeswere carefully confirmed by utilizing a laser beamand a detector. First, the flame centerline at each

Table 1Test conditions

D (mm) m (g/s) V* (m/s) Re** Ri† Hf‡ (m)

4.56 0.48 25.8 13,500 6.7 � 10�5 0.84

* V � 4 m/�D2� where � � 1.14 kg/m3; ** Re � VD/� where � � 8.7 � 10�6 m2/s; † Ri � gD/V2 where g � 9.8 m/s2;‡ Average visible flame height from digital photographs.

96 B. Hu et al. / Combustion and Flame 134 (2003) 93–106

sampling height was found from the symmetry of thetransmission signals measured at various radial loca-tions. Then, the probe was actuated with its fullyextended tip cutting the laser beam into half, as ob-served by an oscilloscope connected to the detector.The oscilloscope trace permitted the evaluation ofsampling times, which were typically 60 to 110 mssuch that particle coverage was generally less thanabout 15% of the total grid area for the present testconditions. Such short exposure of the probe to thesoot-laden flame minimized artificial overlapping ofparticles/aggregates on TEM grids. After positioningthe probe tip relative to the laser beam, the slidingbase carrying the probe was traversed 1.5 mm for-ward in order to match the center of grids with theflame axis. A flame positioning accurate to approxi-mately 0.5 mm was accomplished using this proce-dure.

After sampling, the TEM grids were stored inspecial boxes for subsequent observations with aPhilips EM 430T transmission electron microscopeoperating at 100 kV. The vertical arrow and dia-mond-shape center markings on a TEM grid (see Fig.2) permitted a precise correspondence between theflame location and microscope coordinate. Only par-ticles collected on a few meshes near the center of agrid was considered during the TEM analysis, ignor-ing the rest of the grid surface. Specifically, the actualarea analyzed under the microscope was about 0.1mm2, which was appreciably smaller than the overallsize of a 3-mm grid. Consequently, the present ther-mophoretic sampling experiments had a spatial res-olution of better than 0.5 mm, which was essentiallysufficient to resolve even the smallest soot streaks(�1 mm) that may exist in turbulent flames. TheTEM pictures of randomly chosen aggregates aroundthe center of the specimen grids were taken undervarious magnifications (�7400 to 42,500) and then

digitized using a scanner. The images were stored ona PC before being analyzed using the Image Pro-Plussoftware. The same photography, digitization, andcomputer analysis procedures were also applied tothe latex calibration spheres having a uniform diam-eter of 312 nm in order to achieve an accurate con-version from pixels to nanometers for all magnifica-tions considered. Soot spherule diameters, dp, weremeasured by manually detecting about 20 to 40 ap-parent particle profiles near aggregate edges on rel-atively high-magnification images. For each sample,800 to 2000 aggregates were analyzed using rela-tively low-magnification images to measure pro-jected areas, Aa, and maximum lengths, L, of parti-cles/aggregates within an image. Various imageenhancement tools and automated features integratedinto the computer software helped to process the dataconveniently. With the specific methodologies to beconveyed later on, it should be noted that dp, Aa, andL obtained from the projected (2-D) TEM imageswere sufficient to recover the actual (3-D) aggregatesizes and fractal properties [28].

2.3. Laser extinction experiments

The optical arrangement for laser extinction mea-surements was similar to the one used in this andother laboratories for investigating luminous laminarand turbulent flames. The sending optics and instru-mentation were placed on an optical table outside theplastic enclosure. The light source was a He-Ne laser(JDS Uniphase 1137P) emitting a 500:1 verticallypolarized light at a wavelength of � � 632.8 nm witha power of 7 mW. The laser beam was first choppedat 1015 Hz with a mechanical chopper (Perkin Elmer651) and then focused at the center of the burner witha 1.5 m-focal-length lens. The collecting optics in-cluded a lens and a photodiode detector (Newport

Fig. 2. Sketch of the instrumentation for thermophoretic sampling measurements.

97B. Hu et al. / Combustion and Flame 134 (2003) 93–106

818-SL), which measured the incident and transmit-ted light intensities without and with a flame, I0 andI, respectively. The stability of the laser was con-firmed to be within 1% by checking the referencesignal (I0) before and after each test run. Unwantedradiation from the flame and surroundings were min-imized with a lock-in amplifier (Perkin Elmer 7225)and a laser-line interference filter. Iris diaphragmswere placed along the optical path to eliminate laserreflections from the components and the apparatus.The readings from the lock-in amplifier were trans-ferred to a computer via a data-acquisition system forstorage and subsequent data analysis. Three to fiverealizations of an experimental condition, each in-volving 500–5000 data points over 5–20 s, wereaveraged to achieve repeatability in I/I0 within 2%.

To obtain local soot volume fractions, laser ex-tinction experiments were carried out using either thedeconvolution of line-of sight radial data or a probethat restricts the optical path length. Deconvolution isa classical mathematical method that converts theintegrated absorption measurements at various radiallocations to local soot volume fractions in axisym-metric flames. Despite its wide-spread successful usein both laminar and turbulent flames, the deconvolu-tion method may comprise error buildup as r 3 0(near the flame axis). On the other hand, an opticalprobe has also been successfully employed to mea-sure local soot volume fractions in turbulent flames[11–13]. However, as in the case of any intrusivemethod, concerns regarding the influence of a phys-ical probe on the measurements may be raised.Therefore, both techniques were implemented duringthis study to complement each other.

The deconvolution method involved an Abel in-version of line-of sight transmission data at certainradial intervals of �s across each flame height. On theother hand, the optical probe consisted of two iden-tical tubes of 4.27 mm ID and 6.43 mm OD thatprotected the laser beam as it traveled in and out ofthe desired flame location. The separation distancebetween the probes represented the optical pathlength, �s, over which the average light absorption,I/I0, was measured. Both of these non-intrusive andintrusive versions of the laser transmission techniqueyielded the local extinction coefficients indepen-dently. Then, mean soot volume fractions were de-termined from the following expression by neglect-ing the total scattering contribution to the extinctionsignal:

f� ��

6�EKext �

�

6�E

Ln�I0/I�

�s(1)

The effect of scattering was estimated to be small,especially for lightly sooting fuels like ethylene. Neg-

ligible scattering assumption will be justified later onby applying an approximate aggregate scattering the-ory to our TEM size and morphology data. In Eq. 1,E is a function of soot refractive index, which issurrounded by a dispute in the literature. Fortunately,the range of E functions at the visible wavelengthsfrom different studies is relatively narrow between0.19 and 0.26 [29–31]. For the interpretation of thepresent optical experiments, E was therefore taken tobe 0.23, which was favored by the measurements inthe overfire region of turbulent flames [32,33]. More-over, this value is also consistent with the soot re-fractive index reported by Dalzell and Sarofim [29]that is widely cited and used in the combustion lit-erature. Accordingly, the present fv measurementscan readily be compared to numerous past investiga-tions in this field, and if desired, easily adjusted forany refractive index by a simple conversion factor.

The optical path length, �s, in Eq. 1 should besmall enough so that the measurement volume ishomogeneous. The radial step size of line-of-sightdata and the distance between the probes also deter-mine the spatial resolution achieved during the ex-tinction experiments. As mentioned before, soot canexist in streaks as small as 1 mm in turbulent flames.However, the influence of such smallest scales on theextinction measurements of fv is apparently minor.For example, Magnussen [17] found that the integralscale relevant to soot formation was 4.5–6 mm (1.5to 2 times the burner diameter used) at the axis of anacetylene turbulent flame using light scattering mea-surements. Coppale and Joyeux [13] reported an in-tegral length scale of 7.5 mm at z/D � 85 in anethylene flame that was very similar to the presentone. Recently, Geitlinger et al. [16] also measurednot only the macro scale (characteristic length ofturbulent eddies) but also the micro scale (character-istic length of turbulent dissipation) in an acetyleneturbulent flame using the spatially resolved LII tech-nique. They reported that the macro scale increasedfrom about 2 mm to 6 mm (1 to 3 times the burnerdiameter used) with the height at the flame axis. Theirmeasured micro scale was almost a constant at about2 mm, except at the lowest flame position of z/D �18. Based on these detailed past studies, an opticalpath length equal to approximately half to one burnerdiameter (�s � D/2-D) appeared to be larger than theintegral length scales associated with the soot forma-tion in this turbulent flame. Consequently, two opti-cal lengths of 2.28 mm and 4.56 were adopted in thisstudy to provide a reasonable spatial resolution, es-pecially above z/D � 30. Yet, it must be emphasizedthat this was obviously insufficient to ensure a trulyhomogenous measurement volume.

Although the use of optical probes might be ad-vantageous compared to the deconvolution method

98 B. Hu et al. / Combustion and Flame 134 (2003) 93–106

near the flame axis, the intrusion of physical tubesinto the flame was a potential concern for altering thesoot field. Consequently, various sources of errors inthe measurement of soot volume fraction due to theseprobes were carefully considered. First, frequentcleaning of the probes and limiting each test to lessthan 20 s prevented clogging of inside surfaces bysoot particles. An unblocked laser beam during theextinction experiments was ensured by verifying aconstant output of the detector that observed the laserintensity through the optical guides before and aftereach test. Second, perturbations in the flame proper-ties induced by the probes were inspected by com-paring the measured extinction levels with and with-out the probes at each height. This was accomplishedby measuring I/I0’s across the gap length of theprobes at various radial locations and confirming thatintegration of these local absorption levels to beequal to the total absorption of light through theentire flame axis without the probes. Furthermore,additional preliminary tests involving laser scatteringmeasurements with and without the probes alsoagreed well, supporting a relatively insignificantflame disturbance induced by the probes. This alsoreflected that the errors due to heating up of the tubesin the high-temperature flame environment weresmall. Finally, it should be pointed out that similarprobing techniques have successfully been employedin turbulent flames in the past [11–13].

3. Results and discussion

In the following, the spatially resolved data fromthe TEM measurements are first discussed. The vari-ations of spherule diameter, aggregate morphology,and aggregate size along the flame axis are thenquantified. Coupling these TEM data with the aver-age soot volume fraction, the particle number densityand specific surface area are also determined. Theexperimental uncertainties associated with the sam-pling and optical experiments are addressed. All themeasurements were repeatable within the stated ex-perimental uncertainties that were estimated follow-ing the detailed considerations of [20,21,34].

3.1. TEM observations

Figure 3 shows typical TEM pictures of soot sam-pled from the flame axis at four heights of z/D � 30,60, 80, and 140. Although these pictures were takenat the same magnification of 42,500, note that thesampling times were different. Similar to the pastobservations within laminar flames [23,24], and thefuel-rich [21] and fuel-lean [20] conditions of turbu-lent flames, soot consists of aggregates of nanometer-

scale spherical particles. Aggregate sizes can be seento vary considerably at a particular flame location incontrast to the primary particles (spherules), whichhave nearly uniform diameters. These TEM imagesindicated that soot aggregates within turbulent flameswere polydisperse with a wide size distributionwhereas the soot spherules were almost monodis-perse with a small standard deviation. Both the spher-ule diameter and aggregate size appeared to be in-creasing with height above the burner, as quantifiedbelow. These general TEM observations did notchange from one location to the other on the samegrid. Further inspection of the TEM pictures revealedthat there were some single particles (Fig. 3a and 3b)mainly at the lower flame locations. They were moretransparent than the typical aggregates to the electronbeam, similar to the past observations in laminarflames [23,24]. The existence of such single particlesslowed down with the height above the burner andmostly ceased toward the top of the flame (Fig. 3d).

Before routine measurements, the repeatability ofthe TS/TEM data were verified by taking two or threesamples at selected locations. The results from dif-ferent grids at each of the sampling positions gener-ally agreed well, verifying that the size/morphologydata sets reported in the following sections wererepresentative of the soot populations existing in theflame. It was also necessary to check the effect of theprotective tube on the TEM findings because thepresence of this metal tube in the flame during sam-pling may possibly perturb the flame and conse-quently the soot field. Two flame positions wereconsidered for this evaluation: z/D � 30 and 100,where the soot volume fractions were minimum andmaximum, respectively (see Fig. 8). Because theflame width increased with the height above theburner, the flame centerline (r/D � 0) at z/D � 30while a radial location (r/D � 3) at z/D � 100 werechosen to minimize the transient time of the TS probeas it traveled from the edge of the flame to thecenterline location without the protective tube. Theresults obtained with and without the protective tubestatistically resembled each other, revealing that themetal tube did little to disturb the flame environment.For example, the average particle diameters at z/D �100 were found to be 29 nm and 28 nm with andwithout the protective cover, respectively.

3.2. Spherule diameters

Mean soot spherule diameters along the flamecenterline are illustrated in Fig. 4. At each height, 20to 40 particles were averaged to obtain dp within anexperimental uncertainty (95% confidence interval)of less than 5%. The results indicated that the sootspherule size increased from 19 nm to 35 nm with

99B. Hu et al. / Combustion and Flame 134 (2003) 93–106

increasing height above the burner as the surfacegrowth of particles appeared to continue until z/D �150 in this turbulent flame. Spherule diameter starteddecreasing after this peak axial location down to 25nm at the highest sampling location of z/D � 180.Such a decrease of 10 nm (about 30%) was beyondthe typical experimental uncertainties, indicating theonset of oxidation close to the flame tip. These TEMmeasurements were highly repeatable within thestated experimental uncertainties, supporting thepresence of a peak in dp around z/D � 150.

In general, the observed narrow size range agreedwith the values reported in the literature, which werealso confined to 20 to 50 nm for non-premixedflames; please see Megaridis and Dobbins [23] for acomplete list of previous studies. For example, thesoot primary sizes in laminar flames were in therange of 20 to 35 nm [22–24], while soot emittedfrom various turbulent flames consisted of 30 to 50

Fig. 3. Typical TEM images (42,500x magnification) of soot sampled at four axial positions in the flame.

Fig. 4. Mean spherule (primary particle) diameters along theflame centerline as determined by direct TEM observations

100 B. Hu et al. / Combustion and Flame 134 (2003) 93–106

nm spherules [20]. For the fuel-rich conditions ofnon-premixed turbulent flames, the only direct com-parison to the present results could be made againstFang et al. [21], who also measured the actual spher-ule diameters using the TS/TEM technique at 5 dis-crete locations along the axis of a spray flame. Vary-ing between 10 and 13 nm, their dp’s were noticeablysmaller than the present values and represented thelower range observed in a wide variety of flames.Fang et al. [21] explained this difference to be aconsequence of the low residence times involved intheir complex combustor (liquid-fueled, confined,turbulent, recirculating). Despite this discrepancy,the present findings generally extend the narrow par-ticle size range found in laminar flames to turbulentconditions.

The standard deviation of spherule diameter, �p,at each sampling location was typically 10 to 20% ofthe mean spherule diameters. The relatively smallstandard deviations supported a practical representa-tion of the spherule size distribution with a Gaussian(normal) probability density function, in agreementwith the results from the overfire regions of turbulentflames at long residence times [20]. Similar to[20,23] but in contrast to [21], �p/dp ratios measuredin this study did not exhibit a correlation with theheight above the burner exit.

3.3. Aggregate morphologies

As mentioned before, 800 to 2000 aggregateswere acquired to attain reasonable aggregate statisticsat each sampling location. The 3-D aggregate prop-erties were estimated from the measurements of pro-jected area, Aa, and maximum length, L, of eachaggregate. Following the specific procedures given in[28], the number of spherules in each aggregate, N,were found from the empirical correlation:

N � 1.15� Aa

��dp2/4�

� 1.09

(2)

The complex aggregate morphologies with differentshapes and sizes (see, for example, the TEM picturesin Fig. 3) were characterized using the mass fractalconcept, which implies the following statistical rela-tionship:

N � kf�2 Rg

dp�Df

(3)

where Df is the fractal dimension, and kf is the fractalprefactor (lacunarity) based on the aggregate radiusof gyration, Rg, normalized by the average spheruleradius, dp/2. Because Rg was not directly measurablefrom projected images, Eq. 3 could alternatively beexpressed in terms of L:

N � kL� L

dp�Df

(4)

As demonstrated below, the present TEM measure-ments of soot aggregate morphology were similar tothe ones in laminar [23,24] and turbulent flames[20,21]. This merited the applications of the abovefractal relationships and empirical correlations in thisstudy although they were mainly developed for sootgenerated in laminar flames and post-flame regions ofturbulent flames. Such an extension was justifiedbecause not only experiments but also computer sim-ulations in the past confirmed the applicability ofEqs. 2–4 for aggregates formed by the cluster-clusteraggregation process. This means that as long as thefractal dimension of aggregates is about 1.6–1.8, theactual morphology of aggregates can be expressed bythese relationships. In fact, Eq. 3 is valid for any massfractal object while Eq. 4 is simply an alternativeform for all geometries of aggregated particles.Moreover, the above equations have already beenused by Fang et al. [21] to successfully describe thesoot morphology within the fuel-rich conditions of aturbulent flame.

Based on these considerations, a relationship be-tween N and L/dp was sought in the form of Eq. 4 ona logarithmic scale. The linear least-square fit to suchdata at each sampling position identified the fractaldimensions, Df, in the range 1.63 to 1.85 and thecorrelation constants, kL, in the range 0.8 to 1.3 withno particular trends observed as a function of z/D.These axial variations were considered to be withinthe experimental uncertainties (95% confidence in-terval), which were estimated to be about 5% for Df

and 25% for kL. Due to these invariable fractal prop-erties at various heights, more than 5600 aggregatessampled from all the axial flame positions were con-veniently combined and plotted in Fig. 5. As can beseen in the figure, N generally correlated well withL/dp over three orders of magnitude aggregate sizerange (n � 1 to 1000), yielding a slope of Df � 1.73.This spatially averaged fractal dimension effectivelyrepresented all the individual values (Df � 1.63–1.85) from all the sampling positions. Notice that thescatter in the data for small aggregates was related tothe statistical nature of aggregate correlations. Forexample, Eq. 2 involves the ratio of the projectedareas of an individual aggregate and the average-sizeparticle. Nonetheless, the elimination of small aggre-gates with less than 5 to 8 spherules did not changethe quoted aggregate properties due to the large sam-ple size analyzed.

The present fractal dimensions agreed well withnumerous past measurements [8,18,23,24] in laminarnon-premixed flames within experimental uncertain-ties. Additionally, aggregate fractal dimensions re-

101B. Hu et al. / Combustion and Flame 134 (2003) 93–106

ported within another turbulent flame varied in therange 1.75–1.85 [21], while Df values in the overfireregions of turbulent flames were also similar (1.70 to1.79) and independent of the fuel type. Thus, thepresent results coupled with these previous studiesreveal soot fractal dimension as a universal morpho-logical property (1.74 � 0.11) that is remarkablyinsensitive to flame location, flow condition, and fueltype. It is intriguing to note that other flame-synthe-sized aggregates of different compositions (e.g., ox-ide particles such as silica and alumina) even have anidentical morphology. Numerical simulations vali-dated that this was apparently due to the dominantcluster-cluster agglomeration mechanism in manycombustion environments. Note that the excellentconsistency between the present fractal dimensionsand those from numerous past experimental/compu-tational investigations verified the earlier adoption ofthe analysis method given by Eqs. 2–5.

It has recently been emphasized that both Df andkf were necessary to fully characterize the fractalnature of clustered small particles. In Fig. 5, kL wasequal to 1.1, corresponding to an average fractalprefactor of kf � 2.1 using the relationship

kf

kL� � L

2 Rg�Df

� �Df � 2

Df�Df/ 2

(5)

Equation 5 was based on the experimental and theo-retical considerations of a variety of investigations;as noted by Koylu et al. [28] and references citedtherein. In contrast to a broad agreement on Df in theliterature, however, there is no common consensus

about the value of kf, which may vary between 1.2and 3.4 among a number of investigations. Althoughthe source of this discrepancy is still elusive, thepresent fractal prefactors of kf � 2.2 � 0.4 for dif-ferent flame heights were in very good agreementwith the range 2.0–2.8 that was suggested by themajority of experimental determinations [28]. Inter-estingly, this value was also consistent with a simpletheoretical consideration that is valid for any geom-etry, i.e., Df � 1–3. This was due to the expectationthat kL should be near unity in order to satisfy Eq. 4for any shape, even in the limit of N3 1. Then, Eq.5, which also holds for any fractal dimension, gives akf of 1.73 for straight chains (Df � 1), 2.00 for disks(Df � 2), and 2.15 for spheres (Df � 3).

3.4. Aggregate sizes

Once the mean spherule size at each samplinglocation was determined, the number of primary par-ticles in every aggregate could be calculated from thecorrelation of Eq. 2 using the measurements of ag-gregate projected areas. With the above-mentionedfractal properties, radius of gyration of each aggre-gate within the sample was then found from invertingEq. 3. From the same TEM population of aggregates,statistical information about the size distribution wasobtained at each flame position. Figure 6 illustratesthe mean number of spherules per aggregate, N� , andthe corresponding mean radius of gyration, Rg, forwhich typical experimental uncertainties (95% con-fidence interval) were about 35%, mainly due to finitenumber of samples. For the present test conditions, N�

increased from 10 to 45 between z/D � 30 and 140,where Rg also increased from 17 nm to 53 nm in thesame axial region. This continuous growth was ex-pected because soot aggregates became larger due tothe unavoidable aggregation process as they weresubjected to more residence time when convected upin the flame. The standard deviations of aggregatesizes were always larger than the average values,characteristic of a broad distribution. Consequently,probability density function for N could reasonablybe represented by a lognormal distribution, similar tothe past studies [20,21,23]. Our TEM findings weregenerally consistent with this assessment for aggre-gates comprising more than 8 spherules.

3.5. Soot volume fractions

Various sources of uncertainties during the laser-based measurements of soot volume fractions wereconsidered earlier in the experimental methods. Forexample, there is a considerable disagreement on thevalue of soot refractive index in the literature. For-tunately, the range of reported values of the refractive

Fig. 5. Statistical relationship between the number of spher-ules and characteristic size for 5600 soot aggregates sam-pled from all flame locations for the determination of fractalproperties.

102 B. Hu et al. / Combustion and Flame 134 (2003) 93–106

index function, E, needed in the analysis of fv (seeEq. 1) was quite narrow at visible wavelengths. Ad-ditionally, the contribution of total scattering to theextinction signal was neglected in this study. A max-imum total scattering-to-absorption ratio of 7% wascalculated based on the application of an approximateaggregate scattering theory [8,19,20,32] to our TEMdata, justifying this assumption for the present testconditions. Another source of experimental uncer-tainty results from the finite sampling times, espe-cially at low soot concentrations. When combinedtogether, typical overall experimental uncertainties infv measurements were estimated to be about 30%.Note that higher experimental uncertainties are ex-pected for fv � 0.3 ppm as the measured I/I0’s ap-proached unity, especially low in the flame.

As discussed earlier in the experimental methods,two versions of the laser extinction technique wereconsidered during the present study: non-intrusivedeconvolution of radial line-of-sight measurementsand intrusive optical probes for restricting the pathlength. Both techniques have successfully been im-plemented in laminar and turbulent flames in the past[6-13]. Figure 7 shows typical radial comparisons ofthe deconvolution and probe methods at two heightsabove the burner. Overall, the agreement was excel-lent at z/D � 60, while the results from both tech-

niques at z/D � 90 also agreed to within 15% at allradial locations except near the axis and edge of theflame. As pointed out in the previous paragraph,larger experimental uncertainties were already antic-ipated at r/D � 10 and 11 because of the lower sootconcentrations at these farthest radial locations. Onthe other hand, the discrepancies at r/D � 0 and 1were possibly due to the error propagation of themathematical inversion scheme near the flame axis.However, notice that the disagreements were stillwithin the typical experimental uncertainties (30%)cited above. Consequently, the probe measurementsnear the flame centerline complemented the decon-volution data, which, in turn, verified that the effectof the probe intrusion on the soot field was smallbecause of the general consistency at the radial loca-tions considered here.

Mean soot volume fractions at the flame axis areplotted as a function of normalized height above theburner in Fig. 8. No soot concentration was detect-able below z/D � 20 in this turbulent ethylene flamedue to insufficient laser extinction levels. The mea-sured fv’s constantly increased to a maximum of 1.1ppm until about z/D � 90 to 100 at the flame cen-terline. In this region, most of the soot volume wasformed due to the nucleation and surface growth ofparticles. The maximum in fv was reached noticeablyearlier in the flame than the peak in dp (z/D � 150)for the present test conditions. After z/D � 90, thesoot volume fraction started decreasing and dimin-ished at the highest axial locations. The magnitudesand the trends of the present mean fv’s were generally

Fig. 6. Mean aggregate sizes, represented by average num-ber of spherules per aggregate and average radius of gyra-tion, at the flame axis.

Fig. 7. Radial comparisons of mean soot volume fractions inthe flame measured from the laser extinction experimentsusing the optical probes and deconvoluted line-of-sightdata.

103B. Hu et al. / Combustion and Flame 134 (2003) 93–106

similar to those reported by Coppale and Joyeux [13]in an ethylene-fueled turbulent jet flame. Interest-ingly, while the soot size and morphology were verysimilar in both flame types, mean soot volume frac-tions at the centerline of this turbulent ethylene flamewere much smaller than its laminar counterparts[6,24]. Obviously, such a comparison strictly holdson an average basis because the instantaneous fv’s inturbulent flames could be much higher than the meanvalues.

The soot volume fractions shown in Fig. 8 repre-sented averages over an optical path of 2.28 mm. Asemphasized before, such a measurement length wasnot truly homogeneous and therefore could not spa-tially resolve the smallest soot streaks present inturbulent flames. Nevertheless, it appeared to besmaller than the typical integral length scales mea-sured in various soot-containing turbulent flames[13,16,17]. Therefore, the present laser extinctionexperiments were essentially sufficient to resolve thelength scales primarily associated with soot volumefractions. The presence of this large-scale homoge-neity in the flame was also examined by doubling theoptical path length to 4.56 mm. The measured fvvalues were in agreement in this 2:1 length range,suggesting a relatively minor influence of the small-est scales on the data shown in Fig. 8. Unfortunately,lower separation distances were not feasible in thisstudy because of the low signal-to-noise ratios andconcerns with probe interference.

3.6. Number densities and specific surface areas

By coupling the present laser extinction measure-ments of fv with the TEM determinations of dp, the

number of spherules per unit volume, np, was calcu-lated from

np �6f�

�dp3 (6)

As shown in the top portion of Fig. 9, this resulted inparticle number densities varying in the range of1010–1011 particles/cm3 for the axial locations con-sidered. The number of soot spherules per unit vol-ume reached a maximum at about z/D � 100, with atrend similar to the soot volume fraction. This indi-cated that the nucleation of soot particles continueduntil this height, accompanied by the surface growthprocess (see Fig. 4). The formation of new sootparticles appeared to cease after this location al-though primary particles continued to grow furtherdownstream.

Another fundamental property relevant to sootformation is the particle surface area per unit volume,which is found from

Sp �6f�

dp� np�dp

2 (7)

The above expression is an upper estimate of Sp

because it explicitly assumes point contact of thespherules forming the aggregates. The second part ofFig. 9 illustrates the actual soot specific surface area

Fig. 8. Axial variations of mean soot volume fractions in theflame.

Fig. 9. Mean soot number densities and specific surfaceareas along the flame axis.

104 B. Hu et al. / Combustion and Flame 134 (2003) 93–106

using the spherule diameter found from the directTEM measurements. In general, Sp closely fol-lowed the behavior of soot number density andvolume fraction, increasing to a peak value of 2.5cm2/cm3 near z/D � 100. The increase in the sootsurface area per unit volume up to this height wasin parallel to the simultaneous increases in np anddp. With the end of the nucleation process near z/D� 100, the decline in particle number densityseemed to be sufficient to overcome the sustainedgrowth process such that the soot specific surfacearea overall started declining. Further decrease ofSp after z/D � 150 was mainly due to a dominantoxidation process at these highest positions withthe reduction in spherule diameter (Fig. 4) in thesame region closer to the flame tip. Also shown inthe figure are the Sp estimates based on the vol-

ume-equivalent diameter, deq � dpN1/3, which hasbeen frequently employed in the literature. Notice-ably, the volume-equivalent model, which ignoresthe aggregation process by assuming a spherical mor-phology, underestimated the “real” values by about afactor of three. This was due to the fact that theoverall aggregate size was represented by an equiv-alent diameter, which was substantially larger thanthe actual spherule diameter. This clearly demon-strates that such improper characterizations lead toconsiderable errors in the evaluation of soot surfacearea and, therefore, hinders the reliable understand-ing of various soot processes in non-premixed flames.

4. Summary and conclusions

The evolution of soot particles was quantifiedwithin the fuel-rich conditions of a non-premixedturbulent jet flame burning ethylene in atmospher-ic-pressure air. Thermophoretic sampling followedby transmission electron microscopy and laser ex-tinction techniques were used to measure meansoot properties, including spherule (primary parti-cle) diameter, aggregate size distribution, fractalmorphology, and volume fraction along the flameaxis. In addition to various sources of errors asso-ciated with the sampling/optical experiments,mean number of particles per unit volume andspecific soot surface areas were also determined.The results were compared against past data in theliterature, especially those in the fuel-rich regionsof laminar flames and overfire regions of turbulentflames. The main conclusions can be summarizedas follows:

1. Soot spherule diameter along the flame axisvaried from 19 nm to 35 nm, generally increas-ing with the height above the burner until

about z/D � 150. Although distinctly higherthan those reported by Fang et al. [21] in acomplex turbulent combustor, the present nar-row size range agreed well with the previousdata on soot not only formed in various lami-nar flames but also emitted from turbulentflames.

2. Aggregates of different shapes and sizes wererepresented by Df � 1.74 � 0.11 and kf � 2.2� 0.4 at all sampling locations. Extending theprevious studies in the literature, the presentresults further confirmed these fractal parame-ters as universal morphological properties offlame-generated aggregates, irrespective offlame position, fuel type, flow condition, andchemical composition due to the dominantcluster-cluster agglomeration mechanism.

3. Average aggregate size increased with theheight above the burner (residence time) duethe continuous agglomeration process through-out the flame. In contrast to spherules, aggre-gates were polydisperse with a broad size dis-tribution.

4. Mean soot volume fraction increased to a max-imum of 1.1 ppm around z/D � 90– 100 at theflame axis, following the overall trend in num-ber density, which varied in the range of 1010–1011 particles/cm3. The peak axial locations ofboth soot number density and volume fractionwere found to be lower than that of spherulediameter for the turbulent test conditions con-sidered here.

5. Soot surface area per unit volume reached amaximum value of 2.5 cm2/cm3 near z/D �100 in parallel to the increases in np and dp inthe same axial zone. A volume-equivalentspherical model underestimated specific sur-face areas by a factor of 3, signifying thetypical errors associated with such improperexperimental characterizations.

6. The results identified various axial regions ofdominant soot mechanisms in this non-pre-mixed turbulent flame, that is, particle nucle-ation until z/D � 100, surface growth betweenz/D � 30 and 150, oxidation after z/D � 150,and agglomeration throughout the flame.

Acknowledgments

The authors thank Dr. Scott Miller and Kai Yu ofthe University of Missouri-Rolla for their valuableassistances during the TEM and light extinction mea-surements, respectively. This research was sponsoredby the National Science Foundation, Combustion and

105B. Hu et al. / Combustion and Flame 134 (2003) 93–106

Plasma Systems Program under a CAREER Award(Grant No. CTS-0196012).

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