Soot measurements in combustion systems are ofprimary importance as a control for and monitoringtool of particulate formation. More stringent lawsabout unhealthy exhaust emissions, namely, par-ticulate and polycyclic aromatic hydrocarbons,draw attention to the comprehension of soot forma-tion mechanisms. To this aim, several experimen-tal techniques have been developed, particularlythe nonintrusive optical techniques that are bestsuited to probe the hostile combustion environment.Well-established techniques are based on light ex-tinction and scattering as described in Ref. 1 andreferences therein. By use of the extinction signalonly, the soot volume fraction can be carefully de-termined, whereas a combination of the two signalsalso allowed us to analyze the soot structure char-acteristics, such as the volume-mean diameter andthe primary particle diameter.2–4 This kind of op-tical diagnostic is experimentally simple, but theflame sampling, in general, is quite limited becauseof the point-to-point nature of the measurements.
For the same reason it is nearly impossible to probesoot distribution in nonsteady-state systems as, forexample, in turbulent flames. To overcome thisproblem, we extended the extinction technique totwo-dimensional �2-D� arrangements.5,6 Laser-induced incandescence is one of the most promisingtechniques for 2-D soot distribution measurements,although it still requires a calibration procedure bymeans of extinction7–9 or other techniques.10,11
Here we present one more technique for 2-D visu-alization of soot distribution, based on an extensionto the 2-D case of light emission diagnostics largelydeveloped in luminous flames.12–18 Soot radiationis measured at two different wavelengths and com-pared with a calibrated light source. Both the sootvolume fraction distribution and temperature fieldin the flame can be determined in this way. Bymeans of this simple technique that is not time-consuming a significant amount of data can bedetermined from luminous flames for subsequentanalysis. Here, after we describe the proposedtechnique, we present the validations in both diffu-sion and rich premixed flames by means of acomparison with measurements obtained from ex-tinction and multiwavelength emission.14–19 Fi-nally, the application to alkane diffusion flames ispresented.
2. Theoretical Background
The radiation intensity at wavelength �, Es�, emittedfrom a soot volume characterized by uniform temper-
The authors are with the Istituto per la Technologia dei Mate-riali e dei Processi Energetici, Consiglio Nazionale delle Ricerche�CNR-TeMPE�, via R. Cozzi 53, Milan I-20125, Italy. The e-mailaddress for G. Zizak is [email protected].
Received 19 December 2000; revised manuscript received 30May 2001.
ature Ts and soot volume fraction fv is expressed bythe relationship
Es���, Ts� � �s��� RBB��, Ts�εs��, fv�, (1)
where RBB is the blackbody energy spectral densitygiven by Planck’s law, εS is the soot monochromaticemissivity, and �s��� is an optical parameter that ac-counts for the collection efficiency, solid angle, anddetector sensitivity. The quantity fv is given by
fv ��dp
3N6
, (2)
where N is the number of soot particles per unitvolume and dp is the particle diameter. Assumingthe validity of Kirchhoff ’s law, the monochromaticemissivity of soot is given by12–16
εs��, fv� � �1 � exp��KabsL��, (3)
where L is the length of the soot volume under con-sideration. Using the classical expression for theabsorption coefficient Kabs,14–20
Kabs �36�F��� fv
��
fv
labs���, (4)
we can write the monochromatic emissivity as
εs � 1 � exp� fv Llabs���� . (5)
In Eq. �4� F��� is a function of the real and the imag-inary parts of the refractive index,14,20 and labs��� isthe natural length for absorption at wavelength �,which can be interpreted as the thickness of a layer ofpure soot material � fv 1� with a transmissivity of1�e and depends only on the optical characteristics ofsoot.14 For the refractive-index values we used theexpression given by Chang and Charalampopoulos,21
which takes into account the dependence on wave-length. Moreover such a choice was mandatory for auseful comparison with previous data reported inRefs. 14, 19, and 22.
As shown in Eq. �1�, the soot luminous emission isrelated to both the soot volume fraction and the soottemperature. For the determination of these twoquantities, two equations are required. Soot tem-perature can be obtained from the ratio of the emis-sion intensities at two wavelengths by a comparisonwith the intensity ratio of a calibrated lamp at thesame wavelengths. For the two sets of measure-ments two arrangements must be used, which, forsimplicity, should be identical or should differ byspectrally neutral components. In fact, similar toEq. �1�, the recorded intensity of the calibrated lampemission, EL�, can be expressed as
EL���, TL� � �L��� RBB��, TL�εL��, TL�, (6)
where RBB��, TL� is the blackbody emission at thelamp temperature, εL��, TL� is the emissivity of thelamp material, and �L��� is the optical parameter of
the arrangement used for the lamp. By calculationof the ratio ES�1�ES�2 and by comparison with thesame ratio obtained for the lamp, both optical param-eters cancel because
�s��1�
�s��2��
�L��1�
�L��2�. (7)
After some calculations by using Wien’s equation forthe blackbody emission, we derived the soot temper-ature as given by the following relationship:
TS � �c2� 1�1
�1�2��ln�ES�1
ES�2
EL�2
EL�1
εL��1, TL�
εL��2, TL�
labs��1�
labs��2��
�c2
TL� 1
�2�
1�1���1
, (8)
where c2 is the second Planck constant. After thesoot temperature has been determined, the soot vol-ume fraction can be obtained by the relationship
fv � �labs
Lln�1 � εL��, TL�
�S���
�L���
ES�
EL�
� exp��c2
� � 1TL
�1TS
��� . (9)
Of course, the experimental measurement of ES� isthe result of an integration process over the opticalpath. For nonuniform distributions, such as in aflame, if no knowledge of the local values can beattained, only an average meaning can be attributedto the measured values for the temperature and sootvolume fraction. Such a meaning is, of course, lowerwhen the actual distributions are far from unifor-mity, especially if one takes into consideration that fvand Ts contribute to the signal with different weights.In contrast, for axially symmetric flames it is possibleto calculate the radial distribution of soot radiation atthe two wavelengths through an Abel inversion pro-cedure of laterally acquired measurements. There-fore, with a sufficiently high spatial resolution, theassumption of uniform distribution still holds withinthe probe volume, and the local soot volume fractionand temperature can be achieved from Eqs. �8� and�9�. The latter equation can be evaluated for bothwavelengths as a data consistency test. Because ofthe actual optical arrangement used as discussed inSection 3, the ratio �S��L appears in Eq. �9�, which isto be experimentally determined.
3. Experimental Apparatus
The experimental apparatus is shown in Fig. 1. Toobtain simultaneous images at the two wavelengths,proper optics must be employed. The flame image isdoubled by a two-faceted quartz prism through twosuitably chosen optical filters that face the two sidesof the prism. We used bandpass filters in the blue�BG12� and the red �RG9� regions, with spectral re-gions quite apart from each other to achieve highersensitivity.23 To remove the infrared intensity weadded an anti-heat filter �KG1� and positioned a neu-
tral density filter on the red branch of the collectingoptics to equalize the red and the blue contributionson the detector. The two images obtained in thisway are imaged on a commercial 752�H� 582�V�pixels CCD camera �Ikegami ICD-46E� through anUV 4.5�f Nikon lens. We used the same apparatusfor measuring the emission intensity from a cali-brated tungsten ribbon lamp �Osram Wi 17�g�. Inthis case we positioned a suitable quartz plate ofknown reflectivity �to calculate �s��L� between theflame and the camera to steer a portion of the lampemission to the detector �see Fig. 1�. The opticalpath lengths from the flame and the lamp to thecamera are exactly the same �105 cm�. This ar-rangement allows us to collect the reflected lamp in-tensity with the same solid angle and in the sameexperimental conditions as those used for the flameemission. The two intensities that reach the CCDare at comparable levels.
The use of broadband filters instead of two narrow-band interference filters is dictated by the low signallevel resulting from the spectral region selected �sen-sitivity to temperature� and the high f-number uti-lized �depth of field, see below�, which implies use ofthe effective wavelength concept.23,24 As a conse-quence, the �1 and �2 values in Eq. �8� were deter-mined as the wavelengths at which the ratios of thetwo expressions of Wien’s equation at 1700 and 2100K equal the ratios experimentally obtained from thecalibration lamp for both blue and red spectral re-gions. In doing this, the lamp emissivity variationwith temperature was taken into account, with re-sulting values of 400 and 720 nm. It was also veri-fied that the experimental ratio of the two-coloremissions from the lamp followed the lamp temper-ature within the range of �15 K. Calibration tem-perature TL in Eq. �8� and �9� was chosen to be2000 K.
Some preliminary tests were carried out on thedetection apparatus. Particular care was devoted tothe optical alignment. A first goal was to ensurethat the two images were to be built by two perfectlyequivalent �apart from the filters� optical channels.To this purpose, the prism and the filters were placedon a sliding table, which allowed us to set the prismedge onto the objective axis. We verified this by us-ing a uniform light source �lamp plus diffuser� and by
shifting the prism in front of the camera until theresulting CCD illumination was similar on the twohalves. Furthermore, to avoid any mixture of thetwo chromatic components, an effort was made toobtain widely separated images on the CCD and thebest possible superimposition of the prism edge withthe contact line of the sides of the two filters. In thefinal setup we verified the absence of any cross talkbetween the two optical channels. While lightpasses through the two-faceted prism, a uniform lightfield does not originate a double flat response fromthe CCD. In fact, points closer to the prism edgeexhibit a lower intensity than the ones at the periph-ery �maximum variation of less than 10% across theflame image�. Then, from these tests a software cor-rection mask was derived, which allows flattening theresponse across the two facets �light flat-field mask�.
Although an achromatic objective was used, thewavelength regions considered are quite far fromeach other and, as a consequence, the related images,collected at the same time, were focused on differentplanes. This effect was more evident at large dia-phragm apertures. In our arrangement, however,the light intensities were somewhat high, which in-volved a small diaphragm aperture and highf-number �11–22�. We have verified that the depthsof field at the two wavelengths significantly over-lapped. In practice, by focusing the two images sep-arately and then selecting the midpoint for themeasurements, we solved the problem satisfactorily.
Tests were also carried out to ascertain that thenear and the far layers of the flame contributed in abalanced way to the signal. In fact, results obtainedby bringing into the object plane the near and the farflame layer almost coincided, as will be explainedbelow. The effective linearity range of the 8-bit CCDcamera was also verified in the red and the blueregions separately. To this purpose the light inten-sity from a lamp was measured by the camerathrough calibrated neutral density filters. For com-plete coverage of the experimental conditions, severalcurves were traced to achieve a better balance be-tween exposure time and diaphragm aperture. Theoverall result is shown in Fig. 2. The curve can beconsidered fairly linear up to a CCD output of ap-
Fig. 1. Experimental apparatus.Fig. 2. CCD camera linearity range: {, red and �, blue wave-length regions, ‚, exposure time, and E, diaphragm number.
proximately 160. To take advantage of the full re-sponse of the CCD camera, it would be possible tocorrect for the nonlinearity by use of the second-orderpolynomial expression fitting the data �as shown inFig. 2�. Because of the lengthy processing time, allthe measurements presented here were collected inthe linear range.
We performed the experimental procedure for mea-suring the soot volume fraction and the temperatureas follows:
collection of the two images �blue and red� of theflames;
measurement of the lamp emission under the sameexperimental conditions and settings as those for theflame;
correction of the images �of the lamp and of theflame� by the light flat-field mask �pixel-by-pixel ra-tio�;
introduction of the resulting intensities �Es�1, Es�2,EL�1, EL�2� in the software for the calculation of Tsand fv.
All the images were recorded with a personal com-puter by use of appropriate software �Image Pro Plus3.0 for Windows� and processed by a MATHCAD pro-gram.
4. Tests on Diffusion Flames
A first test of the technique presented here was car-ried out on the widely studied ethylene diffusionflames. Such flames allow meaningful comparisonthanks to the abundance of data in the literature.25–34
We used a coannular circular burner �10-mm i.d. forthe fuel and 100-mm o.d. for the air� with a honey-comb inside the outer tube that allowed us to createan air flow as laminar as possible. The flames weproduced were quite stable and reproducible. Flameheights were chosen in an attempt to repeat the sameexperimental conditions outlined in Refs. 14 and 19.In spite of this, the difference in equipment we usedand in the surrounding environmental conditions didnot allow us to achieve identical reproduction. Mostof the measurements that we report refer to a flameheight of 6.5 cm, with flow rates of 0.155 l�min forethylene and 37.5 l�m for air.
Owing to the axial symmetric geometry of theflame, local values were derived by means of a row-by-row Abel inversion procedure35 on both images�blue and red�. Because of the high resolution of thecamera, the numerical deconvolution had to be ap-plied for a high number of points, possibly leading tohigh noise sensitivity in some cases �oversampling�.A binning on two adjacent pixels was then made be-fore the inversion. This allowed us to reduce therandom error and to decrease the processing time.
Furthermore, radiation self-absorption during thecrossover from the inner flame layers to the detectorwas estimated by the introduction of an appropriate
Fig. 3. Soot volume fraction radial profiles in an ethylene diffu-sion flame of 6.5-cm height. Emission and extinction measure-ments are compared.
Fig. 4. Comparison of the fv radial profiles that we obtained byfocusing on the flame axis �solid curve� and shifting the burner axisby �5 mm �dotted curve� from the detector.
subroutine. This correction was almost negligible inthe flames investigated �a profile modification of lessthan 10% at worst� and, therefore, was omitted frommost of the measurements that we report. For athorough comparison of the soot volume fraction mea-surements with the data of Ref. 19, obtained by theextinction technique, we averaged 100 emission im-ages to keep the number of samples at the same orderof magnitude. As for the duration total of the mea-surement, the two were noticeably different �tens ofseconds for extinction, only a few seconds for emis-sion�. Nevertheless, in both cases, the measure-ment time was conveniently longer than the typicalflame fluctuation times. Comparison of the resultsfor the soot volume fraction on three radial profiles isreported in Fig. 3, where the curves are superim-posed. Some differences appear on the flame axis inthe lower radial profiles, for example, approximately30%, corresponding to 1.5 ppm �parts per million�.Although the absolute value is low, such a differenceis not negligible. Nevertheless, two facts must betaken into account. First, the major discrepanciesare located near the axis, where the Abel inversionprocedure is more prone to errors �see also the dis-cussion of Fig. 5�. Second, the experiments havebeen performed years apart with different equip-ment, thus reasonably involving slightly differentoverall conditions. The lower profile at z�h 0.45�where h is the total flame height and z is the height
above the burner� presents the typical external sootlayer with a minimum along the flame axis. Movingupward in the flame �z�h 0.6�, the profiles tend toflatten. Finally, at z�h 0.75, because the soot ox-idation is stronger on the external layers, a typicalshape is found, with the maximum on the flame axis.With both techniques, the flame tip appears to bethicker than it actually is because of the locally largerflame fluctuations and averaging process. In Fig. 3we also report the error bar in the lower profile thatcorresponds with the maximum value of the emissionmeasurements. This refers to the spread of experi-mental data in the regions where the uncertaintiesare higher �in the outer layers�. The spread reachesa value of 10% in the worst case, which can still beconsidered quite good with reproducible data, espe-cially if we consider the complex procedure that wasapplied. Error bars outside this area have not beenreported, because they are too small to be traced.Error bars for extinction �lower than 1%� are notreported for the same reason. As a general conclu-sion the profiles of Fig. 3 compare well with previ-ously published data.4–6,19
As stated above, during the earlier measurementswe verified that front and back soot layers contributein the same way to the signal. For such tests, wecollected pairs of images by keeping the focus on theflame axis and shifting the burner by �5 and �5 mmfrom its initial position. In this way the rear andfront side of the flame, respectively, were brought onthe focal plane. The corresponding profiles of an8-cm ethylene flame �flow rates of 0.21 l�m for fueland 37.5 l�m for air� are shown in Fig. 4. The gen-eral shapes of the profiles compare well with theshapes of those in Fig. 3 �self-similarity of diffusionflames� with absolute values that are slightly higher.The differences in the profiles of Fig. 4 can be attrib-uted to random flame fluctuations and noise in theCCD acquisition enhanced in the Abel inversion pro-cedure. In any case, the small differences in sootvolume fraction values are consistent with the errorbar reported in Fig. 3. Major differences are at theaxis, because of the numerical inversion procedure.
Fig. 5. Comparison between the temperature radial profiles cal-culated with our technique �filled squares� and those obtained fromthe multiwavelength emission measurements in Ref. 14 �open cir-cles�.
Fig. 6. Chord-integrated radial profile of the soot volume fractionat the maximum fv axial location in a rich premixed flame of Dieseloil by emission and extinction techniques.
The observed flame width variation is due mainly tothe �1 pixel variation at the edges �typical of discretedetectors in the presence of intensity fluctuations andthreshold cut of the background�. This effect is moreobvious when the intensity distribution at the flameboundaries is affected, even slightly, by blurring �de-focusing� and changes in magnification.
As for the validation of temperature measure-ments, we report the comparison of the present re-sults with the data of Ref. 14 obtained by use of amultiwavelength emission technique, which were ingood agreement with previous data reported in theliterature.28,31,32 Figure 5 shows the temperatureprofiles for the same levels in the 6.5-cm-high ethyl-ene diffusion flame. All the curves are characterizedby a maximum in the external region toward theflame reaction zone, where the soot is burned, and aminimum along the axis. At the edges, even higher
temperatures have been detected �above 2000 K� but,being unconvincing, they have not been reported asbeing completely accurate. In fact the small signalsinvolved by the burn out of soot together with thethreshold cut of the CCD background make the pixel-by-pixel ratio erratic in this thin region. The twocurves �from multiwavelength and two-wavelengthemission analysis� exhibit, on average, a shift of ap-proximately 40 K; in the worst case the maximumdifference in temperature is 85 K for one measure-ment point. Nevertheless, the general agreementbetween the two sets of curves is satisfactory.Again, major discrepancies can be explained in termsof a sum of small differences in the experimentalconditions. In particular, the spatial resolution wasnoticeably different. The spectrograph entrance slitin Ref. 14 was 1 mm with a 0.5-mm measurementstep against, in this experiment, approximately 200 m of two binned CCD pixels as imaged into theflame. In these conditions, even with a stiff flame, abroadening of the profile of 0.8 mm per side is ex-pected with the first method. Flame flickering mag-nifies this effect that is particularly visible on thehigher profile. As a final assessment, Figs. 4 and 5overall indicate a satisfactory comparison when threeindependent techniques have been employed.
5. Tests on Diesel Oil Premixed Flames
We performed additional tests on premixed Diesel oilflames. We chose these kinds of flame because datawere readily available for comparison. In fact, in ourlaboratory such flames were under investigation interms of soot volume fraction, by means of the extinc-tion technique, within the framework of another re-search program that studied the soot production fromDiesel oils of targeted composition.22 The flames hada Bunsen-like structure, with a soot distribution that
Fig. 7. Maximum average soot volume fraction versus the equiv-alence ratio in rich premixed flames of Diesel oil by emission andextinction techniques.
Fig. 8. 2-D soot volume fraction distribution for two methanediffusion flames.
Fig. 9. 2-D soot volume fraction distribution for two propanediffusion flames.
depends heavily on the equivalence ratio �� ��fuel�air�work����fuel�air�stoich��. For � slightly above sootinception �i.e., � � 1.83 for these fuels� a thin sootplume appeared, closely confined around the flameaxis. For richer flames, up to � 3, the soot plumeexpanded to nearly the whole flame volume, with theexception of its lower part. The major amount of sootis generally located just above the halfway point of thetotal flame height. Figure 6 shows an integral radialprofile taken in this region with the two techniques,with good agreement. The agreement holds for allvalues of the equivalence ratio, as shown in Fig. 7,where the maximum radially integrated soot volumefraction found along the axis is reported versus theequivalence ratio. The soot load increases almost lin-early with the equivalence ratio. The error bars inFig. 7 refer to a set of ten measurements for bothextinction and emission corresponding to � 2.3.One error bar was shifted aside slightly for clarity.The spread of the emission measurements appears tobe larger than that of the extinction measurements,because the actual measurement time was approxi-mately ten times lower then the time used for theextinction measurements, which involved a reducedaverage of the flame fluctuation and flickering duringemission measurements.
6. Application to Alkane Diffusion Flames
The most peculiar feature of this technique is that,with simple equipment and a minimal amount oftime for the measurements, a significant amount ofinformation can be collected. In fact, temperaturefields and soot distribution can be derived from asingle frame acquisition for a large set of combustionstudies, which would be rather cumbersome if carriedout otherwise. The self-similarity of flames19 andfuel–soot load correlations are meaningful examples.To illustrate these capabilities we report some appli-cations to methane and propane flames. In particu-lar, soot distribution and temperature fields are givenfor two flame structures �5- and 8-cm total height�.In all the figures we included a CCD strip that cor-responds to a 13 mm 80 mm section in the normalflame plane that passes through the burner axis,where the burner mouth coincides with the left end.This allows for a straightforward comparison of thesoot distribution in flames of different heights.
Figure 8 shows the soot volume fraction distribu-tion along the methane flames. For both flameheights a similar soot distribution is exhibited. Sootappears in the lower part only in the external annu-lar region, then tends to fill the entire flame volume.
Fig. 10. Axial profiles of the local values of fv ��a� and �c�� and of the soot load in the flame section ��b� and �d�� compared for methane andpropane.
For a higher flame we obtained a stretched and moreuniform distribution with fv values on the flame sidesapproximately equal to the maximum, still located onthe axis. Soot concentrations are always somewhatlow, with the maximum remaining below 0.4 ppm.
A quite different distribution was obtained for pro-pane flames �Fig. 9�. Here, the general shape is stillthe same, but the absolute maximum of fv is now in theexternal layer, although a local maximum is also foundon the axis toward the flame tip. Propane producesmuch more soot because the fv values are on averageapproximately ten times higher than for methane.
As a further analysis in Fig. 10 we compared theaxial profiles of the local �upper� and radially inte-grated �lower� soot volume fraction. Figures 10�a�
and 10�c� show how soot evolves along the axialstreamline. It should be noted that an increase infuel flow rate involves a higher maximum for meth-ane and a lower maximum for propane. To demon-strate the total behavior of soot production, Figs.10�b� and 10�d� show the integrated values of fv acrossthe flame width as measured at different axial posi-tions. These values are then multiplied by the sootdensity �1.8 g�cm3�, thus obtaining a quantity Fv thatgives an estimate of the soot load �in g�cm� along theflame. A striking difference between methane andpropane is that the variation in soot load �area underthe curves in Figs. 10�b� and 10�d�� is much higher formethane �a factor of 6.9 increase� than for propane �afactor of 2 increase�.
As for the temperature field calculated by meansof this technique, the distribution obtained is sim-ilar for all flames. Here we provide only Fig. 11 asan example. If we bear in mind that the temper-ature measurements, obtained from soot emission,refer to the temperature of soot in the flame, it isobvious that the drop to zero �black area� at theedges is an artifact. In fact, the flame is sur-rounded by hot reacting gases, which give no black-body emission. For the same reason the outline ofthe temperature field closely reproduces that of thesoot distribution. A sharp rise in temperature �ofthe order of 200–500 K, according to the fuel andflow rates� is in any case visible as a white areaalong the flame contour because of soot oxidationespecially in the short methane flame.
7. Conclusions
A two-color emission technique has been developedfor two-dimensional imaging of soot temperature anddistribution in luminous flames. Validation of thetechnique has been performed, and measurements intypical combustion environments show the useful-ness of the proposed method. The technique is sim-ple and robust with the need of only simple optics andwithout a laser, and it delivers a significant amountof data in a short amount of time. The technique canbe profitably applied for both local measurements inlaboratory axially symmetric flames and for line-of-sight measurements in industrial combustion sys-tems, along the lines of the study reported in Ref. 36,where 2-D imaging of soot temperature distributionis shown in a Diesel engine.
During the preparation of this manuscript, we be-came aware of a similar study devoted to laminardiffusion flames at elevated pressures.37,38 The the-oretical background and the experimental system re-ported by Lee and Na37,38 are similar to what wedescribe in this paper. The results they presentedfurther confirm the applicability of the technique fora variety of experimental conditions.
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