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Quantitative comparison of fuel soot formation rates in laminar diffusion flames
Twenty-first Symposium (International) on Combustion/The Combustion Institute, 1986/pp. 1087-1095
Q U A N T I T A T I V E C O M P A R I S O N OF FUEL S O O T F O R M A T I O N RATES IN L A M I N A R D I F F U S I O N FLAMES A. GOMEZ
Department of Chemical Engzneering, Yale University, P.O. Box 2159 YS, New Haven, Ct, 06520 I. GLASSMAN
Department of Mechanical and Aerospace Engineering Princeton University, Princeton NJ, 08544
In order to separate fuel structure from temperature effects, sooting behavior of several gaseous and liquid fuels was studied in diffusion flames whose temperature was controlled by nitrogen dilution. Soot volume fraction was measured by laser scattering/extinction; velocity and residence time information were obtained by laser induced vaporization; temperature was measured in the soot-free region of the flame by thermocouples. The fuels tested were: butene, acetylene, butadiene and benzene. The results show that of all fuels tested, at a given adiabatic flame temperature, benzene has the highest soot formation rate per unit volume and mass flow rate of fuel; butadiene, butene and acetylene follow in decreasing order. This finding holds even if the comparison were made with respect to the (radiatively corrected) measured average peak temperature in the control volume, instead of the calculated temperature. The measured soot formation rates are in agreement with early qualitative indications obtained exclusively by smoke height tests and with purely pyrolytic studies reported in the literature. These results are the first quantitative confirmation of the controlling role of pyrolysis in the soot formation process. Furthermore, for a given fuel, nitrogen dilution decreases the maximum conversion of fuel into soot and the soot formation rates. This effect is attributed to a prevailing temperature decrease which causes a decrease in the pyrolysis rate and, in turn, in the soot formation rate. In all flames the velocity field is confirmed to be buoyancy controlled; the velocity at a given height correlates with the square root of the height, regardless of level of dilution or burner outlet velocity.
1. Introduction B e f o r e the a d v e n t o f sophisticated optical diagnostics, investigations o n soot f o r m a t i o n were c o n f i n e d to the o b s e r v a t i o n o f overall effects. F o r e x a m p l e , in d i f f u s i o n flames, sooting t e n d e n c y was classified on the basis of the smoke h e i g h t 1, the h e i g h t at which the luminous flame b e g a n to emit smoke. T h e smaller this height, the g r e a t e r is the t e n d e n c y o f a given fuel to soot. T h e crucial role that t e m p e r a t u r e has on soot f o r m a t i o n has b e e n l o n g r e c o g n i z e d 2'3. However, no a t t e m p t s were m a d e in early investigations to s e p a r a t e the t e m p e r a t u r e effect f r o m that o f the fuel structure. C o n s e q u e n t l y , misleading conclusions were d r a w n on the factors controlling the soot process. T h e r e f o r e , as part o f a l a r g e r e f f o r t on both p r e m i x e d 4 a n d diffusion flames, sooting be-
havior o f several gaseous and liquid fuels was studied in diffusion flames whose t e m p e r a t u r e was c o n t r o l l e d by n i t r o g e n dilution. N i t r o g e n was a d d e d to the fuel in a coaxial laminar diffusion flame; fuel mass flow rates were m e a s u r e d at the smoke h e i g h t a n d correlated with calculated adiabatic flame t e m p e r a t u r e s 5'6. T h e s e results, a l t h o u g h qualitative, were very h e l p f u l in elucidating fuel structure effects. A r o m a t i c s and dienes showed greater p r o p e n s i t y to soot and lower t e m p e r a t u r e sensitivity as c o m p a r e d to the aliphatics. A m o n g the aliphatics, the high sooting tendency o f acetylene was e x p l a i n e d as a conseq u e n c e o f h i g h e r flame t e m p e r a t u r e s as comp a r e d to the o t h e r fuels; but, at constant adiabatic flame t e m p e r a t u r e , acetylene exhibited lower sooting t e n d e n c y than most o t h e r fuels. M o r e importantly, c o m p a r i s o n of these smoke h e i g h t tests and pyrolysis results in the
1087
1088
COMBUSTION GENERATED POLLUTANTS
literature supported the contention that the fuel pyrolysis plays a controlling role in the sooting tendency of diffusion flames 3. An investigation was initiated in order to verify these conclusions more quantitatively7. Thus, soot particle size, n u m b e r density and volume fraction were measured by the scattering/extinction techniqueS; velocity and residence time information were obtained by laser induced vaporization9; temperature was measured in the soot-free region of the flame by tbermocouples. Some of these measurements are discussed in the present paper with the goal of comparing soot formation rates per unit volume and fuel mass flow rate as a function of temperature and fuel type.
2. Experimental Apparatus, Methods and Conditions T h e experimental apparatus has been described elsewhere6'7'l~ thus, only a brief review will be given here. The b u r n e r consisted of two concentric tubes; fuel and diluent flowed through the i n n e r and air was forced through the outer annulus. An a l u m i n u m chimney with octagonal cross-section was m o u n t e d on the b u r n e r housing; rectangular pyrex windows on the chimney allowed optical access to the flame. The liquid fuel was pressurized by nitrogen and forced into an evaporator t h r o u g h a capillary tube. A preheated stream of nitrogen swept the hydrocarbon vapor into the b u r n e r through heated lines. T h e optical instrumentation consisted of a pulsed N2 laser which p u m p e d an organic dye; the dye laser, turned to a wavelength of 5300 X, was directed to the burner. A photomultiplier tube collected scattered light at 90 o and a photodiode measured the transmitted intensity through the flame. Scattering and extinction signals were processed by gated integrators interfaced via serial A/D converters with a minicomputer. The measured transmittance, resulting from the integration of the local extinction coefficients along the optical path through the flame, was inverted by a Fourier convolution technique 11 in order to yield the local values9 The scattering coefficient of soot was determined by referencing the measurements with respect to Rayleigh scattering from flowing nitrogen and propane at ambient conditions. Particle size, n u m b e r density and volume fraction were determined after comparing the ratio of the referenced scattering signal and the local extinction coefficient to the calculated
values obtained by applying the Mie-Lorenz theory to a monodisperse distribution of spherical soot particles, whose index of refraction was taken from the literature a2 as n = 1.56 - i 0.56. For the velocity measurements a technique based on the laser vaporization of soot 9 was employed. The high energy flux associated with the tightly focused pulsed laser beam causes vaporization of the soot particles with consequent reduction of light scattering and extinction. A He-Ne laser was focused on the flame centerline i m m downstream of the pulsed beam and at a 45 ~ angle with respect to its direction. The average velocity in a small cylindrical volume is d e t e r m i n e d by measuring the time delay between the laser pulse generation and the observation of a decrease in the particle scattered He-Ne laser light, which was detected by another photomultiplier.1 A Hewlett Packard time counter was used to process and average the detected signals. Peak temperatures were measured by silicon oxide coated Pt-6%Rh vs Pt-30%Rh thermocouples whose coated beads measured typically 915 ram. Since soot is formed on the fuel side of the diffusion flame, no soot deposition on the bead takes place at the reaction front where the highest temperatures are attained. No measurements were attempted in the soot laden region of the flame where soot deposition on the bead wotild have complicated the measurements interpretation and alternative laserbased temperature diagnostics 14 could not be implemented with the available equipment. However, even in the soot-free region of the flame these measurements have to be considered only on a qualitative basis, since they are affected by radiative and conductive errors9 A radiative correction was estimated by applying a convective-radiative energy balance to the thermocouple bead whose geometry was assumed cylindrical7; the bead emissivity was assumed equal to .2215. A total of twelve flames were studied9 T h e fuels tested were: butene, acetylene, butadiene and benzene. The experimental conditions are reported in Table I, where, for each experiment, the relative fuel volumetric and mass flow rates, flame height, nitrogen/fuel ratio on a molar basis, calculated adiabatic flame tem-
~In order to eliminate any vaporization effect on the scattering/extinction measurements, during these measurements a N.D. 1 filter was inserted on the path of the incident laser beam, which was thereby attenuated to a laser fluency below the vaporization threshold discussed by Dasch 13.
FUEL SOOT FORMATION RATE
1089
TABLE I Experimental conditions
Fuel
Qf (cc/s)
Mr (mg/s)
H (mm)
Butene Butene Butene Butene Butene* Acetylene Acetylene Acetylene Butadiene Butadiene Butadiene Benzene
.416 .704 1.13 1.48 .712 1.79 2.22 3.85 .600 .909 1.27 ....
.958 1.62 2.60 3.41 1.64 1.91 2.37 4.10 1.33 2.02 2.83 .373
23.0 44.0 70.0 98.0 41.2 42.2 52.0 90.2 41.0 59.2 84.0 10.2
(N2/Fuel)m -0 2.43 4.56 5.70 4.24 2.15 2.53 3.55 4.10 5.58 7.14 2.61
Tad (K)
Symbol --
No. --
2318 2238 2169 2133 2180 2385 2358 2288 2226 2175 2122 2278
~) A 9 V 9 [] 9 + @ * 9 x
1 2 3 4 5 6 7 8 9 10 11 12
*Not at smoke point perature and symbols used in the following figures are listed. All flames, except Flame No.5 of butene, were at the smoke height condition. The temperature field was affected by diluting the fuel with nitrogen. For a given fuel, typically three flames were examined at different levels of dilution and consequently smoke height. By using the NASA CEC p r o g r a m x6, the corresponding adiabatic flame temperatures were calculated for each flame. For the benzene flame, preheating of the fuel/nitrogen mixture to a temperature of 50 ~ C was required and its effect on the adiabatic flame t e m p e r a t u r e calculation was taken into account. T h e selection of flow rate and consequently flame tieight was dictated by considerations of effects caused by heat losses to the b u r n e r on the temperature field 17'14. In a series of measurements on a very similar b u r n e r Boedeker and Dobbs 14 found that the peak temperature measured by CARS in diffusion flames scaled rather well with the adiabatic flame temperature only for relatively tall flames (>25 ram). For these flames, the peak temperature in the lower region of the flame was typically around 85% of the adiabatic flame temperature. Because of significant heat losses to the burner, short flames, like that of, say, undiluted acetylene at its smoke point, showed peak temperatures in the same region not exceeding 73% of the adiabatic temperature. Thus, in order to minimize the b u r n e r effect on the soot-forming region, most of the experiments were performed on flames taller than 40 mm. By focusing on these relatively tall flames, the adiabatic flame temperature could still be used as a temperature parameter, at least in the region where comparisons were to be made,
with complementary information obtained from the qualitative thermocouple measurements. T h e only short flames examined were those of benzene and undiluted butene, which measured 10 and 23 m m in height, respectively.
3. Results and D i s c u s s i o n 3.1 Velocity measurements Preliminary measurements 1~ showed that the velocity within the flame is essentially a function of only the axial coordinate, z, measured from the b u r n e r rim, regardless of an order of magnitude change in the b u r n e r outlet velocity or level of dilution. This finding substantiates the concept of a buoyancy-controlled velocity field, as originally proposed by Roper is and experimentally demonstrated by Mitchell 19, Roper z~ a n d Santoro et al. 21. Within this context, the velocity at a given height can be expressed by u(z) = (u 2 + 2(Ap/p)gz) l/z
(1)
where Ub is the velocity at the b u r n e r outlet, AO/9 is a density factor, g the gravitational acceleration, and the acceleration 2(Ap/p)g has been assumed constant. At the typical conditions of these experiments, Eq. (1) can be approximated to u(z)~- [2(Ap/p)gz] 1/2
(2)
Consequently, the velocity data for all twelve flames are plotted in Fig. 1 versus the square root of the height z; the measurements can be
1090
COMBUSTION GENERATED POLLUTANTS
go
2000
e
1800 1.5
T (K] U
(m/s)
~500
AA A
1
7
.5 •
1400
•
2000
• J
,
,
~
I
2
,
,
,
r
I
i
L
i
4
i
I
6
i
i
i
i
J
I
i
8
1800 T I~J
FIG. 1. Centerline velocity versus the square root of the distance from the burner rim. Symbols as in Table I.
1500
1400
fitted by a straight line whose slope corresponds to an average acceleration of 27 m/s 2. T h e data are poorly fitted in the first few millimeters above the b u r n e r outlet, presumably because the assumption of constant acceleration breaks down due to the particular temperature field of these types of flame. In fact, as a consequence of the location a n d shape of the flame surface in this particular geometry, at lower heights the average t e m p e r a t u r e in the core of the flame changes rapidly as a function of the axial coordinate. As mentioned before, no effects due to fuel dilution are clearly discernable probably because the change in average temperature from flame to flame is fairly modest and, thus, the density factor appearing in the buoyant acceleration is not significantly affected by dilution. Radial scans of the axial velocity were taken at a few selected heights and it was f o u n d that, except for the very first few millimeters above the burner, relatively flat profiles within the flame were obtained. From the velocity information, residence times from the b u r n e r outlet to a height, z, can be evaluated from
t(z) = f~ (llu(z))dz. 3.2 Temperature measurements Figure 2 shows the peak temperature measurements corrected for radiative losses for the butene, acetylene, butadiene and benzene flames plotted as function of z/H, the nondimensional ratio of the axial location of the measurements over the flame height. Comparisons between different fuels show that:
.2
.4
6
.8
z/H
FIG. 2. Peak temperatures versus z/H: a) butene and benzene; b) butadiene and acetylene. Symbols as in Table I. 1.
2.
3.
Acetylene flames exhibit the highest peak temperatures, with maxima above 1980 K at z/H = .1. As reported in Table I, this finding is in accordance with the adiabatic flame temperature calculations, which show that the acetylene flames are on average hotter than the other flames; Benzene has the lowest peak temperatures, even though its adiabatic flame temperature is bracketed by those of the other fuels. Recalling that the benzene flame measures only 10 m m in height, the relative low peak temperatures could be attributed to the role played in short flames by the heat loss to the b u r n e r , as first pointed out by Kent and Wagnerl7; For a given fuel, no clear trends in the peak temperature profiles are discernable as a consequence of nitrogen dilution. It is likely that uncertainties in the radiation correction and conduction errors, which are of the same order of magnitude of the adiabatic temperature range covered for each fuel, are responsible for the inability to discriminate the profiles.
3.3 Soot measurements Particle volume fraction The soot particle field was similar in all flames studied a n d its features confirm the
FUEL SOOT FORMATION RATE results originally reported by Santoro et al. 22 on ethylene flames. Therefore attention will be focused only briefly on one prototype case, labeled as Flame No.2 in Table I, which is a 44 mrn butene flame at the smoke height. For clarity of presentation, the results of only five radial scans at selected heights above the b u r n e r are shown. Starting with the lower half of the flame (Fig.3, a)), we observe that the soot first appears in a toroidal region. As z, the axial coordinate, increases, soot volume fraction also increases and the radial position of the peak values moves toward the axis. Thus, the soot laden region gradually approaches the flame centerline, where soot formation always lags with respect to the region closer to the flame front. Soot formation appears to be a large activation energy process and, as such, requires relatively large temperatures for initiation. Obviously, the hottest region in these flames is the one close to the flame surface, which explains why the first appearance of soot is detected in this annulus. At these heights, the core of the flame is still relatively cold; only at larger distances from the b u r n e r rim, and, consequently, longer residence times, has e n o u g h heat diffused to the core to raise the local temperature to about 1300 K, when soot is first detected on the centerline7. Higher in the flame, as shown by Fig.3 b), quite different trends are noticed. First, the toroidal character of the profiles disappears; second, as z increases in this u p p e r region, the t04
fv 104
1091
soot volume fraction decreases because oxidation takes over with respect to the soot formation which eventually ceases before reaching the tip of the luminous flame envelope. In fact, for the highly sooting flames u n d e r study a cross over between the diffusion flame surface and the luminous greybody radiating region takes place 6. Therefore, this region is probably on the oxidizer side of the flame.
Soot production rate T h e continuity equation for soot in the integral formulation is
fa Psf~ V" da = 2~/+ - ~ / -
(3)
where dA represents an elemental area of the surface A b o u n d i n g the control volume, p~s the soot density, fv, the soot volume fraction, V, the velocity vector, 29I+ and ~ / - are the total production rate and oxidation rate of soot in the volume. It is convenient to choose a frustrum as control volume. T h e n , the surface integral at the left h a n d side (LHS) can be broken up into three terms: an integral over a flame cross-section at height z corresponding to the mass flow rate of soot entering the frustrum; an integral over a flame cross-section at height z+Az, which represents the mass flow rate of soot leaving the control volume of thickness Az, and a third term, the integral over the lateral surface of the frustrum, whose contribution is zero since no soot is measurable outside of this surface and, consequently, no soot is either entering or leaving the flame through this surface. Consequently, after dropping the vectorial notation, Eq. (3) can be rearranged into
Os[u fa f~ dalz + az - u fA f~ dAlz] = ~/+ - 3?/-
(4)
10-:~ t04
f
t04
iO-7 -6
-4
~2
0
2
4
radius (us)
Fic. 3. Soot volume fraction in the lower region of Flame No.2. Symbols: O z = l l mm; [B z=15 ram; o z= 17 mm; V z=27 ram; /X z=35mm.
where u, the axial c o m p o n e n t of the velocity vector, has been taken outside of the integral since, as shown by the velocity measurements, it is i n d e p e n d e n t of the radial position, and p~ also has been assumed constant. Eq. (4) shows that the net production rate in the control volume is equal to the difference in the soot mass flow rates through the flame cross sections binding this volume. The LHS term of this last equation can be readily calculated from the velocity and volume fraction measurements. Thus, the net production rate can be derived. Figure 4 a) shows for all the different butene flames the volume flow rate of soot plotted as function of the residence time calculated from
1092
COMBUSTION GENERATED POLLUTANTS flames also have the largest fuel flow rate, it is not surprising that they show the highest soot flow rate despite dilution. For proper comparison of the tendency to soot, it becomes necessary, then, to normalize the soot flow rate with respect to the fuel flow rate. Dividing Eq. (4) by Mr, the fuel mass flow rate, one obtains
,0002
,00015
f u fv dA [cm~/s] .0001
.00005
0
.2
16
9
= (~/+-~/-)/~//
(6)
=(M+-M->m,
(7)
or, equivalently,
.12
,OB
.04
0 20
40
60
80
Specifying the LHS terms as ~1, one obtains
100
t (msI
v , + d , - v, = (~t + - ~ t - ) / ~ / i
FIG. 4. Butene: a) soot volumetric flow rate as function of residence time in the flame; b) soot mass flow rate per unit mass flow rate of fuel as function of residence time in the flame. Symbols as in Table I. the b u r n e r rim. Since there is a one to one correspondence of axial location, z, and residence time, t, the ordinate is given by
u f f~, d.A], = 2"~u fR~ f~rdrlt
(5)
which is related to the term in bracket on LHS of Eq. (4). All curves show the characteristic rise of the soot flow rate as a function of residence time in correspondence with the region where soot formation dominates, and then a decline where soot oxidation prevails. Fig.ure 4 a) reveals a n o t h e r interesting feature m that if one compares Flames No.2 and No.5 characterized by the same fuel flow rate but different N2 dilution, the most diluted flame has the lowest peak soot flow rate. As one would expect, dilution lowers the temperature in the formation region and consequently soot loading decreases. However, comparisons of the flames at the smoke height show that the flames with highest diluent rate also have the highest peak soot flow rate. At this point, it should be recalled that, as diluent is added to the flame at the smoke point, an increase in fuel flow rate is required in order to restore the smoke height condition. Since the most diluted
(8)
This variable -q, which represents the conversion of fuel into soot, is plotted versus the residence time t for butene (Fig. 4 b), acetylene (Fig. 5 a), butadiene and benzene (Fig. 5 b). Examination of these curves shows that increasing the flame dilution tends to lower the conversion of fuel into soot; the only exception is, as usual, the short, undiluted butene flame. As has been discussed, the importance of heat losses to the b u r n e r makes any comparison with this flame particularly difficult; consequently, it will no longer be considered. For a fair comparison of the various flames studied some criteria should be established to guide the selection of the control volume. If a frustrum is chosen in the lower portion of the flame, it is safe to assume that most of the oxidation taking place is the one at the flame surface, i.e. over the lateral surface of the frustrum. This term, represented by N/-, is probably small compared to the formation rate, A;/+, which takes place over the whole control volume. Thus, in order to obtain soot formation rates, comparisons are made of control volumes positioned by taking the peak of each curves as the reference point and centering the frustrum in each flame at a time interval of 5 ms upstream of this point. From a chemical kinetic standpoint, it is appropriate to compare control volumes characterized by the same residence time. Therefore, the thickness of the frustra was deter-
FUEL SOOT FORMATION RATE
1093 t/Tmeas
* t04(t/K)
.2 5.t
5.3
5.5
5.7
5.9
4.4
4.6
4.8
t0
.t6
.t2
!
.OB
H/cmS)*
.04
0
t 0 -~
.2
9 t6
t 0 -*
4.2
.12
I/Tad ~- '104 (I/K)
.OB
.04
0
. . . . . . . . . . . . . .
20
40
60 t
80
t00
(ms)
Fxc,. 5. Soot mass flow rate per unit mass flow rate of fuel as function of residence time in the flame, a) acetylene; b) butadiene and benzene. Symbols as in Table I. mined by imposing a constant residence time of 5 ms in each control volume. 2 T h e constraints of constant residence time in the frustrum and on its location in the flame resulted in control volumes of different thicknesses; f u r t h e r m o r e , the flames had different characteristic radial dimensions. Thus, since the frustra to be compared have quite different volumes, there is need o f a n o t h e r normalization with respect to the volumes over which the integral balance is performed. Calculating the difference in "q in correspondence with the extrema o f the residence time interval in the frustrum divided by the control volume gives, from Eq. (8), the average soot formation rate per unit volume and mass flow rate of fuel. This quantitative parameter, ~q/V, is the basis
2Adoption of other analogous criteria ~~ for the selection of a reference point or of a different residence time/thickness of the frustra did not significantly affect either the relative position of the curves or the activation energy derived from such plots9
FIG. 6. Arrhenius plot of soot production rate per unit control volume and mass flow rate of fuel versus the inverse of the adiabatic flame temperature (lower abscissa). Symbols: ~) butene; [] acetylene; o butadiene; A benzene. Arrhenius plot of the same function versus the inverse of the measured average peak temperature (upper abscissa). Symbols: # butene; 9 acetylene; 9 butadiene; 9 benzene. for the fuel comparisons to be made and is plotted in Fig. 6 on a semilog scale versus both the inverse o f the calculated adiabatic flame t e m p e r a t u r e (lower abscissa) and the inverse of the average peak temperatur~ in the control volume, measured by t h e r m o c o u p l e and corrected for radiative losses (upper abscissa). T h e selection o f the adiabatic flame temperature for this Arrhenius type of plot is based on the following consideration. In the lower part of the flames, as shown in Fig. 3, most of the soot is f o r m e d in the toroidal region close to the flame surface; thus, a characteristic temperature o f the flame appears to be the peak temperature, in correspondence with the reaction surface. Furthermore, because the radial t e m p e r a t u r e gradients are much steeper than the axial gradients, even in the core of the flame the t em p er at u r e is controlled by the heat feedback from the reaction zone o f the cylindrical portion of the flame. T h e r e f o r e , since, as mentioned before, the peak t e m p e r a t u r e at the reaction surface, is proportional to the adiabatic flame temperature, at least for tall flames, the choice o f the adiabatic flame t em p er at u r e as a t e m p e r a t u r e parameter in the soot formation region remains well justified. Examination of this figure shows that all fuels have the same relative behavior as given
1094
COMBUSTION GENERATED POLLUTANTS
by the qualitative measurements based on smoke height tests previously reported 5'6. Benzene clearly shows the highest normalized soot production rate. 3 Since the heat loss to the b u r n e r is much more important in this flame, the adiabatic flame temperature is no longer a suitable parameter for the comparison. T h e net effect of an additional heat toss would be that of lowering the characteristic temperature for soot formation to be utilized in the comparison and shifting the point somewhere to the right in this figure. A m o n g the other fuels, at a given adiabatic temperature, butadiene has the highest soot production rate, followed by butene and acetylene in decreasing order. As shown in Fig.6, this conclusion holds even if the control volume average peak temperatures had been used for reference. Therefore, acetylene flames unquestionably have higher peak temperatures than the other flames. Consequently, in contrast with other investigators' conclusions23, it is quantitatively proven that the presumed high propensity of acetylene to soot can be attributed to higher flame temperatures, as originally postulated by Glassman~. T h e measured soot formation rates are in agreement, at least on a relative basis, with the early qualitative indications obtained exclusively by smoke height tests and also with purely pyrolytic studies in shock tubes reported in the literature 24'9~'~6. These results are the first quantitative confirmation of the controlling role of pyrolysis in the soot formation process. For a given fuel, nitrogen dilution decreases the maximum conversion of fuel into soot and the soot formation rates. This effect is attributed to a temperature decrease which causes a decrease in the pyrolysis rate and, in turn, in the soot formation rate. Estimates of "activation energies" from Fig. 6 yielded very large values, which indicate that the selection of the reference flame temperature may not be an adequate choice to derive global kinetic data, but is still useful to assess the relative sooting tendency of various fuels in diffusion flames. More reliable temperature measurements by alternative and probably
~For comparison purposes, the only datum pertinent to benzene is shown even though the flame was only 10 mm tall. Attempts were made to study taller flames of benzene to eliminate the problems related to the heat losses to the burner. However, at high dilution, the flames are lifted several millimiters off the burner and the interpretation of the results can be obscured by substantial entrainment of air at the base of the flame.
more involved techniques would be welcome in order to obtain a better reference temperature, possibly by taking into account the temperature history of the formed soot particles, for these volume-averaged soot formation rates.
4. Conclusions Measurements of velocity, temperature and soot volume fraction in nitrogen diluted laminar diffusion flames lead to the following conclusions: T h e velocity field is confirmed to be buoyancy controlled. T h e velocity at a given height is proportional to the square root of the height, regardless of level of dilution or b u r n e r outlet velocity; . Acetylene flames have higher peak temperatures as compared to the other fuels. The benzene flame has the lowest peak temperature because of the very important effect of heat losses to the burner; 3. Of all fuels tested, at a given adiabatic flame temperature, benzene shows the highest soot formation rate; butadiene, butene and acetylene follow in decreasing order. This finding holds whether the comparisons were made with respect to the measured average peak temperatures or the calculated adiabatic flame temperatures. T h e trends observed are in agreement, at least on a relative basis, with qualitative measurements based on smoke height tests and with purely pyrolytic studies reported in the literature. These results are the first quantitative confirmation of the controlling role of pyrolysis in the soot formation process. For a given fuel, nitrogen dilution decreases the m a x i m u m conversion of fuel into soot and the soot formation rates.
Acknowledgements This research was supported by the ONR Contract N00014-81-C-0046,NR094-11 to United Technologies Research Center and Princeton University. Additional funding was provided by a general support grant by the Mobil Research and Development Corporation. Prof. M. Littman's help in the selection and arrangement of the optical configuration and data acquisition system is gratefully acknowlwdged. Stimulating discussions with colleagues at United Technologies Research Center are also gratefully acknowledged. The authors wish to thank Messrs J. Sivo and D. Peoples for their excellent technical assistance.
FUEL SOOT FORMATION RATE
1095
REFERENCES
13. DASCH, C.J.: Appl. Opt. 23, 2209 (1984). 14. BOEDEKER, L.R. AND DOBBS, G.M.: CARS Tem1. SCHALLA, R.L., CLARK, T.P. AND MC DONALD, perature Measurements in Sooting Laminar DifG.E.: Formation and Combustion of Smoke in fusion Flames, Poster No. 67, presented at the Laminar Flames, NACA Report 1186, 1954. 20th Symposium (International) on Combustion, 2. MILLIKAN, R.C.: J. Phys. Chem. 66, 794 (1962). A n n Arbor, MI, 1984; also, Soot Distribution and 3. GLASSMAN, I.: Phenomenological Models of Soot CARS Temperature Measurements in AxisymProcesses in Combustion Systems Princeton Unimetric Laminar Diffusion Flames with Several versity Department of Mechanical and Aerospace Fuels, to be presented at this Symposium. Engineering, Report 1450; also available as 15. KASKAN, W.: Sixth Symposium (International) on AFOSR TR-79-1147 (1979). Combustion, Reinhold, NY, p.134, 1957. 4. TAKAHASHI, F. AND GLASSMAN,I.: Combust. Sci. 16. GORDON, S. AND MC BRIDE, B.J.: Computer Tech. 37, 1(1984). Program for Calculation of Complex Chemical 5. GLASSMAN,I. AND YACCARINO,P.: 18th Symposium Equilibrium Compositions, Rocket Performance, (International) on Combustion, p. 1175, The ComIncident and Reflected Shocks, and Chapmanbustion Institute, 1981. Jouguet Detonations, NASA SP-273, 1971 6. GOMEZ, A., SIDEBOTHAM, G. AND GLASSMAN,I.: 17. KENT, J.H. AND WAGNER, H.Gg.: Comb. Sci. and Combust. Flame 58, 45 (1984). Tech. 41,245 (1984). 7. GOMEZ A.: A Quantitative Investigation of Soot 18. ROPER, F.G., SMITH, C. AND CUNNINGHAM, A.C.: Formation in Laminar Diffusion Flames, Ph.D. Combust. Flame 29, 227 (1977). thesis, Princeton University, 1986. 19. MITCHELL, R.E.: Nitrogen Oxide Formation in 8. D'ALESSlO, A.: Particulate Carbon, (D.C. Siegla and Laminar Methane-Air Diffusion Flames, Sc. D. G.W. Smith, Eds.), p.207, Plenum Press, 1981. Thesis, MIT, 1975. 9. DASCH, c.J.: 20th Symposium (International) on 20. ROPER, F.G.: Comb. Sci. and Tech. 40, 323 (1984). Combustion, p. 1231, T h e Combustion Institute, 21. SANTORO, R.J., YES, T.T. AND SEMERJIAN, H,G.: 1985. T h e Transport and Growth of Soot Particles in 10. GOMEZ, A., LITTMAN, M., AND GLASSMAN, I.: Laminar Diffusion Flames. Paper presented at Comparative Study of the Sooting Tendency of an the 23rd ASME/AIChe National Heat Transfer Aromatic Versus an Aliphatic in Diffusion Conference, Denver, CO, 1985. Flames, Paper presented at the Joint Spring 22. SANTORO, R.J., SEMERJIAN, H.G. AND DOBBINS, Central States/Western States Section Meeting of R.A.: Combust. Flame 51,203 (1983). the Combustion Institute, San Antonio, Tx, 1985. 23. KENT, J.H. AND WAGNER, H.Gg.: Twentieth 11. SANTORO, R.J., SEMERJIAN, H.G., EMMERMAN, Symposium (International) on Combustion, p. P.J., AND GOULARD, R.: Optical Tomography for 1007, The Combustion Institute, 1985. Flow Field Diagnostics, Paper presented at the 24. FRENKLACH, M., TAKI, S. AND MATULA, R.A.: AIAA 15th Thermophysics Conference, SnowCombust. Flame 49, 275 (1983). mass, CO, 1980; also available as AIAA-80-1541, 25. FRENKLACH, M., TAKI, S., DURGAPRASAD, M.B. 1980. AND MATULA, R.A.: Combust. Flame 54, 81 (1983). 12. DALZELL, W.H. AND SAROFIM,A.F.: Trans. ASME 26. FRENRLACH, M.: Private Communication (1984). J. Heat Transf. 91, 100 (1969).
COMMENTS H. G. Wagner, Universitat Gottingen, W. Germany. Could you please comment on the beginning of soot formation on the centerline of your flames, especially for benzene?
relatively short distance. Consequently, this temperature threshold is reached at only 3ram above the b u r n e r rim, much closer to the b u r n e r than for any other flame tested.
Author's Reply. T h e onset of soot on the flame centerline occurs when a characteristic temperature of about 1350 K is measured locally, regardless of fuel type or level of dilution 1. T h e benzene flame is the shortest, measuring only 10ram in height; therefore, its temperature profile is compressed to within a
1. GOMEZ, A., LITTMAN, M. G. AND GLASSMAN,I.: Comparative Study of Soot Formation on the Centerline of Axissymetric Laminar Diffusion Flames: Fuel and T e m p e r a t u r e Effects, submitted to Combust. Flame (1986).
REFERENCE
Q U A N T I T A T I V E C O M P A R I S O N OF FUEL S O O T F O R M A T I O N RATES IN L A M I N A R D I F F U S I O N FLAMES A. GOMEZ
Department of Chemical Engzneering, Yale University, P.O. Box 2159 YS, New Haven, Ct, 06520 I. GLASSMAN
Department of Mechanical and Aerospace Engineering Princeton University, Princeton NJ, 08544
In order to separate fuel structure from temperature effects, sooting behavior of several gaseous and liquid fuels was studied in diffusion flames whose temperature was controlled by nitrogen dilution. Soot volume fraction was measured by laser scattering/extinction; velocity and residence time information were obtained by laser induced vaporization; temperature was measured in the soot-free region of the flame by thermocouples. The fuels tested were: butene, acetylene, butadiene and benzene. The results show that of all fuels tested, at a given adiabatic flame temperature, benzene has the highest soot formation rate per unit volume and mass flow rate of fuel; butadiene, butene and acetylene follow in decreasing order. This finding holds even if the comparison were made with respect to the (radiatively corrected) measured average peak temperature in the control volume, instead of the calculated temperature. The measured soot formation rates are in agreement with early qualitative indications obtained exclusively by smoke height tests and with purely pyrolytic studies reported in the literature. These results are the first quantitative confirmation of the controlling role of pyrolysis in the soot formation process. Furthermore, for a given fuel, nitrogen dilution decreases the maximum conversion of fuel into soot and the soot formation rates. This effect is attributed to a prevailing temperature decrease which causes a decrease in the pyrolysis rate and, in turn, in the soot formation rate. In all flames the velocity field is confirmed to be buoyancy controlled; the velocity at a given height correlates with the square root of the height, regardless of level of dilution or burner outlet velocity.
1. Introduction B e f o r e the a d v e n t o f sophisticated optical diagnostics, investigations o n soot f o r m a t i o n were c o n f i n e d to the o b s e r v a t i o n o f overall effects. F o r e x a m p l e , in d i f f u s i o n flames, sooting t e n d e n c y was classified on the basis of the smoke h e i g h t 1, the h e i g h t at which the luminous flame b e g a n to emit smoke. T h e smaller this height, the g r e a t e r is the t e n d e n c y o f a given fuel to soot. T h e crucial role that t e m p e r a t u r e has on soot f o r m a t i o n has b e e n l o n g r e c o g n i z e d 2'3. However, no a t t e m p t s were m a d e in early investigations to s e p a r a t e the t e m p e r a t u r e effect f r o m that o f the fuel structure. C o n s e q u e n t l y , misleading conclusions were d r a w n on the factors controlling the soot process. T h e r e f o r e , as part o f a l a r g e r e f f o r t on both p r e m i x e d 4 a n d diffusion flames, sooting be-
havior o f several gaseous and liquid fuels was studied in diffusion flames whose t e m p e r a t u r e was c o n t r o l l e d by n i t r o g e n dilution. N i t r o g e n was a d d e d to the fuel in a coaxial laminar diffusion flame; fuel mass flow rates were m e a s u r e d at the smoke h e i g h t a n d correlated with calculated adiabatic flame t e m p e r a t u r e s 5'6. T h e s e results, a l t h o u g h qualitative, were very h e l p f u l in elucidating fuel structure effects. A r o m a t i c s and dienes showed greater p r o p e n s i t y to soot and lower t e m p e r a t u r e sensitivity as c o m p a r e d to the aliphatics. A m o n g the aliphatics, the high sooting tendency o f acetylene was e x p l a i n e d as a conseq u e n c e o f h i g h e r flame t e m p e r a t u r e s as comp a r e d to the o t h e r fuels; but, at constant adiabatic flame t e m p e r a t u r e , acetylene exhibited lower sooting t e n d e n c y than most o t h e r fuels. M o r e importantly, c o m p a r i s o n of these smoke h e i g h t tests and pyrolysis results in the
1087
1088
COMBUSTION GENERATED POLLUTANTS
literature supported the contention that the fuel pyrolysis plays a controlling role in the sooting tendency of diffusion flames 3. An investigation was initiated in order to verify these conclusions more quantitatively7. Thus, soot particle size, n u m b e r density and volume fraction were measured by the scattering/extinction techniqueS; velocity and residence time information were obtained by laser induced vaporization9; temperature was measured in the soot-free region of the flame by tbermocouples. Some of these measurements are discussed in the present paper with the goal of comparing soot formation rates per unit volume and fuel mass flow rate as a function of temperature and fuel type.
2. Experimental Apparatus, Methods and Conditions T h e experimental apparatus has been described elsewhere6'7'l~ thus, only a brief review will be given here. The b u r n e r consisted of two concentric tubes; fuel and diluent flowed through the i n n e r and air was forced through the outer annulus. An a l u m i n u m chimney with octagonal cross-section was m o u n t e d on the b u r n e r housing; rectangular pyrex windows on the chimney allowed optical access to the flame. The liquid fuel was pressurized by nitrogen and forced into an evaporator t h r o u g h a capillary tube. A preheated stream of nitrogen swept the hydrocarbon vapor into the b u r n e r through heated lines. T h e optical instrumentation consisted of a pulsed N2 laser which p u m p e d an organic dye; the dye laser, turned to a wavelength of 5300 X, was directed to the burner. A photomultiplier tube collected scattered light at 90 o and a photodiode measured the transmitted intensity through the flame. Scattering and extinction signals were processed by gated integrators interfaced via serial A/D converters with a minicomputer. The measured transmittance, resulting from the integration of the local extinction coefficients along the optical path through the flame, was inverted by a Fourier convolution technique 11 in order to yield the local values9 The scattering coefficient of soot was determined by referencing the measurements with respect to Rayleigh scattering from flowing nitrogen and propane at ambient conditions. Particle size, n u m b e r density and volume fraction were determined after comparing the ratio of the referenced scattering signal and the local extinction coefficient to the calculated
values obtained by applying the Mie-Lorenz theory to a monodisperse distribution of spherical soot particles, whose index of refraction was taken from the literature a2 as n = 1.56 - i 0.56. For the velocity measurements a technique based on the laser vaporization of soot 9 was employed. The high energy flux associated with the tightly focused pulsed laser beam causes vaporization of the soot particles with consequent reduction of light scattering and extinction. A He-Ne laser was focused on the flame centerline i m m downstream of the pulsed beam and at a 45 ~ angle with respect to its direction. The average velocity in a small cylindrical volume is d e t e r m i n e d by measuring the time delay between the laser pulse generation and the observation of a decrease in the particle scattered He-Ne laser light, which was detected by another photomultiplier.1 A Hewlett Packard time counter was used to process and average the detected signals. Peak temperatures were measured by silicon oxide coated Pt-6%Rh vs Pt-30%Rh thermocouples whose coated beads measured typically 915 ram. Since soot is formed on the fuel side of the diffusion flame, no soot deposition on the bead takes place at the reaction front where the highest temperatures are attained. No measurements were attempted in the soot laden region of the flame where soot deposition on the bead wotild have complicated the measurements interpretation and alternative laserbased temperature diagnostics 14 could not be implemented with the available equipment. However, even in the soot-free region of the flame these measurements have to be considered only on a qualitative basis, since they are affected by radiative and conductive errors9 A radiative correction was estimated by applying a convective-radiative energy balance to the thermocouple bead whose geometry was assumed cylindrical7; the bead emissivity was assumed equal to .2215. A total of twelve flames were studied9 T h e fuels tested were: butene, acetylene, butadiene and benzene. The experimental conditions are reported in Table I, where, for each experiment, the relative fuel volumetric and mass flow rates, flame height, nitrogen/fuel ratio on a molar basis, calculated adiabatic flame tem-
~In order to eliminate any vaporization effect on the scattering/extinction measurements, during these measurements a N.D. 1 filter was inserted on the path of the incident laser beam, which was thereby attenuated to a laser fluency below the vaporization threshold discussed by Dasch 13.
FUEL SOOT FORMATION RATE
1089
TABLE I Experimental conditions
Fuel
Qf (cc/s)
Mr (mg/s)
H (mm)
Butene Butene Butene Butene Butene* Acetylene Acetylene Acetylene Butadiene Butadiene Butadiene Benzene
.416 .704 1.13 1.48 .712 1.79 2.22 3.85 .600 .909 1.27 ....
.958 1.62 2.60 3.41 1.64 1.91 2.37 4.10 1.33 2.02 2.83 .373
23.0 44.0 70.0 98.0 41.2 42.2 52.0 90.2 41.0 59.2 84.0 10.2
(N2/Fuel)m -0 2.43 4.56 5.70 4.24 2.15 2.53 3.55 4.10 5.58 7.14 2.61
Tad (K)
Symbol --
No. --
2318 2238 2169 2133 2180 2385 2358 2288 2226 2175 2122 2278
~) A 9 V 9 [] 9 + @ * 9 x
1 2 3 4 5 6 7 8 9 10 11 12
*Not at smoke point perature and symbols used in the following figures are listed. All flames, except Flame No.5 of butene, were at the smoke height condition. The temperature field was affected by diluting the fuel with nitrogen. For a given fuel, typically three flames were examined at different levels of dilution and consequently smoke height. By using the NASA CEC p r o g r a m x6, the corresponding adiabatic flame temperatures were calculated for each flame. For the benzene flame, preheating of the fuel/nitrogen mixture to a temperature of 50 ~ C was required and its effect on the adiabatic flame t e m p e r a t u r e calculation was taken into account. T h e selection of flow rate and consequently flame tieight was dictated by considerations of effects caused by heat losses to the b u r n e r on the temperature field 17'14. In a series of measurements on a very similar b u r n e r Boedeker and Dobbs 14 found that the peak temperature measured by CARS in diffusion flames scaled rather well with the adiabatic flame temperature only for relatively tall flames (>25 ram). For these flames, the peak temperature in the lower region of the flame was typically around 85% of the adiabatic flame temperature. Because of significant heat losses to the burner, short flames, like that of, say, undiluted acetylene at its smoke point, showed peak temperatures in the same region not exceeding 73% of the adiabatic temperature. Thus, in order to minimize the b u r n e r effect on the soot-forming region, most of the experiments were performed on flames taller than 40 mm. By focusing on these relatively tall flames, the adiabatic flame temperature could still be used as a temperature parameter, at least in the region where comparisons were to be made,
with complementary information obtained from the qualitative thermocouple measurements. T h e only short flames examined were those of benzene and undiluted butene, which measured 10 and 23 m m in height, respectively.
3. Results and D i s c u s s i o n 3.1 Velocity measurements Preliminary measurements 1~ showed that the velocity within the flame is essentially a function of only the axial coordinate, z, measured from the b u r n e r rim, regardless of an order of magnitude change in the b u r n e r outlet velocity or level of dilution. This finding substantiates the concept of a buoyancy-controlled velocity field, as originally proposed by Roper is and experimentally demonstrated by Mitchell 19, Roper z~ a n d Santoro et al. 21. Within this context, the velocity at a given height can be expressed by u(z) = (u 2 + 2(Ap/p)gz) l/z
(1)
where Ub is the velocity at the b u r n e r outlet, AO/9 is a density factor, g the gravitational acceleration, and the acceleration 2(Ap/p)g has been assumed constant. At the typical conditions of these experiments, Eq. (1) can be approximated to u(z)~- [2(Ap/p)gz] 1/2
(2)
Consequently, the velocity data for all twelve flames are plotted in Fig. 1 versus the square root of the height z; the measurements can be
1090
COMBUSTION GENERATED POLLUTANTS
go
2000
e
1800 1.5
T (K] U
(m/s)
~500
AA A
1
7
.5 •
1400
•
2000
• J
,
,
~
I
2
,
,
,
r
I
i
L
i
4
i
I
6
i
i
i
i
J
I
i
8
1800 T I~J
FIG. 1. Centerline velocity versus the square root of the distance from the burner rim. Symbols as in Table I.
1500
1400
fitted by a straight line whose slope corresponds to an average acceleration of 27 m/s 2. T h e data are poorly fitted in the first few millimeters above the b u r n e r outlet, presumably because the assumption of constant acceleration breaks down due to the particular temperature field of these types of flame. In fact, as a consequence of the location a n d shape of the flame surface in this particular geometry, at lower heights the average t e m p e r a t u r e in the core of the flame changes rapidly as a function of the axial coordinate. As mentioned before, no effects due to fuel dilution are clearly discernable probably because the change in average temperature from flame to flame is fairly modest and, thus, the density factor appearing in the buoyant acceleration is not significantly affected by dilution. Radial scans of the axial velocity were taken at a few selected heights and it was f o u n d that, except for the very first few millimeters above the burner, relatively flat profiles within the flame were obtained. From the velocity information, residence times from the b u r n e r outlet to a height, z, can be evaluated from
t(z) = f~ (llu(z))dz. 3.2 Temperature measurements Figure 2 shows the peak temperature measurements corrected for radiative losses for the butene, acetylene, butadiene and benzene flames plotted as function of z/H, the nondimensional ratio of the axial location of the measurements over the flame height. Comparisons between different fuels show that:
.2
.4
6
.8
z/H
FIG. 2. Peak temperatures versus z/H: a) butene and benzene; b) butadiene and acetylene. Symbols as in Table I. 1.
2.
3.
Acetylene flames exhibit the highest peak temperatures, with maxima above 1980 K at z/H = .1. As reported in Table I, this finding is in accordance with the adiabatic flame temperature calculations, which show that the acetylene flames are on average hotter than the other flames; Benzene has the lowest peak temperatures, even though its adiabatic flame temperature is bracketed by those of the other fuels. Recalling that the benzene flame measures only 10 m m in height, the relative low peak temperatures could be attributed to the role played in short flames by the heat loss to the b u r n e r , as first pointed out by Kent and Wagnerl7; For a given fuel, no clear trends in the peak temperature profiles are discernable as a consequence of nitrogen dilution. It is likely that uncertainties in the radiation correction and conduction errors, which are of the same order of magnitude of the adiabatic temperature range covered for each fuel, are responsible for the inability to discriminate the profiles.
3.3 Soot measurements Particle volume fraction The soot particle field was similar in all flames studied a n d its features confirm the
FUEL SOOT FORMATION RATE results originally reported by Santoro et al. 22 on ethylene flames. Therefore attention will be focused only briefly on one prototype case, labeled as Flame No.2 in Table I, which is a 44 mrn butene flame at the smoke height. For clarity of presentation, the results of only five radial scans at selected heights above the b u r n e r are shown. Starting with the lower half of the flame (Fig.3, a)), we observe that the soot first appears in a toroidal region. As z, the axial coordinate, increases, soot volume fraction also increases and the radial position of the peak values moves toward the axis. Thus, the soot laden region gradually approaches the flame centerline, where soot formation always lags with respect to the region closer to the flame front. Soot formation appears to be a large activation energy process and, as such, requires relatively large temperatures for initiation. Obviously, the hottest region in these flames is the one close to the flame surface, which explains why the first appearance of soot is detected in this annulus. At these heights, the core of the flame is still relatively cold; only at larger distances from the b u r n e r rim, and, consequently, longer residence times, has e n o u g h heat diffused to the core to raise the local temperature to about 1300 K, when soot is first detected on the centerline7. Higher in the flame, as shown by Fig.3 b), quite different trends are noticed. First, the toroidal character of the profiles disappears; second, as z increases in this u p p e r region, the t04
fv 104
1091
soot volume fraction decreases because oxidation takes over with respect to the soot formation which eventually ceases before reaching the tip of the luminous flame envelope. In fact, for the highly sooting flames u n d e r study a cross over between the diffusion flame surface and the luminous greybody radiating region takes place 6. Therefore, this region is probably on the oxidizer side of the flame.
Soot production rate T h e continuity equation for soot in the integral formulation is
fa Psf~ V" da = 2~/+ - ~ / -
(3)
where dA represents an elemental area of the surface A b o u n d i n g the control volume, p~s the soot density, fv, the soot volume fraction, V, the velocity vector, 29I+ and ~ / - are the total production rate and oxidation rate of soot in the volume. It is convenient to choose a frustrum as control volume. T h e n , the surface integral at the left h a n d side (LHS) can be broken up into three terms: an integral over a flame cross-section at height z corresponding to the mass flow rate of soot entering the frustrum; an integral over a flame cross-section at height z+Az, which represents the mass flow rate of soot leaving the control volume of thickness Az, and a third term, the integral over the lateral surface of the frustrum, whose contribution is zero since no soot is measurable outside of this surface and, consequently, no soot is either entering or leaving the flame through this surface. Consequently, after dropping the vectorial notation, Eq. (3) can be rearranged into
Os[u fa f~ dalz + az - u fA f~ dAlz] = ~/+ - 3?/-
(4)
10-:~ t04
f
t04
iO-7 -6
-4
~2
0
2
4
radius (us)
Fic. 3. Soot volume fraction in the lower region of Flame No.2. Symbols: O z = l l mm; [B z=15 ram; o z= 17 mm; V z=27 ram; /X z=35mm.
where u, the axial c o m p o n e n t of the velocity vector, has been taken outside of the integral since, as shown by the velocity measurements, it is i n d e p e n d e n t of the radial position, and p~ also has been assumed constant. Eq. (4) shows that the net production rate in the control volume is equal to the difference in the soot mass flow rates through the flame cross sections binding this volume. The LHS term of this last equation can be readily calculated from the velocity and volume fraction measurements. Thus, the net production rate can be derived. Figure 4 a) shows for all the different butene flames the volume flow rate of soot plotted as function of the residence time calculated from
1092
COMBUSTION GENERATED POLLUTANTS flames also have the largest fuel flow rate, it is not surprising that they show the highest soot flow rate despite dilution. For proper comparison of the tendency to soot, it becomes necessary, then, to normalize the soot flow rate with respect to the fuel flow rate. Dividing Eq. (4) by Mr, the fuel mass flow rate, one obtains
,0002
,00015
f u fv dA [cm~/s] .0001
.00005
0
.2
16
9
= (~/+-~/-)/~//
(6)
=(M+-M->m,
(7)
or, equivalently,
.12
,OB
.04
0 20
40
60
80
Specifying the LHS terms as ~1, one obtains
100
t (msI
v , + d , - v, = (~t + - ~ t - ) / ~ / i
FIG. 4. Butene: a) soot volumetric flow rate as function of residence time in the flame; b) soot mass flow rate per unit mass flow rate of fuel as function of residence time in the flame. Symbols as in Table I. the b u r n e r rim. Since there is a one to one correspondence of axial location, z, and residence time, t, the ordinate is given by
u f f~, d.A], = 2"~u fR~ f~rdrlt
(5)
which is related to the term in bracket on LHS of Eq. (4). All curves show the characteristic rise of the soot flow rate as a function of residence time in correspondence with the region where soot formation dominates, and then a decline where soot oxidation prevails. Fig.ure 4 a) reveals a n o t h e r interesting feature m that if one compares Flames No.2 and No.5 characterized by the same fuel flow rate but different N2 dilution, the most diluted flame has the lowest peak soot flow rate. As one would expect, dilution lowers the temperature in the formation region and consequently soot loading decreases. However, comparisons of the flames at the smoke height show that the flames with highest diluent rate also have the highest peak soot flow rate. At this point, it should be recalled that, as diluent is added to the flame at the smoke point, an increase in fuel flow rate is required in order to restore the smoke height condition. Since the most diluted
(8)
This variable -q, which represents the conversion of fuel into soot, is plotted versus the residence time t for butene (Fig. 4 b), acetylene (Fig. 5 a), butadiene and benzene (Fig. 5 b). Examination of these curves shows that increasing the flame dilution tends to lower the conversion of fuel into soot; the only exception is, as usual, the short, undiluted butene flame. As has been discussed, the importance of heat losses to the b u r n e r makes any comparison with this flame particularly difficult; consequently, it will no longer be considered. For a fair comparison of the various flames studied some criteria should be established to guide the selection of the control volume. If a frustrum is chosen in the lower portion of the flame, it is safe to assume that most of the oxidation taking place is the one at the flame surface, i.e. over the lateral surface of the frustrum. This term, represented by N/-, is probably small compared to the formation rate, A;/+, which takes place over the whole control volume. Thus, in order to obtain soot formation rates, comparisons are made of control volumes positioned by taking the peak of each curves as the reference point and centering the frustrum in each flame at a time interval of 5 ms upstream of this point. From a chemical kinetic standpoint, it is appropriate to compare control volumes characterized by the same residence time. Therefore, the thickness of the frustra was deter-
FUEL SOOT FORMATION RATE
1093 t/Tmeas
* t04(t/K)
.2 5.t
5.3
5.5
5.7
5.9
4.4
4.6
4.8
t0
.t6
.t2
!
.OB
H/cmS)*
.04
0
t 0 -~
.2
9 t6
t 0 -*
4.2
.12
I/Tad ~- '104 (I/K)
.OB
.04
0
. . . . . . . . . . . . . .
20
40
60 t
80
t00
(ms)
Fxc,. 5. Soot mass flow rate per unit mass flow rate of fuel as function of residence time in the flame, a) acetylene; b) butadiene and benzene. Symbols as in Table I. mined by imposing a constant residence time of 5 ms in each control volume. 2 T h e constraints of constant residence time in the frustrum and on its location in the flame resulted in control volumes of different thicknesses; f u r t h e r m o r e , the flames had different characteristic radial dimensions. Thus, since the frustra to be compared have quite different volumes, there is need o f a n o t h e r normalization with respect to the volumes over which the integral balance is performed. Calculating the difference in "q in correspondence with the extrema o f the residence time interval in the frustrum divided by the control volume gives, from Eq. (8), the average soot formation rate per unit volume and mass flow rate of fuel. This quantitative parameter, ~q/V, is the basis
2Adoption of other analogous criteria ~~ for the selection of a reference point or of a different residence time/thickness of the frustra did not significantly affect either the relative position of the curves or the activation energy derived from such plots9
FIG. 6. Arrhenius plot of soot production rate per unit control volume and mass flow rate of fuel versus the inverse of the adiabatic flame temperature (lower abscissa). Symbols: ~) butene; [] acetylene; o butadiene; A benzene. Arrhenius plot of the same function versus the inverse of the measured average peak temperature (upper abscissa). Symbols: # butene; 9 acetylene; 9 butadiene; 9 benzene. for the fuel comparisons to be made and is plotted in Fig. 6 on a semilog scale versus both the inverse o f the calculated adiabatic flame t e m p e r a t u r e (lower abscissa) and the inverse of the average peak temperatur~ in the control volume, measured by t h e r m o c o u p l e and corrected for radiative losses (upper abscissa). T h e selection o f the adiabatic flame temperature for this Arrhenius type of plot is based on the following consideration. In the lower part of the flames, as shown in Fig. 3, most of the soot is f o r m e d in the toroidal region close to the flame surface; thus, a characteristic temperature o f the flame appears to be the peak temperature, in correspondence with the reaction surface. Furthermore, because the radial t e m p e r a t u r e gradients are much steeper than the axial gradients, even in the core of the flame the t em p er at u r e is controlled by the heat feedback from the reaction zone o f the cylindrical portion of the flame. T h e r e f o r e , since, as mentioned before, the peak t e m p e r a t u r e at the reaction surface, is proportional to the adiabatic flame temperature, at least for tall flames, the choice o f the adiabatic flame t em p er at u r e as a t e m p e r a t u r e parameter in the soot formation region remains well justified. Examination of this figure shows that all fuels have the same relative behavior as given
1094
COMBUSTION GENERATED POLLUTANTS
by the qualitative measurements based on smoke height tests previously reported 5'6. Benzene clearly shows the highest normalized soot production rate. 3 Since the heat loss to the b u r n e r is much more important in this flame, the adiabatic flame temperature is no longer a suitable parameter for the comparison. T h e net effect of an additional heat toss would be that of lowering the characteristic temperature for soot formation to be utilized in the comparison and shifting the point somewhere to the right in this figure. A m o n g the other fuels, at a given adiabatic temperature, butadiene has the highest soot production rate, followed by butene and acetylene in decreasing order. As shown in Fig.6, this conclusion holds even if the control volume average peak temperatures had been used for reference. Therefore, acetylene flames unquestionably have higher peak temperatures than the other flames. Consequently, in contrast with other investigators' conclusions23, it is quantitatively proven that the presumed high propensity of acetylene to soot can be attributed to higher flame temperatures, as originally postulated by Glassman~. T h e measured soot formation rates are in agreement, at least on a relative basis, with the early qualitative indications obtained exclusively by smoke height tests and also with purely pyrolytic studies in shock tubes reported in the literature 24'9~'~6. These results are the first quantitative confirmation of the controlling role of pyrolysis in the soot formation process. For a given fuel, nitrogen dilution decreases the maximum conversion of fuel into soot and the soot formation rates. This effect is attributed to a temperature decrease which causes a decrease in the pyrolysis rate and, in turn, in the soot formation rate. Estimates of "activation energies" from Fig. 6 yielded very large values, which indicate that the selection of the reference flame temperature may not be an adequate choice to derive global kinetic data, but is still useful to assess the relative sooting tendency of various fuels in diffusion flames. More reliable temperature measurements by alternative and probably
~For comparison purposes, the only datum pertinent to benzene is shown even though the flame was only 10 mm tall. Attempts were made to study taller flames of benzene to eliminate the problems related to the heat losses to the burner. However, at high dilution, the flames are lifted several millimiters off the burner and the interpretation of the results can be obscured by substantial entrainment of air at the base of the flame.
more involved techniques would be welcome in order to obtain a better reference temperature, possibly by taking into account the temperature history of the formed soot particles, for these volume-averaged soot formation rates.
4. Conclusions Measurements of velocity, temperature and soot volume fraction in nitrogen diluted laminar diffusion flames lead to the following conclusions: T h e velocity field is confirmed to be buoyancy controlled. T h e velocity at a given height is proportional to the square root of the height, regardless of level of dilution or b u r n e r outlet velocity; . Acetylene flames have higher peak temperatures as compared to the other fuels. The benzene flame has the lowest peak temperature because of the very important effect of heat losses to the burner; 3. Of all fuels tested, at a given adiabatic flame temperature, benzene shows the highest soot formation rate; butadiene, butene and acetylene follow in decreasing order. This finding holds whether the comparisons were made with respect to the measured average peak temperatures or the calculated adiabatic flame temperatures. T h e trends observed are in agreement, at least on a relative basis, with qualitative measurements based on smoke height tests and with purely pyrolytic studies reported in the literature. These results are the first quantitative confirmation of the controlling role of pyrolysis in the soot formation process. For a given fuel, nitrogen dilution decreases the m a x i m u m conversion of fuel into soot and the soot formation rates.
Acknowledgements This research was supported by the ONR Contract N00014-81-C-0046,NR094-11 to United Technologies Research Center and Princeton University. Additional funding was provided by a general support grant by the Mobil Research and Development Corporation. Prof. M. Littman's help in the selection and arrangement of the optical configuration and data acquisition system is gratefully acknowlwdged. Stimulating discussions with colleagues at United Technologies Research Center are also gratefully acknowledged. The authors wish to thank Messrs J. Sivo and D. Peoples for their excellent technical assistance.
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REFERENCES
13. DASCH, C.J.: Appl. Opt. 23, 2209 (1984). 14. BOEDEKER, L.R. AND DOBBS, G.M.: CARS Tem1. SCHALLA, R.L., CLARK, T.P. AND MC DONALD, perature Measurements in Sooting Laminar DifG.E.: Formation and Combustion of Smoke in fusion Flames, Poster No. 67, presented at the Laminar Flames, NACA Report 1186, 1954. 20th Symposium (International) on Combustion, 2. MILLIKAN, R.C.: J. Phys. Chem. 66, 794 (1962). A n n Arbor, MI, 1984; also, Soot Distribution and 3. GLASSMAN, I.: Phenomenological Models of Soot CARS Temperature Measurements in AxisymProcesses in Combustion Systems Princeton Unimetric Laminar Diffusion Flames with Several versity Department of Mechanical and Aerospace Fuels, to be presented at this Symposium. Engineering, Report 1450; also available as 15. KASKAN, W.: Sixth Symposium (International) on AFOSR TR-79-1147 (1979). Combustion, Reinhold, NY, p.134, 1957. 4. TAKAHASHI, F. AND GLASSMAN,I.: Combust. Sci. 16. GORDON, S. AND MC BRIDE, B.J.: Computer Tech. 37, 1(1984). Program for Calculation of Complex Chemical 5. GLASSMAN,I. AND YACCARINO,P.: 18th Symposium Equilibrium Compositions, Rocket Performance, (International) on Combustion, p. 1175, The ComIncident and Reflected Shocks, and Chapmanbustion Institute, 1981. Jouguet Detonations, NASA SP-273, 1971 6. GOMEZ, A., SIDEBOTHAM, G. AND GLASSMAN,I.: 17. KENT, J.H. AND WAGNER, H.Gg.: Comb. Sci. and Combust. Flame 58, 45 (1984). Tech. 41,245 (1984). 7. GOMEZ A.: A Quantitative Investigation of Soot 18. ROPER, F.G., SMITH, C. AND CUNNINGHAM, A.C.: Formation in Laminar Diffusion Flames, Ph.D. Combust. Flame 29, 227 (1977). thesis, Princeton University, 1986. 19. MITCHELL, R.E.: Nitrogen Oxide Formation in 8. D'ALESSlO, A.: Particulate Carbon, (D.C. Siegla and Laminar Methane-Air Diffusion Flames, Sc. D. G.W. Smith, Eds.), p.207, Plenum Press, 1981. Thesis, MIT, 1975. 9. DASCH, c.J.: 20th Symposium (International) on 20. ROPER, F.G.: Comb. Sci. and Tech. 40, 323 (1984). Combustion, p. 1231, T h e Combustion Institute, 21. SANTORO, R.J., YES, T.T. AND SEMERJIAN, H,G.: 1985. T h e Transport and Growth of Soot Particles in 10. GOMEZ, A., LITTMAN, M., AND GLASSMAN, I.: Laminar Diffusion Flames. Paper presented at Comparative Study of the Sooting Tendency of an the 23rd ASME/AIChe National Heat Transfer Aromatic Versus an Aliphatic in Diffusion Conference, Denver, CO, 1985. Flames, Paper presented at the Joint Spring 22. SANTORO, R.J., SEMERJIAN, H.G. AND DOBBINS, Central States/Western States Section Meeting of R.A.: Combust. Flame 51,203 (1983). the Combustion Institute, San Antonio, Tx, 1985. 23. KENT, J.H. AND WAGNER, H.Gg.: Twentieth 11. SANTORO, R.J., SEMERJIAN, H.G., EMMERMAN, Symposium (International) on Combustion, p. P.J., AND GOULARD, R.: Optical Tomography for 1007, The Combustion Institute, 1985. Flow Field Diagnostics, Paper presented at the 24. FRENKLACH, M., TAKI, S. AND MATULA, R.A.: AIAA 15th Thermophysics Conference, SnowCombust. Flame 49, 275 (1983). mass, CO, 1980; also available as AIAA-80-1541, 25. FRENKLACH, M., TAKI, S., DURGAPRASAD, M.B. 1980. AND MATULA, R.A.: Combust. Flame 54, 81 (1983). 12. DALZELL, W.H. AND SAROFIM,A.F.: Trans. ASME 26. FRENRLACH, M.: Private Communication (1984). J. Heat Transf. 91, 100 (1969).
COMMENTS H. G. Wagner, Universitat Gottingen, W. Germany. Could you please comment on the beginning of soot formation on the centerline of your flames, especially for benzene?
relatively short distance. Consequently, this temperature threshold is reached at only 3ram above the b u r n e r rim, much closer to the b u r n e r than for any other flame tested.
Author's Reply. T h e onset of soot on the flame centerline occurs when a characteristic temperature of about 1350 K is measured locally, regardless of fuel type or level of dilution 1. T h e benzene flame is the shortest, measuring only 10ram in height; therefore, its temperature profile is compressed to within a
1. GOMEZ, A., LITTMAN, M. G. AND GLASSMAN,I.: Comparative Study of Soot Formation on the Centerline of Axissymetric Laminar Diffusion Flames: Fuel and T e m p e r a t u r e Effects, submitted to Combust. Flame (1986).
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