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air flame
ht. Corm Heat Maas 7h&r; Vol. 29, No. 2. pp. 223-231, 2002 cQpyr@tQ2oD2f3kvicrscialceLtd Rittted in the USA. All rights mcrved 073s193mws-ree tiont matter
PII: !3@73!4-1933(02)003135
EXPElUM&NTAL STUDY OF SECONDARYAIR DIFFUSIONh’FECTS ON MMYT CONCEIWRATION ALONG A PARTIALLY PREMIXED ACETYLENFIAIRFLAMFJ
Leona& Goldstein Jr, F&o Luis Faasani and Alex &son Bandeira Santos Department of Fluid and Thermal Engkering College of Mechanical Engineering Carlos Albeato Feamri Depattment of Quantum Electronics Physics Instiite State University of Csmpinas 13083470 Campinas, SP Brazil
(Coxkmmicated
by J.P. Hartnett and W.J. Minkowycz)
ABSTRACT Soot emission is determined by the competition between soot formation and oxidation in a flame. Several factor atkct these processes, including the type of fu& the air-to-fuel ~,flamtempaehue,,andaowpattem.Inthispaper,theinnuenceofanemal airdiiionthesootsxialc4mcultrationandcenterlhleflame tempenmnehawti premixed acctylendair flame was m The flame was generated in a vertical axis bumer in which the discharge of the ikel-oxidant mixture was surrounded by a coaxial am&r flow of nitrogen, which provided a shield which controlled the contact between thetlameandsecom&ryair.Whenprimaryairptemb&gdecreased,theavaqesoot concentration of the flamea without the NZ shield increased more than the concentration of the flames with the shield. The efkct of the shield became more intarse, in general, as partial pnmixmg increassd. 0 2002 Ebvkr S&me Ltd d(tp~h:
Partiilly Premixed
Flame, Secondary Air Effect, Soot Concentration,
Centerline Flame Tempemture
Sooting in flames has been studied not only t?om a scientific point of view but for its practical consequences, such as promotion of radiative heat transfer in boilers and &maces and emission as a pollutant from combustors and diesel engines. Soot was defined by Glassmsn [l] as carbglaceous particulates formed in the gas phase of combustion pmcesses. They consist mainly of carbon, and contain up to 10% hydrogen an a molar basis, and even more when young.
223
224
L. Goldstein Jr., et al.
Vol. 29, No. 2
Soot formation and evolution proceeds in a fbur-step sequence, as reported by Turns [2]: formation of precursor species; soot particle inception; surface growth and particle agglomeration, and particle oxidation. The emission of soot from combustors, or from flames, results from the competition between soot formation and oxidation. Soot emission occurs when fuel is bumt in insufficient oxygen. In the case of a premixed flame of gaseous tire1it is necessary to supply enough oxygen for combustion, while in the diffusion flame, the promotion of mixing between Abeland oxygen is required. Although tlames have been usually defmed into either premixed or difksion flsrnes, many practical combustion devices exhibit intermediate states of fuel-air mixing that result in partially premixed burning. A partially premixed flame is one where the tieI and secondary air are initially separated as in a nonpremixed Same, but the fuel contains sub-stoichiometric amounts of primary air. Many practical combustion devices contain partially premixed flames, including staged combustom and spray burning devices. Many previous computational and/or experimental studies have examined soot formation in laminar partially premixed flames in several physical configurations. Hum and Glassman [3] found that at relatively low levels of partial premixing soot production in ethane flames increased but then decreased sharply as the level of partial premixing was increased and finally no soot was produced. For propane flames, a reduction in soot volume fractions was observed at low levels but an increase was observed at intermediate levels and fmally a sharp reduction was observed at the highest levels of partial premixing leading to no soot. Gore and Zhan [4] studied co-flow jet flames burning ethane and air, and observed that Same luminosity and height decreased with increase in the level of partial premixing. A double flame stntctum was observed for the flames with higher level of partial premixing. The changes in the colors of the flames from yellow to blue indicated that partial premixing reduces and ultimately eliminates soot panicles in methane flames. Mitrovic and Lee [S] measured soot volume S-actions in ethylene flames in a jet flow geometry. The measurements showed that htitial addition of air to the fuel stream causes the overall soot emission to increase, meaning that at high primary equivalence ratios addition of air can have adverse effects in soot behavior of partially premixed flames. The soot volume fraction distributions in the Same were similar to that of laminar diffusion flames for the mnge of equivalence ratio considered, down to 2.5. The soot volume fraction was brought below that of diffusion flames at primary equivalence ratio near 10. Thii nonmonotonic behavior in soot production was attributable to the oxygen addition that enhances the local radical pool production as well as altered hydrocarbon chemistry. Soot was first formed in the annular region, and at downstream locations was observed in the central region. In the limit of premixed flames, there was no observable soot. Ha and Choi [6] investigated the visible spectral characteristics of cross-sectional emissions from a partially premixed methane/air game and a propane/air Same. The blue and yellow flame edges, yellow inside and blue outside, were obtained by processing photographs of the flames. The soot precursor emissions were mainly observed at the blue flame edge in
Vol. 29, No. 2
225
STUDY OF SECONDARY AIR DIFFUSION EFFECTS
the upstream regions, and the soot emissions were observed downstream mainly at regions interior to the yellow tlame edges. Although soot was observed downstream, its production began upstream on the basis of the presence of soot precursors. Soot diibution
in the downstream region was similar to that in a
diffusion frame. McEnally and Pfefferle [7] obtained centerline profiles of gas temperature and soot volume fraction for cotlowing pattially premixed flames with primary equivalence ratios ranging from 24 to 3. Partial premixing was found to increase the maximum soot volume concentration by 7% for the equivalence ratio varying from co to 24. The trends wem similar to those observed by Mitmvic and Lee. The results indicated that the primary effect of partial premixing was not to uniformly increase the concentration of pyrolisis products, but to shift the pyrolisis mechanism towards odd-carbon species. The concentration of benzene was larger in several of the richer partially premixed flames than in the nonpmmixed &me, probably because the shift in pyrolisis mechanism enhances self-reaction of C&is radicals. Increases in soot volume fraction were observed that matched the increases in benzene. Six cotlowing lam&r, partially premixed methane/air flames, varying in primaiy equivalence ratio from a to 2.464 were studied both computationally and expetimentally by Bennett at al. [8]. Heat release rates, as well as those of several species, indicated that the majority of the partially premixed flames contain two game fronts: an inner premixed front whose strength grows with decreasing primary equivalence ratio; and an outer nonpremixed front. As the amount of partial premixing increases, a continual reduction in the amount of flow radiilly inward was predicted; the resulting decrease in mdial transport is responsible for a cooling of the gases near the burner surface. At the same time, radiative losses decrease with increasing amounts of premixing, resulting in higher flame temperatures. The purpose of the present work was to explore the effect of secondary air diffusion on the soot concentration axial distribution and centerline temperature field in partially premixed acetylene/air cotlowing flames. A series of four flames were generated which had a constant total flow rate and a varying primary air flow rate, such that the primary equivalence ratio (PER) varied between 8.50 and 2.38. Tests were performed with and without a nitrogen shield, to assess the effect of secondary air diffusion. The primary equivalence ratio range explored was situated between the premixed flames (PER=l.O), where. secondary air diffusion does not influence soot production, and diffusion flames (PER-),
where
air diffusion is essential to flame existence. In both these extremes the nitrogen shield introduction does not apply.
‘Dte experimental setup is shown in Fig.1. The fuel and oxidant were premixed in PMl, before being fed to the burner QMl, which consisted of two concentric vertical tubes, whose dimensions are shown in
226
L. Goldstein Jr., et al.
Vol. 29, No. 2
FIG.1 Experimental Setup Fig. 2. The t%eland oxidant mixture flowed through the internal tube, while nitrogen flowed through the annular region, pmvidiig a shield that protected the flame Corn the diffusion of external air. Gas flow mlHiEk-2
35
r
50
i
FIG.2 Burner QMl rates were controlled by valves Vl ,V2, V3 and metered by rotametersR1, R2 and R3. Soot concentration was measured along the flame height by means of the laser light extinction technique. The burner was mounted on a step-motor driven vertical translation table, which allowed the beam coming from laser Ll to reach the Same at any desired level. The laser Ll was of He-Ne, with a wavelength of 632.8 nm. Since the power output from the laser was only about ImW, background radiation was blocked from the flame by a narrow band pass interference filter F 1, at the laser wavelength.
Vol. 29, No. 2
227
STUDY OF SECONDARY AIR DIFFUSION EFFECTS
The light was transformed in a electrical current signal by the photodiode FTl, and registered by electrometer ET1. Flame temperatures were measured by an uncoated type S thermocouple Tl (Pt-WlO%Rb) along the central axis of the Eame, and the signals were registered by the temperature meter RGl . The thermocouple tip was cleaned out before every temperature reading. The obtained results were not corrected for radiative and convective losses, considering the uncertainty in evahrating the effect of soot
deposition on the
thermocouple surface. Soot volume fraction, C (ppm), was calculated from the laser light extinction data, using
the
Rayleigh limit of the Mie theory, so that:
(1)
where
A. is the laser wavelength, L tbe optical path length, IOand I the laser beam intensity before and after
traversing the game, and m is the refractive index, adopted as m = 1.90-0.55i, according to Lee and Tien
PI. Some preliminary work was required to find the mixture flow rate which provided a steady flame, avoiding flashback and flame lift. All measuremen ts were taken at a fixed total flow rate
( air +
acetylene) of 0.3 Nm’/h, with PER equals to 8.50, 5.95, 3.97 and 2.38, corresponding to primary air-tofuel-ratios of 1.4,2.0, 3.0 and 5.0. Tests were performed with and without a nitrogen shield. To preserve tlame stability, the same nitrogen flow rate was used in the shield tests with the same superficial gas velocity of the air/acetylene mixture. Both flows were in the laminar regime.
Figure 3 shows the soot volume fraction as a timction of tbe primary equivalence ratio, at four heights above the burner, Z, with and without the nitrogen shield. For the higher values of the primary equivalence ratio, soot formation took place along the flame length, with concentmtion increasing with height, what suggested that there was not enough oxidation of the soot which was formed. As the prhnary
228
L. Goldstein Jr., et al.
Vol. 29, No. 2
AIR-TO-FUEL RATIO
0 1
2
3
4
5
6
7
a
9
IO
PRIMARY EQUIVALENCE RATIO
FIG.3 Soot concentration as a function of the primary equivalence ratio. Tests with and without the nitrogen shield. equivalence ratio decreased, starting at about PER %6, the soot formation process occurred essentially at the flame basis, with no further soot generation or oxidation. This conclusion is supported by the observation of the soot concentration protile below this ratio, which stayed basically leveled at a constant value, with or without the nitrogen shield. One could argue that the induced secondary air oxidii
soot as
it was generated, leveling the concentration, but the same phenomenon was observed when the induction of air was avoided by the introduction of the nitrogen shield. As the shield was brought in, there was an increase in the amount of soot that was generated, because of the lower degree of oxidation which was possible, correspondent basically to the oxygen available in the original tbel-air mixture fed to the burner. In this case, the soot volume traction was higher, but also stayed basically leveled along the flame height, as can be seen in Fig.4, for PER = 5.95,3.97 and 2.38. This fact is an indication that the amount of soot formed at the Same basis remained basically unchanged throughout the flame height.
The flame temperature is shown in Fig. 5, as a function of the premixed air, and in Fig. 4 as a function of Z. Some temperature readings were repeated to check reproducibility, and Td is the adiabatic temperature calculated for each of the primary equivalence ratios, shown in Figure 5 as a mfbrence. As a general trend, the Same temperature increases as the primary equivalence ratio decreases, and this can be
Vol. 29, No. 2
STUDY OF SECONDARY AIR DIFFUSION EFFECTS
229
seen in Fig. 5 for Samea with and without the nitrogen shield, witb the exception of the Same without shield at Z = 5 mm, closer to the burner surface. A similar fact was noticed by M&rally and Pfefkrle [7, lo] for ethylene/air flames without shiekh that is, for a given Z the tempetature increased with PER up to a certain height above the bunter, atIer which it had an inverse behavior, decreasing with PER, although at a smaller rate. Partial premixing reduces the overall height of a game, since less secondary air has to diffise to tbe cent.erhne to create a stoichiometric mixture, and this process compresses the centerline profiles towards the bunter surface, what would expiaht the increase of the temperatum with PER. Near the burner surface the tempemtures of the flames with and without shield exhibited anomalously high readings. The same happened with the data of McEnahy and Pfefferle [lo], who proposed some
FIG. 4 Soot concenttation and centerline flame temperature above the burner for PER = 8.50,5.95,3,97 and 2.38 sort of catalytic ignition on the uncoated thermocouple as a probable cause for such, and omitted the data from further discussion. These authors discovered that, closer to the burner, the reaction rates were
230
L. Goldstein Jr., et al.
Vol. 29, No. 2
negligible, so that temperature and species concentmtion there were controlled by radial diffusion and convection between the centerime and the sunvwrding off-axis flame front. They determined that partial premixing appears to cause a general decrease in radial heat and mass transfer, bringing about a deuease in gas temperature near the burner. This feature can also be noted in Fig. 4. The phenomenon is more evident for flames with the nitrogen shield, which Her
reduces mdial transport
One further observation, for which a possible explanation is not so far propo&, is that the flame seems to be basically isothermal around PER = 4, both with and without the nitrogen shield AIR-TO-FUEL RATIO 10 I
25oor.
8 I
.
.
6 I
4 ,
.
.
1
2
3
4
5
8
.
.
__-...
WITHOUT 0
& l
_---. _..--._
A 0
7
S
2 I
,-I
2 (mm)
0
10
PRIMARY EQUIVALENCE RATIO
FIG. 5 Centerline Flame Temperature as a Functian of Primary Equivalence Ratio. With and Wiiout Niigen Shield As can be noticed in Figs. 4 and 5, flame temperatures were, in general, higher in the tests without the nitrogen shield, because the shield does not permit secondary air flows radially inward, and it destroys the external game front, whose.existence helps to reduce radiative losses from the inner flame. The use of a shield to regulate secondary air diffusion into a partially premixed Same can be a usetirl tool in controlling soot concentration in the flame, as required for radiation heat tmnstbr in equipment design. Table 1 shows the average integrated soot concentration along the
Vol. 29, No. 2
STUDY OF SECONDARY AIR DIFFUSION EFFECTS
231
TABLE 1 Soot Concentration Comparison
No Nz Shield With Nz Shield Relative
PNnary Equivalence Ratio 5.95 2.38 3.97 2.28 1 (Reference) 1.64 3.13 1.86 2.76 37 86 68
8.50 4.93 7.25 47
Increase(%)
tlame relative to the concentration of the flame without the Nr shield at PER = 2.38, atbitrarily taken as a reference. when primaty air premixing decreases, the average soot concentration of the Same without the N2 shield increases more than the concentration of the flame with the shield. The effect of the shield increases, in general, with primary air premixing.
In this paper the e&ct of secondary air diffusion on the soot concentration distribution in a partially premixed acetylene/air flame was studied, for a range of partial premixing. It was observed that the secondary air induced into the flame causes a reduction in soot formation and that the effect of the shield increases, in general, with primary air premixing. As a general trend, the flame temperature increases as the primary equivalence ratio decreases. Flame temperatures are, in general, higher in the tests without the nitrogen shield, because the shield does not permit that secondary air flows radially inward, and destroys the external flame front, whose existence helps to reduce the inner flame radiative losses.
1. I. Glassman, Combustion, 2& ed., Academic Press Inc. New York (1987) 2. S. R. Turns, An Introduction to Combustion, McGraw-Hill, New York (1996). 3. H. S. Hum and I. Glassman, 22”6Symp. (Int.) on Comb., 371(1988) 4. J. P. Gore andN. J. Zhan, Combustion and Flame105 414 (1996) 5. A. Mitrovic and T. W. Lee, Combustion and Flame 115,437 (1998) 6. K. S. Ha and S. Choi, Int. Comm. Heat Mass Transfer 26,8,1139 (1999) 7. C. S. McEnally and L. D. Pfefferle, Combustion and Flame 121,575 (2000) 8. B A. V. Bennett, C. S. McEnally, L. D. Pfefferle and M. D. Smooke, Combustion and Flame 123,522
9. S. C. Lee and C. L. Tien, 18” Symp. (Int) on Comb., 1159 (1981) 10. C. S. McEnally and L. D. Pfefferle, Combustion and Flame 118,619 (1999) Received October 19,200I
PII: !3@73!4-1933(02)003135
EXPElUM&NTAL STUDY OF SECONDARYAIR DIFFUSIONh’FECTS ON MMYT CONCEIWRATION ALONG A PARTIALLY PREMIXED ACETYLENFIAIRFLAMFJ
Leona& Goldstein Jr, F&o Luis Faasani and Alex &son Bandeira Santos Department of Fluid and Thermal Engkering College of Mechanical Engineering Carlos Albeato Feamri Depattment of Quantum Electronics Physics Instiite State University of Csmpinas 13083470 Campinas, SP Brazil
(Coxkmmicated
by J.P. Hartnett and W.J. Minkowycz)
ABSTRACT Soot emission is determined by the competition between soot formation and oxidation in a flame. Several factor atkct these processes, including the type of fu& the air-to-fuel ~,flamtempaehue,,andaowpattem.Inthispaper,theinnuenceofanemal airdiiionthesootsxialc4mcultrationandcenterlhleflame tempenmnehawti premixed acctylendair flame was m The flame was generated in a vertical axis bumer in which the discharge of the ikel-oxidant mixture was surrounded by a coaxial am&r flow of nitrogen, which provided a shield which controlled the contact between thetlameandsecom&ryair.Whenprimaryairptemb&gdecreased,theavaqesoot concentration of the flamea without the NZ shield increased more than the concentration of the flames with the shield. The efkct of the shield became more intarse, in general, as partial pnmixmg increassd. 0 2002 Ebvkr S&me Ltd d(tp~h:
Partiilly Premixed
Flame, Secondary Air Effect, Soot Concentration,
Centerline Flame Tempemture
Sooting in flames has been studied not only t?om a scientific point of view but for its practical consequences, such as promotion of radiative heat transfer in boilers and &maces and emission as a pollutant from combustors and diesel engines. Soot was defined by Glassmsn [l] as carbglaceous particulates formed in the gas phase of combustion pmcesses. They consist mainly of carbon, and contain up to 10% hydrogen an a molar basis, and even more when young.
223
224
L. Goldstein Jr., et al.
Vol. 29, No. 2
Soot formation and evolution proceeds in a fbur-step sequence, as reported by Turns [2]: formation of precursor species; soot particle inception; surface growth and particle agglomeration, and particle oxidation. The emission of soot from combustors, or from flames, results from the competition between soot formation and oxidation. Soot emission occurs when fuel is bumt in insufficient oxygen. In the case of a premixed flame of gaseous tire1it is necessary to supply enough oxygen for combustion, while in the diffusion flame, the promotion of mixing between Abeland oxygen is required. Although tlames have been usually defmed into either premixed or difksion flsrnes, many practical combustion devices exhibit intermediate states of fuel-air mixing that result in partially premixed burning. A partially premixed flame is one where the tieI and secondary air are initially separated as in a nonpremixed Same, but the fuel contains sub-stoichiometric amounts of primary air. Many practical combustion devices contain partially premixed flames, including staged combustom and spray burning devices. Many previous computational and/or experimental studies have examined soot formation in laminar partially premixed flames in several physical configurations. Hum and Glassman [3] found that at relatively low levels of partial premixing soot production in ethane flames increased but then decreased sharply as the level of partial premixing was increased and finally no soot was produced. For propane flames, a reduction in soot volume fractions was observed at low levels but an increase was observed at intermediate levels and fmally a sharp reduction was observed at the highest levels of partial premixing leading to no soot. Gore and Zhan [4] studied co-flow jet flames burning ethane and air, and observed that Same luminosity and height decreased with increase in the level of partial premixing. A double flame stntctum was observed for the flames with higher level of partial premixing. The changes in the colors of the flames from yellow to blue indicated that partial premixing reduces and ultimately eliminates soot panicles in methane flames. Mitrovic and Lee [S] measured soot volume S-actions in ethylene flames in a jet flow geometry. The measurements showed that htitial addition of air to the fuel stream causes the overall soot emission to increase, meaning that at high primary equivalence ratios addition of air can have adverse effects in soot behavior of partially premixed flames. The soot volume fraction distributions in the Same were similar to that of laminar diffusion flames for the mnge of equivalence ratio considered, down to 2.5. The soot volume fraction was brought below that of diffusion flames at primary equivalence ratio near 10. Thii nonmonotonic behavior in soot production was attributable to the oxygen addition that enhances the local radical pool production as well as altered hydrocarbon chemistry. Soot was first formed in the annular region, and at downstream locations was observed in the central region. In the limit of premixed flames, there was no observable soot. Ha and Choi [6] investigated the visible spectral characteristics of cross-sectional emissions from a partially premixed methane/air game and a propane/air Same. The blue and yellow flame edges, yellow inside and blue outside, were obtained by processing photographs of the flames. The soot precursor emissions were mainly observed at the blue flame edge in
Vol. 29, No. 2
225
STUDY OF SECONDARY AIR DIFFUSION EFFECTS
the upstream regions, and the soot emissions were observed downstream mainly at regions interior to the yellow tlame edges. Although soot was observed downstream, its production began upstream on the basis of the presence of soot precursors. Soot diibution
in the downstream region was similar to that in a
diffusion frame. McEnally and Pfefferle [7] obtained centerline profiles of gas temperature and soot volume fraction for cotlowing pattially premixed flames with primary equivalence ratios ranging from 24 to 3. Partial premixing was found to increase the maximum soot volume concentration by 7% for the equivalence ratio varying from co to 24. The trends wem similar to those observed by Mitmvic and Lee. The results indicated that the primary effect of partial premixing was not to uniformly increase the concentration of pyrolisis products, but to shift the pyrolisis mechanism towards odd-carbon species. The concentration of benzene was larger in several of the richer partially premixed flames than in the nonpmmixed &me, probably because the shift in pyrolisis mechanism enhances self-reaction of C&is radicals. Increases in soot volume fraction were observed that matched the increases in benzene. Six cotlowing lam&r, partially premixed methane/air flames, varying in primaiy equivalence ratio from a to 2.464 were studied both computationally and expetimentally by Bennett at al. [8]. Heat release rates, as well as those of several species, indicated that the majority of the partially premixed flames contain two game fronts: an inner premixed front whose strength grows with decreasing primary equivalence ratio; and an outer nonpremixed front. As the amount of partial premixing increases, a continual reduction in the amount of flow radiilly inward was predicted; the resulting decrease in mdial transport is responsible for a cooling of the gases near the burner surface. At the same time, radiative losses decrease with increasing amounts of premixing, resulting in higher flame temperatures. The purpose of the present work was to explore the effect of secondary air diffusion on the soot concentration axial distribution and centerline temperature field in partially premixed acetylene/air cotlowing flames. A series of four flames were generated which had a constant total flow rate and a varying primary air flow rate, such that the primary equivalence ratio (PER) varied between 8.50 and 2.38. Tests were performed with and without a nitrogen shield, to assess the effect of secondary air diffusion. The primary equivalence ratio range explored was situated between the premixed flames (PER=l.O), where. secondary air diffusion does not influence soot production, and diffusion flames (PER-),
where
air diffusion is essential to flame existence. In both these extremes the nitrogen shield introduction does not apply.
‘Dte experimental setup is shown in Fig.1. The fuel and oxidant were premixed in PMl, before being fed to the burner QMl, which consisted of two concentric vertical tubes, whose dimensions are shown in
226
L. Goldstein Jr., et al.
Vol. 29, No. 2
FIG.1 Experimental Setup Fig. 2. The t%eland oxidant mixture flowed through the internal tube, while nitrogen flowed through the annular region, pmvidiig a shield that protected the flame Corn the diffusion of external air. Gas flow mlHiEk-2
35
r
50
i
FIG.2 Burner QMl rates were controlled by valves Vl ,V2, V3 and metered by rotametersR1, R2 and R3. Soot concentration was measured along the flame height by means of the laser light extinction technique. The burner was mounted on a step-motor driven vertical translation table, which allowed the beam coming from laser Ll to reach the Same at any desired level. The laser Ll was of He-Ne, with a wavelength of 632.8 nm. Since the power output from the laser was only about ImW, background radiation was blocked from the flame by a narrow band pass interference filter F 1, at the laser wavelength.
Vol. 29, No. 2
227
STUDY OF SECONDARY AIR DIFFUSION EFFECTS
The light was transformed in a electrical current signal by the photodiode FTl, and registered by electrometer ET1. Flame temperatures were measured by an uncoated type S thermocouple Tl (Pt-WlO%Rb) along the central axis of the Eame, and the signals were registered by the temperature meter RGl . The thermocouple tip was cleaned out before every temperature reading. The obtained results were not corrected for radiative and convective losses, considering the uncertainty in evahrating the effect of soot
deposition on the
thermocouple surface. Soot volume fraction, C (ppm), was calculated from the laser light extinction data, using
the
Rayleigh limit of the Mie theory, so that:
(1)
where
A. is the laser wavelength, L tbe optical path length, IOand I the laser beam intensity before and after
traversing the game, and m is the refractive index, adopted as m = 1.90-0.55i, according to Lee and Tien
PI. Some preliminary work was required to find the mixture flow rate which provided a steady flame, avoiding flashback and flame lift. All measuremen ts were taken at a fixed total flow rate
( air +
acetylene) of 0.3 Nm’/h, with PER equals to 8.50, 5.95, 3.97 and 2.38, corresponding to primary air-tofuel-ratios of 1.4,2.0, 3.0 and 5.0. Tests were performed with and without a nitrogen shield. To preserve tlame stability, the same nitrogen flow rate was used in the shield tests with the same superficial gas velocity of the air/acetylene mixture. Both flows were in the laminar regime.
Figure 3 shows the soot volume fraction as a timction of tbe primary equivalence ratio, at four heights above the burner, Z, with and without the nitrogen shield. For the higher values of the primary equivalence ratio, soot formation took place along the flame length, with concentmtion increasing with height, what suggested that there was not enough oxidation of the soot which was formed. As the prhnary
228
L. Goldstein Jr., et al.
Vol. 29, No. 2
AIR-TO-FUEL RATIO
0 1
2
3
4
5
6
7
a
9
IO
PRIMARY EQUIVALENCE RATIO
FIG.3 Soot concentration as a function of the primary equivalence ratio. Tests with and without the nitrogen shield. equivalence ratio decreased, starting at about PER %6, the soot formation process occurred essentially at the flame basis, with no further soot generation or oxidation. This conclusion is supported by the observation of the soot concentration protile below this ratio, which stayed basically leveled at a constant value, with or without the nitrogen shield. One could argue that the induced secondary air oxidii
soot as
it was generated, leveling the concentration, but the same phenomenon was observed when the induction of air was avoided by the introduction of the nitrogen shield. As the shield was brought in, there was an increase in the amount of soot that was generated, because of the lower degree of oxidation which was possible, correspondent basically to the oxygen available in the original tbel-air mixture fed to the burner. In this case, the soot volume traction was higher, but also stayed basically leveled along the flame height, as can be seen in Fig.4, for PER = 5.95,3.97 and 2.38. This fact is an indication that the amount of soot formed at the Same basis remained basically unchanged throughout the flame height.
The flame temperature is shown in Fig. 5, as a function of the premixed air, and in Fig. 4 as a function of Z. Some temperature readings were repeated to check reproducibility, and Td is the adiabatic temperature calculated for each of the primary equivalence ratios, shown in Figure 5 as a mfbrence. As a general trend, the Same temperature increases as the primary equivalence ratio decreases, and this can be
Vol. 29, No. 2
STUDY OF SECONDARY AIR DIFFUSION EFFECTS
229
seen in Fig. 5 for Samea with and without the nitrogen shield, witb the exception of the Same without shield at Z = 5 mm, closer to the burner surface. A similar fact was noticed by M&rally and Pfefkrle [7, lo] for ethylene/air flames without shiekh that is, for a given Z the tempetature increased with PER up to a certain height above the bunter, atIer which it had an inverse behavior, decreasing with PER, although at a smaller rate. Partial premixing reduces the overall height of a game, since less secondary air has to diffise to tbe cent.erhne to create a stoichiometric mixture, and this process compresses the centerline profiles towards the bunter surface, what would expiaht the increase of the temperatum with PER. Near the burner surface the tempemtures of the flames with and without shield exhibited anomalously high readings. The same happened with the data of McEnahy and Pfefferle [lo], who proposed some
FIG. 4 Soot concenttation and centerline flame temperature above the burner for PER = 8.50,5.95,3,97 and 2.38 sort of catalytic ignition on the uncoated thermocouple as a probable cause for such, and omitted the data from further discussion. These authors discovered that, closer to the burner, the reaction rates were
230
L. Goldstein Jr., et al.
Vol. 29, No. 2
negligible, so that temperature and species concentmtion there were controlled by radial diffusion and convection between the centerime and the sunvwrding off-axis flame front. They determined that partial premixing appears to cause a general decrease in radial heat and mass transfer, bringing about a deuease in gas temperature near the burner. This feature can also be noted in Fig. 4. The phenomenon is more evident for flames with the nitrogen shield, which Her
reduces mdial transport
One further observation, for which a possible explanation is not so far propo&, is that the flame seems to be basically isothermal around PER = 4, both with and without the nitrogen shield AIR-TO-FUEL RATIO 10 I
25oor.
8 I
.
.
6 I
4 ,
.
.
1
2
3
4
5
8
.
.
__-...
WITHOUT 0
& l
_---. _..--._
A 0
7
S
2 I
,-I
2 (mm)
0
10
PRIMARY EQUIVALENCE RATIO
FIG. 5 Centerline Flame Temperature as a Functian of Primary Equivalence Ratio. With and Wiiout Niigen Shield As can be noticed in Figs. 4 and 5, flame temperatures were, in general, higher in the tests without the nitrogen shield, because the shield does not permit secondary air flows radially inward, and it destroys the external game front, whose.existence helps to reduce radiative losses from the inner flame. The use of a shield to regulate secondary air diffusion into a partially premixed Same can be a usetirl tool in controlling soot concentration in the flame, as required for radiation heat tmnstbr in equipment design. Table 1 shows the average integrated soot concentration along the
Vol. 29, No. 2
STUDY OF SECONDARY AIR DIFFUSION EFFECTS
231
TABLE 1 Soot Concentration Comparison
No Nz Shield With Nz Shield Relative
PNnary Equivalence Ratio 5.95 2.38 3.97 2.28 1 (Reference) 1.64 3.13 1.86 2.76 37 86 68
8.50 4.93 7.25 47
Increase(%)
tlame relative to the concentration of the flame without the Nr shield at PER = 2.38, atbitrarily taken as a reference. when primaty air premixing decreases, the average soot concentration of the Same without the N2 shield increases more than the concentration of the flame with the shield. The effect of the shield increases, in general, with primary air premixing.
In this paper the e&ct of secondary air diffusion on the soot concentration distribution in a partially premixed acetylene/air flame was studied, for a range of partial premixing. It was observed that the secondary air induced into the flame causes a reduction in soot formation and that the effect of the shield increases, in general, with primary air premixing. As a general trend, the flame temperature increases as the primary equivalence ratio decreases. Flame temperatures are, in general, higher in the tests without the nitrogen shield, because the shield does not permit that secondary air flows radially inward, and destroys the external flame front, whose existence helps to reduce the inner flame radiative losses.
1. I. Glassman, Combustion, 2& ed., Academic Press Inc. New York (1987) 2. S. R. Turns, An Introduction to Combustion, McGraw-Hill, New York (1996). 3. H. S. Hum and I. Glassman, 22”6Symp. (Int.) on Comb., 371(1988) 4. J. P. Gore andN. J. Zhan, Combustion and Flame105 414 (1996) 5. A. Mitrovic and T. W. Lee, Combustion and Flame 115,437 (1998) 6. K. S. Ha and S. Choi, Int. Comm. Heat Mass Transfer 26,8,1139 (1999) 7. C. S. McEnally and L. D. Pfefferle, Combustion and Flame 121,575 (2000) 8. B A. V. Bennett, C. S. McEnally, L. D. Pfefferle and M. D. Smooke, Combustion and Flame 123,522
9. S. C. Lee and C. L. Tien, 18” Symp. (Int) on Comb., 1159 (1981) 10. C. S. McEnally and L. D. Pfefferle, Combustion and Flame 118,619 (1999) Received October 19,200I