Oxides of nitrogen emissions from turbulent jet flames: Part I—Fuel effects and flame radiation

C O M B U S T I O N A N D F L A M E 87:319-335 (1991)

319

Oxides of Nitrogen Emissions from Turbulent Jet Flames: Part 1--Fuel Effects and Flame Radiation S T E P H E N R. T U R N S and F R A N K L I N H. M Y H R Department of Mechanical Engineering, Propulsion Engineering Research Center, The Pennsylvania State University, University Park, PA 16802 Measurements of oxides of nitrogen emission indices, flame radiant fractions, and visible flame dimensions were made for turbulent jet diffusion flames covering a wide range of flow conditions. Objectives of the study were to explain the observed scaling of NO x emissions with flow variables and to understand the interrelationships among NOx, flow conditions, and flame radiation. The flames were vertical and stabilized with hydrogen pilot flames on straight tube burners. Flow conditions were varied by changing the initial jet velocity and/or the burner tube diameter. Four burner sizes were used, with diameters ranging from 2.18 to 6.17 mm; and four fuel types, having a wide range of sooting tendencies, were employed: methane, ethylene, propane, and a 57% CO/43% H 2 (by volume) mixture. The ranges of Reynolds numbers and Froude numbers explored were 3,130-88,500 and 218 to 2.8 x 106, respectively. The effects of flow parameters and fuel type on radiant losses are shown to be important in determining the NO x emissions from simple jet flames. For high-temperature flames ( T > 2050 K), overall NO x production rates for all four fuels were found to scale with characteristic flame temperatures deduced from the measured radiant fractions in a manner consistent with Zeldovich kinetics. This successful scaling of NO x production rates with global flame temperatures and residence times is consistent with, but does not prove, the view that much of the NO x emitted by jet flames is formed in large-scale eddies at the flame tip. NO x production rates higher than expected from the thermal mechanism alone are observed for the hydrocarbon fuels at lower flame temperatures ( < 2050 K), with the NO x production rates ranking in the same order as sooting tendencies. This suggests that gas-molecular radiation is more relevant than broadband radiation from soot for determining temperatures in NO formation zones. Prompt NO and/or other soot-NO interactions may also be important for the hydrocarbon fuels in this temperature regime. Previously reported Reynolds and Froude number dependencies for NO x production rates are examined and found to be consistent with flame heat loss characteristics.

INTRODUCTION Oxides of nitrogen (NOx) emissions from various combustion processes are important contributors to photochemical smog and acid rain [1, 2]. Also, aircraft emissions of NO x contribute to ozone depletion in the stratosphere [3]. Control of these emissions is obviously of great practical interest and is receiving renewed attention as legislated standards are tightened and more sources are subjected to control. Although NOx has been a subject of study for several decades, much remains to be understood about its formation and destruction in the complex environment found in turbulent hydrocarbon diffusion flames. Recent studies in our laboratories [4], and by others [5], have begun to build an experimental data base that is being utilized to guide modeling efforts [6, 7]. In that regard, an underlying objective of the present study is to greatly expand the data base available to evaluate jet flame NO x models. Copyright © 1991 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc.

The scaling of NO x emissions from jet diffusion flames with geometrical and fluid mechanical parameters has been a long-standing issue [4, 5, 8-14] and has yet to be satisfactorily resolved. In some of the earliest measurements of NO x from jet flames [9], characteristic NO x production rates were found to decrease with Reynolds number, rather than remaining constant, when the jet Froude number was fixed to maintain flame structure similarity. Explaining this seemingly anomalous result has been a theme in many subsequent experimental and analytical studies. For example, the closed-form model of Peters and Donnerhack [13] failed to produce the observed experimental Re and Fr scaling. This failure was attributed to improper turbulence modeling [13]. Bilger [10] and Kent and Bilger [11] developed the idea that superequilibrium O-atom concentrations broadened reaction zones for NOx formation and were responsible for the Re-t/2 scaling. Pursuing this idea further, Drake et al. [14] developed a jet 0010-2180/91153.50

320 flame model that included superequilibrium O atoms and the nonequilibrium effect of NO x being produced in a much wider reaction zone than is associated with an equilibrium distribution of O atoms; however, exercising this model also failed to recover the experimental Re scaling. In an experimental and analytical study of propane jet flames, Buriko and Kuznetsov [12] explained the Re scaling as a consequence of flame radiation. In our initial studies of velocity scaling [4], the results suggested that flame radiation plays an important role in NO x scaling, consistent with the ideas advanced by Buriko and Kuznetsov [12]. That flame radiation can have an important effect on NO x emissions has long been recognized [12, 15]; however, no studies have been conducted for jet flames where both NO x emissions and flame radiation were measured simultaneously. The present study provides measurements of NO x emission indices and radiant fractions for jet diffusion flames using a variety of fuels having differing radiation characteristics. These measurements are used to elucidate the physical processes that cause NO x emissions to scale with flow variables in the manner observed. Understanding NO): production in jet flames requires an understanding of jet flame structure. Figure 1 shows two limiting cases of flame structure thought to exist in jet flames--strained laminar flamelets [16] and homogeneous eddies [17]. Within the framework of the flamelet model, NO is formed in the relatively narrow high-temperature region where the mixture approaches stoichiometric. This picture is complicated by the existence of superequilibrium radical concentrations, in particular OH and O, which can broaden the width of the NO formation zone [10, 14], as discussed above. Drake and Blint [18] have modeled NO production in strained laminar flames and have found that three formation pathways are important: thermal (Zeldovich), prompt, and the N20 intermediate mechanism. They also found that flamefront NO decreased with increasing strain rates because of decreased residence times [18, 19]. Although Drake and Blint [18, 19] did not consider flame radiation in their analyses, Miiller et al. [20], modeling the same flame as Drake and Blint, show that for velocity gradients less than about 50 s-1 gas-molecular radiation can greatly affect NO production in flamelets by

S.R. TURNS AND F. H. MYHR

Xf•/•

~XNOx ,

~s

~I

' COORDINATE

"{'CHAR ~, (STRAIN RATE, Cl )-t (a)

\

AfR

TCHAR "~ ~h/U (b) Fig. I. Conceptual framework for NO x formation. (a) Flame sheets and/or broadened reaction zones. Profiles adapted from Ref. 19. (b) Homogeneous eddies.

lowering temperatures. These strain rates, however, are much smaller than typically found in high-velocity jet flames. Thus radiation may not be important for NO formation in highly strained flamelets. A second model we wish to consider is the homogeneous eddy. In this construct, strained flamelets roll up as large-scale motions mix layers of air and fuel [17], forming an eddy that has dimensions significantly greater than the flame zone thickness. After a turbulent cascade, the fluid in the eddy molecularly mixes. The identity of the eddy persists until such time as it merges with other eddies to form even larger scale structures. In this model, NO is formed in both flamelets and in the core of the eddies. The time available for NO formation in the homogeneous eddies increases significantly as the size of the eddies increases and can greatly exceed the residence times associated with flamelets. Moreover, with long lifetimes, nonequilibrium radical con-

NO x EMISSIONS FROM TURBULENT JET FLAMES centrations are less likely to be important than in flamelets, and more time is available for radiation to cool the eddy. SCALING C O N S I D E R A T I O N S Before proceeding, let us briefly examine the scaling of radiation losses with velocity and length scales in jet flames. We consider the entire flame to be both a uniform source of heat release and radiation. The rate at which energy is lost by radiation can be approximated as

q,(XapVfTf ,

(1)

where ap is the Planck-mean absorption coefficient for an optically thin flame, and Vy and T/ are the flame volume and temperature, respectively. The total rate of heat release by combustion is

qc = thoA H~,

(2)

where #t o and A H c are the fuel mass flowrate and heat of combustion, respectively. Defining the radiant fraction, XR, to be the ratio of qr to qc, we obtain for momentum-dominated flames the scaling

xR- a,r/d/u,

(3)

where the flame volume has been assumed to be proportional to d 3, and the fuel fiowrate to d 2 u. From this crude analysis, we anticipate XR to decrease, and hence, flame temperatures to increase as the convective timescale, d / u , gets smaller. Relating the convective time scale to the jet Reynolds number, ud/~o, and jet exit Froude number, u2/gd, the scaling of radiant fraction with these parameters becomes

XR -

apT~O'oRe)'/3( gFre) -2/3

(4)

Consistent with the global treatment above, we can also define a global residence time associated with the flame,

rt~ -

pyW:L#s 3PodZU '

(5)

321

where p:, Wf, and L f are the flame density, width, and length, respectively, Po is the cold fuel density, and fs is the mass fraction of fuel, both main and pilot, in a stoichiometric mixture. This characteristic time is similar to that defined by Becker and Liang [21], but accounts for the actual characteristic flame density and models the flame volume as a cone. The conical flame volume is consistent with the assumption in Eq. 3 that the volume is proportional to d 3, since it was found experimentally that Wf ~. O. 17 L f for almost all conditions. The use of visual flame dimensions results in flame volumes and global residence times larger than, but proportional to, those that would result from temperature-based flame dimensions [22]. The global residence time is useful in that when used to scale emission indices, the experimental variation of flame length is taken into account, similar to the L3f/d2u normalization implied by Peters and Donnerhack [13] in their theoretical study and adopted in recent experimental studies [4, 5]. Physically, r c represents the time required for a stoichiometric mixture of hot products to pass through a volume equal to that occupied by the visible flame; it is expected to be proportional to the residence time in a final homogeneous eddy. For momentumdominated flames (defined below), it was found experimentally that r G is directly proportional to the convective timescale, d / u . For buoyancydominated flames, the two time scales are no longer directly proportional, with large changes in the d / u time scale producing relatively small changes in r c, as can be seen in Fig. 2. Another useful characteristic time is the time for an optically thin eddy of stoichiometric products to cool from its adiabatic value to the nonadiabatic equilibrium value as a result of radiation. This time is

"/'tad

--

p:G

1

12apo

Tf

T~a3 ,

(6)

where py and Cp are the gas density and specific heat, respectively, o is the Stefan-Boltzmann constant, and Tad is the adiabatic flame temperature. A flame Froude number was used to define momentum-dominated and buoyancy-dominated

322

S.R. TURNS AND F. H. MYHR

regimes, as shown in Fig. 3. Here the flame Froude number, Fry, defined in a manner similar to that of Becker and Liang [23], is the ratio of jet exit momentum flux to the buoyant force experienced by the conical flame volume of combustion products: 12rnou

IrgW/Lf(poo - Of)'

Fr/=

(7)

where g is the gravitational acceleration and p~ is the ambient air density. Measured flame lengths reached their momentum-controlled asymptotic limits for Fr e >_ 2 × l0 s, consistent with Fry > 1.

EXPERIMENTAL METHODS

The overall experimental setup is illustrated in Fig. 4. The apparatus is similar to that used previously [4], but with a few important differ10

i

,

i

,

iiii

A

corn2

0

c~

<>

C2H4

I

i

,

i

i

11,,

I

1

LU /k Z~

A

Z

~

10-1

o0 uJ tr

0 UA 7

0 °102

ences and additions. The screened enclosure (3.5 × 1.2 × 1.2 m) is larger than previously used so that much larger flames ( L f > 2 m) can be studied, and a water-cooled sampling probe has been added to sample the diluted combustion products in the exhaust duct downstream of the collection hood. The use of this sampling location instead of the overfire probe location has several benefits. First, the combustion products were found to be well-mixed with the dilution air. This provides more precise determination of NO x and CO 2 concentrations because the large turbulent concentration fluctuations typically found in the overfire region were essentially eliminated by mixing in the duct. Within the experimental uncertainty, emission indices computed from overfire and duct measurements were identical. Another benefit of the duct location is that the sample is sufficiently diluted to prevent condensation of water vapor in the sample train. This eliminates the need to remove the moisture from the sample, and hence eliminates any measurement uncertainties associated with NO 2 adsorption in the liquid water in an ice bath. Some adsorption-desorption effects were still observed; however, care was taken to allow sufficient time at a given operating condition to achieve a steady state, as evidenced by a constant NO x reading on a strip chart. Other improvements in the experimental system include the use of a cooled photomultiplier tube in the chemiluminescent NO x analyzer and the addition of a high-sensitivity NDIR CO 2 analyzer. The improved NO~ analyzer allows precise measurements of NO x at concentrations below 5 ppm with discrimination to about 0.1 ppm, while the long-path NDIR CO 2 analyzer permits accurate CO 2 measurements in the exhaust duct at the parts-per-million level. Emission indices were calculated solely from measurements of the NO x and CO 2 concentrations in the duct together with a measurement of the ambient CO 2 concentration, i.e.,

EINOx ( g / kg ) MOMENTUM

~

[

~

BUOYANCY

46.0XNo x (12.01 + 1 . 0 0 8 y ) ( X c o 2 - X c o 2 ~ ) 1

10

102

103

GLOBAL RESIDENCE TIME, "[G [ms]

Fig. 2. Convective time scale, flame global residence time.

d/u,

• 1000,

(8)

as a function of the

where X i is the mole fraction of species i and y

NO x EMISSIONS FROM TURBULENT JET FLAMES

323

102

rr I,.IJ co

10 Oo MOMENTUM DOMINATED

z uJ c"l

©

rr IJ._ UJ

BUOYANCY DOMINATED


j

o

O

[]

I I

ix C O / H 2

10-1

0

CH 4

[] C3H 8 0

C2H 4

lo -2 L.. 10 2

10 3

10 4

10 5

10 6

10 7

JET EXIT FROUDE NUMBER Fig. 3. Relationship between jet exit Froude number Fr e and flame Froude number Frf.

is the molar hydrogen-to-carbon ratio (H/C) of the fuel. Note that the molecular weight of NO 2 is used to define the NO x emission index, eliminating any ambiguity associated with NO-to-NO~ conversion in the sampling system. Equation 8 also assumes complete conversion of the fuel to CO s and that the combustion products are significantly diluted with ambient air. For flames in which hydrogen pilot flames were employed, the emission indices were calculated by treating the hydrogen as part of the fuel supplied. Radiant fractions were calculated from radiant heat flux measurements using a Medtherm model 64P-05-24 heat flux transducer having an Irtran II window. The transducer has a 150" field of view and measures the total radiant flux in the wavelength range of 0.35-12 /~m. The transducer was located inside the screened enclosure at a radial position, R (typically 667 mm); the axial position was adjusted to be at approximately half the visible flame height, x = L r~2. This location corresponds closely with the maximum radiant flux and has been found to be the appropriate location to estimate radiant fractions from a single near-field heat-flux measurement [24]. Radi-

ant fractions, XR, were estimated using

XR =

qr"4 7rR 2 rnoA/_/ ,

(9)

where q," is the measured radiant heat flux

fW/m2). Four different sized burners were employed: the 3.86-mm-diameter burner used in previous studies [4], and a family of burners patterned after the design of Sterner and Bilger [25]. Both types of burners provide for pilot flames to keep the jet flames attached to the central tube. In the former [4], hydrogen flows through a narrow annulus surrounding the central tube to produce an H2-air diffusion pilot flame; in the latter, a flame holder for a premixed pilot is contained in a relatively large annulus surrounding the central tube. In the present experiments, stabilization was accomplished using hydrogen diffusion flames as pilots in both burner configurations. We found that the burner design had no effect on NO x emissions. The pilot flames per se did influence the results and are discussed below. Burner parameters are shown in Table 1.

324

S.R. TURNS AND F. H. MYHR

L..\ DUCT FILTER SAHPLE PROBE

SCREENED ENCLOSUR

JET I

FLA~E

WATER TRAP (OPTIONAL)

I L

FILTER

]

VALVE

~>

FLOWMETER SAMPLE PUMP (

.--I FLOWMETER

FLOWMETER ZERO GAS

ZERO GAS

SPAN GAS

NO / NOx ANALYZER

CO2 (?'*) ANALYZER CO 2 (ppm) ANALYZER

I

SPAN GAS

~_IATI T I --1~2

b.

A/Of

6300

PUMP

co (~) ANALYZER

I

Fig. 4. Experimental setup.

Four technical grade fuels were employed providing a range of flame luminosities as a consequence of their differing propensities to form soot. Table 2 ranks the fuels from most to least luminous (sooting) and shows adiabatic flame temperatures for stoichiometric combustion. Fuels were metered using rotameters calibrated for each fuel type. Flame length measurements were obtained from 8-s time exposure photographs of the flames against a black fabric background. A wide range of flow conditions were utilized, with the lower limit set by the

criterion of requiring turbulent flow in the main burner tube, and the upper limit determined by flow metering and/or exhaust handling capabilities (cf. Table 2). Heat release rates for the flames ranged from 5.5 to 170 kW. RESULTS A N D DISCUSSION Pilot Flame Effects Figure 5 shows the amount of H 2 required to stabilize the hydrocarbon flames, expressed as the

NO, EMISSIONS FROM TURBULENT JET FLAMES TABLE

325

1

Burner Specifications Main Jet Central Tube i.d. (mm) o.d. (mm)

Configuration Narrow

annulus [4]

Wide aunulus [25] Wide annulus [2.5] Wide annulus [25]

Central Tube L/d”

Pilot Flame Outer Tube i.d. (mm)

Flameholder Locationb (mm)

3.86

5.59

19.7

7.75

-

2.18 4.12 6.17

3.96 5.54 7.92

99.7 100.0 100.2

15.75 14.83 14.22

5 4 4

’ Based on inside diameter. b Upstream distance from burner exit plane.

fraction of the combined main jet and pilot mass flowrates. The CO/H, flames were not piloted. The general trend of the data shows that more stabilizing gas is required as the jet velocity increases and as the jet diameter decreases. Furthermore, for a fixed burner size, the amount of H, required is inversely related to the laminar flame speed of the fuel (cf. Table 2). All of these trends are consistent with known flame stability trends. Since the amount of hydrogen used could be substantial, the influence of the pilot flame on the various quantities measured was investigated. Tests were performed with CH, and C, H, as fuels, as the liftoff stability of these two fuels bracket that of C,H,. The main jet fuel flow rate was held constant as the hydrogen pilot mass fraction was increased from the minimum necessary to obtain a stable flame to the maximum used in all experiments with that fuel type; all changes

subsequently noted will refer to increases in hydrogen pilot mass fractions from 0.025 to 0.10 for CH,, and from zero to 0.075 for C,H,. Results of these experiments are shown in Fig. 6. Flame lengths increased for both fuels as pilot flow was increased (7% for CH,, 17% for C, H4), probably as a result of the decreased turbulent mixing at the flame base in the presence of the laminar nonpremixed pilot flame. The radiant fractions of the C,H, flames decreased 16% with increasing H, pilot flow, while those of the CH, flames increased by 6%. The decreasing radiant fractions of the C,H, flames result from a decrease in in-flame soot, while the slightly increasing trend of radiant fraction with pilot flow for the CH, flames probably results from the increased flame size. The calculated characteristic flame temperature, defined in a later section, increased by about 70 K for the C,H, flames with the maximum pilot flow, while a much

TABLE 2 Fuel and Flame Properties

Fuel

C,H, C3J-b

CH, 57% cod

Adiabatic Flame Temperature”

Laminar Flame Speed b

Luminosity

(R)

(m/s)

(%)

Reynolds Number Rangee

High Moderate

2370 2267 2226 2371

0.68 0.39 0.34 -

99.5 99.0 98.0 99.0

4350-51,700 4860-88,500 3130-35,400 4864-18,861

LOW

Nil

43% H, d a Calculated from CEC86 [30] for a stoichiometric mixture. b Maximum value using tube method tabulated in Ref. [39]. ’ Manufacturer’s stated minimum purity. d Percent by volume in mixture. e Re = ud/v,, Fr, = u*/gd based on cold fuel properties.

Fuel Purity ’

Froude Number Rangee 2720-2.8 218-7.2. 1930- 1.6 5304-5.6.

. lo6 lo5

. lo6 lo5

326

S.R. TURNS AND F. H. MYHR

0.12 CH4

.~,Jo ,0

0

008

0.04

J /

0 O

° ' ~ ~ o

Z

o 2.18mm ~ 3.86mm

O

L~ < oru_ 03 03

°

o 4 . 1 2 mm

6.17mm 0.00 0.08 C-3H8

ta z LU

8

0.04

~

,

nC3 >0.00

--"

. . . .

,

t

C--~H4 0.04

0.00

: :' 0

'

.

50

.

100

150

200

.

.

.

250

J E T EXIT V E L O C I T Y [m/s]

Fig. 5. Hydrogen pilot flame requirements.

smaller increase of 15 K was observed for the CH 4 flames. For the C2H 4 flames, this temperature increase is the direct result of the reduced radiant fraction; the small temperature increase of the CH 4 flames, however, results from the higher adiabatic flame temperature of hydrogen compared with methane, since the characteristic temperature is based on the total fuel supplied. The NO x emission indices for both fuels increased (20% for CH4, 36% for C2H4) with increasing pilot flow, consistent with increased NOx production rates associated with the higher temperatures and the slightly increased global residence times.

Although the hydrogen pilot flames clearly influence flame lengths, radiant fractions, characteristic flame temperatures, and emission indices, the effects are generally not large enough to affect the conclusions drawn from the data presented below. Emission

Indices

Energy-based emission indices expressed as the mass of NO x produced per unit of fuel energy released are shown as functions of the flame heat release rate for the propane and ethylene flames

NO x EMISSIONS FROM TURBULENT JET FLAMES

327

200 ~oo o 03 L

CE

0

~

0

"

"

+

'

"

o "

"

'

"

o "

"

'

o "

"

•

2200 1.... ~-, 2100

'

+

'

'

o

I.-~2000 1900 1000

10 ~ 03

. . . . .

:

. . . . . . . +. . .

°

4 2

c?

._z

LLI

0 00(

002

0.04

006

010

008

H Y D R O G E N MASS FRACTION IN FUEL

in Fig. 7, which provides an overview of this data set uncomplicated by any scaling relationships. Here we see that the emission indices are relatively insensitive to the test conditions with all variations limited to less than an order of magnirude. Considering the wide variation in time scales and flame temperatures, this result is surely fortuitous and probably obtains by countervailing effects, as will be discussed subsequently. Note that the heat release rates for the largest flames (ca. 170 kW) are in the range associated with commercial space heating devices [1]. Figure 8 shows mass-based emission indices

Fig. 6. Effects of hydrogen pilot flame on jet flame properties for 4.12-mm-diameter jet. Ethylene flowrate 0.314 g/s; methane flowrate 0.783 g/s.

divided by the convective time scale, d / u , which forms an effective rate of NO x production, as functions of the jet exit Froude number. The purpose of Fig. 8 is not to propose that this particular convective time scale and the exit Froude number are necessarily the appropriate parameters to scale NO x emissions, but rather, to allow a convenient and unambiguous presentation of all of our data together with the results of other researchers. The vertical line on the graph shows the exit Froude number corresponding to the region where flame Froude numbers for our hydrocarbon-fueled flames are of order unity, i.e.,

S. R. TURNS AND F. H. MYHR

328

FLAME 01

0 20

,

‘\

HEAT RELEASE

02 ”

”

RATE

03

I

”

[MMBtuh]

04

x I

8

”

,

05 ”

06

“I

I,

”

18

C2H4 218mm

0.00

nn “”

0

50

100

150

200

FLAME HEAT RELEASE RATE [kW]

Fig. ‘7. NO, emission indices (mass of NO, per unit energy released) as functions of flame heat release rate.

108

107 q

m

‘iij’ g

106

9 2 s ‘,

COIH2) CH4

105

PRESENT STUDY

C3H8

P lz

%J+4

/ CH4 ref. El C3H8 ref. PI G3tis ref. [I 21 H2ref. PI

lo4

HZref.ISI 4

lo3 18

103

104

lo5

lo6

lo7

lo*

log

JET EXIT FROUDE NUMBER

Fig. 8. NO, emission indices normalized by the convective time scale, d/u, versus jet exit Froude number.

NO x EMISSIONS FROM TURBULENT JET FLAMES when the initial momentum of the jet and the total flame buoyancy are approximately equal (cf. Fig. 3). The measurements of Chen and Driscoll [5] and Buriko and Kuznetsov [12] for methane and/or propane flames are consistent with our measurements for hydrocarbon flames. The hydrogen-fueled flames of Bilger and Beck [9] and Lavoie and Schlader [8] coincidentally follow a general extrapolation of the data band from the hydrocarbon flames. The CO/H 2 data follow a trend distinct from that of the three hydrocarbon fuels.

329

of each fuel; among the hydrocarbon fuels, the largest XRS are associated with the ethylene flames, and the smallest XRs occur for methane. The radiant fractioas for all fuels essentially collapse to a single trend at the smallest values, of r e, suggesting that nearly all of the radiation is gas molecular for these flames. Both the magnitudes and the trends of the radiant fraction data shown in Fig. 9 agree reasonably well with the measurements of re.hers [26-29]. The radiant fraction data were used with the NASA chemical equilibrium code [30] to calculate the nonadiabatic temperature for a stoichiometric mixture; these global characteristic temperatures are shown in Fig. 10. The lines marked rrad in Fig. 10 will be discussed later. Figure 10 shows that of the hydrocarbon fuels, the ethylene flames exhibited both the highest (2288 K) and lowest (1851 K) characteristic flame temperatures. This is a consequence of this fuel having both the highest adiabatic flame temperature (Tad = 2370 K versus 2267 and 2226 K for propane and methane, respectively) and the largest radiant fraction (XR = 0.332 versus 0.233 and 0.184). For the hydrocarbon fuels in the buoyant regime (large re), the relative temperature levels increased as the propensity to form in-flame soot decreased. Conversely, when the flames become

Radiant Fractions and Temperatures

Interpreting the results shown in Figs. 7 and 8 is the essence of the scaling problem discussed earlier. Adopting the working hypothesis that flame radiation is a major factor in determining NO x emissions from jet flames [4, 12], Fig. 9 shows flame radiant fractions for the four fuels studied as functions of the global residence time, z6. As discussed previously, for optically thin, nonluminous, momentum-dominated flames, radiant fractions should increase with d / u , and therefore ~'c, for a given fuel. In addition to this trend, we see that at the larger residence times the radiant fractions relate strongly to the sooting tendency

0.4 • CO/H 2 O CH 4

0.3

•

/

C3H8 C2H4

0

/

Z

o_ < rr ii

0.2

/

a..a/•

o

-

Z

< < rr 0.1

0.0

I

I

I

I

I

I

I

I I

lO

I

I

I

i

I

I

i

I I

I

I

lOO

G L O B A L F L A M E R E S I D E N C E TIME, x G [ms]

Fig. 9. Flameradiant fractionsas functions of global residencetimes, z o.

i

i i h1

1ooo

330

S.R. TURNS AND F. H. MYHR

2400

E

Tad, (~O/H2 . . . . . . . Tad, G2H4

r

. . . . . . . . .

i

. . . . . . .

2300

2200

<_.,

2100

LL

O

2000

• oo,.2

\

o CH, ~

•

1900

\ \

\ .~

_

\

o C2H4 ,

1800 1

,

,

,

i

,

,I

,

t

,

i

10

,

,

,1

,

,

100

,

,

,

,

1000

T I M E [ms]

Fig. 10. Nonadiabatic characteristic flame temperatures as functions of global residence times, rG, and comparison of characteristic radiation times with global flame residence times for C2H 4 and CH 4 flames.

nonluminous, the rankings of the characteristic temperatures for the various fuels fall in line with their adiabatic flame temperatures. Note that for all of the fuels studied, including the nonluminous C O / H 2 flames, characteristic temperatures fall with increasing r e. N O x Scaling

The preceding results clearly show that flame radiation plays an important role in determining the temperature fields within jet diffusion flames. In addition to lowering temperatures well below their adiabatic values, radiation introduces a somewhat complicated temperature scaling that depends both on flow variables ( d and u) and the composition of the flame gas. Furthermore, the composition of the flame gases, primarily the amount of soot present, also depends on flow variables. Since thermal (Zeldovich) NO x is strongly temperature dependent, this suggests that the dependence of NO x on fluid mechanical variables is closely related to how flow variables affect flame temperatures.

T i m e - Temperature Scaling. Experimental characteristic NO x production rates were calcu-

lated by dividing an average in-flame NO x concentration by the global residence time, r G. The in-flame NO x concentrations (gmol/m 3) were calculated using the measured duct emission indices and assuming flame conditions of a stoichiometric mixture at the characteristic flame temperature and ambient pressure, i.e., [NOx] =

fsPf EINOx/MNo2.

(10)

The in-flame NO x concentrations defined in this manner are global measures of the mean NO;, concentration in a stoichiometric mixture of reaction products, consistent with our global treatment of other flame parameters such as radiant fractions, residence times and flame temperatures. Figure 11 shows an Arrhenius-type plot of these NO x production rates versus the reciprocal of the flame temperature. The inset is a similar plot using the convective time ( d / u ) instead of the global residence time to form a characteristic NO x production rate. For the momentumdominated flames (high temperature), the convective time is directly proportional to the global time, and its use eliminates the scatter arising from flame width measurements. This representa-

NO x EMISSIONS FROM TURBULENT JET FLAMES

1

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0.46 0.48 0.50 0.52 1000 / FLAME TEMPERATURE [K]

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Fig. 11. Emission indices normalized by the global flame residence time, ro, as functions of the reciprocal of the non-adiabetic characteristic flame temperature.

tion (inset) also shows the influence of stoichiometry. Since the stoichiometric fuel-air ratio for a C O / H 2/air mixture is much greater than for the hydrocarbon fuel-air mixtures, the C O / H e flames are much shorter; thus, for the C O / H 2 flames, the NO x emissions scaled by d/u are much lower than for the hydrocarbon flames. In the main plot, the line through the data represents the theoretical NO formation rate via the two primary Zeldovich reactions, O + N 2 --+ NO + N and N + 02 --+ NO + O. The rate of NO formation was calculated assuming equilibrium O atoms, N atoms in steady state, and a value for k 1 of 1.8 • 108 e x p ( - 3 8 , 3 7 0 / T ) [31], where k 1 is the rate constant for the O + N 2 reaction in units of m
able for NO formation reactions. Neglecting turbulent fluctuations in temperature and species concentrations is expected to cause underprediction of NO production rates, although scaling the experimental results with r c, which is derived from visible flame dimensions and is therefore too large, tends to minimize differences between measurements and predictions. The fact that the NO production rate can be scaled with global temperatures and residence times is consistent with the idea that much of the NO is formed in large, homogeneous eddies at the flame tip, although it is certainly not proof. This point is discussed more fully later. Several complications to a universal temperature-time scaling are also evident from Fig. 11. First, there appears to be some effect of fuel type. For example, the C O / H 2 and C 2 H 4 data lie on top of each other, somewhat below the theoretical line, while the C H 4 data are slightly above the calculated rate, with the C3I-I 8 data essentially falling on the theoretical line. Second, we see that at the lower temperatures, the NO production rates of the three hydrocarbon fuels do not drop

332 off as quickly as would be expected for the thermal mechanism alone. This low-temperature deviation from the thermal NO x line seems connected to the sooting propensity of the fuel; the ethylene NO x production rates drop the most slowly, followed by propane and then methane. If one accepts for the time being that simple characteristic times and temperatures suffice to scale NO x emissions at high temperatures, then at least three factors may account for the divergent behavior of the luminous flames at lower temperatures. First, prompt NO and possible soot-NO [32] chemical interactions may cause the fuels to distinguish themselves. Since the prompt mechanism is much less sensitive to temperature [33], this seems a plausible explanation. A second factor may be the relative differences among the radiation time scales for the various flames. To test this hypothesis, radiation time scales (Eq. 6) for methane (without soot) and ethylene (with soot) flames were estimated using absorption coefficients of 0.3 and 1.0 m-~, respectively [34, 35], and are shown in Fig. 10. Here we see that in the low-temperature range the time it takes nonsooting methane products to reach their characteristic nonadiabatic temperature is greater than for ethylene. Thus, actual methane flame temperatures should be somewhat higher than for ethylene during the transient radiant cooling of the gases, and the NO x production rate is expected to be higher for the C H 4 than the C 2 H 4 flames. Since this is the opposite of what is observed in Fig. 11, it appears that the transient nature of the radiant cooling is not a factor in determining NO from these flames. A third possibility is that gas-molecular radiation is more effective at reducing temperatures in NO formation regions than is soot radiation. This seems quite plausible, since soot formation is expected primarily in fuel-rich regions, whereas NO is most likely to be formed in near-stoichiometric or lean regions. Since our measured radiant fractions are in large part due to soot radiation in low-temperature C 3H 8 and C 2H 4 flames, the calculated nonadiabatic flame temperatures would be too low for these flames, if only gas-molecular radiation were important for determining temperatures in NO formation zones. The fact that the deviation from the thermal NO~ line at low temperatures is in proportion to the luminosity of the flames lends credence to this hypothesis.

S.R. TURNS AND F. H. MYHR Considering the potential complexity of the problem, one may wonder whether the relatively straightforward scaling of NO x, time, and temperature demonstrated in Fig. 11 is physically meaningful or simply fortuitous. To address this question, we return to the conceptual models of NOx formation illustrated in Fig. 1. If the dominant route for NO x formation is through strained laminar flamelets, a rather complex situation exists. In flamelets, superequilibrium radical concentrations play an important role, affecting both the temperature profile through the flame and extending the width of NO-formation zone [10, 14]. The situation becomes even more complex when trying to predict the total NO x emission rate from a turbulent flame because strain rates vary throughout the flame and the net NO x production can only be obtained by integrating over all of the flamelets. On the other hand, if the majority of the NO x is formed in large, nearly homogeneous eddies, one can envision a plausible scenario that leads to a simple scaling for total NO x emissions from jet flames. Although the existence of large homogeneous eddies is postulated a priori, the model of Lutz et al. [6] predicts that most of the NOx emitted from a jet flame is produced in the last large eddy associated with the flame. In the framework of their model [6], NO~ formed early in the flame, either in flame sheets or homogeneous zones, mixes with fuel, air and products. Early in the flame, the overall stoichiometry in an eddy is likely to be rich. Hence, little additional NO is formed in the eddy, and, in fact, NO x transported and mixed into the eddy may even be reduced by reactions with hydrocarbons [33]. The merging and growth of eddies continues until the overall mixture strength becomes stoichiometric, which by definition is the end of the flame. This final eddy has all of the characteristics necessary to form large quantities of NOx: near stoichiometric overall mixture ratio, high temperature, and a relatively long lifetime. If indeed this model captures enough of reality, a single characteristic temperature and a single characteristic time may be sufficient to scale NO~ emissions. Moreover, the NO~ chemistry is likely to be quite simple because the large time scales allow the relaxation of temperatures and radical concentrations from nonequilibrium to equilibrium levels, unlike within a flamelet. The measurements of both

NO x EMISSIONS FROM TURBULENT JET FLAMES Drake et al. [36] and Barlow et al. [37] show that OH radicals approach equilibrium in the far downstream region of turbulent jet flames, which suggests that O atoms may behave similarly. Full-kinetics calculations modeling the jet flame as an expanding, well-stirred, adiabatic reactor [38] did not produce the experimental NO x production rate trend shown in Fig. 11. We would like to emphasize that inferring detailed flame structure from global NO x measurements is clearly impossible. It is likely that NO x is formed in several different regions, or regimes, and that the relative contribution of each region to the overall NO~ yield depends on the distribution of local Damk/Shler numbers for both combustion reactions and NO x production. For example, laminar flamelets, partially premixed regions resulting from extinction and subsequent reignition, and large-scale homogeneous eddies are all likely to contribute some fraction of the total NOx yield. The present experiments, however, are instructive in that they show that regardless of the detailed flame structure, radiation must be taken into account. As a consequence of this, the radiation time scales in the regions of NO x formation are comparable to, or smaller than, the chemical time scales associated with the important NO~ formation processes.

Reynolds and Froude Number Scaling. The final issue we would like to address is the scaling of NO~ with Reynolds number at a fixed Froude number. This scaling is illustrated by Fig. 12 and by the series of data points from Bilger and Beck [9] designated with specific values of d~ u on Fig. 8. In Fig. 12, we see that increasing Re at fixed Fr results in a slightly larger measured radiant fraction, XR; hence, characteristic flame temperatures drop slightly causing the normalized NO x to decrease somewhat. For the Bilger and Beck [9] data, increasing Re results in larger values of d / u , which, in turn results in larger XRS and lower temperatures (cf. Figs. 9 and 10). The Re scaling obtained in the present study for CH 4 is essentially the same as that obtained by Bilger and Beck [9], with the normalized NO x proportional to Re raised to the - 0 . 4 5 power for our study and - 0 . 5 for theirs. The scaling obtained for the other hydrocarbon fuels is similar, with somewhat greater dependencies for the more sooting fuels, as shown in Table 3. The CO/H 2

333

flames, however, show no significant Reynolds number dependence. The normalized radiant fraction data were regressed with Re and Fr, and the results are shown in Table 3. Here we see that Reynolds number exponents range from 0.1 to 0.6, bracketing the value of 0.33 in Eq. 4, while the Froude number exponents are generally smaller than the estimated dependence, i.e., -0.21 to -0.61 versus -0.67. Considering the simplistic nature of the analysis in arriving at Eq. 4, the degree of agreement shown between experiment and scaling arguments is quite good. Any better agreement would surely be fortuitous, since many of our flames are buoyancy-dominated, and hence Ly d 3, and the degree of sooting is far from constant, hence at, ~: constant. However, the signs on the exponents are as predicted for all four fuels, i.e., normalized XR increases with Re and decreases with Fr. This dependence of radiant fraction on the Reynolds and Froude numbers may be the explanation of the experimentally observed [4, 9] dependence of EINO, on Re, through the relationship between XR and the flame temperature. Accordingly, the failure of previous analytical models [13, 14] to produce the experimental dependence of EINO x on Re and Fr is not necessarily attributable to inadequate turbulence modeling, but may also relate to neglecting radiative heat loss and its dependence on these flow variables. SUMMARY AND CONCLUSIONS Experimental measurements of postflame NO x emission indices and flame radiant fractions from jet flames were obtained for a wide range of flow conditions and for four fuels having a variety of sooting tendencies. From an analysis of these results, we conclude the following: 1. NO x emission indices normalized by the characteristic time scales, r G or d / u , generally increase with either jet exit or flame Froude numbers for all fuels evaluated. This effect is attributed not to buoyancy per se, but rather to decreasing radiant fractions with decreasing residence times, which results in higher flame temperatures. 2. For characteristic temperatures above approximately 2050 K, a simple NOx-time-tempera-

334

S.R. TURNS AND F. H. MYHR 106 d~mm~ O. 218 ZX3.86 0 412 [3617

CH4

XR =0106 O "&'

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10 s

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JET EXIT R E Y N O L D S NUMBER

Fig. 12. EINO x / ( d / u ) versus jet exit Reynolds number showing lines of constant Froude number for CH 4.

TABLE 3 Power Law Regressions of NO x Production Rates and Radiant Fractions with Reynolds and Froude Numbers Fuel

a*

CH 4 C3H s C2H 4 CO/H 2

-0.45 -0.57 -0.6 0.08

+ + ± ±

b* 0.3* .03 .1 .05

0.740 0.82 0.95 0.41

c*

+ .009 ± .01 +.. .06 _+ .02

0.10 0.23 0.6 0.07

± ± ± ±

d* .02 .07 .2 .08

-0.207 -.26 -0.61 -0.34

± ± ± +

.007 .03 .07 .03

EINO x

* Exponents for fits to ~

oc ReaFreb; ×R oc ReCFred

r;

t Values shown represent least-squares best-fit exponents and their standard deviations.

ture relationship following Zeldovich kinetics with equilibrium oxygen atoms is obtained when the characteristic nonadiabatic flame temperatures and global flame residence times are used as scaling parameters. The present data are insufficient to determine if this is a fortuitous result, or a result of a relatively simple flame structure vis-a-vis NO production. However, the experimental NO x scaling is consistent with the view that much of the NO formed in jet flames has its origins in the last large reacting eddy, as postulated in the modeling results of Lutz et al. [6]. 3. Deviations from thermal NO x predictions for C3H 8 and C2H 4 flames at temperatures below approximately 2050 K suggest that gasmolecular radiation has a greater influence

than soot radiation in determining temperatures in NO formation zones. Prompt NO and/or chemical interactions between NO and soot may also contribute to the observed deviations for these luminous flames at low temperatures. 4. Estimates of the magnitudes and the time scales for radiative heat losses indicate that radiation can indeed play a significant role in NO x formation and should be an essential ingredient in models that seek to predict NO x emissions from jet flames.

This work was supported by the Gas Research Institute under Contract No. 5086-2601308 with T. R. Roose serving as technical

NO x EMISSIONS FROM TURBULENT JET FLAMES

monitor. The authors also would like to thank R. W. Dibble for sharing his kinetic calculations showing superequilibrium effects and acknowledge J. F. Driscoll for many thoughtprovoking discussions enjoyed during the course o f this investigation.

18. 19. 20. 21.

REFERENCES 1. 2. 3. 4. 5.

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335 tion, The Combustion Institute, Pittsburgh, 1984, pp. 303-310. Drake, M. C., and Blint, R. J., Combust. Flame 83:185-203 (1991). Drake, M. C., and Blint, R. J., Combust. Flame 76:151-167 (1989). Mailer, U. C., Man~, F., and Peters, N., personal communication, 1990. Becker, H. A., and Liang, D., Combust. Flame 44:305-318 (1982). Lovett, J. A., Ph.D. thesis, The Pennsylvania State University, 1989. Becker, H. A., and Liang, D., Combust. Flame 32:115-135 (1978). Hamins, A., and Gore, J. P., personal communication, 1991. St~'ner, S. H., and Bilger, R. W., Combust. Flame 61:29-38 (1985). Faeth, G. M., Gore, J. P., Cheuch, S. G., and Jeng, S. M., Annu. Rev. Numer. Fluid Mech. Heat Trans. 1986, pp. 1-38. Gore, J. P., Ph.D. thesis, The Pennsylvania State University, 1986. Delichatsios, M. A., Markstein, G. H., Orloff, L. and deRis, J., GR1 88/0100, 1988. Brzustowski, T. A., et al., ASME 75-HT-4, 1975. Gordon, S., and McBride, B. J., NASA Report SP-273, 1976. Hanson, R. K. and Salimian, S., Combustion Chemistry, Springer-Verlag, New York, 1984, pp. 361-421. Cheng, M. T., Kirsch, M. J., and Lester, T. W., Combust. Flame 77:213-217 (1989). Miller, J. A., and Bowman, C. T., Prog. Ener. Cornbust. Sci. 15(4):287-338 (1989). Gore, J. P., and Jang, J. H., Combust. Sci. Technol. (submitted). Grosshandler, W., and Thurlow, E. M., Third ASME/JSME Thermal Engineering Conference, Reno, NV, 1991. Drake, M. C., et al., Twentieth Symposium (InternationaO on Combustion, The Combustion Institute, Pittsburgh, 1984, pp. 1983-1990. Barlow, R. S., Dibble, R. W., Chen, J.-Y., and Lucht, R. P., Combust. Flame 82:235-251 (1989). Dibble, R. W., personal communication, 1991. Barnett, H. C., and Hibbard, R. R. (eds.), NACA 1300, 1959.

Received 31 May 1991; revised 3 September 1991