Oxides of nitrogen emissions from turbulent jet flames: Part II—Fuel dilution and partial premixing effects

COMBUSTION A N D FL A ME 93: 255-269 (1993)

255

Oxides of Nitrogen Emissions from Turbulent Jet Flames: Part II--Fuel Dilution and Partial Premixing Effects STEPHEN R. TURNS, FRANKLIN H. MYHR, RAMARAO V. BANDARU, and EHREN R. MAUND Propulsion Engineering Research Center, Department of Mechanical Engineering, The Pennsylvania State University, University Park, PA 16802 Measurements of NOx emission indices, flame radiant fractions, and visible flame length were made for turbulent, nonpremixed, jet flames for which various amounts of inert diluent or air were mixed with the fuel. The objective of the study was to explore further the role of flame radiation in NO x production in jet flames. Vertical free jet flames were stabilized on a 4.12 mm diameter straight-tube burner. Four fuels, CH 4, C2H4, C3Hs, and a 95% C O / 5 % H 2 mixture (by mass); three inert diluents, N 2, At, and CO2; and air premixing were employed in parametric tests. Complementary dilution experiments were run with laminar jet flames using the three hydrocarbon fuels and N 2. For the turbulent flames, the results showed that the effects of dilution and premixing were strongly dependent on fuel type. Flame temperatures and NO x emissions increased when the more sooting fuels (C3H 8 and C2H 4) were diluted or partially premixed, resulting in increased NO x emissions. The opposite trend was observed for the nonluminous C O / H 2 flames. Using the results reported here and from Part I [1] of this study, the effects of residence time, flame temperature, and departure from equilibrium on NO x emissions, regardless of what parameter affected the change, were well characterized by regressing characteristic NOx production rates as a function of nonadiabatic characteristic flame temperatures and global residence times. Separate regressions for the hydrocarbon and C O / H 2 flames showed a weaker dependence of NO x on temperature for the hydrocarbons, suggesting that the prompt NO mechanism is quite active in these flames. The laminar flame experiments demonstrated the importance of the relative locations of NOx-producing regions and soot-containing (strongly radiating) regions of the flame.

INTRODUCTION This paper is a companion to our previous work, Part I [1], that focused on understanding how fuel type and flame radiation affect the scaling of NO x emissions from turbulent jet flames. In that study [1], we found that by taking radiation losses into account, a relatively simple global time-temperature relationship served to explain the scaling of NO~ emissions with jet diameter and initial velocity, at least to a first-order approximation. In the present study, we explore this scaling further. Here we investigate how adding either inert diluents or air to the fuel stream affects flame radiation and NO x emissions. Both Parts I and II have as underlying objectives the generation of a comprehensive data base useful to guide and validate NO~ modeling efforts, e.g., Refs. [2-4]. The addition of an inert diluent or air to the fuel stream in a jet flame affects flame structure in several ways, all of which can Copyright © 1993 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc.

affect NOx production. Driscoll et al. [5] used inert and air dilution to study the effects of residence time and Damkohler number on NOx production in hydrogen, methane, and hydrogen-methane jet flames. In their work [5], conditions were deliberately chosen to minimize the role of radiation and prompt NO. Regardless of whether a flame is strongly radiating or not, fuel dilution, either with an inert or air, decreases flame length and moves the reaction zone into regions of higher shear stress [6]. This result is essentially the consequence of changing stoichiometry, there being less fuel to burn for the same initial jet conditions. Dilution thus decreases both global and local residence times, which, if sufficiently short, allows the flame chemistry to be far from equilibrium. It is well known that superequilibrium concentrations of O (and OH) radicals promote NO formation through the extended Zeldovich mechanism [4, 7]. In hydrocarbon flames, fuel dilution also inhibits soot formation. Stfirner et al. [6] report 0010-2180/93/$6.00

256 the disappearance of all yellow luminosity from N 2 or air-diluted turbulent methane flames, while fundamental studies of soot formation seek to understand how inert dilution diminishes soot production in laminar flames [8, 9]. In a comprehensive parametric study of turbulent M-diluted acetylene jet flames, Kent and Bastin [10] showed how dilution affects in-flame soot volume fractions and temperatures. Gollahalli [11] reports decreased radiant losses from inert-diluted propane flames. In the present investigation, we take advantage of this ability of dilution to diminish soot production and thereby alter the radiation characteristics of hydrocarbon flames, with the objective of furthering our understanding of NO x formation in such flames. In addition to our investigation of turbulent jet flames, we also report here results from a limited study of laminar diffusion flames that are chemically identical to the turbulent hydrocarbon flames. These data underscore the importance of flame structure in understanding how radiation relates to NO production.

S . R . TURNS ET AL.

/ / OVERHEAD~ ~

SAMPLE PROBE

SAMPLE PROBE

OPTIONAL

[NO],[NOx],[CO],[COd MEASUREMENT

IN-CAGE/

ISCREENED

OUCT/ \

ENCLOSURE I,CAGE,

HERMO OUPLE PROBE

I

= = -

JET

FLAME

I HEAT I FLUXPROBEI ~~=~ RADIANT

4x R2qr"

XR= rhoAHe

Fig. 1. Schematic of turbulent flame apparatus.

EXPERIMENTAL METHODS Turbulent Flames

An overview of the test set-up is shown in Fig. 1. Details of the apparatus have been described previously [1, 12], so only a brief description is presented here. A chemiluminescent analyzer with a stainless steel converter was used to measure NO and NOx, an NDIR analyzer was employed to measure CO 2 concentrations, and a gas-filter-correlation analyzer was used to measure CO. Emission indices were calculated solely from measurements of NO x and CO 2 concentrations in the duct above the flame and measurements of ambient CO 2 levels. The flame products were found to be well-mixed with the ambient air pulled into the exhaust duct. To increase the sensitivity of the concentration measurements for the smaller C O / H 2 flames, the in-cage duct shown in Fig. 1 was employed. All NO x emission indices are reported as equivalent NO 2, i.e., the molecular weight of NO 2 is used for NO x.

A 4.12-mm-i.d. burner, patterned after the design of Sterner and Bilger [13], was employed. A small concentric flow of hydrogen was used to stabilize the hydrocarbon flames. Pilot flames effects are discussed in Ref. 1. The same four fuels used previously [1], CH4, C3H8, C2H4, and a C O / H 2 mixture, were employed in the present work. These fuels produced flames having a wide range of luminosities. The ethylene flames were the most luminous (sooting), while the C O / H e mixture produced nonluminous blue flames. Radiant fractions were calculated from radiant heat flux measurements obtained at axial and radial locations equal to one half the visible flame height. A broadband (0.35-12 /zm), wide-angle (150 °) radiometer was used. Characteristic nonadiabatic flame temperatures were calculated from the measured radiant fractions, as previously [1]. Global residence times were calculated using visible flame dimensions, and other parameters, also as discussed previously [1]. T i m e - m e a n flame temperature profiles in selected flames were measured using fine-wire

JET FLAME NO x

257

Pt-Pt-10% Rh (type S) thermocouples. To obtain accurate radial positioning, the thermocouple probe was attached to a stepper motor positioner that allowed motion in one direction perpendicular to the flame axis. Traversing in the second orthogonal direction, also perpendicular to the flame axis, was controlled by a unislide to which the burner was attached. The axial position was controlled by raising or lowering the burner. The thermocouples were coated with m1203 to reduce catalytic effects and contamination by flame product gases. Measured temperatures were corrected for radiation losses. A complete description of the procedures used is presented in Ref. 14. Test conditions for the inert dilution sequence are shown in Table 1. The diluent was mixed with the fuel gas well upstream of the burner to assure a uniform composition. Nitrogen was used as the diluent for all fuels, while Ar and CO 2 also were used to dilute the C O / H 2 flames. Various diluent mass fractions were added up to the maximum indicated in Table 1, while maintaining the total mass flow rate constant. The C O / H 2 test series was designed to include the test conditions employed by Drake et al. [15]. Drake et al. [15] found anomalous results with Ar when comparing relative NO yields with predictions. The flame Froude numbers [1] for the CH 4 and the C O / H 2 flames show that these flames are momentum dominated (Frf 5 1), especially the C O / H 2 flames; while the C3H 8 and C2H 4 flames are influenced by both buoyancy and

initial jet momentum. Since the changes in jet momentum flux with dilution are typically less than 25%, the predominant effect of dilution is decreased buoyancy. Test conditions for the partial premixing experiments are shown in Table 2. Again, various quantities of air were added up to the maximum quantity indicated. Maximum air addition was determined by safety considerations. An air equivalence ratio, ~bj*, was defined as the quantity of air supplied divided by the stoichiometric quantity, that is, the inverse of the conventional fuel equivalence ratio, ~b. Numerical values of ~b~* lie between zero (pure fuel) and unity (stoichiometric mixture). Laminar Flames

The laminar flame burner is illustrated in Fig. 2. The bumer design is that used by Santoro and coworkers [9, 16] in soot studies. A fuel jet issues into a low-velocity coannular air flow producing stable conical flames. The products of combustion are mixed by a series of baffle plates at the top of the 230-mm-long glass duct before being sampled. Fuel and air flows are metered by calibrated rotameters. In addition to measuring the product gas species concentrations, visual observations of the flame were recorded. These include the flame length, the length of the nonluminous (blue) zone before soot luminosity first appears, and a subjective measurement of the intensity of the soot radiation.

TABLE 1 Test Conditions for Turbulent F l a m e s - - D i l u t i o n with Inerts Data Set

Fuel

1 2 3 4 5

CH4 b C3H8 b C2H4 c C2H4 c C O / H 2b'd

6 7

C O ~ H 2 b'd C O / H 2 b'd

Diluent (% by mass) N 2 (0%-50%) N 2 (0%-40%) N2 (0%-56%) N2 (0%-48%) N 2 (0%-50%) Ar ( 0 % - 5 9 % ) CO 2 (0%-61%)

Initial Jet Velocity a (m/s) 94.8 63.9 47.5 87.7 61.7 61.4 61.4

(74.5) (77.7) (51.5) (91.8) (61.7) (61.3) (61.2)

" N u m b e r s in parentheses refer to m a x i m u m dilution. hd 0 = 4.12 m m . ,t' d 0 - 3.86 mm. 95% C O / 5 % H 2 by mass.

H 2 Pilot a (% by mass) 2.85 1.55 0.45 1.73

(2.05) (1.58) (0.42) (1.66) ----

Jet Reynolds No. a, Rejc t

Flame Froude No. a, Frf

22,300 (18,100) 56,400 (40,200) 20,000 (16,000) 36,900 (29,800) 9,900 (12,200) 9,900 (12,800) 9,900 (16,950)

1.6 (13.3) 0.5 (2.1) 0.4 (2.2) 0.9 (4.1) 13.8 (76.3) 23 (148) 23 (195)

258

S. R. TURNS ET AL. TABLE 2

Test Conditions for Turbulent Flames--Partial Premixing with Air (do = 4.12 mm) Max. Air Mass Fraction

Fuel CH 4 C3H 8 c2n 4

0.85 0.76 0.49

th*

H 2 Pilot ~ (% by mass)

Initial Jet Velocity a ( m / s )

Jet Reynolds No. =

0.333 0.207 0.066

0.0 (0.99) 0.0 (0.41) 0.0 (0.10)

20.7 (89.2) 11.3 (67.9) 16.3 (31.9)

4900(22400) 10200(23100) 7500(11200)

aNumbers in parentheses refer to maximum premixing.

For the laminar flames, only the effects of inert dilution were investigated. Tests were conducted in two ways. In the first, the hydrocarbon fuel flowrate was fixed and various quantities of nitrogen diluent were added up to 60% by mass. This method results in flame lengths being essentially constant. In the second method, the combined mass flow of the fuel and diluent were fixed as the diluent content was increased. With this procedure, flame lengths decreased as diluent was added. Test conditions are shown in Table 3. Experiments were conducted with the same three hydrocarbon fuels used for the turbulent flames (cf. Table 1).

MIXING CHAMBER GLASS DUCT ( IO5 mm) FUEL TUBE

i,O-;mml .CERAMIC FUEL AIR

GLASS

AIR

GLASS BEADS

Fig. 2. Laminar flame burner schematic.

RESULTS AND DISCUSSION

N2-Diluted Turbulent Flames Figure 3 shows the effects of N 2 dilution on nondimensional visible flame lengths, global residence times, and adiabatic flame temperatures for the four fuels. One of the effects of replacing some of the fuel with diluent is to maintain the mixing rate essentially constant, while the amount of burnable fuel decreases. The flame lengths and global residence times are thus reduced with increasing N 2 dilution. The much larger stoichiometric mixture fraction for the C O / H 2 flames ( f = 0.20) compared with those of the three hydrocarbon fuels ( f = 0.06) results in significantly shorter flame lengths and residence times for this fuel, regardless of the degree of dilution. The adiabatic flame temperatures for all fuels are reduced by N 2 dilution; this effect is seen to be larger for C O / H : than for the three hydrocarbon fuels, again due to the difference in stoichiometric mixture fractions. The effect of N 2 dilution on flame radiant heat loss depends strongly on fuel type. Figure 4 shows that the radiant fraction decreases with dilution for all four fuels, but does so most significantly for the two most luminous flames, c 2 n 4 and C3H 8. For example, the undiluted CzH 4 flames have Xn = 0.27, and increasing the mass fraction of N 2 to 0.56 decreases XR to 0.10; in contrast, the radiant fraction from the CH 4 flames changes only from 0.12 to 0.08 as the N2 mass fraction is increased from 0 to 0.5. The C O / H e flames are nearly adiabatic regardless of the degree of dilution; Xn changes from 0.04 to 0.02 for these flames as the N z mass fraction increases from 0 to 0.5. Increasing the N 2 mass fraction in the fuel was accompanied by a change from yellow to blue flame color, in varying degrees,

JET FLAME NO x

259 TABLE 3 Test Conditions for Laminar Flames--Dilution with Inerts

Fuel

Average

Case

Flowrate (mg/s)

Velocity a (cm/s)

Fuel + Diluent Flowrate (mg/s)

Flowrate (mg/s)

Average Velocity (cm/s)

Oxidizer

A B C

5.0 Variable 5.0

2.9-8.2 -2.9-8.2

Variable 5.0 Variable

850 850 850

8.4 8.4 8.4

Composition A i r

Air Oxygen-enriched air 25% 0 2 (by volume)

"Range represents velocities for C3H s (highest density fuel) and c n 4 (lowest density).

for the three hydrocarbon fuels, while the C O / H 2 flames were blue both with and without dilution. Calculated nonadiabatic characteristic flame temperatures are also shown in Fig. 4; these depend only on the adiabatic flame tempera-

ture and radiant fraction. It is important to note that the characteristic flame temperatures of the C2H 4 and C3H 8 flames increase with N 2 dilution because the increasing flame adiabaticity more than compensates for the decrease in adiabatic flame temperatures. The opposite is true of the relatively nonluminous

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0

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Fig. 4. Effects of N 2 dilution on flame radiant fractions, characteristic nonadiabatic flame temperatures, and N O x emission indices for data sets 1, 2, 3, and 5.

260 CH 4 and C O / H 2 flames; here the decrease in adiabatic flame temperature overpowers the decrease in radiant fraction, and hence, the characteristic temperatures decrease with N 2 dilution. Kent and Bastin [10] found that measured in-flame soot temperatures increased as diluent was added to their highly luminous acetylene flames, which is consistent with our results. The importance of the nonadiabatic flame temperature is demonstrated in the lower plot in Fig. 4, which shows the NOx emission index as a function of N 2 dilution. The NO x emissions indices actually increase with N 2 dilution for the CzH 4 and C3H 8 flames, in spite of decreasing residence times, because of the higher characteristic nonadiabatic flame temperatures for diluted luminous flames. In contrast, the NO x emission indices for the nonluminous CH 4 and C O / H 2 flames decreased with N 2 dilution due to the combined effects of decreasing Tf and decreasing z~. The differing effects of N z dilution on NOx production from luminous and nonluminous flames are thus explained by the effect of radiative heat transfer on characteristic flame temperatures, barring any chemical interactions between NO and soot. Although the effect of dilution on the NO x emission indices for the C3H8 and CH 4 flames is small, the trends are repeatable. Comprehensive temperature measurements were performed in several flames to determine in more detail how dilution affects the thermal environment for NOx formation. Radial traverses were made at five similar axial stations in diluted and undiluted ethylene and methane flames. To provide similarity among all the flames, the axial coordinate was normalized by the thermal flame length, Lrmax, that is, the length based on the axial location of the peak centerline temperature. From the detailed radial profiles, the maximum temperature at each axial station was determined. For axial locations, x, less than the flame length, LTm,x, the maximum temperatures occurred in the annular flame region surrounding the cold flame core. For x >_ LT~,x, maximum temperatures occurred on the centerline, as expected. The detailed radial profiles are available in Ref. 14. Maximum temperatures as functions of axial

S . R . TURNS ET AL.

2500 T ~ (Undil.) T.d (Dil.) Tf (DiI.)

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AXIAL L O C A T I O N (X / L r ~ x )

Fig. 5. Axial distributions of peak temperatures for undiluted (u 0 = 41.5 m / s ) and diluted (56% N 2, u 0 = 44.7 m / s ) ethylene flames. Also shown are characteristic nonadiabatic flame temperatures, Tf, and adiabatic temperatures, Tad.

location are shown in Fig. 5 for the ethylene flames and in Fig. 6 for the methane flames. This presentation of the data clearly reveals how dilution affects the flame thermal structure. In Fig. 5, we see that early in the flames (x/Lrm°x < 0.5) peak temperatures are somewhat lower for the Nz-diluted flame than the pure ethylene flame, with differences ranging from 75 to 120 K. These temperature differences are of the same order as the difference between the adiabatic flame temperatures for these fuels (66 K). This suggests that early in the flame dilution decreases temperatures more or less in accord with its effect on adiabatic flame temperatures. Further downstream, however, a much different behavior is observed. At the end of the flame and beyond (1.0 < X/Lrmox < 1.25) the peak temperatures of the Nz-diluted flames are dramatically higher (400-575 K) than those of the high-luminosity pure-fuel flame. With dilution, soot production is greatly diminished, and hence, radiant losses are much less (XR = 0.07 with 56% dilution versus 0.24, undiluted). With diminished heat

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0.25

0.50

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1.00

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AXIAL L O C A T I O N (X / LTa,,x)

Fig. 6. Axial distributions of peak temperatures for undiluted (u 0 = 94.8 m / s ) and diluted (50% Nz, u o = 74.5 m / s ) m e t h a n e flames. Also shown are characteristic nonadiabatic flame temperatures, Tf, and adiabatic temperatures, Tad.

losses, local temperatures approach their adiabatic values. Corresponding characteristic nonadiabatic flame temperatures were 2201 and 2025 K, respectively, for the diluted and undiluted CzH 4 flames. These temperatures are shown as solid lines cutting across the peak-temperature profiles. We also observe from Fig. 5 that the highest peak temperature occurs much earlier in the undiluted flame, at a location slightly beyond x/Lrm,x = 0.5, compared to the diluted flame, where the maximum peak temperature occurs at the flame tip. The maximum peak temperature for the diluted flame was approximately 160 K higher than for the strongly radiating undiluted flame. From these results, we see that the thermal environments in the diluted and undiluted C z H 4 flames are dramatically different and consistent with the diluted flame producing approximately 1.5 times the NO x of the undiluted flame. These results also confirm the utility of using the heat-loss based nonadiabatic temperature to characterize the flames.

Looking now at the CH 4 flames (Fig. 6), we see some similarities with the C2H 4 flames. The trends are similar, with the diluted-flame temperatures somewhat lower early in the flame and somewhat higher beyond the flame tip in comparison to the undiluted flames; however, the dramatic fall off in peak temperature beyond x/Lrm, x = 0.5 for the undiluted ethylene flame was not observed for the methane flame. In fact, the maximum peak temperatures for both the diluted and undiluted CH 4 flames occurred at the flame tip, with the pure-fuel flame having a slightly higher temperature (AT -_- 115 K). Based on the Tpe~k versus X/LTm,x plot, one would expect slightly higher thermal NO x from the undiluted methane flame, neglecting the influence of residence time differences. This is indeed as was observed, with approximately 12% more NO x produced by the undiluted flame. To show more explicitly the relationship among NO~, residence time, and temperature, Fig. 7 presents characteristic NOx production rates, defined in Ref. 1, as a function of reciprocal characteristic flame temperature for the C2H 4 flames. The solid and shaded symbols are the N2-diluted flames (data sets 3 and 4, cf. Table 1), while the open symbols are for undiluted flames [1] where jet diameter and jet velocity were used to obtain temperature and residence time variations. Here we see that the Nz-diluted data follow the same trend as for the pure-fuel jet flames, which indicates that changes in flame characteristics, whether achieved by dilution or initial conditions, are equivalent in their effect on NO~. The propane flame data, diluted and undiluted, show the same "universal" relationship when viewed on an Arrhenius-type plot. Interestingly, the CH 4 and C O / H 2 flame data fail to show this simple time-temperature scaling. As we will show in the discussion of the partially premixed flames, this lack of correlation is likely the result of strong nonequilibrium effects, since the residence times of the CH 4 and C O / H 2 flames are significantly shorter than those of the C2H 4 and C3H 8 flames. We shall see also that the trend of the data to follow the theoretical NO x production rate based on equilibrium O atoms, at high temperatures (Fig. 7), is a

262

S . R . T U R N S E T AL.

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Fig. 7. Characteristic NOx production rates for undiluted and N2-dilutedC2H 4 flames. fortuitous consequence of the hotter flames having shorter global residence times.

Dilution with Ar and C O 2 The behavior of the C O / H 2 flames when diluted with either Ar or CO 2 was qualitatively similar to that of N 2 dilution. The effect of the various diluents on NO x was consistent with the different heat capacity of each diluent, with the reduction of NO~ at a fixed diluent mass fraction increasing in the same order as diluent heat capacity: Ar, N2, and CO 2. Table 4 illustrates this quantitatively for conditions TABLE 4 Relative Effects of Diluents on NO x Emissions from C O / H 2 / D i l u e n t Flames (Molar Composition: 40% CO/30% H2/30% Diluent)

Diluent

Two-Scalar Model Prediction (Drake et al. [15])

Present Study

N2 Ar CO 2

1.0 1.06 0.26

1.0 1.04 0.39

selected to match the calculations of Drake et al. [15]. More extensive data are available in Ref. 17. In Table 4, we see reasonable agreement between the experiments and the calculations, thus ruling out any substantial chemical effects, or other unusual behavior, associated with diluent type. This is in contrast to the experimental results of Drake et al. [15] that showed Ar to be anomalous. Also, it is unlikely that NO production is affected by changes in in-flame N 2 concentrations, since even in the highly diluted flames the amount of N2 associated with the diluent is very much less than that associated with the air. For example, for C 2 H 4 with 50% dilution, the N 2 content in a stoichiometric mixture increases by just over 1%.

N2-Diluted Laminar Flames Figure 8 shows results for the laminar jet flame experiments. H e r e are shown the length of the yellow (orange) luminous zone, expressed as a fraction of the total flame length; calculated adiabatic flame temperatures; and NO x emission indices, as functions of the N 2 diluent

JET FLAME NO x

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0.3

0.4

0.5

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N2 MASS FRACTION Fig. 8. Luminous zone lengths, adiabatic flame temperatures, and NO x emission indices for N2-diluted laminar jet

flames.

mass fraction. For CH4, addition of 60% N 2 causes the flame to be all blue, while for C3H 8 and C2H4, the luminous lengths decrease about 10%. Adiabatic flame temperatures decrease 85-98 K with the addition of 60% nitrogen. Interestingly, the NO x emission indices depend weakly on dilution, with all fuels showing only a slight decrease at the highest dilutions. The same, but slightly stronger, trend is apparent for the O2-enriched air C2H4 flames. In all cases, these trends were repeatable. The laminar flame NO x results are in strong contrast to those obtained for the turbulent flames, where each fuel clearly exhibited a unique trend. To explain the different behavior for the laminar and turbulent flames, it is useful to examine how dilution affects flame zone residence times and temperatures. Because buoyancy controls the velocity field of axisymmetric, laminar, diffusion flames [18], dilution has only

a small effect on residence times, since relatively small differences in density result with the addition of dilution, that is, approximately 5% based on adiabatic flame temperatures. The fact that the same NO x trends were observed for both constam-fuel-rate conditions, where flame length are essentially unchanged by dilution, and constant-fuel-plus diluent conditions, where flame lengths decrease, supports the idea that dilution has little influence on flame zone residence times. On the other hand, temperature changes can have a relatively large influence on NO production, particularly production resulting from the Zeldovich mechanism. To investigate temperature differences, we plot in Fig. 9 the local maximum temperatures measured at several axial locations for the undiluted and Ar-diluted ethylene flame experiments of Richardson and Santoro [9]. These experiments were run at constant ethylene flowrates and correspond quite closely to the conditions used in the present study: identical burner design, fuel flowrates of 5.0 (present study) and 5.6 m g / s [9], and maximum diluent mass fractions of 0.6 and 0.57 [9]. Figure 9 shows that dilution causes a depression of temperatures low in the flame (x < 35 mm), while further downstream (x > 35 mm) temperatures are higher. This interesting behavior is probably the result of the countervailing effects of dilution on adiabatic flame temperatures and diminished radiation losses (diminished soot production). In the Richardson and Santoro [9] experiments, integrated soot volume fractions increase linearly with axial position, peaking near x = 50 mm, and then decline at a rate similar to the growth rate. Note that the highest temperatures occur in the region of the flame where dilution reduces temperatures; thus, NO production is likely to be diminished by dilution since the highesttemperature regions produce disproportionately large amounts of NO, at least for the Zeldovich contribution. This argument is consistent with the experimental results and explains why NO x emission indices have only a weak decreasing dependence on N: addition. The results with the O2-enriched air, where adiabatic temperatures are higher ( ~ 2400 K) and Zeldovich kinetics are likely to dominate, also are consistent with this reasoning.

264

S . R . TURNS ET AL.

2000

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1800

,,=,

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20

40

60

AXIAL POSITION,

--

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X (mm)

Fig. 9. Comparison of maximum temperatures for diluted and undiluted laminar ethylene flames. Data are from Richardson and Santoro [9].

Partially Premixed Turbulent Flames

Diluting the fuel with air is very effective in reducing flame lengths and residence times, and the flames become significantly momentum-dominated (Fr£ > 1) when the fraction of stoichiometric air (~b7) exceeds about 10%, as can be seen in Fig. 10. The radiant fractions are greatly reduced, with the result that calculated nonadiabatic flame temperatures increase sharply, while the adiabatic flame temperatures remain constant (Fig. 11). The effects of partial premixing on unsealed NOx emission indices are also shown in Fig. 11. A somewhat unusual behavior is observed: NO x rises steeply for C2H 4 flames. The NO x from the C3H 8 flames also rises rather quickly, but somewhat less than for the C2H 4 flames, and then decreases. The CH 4 data mirror the trend for the C3H 8 flames, although the trend is less pronounced. This seemingly complex behavior is a consequence of the opposing effects of increasing temperature and decreasing residence times as more air is added to the fuel. Figure 12 shows how time and temperature effects are unified when characteristic NO x

production rates are plotted versus reciprocal flame temperatures. Figure 12 also shows that the simple time-temperature scaling is actually more complex, with an additional time dependence suggested by the global residence times indicated at selected data points. For example, we see that NO x production rates are significantly affected by residence times at a constant temperature, with approximately an order of magnitude increase associated with a decrease in residence times from 46 to 2 ms at 1000/T = 0.45. With shorter residence times, superequilibrium concentrations of O and OH radicals become increasingly important, causing NO x production rates to rise. It is this effect that results in the appearance of the data following the equilibrium O-atom production rates. These ideas are developed further in the next section. Extension of

Tf and

,re Scaling

To assess the ability of two parameters, Tf and za, to correlate the characteristic NO x production rates, data for all of the flames, both those presented here with dilution and premixing

JET F L A M E N O x 300 . . . .

, ....

, ....

265 , ....

, .........

,...

250

olo

0.25

]~

• %%

NO

c.o

"

100

I~.

looo,

I001

2

lo

i ....

~. . . .

i ....

I .... o

CH 4

0

0'I0

I

0.05

~-

I ....

oct,

O. 15

50o

.~,

I.' " • '1 . . . .

o OH 4

4

C2H 4

•

~

" ,

-

~

-

~

1

looo ~IOO

i

-

<

It. 0.01 ......... 0.00 0.05

' .... 0,10

' .... 0.15

AIR EQUIVALENCE

' .... 0.20 RATIQ

' .... 0.25

' .... 0.30

1900 5

0.35

C j*

....

I ....

I ....

I ....

I ....

I ....

I'''

Q~

4

Fig. 10. Effects of partial premixing on nondimensional flame lengths, global residence times, and flame Froude number. L

and those from Part I [1] of this study, were regressed in the form

I

0 z

i

-

I'-.4

ln[[NOx]/~'G] = A + B In "rG + C / T f ,

(1)

where the characteristic NO x production rate, [ N O x ] / r c, is as defined in Ref. 1. Table 5 shows the regression coefficients and their standard deviations for all of the hydrocarbon-fueled flames regressed as one class and all of the C O / H 2 flames as another. Included also in the second class are data from a few Hz-piloted C O / N 2 flames [14]. To show how well the two-parameter regressions fit the original data (i.e., the actual characteristic NO x production rates, not their logarithms), the percentage difference between regressionbased predictions and the experimental data are presented in Fig. 13 for the 159 data points comprising the hydrocarbon data set. Here we see that the fit is remarkably good, with the bulk of the data (90%) having differences

LIJ 0

,,,I,,,,I

0.00 AIR

....

I ....

0. I 0 EQUIVALENCE

I ....

I ....

0.20 RATIO,

I ....

0.30 ~j~

Fig. 11. Effects of partial premixing on radiant fractions, characteristic nonadiabatic flame temperatures, and NO~ emission indices.

within a band of _+30%. The data are arranged left to right for CH4, C3H8, and C2H 4 fuels. This grouping, with most of the CH 4 data being overpredicted while the C2H 4 data are underpredicted by the regression, suggests that an additional influence of fuel type is not captured by the two-parameter fit. Figure 14 illustrates the regression on an Arrhenius-type plot. Here regression lines for global residence times of 4, 20, and 100 ms are

266

S. R. TURNS ET AL.

• CH 4 + air • C3H 8 + air • C2H 4 + air

e

d[NO] =2 kl [N2] [O]e

E o

g- ms

E LU

.1

bU..I Z W 123 (/) iii rr < £13

G.3 =2.2 ms

XG = 46 ms

-~

INCREASING

.01 I

O .._1 (5

xG

~(r-"~n

=134 ms

=137 ms =236 ms

x

O Z

.001 0.42

0.44

0.46

0.48

0.50

0.52

0.54

1000 / FLAME TEMPERATURE [K] Fig. 12. Characteristic N O x production rates versus reciprocal characteristic flame temperatures for partially premixed turbulent flames.

shown overlayed on the hydrocarbon and CO~H2 data. Also shown on Fig. 14 is the NO production rate for the Zeldovich mechanism assuming equilibrium oxygen atoms. The data shown here corresponds to those of Fig. 11 in Ref. 1, together with the data from the present study. The data, which scatter over the figure since 76 was not a controlled parameter, show the range of validity of the correlation. The goodness-of-fit can only be judged by Fig. 13. Several observations can be made on the results shown in Table 5 and Fig. 14. First, we see that temperature coefficient, C, for the

hydrocarbon fuels, which can be thought of as some overall activation energy, has a magnitude ( ~ 16,400 K) that is perhaps more appropriate for NO x formation through the prompt route rather than the Zeldovich route. The much larger magnitude of the temperature coefficient for the C O / H 2 flames ( ~ 27,000 K), where the prompt mechanism is likely to be relatively less important, supports this hypothesis. The recent evaluations of kinetic rate constants and sensitivity analysis of Heard et al. [19] show that the two most sensitive steps in the prompt mechanism, CH + N 2 ~ N +

TABLE 5

Regression Coefficients a for Fits to Eq. 1 Data Base

A (ln[gmol/cm3-s])

B (I/s)

C (K)

No. of Data Points

g2 Statistic

Hydrocarbon flames, present study and Ref. [1]

1.1146 (0.6328)

- 0 . 7 4 1 0 (0.0230)

- 16,347 (1,210)

159

96.9%

C O / H 2 flames, present study and Ref. [1]

7.324 (0.7131)

- 0.3650 (0.0546)

- 27,219 (1,695)

41

86.3%

aNumbers in parentheses are standard deviations for least-squares curvefits.

JET FLAME NO x

267

50

o~ 0 0m

Ld L)

z

~ 25

ILl = DC " ' "

~

.............d,u,.l ,ll,jl

"r'lilPPliIT" 'iil

X

c~ 0

0

Z

,

z~ ~

A

-25 0

1

,n,l[,i,,i,I ,p-

Z v

<3

-50

.I-

CH4

I~

C3He

TEST

. -. II-

C2H4 ------~

NO.

Fig. 13. Percentage error associated with regression (cf. Table 5) for characteristic NO, production rates. Data are shown for all 159 hydrocarbon flames.

HCN and H + CH2 ~ CH + H2, have activation energies of 11,000 and 6,900 K, respectively. Recent calculations by Drake and Blint [20] are also consistent with our hypothesis. Their calculations [20] show that approximately

°~

two thirds of the NO formed in strained laminar hydrocarbon flames has a promptmechanism origin. It is also instructive to compare the results of the present study with those of Chen and

1F

o

HC

, ~

I

I

0.01

8

g

0.001 0.42

0.44

0.46

0.46

0.50

0.52

0.54

1000 / FLAME TEMPERATURE [K] Fig. 14. Experimental characteristic NO x production rates and curvefits (Eq. 1) for 7c = 4, 20, and 100 ms. Solid lines and open symbols are for hydrocarbon flames, and dashed lines and solid symbols are for C O / H 2 flames.

268 Driscoll [21]. In their study of H z and CH 4 flames, Chen and Driscoll [21] found that normalized NO x emission rates exhibited a clear dependence on U / d for the H 2 flames, while no clear correlation was obtained for the CH 4 flames. They attributed the U / d effect to aerodynamic strain causing departures from equilibrium, that is, a Damkohler number influence. This is equivalent to our usage of r e as an indicator of departures from equilibrium. In light of the success of the two-parameter fit for our hydrocarbon flames, it is likely that if variations in flame temperature (radiation losses) resulting from changes in initial velocity or jet diameter could be taken into account, U / d would be a useful correlating parameter for Chen and Driscoll's CH 4 flames as well. For the H 2 flames in Ref. 21, the power-law exponent for residence time effects, which is equivalent to our coefficient B, is -0.5. This value falls between the regression values for the hydrocarbon flames (B = -0.74) and the C O / H 2 flames (B = - 0.36). Our point in regressing the data (Eq. 1) is not to develop an engineering correlation (although that may still be useful), but rather to test our understanding of what are the important variables or effects associated with NO x production in jet flames. It appears that the use of two parameters to correlate a characteristic NO/ production rate captures, to a large degree, all of the important effects: residence times, temperatures (radiation losses), and strong departures from equilibrium in the radical pool. SUMMARY AND CONCLUSIONS Experimental measurements of postflame NO~ emission indices and flame radiant fractions were performed for inert-diluted and partially premixed jet flames. Four different fuels were utilized to provide a wide range of sooting (radiating) characteristics. From an analysis of these measurements, we draw the following conclusions: 1. Dilution causes characteristic flame temperatures to increase (C2H4, C3H8) , decrease (CO/He), or remain essentially unchanged (CH4) for the range of conditions explored. These effects are a consequence of the

S . R . TURNS ET AL.

2.

3.

4.

5.

countervailing influences of suppressed soot formation, which tends to make the flames more adiabatic, and decreasing adiabatic flame temperatures. Diluting the fuel with an inert or partially premixing with air results in NO x emission indices increasing, decreasing, or remaining relatively constant, consistent with the effects of dilution and premixing on characteristic flame temperatures and residence times. Unless there is a significant chemical interaction between soot and NO within the flame, these results provide strong evidence that flame radiation is a major factor in the scaling of jet-flame NO x emissions. The effects of residence time, flame temperature, and departure from equilibrium on NO x emissions, whether caused by variations in jet diameter, initial velocity, fuel type, fuel dilution, or partial premixing, were well-characterized using two parameters (Tf and r e) to predict characteristic NOx production rates ([NOx]/~-c). As a class, hydrocarbon flames showed less sensitivity to temperature than did C O / H e flames, which is consistent with prompt NO being important in the hydrocarbon flames. The effect of diluent type, either Ar, Ne, or CO2, was consistent with the model predictions of Drake et al. [15]. Inert dilution of laminar jet flames decreased NO x emission indices very slightly for all three hydrocarbon fuels, contrary to the increasing trend observed for the Cell 4 and C3H s turbulent jet flames. These results clearly show the importance of flame structure to NOx emissions, and in particular, the importance of the location of NO x formation zones with respect to sootcontaining and highly radiating regions of the flame.

This work was supported by the Gas Research Institute under Contract No. 5086-260-1308 with T. R. Roose and J. A. Kezerle serving as technical monitors. REFERENCES 1. Turns, S. R., and Myhr, F. H., Combust. Flame 87:319-335 (1991). 2. Lutz, A. E., Kce, R. J., Dibble, R. W., and Broadwell,

JET FLAME

3.

4. 5. 6.

7. 8.

9. 10. 1l.

NO x

J. E., AIAA-91-0478, 29th Aerospace Sciences Meeting, Reno, NV, 1991. R~kke, N. A., Hustad, J. E., Scnju, O. K., and Williams, F. A., Twenty-Fourth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh (in press). Chen, J.-Y., and Kollmann, W., Combust. Flame 88:397-412 (1992). Driscoll, J. F., Chen, R.-H., and Yoon, Y., Combust. Flame 88:37-49 (1992). Sterner, S. H., Bilger, R. W., Dibble, R. W., and Barlow, R. S., Combust. Sci. Technol. 70:111-133 (1990). Miller, J. A., and Bowman, C. T., Prog. Ener. Cornbust. Sci. 15:287-338 (1989). Axelbaum, R. L., and Law, C. K., Twenty-Third Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1991, pp. 1517-1523. Richardson, T. F., and Santoro, R. J., Combust. Sci. Technol. (submitted). Kent, J. H., and Bastin, S. J., Combust. Flame 56:29-42 (1984). Gollahalli, S. R., Combust. Sci. Technol. 15:147-160 (1977).

269 12. Turns, S. R., and Lovett, J. A., Combust. Sci. Technol. 66:233-249 (1989). 13. Sterner, S. H., and Bilger, R. W., Combust. Flame 61:29-38 (1985). 14. Turns, S. R., and Bandaru, R. V., Final Report (January 1990-August 1992), GRI-92/0470, September 1992. 15. Drake, M. C., et al., Combust. Flame 69:347-365 (1987). 16. Santoro, R. J., Semerjian, H. G., and Dobbins, R. A., Combust. Flame 51:203-218 (1983). 17. Turns, S. R., and Myhr, F. H., Semi-Annual Report (July-Dec. 1991) to GRI, Feb. 1992. 18. Roper, F. G., Combust. Flame 29:219-226 (1977). 19. Heard, D. E., Jeffries, J. B., Smith, G. P., and Crosley, D. R., Combust. Flame 88:137-148 (1992). 20. Drake, M. C., and Blint, R. J., Cornbusl. Flame 83:185-203 (1991). 21. Chen, R.-H., and Driscoll, J. F., Twenty-Third Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1991, pp. 281-288. Received 16 June 1992; revised 17 November 1992