Exhaust and in Situ Measurements of Nitric Oxide in Laminar Partially

Sep 19, 1996 - NO formation in laminar partially premixed ethane−air flames is investigated as a ... Combustion Science and Technology 2000 152, 167...
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Energy & Fuels 1996, 10, 1060-1066

Exhaust and in Situ Measurements of Nitric Oxide in Laminar Partially Premixed C2H6-Air Flames: Effect of Premixing Level at Constant Burner Tube Flow Rate Tae Kwon Kim Low Emission Engine Laboratory, Korea Institute of Machinery and Metals, P.O. Box 101, Yuseong, Daejeon 305-600, Korea

B. J. Alder, J. R. Reisel,†,‡ and N. M. Laurendeau*,‡ School of Mechanical Engineering, Purdue University, West Lafayette, Indiana 47907-1288 Received January 31, 1996X

NO formation in laminar partially premixed ethane-air flames is investigated as a function of the amount of air introduced into the central fuel tube of an annular coflow burner. The NOx emission index at the exhaust is determined by chemiluminescent detection while the local NO number density is measured by laser-saturated fluorescence. The measurements are taken in flames with an overall equivalence ratio of 0.9, burner-tube equivalence ratios (φB) varying from 1.1 to 6.7, and a fixed burner-tube flow rate of 0.9 L(STP)/min. Local NO number densities are measured as a function of both radial position and height above the burner. Despite the increase in fuel flow rate with rising φB, an intermediate dual-flame pattern is identified which minimizes the NOx emission index. NO production is found to occur primarily between an inner premixed and an outer nonpremixed flame front, which constitutes the dual-flame structure. These results suggest that the optimum burner-tube equivalence ratio occurs due to a compromise between prompt and thermal formation of NO in the predominantly premixed and nonpremixed flame regions, respectively.

Introduction Lean premixed combustion is currently an active area of research with respect to the reduction of NOx emissions from gas turbine combustors. However, compared to standard nonpremixed designs, the lean premixed approach suffers from the need for a large premixing section, from a lower turndown ratio, and from the dangers of flame instability and flashback. Partially premixed flames, on the other hand, are more stable than ultralean premixed flames and also shorter than the usual nonpremixed flames. Hence, partial premixing has the potential to enhance flame stability compared to lean premixed combustors and also to reduce NOx emissions compared to traditional nonpremixed combustors. In general, NOx emissions are produced through three main reaction pathways:1,2 (1) the Zeldovich, or thermalNO mechanism, (2) the N2O-intermediate route, and (3) the prompt-NO mechanism. The amount of NO formed through each depends on the pressure and residence time, plus the specific distribution in temperature and composition for a particular flame. The production of thermal NO is controlled by the reaction †

Now affiliated with the University of Wisconsin, Milwaukee, WI. E-mail addresses: Dr. Normand M. Laurendeau: laurende@ecn. purdue.edu. Dr. John R. Reisel: [email protected]. X Abstract published in Advance ACS Abstracts, July 15, 1996. (1) Drake, M. C.; Blint, R. J. Calculations of NOx formation pathways in propagating laminar, high pressure premixed CH4/air flames. Combust. Sci. Technol. 1991, 75, 261. (2) Miller, J. A.; Bowman, C. T. Mechanism and Modelling of Nitrogen Chemistry in Combustion. Prog. Energy Combust. Sci. 1989, 15, 287. ‡

N2 + O f NO + N

(R1)

which is favored for lean conditions at T g 1800 K. The N2O-intermediate pathway is more important at high pressures since it involves a three-body reaction to form N2O from N2 and O, with subsequent conversion of N2O to NO. At atmospheric pressure, the prompt-NO pathway dominates for moderately rich conditions and T e 1800 K as it proceeds through the elementary step

CH + N2 f HCN + N

(R2)

Since reaction R2 involves the CH radical, the production of NO during combustion of H2 and CO must occur primarily through the Zeldovich mechanism. Fundamental experimental work on NOx emissions from laminar partially-premixed flames has been reported recently by Gore and Zhan3 and by Kim et al.4 Gore and Zhan3 investigated methane-air flames at a constant fuel flow rate of 42 mg/s (2.1 kW) while Kim et al.4 studied ethane-air flames at a constant fuel flow rate of 3.1 mg/s (0.15 kW). In each case, an annular coflow burner was employed at an overall equivalence ratio of 0.5. Both burners consisted of a central tube that carried fuel plus premixing air and a surrounding annulus for the coflow air stream. Despite the different (3) Gore, J. P.; Zhan, N. J. NOx emission and major species concentrations in partially premixed laminar methane/air co-flow jet flames. Combust. Flame 1996, 105, 414. (4) Kim, T. K.; Alder, B. J.; Laurendeau, N. M.; Gore, J. P. Exhaust and in-situ measurements of nitric oxide for laminar partially premixed C2H6-air flames: Effect of premixing level at constant fuel flow rate. Combust. Sci. Technol. 1996, 110-111, 361.

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fuels and burner sizes, both studies reported that a minimum NOx emission index (NOxEI) is obtained for a central burner-tube equivalence ratio φB ≈ 2.1 at atmospheric pressure. This remarkable result was attributed by Kim et al.4 to a compromise between NO production by the prompt mechanism in premixed regions and the thermal mechanism in nonpremixed regions of the partially premixed flames. This suggestion was based on in situ measurements of NO via lasersaturated fluorescence, which showed that the inner premixed flame front accounts for most of the NO formation at φB < 2.1 while the other nonpremixed flame front accounts for most of the NO production at φB > 2.1. Employing local measurements of C2H2 and C2H4 concentrations, Gore and Zhan3 similarly conjectured that the optimum φB must be associated with changes in intermediate hydrocarbons associated with the prompt mechanism. In the present study, we seek to further understand NOx production in partially premixed flames by employing the same annular co-flow burner used previously by Kim et al.,4 but with a constant total burner-tube flow rate rather than a constant fuel flow rate. Hence, in this case, no change in the overall fuel residence time occurs with changes in the premixing level. As in our previous work, we employ quantitative measurements of NO using chemiluminescent detection and laser-saturated fluorescence, the latter following the technique of Reisel et al.5 In particular, the relationship between NO production and flame appearance is assessed by measuring the local NO number density as a function of the burner-tube equivalence ratio. Emphasis is again placed on the possibility of finding an optimum level of partial premixing for C2H6-air flames.3,4 Experimental Methods Burner and Flow Facilities. The annular coflow burner illustrated in Figure 1 was used for the partial premixing experiments. It consists of a central 4.6 mm i.d. fuel tube and a concentric coflow air tube 25 mm in diameter. Partial premixing is accomplished by adding the desired amount of air to the ethane in the fuel tube at a sufficient distance before the burner assembly so as to obtain molecularly mixed flow. The annular air tube was used to deliver a coflow of oxygen in nitrogen at a dilution ratio of 3.76. Finally, an outer argon guard flow was available to separate the combustion environment from the surrounding room air. Hastelloy honeycomb, glass beads, and sintered metal were used in the annular and guard-flow tubes to ensure uniform laminar flow. For the exhaust NOx measurements, the burner assembly was contained in an airtight pressure vessel which allowed for exhaust removal via a venturi-based system while maintaining the vessel at approximately atmospheric pressure. A 3.2-mm stainless-steel probe was inserted into the exhaust stream, 100 cm downstream from the inlet to the exhaust pipe, in order to draw off NOx samples. The position of the sampling probe corresponded to a length-to-diameter ratio for the exhaust pipe of 40, which is sufficient to ensure a uniform NO concentration across the pipe. A model 4Z026 Dayton Speedaire pump maintained sufficient vacuum in the sampling line to draw the exhaust sample to the sampling system. Resistance heaters extending from the probe to the inlet of the sampling system maintained the exhaust sample above its dew point. (5) Reisel, J. R.; Carter, C. D.; Laurendeau, N. M.; Drake, M. C. Laser-saturated fluorescence measurements of nitric oxide in laminar, flat, C2H6/O2/N2 flames at atmospheric pressure. Combust. Sci. Technol. 1993, 91, 271.

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Figure 1. Schematic diagram of partially premixed annular coflow burner. For the in situ measurements, the burner was located in the high-pressure combustion facility described by Carter et al.6 As these experiments were performed at atmospheric pressure, the pressure vessel was removed, and a 0.6 m × 0.6 m × 0.5 m high, transparent Plexiglas shield was installed around the burner to minimize any effects from room air drafts. To remove the combustion products from the laboratory, a suction hood centered above the flame was employed. Calibrated rotameters were used in a gas delivery system for the C2H6, O2, diluent N2, and Ar guard flows, as well as for the NO/N2 mixture employed for fluorescence signal calibration. Calibration of the rotameters was performed through use of a dry-test volumetric flow meter and a bubble meter. Procedures for Exhaust Measurements. The exhaust levels of nitrogen oxides were determined by using a Thermo Environmental Instruments Model 42 chemiluminescence NO-NO2-NOx analyzer. The chemiluminescent analyzer was calibrated immediately prior to every sequence of NO, NO2, and NOx measurements by using span gases and N2 as a zero gas. The zero gas passed through two filters and a charcoal scrub so as to remove any trace amounts of NO, NO2, H2O, and hydrocarbons. The span and zero gases were reintroduced following each sequential run of measurements to check for any deviation in the calibration. The flow rates of the calibration gases were all determined by rotameters which were in turn calibrated using a dry test volumetric flow meter and a bubble meter. Each experimental run consisted of measuring the exhaust NO and NOx concentrations at 17 selected burner-tube equivalence ratios (φB) from 1.1 to 6.7. An overall equivalence ratio (φO) of 0.9 was maintained for all runs as was a burner tube flow rate of 0.9 L(STP)/min. The overall equivalence ratio was defined as that which would result from premixed combustion with the ethane and net air (annular coflow plus burner tube) for a given run. The overall equivalence ratio and burnertube flow rate were held constant as φB was varied by increasing the fuel and coflow air flow rates while simultaneously decreasing the burner-tube air flow rate. Following every burner adjustment, the analyzers required several minutes to arrive at a steady-state reading. For each condition, measurements of NO and NOx were taken as parts (6) Carter, C. D.; King, G. B.; Laurendeau, N. M. A combustion facility for high-pressure flame studies by spectroscopic methods. Rev. Sci. Instrum. 1989, 60, 2606.

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per million (v/v). Nitrogen diluent was introduced to the exhaust sample immediately prior to the analyzers to prevent condensation within the analyzers. The volumetric dilution ratio was a constant seven to one. All experimental runs were repeated in their entirety over a 2-month period in order to ensure repeatability. The difference among repeated runs averaged ∼2.5% over the entire range of burner-tube equivalence ratios. Expressing concentrations in terms of an emission index eliminates the dilution effects associated with the argon guard flow and also with the excess air required to drive the combustion to fuel-lean conditions. The conversion from parts per million (v/v) after dilution to an NOx emission index (g NO2/kg fuel) is accomplished via

NOXEI ) NOXMNO2 (MFV˙ F + MO2V˙ O2 + MN2V˙ N2 + MARV˙ AR) (1000)MT

MFV˙ F

where

NOX )

(MTV˙ T + MDV˙ D)NOX,d MTV˙ T

Here MNO2, MF, MO2, MN2, MAR, MT, and MD are the molecular weights of NO2, ethane, oxygen, nitrogen, argon, the undiluted exhaust mixture, and the diluent, respectively. The variable V˙ i represents the volumetric flow rate of each of these components. The variable NOx represents the exhaust concentration of NOx before dilution, while NOX,d represents its concentration after dilution. Notice that this formulation follows the standard procedure of using the molecular weight of NO2 for all emission index calculations. Procedures for in Situ Measurements. The laser system and optical layout used to perform the laser-saturated fluorescence (LSF) measurements of NO are shown in Figure 2.5 Excitation of NO is achieved through use of the Q2 (26.5) line of the γ(0,0) band. The laser system producing this wavelength is composed of a Quanta-Ray DCR-3G Nd:YAG laser, with a PDL-2 dye laser and a WEX-1 wavelength extender. The second harmonic (λ ) 532 nm) of the Nd:YAG laser was used to pump the dye laser, which was configured for transverse pumping of the oscillator and longitudinal pumping of the amplifier. The dye laser output (λ ≈ 572 nm) was frequency doubled, and the resulting ultraviolet beam was mixed with the residual infrared beam (1064 nm) to produce radiation at λ ≈ 225.6 nm. A Pellin-Broca prism was used to disperse the collinear beams (with wavelengths of 1064, 572, 286, and 225.6 nm), and the desired beam (225.6 nm) was raised in height with a prism assembly inside the WEX. The typical beam energy leaving the WEX at 225.6 nm was approximately 1.0 mJ/pulse. After the beam left the WEX, a portion was directed to a UV-sensitive photodiode, which produced a triggering pulse for the electronics. The main beam was then focused with a 1000-mm focal length lens, giving a spot size of ∼250 µm over the burner. To block any scattered radiation, an aperture was placed before the burner. The beam was directed toward the burner and raised in height with a two-mirror beam steering assembly. After passing over the burner, the beam was directed toward a beam dump, with a portion of the beam split off with a fused silica plate and directed toward a photodiode. This photodiode was used to monitor the beam energy. Fluorescence from the γ(0,1) band of NO at 234-237 nm was collected at a 90° angle to the incident laser radiation. The fluorescence was collimated with a 200 mm focal length fused silica lens. A mirror assembly rotated the fluorescence by 90°, after which the fluorescence was focused by a 300 mm focal length fused silica lens onto the entrance slit of a 1/2-m monochromator. The detector located after the exit slit was

Figure 2. Schematic diagram of laser system and optical layout: (A) trigger photodiode; (B, G) beamsplitter; (C) 1000 mm focal length lens; (D, K) beam steering assembly; (E) aperture; (F) burner; (H) beam dump; (I) power-monitoring photodiode; (J) 200 mm focal length lens; (L) 300 mm focal length lens; (M) 1/2 m monochromator; (N) PMT. an RCA 1P28B photomultiplier tube (PMT), specially wired for temporal resolution of the fluorescence signal.7 For the LIF measurements of NO number density, the entrance slit width of the 1/2-m monochromator was 120 µm and the entrance slit height was set at 2 mm. With a magnification factor of 1.5 in the collection optics, the resulting image of the entrance slit was 80 µm × 1.33 mm, which caused some spatial averaging of the LIF signal across the central fuel tube. To achieve laser-saturated fluorescence, which produces a minimal dependence of the fluorescence signal on quenching and laser power fluctuations, it is desired to minimize the collection of fluorescence from the wings of the laser beam. To achieve this, the image of the entrance slit (80 µm) was chosen to be smaller than the width of the focused laser beam (∼250 µm) and was positioned at the center of the beam. The PMT and photodiode signals were recorded with Stanford Research Systems equipment. For the LSF experiments, the PMT signal was resolved with an SR255 fast sampler using a 500-ps sampling gate, which was centered on the peak of the fluorescence pulse using an SR200 gate scanner. An SR250 gated integrator was used to capture signals from the photodiode monitoring the UV-beam energy. The output voltages from the fast sampler and the gated integrator were digitized and stored with the SR245 computer interface module and the SR265 software package, respectively. For the measurements of NO number density, the fluorescence signal was generally averaged over 500 laser shots. Measurements were taken in six representative flames, all with an overall equivalence ratio φo ) 0.9 and a fixed burner tube flow rate of 0.9 L(STP)/min. The six flames had burnertube equivalence ratios ranging from 1.3 to 6.7. Measurements (7) Harris, J. M.; Lytle, F. E.; McCain, T. C. Squirrel-cage photomultiplier base design for measurement of nanosecond fluorescence decays. Anal. Chem. 1976, 48, 2095.

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Figure 3. Sketches of six partially premixed flames as a function of the burner tube equivalence ratio; φo ) 0.9. The inner premixed and outer nonpremixed flame fronts are shown, with shading to indicate regions of soot formation. of NO were taken at 95 locations in each flame, i.e., 19 radial locations at each of five different heights (5, 10, 15, 20, and 25 mm) above the burner. The measurements were calibrated in a premixed φ ) 0.95 flame, which was positioned in the central burner tube. The total flow rate of this calibration flame was 3.4 L(STP)/min, and its dilution ratio was 3.31. Calibration was effected by comparing the fluorescence signal at a height of 25 mm in the center of the φ ) 0.95 flame with that at the same position in each of the six partially premixed flames. To convert a fluorescence signal to an NO mole fraction, fluorescence measurements were taken both in the undoped and doped calibration flame using five different NO doping conditions (20-80 ppm). The data formed a straight line which was used to determine the calibration factor for the fluorescence measurements in both the undoped calibration flame and the partially premixed flames.5 To convert from an NO mole fraction to an NO number density, the temperature at the center of the calibration flame was measured at the same height with an uncoated Pt-Pt/ 10% Rh thermocouple (bead diameter ∼ 0.2 mm). The measured temperature (1780 K) was corrected for radiative heat loss by using the procedures found in Bradley and Matthews.8 While the Boltzman fraction of the Q2(26.5) transition is relatively independent of temperature at 9002200 K, significant variation exists at lower temperatures. However, any LSF measurements of NO at such temperatures are sufficiently small as to be of no concern for the purposes of this study. Moreover, the fluorescence measurements were well saturated over the conditions of our flames, as discussed by Reisel et al.5

Results and Discussion Effect of Partial Premixing on Flame Appearance and Temperature. A partially premixed flame is normally dual-structured, consisting of an inner premixed flame and an outer nonpremixed flame in a coflow jet configuration. Figure 3 shows sketches of the partially premixed flames of this investigation as a function of the burner tube equivalence ratio φB. The flame heights are significantly smaller than those obtained for the constant fuel flow rate case4 at φB < 2.8, but increasingly larger than the latter at φB > 2.8. This behavior is shown in Figure 4. As expected, the visible flame height increases with rising burner tube equivalence ratio φB owing to the corresponding increase in fuel flow rate. The influence of φB is especially strong at 1.5 e φB e 3.3. In general, the flame appearance resembles that of a premixed flame at φB < 2.2 and that of a nonpremixed flame at φB > 2.2. These observations suggest a categorization of partially premixed flames into three distinct flame regimes. (8) Bradley, D.; Matthews, K. J. Measurement of high gas temperatures with fine wire thermocouples. J. Mech. Eng. Sci. 1968, 10, 299.

Figure 4. Flame height as a function of burner tube equivalence ratio; constant burner tube flow rate at φo ) 0.9 and constant fuel flow rate at φo ) 0.5.4

The flames at φB < 2.2 display a dual structure with an inner premixed zone and an outer nonpremixed zone. The CO/H2 mixture produced by the fuel-rich inner flame front reacts with available oxygen in the outer nonpremixed flame front.3 Since fuel conversion occurs mainly in the inner premixed zone, the partially premixed flames at φB < 2.2 tend to resemble premixed flames. For such flames, both the inner and the outer flame fronts move downstream with decreased levels of premixing. However, the global structure of the flame remains the same and both flame fronts are blue in appearance. As φB increases from 1.5 to 3.3, the flame structure changes from a premixed-like flame to a nonpremixedlike flame.3,4 In particular, the visible flame height increases monotonically and the flame exhibits a distinct change in structure with increasing φB. Thus, the intermediate flames near φB ) 2.2 represent a hybrid structure existing between the “kinetics-controlled” and “diffusion-controlled” partially premixed flames. A small luminous sooting region appears at the tip of the inner flame at φB ) 2.2, even though the outer flame zone still surrounds the inner flame. At φB > 3.3, the inner flame merges into the outer flame and a much larger portion of the flame tip becomes a luminous sooting region. Thus, the structure of partially premixed flames at φB > 3.3 is similar to that of nonpremixed flames. Figure 5 shows measured radial temperature profiles at a constant height of 20 mm above the burner for the flames at φB ) 1.67, 2.00, and 2.50, as obtained by using a Pt-Pt/10% Rh thermocouple. These radial profiles are representative of those found at other heights above the burner for this range of φB values. The profiles indicate that the peak flame temperatures (1800-1830 K) generally occur between the inner premixed and the outer nonpremixed flame fronts of the partially premixed flames. As expected, the temperature profiles become more uniform closer to the flame tip for all φB values, as shown for the φB ) 1.67 case in Figure 5. Effect of Partial Premixing on NOx Emission Index. The measured emission indices for NOx as a function of burner tube equivalence ratios from 1.0 to 6.7 are shown in Figure 6 for both the constant burner tube flow rate (φo ) 0.9) and the constant fuel flow rate (φo ) 0.5)4 cases. The NOxEI values (>90% NO) rise rather steeply from φB ) 1.0 to φB ) 1.5, then drop off

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Figure 5. Radial temperature profiles 20 mm above the burner as a function of φB at φo ) 0.9. All temperatures are uncorrected for radiative heat losses.

Figure 6. NOxEI as a function of burner tube equivalence ratio; constant burner tube flow rate at φo ) 0.9 and constant fuel flow rate at φo ) 0.5.4

more gradually to a minimum at φB ≈ 2.1 after which they slowly climb to levels greater than those at their previous peak. The NOxEI values at 1.0 < φB < 2.1 are approximately a factor of 3 less than those found in the complementary work,4 partially due to the smaller flame sizes associated with the lower fuel flow rates at φB < 2.1. More generally, however, the dual flame-front region exhibits mean radiation-corrected temperatures near 1830 K for the present case (see Figure 5) as compared to ∼1960 K for the previous case.4 The ∼130 K decrease in temperature for the current as compared to the previous work implies a strong reduction in thermal NO, which is clearly consistent with the results shown in Figure 6. The much more substantial increase in NOxEI with rising φB for φB > 2.1 in this case as compared to the constant fuel flow rate case4 is probably related to the greater flame-front area associated with the larger fuel flow rates at φB > 2.8 (see Figure 4). Even with these higher fuel flow rates, the NOxEI values in the current work fail to reach those for the previous case4 at φB > 2.8 owing to the much lower availability of air at φo ) 0.9 as compared with φo ) 0.5. In other words, less oxygen is available to form thermal NO at the outer nonpremixed flame front in downstream regions of the partially premixed flames at φo ) 0.9. Repetitive experiments show that the reduction in NOxEI at a constant burner tube flow rate of 0.9 L(STP)/ min is approximately 20-30% from its maximum at φB ≈ 1.5 to its minimum at φB ) 2.0. This result compares

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favorably with the 21-25% reduction from φB ) 1.3 to φB ) 2.2 at a constant fuel flow rate of 0.15 L(STP)/ min.4 Hence, as before, the hybrid flame near φB ) 2.1 produces lower NOx emissions in comparison with both the kinetically-controlled flames at lower φB and the diffusion-controlled flames at higher φB. A minimum in NOx emissions is not unreasonable as NOx could easily increase at lower and higher equivalence ratios owing to the greater high-temperature region at both φB < 2.0 (lower stoichiometry) and φB > 2.0 (higher fuel flow rate). Figure 6 confirms the generally premixed nature of these partially premixed flames at φB < 2.2 and the generally nonpremixed nature of such flames at φB > 2.2 (see Figure 3). The NOxEI peak at φB ) 1.5 is consistent with the equivalence ratio (φ ) 1.4) corresponding to peak NO for fully premixed ethane-air flames at 1700 < T < 1900 K.5 However, for the purely premixed flames, the drop in exhaust NO on the fuelrich side of the peak at φ ) 1.4 is much more rapid than the drop on the fuel-lean side.5 Hence, for partially premixed flames, an additional amount of NO is produced at φB > 1.5 by the surrounding diffusion flame. The generally nonpremixed nature of the partially premixed flames at φB > 2.2 is indicated by the rapid rise in NOxEI with increasing φB. As indicated previously, this NOx enhancement probably arises from the higher flame-front area which accompanies the rising fuel flow rates at greater equivalence ratios. This enhancement would be even more pronounced if substantial sooting had not occurred at φB > 2.5. While prompt NO is responsible for the NOxEI peak at φB ) 1.5 for the kinetically-controlled flames,5 thermal NO is apparently responsible for the NOxEI enhancement at φB > 2.2 for the diffusion-controlled flames. In fact, the minimum at φB ) 2.0 suggests that ∼60% of the NOx produced at φB ) 6.0 comes from the Zeldovich mechanism. Prompt NO is associated with the premixed inner flame front for C2H6-air, while thermal NO is associated with the nonpremixed outer flame front for CO/H2-air. Thus, the minimum NOxEI at φB ) 2.0 again represents a compromise between the prompt and Zeldovich mechanisms for NO formation, although here the behavior at φB > 2.2 is unaffected by changes in the global residence time. This compromise arises from the dual-flame structure of partially premixed flames. Moreover, the optimum φB value found here is generally consistent with those found previously by Gore and Zhan3 and by Kim et al.4 for a constant fuel flow rate. Effect of Partial Premixing on Local NO Concentration. Further confirmation for the observations based on the exhaust data was sought through detailed in situ measurements of NO number density in six of the above partially-premixed flames at φB ) 1.3-6.7. Figure 7 shows the NO number density as a function of radial position at five different heights above the burner for these six flames. In each case, the NO concentrations tend to increase slightly with increasing height above the burner. The radial NO profiles are generally double-humped at lower positions relative to the flame height and parabolic at higher positions relative to flame height. The double-humped structure dominates as φB rises owing to the greater flame heights associated with

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Figure 7. NO number density (cm-3) as a function of radial position and height above the burner at six burner tube equivalence ratios; φo ) 0.9.

lower levels of premixing (see Figure 4). The doublehumped structure denotes the annular NO production region resulting from the dual-flame fronts. The shorter flames at lower φB values exhibit parabolic NO profiles which result from radial mixing downstream of the initial annular regions of NO production. The downstream formation of thermal NO along the centerline of the lower-velocity, dual-flame region strengthens the parabolic profile. Any additional thermal NO generated at the flame peak is consistent with the anticipated higher flame temperatures in this zone owing to fuelair preheating lower in the flame by the surrounding dual-flame fronts (see Figure 5). For 1.3 e φB e 2.2, the maximum NO depicted by the parabolic radial profiles are all obtained between the inner and outer flame zones, which suggests that the major source of NO production occurs in the premixed flame region. For the φB ) 2.2 flame, the maximum NO decreases as compared with the φB ) 1.5 flame, in agreement with the minimum in NOxEI at φB ) 2.0. The onset of soot formation at the tip of the inner flame near φB ) 2.2 suggests a correlation between incipient soot formation and reduced formation of NO. The diffusioncontrolled flames at φB g 3.3 are characterized by peak radial NO values near the visible flame fronts. As φB increases from 3.3 to 6.7, the maximum NO concentrations increase, in concert with the rising NOx emission indices.

An important feature of the in situ data is described in Figure 8, which shows the relationship between the location of the visible flame zones and that of the maximum NO number density at different heights above the burner. The width of the flame region between the inner premixed and the outer nonpremixed flame fronts gradually decreases with increasing φB. Furthermore, the position of maximum NO is essentially located between these inner and outer flame fronts, in agreement with our previous results for the constant fuel flow rate case.4 At lower heights above the burner, the maximum NO contour lies closer to the inner flame front, while further downstream, the maximum NO contour moves toward the outer flame front. This implies that NO formation shifts from premixedcontrolled to diffusion-controlled with increasing height above the burner, and also with increasing φB. In fact, for the φB ) 6.7 case, the downstream maximum NO contour is located just outside of the outer flame front. Hence, the outer diffusion flame dominates so that NO formation is now essentially controlled by nonpremixed combustion. Recent research on the diffusion flame structure of hydrocarbon flames by Kim and Shin9 reports that the position of maximum temperature is located just outside the flame front. Therefore, since (9) Kim, S. J.; Shin, H. D. A visual study of the structure of turbulent nonpremixed flames near jet exit. Combust. Sci. Technol. 1994, 99, 37.

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Figure 8. Visible flame width and location of maximum NO as a function of burner tube equivalence ratio at 5, 15, and 25 mm heights above the burner; φo ) 0.9.

the thermal mechanism contributes significantly in diffusion flames, we would indeed expect that the maximum NO contour should exist just outside of the outer flame front for φB ) 6.7. As discussed by Nishioka et al.,10 NO emissions from counterflow partially premixed flames are controlled by a combination of the thermal and prompt NO mechanisms. For premixed flames, thermal NO peaks at φ ) 1.010 while prompt NO peaks at φ ) 1.4.5 For the partially premixed flames of this study, the burner-tube equivalence ratio corresponding to peak NO occurs near φB ) 1.5 at upstream locations (see Figure 7), which accentuates the premixed nature of those flames having φB e 2.2. This result confirms that prompt NO plays a significant role in the total production of NOx emissions for these low-temperature flames. However, the rapid rise in peak NO at higher φB values must be related to the enhanced temperature available for formation of thermal NO. The minimum in the NO number density profiles measured at φB ) 2.2 is clearly consistent with the independent determination of a minimum NOxEI at φB ) 2.0. Conclusions We have employed both exhaust and in situ measurements of NOx to investigate the flame structure and pollutant characteristics of laminar partially-premixed C2H6-air flames having a constant burner-tube flow rate. For the most part, the conclusions from this study are consistent with those found for partially-premixed flames at a constant fuel flow rate:4 1. The flame height decreases with increasing levels of partial premixing. As the burner-tube equivalence (10) Nishioka, M.; Nakagawa, S.; Ishikawa, Y.; Takeno, T. NO emission characteristics of methane-air double flame. Combust. Flame 1994, 98, 127.

ratio is reduced, the partially premixed flames shift from a nonpremixed flame appearance to a premixed flame appearance. In particular, the sooting region at the flame tip is reduced and for our conditions eventually disappears at a burner tube equivalence ratio φB ≈ 2.2. 2. An intermediate level of premixing apparently related to the initial appearance of soot at the flame tip offers a minimum NOx emission index. For our flame conditions, this optimum condition occurs at φB ≈ 2.0 and provides an approximately 24% reduction in exhaust NOx as compared to the peak emission index. 3. Measurements of in situ NO via laser-induced fluorescence show that NO production occurs mainly in the high-temperature region between the inner premixed and outer nonpremixed flame fronts. The inner premixed flame front apparently accounts for most of the NO formation at lower φB values while the outer nonpremixed flame front accounts for most of the NO production at high φB values. 4. Prompt NO is primarily responsible for NO formation at φB e 2.0. On the other hand, for our flame conditions, thermal NO makes a major contribution at φB g 2.0. Hence, the minimum NOx emission index at φB ) 2.0 probably represents a compromise between NO production by the prompt mechanism at the premixed flame front and the thermal mechanism at the nonpremixed flame front. Acknowledgment. T.K.K. was supported by the Korea Science and Engineering Foundation (KOSEF), the Korea Institute of Machinery and Metals (KIMM), and also the G-7 Highly Advanced Project of Korea. This research was funded by the Office of Naval Research, with Dr. Gabriel Roy as technical monitor. EF960020F