Measurements and Modeling of OH and NO in Premixed C2

1092

Energy & Fuels 1997, 11, 1092-1100

Measurements and Modeling of OH and NO in Premixed C2H6/O2/N2 Flames at Atmospheric Pressure John R. Reisel* Mechanical Engineering Department, University of WisconsinsMilwaukee, Milwaukee, Wisconsin 53201-0784

Campbell D. Carter† Innovative Scientific Solutions, Inc., Dayton, Ohio 45440-3638

Normand M. Laurendeau‡ School of Mechanical Engineering, Purdue University, West Lafayette, Indiana 47907-1288 Received March 3, 1997X

We present laser-induced fluorescence (LIF) measurements of OH and NO concentrations in six premixed, atmospheric-pressure, laminar, flat C2H6/O2/N2 flames. The flames have a similar dilution ratio and total flow rate, while the equivalence ratios vary between 0.6 and 1.6. Using the data, we evaluate the predictions from two chemical kinetics models. These two models are the Glarborg, Miller, and Kee mechanism as modified by Drake and Blint (GMK-DB) and the GRI-Mech mechanism, version 2.11 (GRI). Two temperature profiles are used to generate the predictions from each model: a measured temperature profile and a predicted profile based on the energy equation. For the GMK-DB model, the measured temperature profile tends to give better results than the calculated temperature profile for the OH concentrations. Both the NO and OH concentration profiles are well predicted by the GMK-DB model in lean flames, while poorer agreement is generally obtained between measurements and modeling in the rich flames. The predictions for the GRI mechanism are satisfactory for both OH and NO in the lean flames but also become poorer in the richer flames. The two temperature profiles do not give significantly different results when using the GRI model in lean flames, but in rich flames, the measured temperature profile tends to give better agreement with the LIF measurements. The results indicate that while improved temperature measurements would be beneficial, further refinement of the chemical kinetics is required to improve the agreement between the predicted NO and OH concentration profiles in the rich flames. In particular, it is important that the rate coefficient for the reaction CH + N2 T HCN + N be more firmly established.

Introduction Nitric oxide (NO) is a major atmospheric pollutant formed during all practical combustion processes and which contributes to a variety of environmental problems. Attempts to alleviate the environmental problems to which NO contributes have resulted in initiatives to minimize NOx emissions from combustion processes. The achievement of this goal by combustion designers requires a thorough understanding of the chemical kinetics involved in the production of NO. In turn, such understanding mandates accurate in situ measurements of NO concentration for the purpose of refining chemical kinetics models. Nitric oxide is formed through three main reaction pathways:1 (1) the Zeldovich, or thermal-NO, pathway,2 * Address communications to this author at the Department of Mechanical Engineering, P.O. Box 784, University of Wisconsins Milwaukee, Milwaukee, WI 53201. Fax: (414) 229-6958. E-mail: [email protected]. † E-mail: [email protected]. ‡ E-mail:[email protected]. X Abstract published in Advance ACS Abstracts, September 1, 1997. (1) Drake, M. C.; Blint, R. J. Combust. Sci. Technol. 1991, 75, 261285. (2) Zeldovich, J. Acta Physiochem. URSS 1946, 21, 577-628.

S0887-0624(97)00034-0 CCC: $14.00

(2) the N2O-intermediate pathway,3,4 and (3) the promptNO pathway.5 The amount of NO formed through each depends on the temperature, pressure, and equivalence ratio of the flame. By maintaining lower flame temperatures (T < 1900 K), one may minimize the amount of NO formed in the postflame zone through the thermal-NO mechanism.6 This strategy permits analysis of the pathways which are particularly important in flamefront chemical kinetics; i.e., the N2O-intermediate and prompt-NO pathways. The N2O-intermediate pathway involves a three-body reaction resulting in the formation of N2O from N2 and O; the N2O subsequently reacts with O or H to form NO. The prompt-NO pathway is thought to proceed through the reaction7 (3) Wolfrum, J. Chem. Ing. Tech. 1972, 44, 656-659. (4) Malte, P. C.; Pratt, D. T. Combust. Sci. Technol. 1974, 9, 221231. (5) Fenimore, C. P. Thirteenth Symposium (International) on Combustion, [Proceedings]; The Combustion Institute: Pittsburgh, PA, 1971; pp 373-379. (6) Corr, R. A.; Malte, P. C.; Marinov, N. M. ASME J. Eng. Gas Turbines Power 1992, 114, 425-434. (7) Miller, J. A.; Bowman, C. T. Prog. Energy Combust. Sci. 1989, 15, 287-338.

© 1997 American Chemical Society

OH and NO in Premixed C2H6/O2/N2 Flames

CH + N2 T HCN + N

Energy & Fuels, Vol. 11, No. 5, 1997 1093

(R1)

Knowledge of the concentration profiles for OH and O is critical to an accurate prediction of the NO formed in the flamefront. Because of partial equilibrium between OH and O,8 accurate predictions of the OH concentration should lead to a good estimate of the O-atom concentration. Both the Zeldovich and N2Ointermediate pathways are enhanced by superequilibrium concentrations of oxygen atoms.1 Therefore, accurate prediction of the O-concentration profile is essential for good predictions of NO concentration. The OH and O radical concentrations are also important in the oxidation of hydrocarbon fuel, which directly affects the amount of prompt-NO formed. Moreover, it has been suggested that OH plays an important role in the behavior of prompt-NO formation at high pressure.9 Many practical combustion devices, such as gasturbine combustors, operate at pressures above atmospheric. However, analyzing the flamefront chemical kinetics of high-pressure flames is difficult, due to the smaller thickness of the flamefront at higher pressure, and also the accompanying reduction in distance between the flamefront and the burner surface. While it is possible to obtain measurements of OH concentration as a function of height above the burner in highpressure, low-temperature (T < 1800 K) flames,10 the peak of the profile is often not observable.11 In addition, the NO in such high-pressure flames is formed so near to the burner surface that the measurable profiles of the NO concentration as a function of height above the burner are essentially flat.12 Evaluation of the flamefront region is possible at atmospheric pressure, however.8 In this paper, we present laser-induced fluorescence (LIF) measurements of NO and OH concentrations in a series of premixed, atmospheric-pressure, flat, laminar, C2H6/O2/N2 flames. The flame conditions are given in Table 1. Some of the LIF data has appeared elsewhere.13,14 Using this data, we evaluate the predictions from two comprehensive chemical kinetics mechanisms: (1) the Glarborg, Miller, and Kee15 mechanism as modified by Drake and Blint (GMK-DB),1 and (2) the GRI (version 2.11) mechanism (GRI).16 We can thus evaluate the ability of these two models to predict NO and OH concentration profiles as a function of equivalence ratio (φ). Previously, we have looked in detail at the predictions of the GMK-DB mechanism.17 In this paper, we compare predictions from both models to the (8) Drake, M. C.; Ratcliffe, J. W.; Blint, R. J.; Carter C. D.; Laurendeau N. M. Twenty-third Symposium (International) on Combustion,[Proceedings]; The Combustion Institute: Pittsburgh, PA, 1991; pp 387-395. (9) Reisel, J. R.; Laurendeau, N. M. Combust. Sci. Technol. 1994, 98, 137-160. (10) Carter, C. D.; King, G. B.; Laurendeau, N. M. Appl. Opt. 1992, 31, 1511-1522. (11) Carter, C. D.; King, G. B.; Laurendeau, N. M. Combust. Sci. Technol. 1990, 71, 263-273. (12) Reisel, J. R.; Carter, C. D.; Laurendeau, N. M. Combust. Flame 1993, 92, 485-489. (13) Reisel, J. R.; Carter, C. D.; Laurendeau, N. M.; Drake, M. C. Combust. Sci. Technol. 1993, 91, 271-295. (14) Carter, C. D.; Laurendeau, N. M. Appl. Phys. B 1994, 58, 519528. (15) Glarborg, P.; Miller, J. A.; Kee, R. J. Combust. Flame 1986, 65, 177-202. (16) Bowman, C. T.; Hanson, R. K.; Davidson, D. F.; Gardiner, Jr., W. C.; Lissianski, V.; Smith, G. P.; Golden, D. A.; Frenklach, M.; Goldenberg, M. http://www.me.berkeley.edu/gri mech/, 1995.

LIF data and to each other so as to determine which model is best at predicting NO and OH concentrations in both lean and rich, atmospheric-pressure, premixed ethane flames. Experimental Methods Two different sintered-bronze, water-cooled, McKenna flatflame burners were used for this experiment. The NO measurements were taken using a 6 cm diameter burner, while the OH measurements were obtained using a 2.5 cm diameter burner. The total flow rates of the inlet gases were scaled, such that the mass flux across the burner surface was constant for the two burners. We have found that for the same mass flux, the smaller burner tends to produce slightly higher temperatures (∼25 K), but this temperature difference is not significant when uncertainties in the temperature measurements ((70 K) are considered. The gases were delivered and metered with electronic mass flow controllers which were calibrated with a bubble meter, as well as with wet and dry test meters. The NO measurements were performed with a Nd:YAGpumped dye laser (Quanta-Ray DCR-3G/PDL-2), which generated ultraviolet light at ∼225.5 nm. The short temporal pulses (∼10 ns) and narrow linewidths (∼0.6 cm-1) (fwhm) can lead to substantial saturation of the excited rovibronic transition in atmospheric-pressure flames. The advantage of saturation is that the fluorescence signal is insensitive to variations in both the collisional quenching rate coefficient and the laser power. However, the NO concentrations for the flames in this study are small (1-40 ppm); therefore, fluorescence from other species, which can be induced by the large laser powers, can interfere with the NO fluorescence signal, especially for rich conditions. To minimize such interferences, one must avoid regions of the excitation spectrum associated with species other than NO. In addition, when broadband fluorescence from such species interferes with the desired signal, the measurement can be improved by subtracting the off-resonance signal from the on-resonance signal to obtain an unbiased measurement.13 However, making an off-resonance measurement in the presence of saturation broadening is complicated by the spectral density of the NO lines. Consequently, for the measurements in the present investigation, we attenuated the laser energy (∼25 µJ/pulse) to ensure a large ratio (>10:1) of NO fluorescence to background in the flames, while still producing sufficient signal for meaningful NO measurements. Because its ground energy level is insensitive to variations in temperature at 1500-2000 K, we excited the Q2(27) line of the γ(0,0) band of NO, while monitoring the γ(0,1) band (∼236.5 nm). The γ(0,1) band is the strongest band emanating from the v′ ) 0 level18 and is well isolated from scattered light at 225.5 nm. The spatial resolution was 200 µm × 6.7 mm (while the beam width was ∼250 µm), with the minor dimension located in the streamwise direction (where the gradients of temperature and species concentrations are large). A temporal gate width of 6 ns was used for fluorescence detection. To isolate the γ(0,1) band, we used a 1/2 m monochromator with a passband (∼3 nm) sufficient to capture most of the γ(0,1) fluorescence. In addition, we blocked the lower half of the fluorescence beam to eliminate fluorescence vignetting by the burner.10 The calibration procedure was based on the observation that NO is not destroyed when added to lean flames. Thus, to calibrate the NO fluorescence voltages, we first measured the NO fluorescence signal in the post-flame zone of a φ ) 0.8 flame. Next, we replaced some of the pure N2 in the flame (17) Reisel, J. R.; Carter, C. D.; Laurendeau, N. M. Transport Phenomena in Combustion; Taylor and Francis: Washington, DC, 1996; pp 317-328. (18) Piper, L. G.; Cowles, L. M. J. Chem. Phys. 1986, 85, 24192422.

1094 Energy & Fuels, Vol. 11, No. 5, 1997

Reisel et al.

Table 1. Flame Conditions for This Investigation, Including Measured and Predicted Temperatures in the Postflame Zone (5 mm above the Burner Surface), and the Calculated Electronic Quenching Rate Coefficient for NO at the Measured Temperatures

φ

ψ

cold-flow velocity (cm/s)

0.6 0.8 1.0 1.2 1.4 1.6

3.1 3.1 3.1 3.1 3.1 3.1

10.4 10.4 10.4 10.4 10.4 10.4

measd temp (K)

GMK-DB predicted temp (K)

GRI predicted temp (K)

QNO (×108 s-1)

1633 1675 1814 1774 1814 1845

1621 1710 1813 1700 1708 1729

1644 1735 1850 1769 1796 1825

7.15 7.92 8.38 7.49 6.58 5.82

with N2 doped with a known concentration of NO, and again measured the fluorescence signal. A calibration factor was determined by using three levels of doping from ∼30 to ∼100 ppm. The calibration curve was linear within the measurement precision. We also monitored NO fluorescence signals in doped φ ) 0.6 and φ ) 1.4 flames. The φ ) 0.6 flame gave nearly the same calibration factor as obtained in the φ ) 0.8 flame (after accounting for different flame temperatures), while the results from the φ ) 1.4 flame indicated some destruction of the added NO. The measurements have not been corrected for changes in the quenching environment, since the electronic quenching rate coefficients do not vary significantly over the present range of experimental conditions. We have confirmed this assertion by calculating the electronic quenching rate coefficient for NO in the postflame zone of each flame.19 The associated values are listed in Table 1; on this basis, only the φ ) 1.6 flame lies slightly outside the (25% accuracy of the NO measurements. The OH measurements were obtained using the saturated fluorescence method.10 We excited the Q1(8) transition (309.3 nm) with ∼12 mJ/pulse of laser energy. The Q1(8) line was chosen to minimize changes in the Boltzmann fraction of the ground rovibronic level with varying temperature. Fluorescence from the P1(9) transition (312.5 nm) was isolated using the 1/2 m monochromator and detected with a fast-response photomultiplier tube and a 500 ps sampling gate (which was located at the peak of the fluorescence pulse). The spatial resolution was 83 µm × 1.7 mm (while the beam width was ∼200 µm). Potential sources of measurement biasing include fluorescence vignetting and self-absorption. As with the NO measurements, we blocked the lower half of the fluorescence beam to avoid vignetting of the signal by the burner surface. In addition, self-absorption was reduced by decreasing the fluorescence path length in the flame; i.e., we sampled nearer the burner edge at larger OH concentrations. Hence, as we sampled further from the flamefront (i.e., at greater heights above the burner), we translated the burner horizontally to maintain the one-dimensionality of the flame. As shown by Carter et al.,10 the variation in OH concentration horizontally across the burner is minimal, but the length across the burner not affected by edge effects does become smaller with increasing distance above the burner. Therefore, in order to obtain measurements approximating a one-dimensional flame, it was necessary to shift the probe volume closer to the burner centerline with increasing distance above the burner surface. Thus, for the data points at y ) 0.5 mm, the distance from the centerline was 8.5 mm, while for those at 9 mm, this distance was only 2 mm. Note that the choice of these sampling positions was based on horizontal OH LIF profiles detecting the weak O12(10) satellite transition. In spite of these efforts, a significant source of uncertainty in the OH measurements is from fluorescence trapping (∼10%) near the surface (under lean to stoichiometric conditions) and departure from one-dimensionality downstream (∼10%). (19) Paul, P. H.; Carter, C. D.; Gray, J. A.; Durant Jr., J. L.; Furlanetto, M. R., Sandia Report SAND94-8237; Sandia National Laboratories: Livermore, CA, 1994.

The fluorescence voltages were calibrated by comparison to laser absorption measurements in the φ ) 0.8 flame at heights above the burner of 1, 2, and 4 mm. At a height of 1 mm, five lines were employed for the absorption measurement, while only the Q1(8) transition was used at 2 and 4 mm above the burner. To obtain a path length for absorption, we recorded the OH fluorescence as a function of burner horizontal position. Self-absorption necessitated using a weak fluorescence line; thus we used the O12(10) transition. These measurements were obtained for heights of 1 and 4 mm. As with the NO measurements, we used the same calibration factor for all of the OH fluorescence voltages. This strategy is valid since the sensitivity of the fluorescence to collisional deexcitation (rotational energy transfer and electronic quenching) is only 18% for our saturated fluorescence conditions.10,14

Chemical Kinetics Modeling All of the experimental flames in this study were investigated through computer modeling. The modeling of the chemical kinetics was performed using the Sandia steady, laminar, one-dimensional, premixed flame code.20 In addition, the CHEMKIN-II computer program library21 was used to process the reaction mechanisms into forms appropriate for use by the flame code. In this work, we are evaluating the capabilities of the GMK-DB and the GRI chemical kinetics mechanisms with respect to prediction of the NO and OH concentration profiles. We are also evaluating the results for two different temperature profiles for each mechanism. First, we use a temperature profile as measured with radiation-corrected, silica-coated, Pt/Pt-10% Rh thermocouples. To assess the capabilities of the models when measured temperature profiles are not available, we also use the coupled species-energy equations to determine predicted temperature profiles for each model. This strategy is important for application to highpressure flames, for which the rapid temperature rise through the much thinner flamefront is not as easily measured. A burner surface temperature of 300 K is used as a boundary condition for the modeling. The first mechanism used in the modeling is taken from the set of elementary reactions listed by Drake and Blint.1 This reaction mechanism considers 49 species and over 200 reactions. The thermodynamic and transport properties, required by the Sandia flame code for calculation of the species concentration profiles, were provided by a thermodynamic property database22 and a transport property database.23 Drake and Blint1 adopted most of the reaction mechanism from Glarborg et al.15 though with some modifications. These include the addition of a C3H8 reaction mechanism, and the introduction of rate parameters (units of moles, cm3, K, cal/mol) for reaction R1 based on measurements in a high-temperature shock tube:24 (20) Kee, R. J.; Grcar, J. F.; Smooke, M. D.; Miller, J. A. Sandia Report SAND85-8240; Sandia National Laboratories: Livermore, CA, 1985. (21) Kee, R. J.; Rupley, F. M.; Miller, J. A. Sandia Report SAND898009; Sandia National Laboratories: Livermore, CA, 1989. (22) Kee, R. J.; Rupley, F. M.; Miller, J. A. Sandia Report SAND878215; Sandia National Laboratories: Livermore, CA, 1987. (23) Kee, R. J.; Dixon-Lewis Jr.; G., Warnatz, J.; Coltrin, M. E.; Miller, J. A. Sandia Report SAND86-8246; Sandia National Laboratories: Livermore, CA, 1986. (24) Dean, A. J.; Davidson, D. F.; Hanson, R. K.; Bowman, C. T. Development and application of CH laser absorption diagnostic for shock tube kinetic studies. Western States Section/The Combustion Institute, Paper 88-91, 1988.



Table 2. Measured Temperature Profiles for the Six C2H6/O2/N2 Flames of This Studya

Table 3. Calculated Temperature Profiles Using the Energy Solution of the GMK-DB Mechanism

temperature (K)

calculated temperature (K)

height (mm) φ ) 0.6 φ ) 0.8 φ ) 1.0 φ ) 1.2 φ ) 1.4 φ ) 1.6 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0

1069 1129 1211 1289 1357 1415 1476 1535 1569 1590 1605 1614 1622 1629 1634 1636 1638 1639 1639 1641 1642 1641 1639 1638 1636 1633 1630 1629 1629 1626 1624 1621 1618 1617 1613 1612

1204 1315 1432 1525 1583 1622 1644 1659 1665 1672 1676 1680 1680 1683 1683 1681 1683 1684 1684 1684 1683 1683 1681 1679 1675 1675 1672 1665 1668 1665 1663 1662 1658 1656 1655 1651

1211 1370 1473 1580 1662 1715 1737 1753 1765 1773 1780 1786 1791 1793 1795 1798 1800 1802 1805 1806 1811 1812 1815 1815 1814 1814 1812 1810 1809 1807 1805 1804 1801 1800 1797 1795

1227 1372 1504 1579 1663 1716 1758 1776 1782 1786 1789 1790 1792 1790 1792 1792 1792 1792 1790 1790 1789 1786 1784 1780 1777 1774 1770 1766 1763 1761 1757 1754 1750 1747 1744 1739

814 1088 1339 1438 1551 1641 1720 1773 1811 1834 1843 1847 1847 1846 1845 1843 1841 1841 1838 1837 1830 1826 1822 1819 1815 1814 1814 1814 1814 1812 1810 1806 1803 1800 1797 1796

840 894 959 1039 1127 1255 1379 1461 1537 1626 1705 1760 1798 1825 1844 1852 1860 1861 1863 1863 1860 1856 1853 1850 1848 1845 1842 1841 1838 1836 1833 1830 1829 1826 1825 1821

a The temperatures were measured with a radiation-corrected Pt/Pt-10% Rh thermocouple.

k1,f ) 4.2 × 1012 exp(-20400/RT) This model was chosen for evaluation because it has been found previously to predict fairly well the postflame NO concentrations in high-pressure flames.9 The second mechanism used in the modeling is the GRI mechanism, version 2.11.16 The GRI mechanism contains 49 species and 277 reactions. This mechanism was employed as presented, using the thermodynamic and transport properties in files provided with the GRI mechanism. The evolving GRI mechanism is becoming increasingly popular for use with methane flames. It was chosen for evaluation in this work in order to provide guidance for those who may attempt to use the mechanism for the modeling of ethane flames. Results and Discussion Figures 1-6 present measurements and predictions of OH concentrations for a series of laminar, flat, premixed, atmospheric-pressure C2H6/O2/N2 flames with the same constant total flow rate. Figures 7-12 present measurements and predictions of NO concentrations for the same flames. The flames each have a dilution ratio (N2 to O2 volumetric flow rates) of ψ ) 3.1. The equivalence ratios of the flames are in the range 0.6 e φ e 1.6. The figures display, for either OH or NO, the LIF measurements and the results obtained from modeling these flames using the thermocouple-based

height (mm) φ ) 0.6 φ ) 0.8 φ ) 1.0 φ ) 1.2 φ ) 1.4 φ ) 1.6 0.00 0.13 0.25 0.38 0.50 0.63 0.75 0.88 1.00 1.25 1.50 1.75 2.00 2.50 3.75 5.00 7.50 10.0

300 495 701 917 1138 1319 1414 1464 1500 1545 1572 1587 1598 1608 1618 1621 1623 1624

300 656 967 1239 1403 1481 1530 1567 1593 1630 1652 1668 1678 1691 1704 1710 1714 1715

300 758 1116 1367 1492 1555 1597 1629 1653 1689 1714 1733 1747 1768 1795 1813 1830 1840

300 730 1074 1339 1491 1561 1595 1615 1629 1648 1660 1669 1676 1684 1695 1700 1705 1707

300 632 921 1189 1395 1523 1590 1625 1644 1665 1677 1684 1689 1696 1704 1708 1711 1713

300 547 778 1008 1230 1408 1526 1596 1637 1676 1694 1704 1710 1718 1726 1729 1731 1732

Table 4. Calculated Temperature Profiles Using the Energy Solution of the GRI-Mech temperature (K) height (mm) φ ) 0.6 φ ) 0.8 φ ) 1.0 φ ) 1.2 φ ) 1.4 φ ) 1.6 0.00 0.13 0.25 0.38 0.50 0.63 0.75 0.88 1.00 1.25 1.50 1.75 2.00 2.50 3.75 5.00 7.50 10.0

300 461 640 833 1038 1245 1409 1486 1525 1572 1599 1614 1624 1634 1641 1644 1646 1647

300 642 946 1228 1435 1526 1572 1606 1630 1664 1684 1698 1708 1719 1730 1735 1739 1740

300 741 1096 1385 1545 1611 1650 1679 1701 1734 1758 1775 1789 1809 1835 1850 1866 1876

300 700 1031 1322 1532 1636 1678 1698 1709 1724 1734 1741 1747 1755 1764 1769 1774 1776

300 596 865 1125 1365 1548 1656 1713 1743 1770 1781 1786 1790 1793 1795 1796 1796 1796

300 481 667 862 1063 1264 1444 1580 1669 1756 1790 1805 1813 1819 1824 1825 1826 1826

temperature profile and the predicted temperature profile for both the GMK-DB and GRI chemical kinetics mechanisms. Table 1 includes a list of the temperatures found via solution of the coupled species-energy equations at a height of 5 mm above the burner surface for both models. Table 2 contains a listing of the thermocouple measurements for these flames, as measured along the centerline in the 6 cm burner. The thermocouple was treated as a cylinder with a measured diameter of 100 µm. The radiation correction was accomplished using the correlation of Bradley and Matthews25 and considering the emissivity equal to 0.22. We estimate the temperature uncertainty at the 95% confidence level to be (70 K. Table 3 lists the calculated temperature profiles as found from the energy solution using the GMK-DB model, while Table 4 lists the calculated temperature profiles found using the GRI model. As can be seen, the GMK-DB model predicts the postflame temperature well for the lean and stoichiometric flames, but tends to underpredict these temperatures by as much as 100 K for the rich flames. The GRI model produces generally good postflame temperature predictions at all stoichiometries, although the (25) Bradley, D.; Matthews, K. J. J. Mech. Eng. Sci. 1968, 10, 299305.


Reisel et al.

Table 5. Measured OH Number Densities (cm-3) in the Six Atmospheric-Pressure Premixed C2H6/O2/N2 Flamesa equivalence ratio

a

height (mm)

0.6

0.8

1.0

1.2

1.4

1.6

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5

1.70 E13b 1.87 E14 1.01 E15 3.62 E15 7.67 E15 1.05 E16 1.14 E16 1.14 E16 1.14 E16 1.10 E16 1.06 E16 9.89 E15 9.06 E15 7.98 E15 7.01 E15 6.44 E15 5.86 E15 5.25 E15 4.89 E15 4.35 E15 3.63 E15 3.02 E15 2.63 E15 2.32 E15 2.13 E15 1.91 E15 1.78 E15 1.68 E15 1.56 E15 1.52 E15 1.48 E15 1.40 E15 1.32 E15

1.89 E15 5.90 E15 1.01 E16 1.27 E16 1.34 E16 1.36 E16 1.32 E16 1.30 E16 1.23 E16 1.19 E16 1.17 E16 1.08 E16 1.01 E16 9.35 E15 8.45 E15 8.03 E15 7.39 E15 6.85 E15 6.34 E15 6.12 E15 5.39 E15 4.73 E15 4.30 E15 3.91 E15 3.63 E15 3.38 E15 3.15 E15 2.97 E15 2.81 E15 2.71 E15 2.56 E15 2.46 E15 2.38 E15


1.58 E15 4.01 E15 6.44 E15 7.98 E15 8.36 E15 8.26 E15 7.94 E15 7.59 E15 7.17 E15 6.70 E15 6.52 E15 6.18 E15 5.60 E15 5.23 E15 4.60 E15 4.27 E15 4.02 E15 3.63 E15 3.45 E15 3.06 E15 2.56 E15 2.21 E15 1.95 E15 1.64 E15 1.48 E15 1.35 E15 1.20 E15 1.09 E15 9.66 E14 9.27 E14 8.29 E14 7.65 E14


4.59 E13 1.05 E14 2.97 E14 6.76 E14 1.12 E15 1.41 E15 1.47 E15 1.40 E15 1.32 E15 1.17 E15 1.06 E15 9.35 E14 7.55 E14 6.21 E14 5.28 E14 4.53 E14 3.82 E14 3.55 E14 3.11 E14 2.86 E14 2.40 E14 2.00 E14 1.79 E14 1.67 E14 1.46 E14 1.40 E14 1.30 E14 1.27 E14 1.18 E14 1.14 E14 9.97 E13 9.04 E13

The dilution ratio of each flame is 3.1. The estimated accuracy of the measurements is (20%. b 1.70 E13 ≡ 1.70 × 1013. Table 6. Measured NO Number Densities (cm-3) in the Six Atmospheric-Pressure Premixed C2H6/O2/N2 Flamesa equivalence ratio

a

height (mm)

0.6

0.8

1.0

1.2

1.4

1.6

0.5 0.75 1.0 1.5 2.0 2.5 3.0 3.5 4.0 5.0 6.0 7.0 8.0 9.0 10.0

7.56 E12 9.20 E12 9.54 E12 1.06 E13 1.16 E13 1.13 E13 1.19 E13 1.22 E13 1.15 E13 1.22 E13 1.19 E13 1.22 E13 1.24 E13 1.21 E13 1.21 E13






The dilution ratio of each flame is 3.1. The estimated accuracy of the measurements is (25%.

prediction is somewhat high for the φ ) 0.80 flame. In addition, for both models the measured temperatures are higher than the predicted temperatures near the burner surface. In part, this may reflect the difficulty of resolving the steep temperature gradient near the burner surface with a thermocouple. Table 5 contains a listing of the OH measurements found through LIF for the six flames at various heights above the burner. Table 6 contains a similar presentation of the experimental data found with LIF for the NO measurements. This data is provided to facilitate the comparison of future chemical kinetics models to these ethane flames. Figures 1-3 present the results for the OH measurements and modeling in the lean and stoichiometric

flames. The LIF measurements of OH have a precision of ∼4% and an estimated uncertainty of ∼20%. For these flames, both the GMK-DB and GRI mechanisms give excellent predictions of the OH concentration profiles when using both the measured and predicted temperature profiles. The higher temperatures near the burner surface for calculations using the measured temperature profiles lead to a slightly more rapid increase in the OH concentration than for the calculated temperature profiles, but this distinction does not appear to significantly affect the predictions of OH concentration. Figures 4-6 present the results for the OH measurements and modeling for the three rich ethane flames. Unlike the lean and stoichiometric flames, the predicted


Figure 1. Measurements and predictions of [OH] in a φ ) 0.60, ψ ) 3.1, C2H6/O2/N2 flame. Shown are the LIF measurements of OH, and the predicted OH as found with the GMKDB and the GRI mechanisms for both the thermocouple (TC) and energy solution temperature profiles.


Figure 3. Measurements and predictions of [OH] in a φ ) 1.00, ψ ) 3.1, C2H6/O2/N2 flame.

Figure 4. Measurements and predictions of [OH] in a φ ) 1.20, ψ ) 3.1, C2H6/O2/N2 flame. Figure 2. Measurements and predictions of [OH] in a φ ) 0.80, ψ ) 3.1, C2H6/O2/N2 flame.

OH concentration profiles in the rich flames differ considerably when using the two temperature profiles, especially for the GMK-DB mechanism. The results for the φ ) 1.2 flame, while not as good as those for the lean flames, are still satisfactory. However, the agreement between the OH predictions and measurements becomes significantly worse with increasing equivalence ratio. The best predictions for the OH concentration profiles in the rich flames come consistently from using the measured temperature profiles and the GMK-DB mechanism; in fact, the predictions when using the measured temperature profiles and the GMK-DB model

are excellent for the φ ) 1.2 and φ ) 1.4 flames. For the φ ) 1.4 and φ ) 1.6 flames, the energy solution with the GMK-DB model and both temperature profiles with the GRI mechanism tend to give OH predictions considerably lower than the measured OH concentrations. The results found with the GMK-DB mechanism indicate that the measured temperature profile leads to more accurate OH predictions that those obtained when using the GMK-DB predicted temperatures in the modeling of the rich flames. Due to the uncertainties of the temperatures in these flames, it is difficult to tell whether the measured temperature profile is better than the GMK-DB predicted temperature profile, or if there are errors in both the measured temperatures and the chemical kinetics which cancel out. The results



Reisel et al.

Figure 7. Measurements and predictions of [NO] in a φ ) 0.60, ψ ) 3.1, C2H6/O2/N2 flame. Shown are the LIF measurements of NO, and the predicted NO as found with the GMKDB and the GRI mechanisms for both the thermocouple (TC) and energy solution temperature profiles.


when using the GRI model suggest that this mechanism does not handle the chemical kinetics of ethane combustion in rich flames well (at least not as well as the GMKDB mechanism). Further refinement of the GRI mechanism is thus required for applications to ethane combustion at φ g 1.2. Both models are very deficient for the richest flame (φ ) 1.6). The best predictions for the φ ) 1.6 flame appear to be from the GMK-DB mechanism with the measured temperature profile. Although the match between magnitudes of the predicted and measured OH concentrations is reasonable, the predicted profile appears shifted ∼0.5 mm downstream in comparison to the LIF measurements. In summary, both models predict the OH concentration profiles in lean, stoichiometric, and slightly rich

Figure 8. Measurements and predictions of [NO] in a φ ) 0.80, ψ ) 3.1, C2H6/O2/N2 flame.

flames (0.6 < φ < 1.2) well. As the flames become richer, however, the two mechanisms (except for the GMK-DB model with the measured temperature profile) tend to strongly underpredict the measured OH concentrations. Moreover, the temperature profiles predicted from the energy equation are generally inadequate for accurate modeling of the richer flames. Figures 7-9 present the results for the NO measurements and modeling in the lean and stoichiometric flames. The LIF measurements of NO have a precision of ∼4% and an estimated uncertainty of ∼25%. For the φ ) 0.60 and φ ) 1.0 flames, both models with either



the measured or predicted temperature profiles give good predictions for the NO concentrations, particularly in the postflame zone. The predictions tend to be a somewhat low in the φ ) 0.80 flame. The biggest effect of the various temperature profiles occurs in the region near the burner surface. In particular, for these three flames, the predicted NO concentrations increase more rapidly near the burner surface when using the measured temperature profiles. This more accurately corresponds to the measured NO. That is, the resulting higher NO reaction rates at the flamefront lead to larger predicted NO concentrations near the burner. Regardless of the temperature profile employed, the measured NO concentrations near the burner surface are higher than the predicted values. Figures 10-12 present the results for the NO measurements and modeling in the rich flames. Here there is a substantial difference in the NO predictions between the two models. For the GMK-DB model, the predicted NO concentration profiles found when using the measured temperature profiles typically follow the shape of the measured NO profiles slightly better than those based on the energy solution. However, in this case, the energy solution provides better quantitative agreement with the NO measurements. For all three rich flames, the NO predictions from the GRI model are significantly lower than the correspondingly measured NO profiles. However, unlike the GMK-DB model, the measured temperature profile in conjunction with the GRI mechanism seems to give better NO predictions as compared to the energy solution. This arises from the higher measured temperatures, which yield somewhat higher predictions for the NO concentrations. Because of the strong influence of prompt-NO in these rich flames9 and the subsequent dependence of prompt-NO on hydrocarbon chemistry, further work appears to be necessary with respect to the hydrocarbon chemical kinetics in both models. As these models were developed primarily for use in methane flames, the unfavor-




able results for the rich ethane flames are not completely unexpected. On the other hand, much of the quantitative error in these rich flames could be alleviated through refinement of the rate coefficient for reaction R1. As indicated by Drake and Blint,1 substantial uncertainty exists in the rate coefficient for reaction R1; moreover, the amount of prompt NO formed is very sensitive to this rate coefficient. In fact, much of the quantitative difference between the results of the GMK-DB and GRI models can be attributed to the difference in the rate coefficients employed for reaction R1. Substitution of the rate coefficient used for reaction R1 in the GMK-DB model into the GRI model yields NO predictions from the GRI


Reisel et al.

tion of NO in lean flames. In addition, the flame temperatures are low enough so as to never result in significant thermal-NO, in spite of the rather large superequilibrium O-atom concentrations. As expected, the prompt-NO pathway gains in importance with increasing equivalence ratio. Finally, note that the temperature profiles used in the modeling, while changing the quantitative NO concentrations formed through each pathway, exert less influence on the relative amounts of NO formed through these pathways. This behavior is related to the similarity of the measured and calculated temperatures in the postflame region. Conclusions

Figure 12. Measurements and predictions of [NO] in a φ ) 1.60, ψ ) 3.1, C2H6/O2/N2 flame. Table 7. Calculated Amounts of NO (ppm) Formed at 7.5 mm through Each Pathway for the Lean and Stoichiometric Flames, by Using the GMK-DB Mechanisma φ

temp profile

0.6 0.8 1.0 0.6 0.8 1.0

energy solution energy solution energy solution thermocouple thermocouple thermocouple

thermal N2O-intermediate prompt-NO 0.17 0.58 1.91 0.25 0.68 2.97

2.04 3.20 5.18 2.68 4.12 7.29

0.15 0.72 6.26 0.25 0.88 7.00

a Both the energy solution and the measured temperature profiles are employed.

mechanism substantially closer to those from the GMKDB mechanism. For example, the GRI model with the substituted (R1) rate coefficient increases the postflame NO concentration by ∼25% in the φ ) 0.80 flame and by ∼300% in the φ ) 1.4 flame. Moreover, other complications remain; in particular, the CH kinetics remain uncertain and could prove substantially different for ethane as compared to methane flames. Finally, we have used the GMK-DB predictions to estimate the amount of NO formed through each of the three pathways.9 The results of this study for the lean flames when using both temperature profiles are shown in Table 7. The calculated distribution represents the NO produced in the first 7.5 mm above the burner. Results for the rich flames are not shown, for while almost all of the NO should arise from the prompt-NO mechanism, the accuracy of the chemical kinetics models in the rich flames is not good enough to warrant definitive conclusions. As indicated in Table 7, the N2Ointermediate pathway is quite important to the forma-

A series of flat, laminar, premixed, atmosphericpressure, C2H6/O2/N2 flames has been modeled using two comprehensive chemical kinetics mechanisms, and the predicted NO and OH concentration profiles have been compared to those measured using LIF. The GMK-DB and GRI chemical kinetics models were employed to provide NO and OH concentration profiles when using both measured and predicted temperature profiles. For the GMK-DB mechanism, we found that using the measured temperature profiles in the modeling tended to give better agreement with experiment in comparison to using the predicted temperature profiles. Both the NO and OH concentration profiles were well predicted in lean flames, while there tended to be worse agreement between measurements and modeling in the rich flames. Although the GMK-DB mechanism predicted the shape of the NO profiles in rich flames better when using the measured temperature profiles, improved quantitative agreement occurred when using the predicted temperature profiles; however this agreement may be fortuitous rather than an indication that the model truly describes the chemistry of these flames. For the GRI mechanism, the predictions of NO and OH in the lean flames were satisfactory when using both temperature profiles, but problems existed in the rich flames. The prediction of OH in the φ ) 1.2 flame was still acceptable for the GRI mechanism, but the predictions of NO and of OH in the richer flames were unsatisfactory. Finally, while improved temperature measurements would be helpful, we believe that refinement of the chemical kinetics is required to improve the agreement between the predicted and measured NO and OH concentrations, particularly in the rich flames. In particular, it is important that the rate coefficient for reaction R1 be more firmly established. Acknowledgment. The authors thank the NASALewis Research Center for funding this investigation, and Dr. Michael C. Drake of the General Motors Research Laboratories for his assistance and suggestions. EF9700341

Measurements and Modeling of OH and NO in Premixed C2

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