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CH∗ chemiluminescence modeling for combustion diagnostics
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Proceedings of the
Combustion Institute
Proceedings of the Combustion Institute 32 (2009) 895–903
www.elsevier.com/locate/proci
CH* chemiluminescence modeling for combustion diagnostics Venkata N. Nori *, Jerry M. Seitzman School of Aerospace Engineering, Georgia Institute of Technology, 270 Ferst Drive, Guggenheim 364, Atlanta, GA 30332-0150, USA
Abstract CH* chemiluminescence is often employed in combustion diagnostics, for example as a measure of heat release rate and for equivalence ratio sensing. However, most interpretations of CH* chemiluminescence rely either on heuristic arguments or empirical data gathered under limited conditions. More rigorous analysis is required to understand the effects of combustion conditions, e.g., pressure, reactant composition and preheat, strain and exhaust gas recirculation, on CH* chemiluminescence. Chemiluminescence modeling holds promise in this regard. The predictive accuracy of four proposed CH* chemiluminescence formation models were experimentally tested in premixed, methane–air and prevaporized Jet-A–air flames. Two of the models, based on CH* formation via reactions between C2H and O or O2, are able to predict the experimental data within the experimental uncertainty, in both room temperature methane and preheated Jet-A flames. The utility of CH* chemiluminescence for sensing heat release rate and equivalence ratio (/), when combined with OH* chemiluminescence, is then analyzed in methane flames for varying pressure, preheat temperature and strain. The CH*/OH* chemiluminescence ratio is found to be useful for sensing equivalence ratio in lean methane systems, but only at certain pressure and reactant temperature conditions. The relationship between CH* chemiluminescence and heat release varies with /, pressure, temperature and strain. At high pressures, however, the dependence on / and strain are small, making CH* attractive for heat release sensing applications in gas turbine combustors. Ó 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: CH* chemiluminescence; Equivalence ratio sensing; Heat release rate sensing; Combustion diagnostics; Chemiluminescence modeling
1. Introduction Combustion diagnostics based on flame chemiluminescence are of great interest for their simplicity and non-intrusive nature. Chemiluminescence is light emitted by molecules chemically created in an excited energy state when they radiatively relax to a *
Corresponding author. Fax: +1 404 463 0888. E-mail address: [email protected] (V.N. Nori).
lower energy state. Common chemiluminescent species in hydrocarbon–air flames are CH*, OH*, C2 and CO2 [1]. Chemiluminescence can potentially provide information about reaction zone conditions that relate to combustor performance, pollutant formation and combustor health. For example, chemiluminescence has been used for equivalence ratio (/) sensing in both gaseous and liquid fueled systems [2–4], in flame front location [5] and in characterizing temporal and spatial heat release fluctuations in combustion dynamics
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studies [6]. However, quantitative interpretation of chemiluminescence signals has been based historically on heuristic arguments or empirical data gathered under limited conditions. Additionally, care must be taken to interpret these line of sight global measurements for flames with widely varying conditions [7]. Therefore, a number of experimental investigations have examined chemiluminescence, primarily from OH* and CH* in premixed methane–air systems, while varying conditions such as pressure [8,9] and strain [8,10]. The ability to model and predict chemiluminescence would provide an alternate, more flexible approach to understanding the dependence of chemiluminescence signals on key combustion parameters. Such models would also be of great utility in developing combustion diagnostics/sensors and interpreting the resulting data. Reliable chemical kinetics simulations of chemiluminescence require accurate modeling of the formation rates of the excited state species. Thus the chemiluminescence reactions and their rate constants must be known. In addition, the need to simulate the concentrations of the chemiluminescence precursor and quencher species requires a reliable detailed chemical kinetic mechanism for the particular fuel-oxidizer system of interest. While a number of studies have focused on identifying formation reactions and their rate parameters for OH* and CH*, only a few studies have attempted to validate proposed models in combustion systems [11,12]. Most of these studies were limited to methane–air mixtures. However, OH* and CO2 chemiluminescence models were recently validated in methane and CO/H2 (syngas) flames at atmospheric pressure under varying operating conditions such as equivalence ratio, syngas fuel composition, preheat and dilution [12]. Therefore, the goal of this work is to examine existing chemiluminescence mechanisms for CH*, by incorporating them into accepted flame chemistry models, and validate them with experimental data using different fuels. Methane and Jet-A were chosen for this evaluation, because they are highly relevant to many combustion systems in power generation and aeropropulsion applications. Moreover, this paper focuses on lean combustion as it is interesting for low NOx gas turbines. Implications of using CH* chemiluminescence for sensing heat release rate and equivalence ratio are also discussed. 2. CH* mechanisms The primary CH* emission in the ultraviolet and visible region of the spectrum is due to the A2D ? X2P (431 nm) and B2R ? X2P (390 nm) transitions, with the former usually dominating [1]. To simulate CH* emission, a chemiluminescence mechanism requires excited
state formation reactions and their rates; and the removal rates by collisional quenching and radiative relaxation. Radiative rates and CH* quenching data for major species of hydrocarbon combustion are available [13]. Of the various proposed sources for CH*, research has focused on the following: C2 þ OH ! CH þ CO C2 H þ O ! CH þ CO C2 H þ O2 ! CH þ CO2
ðR1Þ ðR2Þ ðR3Þ
Gaydon [1] suggested R1, which was later challenged by Brenig [14] and Grebe and Homann [15]. Brenig’s experiments suggested that CH* formed from the reaction of ground state ethynyl radicals (C2H) with O atoms, as proposed earlier [16]. A later shock tube study with methane–hydrogen mixtures supports R1 and R2 as the dominant CH* formation pathways for conditions in the range 1200–2300 K and 0.6–2.2 atm [17]. R1 may also be important in higher order hydrocarbon (liquid) fuel systems where C2 is relatively more abundant. Devriendt et al. [18] in a pulsed laser photolysis study at low pressure determined the temperature dependence of R2 and concluded that the majority of CH* is produced by that reaction. Renlund et al. [19] suggested the importance of C2H reacting with O2 (R3) rather than O. In a flash photolysis study of acetylene at low pressure [20], the temperature dependence of R3 was evaluated, and the relevance of R3 in hot flames and fuel lean conditions was suggested. Further studies measured absolute excited state concentrations of CH* at low pressure in methane–air premixed flames, and rate parameters for R2 and R3 were determined [21,22]. In this study, four models proposed in the literature (designated by the last author of the reference source) were used to model CH* in lean flames. Each is based on two of the formation reactions (R1–R3); the reaction rate parameters associated with each are listed in Table 1. In flames, electronically excited species in general have low concentrations due to their low production rates and their rapid removal by collisional quenching. They therefore often have little impact on the overall flame chemistry. For these reasons, CH* is typically assumed to be in quasi-steady state and limited by the formation rate [12]. Under these conditions, the concentration of CH* (e.g., moles/cm3) is given by ½CH ¼
k 1 ½C2 ½OH þ k 2 ½C2 H½O þ k 3 ½C2 H½O2 P j k Q;j ½M j þ A ð1Þ
where k1–k3 are the rate constants for the corresponding formation reactions, kQ,j is the quenching rate constant for CH* by species j and A is the Einstein coefficient for spontaneous emission for the
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Table 1 Chemiluminescence reaction mechanism to model CH* formation #
Reaction
A
R1 R2
C2 + OH M CH* + CO C2H + O M CH* + CO
R3
C2H + O2 M CH* + CO2
R4 R5
H + O + M M OH* + M CH + O2 M OH* + CO
2 1014 2.5(±0.8) 1012 5.2 1011 1.08(±0.4) 1013 6.023(±3.0) 1012 3.2(±1.0) 1011 2.17(±0.8) 1010 6.023 104 6 1014 1.173 1014
b 0 0 0 0 0 0 0 4.4 0.0 0.4
Ea 0 0 2600 0 457 1599 0 2285 6940 4150
Ref. Petersen [17] Smith [22] Petersen [17] Peeters [18] Carl [20] Smith [22] Peeters [18] Carl [20] [12] [12]
Rate coefficients are expressed as k = ATbexp(Ea/RT) in units of cal, mol, cm and s. Quenching reactions are given in [13]. Einstein A coefficient for CH* is 1.85 106 s1 [13].
CH* transition. The photon emission rate i (mole photons/cm3/s) can then be found from iCH ¼ A½CH
ð2Þ
3. Experimental setup 3.1. Combustors Chemiluminescence spectra were obtained in two atmospheric premixed fuel systems: a laminar jet flame (methane and Jet-A fuels) and a swirlstabilized combustor (methane only). The laminar burner is a straight cylindrical stainless steel tube (Fig. 1). For the methane experiments, two different tubes were used, with inner diameters of 18 and 25 mm. For flames with / < 0.8, the 25 mm burner with pilot-stabilization was used. The smaller burner was employed for near stoichiometric mixtures (high flame speeds) to maintain laminar flow. The burner lengths (>1 m) were
Fiberhead
L
12.5º
To Spectrometer
CH 4-Air pilot
TC1 TC2 Calibrated Air
N2 pressurized Jet-A
3.2. Detection optics Temperature controller
Pre-vaporized Jet-A/Air TC 3
chosen to ensure a fully developed laminar exit velocity profile. Calibrated rotameters determined the flow rates with ±1% accuracy, or a 1.5% uncertainty in /. For Jet-A, the 25 mm tube, with no pilot, was used for mixtures with / < 0.7; otherwise a 10 mm tube was employed. Being a liquid fuel, the Jet-A was first vaporized to obtain a laminar premixed flame. As shown in Fig. 1, the liquid fuel, pressurized with N2 and monitored with a calibrated rotameter, was vaporized in a straight tube maintained at a wall temperature of 573 K, and then mixed with preheated air at 393 K to prevent fuel condensation. Further heating of the mixture was feedback controlled with a thermocouple placed close to the burner exit. Electrical resistance tape surrounding the burner tube provided heating. The fuel rotameter had a ±3% accuracy, resulting in a maximum uncertainty of 4% in /. The swirl-dump combustor, the same employed in an earlier study [3], was formed from a 70 mm inner diameter, 127 mm long quartz tube. The reactants enter from a 23 mm diameter tube, with swirl vanes upstream producing a flow with a theoretical inlet swirl number of 0.66. The data presented here corresponds to a bulk average (cold) velocity of 4–5 m/s in the test section and a Reynolds number of 20,000.
CH 4 /Air Main Flow
Tape-Heater
Fig. 1. Schematic of Jet-A and methane experimental setup (TC, thermocouple). The pilot was used to stabilize methane–air flames in the 25 mm burner.
Optical emission was detected with a fiberoptic based collection system coupled to an imaging spectrometer. Detailed description of the collection optics, spectrometer and the intensified CCD camera can be found in [3]. In most cases, 10 images were acquired at each operating condition, with a camera exposure time of 100 ms. The 10 long exposure images were averaged and background corrected before extracting the CH* chemiluminescence signal, S CH , which
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was found by integrating the background corrected A2D ? X 2P transition over a bandwidth of ±5 nm. In the laminar jet flames, the total chemiluminescence was captured by placing the fiber along a line perpendicular to the flame axis. The distance (L) from the flame to the fiber was chosen such that the solid angle variation across the flame as seen by the fiber becomes independent of L. This is achieved at L P 11h, where h is the flame height. For the swirl combustor, the fiber was placed at a fixed location 9.5 cm from the combustor wall and 6.5 cm above the base of the combustor. This allowed the fiber to collect light from most of the flame region. Due to the swirl-stabilization, the flame height was nearly the same for different /. The detected signal, S CH (counts), is related to the total CH* photon emission rate, P CH (photons s1) from the flame by, X Rk DtE ð3Þ 4p where X is the collection solid angle, Rk (counts/ photon) is the wavelength dependent responsivity of the detection system and DtE is the exposure time of the camera. The change in fiber location alters the solid angle, which is approximately given by pD2/4L2 where D is the diameter of the fiber core. S CH ¼ P CH
4. Chemiluminescence modeling Detailed chemical kinetic calculations were performed for one-dimensional (1-d), adiabatic, unstrained flames with the PREMIX algorithm of CHEMKIN 4.1 and for adiabatic, opposed (strained) flames with OPPDIFF. The GRI-Mech 3.0 chemical mechanism was used for methane [23], while the Jet-A simulations employed a reduced mechanism (167 reactions and 63 species) [24]. Multi-component and thermal diffusion were included in all simulations. CH* intensities were determined by post-processing the Chemkin output using Eq. (1) and the rate parameters given in Table 1. Of the four CH* mechanisms considered, three (Peeters [18], Carl [20] and Smith [22]) are based on formation steps R2 and R3. The Peeters model has temperature independent rate constants, while the Carl model is an updated version that adds temperature dependence to both formation rates and matches, at 650 K, the large rate constant ratio of Peeters, k2/k3 500. While k2/k3 in the Peeters model is a constant, the Carl ratio drops from 420 at 700 K to 15 at 2000 K. The Smith model includes temperature dependence only for the O2 step and has a lower k2/k3, decreasing from 25 to 12 for the same 700–2000 K range. The fourth model (Petersen [17]) uses R1 and R2, and
requires the addition of C2 reactions to use it with the GRI mechanism, as outlined in [17]. To simulate the spatially integrated chemiluminescence from the experiments, the profiles through the 1-d flame simulations are integrated, Z LI I CH ¼ iCH dx ð4Þ 0
where I CH is the emission intensity per unit flame area, and LI is the integration length. As CH* is formed predominantly in the primary reaction zone for lean flames, LI can be any value that exceeds the flame thickness. Here, 10 mm was used. Entrainment of cold-air into the products downstream of the flame occurs in the open (laminar) flames, which is not included in the simulations. However, the reaction zone, and thus the CH* emission, is unaffected. The calculated emission intensity I CH (photons cm2 s1) is related to the total photon emission rate P CH by the relation, m_ ð5Þ P CH ¼ I CH Af ¼ I CH qu S L where Af is the flame area, m_ is the mass flow rate, qu is the (unburned) reactant density and SL is the laminar flame speed. Finally, the chemiluminescence normalized by the fuel mass flow rate is given by P CH 1 þ m_ a =m_ f ð6Þ ¼ I CH m_ f qu S L where m_ a and m_ f are the air and fuel mass flow rates. 5. Results and discussion Two studies were performed. First, the various proposed mechanisms for CH* formation are evaluated in the methane and Jet-A flames. The second study uses the identified model to examine the usefulness of chemiluminescence for combustion sensing/diagnostic applications under various conditions. 5.1. Validation tests We begin with comparisons for methane, by comparing experimental and simulated CH* chemiluminescence, normalized by fuel flow rate. In this context, Eqs. (3) and (6) for the 1-d chemiluminescence simulations, are combined as follows: S CH =DtE S_ CH P CH X Rk ¼ m_ f m_ f m_ f 4p 1 þ m_ a =m_ f CCH ¼ I CH qu S L L2
ð7Þ
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where S_ CH is the chemiluminescence signal rate and all the constants and unchanging variables are combined into CCH . With Eq. (7), the simulations can be compared to the experimental results for total chemiluminescence signal. Experimental and simulation results for the laminar methane–air flames are shown in Fig. 2. The simulation results for each model were multiplied with a CCH chosen to produce the least square deviation from the experiments. The best agreement is provided by the Peeters and Carl mechanisms, which match the measured results within the experimental uncertainty. In addition, both result in nearly the same CCH value. The Smith rate constants produce a poor match to the data, which argues against its lower k2/k3 ratio. The mechanism that includes the C2 formation step (Petersen) provides the poorest result. While we added the same C2 kinetics rates to GRI-Mech as suggested by Petersen, inaccuracies in the C2 kinetics could account for some of this disagreement. Still, the results support the importance of the C2H oxidation steps for CH* formation in lean methane flames. The agreement between the Peeters and Carl mechanisms, with their different temperature dependence for k2/k3 occurs due to an offsetting change in the concentration ratio [O]/[O2] and k2/k3 for the Carl and Peeters mechanisms in the methane flames studied. For the Peeters mechanism, R2 dominates CH* production in the lean (atmospheric) methane flames, and the formation of CH* is essentially given by k2,P[C2H][O]. For the Carl values (k 2,C and k3,C), both reactions are important; thus the ratio of the CH* chemiluminescence for the two models scales like . k 2;P k 2;C
k
½O2 1 þ k3;C . Since k2,P/k2,C is a constant 2;C ½O (1.8), the two models will give the same result if k2/k3 for the Carl model varies with temperature
899
in a similar fashion to [O2]/[O], which turns out to be the case in our flames. In the above comparisons, we have implicitly assumed that the jet flames can be approximated with 1-d laminar flame simulations. While this may be reasonable for estimating the species profiles within the reaction zone, there can be errors associated with the curvature and stretch experienced by the conical flame [25]. This would primarily affect the predicted flame speed in Eq. (7). To test this, we compared the ratio of two chemiluminescence species produced in the reaction zone, OH* and CH*. From Eqs. (3) and (5), the measured chemiluminescence signal ratio is related to the modeled ratio through the expression S CH I ¼ S OH I
CH OH
Rk;CH I ¼ C det Rk;OH I
CH
ð8Þ
OH
Thus the ratio is independent of all the experimental parameters, including flame speed or flame area, except for the change in detection system responsivity with wavelength ( Cdet), which is independent of changes in /. The experimental OH* signal was found by integrating the background corrected (0, 0) band emission over a 10 nm bandwidth centered at the OH* peak. I OH was determined with the same process applied for I CH and a previously validated chemiluminescence mechanism [12]. The OH* formation reactions along with their associated rate parameters are included in Table 1; quenching data for typical colliders in methane flames can be found in [13]. The chemiluminescence ratio results are shown in Fig. 3, with Cdet chosen to match the results at / = 1. As before, the Smith and Petersen mechanisms poorly predict the data, though the disagreement is even more noticeable here due to the reduced range of the chemiluminescence ratio
0.5 6
4
.
S CH *
Carl
Carl Smith Peeters Petersen Laminar
Smith
0.4
Peeters
.
SCH *
Petersen
0.3
Laminar
.
.
SOH *
mf
0.2
2 0.1
0 0.5
0.7
φ
0.9
1.1
Fig. 2. Comparison between experimental and simulation results for normalized CH* chemiluminescence in laminar methane–air flames (1 atm, Treactants = 300 K); measurements (symbols); mechanisms (lines).
0.0 0.5
0.7
0.9
1.1
φ Fig. 3. Comparison between experimental and simulation results for the chemiluminescence ratio in laminar methane–air flames; measurements (symbols); mechanisms (lines).
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compared to the data of Fig. 2. The simulations with the Carl and Peeters values again reproduce the experimental data, though the reduced range shows the Carl values deviate somewhat below / 0.75. Thus, we see little evidence that the 1-d simulation approach is inappropriate for these flames. Now we consider CH* chemiluminescence in a more complex system, premixed Jet-A combustion. Results from the experiments and simulations (with the C2H-based mechanisms) are shown in Fig. 4. For the simulations, the initial temperature of the reactants was matched to the measured preheat (450 K). The Peeters and Carl mechanisms again predict the experimental data well. The two provide essentially identical results, except below / 0.7, where the Peeters model now provides a noticeably better match to the data. In summary, tests at atmospheric pressure in two very different hydrocarbon fuel–air systems (methane and Jet-A), with and without preheat, indicate that the CH* chemiluminescence can be accurately predicted with models employing the C2H-based formation reactions. The best agreement with the experimental results is provided by the Peeters (temperature independent) rate constants, though simulations with the Carl (temperature dependent) rate constants are nearly as good. The agreement between the two models is due to the counterbalancing change in [O]/[O2] and the Carl k2/k3 ratio in the validation flames. As the [O]/[O2] relationship should change with pressure, we expect predictions using the two models will no longer agree in high pressure flames. To select one of the two mechanisms will require experiments at elevated pressures, or possibly with a different oxidizer, e.g., O2/N2O (this may call for another validated combustion mechanism). 5.2. Chemiluminescence interpretation
preheat and strain on chemiluminescence sensing in methane–air combustion. First, we consider chemiluminescence-based equivalence ratio sensing. Previous experimental studies [2–4] report that the chemiluminescence ratio I CH =I OH increases monotonically with / in methane flames, providing a means to estimate flame zone equivalence ratio. This is the same behavior observed in the results presented in Fig. 3. The influence of pressure and reactant heating on this relationship is shown in Fig. 5. Simulation results are shown for a single mechanism (Carl) at lower pressures, and two mechanisms (Carl and Peeters) for 15 atm. The most striking result is the change in the / dependence of the chemiluminescence ratio at high pressures. At 5 and 15 atm, I CH =I OH increases as the mixture is made less lean, in contrast to the atmospheric pressure results. In addition, the absolute value of the ratio changes, increasing with pressure especially for leaner mixtures. The cause is the decrease in OH* chemiluminescence per unit heat release as pressure rises, with a more pronounced decrease occurring for lean mixtures. On the other hand, the CH* signal does not drop significantly with pressure, because the concentration of its fuel precursor (C2H) is not as sensitive to pressure and temperature changes as the OH* precursors (e.g., CH). As predicted, the Carl and Peeters mechanisms produce different results at high pressure. At 15 atm, the Peeters mechanism predicts a lower value for I CH =I OH (since it favors the C2H + O step and the [O]/[O2] ratio drops at high pressure). Nevertheless, it qualitatively agrees with the Carl predictions; the relationship between I CH =I OH and / is a strong function of pressure. The simulations also indicate that the influence of reactant preheating is small, especially compared to the pressure effect.
At this point, we proceed to use the validated CH* mechanisms to study the effects of pressure, 5
0.5 x 1/3 15 12
. S CH * . mf
0.4
Carl Peeters Smith
I CH* I OH*
9
0.3
1atm-300K 1atm-500K 5atm-500K 5atm-700K 15atm-700K 15atm-700K (P)
4 3
0.2
2
0.1
1
6 3 0 0.5
0.0 0.45 0.7
φ
0.9
1.1
Fig. 4. Normalized CH* chemiluminescence in laminar Jet-A–air flames (1 atm, Treactants = 450 K); measurements (symbols); mechanisms (lines).
0.65
φ
0.85
0 1.05
Fig. 5. CH*–OH* chemiluminescence ratio for varying pressure, preheat conditions in methane–air flames; the high pressure results are scaled to the right axis. (P) in the legend identifies Peeters mechanism.
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To examine the use of CH* for heat release sensing, Fig. 6 shows the CH* signal normalized by total heat release per unit flame area (q) (again, the Carl and Peeters mechanism are compared at 15 atm). I CH =q is seen to vary significantly with /; thus we conclude that changes in fuel–air ratio can lead to noticeable errors in measured heat release rate. The simulations also indicate that reactant preheating increases the CH* signal for a given heat release. Increasing pressure provides help, as the variation with / decreases. As an example, consider heat release sensing in a combustor with a 5% fluctuation in /, nominally at 0.8, but with fixed heat release. This would produce a roughly 30% fluctuation in CH* signal at 1 atm/298 K. This would be reduced to a 12% variation at 15 atm/700 K. As before, the simulations with the Peeters mechanism differ from those using the Carl rates, but the qualitative behavior is similar. In many practical combustors, aerodynamic strain can potentially alter the flame zone and thus lead to a change in chemiluminescence. To investigate this, simulation results for I CH =q (Carl mechanism) are shown in Fig. 7 for twin, premixed opposed jet flames at two conditions: / = 0.7 and 1. The CH* chemiluminescence and volumetric heat release rate were integrated along the center streamline of the system, up to the stagnation plane. For the 1 atm, 298 K, / = 0.7 case, the normalized chemiluminescence increases by as much as 50% as strain is raised to 400 s1. An increase in preheat temperature (to 500 K) reduces the strain sensitivity slightly. At higher pressures, the dependence on strain is reduced considerably. Furthermore, the stoichiometric mixture shows less strain dependence under most conditions. The increase in normalized CH* chemiluminescence with strain occurs because the integrated heat release decreases with strain, while the integrated CH* chemiluminescence either increases or remains roughly constant.
I CH * q
901
2.5
20
2.0
16
1.5
1atm-300K 1atm-500K
1.0
5atm-500K
8 4
0.5 0.0
12
0
500
1000
1500
0 2000
Strain Rate, s-1 Fig. 7. Normalized CH* chemiluminescence for various strain rates in methane–air flames at U = 0.7 (thin lines) and U = 1.0 (thick lines). Stoichiometric results are scaled to the right axis.
Overall, we find that strain and preheating effects on CH* chemiluminescence-based heat release sensing are generally less pronounced than the signal’s dependence on equivalence ratio and pressure. The strain conclusion is supported by previous experiments that specifically investigated aerodynamic strain on CH* in stagnation flame configurations; the authors of those studies found no systematic dependence on strain, within a ±10% uncertainty at high pressures [8] and within a much large spread of the data for atmospheric pressures [10]. The weak strain dependence suggests that the laminar simulation approach may even be applicable to a number of turbulent combustion systems. To test this, Fig. 8 shows results for normalized CH* chemiluminescence obtained in the methane–fueled swirl combustor. The turbulent swirl combustor results are in excellent agreement with the laminar experiments and
6 12
Carl Laminar Swirl
1atm-300K 9
1atm-500K 5atm-500K
I CH * q
6
4
.
S CH *
5atm-700K
.
15atm-700K 15atm-700K (P)
mf 2
3
0 0.45
0 0.5 0.65
φ
0.85
1.05
Fig. 6. Normalized CH* chemiluminescence in methane–air flames at various pressure and preheat conditions. (P) in the legend identifies Peeters mechanism.
0.7
φ
0.9
1.1
Fig. 8. Normalized CH* chemiluminescence in methane–air laminar and swirl flames (p = 1 atm, T = 300 K); measurements (symbols); mechanisms (lines).
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simulations, except near the lean blow out limit of the combustor (/LBO 0.66), where strain effects are more important. The similarity between the turbulent and laminar results may partly result from the spatial (and temporal) averaging of the signal. While the instantaneous CH* emission from specific locations, for example in the highly strained shear layer near the injector, might exhibit significant variations, the global CH* emission is likely less effected. Also, the conditions in this combustor are estimated to have a Karlovitz number near or below one, so laminar flamelet modeling is likely relevant to the reaction zone. 6. Conclusions Four CH* mechanisms were investigated for predicting flame chemiluminescence in lean, premixed, hydrocarbon–air flames. To validate the mechanisms, chemiluminescence spectra were obtained in atmospheric pressure, premixed laminar jet flames with non-preheated methane–air and with prevaporized, preheated Jet-A–air mixtures. The data were compared to predictions obtained from numerical simulations of 1-d laminar flames using appropriate detailed chemical mechanisms. The simulation outputs were postprocessed using a quasi-steady state approximation for CH* concentration. The validation tests indicate that reactions R1 and R2 (the ethynyl precursor) are the source of CH*, at least in lean flames. Two pairs of rate constants, [18] and [20], produce excellent agreement with the atmospheric pressure data from both flames, though the rate constants proposed by the Peeters group [18] provide a better match for very lean mixtures. The use of CH*/OH* chemiluminescence ratios for equivalence ratio (/) sensing in methane flames is shown to be highly dependent on pressure, with the sensitivity to / reversing as pressure increases from 1 to 15 atm. Thus at some intermediate pressure, the CH*/OH* ratio should become insensitive to /. With regard to heat release rate sensing, CH* chemiluminescence scales with heat release, but it also depends strongly on local equivalence ratio and pressure. At high pressures, the dependence on / is reduced. Thus a ±5% change in / could easily lead to a ±30% error in heat release at atmospheric pressure, and a smaller ±10% error at 15 atm. For high pressures, predictions with the two rate constant models no longer agree. Thus further validation experiments at high pressure or with a different oxidizer would be necessary to determine the best mechanism. Also, various comparisons presented here support the use of the 1-d flame modeling approach for the laminar jet flames used in the validations. Furthermore, results from an atmospheric pressure swirl burner
indicate that the laminar results may be useful in some turbulent flames, at least where the reaction zone profiles are flamelet like. For other cases, the CH* mechanism will have to be added to more complex flame modeling codes to predict or interpret chemiluminescence sensing.
Acknowledgments This research was supported by NASA through the University Research, Engineering, and Technology Institute for Aeropropulsion and Power, under NCC3-982. The authors thank Dr. Adrian Kyne for providing the reduced mechanism for Jet-A, and Prof. Eric Petersen for the C2-mechanism modifications for GRI-Mech. References [1] A.G. Gaydon, H.G. Wolfhard, Flames: Their Structure, Radiation, and Temperature, fourth ed., Chapman and Hall, 1978. [2] N. Docquier, F. Lacas, S. Candel, Proc. Combust. Inst. 29 (2002) 139–145. [3] T.M. Muruganandam, B.H. Kim, M.R. Morrell, et al., Proc. Combust. Inst. 30 (2005) 1601–1609. [4] M. Yamazaki, M. Ohya, K. Tsuchiya, Int. Chem. Eng. 30 (1992) 160–168. [5] B.D. Stojkovic, T.D. Fansler, M.C. Drake, V. Sick, Proc. Combust. Inst. 30 (2005) 2657–2665. [6] J.G. Lee, K. Kim, D.A. Santavicca, Proc. Combust. Inst. 28 (2000) 415–421. [7] H.N. Najm, P.H. Paul, C.J. Mueller, P.S. Wyckoff, Combust. Flame 113 (3) (1998) 312–332. [8] B. Higgins, M.Q. McQuay, F. Lacas, S. Candel, Fuel 80 (11) (2001) 1583–1591. [9] N. Docquier, S. Belhafaoui, F. Lacas, N. Darabiha, J.-C. Rolon, Proc. Combust. Inst. 28 (2000) 1765– 1774. [10] Y. Hardalupas, M. Orain, Combust. Flame 139 (2004) 188–207. [11] M. De Leo, A. Saveliev, L.A. Kennedy, S.A. Zelepouga, Combust. Flame 149 (2007) 435–447. [12] V.N. Nori, J.M. Seitzman, Combust. Flame, submitted for publication. [13] M. Tamura, P.A. Berg, J.E. Harrington, et al., Combust. Flame 144 (1998) 502–514. [14] H.H. Brenig, Ph.D. Thesis, Wuppertal, 1981. [15] J. Grebe, K.H. Homann, Ber. Bunsenges. Phys. Chem. 86 (1982) 587. [16] G.P. Glass, G.B. Kistiakowsky, J.V. Michael, H. Niki, Proc. Combust. Inst. 10 (1965) 513–522. [17] J.M. Hall, J. De Vries, A.R. Amadio, E.L. Petersen, AIAA Paper 2005-1318, 43rd AIAA Aerospace Sciences Meeting and Exhibit, Reno, Nevada, 2005. [18] K. Devriendt, H. Van Look, B. Ceursters, J. Peeters, Chem. Phys. Lett. 261 (1996) 450–456. [19] A.M. Renlund, F. Shokoohi, H. Reisler, C. Wittig, J. Phys. Chem. 86 (21) (1982) 4165–4170. [20] R.M.I. Elsamra, S. Vranckx, S.A. Carl, J. Phys. Chem. A 109 (45) (2005) 10287–10293. [21] G.P. Smith, J. Luque, C. Park, J.B. Jeffries, D.R. Crosley, Combust. Flame 131 (2002) 59–69.
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[24] L. Elliott, D.B. Ingham, A.G. Kyne, et al., Paper GT2004-53053, Proc. ASME Turbo Expo, Vienna, Austria, 2004. [25] J. Natarajan, T. Lieuwen, J. Seitzman, Combust. Flame 151 (2007) 104–119.
Proceedings of the
Combustion Institute
Proceedings of the Combustion Institute 32 (2009) 895–903
www.elsevier.com/locate/proci
CH* chemiluminescence modeling for combustion diagnostics Venkata N. Nori *, Jerry M. Seitzman School of Aerospace Engineering, Georgia Institute of Technology, 270 Ferst Drive, Guggenheim 364, Atlanta, GA 30332-0150, USA
Abstract CH* chemiluminescence is often employed in combustion diagnostics, for example as a measure of heat release rate and for equivalence ratio sensing. However, most interpretations of CH* chemiluminescence rely either on heuristic arguments or empirical data gathered under limited conditions. More rigorous analysis is required to understand the effects of combustion conditions, e.g., pressure, reactant composition and preheat, strain and exhaust gas recirculation, on CH* chemiluminescence. Chemiluminescence modeling holds promise in this regard. The predictive accuracy of four proposed CH* chemiluminescence formation models were experimentally tested in premixed, methane–air and prevaporized Jet-A–air flames. Two of the models, based on CH* formation via reactions between C2H and O or O2, are able to predict the experimental data within the experimental uncertainty, in both room temperature methane and preheated Jet-A flames. The utility of CH* chemiluminescence for sensing heat release rate and equivalence ratio (/), when combined with OH* chemiluminescence, is then analyzed in methane flames for varying pressure, preheat temperature and strain. The CH*/OH* chemiluminescence ratio is found to be useful for sensing equivalence ratio in lean methane systems, but only at certain pressure and reactant temperature conditions. The relationship between CH* chemiluminescence and heat release varies with /, pressure, temperature and strain. At high pressures, however, the dependence on / and strain are small, making CH* attractive for heat release sensing applications in gas turbine combustors. Ó 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: CH* chemiluminescence; Equivalence ratio sensing; Heat release rate sensing; Combustion diagnostics; Chemiluminescence modeling
1. Introduction Combustion diagnostics based on flame chemiluminescence are of great interest for their simplicity and non-intrusive nature. Chemiluminescence is light emitted by molecules chemically created in an excited energy state when they radiatively relax to a *
Corresponding author. Fax: +1 404 463 0888. E-mail address: [email protected] (V.N. Nori).
lower energy state. Common chemiluminescent species in hydrocarbon–air flames are CH*, OH*, C2 and CO2 [1]. Chemiluminescence can potentially provide information about reaction zone conditions that relate to combustor performance, pollutant formation and combustor health. For example, chemiluminescence has been used for equivalence ratio (/) sensing in both gaseous and liquid fueled systems [2–4], in flame front location [5] and in characterizing temporal and spatial heat release fluctuations in combustion dynamics
1540-7489/$ - see front matter Ó 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2008.05.050
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studies [6]. However, quantitative interpretation of chemiluminescence signals has been based historically on heuristic arguments or empirical data gathered under limited conditions. Additionally, care must be taken to interpret these line of sight global measurements for flames with widely varying conditions [7]. Therefore, a number of experimental investigations have examined chemiluminescence, primarily from OH* and CH* in premixed methane–air systems, while varying conditions such as pressure [8,9] and strain [8,10]. The ability to model and predict chemiluminescence would provide an alternate, more flexible approach to understanding the dependence of chemiluminescence signals on key combustion parameters. Such models would also be of great utility in developing combustion diagnostics/sensors and interpreting the resulting data. Reliable chemical kinetics simulations of chemiluminescence require accurate modeling of the formation rates of the excited state species. Thus the chemiluminescence reactions and their rate constants must be known. In addition, the need to simulate the concentrations of the chemiluminescence precursor and quencher species requires a reliable detailed chemical kinetic mechanism for the particular fuel-oxidizer system of interest. While a number of studies have focused on identifying formation reactions and their rate parameters for OH* and CH*, only a few studies have attempted to validate proposed models in combustion systems [11,12]. Most of these studies were limited to methane–air mixtures. However, OH* and CO2 chemiluminescence models were recently validated in methane and CO/H2 (syngas) flames at atmospheric pressure under varying operating conditions such as equivalence ratio, syngas fuel composition, preheat and dilution [12]. Therefore, the goal of this work is to examine existing chemiluminescence mechanisms for CH*, by incorporating them into accepted flame chemistry models, and validate them with experimental data using different fuels. Methane and Jet-A were chosen for this evaluation, because they are highly relevant to many combustion systems in power generation and aeropropulsion applications. Moreover, this paper focuses on lean combustion as it is interesting for low NOx gas turbines. Implications of using CH* chemiluminescence for sensing heat release rate and equivalence ratio are also discussed. 2. CH* mechanisms The primary CH* emission in the ultraviolet and visible region of the spectrum is due to the A2D ? X2P (431 nm) and B2R ? X2P (390 nm) transitions, with the former usually dominating [1]. To simulate CH* emission, a chemiluminescence mechanism requires excited
state formation reactions and their rates; and the removal rates by collisional quenching and radiative relaxation. Radiative rates and CH* quenching data for major species of hydrocarbon combustion are available [13]. Of the various proposed sources for CH*, research has focused on the following: C2 þ OH ! CH þ CO C2 H þ O ! CH þ CO C2 H þ O2 ! CH þ CO2
ðR1Þ ðR2Þ ðR3Þ
Gaydon [1] suggested R1, which was later challenged by Brenig [14] and Grebe and Homann [15]. Brenig’s experiments suggested that CH* formed from the reaction of ground state ethynyl radicals (C2H) with O atoms, as proposed earlier [16]. A later shock tube study with methane–hydrogen mixtures supports R1 and R2 as the dominant CH* formation pathways for conditions in the range 1200–2300 K and 0.6–2.2 atm [17]. R1 may also be important in higher order hydrocarbon (liquid) fuel systems where C2 is relatively more abundant. Devriendt et al. [18] in a pulsed laser photolysis study at low pressure determined the temperature dependence of R2 and concluded that the majority of CH* is produced by that reaction. Renlund et al. [19] suggested the importance of C2H reacting with O2 (R3) rather than O. In a flash photolysis study of acetylene at low pressure [20], the temperature dependence of R3 was evaluated, and the relevance of R3 in hot flames and fuel lean conditions was suggested. Further studies measured absolute excited state concentrations of CH* at low pressure in methane–air premixed flames, and rate parameters for R2 and R3 were determined [21,22]. In this study, four models proposed in the literature (designated by the last author of the reference source) were used to model CH* in lean flames. Each is based on two of the formation reactions (R1–R3); the reaction rate parameters associated with each are listed in Table 1. In flames, electronically excited species in general have low concentrations due to their low production rates and their rapid removal by collisional quenching. They therefore often have little impact on the overall flame chemistry. For these reasons, CH* is typically assumed to be in quasi-steady state and limited by the formation rate [12]. Under these conditions, the concentration of CH* (e.g., moles/cm3) is given by ½CH ¼
k 1 ½C2 ½OH þ k 2 ½C2 H½O þ k 3 ½C2 H½O2 P j k Q;j ½M j þ A ð1Þ
where k1–k3 are the rate constants for the corresponding formation reactions, kQ,j is the quenching rate constant for CH* by species j and A is the Einstein coefficient for spontaneous emission for the
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897
Table 1 Chemiluminescence reaction mechanism to model CH* formation #
Reaction
A
R1 R2
C2 + OH M CH* + CO C2H + O M CH* + CO
R3
C2H + O2 M CH* + CO2
R4 R5
H + O + M M OH* + M CH + O2 M OH* + CO
2 1014 2.5(±0.8) 1012 5.2 1011 1.08(±0.4) 1013 6.023(±3.0) 1012 3.2(±1.0) 1011 2.17(±0.8) 1010 6.023 104 6 1014 1.173 1014
b 0 0 0 0 0 0 0 4.4 0.0 0.4
Ea 0 0 2600 0 457 1599 0 2285 6940 4150
Ref. Petersen [17] Smith [22] Petersen [17] Peeters [18] Carl [20] Smith [22] Peeters [18] Carl [20] [12] [12]
Rate coefficients are expressed as k = ATbexp(Ea/RT) in units of cal, mol, cm and s. Quenching reactions are given in [13]. Einstein A coefficient for CH* is 1.85 106 s1 [13].
CH* transition. The photon emission rate i (mole photons/cm3/s) can then be found from iCH ¼ A½CH
ð2Þ
3. Experimental setup 3.1. Combustors Chemiluminescence spectra were obtained in two atmospheric premixed fuel systems: a laminar jet flame (methane and Jet-A fuels) and a swirlstabilized combustor (methane only). The laminar burner is a straight cylindrical stainless steel tube (Fig. 1). For the methane experiments, two different tubes were used, with inner diameters of 18 and 25 mm. For flames with / < 0.8, the 25 mm burner with pilot-stabilization was used. The smaller burner was employed for near stoichiometric mixtures (high flame speeds) to maintain laminar flow. The burner lengths (>1 m) were
Fiberhead
L
12.5º
To Spectrometer
CH 4-Air pilot
TC1 TC2 Calibrated Air
N2 pressurized Jet-A
3.2. Detection optics Temperature controller
Pre-vaporized Jet-A/Air TC 3
chosen to ensure a fully developed laminar exit velocity profile. Calibrated rotameters determined the flow rates with ±1% accuracy, or a 1.5% uncertainty in /. For Jet-A, the 25 mm tube, with no pilot, was used for mixtures with / < 0.7; otherwise a 10 mm tube was employed. Being a liquid fuel, the Jet-A was first vaporized to obtain a laminar premixed flame. As shown in Fig. 1, the liquid fuel, pressurized with N2 and monitored with a calibrated rotameter, was vaporized in a straight tube maintained at a wall temperature of 573 K, and then mixed with preheated air at 393 K to prevent fuel condensation. Further heating of the mixture was feedback controlled with a thermocouple placed close to the burner exit. Electrical resistance tape surrounding the burner tube provided heating. The fuel rotameter had a ±3% accuracy, resulting in a maximum uncertainty of 4% in /. The swirl-dump combustor, the same employed in an earlier study [3], was formed from a 70 mm inner diameter, 127 mm long quartz tube. The reactants enter from a 23 mm diameter tube, with swirl vanes upstream producing a flow with a theoretical inlet swirl number of 0.66. The data presented here corresponds to a bulk average (cold) velocity of 4–5 m/s in the test section and a Reynolds number of 20,000.
CH 4 /Air Main Flow
Tape-Heater
Fig. 1. Schematic of Jet-A and methane experimental setup (TC, thermocouple). The pilot was used to stabilize methane–air flames in the 25 mm burner.
Optical emission was detected with a fiberoptic based collection system coupled to an imaging spectrometer. Detailed description of the collection optics, spectrometer and the intensified CCD camera can be found in [3]. In most cases, 10 images were acquired at each operating condition, with a camera exposure time of 100 ms. The 10 long exposure images were averaged and background corrected before extracting the CH* chemiluminescence signal, S CH , which
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was found by integrating the background corrected A2D ? X 2P transition over a bandwidth of ±5 nm. In the laminar jet flames, the total chemiluminescence was captured by placing the fiber along a line perpendicular to the flame axis. The distance (L) from the flame to the fiber was chosen such that the solid angle variation across the flame as seen by the fiber becomes independent of L. This is achieved at L P 11h, where h is the flame height. For the swirl combustor, the fiber was placed at a fixed location 9.5 cm from the combustor wall and 6.5 cm above the base of the combustor. This allowed the fiber to collect light from most of the flame region. Due to the swirl-stabilization, the flame height was nearly the same for different /. The detected signal, S CH (counts), is related to the total CH* photon emission rate, P CH (photons s1) from the flame by, X Rk DtE ð3Þ 4p where X is the collection solid angle, Rk (counts/ photon) is the wavelength dependent responsivity of the detection system and DtE is the exposure time of the camera. The change in fiber location alters the solid angle, which is approximately given by pD2/4L2 where D is the diameter of the fiber core. S CH ¼ P CH
4. Chemiluminescence modeling Detailed chemical kinetic calculations were performed for one-dimensional (1-d), adiabatic, unstrained flames with the PREMIX algorithm of CHEMKIN 4.1 and for adiabatic, opposed (strained) flames with OPPDIFF. The GRI-Mech 3.0 chemical mechanism was used for methane [23], while the Jet-A simulations employed a reduced mechanism (167 reactions and 63 species) [24]. Multi-component and thermal diffusion were included in all simulations. CH* intensities were determined by post-processing the Chemkin output using Eq. (1) and the rate parameters given in Table 1. Of the four CH* mechanisms considered, three (Peeters [18], Carl [20] and Smith [22]) are based on formation steps R2 and R3. The Peeters model has temperature independent rate constants, while the Carl model is an updated version that adds temperature dependence to both formation rates and matches, at 650 K, the large rate constant ratio of Peeters, k2/k3 500. While k2/k3 in the Peeters model is a constant, the Carl ratio drops from 420 at 700 K to 15 at 2000 K. The Smith model includes temperature dependence only for the O2 step and has a lower k2/k3, decreasing from 25 to 12 for the same 700–2000 K range. The fourth model (Petersen [17]) uses R1 and R2, and
requires the addition of C2 reactions to use it with the GRI mechanism, as outlined in [17]. To simulate the spatially integrated chemiluminescence from the experiments, the profiles through the 1-d flame simulations are integrated, Z LI I CH ¼ iCH dx ð4Þ 0
where I CH is the emission intensity per unit flame area, and LI is the integration length. As CH* is formed predominantly in the primary reaction zone for lean flames, LI can be any value that exceeds the flame thickness. Here, 10 mm was used. Entrainment of cold-air into the products downstream of the flame occurs in the open (laminar) flames, which is not included in the simulations. However, the reaction zone, and thus the CH* emission, is unaffected. The calculated emission intensity I CH (photons cm2 s1) is related to the total photon emission rate P CH by the relation, m_ ð5Þ P CH ¼ I CH Af ¼ I CH qu S L where Af is the flame area, m_ is the mass flow rate, qu is the (unburned) reactant density and SL is the laminar flame speed. Finally, the chemiluminescence normalized by the fuel mass flow rate is given by P CH 1 þ m_ a =m_ f ð6Þ ¼ I CH m_ f qu S L where m_ a and m_ f are the air and fuel mass flow rates. 5. Results and discussion Two studies were performed. First, the various proposed mechanisms for CH* formation are evaluated in the methane and Jet-A flames. The second study uses the identified model to examine the usefulness of chemiluminescence for combustion sensing/diagnostic applications under various conditions. 5.1. Validation tests We begin with comparisons for methane, by comparing experimental and simulated CH* chemiluminescence, normalized by fuel flow rate. In this context, Eqs. (3) and (6) for the 1-d chemiluminescence simulations, are combined as follows: S CH =DtE S_ CH P CH X Rk ¼ m_ f m_ f m_ f 4p 1 þ m_ a =m_ f CCH ¼ I CH qu S L L2
ð7Þ
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where S_ CH is the chemiluminescence signal rate and all the constants and unchanging variables are combined into CCH . With Eq. (7), the simulations can be compared to the experimental results for total chemiluminescence signal. Experimental and simulation results for the laminar methane–air flames are shown in Fig. 2. The simulation results for each model were multiplied with a CCH chosen to produce the least square deviation from the experiments. The best agreement is provided by the Peeters and Carl mechanisms, which match the measured results within the experimental uncertainty. In addition, both result in nearly the same CCH value. The Smith rate constants produce a poor match to the data, which argues against its lower k2/k3 ratio. The mechanism that includes the C2 formation step (Petersen) provides the poorest result. While we added the same C2 kinetics rates to GRI-Mech as suggested by Petersen, inaccuracies in the C2 kinetics could account for some of this disagreement. Still, the results support the importance of the C2H oxidation steps for CH* formation in lean methane flames. The agreement between the Peeters and Carl mechanisms, with their different temperature dependence for k2/k3 occurs due to an offsetting change in the concentration ratio [O]/[O2] and k2/k3 for the Carl and Peeters mechanisms in the methane flames studied. For the Peeters mechanism, R2 dominates CH* production in the lean (atmospheric) methane flames, and the formation of CH* is essentially given by k2,P[C2H][O]. For the Carl values (k 2,C and k3,C), both reactions are important; thus the ratio of the CH* chemiluminescence for the two models scales like . k 2;P k 2;C
k
½O2 1 þ k3;C . Since k2,P/k2,C is a constant 2;C ½O (1.8), the two models will give the same result if k2/k3 for the Carl model varies with temperature
899
in a similar fashion to [O2]/[O], which turns out to be the case in our flames. In the above comparisons, we have implicitly assumed that the jet flames can be approximated with 1-d laminar flame simulations. While this may be reasonable for estimating the species profiles within the reaction zone, there can be errors associated with the curvature and stretch experienced by the conical flame [25]. This would primarily affect the predicted flame speed in Eq. (7). To test this, we compared the ratio of two chemiluminescence species produced in the reaction zone, OH* and CH*. From Eqs. (3) and (5), the measured chemiluminescence signal ratio is related to the modeled ratio through the expression S CH I ¼ S OH I
CH OH
Rk;CH I ¼ C det Rk;OH I
CH
ð8Þ
OH
Thus the ratio is independent of all the experimental parameters, including flame speed or flame area, except for the change in detection system responsivity with wavelength ( Cdet), which is independent of changes in /. The experimental OH* signal was found by integrating the background corrected (0, 0) band emission over a 10 nm bandwidth centered at the OH* peak. I OH was determined with the same process applied for I CH and a previously validated chemiluminescence mechanism [12]. The OH* formation reactions along with their associated rate parameters are included in Table 1; quenching data for typical colliders in methane flames can be found in [13]. The chemiluminescence ratio results are shown in Fig. 3, with Cdet chosen to match the results at / = 1. As before, the Smith and Petersen mechanisms poorly predict the data, though the disagreement is even more noticeable here due to the reduced range of the chemiluminescence ratio
0.5 6
4
.
S CH *
Carl
Carl Smith Peeters Petersen Laminar
Smith
0.4
Peeters
.
SCH *
Petersen
0.3
Laminar
.
.
SOH *
mf
0.2
2 0.1
0 0.5
0.7
φ
0.9
1.1
Fig. 2. Comparison between experimental and simulation results for normalized CH* chemiluminescence in laminar methane–air flames (1 atm, Treactants = 300 K); measurements (symbols); mechanisms (lines).
0.0 0.5
0.7
0.9
1.1
φ Fig. 3. Comparison between experimental and simulation results for the chemiluminescence ratio in laminar methane–air flames; measurements (symbols); mechanisms (lines).
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compared to the data of Fig. 2. The simulations with the Carl and Peeters values again reproduce the experimental data, though the reduced range shows the Carl values deviate somewhat below / 0.75. Thus, we see little evidence that the 1-d simulation approach is inappropriate for these flames. Now we consider CH* chemiluminescence in a more complex system, premixed Jet-A combustion. Results from the experiments and simulations (with the C2H-based mechanisms) are shown in Fig. 4. For the simulations, the initial temperature of the reactants was matched to the measured preheat (450 K). The Peeters and Carl mechanisms again predict the experimental data well. The two provide essentially identical results, except below / 0.7, where the Peeters model now provides a noticeably better match to the data. In summary, tests at atmospheric pressure in two very different hydrocarbon fuel–air systems (methane and Jet-A), with and without preheat, indicate that the CH* chemiluminescence can be accurately predicted with models employing the C2H-based formation reactions. The best agreement with the experimental results is provided by the Peeters (temperature independent) rate constants, though simulations with the Carl (temperature dependent) rate constants are nearly as good. The agreement between the two models is due to the counterbalancing change in [O]/[O2] and the Carl k2/k3 ratio in the validation flames. As the [O]/[O2] relationship should change with pressure, we expect predictions using the two models will no longer agree in high pressure flames. To select one of the two mechanisms will require experiments at elevated pressures, or possibly with a different oxidizer, e.g., O2/N2O (this may call for another validated combustion mechanism). 5.2. Chemiluminescence interpretation
preheat and strain on chemiluminescence sensing in methane–air combustion. First, we consider chemiluminescence-based equivalence ratio sensing. Previous experimental studies [2–4] report that the chemiluminescence ratio I CH =I OH increases monotonically with / in methane flames, providing a means to estimate flame zone equivalence ratio. This is the same behavior observed in the results presented in Fig. 3. The influence of pressure and reactant heating on this relationship is shown in Fig. 5. Simulation results are shown for a single mechanism (Carl) at lower pressures, and two mechanisms (Carl and Peeters) for 15 atm. The most striking result is the change in the / dependence of the chemiluminescence ratio at high pressures. At 5 and 15 atm, I CH =I OH increases as the mixture is made less lean, in contrast to the atmospheric pressure results. In addition, the absolute value of the ratio changes, increasing with pressure especially for leaner mixtures. The cause is the decrease in OH* chemiluminescence per unit heat release as pressure rises, with a more pronounced decrease occurring for lean mixtures. On the other hand, the CH* signal does not drop significantly with pressure, because the concentration of its fuel precursor (C2H) is not as sensitive to pressure and temperature changes as the OH* precursors (e.g., CH). As predicted, the Carl and Peeters mechanisms produce different results at high pressure. At 15 atm, the Peeters mechanism predicts a lower value for I CH =I OH (since it favors the C2H + O step and the [O]/[O2] ratio drops at high pressure). Nevertheless, it qualitatively agrees with the Carl predictions; the relationship between I CH =I OH and / is a strong function of pressure. The simulations also indicate that the influence of reactant preheating is small, especially compared to the pressure effect.
At this point, we proceed to use the validated CH* mechanisms to study the effects of pressure, 5
0.5 x 1/3 15 12
. S CH * . mf
0.4
Carl Peeters Smith
I CH* I OH*
9
0.3
1atm-300K 1atm-500K 5atm-500K 5atm-700K 15atm-700K 15atm-700K (P)
4 3
0.2
2
0.1
1
6 3 0 0.5
0.0 0.45 0.7
φ
0.9
1.1
Fig. 4. Normalized CH* chemiluminescence in laminar Jet-A–air flames (1 atm, Treactants = 450 K); measurements (symbols); mechanisms (lines).
0.65
φ
0.85
0 1.05
Fig. 5. CH*–OH* chemiluminescence ratio for varying pressure, preheat conditions in methane–air flames; the high pressure results are scaled to the right axis. (P) in the legend identifies Peeters mechanism.
V.N. Nori, J.M. Seitzman / Proceedings of the Combustion Institute 32 (2009) 895–903
To examine the use of CH* for heat release sensing, Fig. 6 shows the CH* signal normalized by total heat release per unit flame area (q) (again, the Carl and Peeters mechanism are compared at 15 atm). I CH =q is seen to vary significantly with /; thus we conclude that changes in fuel–air ratio can lead to noticeable errors in measured heat release rate. The simulations also indicate that reactant preheating increases the CH* signal for a given heat release. Increasing pressure provides help, as the variation with / decreases. As an example, consider heat release sensing in a combustor with a 5% fluctuation in /, nominally at 0.8, but with fixed heat release. This would produce a roughly 30% fluctuation in CH* signal at 1 atm/298 K. This would be reduced to a 12% variation at 15 atm/700 K. As before, the simulations with the Peeters mechanism differ from those using the Carl rates, but the qualitative behavior is similar. In many practical combustors, aerodynamic strain can potentially alter the flame zone and thus lead to a change in chemiluminescence. To investigate this, simulation results for I CH =q (Carl mechanism) are shown in Fig. 7 for twin, premixed opposed jet flames at two conditions: / = 0.7 and 1. The CH* chemiluminescence and volumetric heat release rate were integrated along the center streamline of the system, up to the stagnation plane. For the 1 atm, 298 K, / = 0.7 case, the normalized chemiluminescence increases by as much as 50% as strain is raised to 400 s1. An increase in preheat temperature (to 500 K) reduces the strain sensitivity slightly. At higher pressures, the dependence on strain is reduced considerably. Furthermore, the stoichiometric mixture shows less strain dependence under most conditions. The increase in normalized CH* chemiluminescence with strain occurs because the integrated heat release decreases with strain, while the integrated CH* chemiluminescence either increases or remains roughly constant.
I CH * q
901
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0
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1500
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Strain Rate, s-1 Fig. 7. Normalized CH* chemiluminescence for various strain rates in methane–air flames at U = 0.7 (thin lines) and U = 1.0 (thick lines). Stoichiometric results are scaled to the right axis.
Overall, we find that strain and preheating effects on CH* chemiluminescence-based heat release sensing are generally less pronounced than the signal’s dependence on equivalence ratio and pressure. The strain conclusion is supported by previous experiments that specifically investigated aerodynamic strain on CH* in stagnation flame configurations; the authors of those studies found no systematic dependence on strain, within a ±10% uncertainty at high pressures [8] and within a much large spread of the data for atmospheric pressures [10]. The weak strain dependence suggests that the laminar simulation approach may even be applicable to a number of turbulent combustion systems. To test this, Fig. 8 shows results for normalized CH* chemiluminescence obtained in the methane–fueled swirl combustor. The turbulent swirl combustor results are in excellent agreement with the laminar experiments and
6 12
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.
S CH *
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.
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Fig. 6. Normalized CH* chemiluminescence in methane–air flames at various pressure and preheat conditions. (P) in the legend identifies Peeters mechanism.
0.7
φ
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Fig. 8. Normalized CH* chemiluminescence in methane–air laminar and swirl flames (p = 1 atm, T = 300 K); measurements (symbols); mechanisms (lines).
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V.N. Nori, J.M. Seitzman / Proceedings of the Combustion Institute 32 (2009) 895–903
simulations, except near the lean blow out limit of the combustor (/LBO 0.66), where strain effects are more important. The similarity between the turbulent and laminar results may partly result from the spatial (and temporal) averaging of the signal. While the instantaneous CH* emission from specific locations, for example in the highly strained shear layer near the injector, might exhibit significant variations, the global CH* emission is likely less effected. Also, the conditions in this combustor are estimated to have a Karlovitz number near or below one, so laminar flamelet modeling is likely relevant to the reaction zone. 6. Conclusions Four CH* mechanisms were investigated for predicting flame chemiluminescence in lean, premixed, hydrocarbon–air flames. To validate the mechanisms, chemiluminescence spectra were obtained in atmospheric pressure, premixed laminar jet flames with non-preheated methane–air and with prevaporized, preheated Jet-A–air mixtures. The data were compared to predictions obtained from numerical simulations of 1-d laminar flames using appropriate detailed chemical mechanisms. The simulation outputs were postprocessed using a quasi-steady state approximation for CH* concentration. The validation tests indicate that reactions R1 and R2 (the ethynyl precursor) are the source of CH*, at least in lean flames. Two pairs of rate constants, [18] and [20], produce excellent agreement with the atmospheric pressure data from both flames, though the rate constants proposed by the Peeters group [18] provide a better match for very lean mixtures. The use of CH*/OH* chemiluminescence ratios for equivalence ratio (/) sensing in methane flames is shown to be highly dependent on pressure, with the sensitivity to / reversing as pressure increases from 1 to 15 atm. Thus at some intermediate pressure, the CH*/OH* ratio should become insensitive to /. With regard to heat release rate sensing, CH* chemiluminescence scales with heat release, but it also depends strongly on local equivalence ratio and pressure. At high pressures, the dependence on / is reduced. Thus a ±5% change in / could easily lead to a ±30% error in heat release at atmospheric pressure, and a smaller ±10% error at 15 atm. For high pressures, predictions with the two rate constant models no longer agree. Thus further validation experiments at high pressure or with a different oxidizer would be necessary to determine the best mechanism. Also, various comparisons presented here support the use of the 1-d flame modeling approach for the laminar jet flames used in the validations. Furthermore, results from an atmospheric pressure swirl burner
indicate that the laminar results may be useful in some turbulent flames, at least where the reaction zone profiles are flamelet like. For other cases, the CH* mechanism will have to be added to more complex flame modeling codes to predict or interpret chemiluminescence sensing.
Acknowledgments This research was supported by NASA through the University Research, Engineering, and Technology Institute for Aeropropulsion and Power, under NCC3-982. The authors thank Dr. Adrian Kyne for providing the reduced mechanism for Jet-A, and Prof. Eric Petersen for the C2-mechanism modifications for GRI-Mech. References [1] A.G. Gaydon, H.G. Wolfhard, Flames: Their Structure, Radiation, and Temperature, fourth ed., Chapman and Hall, 1978. [2] N. Docquier, F. Lacas, S. Candel, Proc. Combust. Inst. 29 (2002) 139–145. [3] T.M. Muruganandam, B.H. Kim, M.R. Morrell, et al., Proc. Combust. Inst. 30 (2005) 1601–1609. [4] M. Yamazaki, M. Ohya, K. Tsuchiya, Int. Chem. Eng. 30 (1992) 160–168. [5] B.D. Stojkovic, T.D. Fansler, M.C. Drake, V. Sick, Proc. Combust. Inst. 30 (2005) 2657–2665. [6] J.G. Lee, K. Kim, D.A. Santavicca, Proc. Combust. Inst. 28 (2000) 415–421. [7] H.N. Najm, P.H. Paul, C.J. Mueller, P.S. Wyckoff, Combust. Flame 113 (3) (1998) 312–332. [8] B. Higgins, M.Q. McQuay, F. Lacas, S. Candel, Fuel 80 (11) (2001) 1583–1591. [9] N. Docquier, S. Belhafaoui, F. Lacas, N. Darabiha, J.-C. Rolon, Proc. Combust. Inst. 28 (2000) 1765– 1774. [10] Y. Hardalupas, M. Orain, Combust. Flame 139 (2004) 188–207. [11] M. De Leo, A. Saveliev, L.A. Kennedy, S.A. Zelepouga, Combust. Flame 149 (2007) 435–447. [12] V.N. Nori, J.M. Seitzman, Combust. Flame, submitted for publication. [13] M. Tamura, P.A. Berg, J.E. Harrington, et al., Combust. Flame 144 (1998) 502–514. [14] H.H. Brenig, Ph.D. Thesis, Wuppertal, 1981. [15] J. Grebe, K.H. Homann, Ber. Bunsenges. Phys. Chem. 86 (1982) 587. [16] G.P. Glass, G.B. Kistiakowsky, J.V. Michael, H. Niki, Proc. Combust. Inst. 10 (1965) 513–522. [17] J.M. Hall, J. De Vries, A.R. Amadio, E.L. Petersen, AIAA Paper 2005-1318, 43rd AIAA Aerospace Sciences Meeting and Exhibit, Reno, Nevada, 2005. [18] K. Devriendt, H. Van Look, B. Ceursters, J. Peeters, Chem. Phys. Lett. 261 (1996) 450–456. [19] A.M. Renlund, F. Shokoohi, H. Reisler, C. Wittig, J. Phys. Chem. 86 (21) (1982) 4165–4170. [20] R.M.I. Elsamra, S. Vranckx, S.A. Carl, J. Phys. Chem. A 109 (45) (2005) 10287–10293. [21] G.P. Smith, J. Luque, C. Park, J.B. Jeffries, D.R. Crosley, Combust. Flame 131 (2002) 59–69.
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