- Email: [email protected]
air flames
Combustion and Flame 159 (2012) 2563–2575
Contents lists available at SciVerse ScienceDirect
Combustion and Flame j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o m b u s t fl a m e
Effects of preferential transport in turbulent bluff-body-stabilized lean premixed CH4/air flames Robert S. Barlow a,⇑, Matthew J. Dunn a, Mark S. Sweeney b, Simone Hochgreb b a b
Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94550, USA Engineering Department, University of Cambridge, Cambridge CB2 1PZ, UK
a r t i c l e
i n f o
Article history: Available online 14 December 2011 Keywords: Turbulent premixed flames Bluff-body flames Differential diffusion Preferential transport Multiscalar diagnostics
a b s t r a c t Preferential species diffusion is known to have important effects on local flame structure in turbulent premixed flames, and differential diffusion of heat and mass can have significant effects on both local flame structure and global flame parameters, such as turbulent flame speed. However, models for turbulent premixed combustion normally assume that atomic mass fractions are conserved from reactants to fully burnt products. Experiments reported here indicate that this basic assumption may be incorrect for an important class of turbulent flames. Measurements of major species and temperature in the near field of turbulent, bluff-body stabilized, lean premixed methane–air flames (Le = 0.98) reveal significant departures from expected conditional mean compositional structure in the combustion products as well as within the flame. Net increases exceeding 10% in the equivalence ratio and the carbon-to-hydrogen atom ratio are observed across the turbulent flame brush. Corresponding measurements across an unstrained laminar flame at similar equivalence ratio are in close agreement with calculations performed using Chemkin with the GRI 3.0 mechanism and multi-component transport, confirming accuracy of experimental techniques. Results suggest that the large effects observed in the turbulent bluff-body burner are cause by preferential transport of H2 and H2O through the preheat zone ahead of CO2 and CO, followed by convective transport downstream and away from the local flame brush. This preferential transport effect increases with increasing velocity of reactants past the bluff body and is apparently amplified by the presence of a strong recirculation zone where excess CO2 is accumulated. Ó 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
1. Introduction It is well established in combustion literature that the structure and propagation rate of turbulent premixed flames can be strongly affected by preferential molecular transport. Lipatnikov and Chomiak [1] provide a comprehensive review on this topic. They focus in particular on evidence from both experiments and Direct Numerical Simulation (DNS) that differences between molecular diffusion coefficients for fuel, oxidizer, and heat can have significant influence on flame structure and propagation even in moderately and highly turbulent flames, and they review theoretical and modeling approaches developed to address these phenomena. From DNS of turbulent premixed flames it is known that local flame structure is strongly altered by focusing or defocusing of fast diffusing species [2–4]. Both experimental and computational work has shown that super-adiabatic pockets can be formed in turbulent premixed flames as a result of preferential diffusion. This has been observed in lean hydrogen–air flames, using planar
⇑ Corresponding author. E-mail address: [email protected] (R.S. Barlow).
Rayleigh scattering [5] and 3D DNS with detailed chemistry [6,7]. Super-adiabatic temperatures have also been observed in rich hydrocarbon–air flames [8,9]. The Lewis number, Le, defined as the ratio of thermal to fuel mass diffusivity, is an important parameter in characterizing molecular diffusion effects in premixed combustion. For Le 1, as for premixed hydrogen–air flames, global flame characteristics including topology and propagation speed are strongly affected [1,5–7], and these effects have been shown to extend to high Reynolds number [1,10]. The development of models to account for effects of non-unity Lewis number, as well as effects of turbulence parameters, flame strain and curvature is an active area of research. However, even when accounting for effects of preferential transport, theory and models for turbulent premixed combustion generally assume that atom balances are conserved in going from reactants to fully burnt products across a turbulent flame brush [1,10–12]. The present paper will show that this basic assumption does not necessarily hold true for an important class of turbulent flames. It is also well established in the combustion literature that differential molecular diffusion can have significant effects on the local and global scalar structure of turbulent non-premixed flames, especially in the near field of flames at low to moderate Reynolds
0010-2180/$ - see front matter Ó 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.combustflame.2011.11.013
2564
R.S. Barlow et al. / Combustion and Flame 159 (2012) 2563–2575
number [13–16]. Experimental evidence for this has come primarily from application of spontaneous Raman scattering to measure major species mass fractions, often combined with Rayleigh scattering to measure temperature. The effects of differential diffusion in non-premixed or partially premixed jet flames tend to become less important with increasing Reynolds number and are most often neglected in modeling such flames. While Raman/Rayleigh diagnostics have been applied in a large number of studies on non-premixed and partially premixed flames [17,18], sometimes combined with laser induced fluorescence (LIF) measurements of minor species, relatively few applications of these techniques to premixed turbulent flames have been reported. Chen et al. [19] applied line-imaged Raman/Rayleigh/OH-LIPF diagnostics to turbulent, piloted methane–air Bunsen flames, but reported only profiles mean species mass fractions and did not consider the effects of preferential diffusion. Nandula et al. [20] obtained single point measurements of major species, temperature, OH and NO in a turbulent, enclosed, bluff-body stabilized lean methane–air flame. They reported radial profiles of mean and rms temperature and mole fractions, as well as scatter plots of species vs. temperature at selected locations, but effects of preferential transport were not considered. Frank and Barlow [21] applied Raman/Rayleigh combined with LIF of CO, OH, and NO to piloted, lean methane–air Bunsen flames. They compare scatter plots of measured mole fractions vs. temperature to results from laminar calculations. However, because the spatial resolution of these point measurements was 0.75 mm, the internal flame structure could not be accurately resolved. Measured equivalence ratio is also reported, with values within reactant and products are in agreement within experimental uncertainty. Meier et al. [22] investigated a swirl stabilized, technically premixed (not perfectly premixed) lean methane–air flames and reported measurements of temperature and mixture fraction based on pointwise Raman scattering. Incomplete mixing of reactants near the flame base precludes interpretation of preferential transport effects. Gregor et al. [23] obtained line-imaged Raman/Rayleigh measurements in the premixed TECFLAM bluff-body stabilized swirl burner. Scatter plots of equivalence ratio vs. temperature are presented but not conditional means. It appears that the mean equivalence ratio in the products is slightly higher than in the reactants, but the difference is small, and the authors attribute this to experimental uncertainty. Premixed and stratified methane–air V-flames in weak turbulent (u0 /SL near unity) were studied in [24] using Raman/Rayleigh/LIF diagnostics with improved spatial resolution and precision compared to the studies cited above. The measured equivalence ratio in the products was reported to be about 3% too high, and this was attributed to uncertainty in measurement of H2. Results also show non-zero (unphysical) CH4 mass fraction in the products, due to under correction of background signal, which would also have contributed to the elevated results for equivalence ratio. This present paper provides strong experimental evidence, based on Raman/Rayleigh/LIF diagnostics, of a potentially important effect of preferential molecular diffusion in premixed flames that, as far as we are aware, has not been reported previously in
Table 1 Representative uncertainties in scalar measurements at flame conditions. Scalar
Precision, r (%)
Accuracy (%)
Premixed flame
T N2 CO2 H2O / CO H2
0.5 0.7 3.2 2.1 1.6 5.0 6.0
2 2 4 3 4 10 10
/ = 0.97, / = 0.97, / = 0.97, / = 0.97, / = 0.97, / = 1.28, / = 1.28,
T = 2185 T = 2185 T = 2185 T = 2185 T = 2185 T = 2045 T = 2045
Fig. 1. Co-annular swirl burner diagram (dimensions in mm), shown without the 20-cm OD laminar coflow assembly.
the combustion literature. Specifically, measurements of major species and temperature in the near field of bluff-body-stabilized, turbulent, lean premixed methane–air flames (Le = 0.98) have revealed significant increases in the equivalence ratio / and the atom ratio C/H when crossing a relatively thin turbulent flame brush from reactants to products. The measured mass fraction of CO2 in the burnt products was significantly higher than expected for the reactant mixture, and the mass fraction of O2 in the products was lower than expected. It will be shown that these observations cannot be attributed to experimental error and that they appear to result from preferential molecular transport, the effects of which are amplified by the presence of the bluff-body recirculation zone. Furthermore, the amplitude of the effect measured in the bluffbody configuration increases with increasing reactant velocity past the bluff body. This runs counter to the general concept in the field of combustion that the influence of preferential diffusion on flame structure decreases as the level of turbulence increases. This preferential transport phenomenon was observed during the course of a study on the influence of mixture stratification on the structure of turbulent methane–air flames. The complete study involves measurements of a dozen different premixed and stratified flames stabilized on a co-annular bluff-body burner with variable swirl [25]. In the present paper we focus only on uniformly premixed flames and on the apparent effects of preferential transport on the global scalar structure of these turbulent bluff-body stabilized flames. Multiscalar results on effects of mixture stratification will be reported separately. In the following sections the measurement techniques and experimental conditions are described, and the accuracy of the system is demonstrated for an unstrained laminar premixed flame. Results from turbulent premixed flames stabilized on the Cambridge/Sandia co-annular bluff-body burner are then presented, showing that atom balances are not conserved through the turbulent flame brush in this configuration at the flow conditions
R.S. Barlow et al. / Combustion and Flame 159 (2012) 2563–2575
Fig. 2. (a) Photograph of the laminar ‘‘vertical’’ premixed flame burner (left), with illustration of the near-normal intersection of the laser beam axis with the flame. (b) Photograph of case SwB1, the non-swirling premixed flame with / = 0.75 in both annular streams, with the green line segment illustrating the location of the 6-mm laser probe volume at z = 10 mm downstream of the bluff body. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
investigated. Results from laminar unstrained flame calculations and from subsequent experiments of four simple premixed burners of different geometry are used to provide insight on physical mechanisms contributing to the phenomenon observed in the co-annular burner experiments.
2. Measurement techniques The facility for multiscalar measurements at Sandia [24,26] combined line imaging of spontaneous Raman scattering, Rayleigh scattering, and two-photon LIF of CO, in order to obtain single-shot profiles of temperature and the concentrations of major species (CO2, O2, CO, N2, CH4, H2O, and H2) along a 6-mm segment. The beams from four frequency-doubled Nd:YAG lasers were used for Raman and Rayleigh line imaging, yielding a total energy of 1.8 J/pulse in the probe volume. Optical delay lines were used to temporally stretch each pulse to avoid optical breakdown at the focus, which had a diameter of 0.22 mm (1/e2). CO was excited at 230.1 nm (two photons), with the UV laser beam aligned on the same axis as the Nd:YAG laser beams. CCD cameras for line-imaged Raman scattering, Rayleigh scattering, and CO-LIF were mounted onto a single detection unit. A pair of 150-mm diameter achromats (Linos Photonics, f/2 and f/4) imaged a portion of the laser beam into this detection unit. The main internal components included two custom-built, motor-driven chopper wheels, six commercial camera lenses, a custom transmission grating (1200 lines/mm, Kaiser Optical), and mirrors and filters to separate and isolate the light signals for Raman
2565
scattering (550–700 nm), Rayleigh scattering (532 nm), and CO fluorescence (480–488 nm). The CCD cameras were the same as used in the previous collection/detection setup, with the non-intensified, low-noise, cryogenically-cooled Raman detection array (Princeton Instruments VersArray 1300B with CryoTiger cooling unit, 110 C operating temperature) being most critical for the overall system, due to its low noise characteristics. Gating for the Raman camera was 4.9 ls (FWHM). The projected (object plane) spacing of binned superpixels along the laser axis was 0.103 mm, 0.020 mm, and 0.101 mm, for the Raman, Rayleigh, and CO-LIF measurements, respectively. The optical resolution of 0.04–0.06 mm across most of the detection region was limited primarily by the pair of achromatic lenses at the front end of the collection system. Focusing and common alignment of each of the three cameras was accomplished by placing a target (linear pattern of 0.050-mm laser drilled holes on 0.200mm centers) at the object plane and back-illuminating the target through a diffuser with light of appropriate wavelength. Data processing was performed using a recently developed method [26] that integrates theoretically calculated Raman spectral libraries to determine the temperature dependence of system response for each molecule, excluding methane. Calibrations were based on cold flows and flat CH4/air flames of known composition. Table 1 lists uncertainties at representative conditions, based on the standard deviations (precision) and estimated accuracy of averaged measurements. Estimates of accuracy were based on uncertainties in calibration flow conditions, repeatability of measurement, and variation of measured results along the 6-mm probe length. All flows were measured using mass flow controllers that were calibrated (within 1% of reading) against laminar flow elements. Preliminary velocity measurements on the co-annular burner were conducted using particle image velocimetry (PIV), and they are used here to only to illustrate the shape of the bluff-body recirculation zone and show the locations of shear layers. The flows were seeded with 1 lm calcined aluminum oxide particles. These particles were illuminated using a double-pulsed Nd:YAG laser (Litron Nano PIV) operating at 532 nm. The vertical laser sheets had sub-millimeter thickness at the burner centerline. Pulses were separated by a time delay of Dt of 16 ls to optimize overall performance for the range of velocity encountered. The light scattered by the seed particles was imaged using a CCD camera (LaVision Imager Pro X 4M) fitted with a Nikon AF Micro Nikkor 60 mm lens (f/ 4) and a 50 mm interference filter centered at 532 nm (0.5 nm FWHM). The image resolution was 37.1 lm/pixel. The PIV system was run at 7.5 Hz, and the raw images were processed using LaVision software (DaVis 7.4). Algorithmic mask generation was used to constrain calculations to relevant areas in each image, and vectors were calculated using single-pass cross-correlation with a 32 32 window size and 50% overlap, giving a vector field resolution of 0.61 mm/vector.
3. Burners and flow conditions The Cambridge/Sandia burner [25], shown in Fig. 1, was designed to stabilize premixed and stratified flames over a range of flow rates, stratification levels, and swirl numbers in order to investigate the effects of these parameters on turbulent flame structure. The burner consists of two concentric annular flow passages, with a ceramic central bluff body to aid flame stabilization. The inner annulus provided an axial flow at an equivalence ratio, /2, equal or higher than the outer annular flow. The outer annulus, /1, was operated with either axial flow or an adjustable swirl flow. Swirl was generated by directing a metered fraction of the total
2566
R.S. Barlow et al. / Combustion and Flame 159 (2012) 2563–2575
Fig. 3. Comparison of measured results from the laminar vertical flame for species mass fractions, equivalence ratio, and selected atom ratios plotted vs. temperature and calculated results for an unstrained flame at / = 0.73. Error bars plotted around the measured conditional means represent the conditional fluctuation (±r) in the measurements.
outer annular flow through a swirl-flow plenum with eight injector holes orientated upwards at 30° to the horizontal and 30° to the annulus itself. The extended flow section between injectors and the burner exit plane allowed for well developed turbulent flow. A large annular laminar co-flow (diameter 19.2 cm, velocity Uc = 0.4 m/s) of filtered air was used to prevent environmental entrainment and to provide known boundary conditions. Twelve premixed and stratified flames were measured in detail, varying stratification ratio, swirl strength, and overall equivalence
ratio. In all cases the inner annulus (5.46-mm gap) was operated with bulk axial velocity U2 = 8.31 m/s (based on volumetric flow rate and flow area) and Re = 5960 (based on hydraulic diameter). In all cases the outer annulus (4.70-mm gap) was run with bulk axial velocity U1 = 18.7 m/s and Re = 11,540. The complete study on the effects of stratification on turbulent flame structure will be reported elsewhere. In the present work, the focus is on preferential transport effects, using results from a non-swirling premixed flame, SwB1, with /1 = /2 = 0.75 in the two annular streams.
R.S. Barlow et al. / Combustion and Flame 159 (2012) 2563–2575
4. Laminar flame results In order to assess the performance of the multiscalar diagnostic system in resolving the internal structure of premixed CH4/air flames, a burner was constructed to produce a vertical and nearly planar, unstrained laminar premixed flame. As shown in Fig. 2a, this was a rod-stabilized V-flame, but with the rod mounted near the edge of the 50-mm diameter premixed laminar flow and with the whole burner tilted to bring the longer leg of the V-flame to a near vertical orientation, normal to the laser axis. Figure 3 compares measurements from this ‘‘vertical’’ flame against results from Chemkin Pro (R 15082 from Reaction Design) for an unstrained premixed CH4/air flame at / = 0.73, using GRI Mech 3.0 [27] and multi-component transport with the Soret effect included. (All laminar calculations in the present paper used the above mechanism and transport models, unless otherwise stated.) Results for species mass fractions, equivalence ratio, and the atom ratios C/ H, C/O, C/N, and H/N are plotted vs. temperature. The measurements are plotted as conditional mean values with error bars indicating ±1 standard deviation, r, in the conditional fluctuation. Conditional statistics for this laminar flame are determined from 300 laser shots where the complete flame is always included within the 6-mm probe length. Note that the statistical uncertainty in
2567
measured conditional means is significantly smaller than the indicated conditional fluctuations. In the present work, equivalence ratio from the experiments and from Chemkin was calculated from the fuel/oxygen atom balance using only the measured species (CH4, O2, CO2, H2O, CO, and H2), according to the formula:
/¼
½ðX H2 þ X H2O Þ=2 þ X CO2 þ X CO þ 2X CH4 X O2 þ X CO2 þ ½ðX H2O þ X CO Þ=2
where Xi are the mole fractions of the measured major species. The influence of minor species on the calculated local equivalence ratio is negligible in the context of the comparisons and conclusions of the present paper. The agreement between measured and calculated flame structure is excellent, with only minor differences apparent in some scalars. The small deficit in the measurement of the CO mass fraction YCO near the peak value is an expected effect of spatial averaging, as the spatial profile of CO has a narrow peak that cannot be fully resolved with 0.10 mm data spacing. In order to illustrate the spatial resolution of the laminar flame structure, example single shot profiles from the laminar vertical flame are plotted in Fig. 4, along with results from the Chemkin calculation. To facilitate visual comparison, each single-shot profile has been shifted to align the location of maximum YCO with the origin. This is necessary because the
Fig. 4. Four single shot measurements from the laminar vertical flame with corresponding spatial profiles from the Chemkin calculation at / = 0.73 (dashed line). The spatial coordinate for each profile has been shifted to align the maximum CO mass fraction at the origin.
2568
R.S. Barlow et al. / Combustion and Flame 159 (2012) 2563–2575
Fig. 5. Comparison of measured results for / and the atom ratios C/H and H/N against unstrained flame calculations using multi-component transport (upper row) or mixture-averaged transport (lower row), with the Soret effect included in both. Error bars plotted around the measured conditional means represent the conditional fluctuation (±r) in the measurements.
position of the laminar flame fluctuated somewhat due to room air currents. The laminar measurements show a thicker flame than is calculated. This difference is greater than can be attributed to spatial averaging and may reflect differences in actual boundary conditions from the ideal freely propagating flame. However, it is clear that the internal structure of the flame is reasonably well resolved by the experiment. The region of CO burnout extends more than 2 mm beyond the location of maximum YCO, so there is no issue of spatial averaging in that part of the flame. Noise in each scalar profile is consistent with levels of conditional fluctuation in Fig. 3. The relative noise level for the H2 measurement is high at these very low concentrations because at high temperature a significant portion of the signal on the H2 channel comes from background luminosity, which must be subtracted. However, this does not translate to excessive noise or significant bias in / or atom ratios because the H2 levels are so low. There are two observations to be made relevant to the following section on turbulent premixed flame structure. First, the equivalence ratio and the atom ratios are not constant through the flame. In this planar flame geometry (1D calculation) these scalars must have the same values at the product and reactant boundaries far from the reaction zone. However, differential diffusion of species causes variation of /, C/H, and other atomic ratios within the flame. Second, the initial trajectory of YH2 vs. T, starting from room temperature, is vertical. This is also a result of preferential diffusion of H2, which moves into the reactants ahead of the thermal profile. Deviations of the measured conditional mean scalar profiles through this laminar flame are small compared to differences in the turbulent flame results reported in the next section. The calculation shown in Figs. 3 and 4 used multi-component transport, whereas most DNS of turbulent flames apply mixture averaged transport or even simpler levels of transport modeling in order to control computational cost. Figure 5 compares measured profiles of / and the atom ratios C/H and H/N against Chemkin calculations using multi-component (upper row) or
mixture-averaged (lower row) transport. The Soret effect was included in both calculations. The multi-component results are clearly more consistent with the measurements. In their review of the influence of detailed chemistry and transport models in combustion simulations, Hilbert et al. [28] point to numerous cases where preferential transport can significantly alter the local structure of premixed flames and where different levels of transport modeling can produce quantitative and even qualitative differences among simulation results. They conclude that the level of transport modeling can be as important as the level of chemical kinetic modeling in simulations of turbulent flames. Grcar et al. [29] investigated the effect of transport models in DNS naturally propagating lean premixed hydrogen–air flames, and they reported significant differences in flame structure and propagation speed between results using the multi-component transport model, with Dufort and Soret effects, and those using the mixture averaged approximation without cross diffusion. Differences may be less important in simulations of hydrocarbon combustion, but it would be prudent to make the comparison if detailed simulations are undertaken to further investigate the preferential transport effects identified in present experimental study.
5. Apparent effects of preferential molecular transport in turbulent flames Measurements in turbulent premixed CH4/air flames stabilized on the bluff-body burner reveal what appear to be significant effects of preferential transport on the mean spatial profiles and conditional mean results for certain scalars. Figure 6 shows measured conditional means and conditional fluctuations of species mass fractions, equivalence ratio, and the atom ratios C/H, C/O, C/N, and H/N from case SwB1 (shown in Fig. 2b), with the 6-mm probe volume aligned radially at z = 10 mm above the burner exit and centered in the middle of the turbulent flame brush. Conditional statistics are calculated
R.S. Barlow et al. / Combustion and Flame 159 (2012) 2563–2575
2569
Fig. 6. Comparison of measured results for species mass fractions, equivalence ratio, and selected atom ratios from z = 10 mm above the bluff body surface in the turbulent flame SwB1 plotted vs. temperature and compared with calculated results for an unstrained laminar flame at / = 0.75. Error bars plotted around the measured conditional means represent the conditional fluctuation (±r) in the measurements.
from 1500 laser shots (90,000 total samples) using a minimum of 300 samples in each temperature bin. Bin width is inversely proportional to the temperature gradient, with a maximum bin width of 47 K. Results from an unstrained laminar flame calculation at / = 0.75 are shown in each graph for comparison. Several observations can be made. The measured curve of conditional mean equivalence ratio within the flame is significantly higher than the calculation, and the final value in the products is roughly 13% higher than the initial value in the reactants. Similarly,
the C/H, C/O, and C/N atom ratios are not conserved going from reactants to products across the flame brush, but end at values more than 10% higher than in the reactants. This is at a condition where H2 and CO mass fractions are very small, so burnout is nearly complete. The final value of CO2 in the products is higher than expected based on the equivalence ratio of the reactant stream, and the O2 mass fraction is lower. The conditional mean curve of H2 mass fraction in the turbulent flame does not follow the laminar calculation in the low temperature range but is more
2570
R.S. Barlow et al. / Combustion and Flame 159 (2012) 2563–2575
Fig. 7. Expanded view of the behavior of conditional mean H2 mass fraction in the low temperature range of the laminar and turbulent flames. Error bars plotted around the measured conditional means represent the conditional fluctuation (±r) in the measurements.
linear, suggesting an effect of turbulent transport within the preheat zone. Figure 7 shows an expanded view of the measured results for YH2 up to 400 K in the laminar and turbulent flames, along with results from laminar calculations at the corresponding equivalence ratios. Figure 8 shows radial profiles of measured mean and rms temperature and equivalence ratio and the mean C/H atom ratio in the same non-swirling flame, SwB1, at three downstream locations, z = 10, 20, 30 mm. The changes in measured / and C/H going
through the flame brush are greatest at z = 10 mm. The magnitude of the effects decrease with downstream distance but are still present at z = 30 mm. Farther downstream (z P 40 mm) the flame becomes stratified as it begins to interact with the mixing layer between the outer annular CH4/air flow and the coflowing air. In Fig. 6 the conditional means of YCO2, YO2, /, and C/H in the products at maximum temperature differ from the laminar calculations by four or more standard deviations in the conditional fluctuations. Furthermore, the level of conditional fluctuation of each measured scalar on the product side of the turbulent flame brush is comparable to that in the laminar flame results. This indicates that the change in scalar structure is expressed not only in the mean but in essentially every instantaneous realization in the turbulent flame. Figure 9 displays example single shot spatial profiles from the turbulent flame SwB1 (z = 10 mm), along with results from the Chemkin laminar flame calculation at / = 7.5. Again, the origin in each single-shot profile has been shifted to the location of maximum YCO. These example profiles are typical of the complete data set at this location in that obvious effects of turbulent eddies appear to be limited to the preheat zone, and distortion of the thermal ramp at higher temperatures is rare. It is clear that the profiles of YCO2, YO2, /, and C/H depart significantly from the laminar calculation in each realization. This departure can start very early in the preheat zone and extends through the CO burnout zone. These turbulent flame results, showing significant changes in atom balances across a relatively thin flame brush, were unexpected and would be difficult to believe without confirmation from the previous section that the measured scalar structure of laminar premixed flames is in excellent agreement with calculations. Care-
Fig. 8. Radial profiles of mean and rms temperature and equivalence ratio and the mean of the C/H atom ratio measured in flame SwB1 (non-swirling premixed flame, /1 = /2 = 0.75) at axial locations of z = 10, 20, 30 mm.
R.S. Barlow et al. / Combustion and Flame 159 (2012) 2563–2575
2571
Fig. 9. Four single shot measurements from the turbulent flame SwB1 at z = 10 mm with corresponding spatial profiles from the Chemkin calculation at / = 0.75 (dashed line). The spatial coordinate for each profile has been shifted to align the maximum CO mass fraction at the origin.
ful examination of all the calibration and data processing methods did not reveal any significant errors or inconsistencies. Differences between measured values of YCO2, YO2, /, and atom balances in the turbulent flame products and the values expected for the equivalence ratio of the reactants are well outside the limits of experimental uncertainty. Therefore, a physical explanation of the observed phenomenon is needed.
6. Analysis and further experiments to identify physical mechanisms A possible explanation for these turbulent flame results comes in part from consideration of the effects of preferential diffusion in the laminar flame calculations. The initial trajectory of the calculated curve for YH2 vs. T is nearly vertical (see Fig. 7), pointing to the potential importance of preferential diffusion of molecular hydrogen through the preheat zone and into the axially oriented flow. Spatial profiles of temperature and the mole fractions of H2, H2O, CO2, and CO are shown in Fig. 10a for the laminar flame calculation at / = 0.73. It is clear that H2 diffuses well ahead of the thermal profile and the other species profiles. It is also apparent that H2O has higher mole fraction than CO2 and CO near the front of the
preheat zone, possibly due to a combination of preferential diffusion and lower temperature oxidation of H2. In the calculations the spatial profile of H2O leads that of CO2 by nearly 0.3 mm in this freely propagating flame. Figure 10b shows the ratio of mole fractions (XH2O + XH2)/(XCO2 + XCO), which increases rapidly toward the reactant side of the flame. This determines the behavior of the C/H ratio in the preheat portion of the laminar flame (Fig. 3). The rise in this mole fraction ratio begins near the location of maximum heat release rate, which is marked by the vertical line in Fig. 10b. Also, H2 becomes more important than H2O near the zero location in the plotted spatial profile, as it diffuses ahead of other scalars. We hypothesize that in the turbulent flames H2 and H2O are preferentially transported through the preheat zone into the reactants. Then, due to turbulent mixing and the high angle between the mean velocity vector in the reactants and the flame brush normal, these species are further transported downstream and away from the local flame, increasing the C/H atom ratio within the flame and in the products. H2 and H2O mole fractions in the reactants remain below the detection limit of the experiment, so this downstream transport process cannot be tracked directly by the measurements. Furthermore, based on the above analysis, it was hypothesized that it should be possible to observe a similar effect of preferential transport on the C/H atom ratio in an anchored
2572
R.S. Barlow et al. / Combustion and Flame 159 (2012) 2563–2575
Fig. 10. Calculated spatial profiles of temperature and (a) mole fractions of H2, H2O, CO2, and CO; and (b) mole fraction ratios of these species, showing the effect of differential molecular diffusion through the preheat zone of an unstrained laminar premixed CH4/air flame. The vertical line at 0.8 mm in (b) marks the location of maximum heat release rate.
laminar flame oriented at a high angle to the direction of a reactant flow (high ratio of streamwise velocity to laminar flame speed). Two simple laminar burners were constructed to test this hypothesis and gain insight on physical mechanisms contributing to the observed behavior in the Cambridge/Sandia burner flames. These two burners were constructed from 25.4 mm by 12.7 mm aluminum channel (3.2 mm wall thickness) placed on either side
of a thin (0.43 mm) stainless steel splitter plate that extended 5 mm past the channel exit plane. Screens and a 50-mm length of honeycomb were mounted at the base of each rectangular channel, and the flow developed over roughly 300 mm to produce a laminar velocity gradient (not measured) normal to the splitter plate. For the burner shown in Fig. 11a a perforated plate was installed at the exit of one channel (left side) to stabilize a pilot flame in order to anchor a laminar premixed flame. The burner was mounted at an angle, so that the center portion of the flame was nearly vertical and normal to the axis of the laser beam. This piloted burner was operated with lower reactant flow rate through the pilot side than through the open channel. The burner in Fig. 11b was of similar construction, with two channels separated by a splitter plate. However, this flame was stabilized by a 1-mm diameter rod located at the end of the splitter plate. This V-flame burner, with laminar velocity gradients on both sides of the splitter plane, was also mounted at an angle to align one side of the Vflame normal to the laser beam. Reactant flow was split roughly evenly between the two sides of this V-flame burner. Measurements in these flames did not reveal significant differences from the results in Fig. 3 from the laminar ‘‘vertical’’ flame shown in Fig. 2a, for which the angle between the approach flow and the flame normal vector was much smaller and there was no velocity gradient across the approach flow. Therefore, if there is any effect of preferential transport on atom balances in the products for these two burner geometries, the effect is too small to be measured with confidence using the present system. Detailed measurements are not shown. Direct simulation of a similar laminar configuration would be needed to further test the above hypothesis. Simulation of laminar bluff-body stabilized laminar flames would also be informative. Subsequently, because the co-annular burner was no longer at Sandia, a simple bluff-body burner was built with the same center body diameter, 12.7 mm, and a single annular non-swirling premixed flow, as shown in Fig. 11c. The annular gap of 5.0 mm was close to that for the inner annulus of the Cambridge/Sandia burner. A perforated plate was mounted in the annular gap 35 mm upstream of the exit plane to act as a turbulence generator. The bulk velocity was varied (Ub = 1.2, 1.8, 2.4, 3.6, 4.8, 6.0, 7.2 m/s). The intended equivalence ratio for these flames was / = 0.75 match that of the SwB1 flame. However, the measured equivalence ratio in the reactants was 0.77, which is within the combined uncertainty in the measurement and flow rates. Measurements were obtained
Fig. 11. Photos and diagrams of three simple burners constructed to gain qualitative insights on contributing physical mechanisms: (a) piloted splitter plate burner in which combustion products from the pilot anchor a nearly planar laminar flame that burns into the reactant flow on the opposite side of the splitter plate; (b) symmetric splitter plate burner with laminar V-flame anchored by a 1-mm rod at the end of the splitter plate; (c) simple annular burner with a central bluff-body.
R.S. Barlow et al. / Combustion and Flame 159 (2012) 2563–2575
2573
Fig. 12. Measured conditional mean mass fractions and atom ratios in a series of premixed CH4/air flames stabilized on the simple bluff-body burner (Fig. 10d) for increasing bulk velocity of the annular flow: 2.4 m/s (dash), 4.8 m/s (dot-dash), 7.2 m/s (solid). Chemkin results for an unstrained laminar 1D flame at / = 0.77 (bold line) are included in each plot.
at z = 10 mm above the burner exit, as for the Cambridge/Sandia burner results shown in Fig. 6. Figure 12 shows conditional mean curves for YCO2, YO2, YH2, YCO, /, C/H, C/O, C/N, and H/N based on 500 shots from each of three flames (Ub = 2.4, 4.8, 7.2 m/s) in comparison with a Chemkin calculation at / = 0.77. A significant trend is revealed. The lower velocity cases, Ub 6 2.4, are consistent with the 1D laminar calculation, within measurement uncertainty, while results from the Ub = 7.2 m/s case are similar to those from the Cambridge/Sandia burner (8.3 m/s inner annular flow). The intermediate velocity cases follow a roughly evenly spaced progression, with only the results from the Ub = 4.8 m/s flame being shown for clarity of the figure. In addition to the strong effects of preferential transport on YCO2, YO2, /, C/H, C/O, C/N shown in Fig. 12, there is an apparent reduction in product temperature with increasing reactant velocity. However, heat loss to the bluff-body surface may be a contributing factor in this temperature trend, so it is not possible to make a quantitative conclusion regarding the effect of preferential transport on mean temperature in the burnt products. Mass fractions of H2 and CO decrease mildly with increasing reactant velocity, and this trend may be due to increasing strain. Also, changes in the trajectory of the YH2 curves through the low temperature region
suggest an increasing effect of turbulent transport in the preheat zone, as discussed above. The Reynolds number based on the bluff-body diameter and cold flow properties of the reactants is 5910 for the highest velocity case, and that based on hydraulic diameter of the annular flow is 4670. However, it is not clear at this point what dimensionless quantities will prove most appropriate for describing the observed behavior of scalar properties. The results in Fig. 12 suggest that the presence of a recirculation zone is a critical factor and that the effects of preferential transport are amplified as the strength of the recirculation zone increases. The importance of the recirculation zone was confirmed by testing a fourth burner (not shown) with the bluff body replaced by a pilot flame, which was stabilized on a perforated plate just below the exit of a 12.7-mm OD tube. When operated at the same 7.2 m/s bulk annular flow velocity, measurements did not show the strong effects of preferential transport observed in the bluff body burners. The shape of the recirculation zone is shown in Fig. 13, which represents the two-dimensional mean velocity field for the SwB1 flame on the Cambridge/Sandia burner, as measured by PIV. Interpolated streamlines are displayed with a color code corresponding to the magnitude of velocity in the measurement plane. The
2574
R.S. Barlow et al. / Combustion and Flame 159 (2012) 2563–2575
Fig. 13. Mean velocity field in the axial–radial plane measured by PIV in flame SwB1 (no swirl). Interpolated streamlines are shown, while the color scale corresponds to velocity magnitude in the PIV plane.
turbulent flame brush is adjacent to the edge of the recirculation zone, such that there is a significant gradient in streamwise velocity within the reactants near the flame and, presumably, across the flame brush. A possible explanation for the apparent influence of the recirculation zone on the magnitude of the observed transport effect is illustrated conceptually in Fig. 14, which shows the recirculation zone and flame locations. It is proposed that H2 and H2O diffuse preferentially ahead of CO2 and CO toward the reactants and are subsequently transported downstream and away from the local flame brush. Products are depleted of hydrogen relative to carbon, and because the fluid within the recirculation zone has some residence time, the effect is amplified or integrated over the effective residence time. As velocity increases the flame brush becomes more tightly constrained between the reactant flow and the recirculation zone. In this context one could expect that the ratio of reactant flow velocity to turbulent flame speed may be an important parameter. The explanation above is speculative but well supported by the experimental results. There is more to be done to understand the physical mechanisms contributing to the experimental results reported here. Dependence on stoichiometry, fuel type, and flow parameters should be further studied, and application of direct simulation to investigate these effects could be very informative. It is not yet known whether this apparent effect of preferential transport in turbulent premixed flames is important with regard to accurate modeling of practical combustion systems. It is worth noting, however, that available evidence suggests that the effect increases with increasing velocity (Reynolds number). Furthermore, while this preferential transport effect was observed in bluff-body stabilized lean premixed CH4/air flames at atmospheric pressure, it might be present in any premixed or stratified hydrocarbon flame that is stabilized by a strong recirculation zone at the edge of a high shear region. Additionally, there is significant
Fig. 14. Conceptual diagram illustrating the proposed process of preferential transport of H2 and H2O into the reactant stream, combined with accumulation of excess CO2 within the recirculation zone.
current research interest in combustion of high hydrogen content fuels, and such fuels blends may exhibit even larger effects of preferential transport than observed in the present methane flames.
7. Conclusions Multiscalar measurements in bluff-body-stabilized, turbulent, lean premixed CH4/air flames revealed that atom balances (atomic mass fractions) were not conserved across the flame brush going from reactants to products. Increases exceeding 10% in measured mean values of the equivalence ratio and the carbon-to-hydrogen atom ratio were observed, along with corresponding changes in the mass fractions of CO2 and O2 in the burnt products. Measurements in an unstrained laminar flame at similar equivalence ratio showed close agreement with calculations using Chemkin with GRI Mech 3.0 and multicomponent transport, confirming the accuracy of the laser diagnostic techniques. Additional experiments on laminar and turbulent burners suggested that the observed effects result from preferential diffusive transport of H2 and H2O toward the reactants ahead of CO2 and CO, followed by convective transport downstream and away from the local flame brush. The presence of a recirculation zone was identified as a key factor, apparently causing the preferential transport effect to be amplified. In this bluff-body stabilized configuration, the magnitude of the
R.S. Barlow et al. / Combustion and Flame 159 (2012) 2563–2575
preferential transport effect increased with increasing velocity of the reactant flow, suggesting that the effect may be present in practical combustion systems where premixed flames are stabilized by strong recirculation zones. Acknowledgments Sandia authors were supported by the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences, US Department of Energy. Sandia National Laboratories is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94-AL85000. Cambridge authors were supported by EPSRC and Rolls-Royce. Contributions by Bob Harmon in support of these experiments are gratefully acknowledged. Forman Williams and Ed Richardson contributed insightful comments regarding the potential importance of H2O transport and the presence of the recirculation zone, respectively. References [1] [2] [3] [4] [5] [6]
A.N. Lipatnikov, J. Chomiak, Prog. Energy Combust. Sci. 31 (2005) 1–73. J.H. Chen, T. Echekki, W. Kollmann, Combust. Flame 116 (1998) 15–48. H.G. Im, J.H. Chen, Combust. Flame 131 (2002) 246–258. J. de Charentenay, A. Ern, Combust. Theory Modell. 6 (2002) 463–478. Y.C. Chen, R.W. Bilger, Combust. Flame 138 (2004) 155–174. J.B. Bell, R.K. Cheng, M.S. Day, I.G. Shepherd, Proc. Combust. Inst. 31 (2007) 1309–1317.
2575
[7] M. Day, J. Bell, P.-T. Bremer, V. Pascucci, V. Beckner, M. Lijewski, Combust. Flame 156 (2009) 1035–1045. [8] F. Liu, Ö.L. Gülder, Combust. Flame 143 (2005) 264–281. [9] F. Liu, Ö.L. Gülder, Combust. Sci. Technol. 180 (2008) 437–452. [10] F. Dinkelacker, B. Manickam, S.P.R. Muppala, Combust. Flame 158 (2011) 1742–1749. [11] D. Veynante, L. Vervisch, Prog. Energy Combust. Sci. 28 (2002) 193–266. [12] N. Chakraborty, R.S. Cant, Combust. Flame 156 (2009) 1427–1444. [13] M.C. Drake, R.W. Bilger, S.H. Stårner, Proc. Combust. Inst. 19 (1982) 459–467. [14] R.W. Dibble, M.B. Long, Combust. Flame 143 (2005) 644–649. [15] R.S. Barlow, J.H. Frank, A.N. Karpetis, J.-Y. Chen, Combust. Flame 143 (2005) 433–449. [16] H. Pitsch, Combust. Flame 123 (2000) 358–374. [17] R.S. Barlow, Proc. Combust. Inst. 31 (2007) 49–75. [18] E.P. Hassel, S. Linow, Meas. Sci. Technol. 11 (2000) R37–R57. [19] Y.C. Chen, N. Peters, G.A. Schneemann, N. Wruck, U. Renz, M.S. Mansour, Combust. Flame 107 (1996) 223–244. [20] S.P. Nandula, R.W. Pitz, R.S. Barlow, G.J. Fiechtner, AIAA 96-0937 (1996). [21] J.H. Frank, R.S. Barlow, Proc. Combust. Inst. 27 (1998) 759–766. [22] W. Meier, P. Weigand, X.R. Duan, R. Giezendanner-Thoben, Combust. Flame 150 (2007) 2–26. [23] M.A. Gregor, F. Seffrin, F. Fuest, D. Geyer, A. Dreizler, Proc. Combust. Inst. 32 (2009) 1739–1746. [24] R.S. Barlow, G.H. Wang, P. Anselmo-Filho, M.S. Sweeney, S. Hochgreb, Proc. Combust. Inst. 32 (2009) 945–953. [25] M.S. Sweeney, S. Hochgreb, M.J. Dunn, R.S. Barlow, Proc. Combust. Inst. 33 (2011) 1419–1427. [26] F. Fuest, R.S. Barlow, D. Geyer, F. Seffrin, A. Dreizler, Proc. Combust. Inst. 33 (2011) 815–822. [27] G.P. Smith et al., GRI-Mech 3.0.. [28] R. Hilbert, F. Tap, H. El-Rabii, D. Thévenin, Prog. Energy Combust. Sci. 30 (2004) 61–117. [29] J.F. Grcar, J.B. Bell, M.S. Day, Proc. Combust. Inst. 32 (2009) 1173–1180.
Contents lists available at SciVerse ScienceDirect
Combustion and Flame j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o m b u s t fl a m e
Effects of preferential transport in turbulent bluff-body-stabilized lean premixed CH4/air flames Robert S. Barlow a,⇑, Matthew J. Dunn a, Mark S. Sweeney b, Simone Hochgreb b a b
Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94550, USA Engineering Department, University of Cambridge, Cambridge CB2 1PZ, UK
a r t i c l e
i n f o
Article history: Available online 14 December 2011 Keywords: Turbulent premixed flames Bluff-body flames Differential diffusion Preferential transport Multiscalar diagnostics
a b s t r a c t Preferential species diffusion is known to have important effects on local flame structure in turbulent premixed flames, and differential diffusion of heat and mass can have significant effects on both local flame structure and global flame parameters, such as turbulent flame speed. However, models for turbulent premixed combustion normally assume that atomic mass fractions are conserved from reactants to fully burnt products. Experiments reported here indicate that this basic assumption may be incorrect for an important class of turbulent flames. Measurements of major species and temperature in the near field of turbulent, bluff-body stabilized, lean premixed methane–air flames (Le = 0.98) reveal significant departures from expected conditional mean compositional structure in the combustion products as well as within the flame. Net increases exceeding 10% in the equivalence ratio and the carbon-to-hydrogen atom ratio are observed across the turbulent flame brush. Corresponding measurements across an unstrained laminar flame at similar equivalence ratio are in close agreement with calculations performed using Chemkin with the GRI 3.0 mechanism and multi-component transport, confirming accuracy of experimental techniques. Results suggest that the large effects observed in the turbulent bluff-body burner are cause by preferential transport of H2 and H2O through the preheat zone ahead of CO2 and CO, followed by convective transport downstream and away from the local flame brush. This preferential transport effect increases with increasing velocity of reactants past the bluff body and is apparently amplified by the presence of a strong recirculation zone where excess CO2 is accumulated. Ó 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
1. Introduction It is well established in combustion literature that the structure and propagation rate of turbulent premixed flames can be strongly affected by preferential molecular transport. Lipatnikov and Chomiak [1] provide a comprehensive review on this topic. They focus in particular on evidence from both experiments and Direct Numerical Simulation (DNS) that differences between molecular diffusion coefficients for fuel, oxidizer, and heat can have significant influence on flame structure and propagation even in moderately and highly turbulent flames, and they review theoretical and modeling approaches developed to address these phenomena. From DNS of turbulent premixed flames it is known that local flame structure is strongly altered by focusing or defocusing of fast diffusing species [2–4]. Both experimental and computational work has shown that super-adiabatic pockets can be formed in turbulent premixed flames as a result of preferential diffusion. This has been observed in lean hydrogen–air flames, using planar
⇑ Corresponding author. E-mail address: [email protected] (R.S. Barlow).
Rayleigh scattering [5] and 3D DNS with detailed chemistry [6,7]. Super-adiabatic temperatures have also been observed in rich hydrocarbon–air flames [8,9]. The Lewis number, Le, defined as the ratio of thermal to fuel mass diffusivity, is an important parameter in characterizing molecular diffusion effects in premixed combustion. For Le 1, as for premixed hydrogen–air flames, global flame characteristics including topology and propagation speed are strongly affected [1,5–7], and these effects have been shown to extend to high Reynolds number [1,10]. The development of models to account for effects of non-unity Lewis number, as well as effects of turbulence parameters, flame strain and curvature is an active area of research. However, even when accounting for effects of preferential transport, theory and models for turbulent premixed combustion generally assume that atom balances are conserved in going from reactants to fully burnt products across a turbulent flame brush [1,10–12]. The present paper will show that this basic assumption does not necessarily hold true for an important class of turbulent flames. It is also well established in the combustion literature that differential molecular diffusion can have significant effects on the local and global scalar structure of turbulent non-premixed flames, especially in the near field of flames at low to moderate Reynolds
0010-2180/$ - see front matter Ó 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.combustflame.2011.11.013
2564
R.S. Barlow et al. / Combustion and Flame 159 (2012) 2563–2575
number [13–16]. Experimental evidence for this has come primarily from application of spontaneous Raman scattering to measure major species mass fractions, often combined with Rayleigh scattering to measure temperature. The effects of differential diffusion in non-premixed or partially premixed jet flames tend to become less important with increasing Reynolds number and are most often neglected in modeling such flames. While Raman/Rayleigh diagnostics have been applied in a large number of studies on non-premixed and partially premixed flames [17,18], sometimes combined with laser induced fluorescence (LIF) measurements of minor species, relatively few applications of these techniques to premixed turbulent flames have been reported. Chen et al. [19] applied line-imaged Raman/Rayleigh/OH-LIPF diagnostics to turbulent, piloted methane–air Bunsen flames, but reported only profiles mean species mass fractions and did not consider the effects of preferential diffusion. Nandula et al. [20] obtained single point measurements of major species, temperature, OH and NO in a turbulent, enclosed, bluff-body stabilized lean methane–air flame. They reported radial profiles of mean and rms temperature and mole fractions, as well as scatter plots of species vs. temperature at selected locations, but effects of preferential transport were not considered. Frank and Barlow [21] applied Raman/Rayleigh combined with LIF of CO, OH, and NO to piloted, lean methane–air Bunsen flames. They compare scatter plots of measured mole fractions vs. temperature to results from laminar calculations. However, because the spatial resolution of these point measurements was 0.75 mm, the internal flame structure could not be accurately resolved. Measured equivalence ratio is also reported, with values within reactant and products are in agreement within experimental uncertainty. Meier et al. [22] investigated a swirl stabilized, technically premixed (not perfectly premixed) lean methane–air flames and reported measurements of temperature and mixture fraction based on pointwise Raman scattering. Incomplete mixing of reactants near the flame base precludes interpretation of preferential transport effects. Gregor et al. [23] obtained line-imaged Raman/Rayleigh measurements in the premixed TECFLAM bluff-body stabilized swirl burner. Scatter plots of equivalence ratio vs. temperature are presented but not conditional means. It appears that the mean equivalence ratio in the products is slightly higher than in the reactants, but the difference is small, and the authors attribute this to experimental uncertainty. Premixed and stratified methane–air V-flames in weak turbulent (u0 /SL near unity) were studied in [24] using Raman/Rayleigh/LIF diagnostics with improved spatial resolution and precision compared to the studies cited above. The measured equivalence ratio in the products was reported to be about 3% too high, and this was attributed to uncertainty in measurement of H2. Results also show non-zero (unphysical) CH4 mass fraction in the products, due to under correction of background signal, which would also have contributed to the elevated results for equivalence ratio. This present paper provides strong experimental evidence, based on Raman/Rayleigh/LIF diagnostics, of a potentially important effect of preferential molecular diffusion in premixed flames that, as far as we are aware, has not been reported previously in
Table 1 Representative uncertainties in scalar measurements at flame conditions. Scalar
Precision, r (%)
Accuracy (%)
Premixed flame
T N2 CO2 H2O / CO H2
0.5 0.7 3.2 2.1 1.6 5.0 6.0
2 2 4 3 4 10 10
/ = 0.97, / = 0.97, / = 0.97, / = 0.97, / = 0.97, / = 1.28, / = 1.28,
T = 2185 T = 2185 T = 2185 T = 2185 T = 2185 T = 2045 T = 2045
Fig. 1. Co-annular swirl burner diagram (dimensions in mm), shown without the 20-cm OD laminar coflow assembly.
the combustion literature. Specifically, measurements of major species and temperature in the near field of bluff-body-stabilized, turbulent, lean premixed methane–air flames (Le = 0.98) have revealed significant increases in the equivalence ratio / and the atom ratio C/H when crossing a relatively thin turbulent flame brush from reactants to products. The measured mass fraction of CO2 in the burnt products was significantly higher than expected for the reactant mixture, and the mass fraction of O2 in the products was lower than expected. It will be shown that these observations cannot be attributed to experimental error and that they appear to result from preferential molecular transport, the effects of which are amplified by the presence of the bluff-body recirculation zone. Furthermore, the amplitude of the effect measured in the bluffbody configuration increases with increasing reactant velocity past the bluff body. This runs counter to the general concept in the field of combustion that the influence of preferential diffusion on flame structure decreases as the level of turbulence increases. This preferential transport phenomenon was observed during the course of a study on the influence of mixture stratification on the structure of turbulent methane–air flames. The complete study involves measurements of a dozen different premixed and stratified flames stabilized on a co-annular bluff-body burner with variable swirl [25]. In the present paper we focus only on uniformly premixed flames and on the apparent effects of preferential transport on the global scalar structure of these turbulent bluff-body stabilized flames. Multiscalar results on effects of mixture stratification will be reported separately. In the following sections the measurement techniques and experimental conditions are described, and the accuracy of the system is demonstrated for an unstrained laminar premixed flame. Results from turbulent premixed flames stabilized on the Cambridge/Sandia co-annular bluff-body burner are then presented, showing that atom balances are not conserved through the turbulent flame brush in this configuration at the flow conditions
R.S. Barlow et al. / Combustion and Flame 159 (2012) 2563–2575
Fig. 2. (a) Photograph of the laminar ‘‘vertical’’ premixed flame burner (left), with illustration of the near-normal intersection of the laser beam axis with the flame. (b) Photograph of case SwB1, the non-swirling premixed flame with / = 0.75 in both annular streams, with the green line segment illustrating the location of the 6-mm laser probe volume at z = 10 mm downstream of the bluff body. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
investigated. Results from laminar unstrained flame calculations and from subsequent experiments of four simple premixed burners of different geometry are used to provide insight on physical mechanisms contributing to the phenomenon observed in the co-annular burner experiments.
2. Measurement techniques The facility for multiscalar measurements at Sandia [24,26] combined line imaging of spontaneous Raman scattering, Rayleigh scattering, and two-photon LIF of CO, in order to obtain single-shot profiles of temperature and the concentrations of major species (CO2, O2, CO, N2, CH4, H2O, and H2) along a 6-mm segment. The beams from four frequency-doubled Nd:YAG lasers were used for Raman and Rayleigh line imaging, yielding a total energy of 1.8 J/pulse in the probe volume. Optical delay lines were used to temporally stretch each pulse to avoid optical breakdown at the focus, which had a diameter of 0.22 mm (1/e2). CO was excited at 230.1 nm (two photons), with the UV laser beam aligned on the same axis as the Nd:YAG laser beams. CCD cameras for line-imaged Raman scattering, Rayleigh scattering, and CO-LIF were mounted onto a single detection unit. A pair of 150-mm diameter achromats (Linos Photonics, f/2 and f/4) imaged a portion of the laser beam into this detection unit. The main internal components included two custom-built, motor-driven chopper wheels, six commercial camera lenses, a custom transmission grating (1200 lines/mm, Kaiser Optical), and mirrors and filters to separate and isolate the light signals for Raman
2565
scattering (550–700 nm), Rayleigh scattering (532 nm), and CO fluorescence (480–488 nm). The CCD cameras were the same as used in the previous collection/detection setup, with the non-intensified, low-noise, cryogenically-cooled Raman detection array (Princeton Instruments VersArray 1300B with CryoTiger cooling unit, 110 C operating temperature) being most critical for the overall system, due to its low noise characteristics. Gating for the Raman camera was 4.9 ls (FWHM). The projected (object plane) spacing of binned superpixels along the laser axis was 0.103 mm, 0.020 mm, and 0.101 mm, for the Raman, Rayleigh, and CO-LIF measurements, respectively. The optical resolution of 0.04–0.06 mm across most of the detection region was limited primarily by the pair of achromatic lenses at the front end of the collection system. Focusing and common alignment of each of the three cameras was accomplished by placing a target (linear pattern of 0.050-mm laser drilled holes on 0.200mm centers) at the object plane and back-illuminating the target through a diffuser with light of appropriate wavelength. Data processing was performed using a recently developed method [26] that integrates theoretically calculated Raman spectral libraries to determine the temperature dependence of system response for each molecule, excluding methane. Calibrations were based on cold flows and flat CH4/air flames of known composition. Table 1 lists uncertainties at representative conditions, based on the standard deviations (precision) and estimated accuracy of averaged measurements. Estimates of accuracy were based on uncertainties in calibration flow conditions, repeatability of measurement, and variation of measured results along the 6-mm probe length. All flows were measured using mass flow controllers that were calibrated (within 1% of reading) against laminar flow elements. Preliminary velocity measurements on the co-annular burner were conducted using particle image velocimetry (PIV), and they are used here to only to illustrate the shape of the bluff-body recirculation zone and show the locations of shear layers. The flows were seeded with 1 lm calcined aluminum oxide particles. These particles were illuminated using a double-pulsed Nd:YAG laser (Litron Nano PIV) operating at 532 nm. The vertical laser sheets had sub-millimeter thickness at the burner centerline. Pulses were separated by a time delay of Dt of 16 ls to optimize overall performance for the range of velocity encountered. The light scattered by the seed particles was imaged using a CCD camera (LaVision Imager Pro X 4M) fitted with a Nikon AF Micro Nikkor 60 mm lens (f/ 4) and a 50 mm interference filter centered at 532 nm (0.5 nm FWHM). The image resolution was 37.1 lm/pixel. The PIV system was run at 7.5 Hz, and the raw images were processed using LaVision software (DaVis 7.4). Algorithmic mask generation was used to constrain calculations to relevant areas in each image, and vectors were calculated using single-pass cross-correlation with a 32 32 window size and 50% overlap, giving a vector field resolution of 0.61 mm/vector.
3. Burners and flow conditions The Cambridge/Sandia burner [25], shown in Fig. 1, was designed to stabilize premixed and stratified flames over a range of flow rates, stratification levels, and swirl numbers in order to investigate the effects of these parameters on turbulent flame structure. The burner consists of two concentric annular flow passages, with a ceramic central bluff body to aid flame stabilization. The inner annulus provided an axial flow at an equivalence ratio, /2, equal or higher than the outer annular flow. The outer annulus, /1, was operated with either axial flow or an adjustable swirl flow. Swirl was generated by directing a metered fraction of the total
2566
R.S. Barlow et al. / Combustion and Flame 159 (2012) 2563–2575
Fig. 3. Comparison of measured results from the laminar vertical flame for species mass fractions, equivalence ratio, and selected atom ratios plotted vs. temperature and calculated results for an unstrained flame at / = 0.73. Error bars plotted around the measured conditional means represent the conditional fluctuation (±r) in the measurements.
outer annular flow through a swirl-flow plenum with eight injector holes orientated upwards at 30° to the horizontal and 30° to the annulus itself. The extended flow section between injectors and the burner exit plane allowed for well developed turbulent flow. A large annular laminar co-flow (diameter 19.2 cm, velocity Uc = 0.4 m/s) of filtered air was used to prevent environmental entrainment and to provide known boundary conditions. Twelve premixed and stratified flames were measured in detail, varying stratification ratio, swirl strength, and overall equivalence
ratio. In all cases the inner annulus (5.46-mm gap) was operated with bulk axial velocity U2 = 8.31 m/s (based on volumetric flow rate and flow area) and Re = 5960 (based on hydraulic diameter). In all cases the outer annulus (4.70-mm gap) was run with bulk axial velocity U1 = 18.7 m/s and Re = 11,540. The complete study on the effects of stratification on turbulent flame structure will be reported elsewhere. In the present work, the focus is on preferential transport effects, using results from a non-swirling premixed flame, SwB1, with /1 = /2 = 0.75 in the two annular streams.
R.S. Barlow et al. / Combustion and Flame 159 (2012) 2563–2575
4. Laminar flame results In order to assess the performance of the multiscalar diagnostic system in resolving the internal structure of premixed CH4/air flames, a burner was constructed to produce a vertical and nearly planar, unstrained laminar premixed flame. As shown in Fig. 2a, this was a rod-stabilized V-flame, but with the rod mounted near the edge of the 50-mm diameter premixed laminar flow and with the whole burner tilted to bring the longer leg of the V-flame to a near vertical orientation, normal to the laser axis. Figure 3 compares measurements from this ‘‘vertical’’ flame against results from Chemkin Pro (R 15082 from Reaction Design) for an unstrained premixed CH4/air flame at / = 0.73, using GRI Mech 3.0 [27] and multi-component transport with the Soret effect included. (All laminar calculations in the present paper used the above mechanism and transport models, unless otherwise stated.) Results for species mass fractions, equivalence ratio, and the atom ratios C/ H, C/O, C/N, and H/N are plotted vs. temperature. The measurements are plotted as conditional mean values with error bars indicating ±1 standard deviation, r, in the conditional fluctuation. Conditional statistics for this laminar flame are determined from 300 laser shots where the complete flame is always included within the 6-mm probe length. Note that the statistical uncertainty in
2567
measured conditional means is significantly smaller than the indicated conditional fluctuations. In the present work, equivalence ratio from the experiments and from Chemkin was calculated from the fuel/oxygen atom balance using only the measured species (CH4, O2, CO2, H2O, CO, and H2), according to the formula:
/¼
½ðX H2 þ X H2O Þ=2 þ X CO2 þ X CO þ 2X CH4 X O2 þ X CO2 þ ½ðX H2O þ X CO Þ=2
where Xi are the mole fractions of the measured major species. The influence of minor species on the calculated local equivalence ratio is negligible in the context of the comparisons and conclusions of the present paper. The agreement between measured and calculated flame structure is excellent, with only minor differences apparent in some scalars. The small deficit in the measurement of the CO mass fraction YCO near the peak value is an expected effect of spatial averaging, as the spatial profile of CO has a narrow peak that cannot be fully resolved with 0.10 mm data spacing. In order to illustrate the spatial resolution of the laminar flame structure, example single shot profiles from the laminar vertical flame are plotted in Fig. 4, along with results from the Chemkin calculation. To facilitate visual comparison, each single-shot profile has been shifted to align the location of maximum YCO with the origin. This is necessary because the
Fig. 4. Four single shot measurements from the laminar vertical flame with corresponding spatial profiles from the Chemkin calculation at / = 0.73 (dashed line). The spatial coordinate for each profile has been shifted to align the maximum CO mass fraction at the origin.
2568
R.S. Barlow et al. / Combustion and Flame 159 (2012) 2563–2575
Fig. 5. Comparison of measured results for / and the atom ratios C/H and H/N against unstrained flame calculations using multi-component transport (upper row) or mixture-averaged transport (lower row), with the Soret effect included in both. Error bars plotted around the measured conditional means represent the conditional fluctuation (±r) in the measurements.
position of the laminar flame fluctuated somewhat due to room air currents. The laminar measurements show a thicker flame than is calculated. This difference is greater than can be attributed to spatial averaging and may reflect differences in actual boundary conditions from the ideal freely propagating flame. However, it is clear that the internal structure of the flame is reasonably well resolved by the experiment. The region of CO burnout extends more than 2 mm beyond the location of maximum YCO, so there is no issue of spatial averaging in that part of the flame. Noise in each scalar profile is consistent with levels of conditional fluctuation in Fig. 3. The relative noise level for the H2 measurement is high at these very low concentrations because at high temperature a significant portion of the signal on the H2 channel comes from background luminosity, which must be subtracted. However, this does not translate to excessive noise or significant bias in / or atom ratios because the H2 levels are so low. There are two observations to be made relevant to the following section on turbulent premixed flame structure. First, the equivalence ratio and the atom ratios are not constant through the flame. In this planar flame geometry (1D calculation) these scalars must have the same values at the product and reactant boundaries far from the reaction zone. However, differential diffusion of species causes variation of /, C/H, and other atomic ratios within the flame. Second, the initial trajectory of YH2 vs. T, starting from room temperature, is vertical. This is also a result of preferential diffusion of H2, which moves into the reactants ahead of the thermal profile. Deviations of the measured conditional mean scalar profiles through this laminar flame are small compared to differences in the turbulent flame results reported in the next section. The calculation shown in Figs. 3 and 4 used multi-component transport, whereas most DNS of turbulent flames apply mixture averaged transport or even simpler levels of transport modeling in order to control computational cost. Figure 5 compares measured profiles of / and the atom ratios C/H and H/N against Chemkin calculations using multi-component (upper row) or
mixture-averaged (lower row) transport. The Soret effect was included in both calculations. The multi-component results are clearly more consistent with the measurements. In their review of the influence of detailed chemistry and transport models in combustion simulations, Hilbert et al. [28] point to numerous cases where preferential transport can significantly alter the local structure of premixed flames and where different levels of transport modeling can produce quantitative and even qualitative differences among simulation results. They conclude that the level of transport modeling can be as important as the level of chemical kinetic modeling in simulations of turbulent flames. Grcar et al. [29] investigated the effect of transport models in DNS naturally propagating lean premixed hydrogen–air flames, and they reported significant differences in flame structure and propagation speed between results using the multi-component transport model, with Dufort and Soret effects, and those using the mixture averaged approximation without cross diffusion. Differences may be less important in simulations of hydrocarbon combustion, but it would be prudent to make the comparison if detailed simulations are undertaken to further investigate the preferential transport effects identified in present experimental study.
5. Apparent effects of preferential molecular transport in turbulent flames Measurements in turbulent premixed CH4/air flames stabilized on the bluff-body burner reveal what appear to be significant effects of preferential transport on the mean spatial profiles and conditional mean results for certain scalars. Figure 6 shows measured conditional means and conditional fluctuations of species mass fractions, equivalence ratio, and the atom ratios C/H, C/O, C/N, and H/N from case SwB1 (shown in Fig. 2b), with the 6-mm probe volume aligned radially at z = 10 mm above the burner exit and centered in the middle of the turbulent flame brush. Conditional statistics are calculated
R.S. Barlow et al. / Combustion and Flame 159 (2012) 2563–2575
2569
Fig. 6. Comparison of measured results for species mass fractions, equivalence ratio, and selected atom ratios from z = 10 mm above the bluff body surface in the turbulent flame SwB1 plotted vs. temperature and compared with calculated results for an unstrained laminar flame at / = 0.75. Error bars plotted around the measured conditional means represent the conditional fluctuation (±r) in the measurements.
from 1500 laser shots (90,000 total samples) using a minimum of 300 samples in each temperature bin. Bin width is inversely proportional to the temperature gradient, with a maximum bin width of 47 K. Results from an unstrained laminar flame calculation at / = 0.75 are shown in each graph for comparison. Several observations can be made. The measured curve of conditional mean equivalence ratio within the flame is significantly higher than the calculation, and the final value in the products is roughly 13% higher than the initial value in the reactants. Similarly,
the C/H, C/O, and C/N atom ratios are not conserved going from reactants to products across the flame brush, but end at values more than 10% higher than in the reactants. This is at a condition where H2 and CO mass fractions are very small, so burnout is nearly complete. The final value of CO2 in the products is higher than expected based on the equivalence ratio of the reactant stream, and the O2 mass fraction is lower. The conditional mean curve of H2 mass fraction in the turbulent flame does not follow the laminar calculation in the low temperature range but is more
2570
R.S. Barlow et al. / Combustion and Flame 159 (2012) 2563–2575
Fig. 7. Expanded view of the behavior of conditional mean H2 mass fraction in the low temperature range of the laminar and turbulent flames. Error bars plotted around the measured conditional means represent the conditional fluctuation (±r) in the measurements.
linear, suggesting an effect of turbulent transport within the preheat zone. Figure 7 shows an expanded view of the measured results for YH2 up to 400 K in the laminar and turbulent flames, along with results from laminar calculations at the corresponding equivalence ratios. Figure 8 shows radial profiles of measured mean and rms temperature and equivalence ratio and the mean C/H atom ratio in the same non-swirling flame, SwB1, at three downstream locations, z = 10, 20, 30 mm. The changes in measured / and C/H going
through the flame brush are greatest at z = 10 mm. The magnitude of the effects decrease with downstream distance but are still present at z = 30 mm. Farther downstream (z P 40 mm) the flame becomes stratified as it begins to interact with the mixing layer between the outer annular CH4/air flow and the coflowing air. In Fig. 6 the conditional means of YCO2, YO2, /, and C/H in the products at maximum temperature differ from the laminar calculations by four or more standard deviations in the conditional fluctuations. Furthermore, the level of conditional fluctuation of each measured scalar on the product side of the turbulent flame brush is comparable to that in the laminar flame results. This indicates that the change in scalar structure is expressed not only in the mean but in essentially every instantaneous realization in the turbulent flame. Figure 9 displays example single shot spatial profiles from the turbulent flame SwB1 (z = 10 mm), along with results from the Chemkin laminar flame calculation at / = 7.5. Again, the origin in each single-shot profile has been shifted to the location of maximum YCO. These example profiles are typical of the complete data set at this location in that obvious effects of turbulent eddies appear to be limited to the preheat zone, and distortion of the thermal ramp at higher temperatures is rare. It is clear that the profiles of YCO2, YO2, /, and C/H depart significantly from the laminar calculation in each realization. This departure can start very early in the preheat zone and extends through the CO burnout zone. These turbulent flame results, showing significant changes in atom balances across a relatively thin flame brush, were unexpected and would be difficult to believe without confirmation from the previous section that the measured scalar structure of laminar premixed flames is in excellent agreement with calculations. Care-
Fig. 8. Radial profiles of mean and rms temperature and equivalence ratio and the mean of the C/H atom ratio measured in flame SwB1 (non-swirling premixed flame, /1 = /2 = 0.75) at axial locations of z = 10, 20, 30 mm.
R.S. Barlow et al. / Combustion and Flame 159 (2012) 2563–2575
2571
Fig. 9. Four single shot measurements from the turbulent flame SwB1 at z = 10 mm with corresponding spatial profiles from the Chemkin calculation at / = 0.75 (dashed line). The spatial coordinate for each profile has been shifted to align the maximum CO mass fraction at the origin.
ful examination of all the calibration and data processing methods did not reveal any significant errors or inconsistencies. Differences between measured values of YCO2, YO2, /, and atom balances in the turbulent flame products and the values expected for the equivalence ratio of the reactants are well outside the limits of experimental uncertainty. Therefore, a physical explanation of the observed phenomenon is needed.
6. Analysis and further experiments to identify physical mechanisms A possible explanation for these turbulent flame results comes in part from consideration of the effects of preferential diffusion in the laminar flame calculations. The initial trajectory of the calculated curve for YH2 vs. T is nearly vertical (see Fig. 7), pointing to the potential importance of preferential diffusion of molecular hydrogen through the preheat zone and into the axially oriented flow. Spatial profiles of temperature and the mole fractions of H2, H2O, CO2, and CO are shown in Fig. 10a for the laminar flame calculation at / = 0.73. It is clear that H2 diffuses well ahead of the thermal profile and the other species profiles. It is also apparent that H2O has higher mole fraction than CO2 and CO near the front of the
preheat zone, possibly due to a combination of preferential diffusion and lower temperature oxidation of H2. In the calculations the spatial profile of H2O leads that of CO2 by nearly 0.3 mm in this freely propagating flame. Figure 10b shows the ratio of mole fractions (XH2O + XH2)/(XCO2 + XCO), which increases rapidly toward the reactant side of the flame. This determines the behavior of the C/H ratio in the preheat portion of the laminar flame (Fig. 3). The rise in this mole fraction ratio begins near the location of maximum heat release rate, which is marked by the vertical line in Fig. 10b. Also, H2 becomes more important than H2O near the zero location in the plotted spatial profile, as it diffuses ahead of other scalars. We hypothesize that in the turbulent flames H2 and H2O are preferentially transported through the preheat zone into the reactants. Then, due to turbulent mixing and the high angle between the mean velocity vector in the reactants and the flame brush normal, these species are further transported downstream and away from the local flame, increasing the C/H atom ratio within the flame and in the products. H2 and H2O mole fractions in the reactants remain below the detection limit of the experiment, so this downstream transport process cannot be tracked directly by the measurements. Furthermore, based on the above analysis, it was hypothesized that it should be possible to observe a similar effect of preferential transport on the C/H atom ratio in an anchored
2572
R.S. Barlow et al. / Combustion and Flame 159 (2012) 2563–2575
Fig. 10. Calculated spatial profiles of temperature and (a) mole fractions of H2, H2O, CO2, and CO; and (b) mole fraction ratios of these species, showing the effect of differential molecular diffusion through the preheat zone of an unstrained laminar premixed CH4/air flame. The vertical line at 0.8 mm in (b) marks the location of maximum heat release rate.
laminar flame oriented at a high angle to the direction of a reactant flow (high ratio of streamwise velocity to laminar flame speed). Two simple laminar burners were constructed to test this hypothesis and gain insight on physical mechanisms contributing to the observed behavior in the Cambridge/Sandia burner flames. These two burners were constructed from 25.4 mm by 12.7 mm aluminum channel (3.2 mm wall thickness) placed on either side
of a thin (0.43 mm) stainless steel splitter plate that extended 5 mm past the channel exit plane. Screens and a 50-mm length of honeycomb were mounted at the base of each rectangular channel, and the flow developed over roughly 300 mm to produce a laminar velocity gradient (not measured) normal to the splitter plate. For the burner shown in Fig. 11a a perforated plate was installed at the exit of one channel (left side) to stabilize a pilot flame in order to anchor a laminar premixed flame. The burner was mounted at an angle, so that the center portion of the flame was nearly vertical and normal to the axis of the laser beam. This piloted burner was operated with lower reactant flow rate through the pilot side than through the open channel. The burner in Fig. 11b was of similar construction, with two channels separated by a splitter plate. However, this flame was stabilized by a 1-mm diameter rod located at the end of the splitter plate. This V-flame burner, with laminar velocity gradients on both sides of the splitter plane, was also mounted at an angle to align one side of the Vflame normal to the laser beam. Reactant flow was split roughly evenly between the two sides of this V-flame burner. Measurements in these flames did not reveal significant differences from the results in Fig. 3 from the laminar ‘‘vertical’’ flame shown in Fig. 2a, for which the angle between the approach flow and the flame normal vector was much smaller and there was no velocity gradient across the approach flow. Therefore, if there is any effect of preferential transport on atom balances in the products for these two burner geometries, the effect is too small to be measured with confidence using the present system. Detailed measurements are not shown. Direct simulation of a similar laminar configuration would be needed to further test the above hypothesis. Simulation of laminar bluff-body stabilized laminar flames would also be informative. Subsequently, because the co-annular burner was no longer at Sandia, a simple bluff-body burner was built with the same center body diameter, 12.7 mm, and a single annular non-swirling premixed flow, as shown in Fig. 11c. The annular gap of 5.0 mm was close to that for the inner annulus of the Cambridge/Sandia burner. A perforated plate was mounted in the annular gap 35 mm upstream of the exit plane to act as a turbulence generator. The bulk velocity was varied (Ub = 1.2, 1.8, 2.4, 3.6, 4.8, 6.0, 7.2 m/s). The intended equivalence ratio for these flames was / = 0.75 match that of the SwB1 flame. However, the measured equivalence ratio in the reactants was 0.77, which is within the combined uncertainty in the measurement and flow rates. Measurements were obtained
Fig. 11. Photos and diagrams of three simple burners constructed to gain qualitative insights on contributing physical mechanisms: (a) piloted splitter plate burner in which combustion products from the pilot anchor a nearly planar laminar flame that burns into the reactant flow on the opposite side of the splitter plate; (b) symmetric splitter plate burner with laminar V-flame anchored by a 1-mm rod at the end of the splitter plate; (c) simple annular burner with a central bluff-body.
R.S. Barlow et al. / Combustion and Flame 159 (2012) 2563–2575
2573
Fig. 12. Measured conditional mean mass fractions and atom ratios in a series of premixed CH4/air flames stabilized on the simple bluff-body burner (Fig. 10d) for increasing bulk velocity of the annular flow: 2.4 m/s (dash), 4.8 m/s (dot-dash), 7.2 m/s (solid). Chemkin results for an unstrained laminar 1D flame at / = 0.77 (bold line) are included in each plot.
at z = 10 mm above the burner exit, as for the Cambridge/Sandia burner results shown in Fig. 6. Figure 12 shows conditional mean curves for YCO2, YO2, YH2, YCO, /, C/H, C/O, C/N, and H/N based on 500 shots from each of three flames (Ub = 2.4, 4.8, 7.2 m/s) in comparison with a Chemkin calculation at / = 0.77. A significant trend is revealed. The lower velocity cases, Ub 6 2.4, are consistent with the 1D laminar calculation, within measurement uncertainty, while results from the Ub = 7.2 m/s case are similar to those from the Cambridge/Sandia burner (8.3 m/s inner annular flow). The intermediate velocity cases follow a roughly evenly spaced progression, with only the results from the Ub = 4.8 m/s flame being shown for clarity of the figure. In addition to the strong effects of preferential transport on YCO2, YO2, /, C/H, C/O, C/N shown in Fig. 12, there is an apparent reduction in product temperature with increasing reactant velocity. However, heat loss to the bluff-body surface may be a contributing factor in this temperature trend, so it is not possible to make a quantitative conclusion regarding the effect of preferential transport on mean temperature in the burnt products. Mass fractions of H2 and CO decrease mildly with increasing reactant velocity, and this trend may be due to increasing strain. Also, changes in the trajectory of the YH2 curves through the low temperature region
suggest an increasing effect of turbulent transport in the preheat zone, as discussed above. The Reynolds number based on the bluff-body diameter and cold flow properties of the reactants is 5910 for the highest velocity case, and that based on hydraulic diameter of the annular flow is 4670. However, it is not clear at this point what dimensionless quantities will prove most appropriate for describing the observed behavior of scalar properties. The results in Fig. 12 suggest that the presence of a recirculation zone is a critical factor and that the effects of preferential transport are amplified as the strength of the recirculation zone increases. The importance of the recirculation zone was confirmed by testing a fourth burner (not shown) with the bluff body replaced by a pilot flame, which was stabilized on a perforated plate just below the exit of a 12.7-mm OD tube. When operated at the same 7.2 m/s bulk annular flow velocity, measurements did not show the strong effects of preferential transport observed in the bluff body burners. The shape of the recirculation zone is shown in Fig. 13, which represents the two-dimensional mean velocity field for the SwB1 flame on the Cambridge/Sandia burner, as measured by PIV. Interpolated streamlines are displayed with a color code corresponding to the magnitude of velocity in the measurement plane. The
2574
R.S. Barlow et al. / Combustion and Flame 159 (2012) 2563–2575
Fig. 13. Mean velocity field in the axial–radial plane measured by PIV in flame SwB1 (no swirl). Interpolated streamlines are shown, while the color scale corresponds to velocity magnitude in the PIV plane.
turbulent flame brush is adjacent to the edge of the recirculation zone, such that there is a significant gradient in streamwise velocity within the reactants near the flame and, presumably, across the flame brush. A possible explanation for the apparent influence of the recirculation zone on the magnitude of the observed transport effect is illustrated conceptually in Fig. 14, which shows the recirculation zone and flame locations. It is proposed that H2 and H2O diffuse preferentially ahead of CO2 and CO toward the reactants and are subsequently transported downstream and away from the local flame brush. Products are depleted of hydrogen relative to carbon, and because the fluid within the recirculation zone has some residence time, the effect is amplified or integrated over the effective residence time. As velocity increases the flame brush becomes more tightly constrained between the reactant flow and the recirculation zone. In this context one could expect that the ratio of reactant flow velocity to turbulent flame speed may be an important parameter. The explanation above is speculative but well supported by the experimental results. There is more to be done to understand the physical mechanisms contributing to the experimental results reported here. Dependence on stoichiometry, fuel type, and flow parameters should be further studied, and application of direct simulation to investigate these effects could be very informative. It is not yet known whether this apparent effect of preferential transport in turbulent premixed flames is important with regard to accurate modeling of practical combustion systems. It is worth noting, however, that available evidence suggests that the effect increases with increasing velocity (Reynolds number). Furthermore, while this preferential transport effect was observed in bluff-body stabilized lean premixed CH4/air flames at atmospheric pressure, it might be present in any premixed or stratified hydrocarbon flame that is stabilized by a strong recirculation zone at the edge of a high shear region. Additionally, there is significant
Fig. 14. Conceptual diagram illustrating the proposed process of preferential transport of H2 and H2O into the reactant stream, combined with accumulation of excess CO2 within the recirculation zone.
current research interest in combustion of high hydrogen content fuels, and such fuels blends may exhibit even larger effects of preferential transport than observed in the present methane flames.
7. Conclusions Multiscalar measurements in bluff-body-stabilized, turbulent, lean premixed CH4/air flames revealed that atom balances (atomic mass fractions) were not conserved across the flame brush going from reactants to products. Increases exceeding 10% in measured mean values of the equivalence ratio and the carbon-to-hydrogen atom ratio were observed, along with corresponding changes in the mass fractions of CO2 and O2 in the burnt products. Measurements in an unstrained laminar flame at similar equivalence ratio showed close agreement with calculations using Chemkin with GRI Mech 3.0 and multicomponent transport, confirming the accuracy of the laser diagnostic techniques. Additional experiments on laminar and turbulent burners suggested that the observed effects result from preferential diffusive transport of H2 and H2O toward the reactants ahead of CO2 and CO, followed by convective transport downstream and away from the local flame brush. The presence of a recirculation zone was identified as a key factor, apparently causing the preferential transport effect to be amplified. In this bluff-body stabilized configuration, the magnitude of the
R.S. Barlow et al. / Combustion and Flame 159 (2012) 2563–2575
preferential transport effect increased with increasing velocity of the reactant flow, suggesting that the effect may be present in practical combustion systems where premixed flames are stabilized by strong recirculation zones. Acknowledgments Sandia authors were supported by the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences, US Department of Energy. Sandia National Laboratories is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94-AL85000. Cambridge authors were supported by EPSRC and Rolls-Royce. Contributions by Bob Harmon in support of these experiments are gratefully acknowledged. Forman Williams and Ed Richardson contributed insightful comments regarding the potential importance of H2O transport and the presence of the recirculation zone, respectively. References [1] [2] [3] [4] [5] [6]
A.N. Lipatnikov, J. Chomiak, Prog. Energy Combust. Sci. 31 (2005) 1–73. J.H. Chen, T. Echekki, W. Kollmann, Combust. Flame 116 (1998) 15–48. H.G. Im, J.H. Chen, Combust. Flame 131 (2002) 246–258. J. de Charentenay, A. Ern, Combust. Theory Modell. 6 (2002) 463–478. Y.C. Chen, R.W. Bilger, Combust. Flame 138 (2004) 155–174. J.B. Bell, R.K. Cheng, M.S. Day, I.G. Shepherd, Proc. Combust. Inst. 31 (2007) 1309–1317.
2575
[7] M. Day, J. Bell, P.-T. Bremer, V. Pascucci, V. Beckner, M. Lijewski, Combust. Flame 156 (2009) 1035–1045. [8] F. Liu, Ö.L. Gülder, Combust. Flame 143 (2005) 264–281. [9] F. Liu, Ö.L. Gülder, Combust. Sci. Technol. 180 (2008) 437–452. [10] F. Dinkelacker, B. Manickam, S.P.R. Muppala, Combust. Flame 158 (2011) 1742–1749. [11] D. Veynante, L. Vervisch, Prog. Energy Combust. Sci. 28 (2002) 193–266. [12] N. Chakraborty, R.S. Cant, Combust. Flame 156 (2009) 1427–1444. [13] M.C. Drake, R.W. Bilger, S.H. Stårner, Proc. Combust. Inst. 19 (1982) 459–467. [14] R.W. Dibble, M.B. Long, Combust. Flame 143 (2005) 644–649. [15] R.S. Barlow, J.H. Frank, A.N. Karpetis, J.-Y. Chen, Combust. Flame 143 (2005) 433–449. [16] H. Pitsch, Combust. Flame 123 (2000) 358–374. [17] R.S. Barlow, Proc. Combust. Inst. 31 (2007) 49–75. [18] E.P. Hassel, S. Linow, Meas. Sci. Technol. 11 (2000) R37–R57. [19] Y.C. Chen, N. Peters, G.A. Schneemann, N. Wruck, U. Renz, M.S. Mansour, Combust. Flame 107 (1996) 223–244. [20] S.P. Nandula, R.W. Pitz, R.S. Barlow, G.J. Fiechtner, AIAA 96-0937 (1996). [21] J.H. Frank, R.S. Barlow, Proc. Combust. Inst. 27 (1998) 759–766. [22] W. Meier, P. Weigand, X.R. Duan, R. Giezendanner-Thoben, Combust. Flame 150 (2007) 2–26. [23] M.A. Gregor, F. Seffrin, F. Fuest, D. Geyer, A. Dreizler, Proc. Combust. Inst. 32 (2009) 1739–1746. [24] R.S. Barlow, G.H. Wang, P. Anselmo-Filho, M.S. Sweeney, S. Hochgreb, Proc. Combust. Inst. 32 (2009) 945–953. [25] M.S. Sweeney, S. Hochgreb, M.J. Dunn, R.S. Barlow, Proc. Combust. Inst. 33 (2011) 1419–1427. [26] F. Fuest, R.S. Barlow, D. Geyer, F. Seffrin, A. Dreizler, Proc. Combust. Inst. 33 (2011) 815–822. [27] G.P. Smith et al., GRI-Mech 3.0.