air flames II: Swirling flows

Combustion and Flame 159 (2012) 2912–2929

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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

The structure of turbulent stratified and premixed methane/air flames II: Swirling flows Mark S. Sweeney a,⇑, Simone Hochgreb a, Matthew J. Dunn b, Robert S. Barlow b a b

Department of Engineering, University of Cambridge, UK Sandia National Laboratories, California, USA

a r t i c l e

i n f o

Article history: Available online 5 July 2012 Keywords: Turbulent combustion Lean stratified combustion Laser diagnostics Co-annular jet burner Scalar dissipation rate

a b s t r a c t Experimental results are presented from a series of turbulent methane/air stratified flames stabilized on a swirl burner. Nine operating conditions are considered, systematically varying the level of stratification  ¼ 0:75. Scalar data are obtained and swirl while maintaining a lean global mean equivalence ratio of / from Rayleigh/Raman/CO laser induced fluorescence (CO-LIF) line measurements at 103 lm resolution, allowing the behavior of the major combustion species—CH4, CO2, CO, H2, H2O and O2—to be probed within the instantaneous flame front. The corresponding three-dimensional surface density function and thermal scalar dissipation rate are investigated, along with geometric characteristics of the flame such as curvature and flame thickness. Hydrogen and carbon monoxide levels within the flame brush are raised by stratification, indicating models with laminar premixed flame chemistry may not be suitable for stratified flames. However, flame surface density, scalar dissipation and curvature all appear insensitive to the degree of stratification in the flames surveyed. Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction The fuel/oxidizer fields in practical combustion devices typically have spatial gradients in equivalence ratio (cf. mixture fraction) [1–5]. In such cases, the operating conditions are said to be partially premixed or stratified. The use of richer pilots in flows that are globally lean for emission and performance reasons is a common source of stratification in industrial applications such as gas turbine engines. A detailed review of the stratification literature in weakly and moderately turbulent combustion is given in the previous paper in the present series [6], and the brief synopsis provided here is simply for context. The effects of stratification on weakly turbulent ðu0 =SL  Oð1ÞÞ combustion have been investigated over the past decade using various test geometries, from bomb combustors [7] to slot burners [8]. More recently, research has focused on stratified flames with higher levels of turbulence [9–13]. The main findings in the low turbulence flows have been an enhancement of the burning rate [7,14] and a broadening of curvature distributions [8,14,15] under stratified conditions. The influence of stratification on curvature distributions in flames subject to moderate turbulence is less clear cut; the similar trends reported in [10] are contrasted by far less significant effects described in [6,13]. Attenuated turbulent flame speeds have been ⇑ Corresponding author. E-mail address: [email protected] (M.S. Sweeney).

observed under stratified conditions [10], indicating a degree of turbulence sensitivity in the results and highlighting the need to investigate flames subject to a range of turbulence intensities. Swirling flow fields are also common in industrial combustors for a number of reasons. Swirling flows can assist flame stabilization [16–19] by inducing or enhancing the recirculation of products. Greater turbulence intensities are also achievable in swirling flow fields, which can increase the heat release rate in combustion applications [20] due to the creation of flame surface area. Mixing can also be enhanced [16–18,21], an important factor where premixing of fuel and oxidizer is constrained or precluded prior to entering the combustion chamber. It is worth noting that the addition of swirl can also push combustors into unstable modes of operation [22]; such instabilities can cause significant damage and need to be avoided wherever possible. The effect of swirl on reacting flow fields has been researched extensively over the years, and was comprehensively reviewed by Syred and Beér [18]. However, despite the prevalence of stratified conditions in combustion devices, there have been relatively few studies of stratified combustion in open swirl combustors. The bulk of the work to date in this area has arisen from investigations in low swirl burners [23–27]. The present work is the second study of three investigating the effects of stratification on combustion, using experimental results from a series of methane/air flames in a turbulent swirl burner introduced in full in Part I [6]. This burner was designed to have an open flame, facilitating high fidelity scalar measurements. The

0010-2180/$ - see front matter Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.combustflame.2012.05.014

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design incorporates a central bluff-body and variable swirl to emulate in part the flow complexity and flame stabilization methods seen in practical systems. These design decisions were also made in the context of providing model test cases progressively more complex and therefore complementary to those of the Darmstadt stratified burner [9,10]. Results obtained for lean non-swirling flames were presented in Part I [6], considering a premixed case, SwB1; a moderately stratified case, SwB5; and a highly stratified case SwB9. The main finding was that the behavior of the key combustion species, excepting H2, is well captured on the mean in temperature space by laminar flame calculations, and shows no influence of stratification. H2 was observed at elevated levels in the stratified flames. Values for progress variable gradients and by extension thermal scalar dissipation rate were found to be substantially lower than laminar values; this was attributed to the thickening of the flame due to turbulence, which dominated the effect of increased strain. These findings held for both premixed and stratified flames. Local curvature distributions (derived from OH-PLIF images) showed negligible dependence on the degree of stratification, counter to many of the studies detailed previously [7,8,14,15] under less turbulent conditions. The stratification independence of the curvature distributions in the non-swirling cases shown in Part I [6], which holds at multiple locations downstream of the burner exit, was attributed to the higher turbulence levels in the stratified swirl burner than those in the aforementioned studies. The present study considers the combined effects of stratification and swirl, and results from the flames surveyed are presented as follows. The thermal and compositional structure of the flames are surveyed using radial profiles of the Favre-averaged temperature and equivalence ratio at a range of axial locations. Maps of mean and fluctuating velocity obtained from PIV measurements are used to characterize the flow field above the burner exit in reacting and non-reacting flows. Near-exit profiles of mean and fluctuating components of radial and axial velocities in non-reacting conditions are also shown so as to provide approximate boundary conditions. The behavior of species mass fractions in temperature space is then presented. Data are taken from the intersection of the mixing layer and the mean flame brush to highlight the effects of stratification on the mean behavior of these quantities. Here the mixing layer is defined by the locus of / = 0.5(/i + /o) = /g, where the subscripts i, o and g indicate inner annulus, outer annulus, and global equivalence ratios, respectively, and the mean flame brush is defined by the locus of peak RMS fluctuation of temperature. Scalar gradients, in the form of surface density function and scalar dissipation rate, are shown in progress variable space. The influence of stratification on topological quantities such as flame thickness and curvature is also investigated. The experimental data are available on request.

2. Experimental details Brief synopses of the swirl burner, operating conditions, and experimental methods are provided in the present work; detailed accounts are given in Part I [6].

2.1. Cambridge stratified swirl burner The swirl burner, referred to as SwB throughout the present series of papers, is shown in elevation in Fig. 1. It consists of two annular channels through which fuel/oxidizer mixtures can flow, and a large air co-flow. The innermost tube is terminated in a ceramic cap, providing a central bluff body to aid flame stabilization.

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Fig. 1. Elevation of the stratified swirl burner (SwB). (A) Inner annulus plenum; (B) outer annulus axial flow plenum; (C) outer annulus swirl flow plenum; (D) locating collar; (E) outer tube; (F) middle tube; (G) inner tube; (H) flow straighteners; (I) swirl generating collar; (J) ceramic cap; (K) wire mesh; (L) honeycomb section; (M) perforated disk. Flow fittings are omitted for clarity. All dimensions are to scale and in mm.

Flow to the inner annulus is metered independently of that to the outer annulus, through a plenum at the base of the burner. All flows are metered using mass flow controllers calibrated at the beginning of the experimental campaign to within ±1% of reading in reference to laminar flow elements. The outer annulus is supplied with axial or swirling flow by metering fuel/oxidizer through two plenums, one of which is connected to the outer annulus through eight 2 mm diameter inlets inclined at a 30° angle to both the horizontal and the surface of the annulus. The flow to each of these plenums is metered independently, enabling a variable degree of swirl in the outer annulus. More detailed descriptions of the swirl burner are given in Part I [6], and detailed drawings are available online [28].

2.2. Operating conditions The swirl burner was tested using a matrix of 16 operating conditions [6], though only the nine conditions considered in the present work are shown in Table 1. Two levels of swirl were investigated,1 while the fuel/oxidizer field was either homogeneously premixed, moderately stratified or highly stratified. The total power load P varies from 25.8 kW in the premixed cases (split approximately 1:3 between the inner and outer flows), through 21.5 kW in the moderately stratified cases (5:8), to 19.3 kW in the highly stratified cases (11:10). The swirl number was calculated as the ratio of measured mean tangential velocity to axial velocity, S = Utg/Uz, above the center of the outer annulus and values are listed in Table 1. This was done by aligning the PIV plane tangent to the burner centerline and above the center of the outer annulus. The corresponding flow angles, g = tan1 S, are also tabulated. These preliminary measurements of swirl strength were obtained in the non-reacting flows by aligning the PIV tangent to the burner centerline. The swirl flow ratio, SFR, listed in Table 1 is defined as the ratio between the volumetric flow rate through the swirl plenum and the total volumetric flow 1 The overall level of swirl is low compared to industrial applications in all cases; the terms ‘‘moderate’’ and ‘‘high’’ are used to simplify description of the conditions further on in the present work.

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Table 1 Operating conditions for Cambridge Stratified Swirl Burner. In all cases /g = 0.75, Ui = 8.31 m/s, and Ucoflow = 0.4 m/s. Swirling flows are highlighted in bold font. Flame

SFR

S

g(°)

/o//i

/i

/o

SwB1 SwB2 SwB3 SwB5 SwB6 SwB7 SwB9 SwB10 SwB11

0 0.25 0.33 0 0.25 0.33 0 0.25 0.33

0 0.45 0.79 0 0.45 0.79 0 0.45 0.79

0 24.4 38.5 0 24.4 38.5 0 24.4 38.5

1 1 1 2 2 2 3 3 3

0.75 0.75 0.75 1.0 1.0 1.0 1.125 1.125 1.125

0.75 0.75 0.75 0.5 0.5 0.5 0.375 0.375 0.375

rate in the outer annulus. These values should allow detailed simulation of the inlet flows and subsequent flow development through the annular channels to be checked. The swirl number obtained for the higher swirl case (S = 0.79) indicates that the burner can operate at practically relevant levels of swirl (S P 0.6 [18]). More complete measurements of the three dimensional velocity field are in progress. Moderately swirling (SwB2, premixed; SwB6, moderately stratified; and SwB10, highly stratified) and highly swirling (SwB3, premixed; SwB7, moderately stratified; and SwB11, highly stratified) results are presented in the current work. Selected results from the non-swirling cases (SwB1, SwB5 and SwB9) are included to highlight the effects of swirl on velocity and scalar fields. Each case is nominally lean in so far as the global equivalence ratio at the intersection of the mixing layer and the mean flame brush is given by /g = 0.75, though the inner annulus streams are stoichiometric and rich for the moderately and highly stratified cases respectively. Photographs of the nine cases listed in Table 1 are shown in Fig. 2. In all cases the flames are attached and stabilize on the perimeter of the central bluff body. The angle of the flame as it spreads from the edge of the bluff body is affected by swirl and to a lesser degree by the equivalence ratio of the inner annular flow. The flame becomes broader with increasing swirl (moving down each column of photographs), and this effect is substantially greater than that associated with stratification. It is also apparent that the lower portion of flame SwB1(/i = 0.75) is narrower than for SwB5 or SwB9, where the flame speed is higher. The increase in luminosity seen under stratified conditions (across the rows) is due to the increase in equivalence ratio in the inner annulus. When referring to results from case N at a distance of z downstream of the burner exit, the shorthand SwBNz is used; for example, the highly swirling lean premixed case at z = 30 mm downstream of the burner exit is denoted SwB330. 2.3. Velocity characterization Velocity characterization in the swirl burner was performed using two-dimensional particle image velocimetry (PIV). The resulting vector fields have a vector resolution of 0.55 mm. Details of the experiments are supplied in Part I [6]. The present PIV measurements reveal the main features of the axial and radial flow fields, and are useful for interpretation of some features of the scalar results. The swirling flows considered in the present work feature significant tangential velocity fields, which cannot be captured by this experimental setup. This should be borne in mind in the interpretation of velocity results presented further on. Future work on the swirl burner will investigate the tangential flow field using three-component LDV or stereoscopic PIV. 2.4. Multi-scalar diagnostics Multi-scalar laser diagnostics were applied at the Turbulent Combustion Laboratory in Sandia National Laboratories. The diag-

Fig. 2. Photographs of flame conditions surveyed in the present work and Part I [6].

nostics setup [6,29,30] allows for the line measurement of temperature (Rayleigh scattering) and major species (Raman scattering and CO-LIF) at 103 lm projected pixel resolution with simultaneous cross planar OH-PLIF at 48 lm projected pixel resolution. Signal to noise ratios for measurements in the current experiment are detailed in Table 2. As the optical resolution of the Raman-Rayleigh-LIF measurements is smaller than the spatial sampling rate, the resolution of the temperature and major species measurements is limited by the sampling resolution (103 lm) and the laser beam diameter (0.22 mm, 1/e2). The optical resolution (1/e2) of the OH-PLIF measurement is between 98 and 144 lm and therefore the spatial resolution of the OH-PLIF measurements is limited by the optical resolution rather than the sampling resolution. Radial profiles were obtained by moving the burner horizontally in 4 mm steps, producing overlapping steps in the relative position of the 6 mm wide measurement window, with 300 laser shots taken at each step. A further 1200 shots were taken across the mean flame brush. Radial profiles were taken at axial increments of 10 mm above the burner exit to capture changes

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M.S. Sweeney et al. / Combustion and Flame 159 (2012) 2912–2929 Table 2 Uncertainties (accuracy, D, and precision, r) and signal to noise ratios (SNR) in scalar measurements in SwB based on measurements in the products of a CH4/air flat calibration flame (T = 2050 K, / = 1.28), arranged in order of decreasing SNR.

Table 3 Table of propagated uncertainties, expressed as a percentage of the mean value of the scalar at the location of peak jrcj, for derived quantities in the highly swirling lean cases at z = 10 mm downstream of the burner exit.

Scalar

D (%)

r (%)

SNR

Flame

rT eq

rc

ra

rjrcj

rvc

T Y N2 Y H2 O / Y CO2 YCO Y H2

2 2 3 5 4 10 10

0.7 0.8 2.0 1.5 3.5 5.9 6.3

150 130 50 65 29 16 17

SwB210 SwB310 SwB610 SwB710 SwB1010 SwB1110

0.9 0.9 0.5 0.6 0.1 0.4

1.4 1.4 1.1 1.1 0.9 1.0

1.2 1.2 1.2 1.2 1.2 1.2

3.9 5.6 2.4 2.7 2.8 2.9

7.8 9.9 4.9 5.4 5.6 5.7

cðx; /Þ ¼

TðxÞ  T u T e ð/Þ  T u

in flame structure with axial distance. Substantially larger datasets (5000–30,000 shots) were taken at the intersection of the mean flame brush and the mixing layer using a technique of spatial oversampling and wavelet denoising [31], and these data will be the focus of future work investigating conditional statistics in the flames surveyed (to be presented in Part III of the present series). This is shown schematically in Part I [6].

ð2Þ

where T(x) is the local temperature, Tu is the temperature in the unburned reactants, and Te(/) is the equilibrium flame temperature as a function of /. 3.4. Scalar gradients

3. Data analysis A brief synopsis of the data analysis methodologies employed to calculate derived quantities from the experimental measurements is provided for reference. 3.1. Flame front geometry Flame fronts were extracted from the OH-PLIF images using a threshold-based methodology [11], as described in [6]. Flame front geometry and the angle between the crossed planes were used to extract the three-dimensional flame normal, n, at the line measurement axis [32,33]. The discrete two-dimensional local curvature, jd, was calculated using first and second derivatives obtained from third-order polynomial fits to the flame front contours, as in [30]. jd is taken as positive if the flame front is convex towards the reactants.

Scalar gradients are calculated using second-order central differencing. These gradients are corrected for the effects of finite resolution using a lookup table of correction factors, as described in [6]. The mean magnitude of the correction for peak progress variable gradient jrcj in the premixed SwB310 dataset is 5  102 mm1, with a maximum correction of 1.1  101 mm1, which is small relative to the peak values of jrcj  1:1 mm1 . The gradient of temperature or progress variable along the line measurement axis is the projection of the three-dimensional value along the flame normal, n. The value along the normal is calculated by dividing the gradients by the cosine of the solid angle h between the flame normal and the line measurement axis (e.g., dc/dn = (dc/dx)/cos h). Spurious angle-corrected gradients [30,32] were minimized by only considering data where h 6 50° (as in Part I [6]), reducing the datasets by approximately 20%. Three-dimensional surface density function (jrcj) and scalar dissipation rate2 (vc = ajrcj2) are investigated further on in the present work.

3.2. Equivalence ratio and mixture fraction 3.5. Error analysis The present work makes use of an equivalence ratio derived from the following atomic balance:

/¼

X CO2 þ 2X CH4 þ X CO þ 0:5ðX H2 O þ X H2 Þ X CO2 þ X O2 þ 0:5ðX CO þ X H2 O Þ

ð1Þ

This definition of equivalence ratio yields values that are close to those calculated using the scaled Bilger mixture fraction Z [34]. Both / and Z are conserved from reactants to products across a planar laminar or turbulent premixed flame. However, equivalence ratio and mixture fraction vary within the thermal ramp of the flame due to effects of differential species diffusion. The combination of differential diffusion and bulk transport effects associated with the bluff-body stabilized burner geometry can cause a significant net increase in equivalence ratio across the turbulent flame brush going from reactants to products, especially in the near field of the stabilized flame [35]. This has implications for the determination of the local gradient of equivalence ratio and the conditioning of data on local equivalence ratio, particularly in stratified flames, and is discussed in [35] and in sections that follow.

The precision of derived quantities, r, is determined through the application of error propagation in conjunction with the experimental uncertainties listed in Table 2. The formulas used are given in Part I [6]. Uncertainties at z = 10 mm in the swirling cases are presented in Table 3; these values are indicative of the propagated uncertainties at other axial locations. 4. Results and discussion Presentation and discussion of results are structured as follows. First, the mean and fluctuating velocity fields are shown, along with near-exit profiles. Next, the thermal and compositional evolution of the flames are investigated using radial profiles of Favreaveraged temperature and equivalence ratio at various distances downstream of the burner exit. The influence of stratification on scalar structure near the intersection of the mixing layer and mean flame brush is considered in detail, including species-temperature state space, surface density function, and scalar dissipation rate. Finally, turbulent flame thickness and curvature are considered at various downstream distances.

3.3. Stratified progress variable The present work makes use of a thermal progress variable, c(T), suitable for application to both premixed and stratified flames:

2 The thermal diffusivity, a, is obtained by interpolating a lookup table of laminar flame calculations (CHEMKIN [36], GRI-Mech 3.0 [37]) using the local temperature and equivalence ratio.

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Fig. 3. Mean (r < 0 mm) and fluctuating (r > 0 mm) velocities for non-swirling (top row), moderately swirling (middle row) and highly swirling (bottom row) flows. NR: nonreacting (all others are reacting). Streak lines are shown in white, with the mixing layer and mean flame brush marked by short- and long-dashed lines respectively.

4.1. Velocity characterization pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Maps of mean velocity, U ¼ U z2 þ U r2 , and rms velocity, pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi 0 0 u ¼ uz2 þ ur2 , are shown in Fig. 3 for non-reacting (leftmost column) and reacting states (remaining columns) for non-swirling (top row), moderately swirling (middle row) and highly swirling (bottom row) cases. The non-swirling cases investigated in Part I [6] are shown to assist comparison (top row).The U and u0 measurements presented are derived from the same side of the centerline of the experimental images. Under the assumption of radial symmetry, U is shown (mirrored) for r < 0 mm and u0 is shown for r > 0 mm, with white streak lines to highlight the flow patterns. The mixing layer (short-dashed lines) and mean flame brush (longdashed lines) are plotted to illustrate the shape of the mixture and thermal fields with respect to the flow field. Both u0 and U are underestimated as the present measurements do not capture the tangential components of mean and fluctuating velocity ðU tg ; u0tg Þ. The two-dimensional flow fields, whether reacting or not, demonstrate features typical of co-annular jet flow with a central bluff body: the peak velocities are found between the annuli, recirculation zones (the areas enclosed by the points where Uz switches from positive to negative) form over the bluff body, and the peak turbulent intensities coincide with the locations of the shear layers. The loci of peak U move radially outwards in the reacting cases due to the expansion of the hot products; the effect is more pronounced in the stratified cases due to the higher equivalence ratios and corresponding higher temperatures. Shear layers between the recirculation zone, the inner flow, the outer flow, and the co-flow are evident in the plots of u0 . The breadth and magnitude of these zones indicate that significant turbulent mixing takes place between the streams in all cases. The velocity fluctuations seen in the recirculation zone are much greater in the moderately swirling cases than the highly swirling cases. Furthermore the sound levels experienced while running the burner under moderate swirl were substantially higher than under high swirl. These observations indicate the presence of coherent instabilities at the moderate swirl level, and that the flow shifts back into a more stable operating regime by further increasing the degree of swirl. Interestingly, the behavior of the recirculation zone above the central bluff body varies substantially depending on whether or not the flow is reacting, the stoichiometry, and the degree of swirl. The non-reacting cases (leftmost column) exhibit a narrow closed 0

recirculation zone typical of annular axial flows past a flat central bluff body, regardless of the level of swirl. These effects are illustrated more clearly in Fig. 4, which provides the axial velocity field and streamlines for each case, with regions of negative mean Uz shown in blue (dark gray in print edition). As in Fig. 3, data are taken from the same side of the centerline and are mirrored under the assumption of radial symmetry. Under non-swirl conditions (Fig. 4, top row), stratification leads to a shorter recirculation zone, with the smallest in the case of moderate stratification. This may be associated with the fact that under these conditions, the flame speed is fastest at the central annulus, thus the flame zone is shortest. Considering the moderately swirling reacting conditions (Fig. 4, middle row), the premixed recirculation zone (SwB2) is much longer (+75% to +130%) than in the corresponding stratified cases (SwB6 and SwB10). This pattern is the same as that seen for the non-swirling cases (top row). In contrast to this, the highly swirling reacting cases (bottom row) exhibit significantly different behavior, with recirculation zones that are unclosed within the imaged window. The premixed SwB3 case forms a V-shaped recirculation zone, while the stratified cases (SwB7 and SwB11) show signs of closing as the downstream distance increases before broadening again. The differences between the premixed and stratified cases are attributed to the balance between the non-reacting flow field and the influences of flame speed, heat release ratio, and the expansion of hot products, all of which are a result of stoichiometry. In the non-swirling (top row) and moderately swirling (middle row) cases shown in Fig. 4 the recirculation zones are significantly more compact under stratified conditions. In the swirling cases, centrifugal forces tend to pull low density products towards the burner centerline, while high density reactants move radially outwards, and this effect dominates in the highly swirling cases (bottom row) leading to the open recirculation zones seen. However, the stoichiometry effects seen for the non-swirling and moderately swirling cases are still present, and are evident in the narrowing of the recirculation zones in SwB7 and SwB11 relative to the premixed case. Profiles of Uz and u0z near the exit of the burner (z = 5 mm) are shown for non-reacting conditions in Fig. 5. Uz for the non-reacting, non-swirling case is shown with dashed lines to highlight changes with the addition of swirl. Profiles of the radial components of mean and fluctuating velocity are also shown, though these are substantially smaller than the axial components.

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Fig. 4. Mean axial velocities Uz for non-swirling (top row), moderately swirling (middle row) and highly swirling (bottom row) flows. NR: non-reacting (all others are reacting). Streak lines are shown in white, with the mixing layer and mean flame brush marked by short- and long-dashed lines respectively.

Fig. 5. Near-exit (z = 5 mm) velocities and fluctuations for non-reacting moderately swirling (left, SwB25, SwB65 and SwB105) and highly swirling (right, SwB35, SwB75 and SwB115) cases. Uz for the non-reacting, non-swirling case is shown with dashed lines. Symbols plotted every four data points for clarity.

The exit profiles demonstrate behavior consistent with fully developed channel flow, with peaks in Uz corresponding to flow from the inner and outer annuli. The inner annulus peaks are essentially unaltered by the addition of swirl, while the locations of peak velocity in the outer annulus shift radially outwards by 1 mm in both cases. The roll-off to the co-flow velocity is gentler with the addition of swirl. Uz is negative for r < 5 mm, corresponding to the recirculation zone seen in Figs. 3 and 4. The radial component of mean velocity, though small, is positive from the burner centerline to r  18 mm, where it becomes negative (corresponding to the entrainment of the co-flow). The extent and strength of the turbulent shear layers between the inner- and outer-flows and the outer-flow and the co-flow air are shown by u0z . The radial fluctuations u0r are similar to their axial counterpart, showing peaks in the shear layers between the innerand outer-flows and also the outer- and co-flow. The fluctuations in the moderately swirling case are much higher than those seen in the highly swirling case; this is attributed to the unstable mode the burner is operating in. Turbulence parameters were calculated for non-reacting measurements using the equations given in Part I [6]. These parameters were calculated near the exit of the burner (z = 5 mm) at the center of the inner and outer annular gaps, and at the center of the mixing layer between the annular flows at the height at which mean flame brush intersects the mixing layer in reacting flows (z = 40 mm for

SFR = 0.25, z = 30 mm for SFR = 0.33). sL and dL values from laminar flame calculations at / = 0.75 were used to calculate Da, Ka, and u0 / sL at the downstream locations. The values shown in Table 4 indicate that the swirling flames fall within the thin reaction zone regime. This is corroborated by the absence of broken flame fronts in the OH images. The modified Borghi diagram shown in Part I [6] indicates that the turbulent conditions in the swirling cases are similar to those in the non-swirling case. It is stressed that U and u0 are underestimated as the tangential components of velocity, Utg and u0tg , are not measured in the present experiments. 4.2. Favre-averaged structure Radial profiles of Favre-averaged temperature and equivalence ratio at a range of distances downstream of the burner exit (z = 10, 30, 50 mm) are shown in Figs. 6 and 7 for the two levels e at z = 10 mm are similar within of swirl. The thermal ramps in T the FWHM envelope of temperature fluctuation, despite the sub~ within the flame brush. The differences stantial differences in / in stoichiometry influence the peak temperatures attained, with the stratified cases showing substantially higher values. Under e decays considerthe highly swirling conditions shown in Fig. 7, T ably after these peaks, particularly in the stratified cases, due to the recirculation of leaner, cooler products and air from farther downstream. This effect is not evident to any significant degree in the

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Table 4 Key non-reacting turbulence parameters near the burner exit (z = 5 mm at the center of each annular gap, and at the center of the mixing layer between the annular flows at the height at which mean flame brush intersects the mixing layer in reacting flows (z = 40 mm for SFR = 0.25, z = 30 mm for SFR = 0.33). SFR

z (mm)

r (mm)

U (m/s)

u0 (m/s)

I (%)

Ret

Lturb (mm)

gK (lm)

sK (ls)

mK (m/s)

Da

Ka

u0 /sL

0.25 0.25 0.25 0.33 0.33 0.33

5 5 40 5 5 30

9.1 15.1 7.8 9.1 15.1 9.6

8.81 19.11 8.99 9.20 17.70 9.91

4.40 4.87 3.67 2.00 3.40 3.05

50 26 41 22 19 31

298 466 407 115 260 255

1.10 1.56 1.80 0.94 1.24 1.36

15 16 20 27 19 21

15 15 24 44 23 28

1.06 1.05 0.82 0.61 0.85 0.76

– – 0.21 – – 0.17

– – 876 – – 762

– – 14.8 – – 12.3

Fig. 6. Radial profiles of Favre-averaged temperature and equivalence ratio in the SwB2, SwB6 and SwB10 datasets at various downstream distances (z = 10,30,50 mm). The FWHM locations of the Favre-averaged temperature fluctuations are marked on each plot by the symbols in the legend.

Fig. 7. Radial profiles of Favre-averaged temperature and equivalence ratio in the SwB3, SwB7 and SwB11 datasets at various downstream distances (z = 10, 30,50 mm). The FWHM locations of the Favre-averaged temperature fluctuations are marked on each plot by the symbols in the legend.

moderately swirling cases, which have more compact recirculation zones. The temperature profiles (and thus location of the flame brush) in the stratified flames are similar to the corresponding profiles in the premixed flames as downstream distance increases in moderately swirling conditions, but move progressively radially outwards in the highly swirling conditions. The flame position is a function of the expansion of the hot products and the flame burn-

ing rate, which are influenced by the equivalence ratio distribution (and in the case of the burning rate, by the turbulence experienced). As the laminar flame speeds for / = 1.0 and / = 1.125 are nearly equal [38], the initial spreading of the stratified flames (where the flame is burning through homogeneous mixture from the inner annulus) should be commensurate. ~ in the moderately swirling cases (Fig. 6) show disThe plots of / tinct jumps in equivalence ratio near to the burner exit (z = 10 mm, bottom row). The premixed and stratified equivalence ratio rises sharply from zero in the co-flow air to the nominal value in the outer annulus from r 6 20 mm. The two stratified cases show a second ramp in equivalence ratio corresponding to the mixing layer between richer mixture in the inner annulus and the leaner flow ~ occurs for r  9 mm from the outer annulus. A final jump in / regardless of stratification. This increase in equivalence ratio is due to the effects of preferential molecular transport, as described in [35]. Briefly, H2 and H2O diffuse preferentially toward the reactants and are subsequently transported downstream in this highly sheared configuration. Simultaneously, excess CO2 is trapped and accumulated within the recirculation zone, causing an increase in the C/H atom ratio and an increase in the equivalence ratio of the products. ~ profiles in Fig. 7 show significant differThe highly swirling / ences from their moderately swirling equivalents at z = 10 mm. The steps in equivalence ratio between the co-flow air and the outer annulus flow, and between the outer and inner annulus flows are still apparent, but the plateaus at the outer annulus equivalence ratio value seen in the moderately swirling stratified cases are mostly smeared out in the corresponding highly swirling cases, indicating greater levels of turbulent mixing between the inner and outer annulus streams due to the additional swirl. Furthermore, the equivalence ratio decreases substantially towards the center of the recirculation zone. This is attributed to the entrainment of lower equivalence ratio products from a large region due to the large recirculation zones in the highly swirling cases, as seen in Fig. 4. This attenuation is more pronounced in the highly swirling stratified cases due to the entrainment of mixture from the leaner outer annulus stream (which has mixed with the co-flow air) into the recirculation zone from farther downstream. There is no indication of a step increase in equivalence ratio going through the flame brush in the highly swirling cases. In [35] it was proposed that the increase in / through the reaction zone in the near field of non-swirling bluff body stabilized flames results from the combination of preferential transport of H2 and H2O through the preheat zone and the presence of a strong recirculation zone that accumulates excess CO2. The same effect was not observed in [35] when the annular premixed flame was anchored by a central pilot flame, rather than the bluff body. Here, in the highly swirling cases, the expanded recirculation zone extends far enough downstream to bring in dilute products having equivalence ratio well below that of the reactants. Apparently, this short circuits the accumulation effect, such that any effect of preferential transport of H2 and H2O through the preheat zone remains below a measurable level.

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Fig. 8. Scatter plots of near-field (z = 10 mm) scalar structure (T; Y CH4 , and YCO) against equivalence ratio from complete radial profiles across SwB610 and SwB710, colored by radial position. /o = 0.5 and /i = 1 are shown by black and gray dashed lines respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 9. Radial profiles of Favre-averaged scalars in the SwB240, SwB640 and SwB1040 datasets. The FWHM locations of the Favre-averaged temperature fluctuations are marked on each plot by the symbols in the legend.

Fig. 10. Radial profiles of fluctuation of Favre-averaged scalars in the SwB240, SwB640 and SwB1040 datasets. The FWHM locations of the Favre-averaged temperature fluctuations are marked on each plot by the symbols in the legend.

This difference in near-field scalar structure between the highly swirling cases and the moderately swirling (and non-swirling) cases is investigated further by considering the scatter plots in Fig. 8, which show measurements of T, Y CH4 , and YCO from complete

radial profiles across cases SwB610 and SwB710 at the downstream location z = 10 mm, with each scalar plotted versus equivalence ratio. In each case the reactants entering the flame brush are at the equivalence ratio of the inner annular flow. Each scatter plot pro-

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Fig. 11. Radial profiles of Favre-averaged scalars in the SwB330, SwB730 and SwB1130 datasets. The FWHM locations of the Favre-averaged temperature fluctuations are marked on each plot by the symbols in the legend.

Fig. 12. Radial profiles of fluctuation of Favre-averaged scalars in the SwB330, SwB730 and SwB1130 datasets. The FWHM locations of the Favre-averaged temperature fluctuations are marked on each plot by the symbols in the legend.

vides a complementary view of the variation in equivalence ratio with progress of reaction. In the moderately swirling case SwB6 (top row) there is no evidence of low equivalence ratio fluid from farther downstream being mixed into the recirculation zone. As CO burns out, temperature continues to increase slightly, and equivalence ratio passes through a maximum near 2000 K. At the highest values of temperature in the products, the measured equivalence ratio is higher than that of the reactants by roughly 8% on the mean. In contrast, the scatter plots for the highly swirling case, SwB7, shows that reaction through the flame brush is followed by mixing with fluid at much lower equivalence ratio (also lower T and YCO), including samples below / = 0.6. In SwB7, the scatter data show that the post flame products are at the same equivalence ratio as reactants (within experimental uncertainty), as would be the case with a freely propagating turbulent premixed flame. Thus, it appears that the accumulation effect of the compact recirculation zone is no longer present and the burnout process merges into a mixing process with dilute products pulled into an extended recirculation zone from farther downstream. Differential diffusion still causes variation of equivalence ratio within the flame, but the products return to an equivalence ratio of the reactants. Similar results are found in comparing cases SwB10 and SwB11 (not shown). Moving farther downstream, the sharp steps in Favre average equivalence ratio seen in both the moderately (Fig. 6) and highly swirling (Fig. 7) cases become broader and more gradual as the mixing layers between the recirculation zone, inner annulus flow, outer annulus flow and co-flow become larger. The mixing layers

between the inner and outer annuli and the outer annulus and co-flow have merged in all cases by the time the mean equivalence ratio burned through by the mean flame brush (peak T0 ) is equal to ~ ¼ / ¼ 0:75. the global equivalence ratio / g Favre averages and fluctuations of measured scalars are available at increments of 10 mm above the burner exit (z = 10, 20, . . . , 60 mm), and are presented at the intersection of the mixing layer and the mean flame brush in Figs. 9 and 10 (moderate swirl) and Figs. 11 and 12 (high swirl). These plots demonstrate the structural differences in physical space brought about due to stratification of the inlet flows. The moderate swirl profiles in Fig. 9 show little difference in shape or magnitude relative to the nonswirling cases investigated previously. Products of combustion appear on the inner zone, in line with the change in stoichiometry, and the fuel concentrations decreases on the outside due to mixing with the surrounding stream and co-flow air. The most notable change is that both the average and fluctuating profiles are shifted radially outwards relative to non-swirling profiles at z = 40 mm. As mentioned previously, stratification does not appear to have a strong influence on the location of the flame brush in the moderately swirling cases. The profiles of the fluctuations are also similar to those seen in the non-swirling cases, again shifted radially outwards, with slightly elevated levels at the z = 10 mm location for most species. The highly swirling profiles shown in Fig. 11 demonstrate more significant departure from the non-swirling behavior than the e O and Y e CH show that the mixture moderately swirling cases. Y 2 4 in the stratified recirculation zones contains significant levels of

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Fig. 13. Single-shot profiles of temperature and equivalence ratio at the intersection of the mixing layer and the mean flame brush, randomly selected from the SwB2, SwB6 and SwB10 datasets.

Fig. 14. Single-shot profiles of temperature and equivalence ratio at the intersection of the mixing layer and the mean flame brush, randomly selected from the SwB3, SwB7 and SwB11 datasets.

oxygen and no methane, confirming that it is largely composed of the products of lean combustion rather than reactants. The substantial cooling seen in the thermal profiles on the inner side of the temperature peaks would suggest that the products are entrained substantially farther downstream, and may have mixed with co-flow air. The degree of mixing of reactants with the co-flow at z = 30 mm for the highly swirling premixed flame is significantly more pronounced than in the non-swirling case. The mixing seen in the moderately swirling premixed case at z = 40 mm is less pronounced, but still noticeable. This suggests that careful conditioning of the data at this location is required in order to make sensible comparisons with the stratified conditions. 4.3. Instantaneous flame profiles Instantaneous sample profiles of T and / (selected at random) for the moderately and highly swirling cases are shown in Figs. 13 and 14, respectively. Profiles are presented near the burner exit (z = 10 mm) and at the intersection of the mixing layer and the mean flame brush (z = 40 mm for moderate swirl and z = 30 mm

for high swirl). The corresponding images of OH-PLIF are shown in Figs. 15 and 16. The premixed profiles for each flow condition are plotted with markers at the experimental data spacing (103 lm) to demonstrate the fidelity of the current scalar measurements within the instantaneous flame brush. The other profiles are plotted with markers at larger distances to aid clarity. The temperature profiles for all cases resemble error functions near the burner exit (z = 10 mm), similar to the thermal ramps seen in laminar flames and the non-swirling flows from Part I [6]. There is little evidence of turbulence in the thermal records close to the burner exit. Turbulence in this region of initial shear layer growth may be suppressed by heat release. Farther downstream the influence of swirl and shear-generated vortices lead to higher turbulence with a wider range of scales, thus the variation seen in the profiles at z = 40 mm and z = 30 mm. The instantaneous profiles of equivalence ratio at z = 10 mm behave differently depending on the degree of swirl, as for the Favre average profiles. The moderately swirling cases exhibit similar behavior to the non-swirling cases studied previously, with elevated equivalence ratios near the burner centerline due to the

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Fig. 15. Instantaneous shots of OH-PLIF corresponding to the profiles presented in Fig. 13. The axis of intersection with the other OH-PLIF plane is marked by a dashed line.

Fig. 16. Instantaneous shots of OH-PLIF corresponding to the profiles presented in Fig. 14. The axis of intersection with the other OH-PLIF plane is marked by a dashed line.

effects of differential diffusion [35]. In contrast, the profiles for SwB310, SwB710 and SwB1110 decay steadily towards the centerline from r 6 10 mm. Again, this is attributed to the differences in the shape of the recirculation zones. In the highly swirling flows, the recirculation zone extends farther downstream than in either the non-swirling or moderately swirling cases. The products entrained into the highly swirling recirculation zone are sampled from a much larger region of the overall flame than for the other flow patterns, including downstream regions burning low equivalence ratio mixtures. The effect of this larger recirculation zone dominates the effects of preferential transport on equivalence ratio in the highly swirling cases. Farther downstream, at z = 40 mm, the instantaneous equivalence ratio profiles for the moderately swirling cases are consistent with the expected behavior based on the stoichiometries at exit; the premixed case is largely constant, and the two stratification levels show increasing equivalence ratio gradients. More interesting behaviors are highlighted by the sample profiles from the highly swirling cases (z = 30 mm). A large dip is seen in the SwB1130 profile, which from the corresponding OH-PLIF image in Fig. 16 appears to be due to richer burned mixture wrapping around a leaner region. This gives very sharp / gradients in the two flame fronts observed in the thermal profiles. The gradients in the moderately stratified example are more representative of the spatial variations in equivalence ratio typically experienced by the stratified flames. The change in equivalence ratio across the instantaneous flame front is examined statistically in the next section to quantify the extent of stratification.

The OH-PLIF images shown are typical, and demonstrate qualitatively the evolution of the flame topology from relatively smooth near the burner exit (z = 10 mm) to a more convoluted state at the intersection of the mixing layer and the mean flame brush. This increase in wrinkling is much more pronounced in the highly swirling cases. The SwB330 example shows that spurs of hot products may form at this location. In general, however, the addition of swirl results in a greater degree of wrinkling than was seen in the nonswirling flames described in Part I [6].

4.4. Extent of stratification The instantaneous profiles of equivalence ratio, and the corresponding local gradients, within the thermal ramp of the flame are governed not only by local mixing but also by differential diffusion. This complicates the quantification of stratification using r/ because equivalence ratio gradients due to stratification can be dominated by those due to differential diffusion (discussed in more detail in Part I [6]), with no simple method to separate them. Accordingly, the change in equivalence ratio, D /, across the thermal ramp of the instantaneous flame front is used to quantify the extent of stratification as the effect of differential diffusion on equivalence ratio is small outside of the thermal ramp. This metric requires the thermal ramp to be encompassed in full by the line measurement window, resulting in 2–18% of the data being discarded depending on the degree to which the position of the flame front oscillates radially at a given downstream location.

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M.S. Sweeney et al. / Combustion and Flame 159 (2012) 2912–2929 Table 5 Table of mean and rms values of D/ in swirling cases at the intersection of the mean flame brush and the mixing layer.

Fig. 17. Probability density functions of the equivalence ratio spanned by the instantaneous thermal ramp, D/, for moderately (left column) and highly swirling (right column) datasets at various downstream distances (z = 10, 30, 50 mm).

The distributions of D/ are shown in Fig. 17 for the moderately and highly swirling flames at downstream locations z = 10, 30, and 50 mm. Mean and rms values for D/ are listed in Table 5 at the intersection of the mean flame brush and the mixing layer. At z = 10 mm, the moderately swirling cases tend to exhibit small positive changes, with the distributions centered about D/ = 0.07. The positive mean change in equivalence ratio across the flame brush in the moderately swirling cases corresponds to the steps in equivalence ratio associated with preferential transport seen in Fig. 6 at z = 10 mm. As discussed above, the highly swirling cases have extended recirculation zones, preventing amplification of the preferential transport effect [35], such that the distributions of D/ are centered around zero for the z = 10 mm location in each of the three highly swirling flames. The stratified SwB710 and SwB1110 flames burn through rich premixed mixture from the inner annulus as the two fuel/oxidizer streams are still segregated at these locations (Fig. 7), giving similar D/ distributions to that for the premixed SwB310 flame. The premixed (SwB2 and SwB3) distributions remain relatively narrow until the mixing layer between the outer annulus and the co-flow air intersects the flame brush (z > 30 mm). This effect is more pronounced under highly swirling conditions, as shown by the broader distribution for SwB350 than SwB250 in the top row of Fig. 17. The stratified distributions broaden by over 300% moving downstream. The stratified distributions broaden more rapidly in the highly swirling cases, due to the increased turbulent mixing. The mean values of the stratified distributions are greater than their non-swirling equivalents for similar reasons. The highly stratified distributions are broader and have higher means than the equivalent moderately stratified cases for z > 10 mm. 4.5. The thermal evolution of major species The mean behavior of various species mass fractions can be tracked as a function of local temperature, representative of the ex-

Flame

D/

rD/

SwB240 SwB640 SwB1040 SwB330 SwB730 SwB1130

0.04 0.18 0.22 0.04 0.23 0.28

0.05 0.10 0.13 0.06 0.12 0.15

tent of reaction, to identify the influence of stratification on the flame structure. Figure 18 provides scatter plots of various species mass fractions against temperature at z = 40 mm, which marks the intersection of the mixing layer and the mean flame brush for the moderately stratified cases SwB6 and SwB10, as seen in Section 4.2. Points in the premixed SwB240 dataset where the local equivalence ratio was significantly (7.5%) below the mean value at that temperature (due to mixing with co-flow air) were eliminated to ensure these data can be considered homogeneously premixed in the present analysis. This removed 4% and 11% of the data in the moderately and highly swirling premixed cases respectively. Figure 19 shows the thermal evolution of major species in the highly swirling cases at z = 30 mm using the same format. Mean fits and standard deviations are generated by binning the species data in temperature space in steps of 20 K, and mean values from the premixed data are shown by open circles in all plots to facilitate comparisons with the stratified cases. Laminar calculations at the nominal mean equivalence ratio (/ = 0.75) are also shown. Note that the experimental data sets are too large to plot all points in a scatter plot; instead 100 points were randomly selected from each temperature bin, and only these are plotted in the scatter plots. In general there is an order of magnitude more points on the reactant and product side than within the thin reaction zone, with the result that a variable percentage of the total data set is shown. Black points correspond to those below the lean flammability limit3 (/ = 0.47). The black points are all at temperatures under 1 000 K, suggesting that these points have simply been warmed by mixing with nearby mixture at higher equivalence ratio that can react fully, rather than indicating an extension of the lean flammability limit due to back support (as posited in [43–45]). Stratification demonstrates minimal effect on the behavior of CO2, H2O and O2 mass fractions on the mean. Mean methane levels in the highly swirling stratified cases are slightly reduced relative to the premixed state at low temperatures due to the broader range (higher and lower) of methane values experienced in the mixing layer between the co-flow air and the inner/outer annulus flows. This contributes to the increased density of black points in Fig. 18. Carbon monoxide and hydrogen mole fractions show a greater degree of variation between premixed and stratified conditions. These differences can be quantified by considering the peak mole fraction in the premixed cases and making comparisons against the values in the corresponding stratified cases at the same location in the thermal profile. The values of YCO for moderate swirl are elevated (12%) at both levels of stratification relative to the corresponding premixed case using this rationale. Similarly, peak hydrogen levels are significantly raised (22–54%) under stratified

3 The lean flammability limit is taken to be / = 0.47 based on the experimental findings of Shoshinet al. [39]. There is a degree of variation in the lean limit found, depending on the experimental configuration used, but typically it has been found to be in the range 0.45 < / < 0.51 [39–42].

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Fig. 18. Scatter plots of key combustion species in temperature space for the SwB240, SwB640 and SwB1040 datasets. Points where / < 0.47 are plotted in black. Mean fits are plotted in red, while vertical red bars denote ± one standard deviation. Laminar flame calculations at the nominal equivalence ratio (/n = 0.75) are plotted in gray. The premixed profile is marked in all cases by open circles. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

conditions. This behavior deviates substantially from that seen in the non-swirling cases [6], where the hydrogen profiles were generally commensurate below 1700 K. The deviations seen in both species in the stratified conditions increase rapidly on the product side where the mean equivalence ratio exceeds the premixed value. The most significant effects of stratification on the highly swirling cases are also seen for YCO and Y H2 . The hydrogen mass fractions are higher for the SwB730 and SwB1130 cases (+46% and +38% respectively) at the temperature corresponding to peak Y H2 in the premixed SwB330 profile. Carbon monoxide demonstrates similar trends, with the moderately stratified SwB730 peaking 25% higher than the premixed case, and the highly stratified SwB1130 exceeding SwB330 by 18%. The calculated mass fractions of O2 and H2O at the nominal equivalence ratio show good agreement with the experimental means regardless of stratification. Calculated methane mass frac-

tions agree well with the premixed mean values for all temperatures, and for temperatures above 1 000 K in the stratified cases. The disparity at lower temperatures is due to the mean equivalence ratio in the stratified cases being lower than the nominal mean used for the laminar flame calculations in this portion of the thermal profile. Carbon monoxide values from laminar flame calculations at the nominal mean equivalence ratio are in good agreement with the premixed SwB240 case, but peak 14% higher than the highly swirling SwB330 case. The calculated hydrogen values show excellent agreement with the premixed SwB240 and SwB330 cases, but peak significantly lower (by 23–52%) than the stratified averages in swirling conditions. The experimental CO2 mean mass fractions are higher than the calculated laminar flame values through much of temperature space, as observed in the non-swirling cases [6]. This may be attributable to the influence of differential diffusion.

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Fig. 19. Scatter plots of key combustion species in temperature space for the SwB330, SwB730 and SwB1130 datasets. Points where / < 0.47 are plotted in black. Mean fits are plotted in red, while vertical red bars denote ± one standard deviation. Laminar flame calculations at the nominal equivalence ratio (/n = 0.75) are plotted in gray. The premixed profile is marked in all cases by open circles. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Figure 20 shows mean fits of the various species shown in Figs. 18 and 19 where the data are conditioned on equivalence ratio. First, the premixed data (SwB240 and SwB330) are binned in temperature space using 40 equally spaced bins. The mean equivalence ratio within each bin is then determined. The data shown in Fig. 20 are conditioned to be within ±2.5% of the mean premixed equivalence ratio in each temperature bin. This conditioning results in very high levels of data rejection; 39–54% of points are eliminated in the premixed cases, while 90–95% are removed in the stratified cases. As a result of this the number of points considered within each bin can be as low as 125 points in the flame brush of the highly swirling, highly stratified case. However, convergence tests indicate that the result for the mean values are statistically robust, in spite of the relatively small sample size. Future work will focus on larger datasets to allow more detailed conclusions to be obtained from scalars subject to multiple levels of conditioning.

The equivalence ratio conditioned results indicate that stratification has no influence on the behavior of carbon dioxide, water, oxygen or methane across the thermal profile of the flame, regardless of the degree of swirl. The mole fractions of both carbon monoxide and hydrogen are elevated under stratified conditions relative to the corresponding premixed values. Given that the stratified data have been conditioned on local equivalence ratio, this enhancement is directly attributed to equivalence ratio gradients. Similar CO enhancement in flames subject to temporal, rather than spatial, gradients of equivalence ratio have been seen in simulations of premixed and back-supported (burning from stoichiometric to lean) stratified flames by Marzouk et al. [43], which they attributed to increased radical concentration from the richer products. Additionally, da Cruz et al. [46] reported elevated hydrogen levels under stratified conditions in back-supported laminar flames which they attributed to increased radical concentration

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4.6. Stratification and scalar gradients Surface density function, jrcj, thermal scalar dissipation rate,

vc, and turbulent flame thickness, dt, and their sensitivity to strat-

Fig. 20. Mean fits of key combustion species in temperature space where data are conditioned on equivalence ratio to be within ±2.5% of the mean premixed value in the corresponding temperature bin. Vertical bars denote ± one standard deviation.

from the richer products. Both the results in the present work and those highlighted from the literature have important implications for the modeling of stratified flames; tabulated chemistry based on homogeneously premixed flames may not be sufficient to capture the behavior of all major species under stratified conditions.

ification are considered in this section. Data have been corrected for spatial averaging and have been angle-corrected to their three-dimensional values. A maximum permissible solid angle between line measurement axis and the three-dimensional flame normal of hc = 50° has been applied to exclude data where the angle-correction process is liable to introduce large systemic error. Surface density function (top row) and scalar dissipation rate (bottom row) results are shown in progress variable space in Fig. 21 for the SwB240 (left column), SwB640 (central column), and SwB1040 (right column) cases. The format is similar to that used to show the thermal evolution of major species in Fig. 18, with mean fits and standard deviations of the experimental data obtained by binning in steps of progress variable of 0.05. Figure 22 shows the results for the SwB330 (left column), SwB730 (central column) and SwB1130 (right column) cases. There is a high degree of scatter in the surface density function data (top) within each bin regardless of the degree of stratification, comparable to the levels seen in the non-swirling cases [6]. As discussed in the previous paper in this series, this scatter indicates the presence of local maxima and minima at intermediate temperatures in the instantaneous thermal ramp. The wide range of progress variable gradients resulting at intermediate values of c leads to the scatter seen in Fig. 22. Stratification does not seem to significantly influence the mean surface density function or thermal scalar dissipation rate, in agreement with the results seen for the non-swirling data [6]. There is little difference in these quantities between the previous investigation and the present, indicating that the rate of reaction at the intersection of the mixing layer and the mean flame brush is similar in the surveyed flow conditions, despite the differences in turbulence characteristics at the respective locations. The experimental means for both quantities in Fig. 22 are significantly lower than the corresponding laminar flame calculations (30–40% less for jrcj, 16–31% less for vc. As in the previous work, this is attributed to the thickening of the flame relative to the laminar value due to small eddies penetrating the pre-heat zone (Fig. 4 indicates that the Kolmogorov length scale is much smaller than the thermal flame thickness). These results are in agreement with

Fig. 21. Scatter plots of surface density function (top) and scalar dissipation rate (bottom row) in progress variable space for the SwB240, SwB640 and SwB1040 datasets. Points where / < 0.47 are plotted in black. Mean fits are plotted in red, while vertical red bars denote ± one standard deviation. Laminar flame calculations at the nominal equivalence ratio (/n = 0.75) are plotted in gray. The premixed profile is marked in all cases by open circles. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 22. Scatter plots of surface density function (top) and scalar dissipation rate (bottom row) in progress variable space for the SwB330, SwB730 and SwB1130 datasets. Points where / < 0.47 are plotted in black. Mean fits are plotted in red, while vertical red bars denote ± one standard deviation. Laminar flame calculations at the nominal equivalence ratio (/n = 0.75) are plotted in gray. The premixed profile is marked in all cases by open circles. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

DNS simulations of turbulent flames by Sankaran et al. [47], as well as experimental investigations by O’Young et al. [48] and Chen et al. [49]. Equivalence ratio conditioned results for jrcj and vc are shown in Fig. 23. The data are conditioned to be within ±2.5% of the premixed equivalence ratio (as in Fig. 20) which results in a large reduction in the number of points considered, as outlined previously. Stratification appears to have little influence on the conditional means of either surface density function or scalar dissipation rate, regardless of the degree of swirl. This indicates that the commensurate lack of influence seen for the unconditioned data (Figs. 21 and 22) cannot be attributed to equivalence ratio effects to any significant degree. The high degree of variance seen in the unconditioned data remains after conditioning on equivalence ratio, as indicated by the vertical bars in Fig. 23. This is again attributed to the effects of local peaks or troughs in the thermal profile resulting in a range of jrcj at intermediate c values. It is difficult to draw strong conclusions from the conditioned results given the small number of data points used to determine the mean and standard deviation within each progress variable bin. However it appears that the highly swirling

Fig. 23. Mean fits of surface density function and scalar dissipation rate in progress variable space where data are conditioned on equivalence ratio to be within ± 2.5% of the mean premixed value in the corresponding progress variable bin. Vertical bars denote ± one standard deviation.

cases have slightly lower conditional mean surface density function and thermal scalar dissipation rate than the moderately swirling cases; this may result from higher turbulence levels and the corresponding increase in the frequency of local peaks and troughs. Figure 24 shows probability density functions of flame thickness, defined as [Tb  Tu]/jrTjmax, at various axial distances downstream of the burner exit (z = 10, 30, and 50 mm) for the swirling cases. The behavior of the distributions is not consistent between the two swirl levels. In the moderately swirling conditions, the

Fig. 24. Probability density functions of flame thickness at various downstream distances (z = 10, 30, 50 mm) for the moderately swirling (left column) and highly swirling (right column) datasets. Data are binned in 30 equally sized bins between 0 mm and 1.6 mm. Shots where the temperature range is less than 1200 K are not considered.

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peak of the premixed distribution remains constant with axial distance, while highly stratified case becomes thinner with distance. The moderately stratified SwB6 case peaks at a similar value at both z = 10 mm and z = 30 mm, and then shifts to thicker values at z = 50 mm. Caution is advised in interpreting these results; as discussed in Part I [6], the flame thickness equation used takes no account of local peaks or troughs within the instantaneous flame brush, which may bias dt to values lower than the actual physical thickness of the thermal ramp. The behavior exhibited by the highly swirling cases is noticeably different. The distributions for the premixed case shift towards smaller thicknesses as the axial distance increases, while the stratified cases tend to thicken. dt is similar regardless of stratification at the intersection of the mean flame brush and the mixing layer (z = 30 mm), as was the case for the non-swirling flows. The range of flame thicknesses experienced tends to increase with distance, as shown by the broadening of the flame thickness distributions, and this is attributed to the increasing effects of turbulence on the preheat zone of the flame. Similar effects are observed for the moderately swirling cases. 4.7. Influence of stratification on curvature Probability density functions of local curvature in the swirling cases are shown for z = 10, 30, 40, and 50 mm (Fig. 25). The distributions are commensurate at z = 10 mm under both swirl levels as the combustion is stabilized in premixed reactants from the inner annulus at this axial location. Local curvature appears fairly insensitive to the level of stratification in both the moderately swirling

Fig. 25. Probability density functions of local curvature at various downstream distances (z = 10, 30, 50 mm) for the moderately swirling (left column) and highly swirling (right column) datasets. Data are taken from both OH-PLIF planes and is binned in 50 equally sized bins between 3 mm1 and 3 mm1. Points where jjj > 3 mm1 are not considered.

and highly swirling flows, with the distributions showing little differences for z > 30 mm. There is a slight broadening under stratified conditions at z = 30 mm in both swirl levels, though the effect observed is smaller than that reported typically reported in the literature [14,8,11,30]. The general behavior seen in Fig. 25 parallels that exhibited by the non-swirling cases, though the swirling distributions are broader (indicating more wrinkling on average). There is a noticeable increase in the degree of wrinkling moving from the moderately swirling state to the highly swirling state for z < 50 mm. This is shown by the decrease in the peak amplitude of the distributions looking across the columns of Fig. 25. By z = 50 mm there is no significant broadening of the curvature distributions with the further addition of swirl. This indicates that the convolution of the flame surface due to the turbulence field is comparable by this axial location. As posited in Part I [6], the disparity between the findings in the weakly turbulent literature [8,11,14,30] (where stratification broadens curvature distributions) and the present results is attributed to the increase in turbulence intensity. Turbulence levels in the present burner are still modest compared to those in most practical combustion applications, so the implication is that stratification is unlikely to have any significant influence on flame curvature statistics in practical combustors.

5. Conclusions A double annular, bluff-body burner was developed in order to systematically investigate effects of fuel–air stratification on flame structure in a turbulent flow field having features relevant to practical combustion applications, including swirl and recirculation. Velocity and scalar results were presented for nine flames in the thin reaction zone regime, each having different levels of stratification and swirl: homogeneously premixed (/i = /o = 0.75), moderately stratified (/i = 1.0, /o = 0.5), and highly stratified (/i = 1.125, /o = 0.375); non-swirling, moderately swirling (S = 0.45), and highly swirling (S = 0.79). Axial and radial flow fields were revealed using two dimensional PIV, while the thermochemical structure was probed within the flame front using Rayleigh/Raman/CO-LIF line measurements with simultaneous cross-planar OH-PLIF. The main conclusions regarding stratification are as follows: (i) Stratification elevates H2 and CO levels relative to the premixed flames both on the mean and when further conditioned on local equivalence ratio in temperature space. This has implications for the applicability of premixed laminar flamelets within stratified combustion models. Stratification has no significant influence on mass fractions of the main reactant and product species (CH4, O2, CO2, H2O) when considered in state space as mean values conditional on temperature. (ii) Surface density function and scalar dissipation rate show little evidence of being stratification dependent. These quantities are attenuated in all measured cases relative to laminar calculations, and this is attributed to the presence of local minima and maxima at intermediate temperatures in the instantaneous thermal profiles. (iii) Curvature in the swirling flames exhibits little stratification sensitivity, similar to the behavior in the non-swirling cases. This contrasts with findings in mildly turbulent stratified flames and suggests that any influence of stratification of curvature is quickly overcome when turbulence levels increase. These conclusions show that it should be largely possibly to make predictions for both non-swirling and swirling strati-

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fied flames based on premixed flame models, though modifications would be required to capture the behavior of CO and H2. Regarding the effects of swirl: (iv) The moderately swirling flames retain relatively compact recirculation zones comparable to those of the corresponding premixed or stratified non-swirling flames. Consequently, effects of preferential species transport are amplified as in the non-swirling flames, such that the measured equivalence ratio of products within the recirculation zone is higher that of the adjacent reactants. (v) The shape of the recirculation zone changes significantly with high swirl, extending farther downstream and entraining dilute products of combustion with Favre-averaged equivalence ratio significantly lower the adjacent reactants. This opening of the recirculation zone prevents amplification of preferential transport effects, such that there is no net increase in equivalence ratio across the flame. This will make it easier to conduct model comparisons on the premixed and stratified, highly swirling cases. (vi) The addition of swirl results in higher levels of stratification than were seen for the corresponding non-swirling cases in Part I [6]; for example, the SwB11 case experiences a mean change in / across the thermal ramp that is approximately three times that for the non-swirling SwB9 case at the intersection of the mean flame brush and the mixing layer. Therefore, the effect of swirl is important not only on the fluid mechanics and stabilization, as previously known, but also in enhancing mean stratification gradients (D/) as well as influencing the extent of preferential transport effects. Acknowledgments The authors thank the EPSRC, the Leverhulme Trust, and Rolls Royce for their financial contributions to this work. Work at Sandia was supported by the United States Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences. 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. The authors also thank Bob Harmon for his contributions to the experiments. References [1] A. Mansour, Proc. ASME Turbo Expo 2 (2005) 141–149. [2] D.A. Nickolaus, D.S. Crocker, D.L. Black, C.E. Smith, ASME Conf. Proc. 2002 (36061) (2002) 713–720. [3] H.J. Bauer, Prog. Comput. Fluid Dyn. 4 (3–5) (2004) 130–142. [4] A.C. Alkidas, Energy Conserv. Manage. 48 (11) (2007) 2751–2761. [5] M.C. Drake, D.C. Haworth, Proc. Combust. Inst. 31 (2007) 99–124. [6] M.S. Sweeney, S. Hochgreb, M.J. Dunn, R.S. Barlow, Combust. Flame, accepted for publication. [7] N. Pasquier, B. Lecordier, M. Trinite, A. Cessou, Proc. Combust. Inst. 31 (1) (2007) 1567–1574.

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