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Tomographic reconstruction of OH* chemiluminescence in two interacting turbulent flames
This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2013 Meas. Sci. Technol. 24 024013 (http://iopscience.iop.org/0957-0233/24/2/024013) View the table of contents for this issue, or go to the journal homepage for more
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IOP PUBLISHING
MEASUREMENT SCIENCE AND TECHNOLOGY
Meas. Sci. Technol. 24 (2013) 024013 (11pp)
doi:10.1088/0957-0233/24/2/024013
Tomographic reconstruction of OH∗ chemiluminescence in two interacting turbulent flames Nicholas A Worth and James R Dawson Department of Engineering, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, UK E-mail:
[email protected]
Received 5 April 2012, in final form 5 October 2012 Published 20 December 2012 Online at stacks.iop.org/MST/24/024013 Abstract The tomographic reconstruction of OH∗ chemiluminescence was performed on two interacting turbulent premixed bluff-body stabilized flames under steady flow conditions and acoustic excitation. These measurements elucidate the complex three-dimensional (3D) vortex–flame interactions which have previously not been accessible. The experiment was performed using a single camera and intensifier, with multiple views acquired by repositioning the camera, permitting calculation of the mean and phase-averaged volumetric OH∗ distributions. The reconstructed flame structure and phase-averaged dynamics are compared with OH planar laser-induced fluorescence and flame surface density measurements for the first time. The volumetric data revealed that the large-scale vortex–flame structures formed along the shear layers of each flame collide when the two flames meet, resulting in complex 3D flame structures in between the two flames. With a fairly simple experimental setup, it is shown that the tomographic reconstruction of OH∗ chemiluminescence in forced flames is a powerful tool that can yield important physical insights into large-scale 3D flame dynamics that are important in combustion instability. Keywords: tomographic reconstruction, turbulent flames, vortex–flame interactions
(Some figures may appear in colour only in the online journal)
1. Introduction
understanding of the flame dynamics, planar laser-induced fluorescence (PLIF) can be used to provide spatial information in two dimensions. In premixed flames, OH planar laserinduced fluorescence (OH-PLIF) has been shown to provide an accurate marker of the instantaneous flame front location from which the change in flame surface area can be estimated (Lee and Santavicca 2003). It has also been demonstrated that the nonlinear response of the heat release is often associated with the formation of large-scale vortex structures (Balachandran et al 2005, Bellows et al 2007, Thumuluru and Lieuwen 2009). A more accurate estimate of the reaction rate can be obtained from the product of OH and CH2 O PLIF but this requires two dye laser systems to image each species simultaneously (Balachandran et al 2005). In addition, most studies in combustion instability are carried out in axisymmetric or nominally two-dimensional (2D) flames. However, in practical applications such as annular gas turbine combustion chambers,
Self-excited combustion instabilities are a well-known problem in practical combustion systems, particularly for low-emission gas turbine combustion concepts. They occur when velocity fluctuations imposed by acoustic waves induce fluctuations in the heat release rate which, under the right conditions, feed back causing the amplitude of the instability to grow until nonlinear effects dominate and a limit cycle is established. Most theoretical approaches are based on a linear stability analysis and require modelling of the nonlinear flame dynamics. A black box approach is often used to characterize the nonlinear response by simply measuring global CH∗ or OH∗ chemiluminescence as a surrogate for the heat release rate over a range of forcing frequencies and amplitudes to construct a flame transfer or describing function (Balachandran et al 2005, Palies et al 2010). To obtain a more physical 0957-0233/13/024013+11$33.00
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there are numerous flames arranged around an annulus which can interact with each other depending on separation distance. Planar imaging techniques offer some advantages compared with global chemiluminescence measurements, for example spatial distribution and local curvature, but only in two dimensions. This is particularly restrictive when flames are not axisymmetric and or exhibit complex large-scale threedimensional (3D) flame dynamics. A recent study of the interactions between two bluffbody stabilized premixed flames by Worth and Dawson (2012) demonstrated that, depending on their separation distance, large-scale flame front merging occurs where the annular jets impinge upon each other resulting in a 3D, asymmetric flame structure. In an effort to further understand the structure of these flames and the complex vortex–flame interactions that occur when subjected to acoustic forcing, it is necessary to resolve their full 3D unsteady structure, a task particularly suited to computed tomography (CT) techniques. Although not widely used, tomography has been applied to flames to study laser absorption phenomena, with scanned line of sight projections allowing the reconstruction of 2D slices (McNesby et al 1995, Best et al 1991, Santoro et al 1981), and has been applied to the reconstruction of temperature information through refractive index-based laser speckle tomography (Liu et al 1988, Ko and Kihm 1999). It has also been used to reconstruct the distribution of chemiluminescence in 2D slices (Anikin et al 2010). However, it is only relatively recently that it has been used to reconstruct large volumes with an emphasis on understanding the 3D flame structure in stagnation-, jet- and bunsen-type flames (Ishino and Ohiwa 2005, Ishino et al 2009, Upton et al 2011, Floyd and Kempf 2011, Floyd et al 2011, Hossain et al 2012). A common approach is to record line integral projections (camera images) of chemiluminescence from a number of viewing angles, which are then processed using a tomographic algorithm in order to reconstruct the volumetric distribution of the chemiluminescence. Floyd and Kempf (2011) used CT to reconstruct the mean flame structure in a laminar matrix burner using 48 mean CH∗ chemiluminescence images taken from a single repositioned camera. As noted by the authors, this is an extremely cost-efficient method of obtaining the 3D flame structure, as only a single camera is required. Simultaneous imaging was also used to capture the instantaneous matrix burner flame structure, demonstrating that with only ten views, it was possible to capture the main features of the flame with reasonable accuracy, despite the use of an inadequate camera calibration model. This work was extended to analyse a turbulent-opposed jet flame, again demonstrating the ability of the technique to capture the main features (Floyd et al 2011). Ishino and Ohiwa (2005) use a custom-built 40-lens film camera to image and reconstruct the broadband (200–400 nm) chemiluminescence distribution in a turbulent propane flame. The 3D flame structure is well described, allowing variations in the local concentration of chemiluminescence to be qualitatively related to a local flame surface curvature. This work was extended by taking two images in a rapid succession in order to estimate the local flame velocity (Ishino et al 2009). Upton et al (2011) used a slightly different approach, recording
12 simultaneous views of flash lamp illuminated oil droplets in a turbulent propane flame. The 3D distribution of light scattered from the oil droplets is then reconstructed giving an indication flame front location. While this approach yields a high-resolution reconstruction of a 3D turbulent surface, it does not resolve the actual flame surface as the droplets evaporate in the preheat zone and do not reach the flame front. Nevertheless, the potential for 3D quantitative measurements of flames is certainly convincing. In previous studies, the accuracy of the tomographic reconstructions was typically assessed by comparing reconstructions of known artificially generated objects with the objects themselves (Anikin et al 2010, Floyd et al 2011). Although this approach is useful, it is difficult to define the level at which the reconstructions accurately capture the relevant flow features, and as yet this technique has not been compared with more well-established, independent measurement techniques. In this study, the tomographic PIV code developed by Worth and Nickels (2008) and Worth et al (2010) has been modified to reconstruct the 3D OH∗ chemiluminescence field of two acoustically excited, closely spaced turbulent premixed bluff-body flames. The aim is to demonstrate the ability of tomography to elucidate complex 3D flame–vortex interactions that occur during thermoacoustic instability in two interacting turbulent flames which is of importance to the gas turbine community. The results are benchmarked against OH-PLIF measurements which describe the 2D dynamics along a centreline slice in a previous study (Worth and Dawson 2012). This back-to-back comparison of the reconstructions against the well-established technique of OH-PLIF has not been performed before. By only considering the mean unforced flame structure and the phase-averaged forced flame structure, it is possible to capture the necessary information using only a single camera and intensifier following a similar approach to Floyd et al (2011). This paper begins by detailing the experimental apparatus and methodology, after which a novel comparison between the tomographic OH∗ reconstruction and OH-PLIF demonstrates for the first time the ability of the technique to reproduce the large-scale features of the heat release in both the mean and phase-averaged fields. An analysis of the volumetric data provides unique physical insight into the large-scale features of the 3D asymmetric heat release structure in the central interacting region both in the absence and during acoustic excitation.
2. Experimental methods 2.1. Apparatus The apparatus is identical to that used by Worth and Dawson (2012) and is shown in figure 1(a). It consists of two identical bluff-body stabilized turbulent premixed flames with a flame separation distance of S = 1.14D, where S is defined as the distance between the bluff-body centres and D is the tube diameter. Methane–air mixtures were premixed upstream and flowed into two cylindrical plenum chambers of length 200 mm and inner diameter 100 mm containing flow straighteners. Each 2
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Figure 1. Schematics of the experimental setup. (a) Photo showing the bluff-body flame holders, loudspeaker arrangement and pressure sensors. (b) Configuration of the high-speed OH-PLIF system. (c) Viewing positions for the high-speed intensified camera. (d) Example mean images from four viewing positions.
forcing frequency of 160 Hz and amplitude of A = 0.4 were chosen to illustrate the large-scale flame–vortex interactions.
flame holder consisted of a 400 mm long circular tube of inner diameter D = 35 mm with a 25 mm centrally located conical bluff-body giving a blockage ratio of 50%. In order to position the flames close together and in parallel whilst accommodating the plenum chambers, the inlet tube required a 22.5◦ bend which was located 300 mm upstream of the bluff bodies. Air and fuel flow rates were independently controlled for each of the flames using four Alicat mass flow controllers. The reactant exit velocity was set to 10 ms−1 which gave a Reynolds number of 1.7 × 104 based on the bluff-body diameter. The equivalence ratio was φ = 0.8. Each plenum was fitted with a pair of loudspeakers positioned diametrically opposite each other which were used to generate sinusoidal velocity oscillations at the bluff-body exit. Two Kulite XCS-093 pressure transducers were mounted flush with the inside tube walls at two upstream locations. These pressure signals were amplified and filtered before being used to calculate the velocity fluctuation magnitude using the two-microphone technique. Data were acquired at 10 kHz with sample lengths of 3.2 s, and digitized using a National Instruments 16-bit PCI 6251 card, providing a frequency resolution of 0.3 Hz. The signals were analysed spectrally using the fast Fourier transform in order to determine the complex amplitude of the velocity, A = u ( f )/U. A single
2.2. OH∗ imaging and tomographic reconstruction Image reconstruction from projections is the process of producing an image or volume distribution of some physical property, in this case a scalar field, from the estimates of its line integrals along a finite number of lines at known locations (Herman and Lent 1976). In the present investigation, the flames’ natural OH∗ chemiluminescence was recorded by an intensified camera over a range of viewing angles and was used to reconstruct the 3D OH∗ chemiluminescence distribution. The following section describes this setup in detail. OH∗ chemiluminescence images were captured with a single Photron SA1.1 high-speed CMOS camera with a 12-bit 10242 pixel resolution coupled with a LaVision IRO highspeed two-stage intensifier, fitted with a Nikon Rayfact PF10545MF-UV lens and an OH filter (300–325 nm). An aperture setting of f = 1/16 and an object distance of approximately 1100 mm were used to ensure a sufficient depth of field, resulting in a final image resolution of approximately 6002 pixels at a resolution of approximately 5 pixels mm−1 . As only a single camera and intensifier were used, after each imaging sequence, the camera angle was rotated around the 3
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Figure 2. Locations of the calibration plate (a) at the back and (b) front of the field of view. (c) Variation of pixel response of the high-speed intensifier and CMOS camera.
burner in 10◦ increments from θim = 20◦ to 160◦ , producing 15 independent views as shown in figure 1(c). A total of 5277 images were obtained for the forced flame at each camera angle at a frame rate of 2880 Hz. At this frame rate, 18 images were obtained per oscillation cycle and each averaged over 293 oscillation cycles at each camera angle. A total of 500 images were used to obtain mean images of the unforced flame at each camera angle at a frame rate of 50 Hz. The advantage of using a single camera is low cost and therefore feasibility, allowing 15 independent views of the flame without the need of 15 intensified cameras or multiple cameras with image splitters. However, the main drawback of this approach is that the instantaneous volumetric distribution of OH∗ cannot be recovered which would be of interest to estimate the 3D flame surface density (FSD) for example. Nevertheless, the single camera approach is particularly suited to the problem at hand as the unsteady flow is periodic and dominated by the formation and complex interaction of largescale vortical structures (Worth and Dawson 2012) whose phase-averaged evolution is of significant interest. Camera calibrations were taken at each angle using a custom-made calibration plate. Calibration images were recorded at two translation positions corresponding to the front and back of the measurement volume as shown in figures 2(a) and (b), respectively. To ensure accurate positioning of the calibration plate, locating screws were manufactured at the front and back of the measurement volume. Real and image space co-ordinates were related through a third-order polynomial calibration function (Soloff et al 1997), which is defined for each camera angle and in each depth plane, with a mean calibration error of around 0.10 pixels. As reported by Weber et al (2011), when using a highspeed intensifier and CMOS camera for scalar imaging, care must be taken to calibrate the individual pixel response to variations in light intensity. Following the recommendations of Weber et al (2011), care was taken to ensure that the imaging system had reached thermal equilibrium and a pixelwise intensity calibration was performed using a custommade Ulbricht sphere with a UV light source. The spatial variation in pixel intensity is shown in figure 2(c). The dark image subtraction was employed, and following this, the spatial variation in pixel intensity was normalized. Within the measured range, the intensity response was found to vary linearly, and therefore, the pixel-wise nonlinear intensity
calibration step suggested by Weber et al (2011) was not required. In addition to hardware calibrations, a rudimentary correction procedure was devised to account for the absorption OH∗ chemiluminescence from signal trapping (Sadanandan et al 2012). The magnitude of signal trapping was estimated by imaging both flames from the side, i.e. from a viewing angle perpendicular to camera position 8 in figure 1(c), with a flame separation distance large enough to ensure that both flames were axisymmetric. First, both flames were imaged simultaneously, then each flame was imaged separately and their intensity summed. By comparing the OH∗ intensities of each case, it was found that the intensity of the former was attenuated. This is ascribed to signal trapping as OH∗ from the rear flame is trapped during its passage through the front flame along the line of sight. It is assumed that the amount of trapping is proportional to the concentration of OH∗ along the line of sight, and that the measured signal strength at any point is a representative of this OH∗ concentration, allowing us to construct a rudimentary nonlinear correction. Following the intensity correction, image smoothing is performed using a 3 × 3 top-hat kernel followed by the background intensity subtraction to reduce reconstruction noise through an improvement in the signal-to-noise ratio (SNR). A maximum image intensity of 2000 counts after image intensity correction and background subtraction sets the dynamic range. After ensemble averaging, standard pixel intensity errors of 2.3 and 2.9 intensity counts are present in the unforced and forced images, respectively, representing percentage errors of 0.1% and 0.15% of the full-scale intensity. Furthermore, the high SNR of 30 dB in the current investigation is sufficient to ensure high accuracy reconstructions, as demonstrated previously by Floyd and Kempf (2011). The tomographic reconstruction was performed using a FORTRAN 90 implementation of the MART algorithm (Herman and Lent 1976), employing similar basis functions and pixel/voxel intersection relationships to Elsinga et al (2006). Thus, the volume is discretized into voxels which take a non-zero value inside and zero outside. Weighting values between pixels and voxels are determined by the distance between their centres and their relative sizes, based on the approximation of the intersecting area between a circle and a rectangle which are a simplified representation 4
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Figure 3. Rendered 3D volume of the mean OH∗ chemiluminescence distribution at two different projections.
regions. Similar to the OH∗ , the FSD and OH-PLIF images are normalized by the peak cycle intensity and scaled to vary over an arbitrary six intensity counts.
of the voxel volume and pixel line of sight, respectively. A 120 mm×120 mm×90 mm volume of interest was discretized at 5 voxels mm−1 to create a 600 × 600 × 450 voxel volume, with a pixel-to-voxel ratio of approximately 1. An underrelaxation parameter of 1 is used in the MART algorithm, which after the multiplicative first guess technique of Worth and Nickels (2008) is applied iteratively over five passes. Volume smoothing using a 3 × 3 × 3 top-hat kernel and background subtraction are performed to reduce the effect of reconstruction artefacts. The intensity distribution was normalized by the peak cycle intensity and scaled to vary over an arbitrary six intensity counts.
3. Results and discussion In the following section, the results of the tomographic OH∗ reconstruction for both steady and acoustically forced flames are presented. It must be stressed that while these measurements do not provide a quantitative measure of heat release rate, they do provide an indication of the relative rate of heat release through which the flame structure and periodic dynamics may be described. Comparisons are also drawn in this section between the tomographic OH∗ reconstruction and OH-PLIF techniques. While inherent differences between these techniques preclude a quantitative comparison, the FSD results can be used qualitatively in order to assess the ability of the tomographic technique to capture large-scale features of the flame structure and dynamics, features of significant interest in the study of thermoacoustic instabilities (Balachandran et al 2005, Worth and Dawson 2012).
2.3. OH-PLIF Time-resolved OH-PLIF was used to provide a comparative measure of the flame structure along the burner centreline. The imaging system consisted of a 15 W JDSU Q201-HD laser, which was used to pump a high-speed Sirah Credo 2400 dye laser, achieving ≈60 μJ per pulse at 5 kHz. A series of optics were used to produce a thin 40 mm high sheet whose path traversed the centres of both bluff bodies as shown in figure 1(b). The camera and intensifier were fitted with a Cerco 2178 UV lens 100 F/2.8 and OH filter (300–325 nm). A minimum of 5400 images were obtained for each case. After correcting for beam profile inhomogeneities, the FSD was computed following a similar approach to Balachandran et al (2005), using an interrogation window size of 5 pixels. The FSD is used qualitatively as a flame front marker in the current investigation. The high-speed OH-PLIF system was not powerful enough to illuminate the entire combustion region with an adequate signal. To overcome this drawback, the upper and lower halves of the combustor were imaged separately with a 10 mm overlap region. Through the use of a calibrated common co-ordinate system, the mean and phase-averaged FSD images were stitched together to illustrate the flame response over the entire combustion zone. The overlapping region between the stitched images comprises the average of both upper and lower
3.1. Steady flames A rendering of the volumetric mean unforced flame structure is shown in figure 3 for different viewing angles. The mean structure of the two flames appears to be well captured showing the mean flame stabilized on the bluff-body-generated shear layers which are angled due to the bluff-body geometry. The otherwise axisymmetric flame structure is broken in the central interacting region where the two flame fronts merge together and extend downstream as a consequence of the acceleration produced by the merging of the annular jets (Worth and Dawson 2012). The reconstruction captures the extent of the merged flame in 3D. To examine the flame structure and particularly the flame merging region, a selection of 2D slices in the x–y plane from z = 10 to 60 mm is shown in figure 4. Close to the bluff body, 5
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the two flame structures remain axisymmetric and isolated from each other with a turbulent flame brush thickness of approximately 1 mm. At this position, several gaps in the flame sheet are observed. These gaps are not due to the absence of flame but due to the low OH∗ signal strength near the flame base and due to the thinner flame brush (as shown in figure 1(d)). A similar problem was found by Anikin et al (2010) due to short exposure times and also the high-speed OH-PLIF measurements in Worth and Dawson (2012). Further downstream shows the flame brushes’ thickening to 3 to 4 mm as well as flame cross-sections as the flow develops and becomes more turbulent. Eventually, the two flames approach each other in the central region at z = 30 mm. At z = 40 mm, clear evidence of merging is seen in the central region with a new fully merged flame structure by z = 50 mm. The intensity of this merging region is over twice that of the surrounding regions and extends significantly further downstream. This is consistent with the OH-PLIF results reported in Worth and Dawson (2012) which showed that the merging region produced the majority of the FSD.
Figure 5 shows 2D slices in the y–z plane through the centre of each flame and merging region, i.e. at x = −25, 25 and 0 mm. Each of the flames along the bluff-body centrelines at x = −25 and 25 mm appears reasonably symmetric showing the development of the flame brush with a downstream distance. A slice through the merging region between the two flames is shown in figure 5(b). This shows that the merged flame structure begins around z = 30 mm with the peak OH∗ occurring further downstream. The rate of spread of the merged flame in the y-direction results in a jet-flame-like structure. The increase in turbulent intensity is most likely responsible for the observed increase in the global heat release through an increase in the flame surface area. In order to assess the ability of the technique to capture the large-scale flame structure, figure 6 compares centreline slices of the tomographically reconstructed volume with the mean FSD and the mean OH fluorescence from the OH-PLIF measurements, all of which are in the x–z plane passing through both bluff-body centrelines. Good agreement is found between the volumetric OH∗ in figure 6(a) and the FSD in figure 6(b) as both show similar overall flame structures. 6
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was optimized to capture the large-scale flame dynamics rather than the anchoring structure close to the flame base, and as such the structure in this region may not be fully resolved through the current measurements. Although the OH-PLIF technique suffers from a similar problem, requiring the lowest part of the domain to be masked, in this case, it was possible to obtain data at around z = 3 mm. As noted by Upton et al (2011), PLIF excites OH present in both combustion products and at the flame front, whereas OH∗ chemiluminescence only occurs near the flame front. However, comparing the locations of peak OH with the OH-PLIF in figure 6(c) shows good agreement with the reconstructed OH∗ in figure 6(a), especially in the merging region which was poorly captured by the FSD.
Importantly, both methods show the flame merging location occurs around z = 27 mm in the central interacting region. Nevertheless, a number of differences exist between these two metrics. Firstly, the FSD underestimates the heat release in the merging region and secondly, a small shift in the flame position in the x-direction can be observed with the FSD contours located slightly further from the bluff-body centre. Both these discrepancies are most likely an artefact of the edge detection algorithm which strongly depends on the gradients of OH. The OH∗ signal is significantly weaker near the jet exit plane which results in no information below z = 6 mm for the volumetric OH∗ . The weaker signal strength close to the flame base can be observed in figure 1, and may be caused by the thin shear layer and intermittency in this region. The current experiment 7
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flame sheet into two large annular vortices which surround these structures. The outer vortices break down before the long tube-like structures, as shown from θ = 240◦ , which are linked across the merging region and anchored in the bluff-body wakes. Clearly, the tomographic reconstruction of OH∗ chemiluminescence reveals a rich complexity in flame dynamics not accessible from planar imaging techniques. Figure 8 shows cross-sections of the phase-averaged flame in the x–y plane at a single downstream location (z = 20 mm). The development of inner and outer vortices, and long tubelike flame structures formed later in the cycle and anchored in the bluff-body wakes, as described previously, is much easier to distinguish. The deformation of the flame front in the interacting region is particularly evident from these slices as shown by the flattened flame structures in the interacting region between the two flames. The concentric vortex rings formed by the roll-up of the outer and inner shear layers are captured at θ = 120◦ . The quality of the OH∗ reconstruction is again shown to be poor near the flame base, particularly at θ = 0◦ , where large sections of the flame sheet are not captured due to the comparably weak OH∗ signal. The streaking patterns of light and dark intensity are most likely to be reconstruction artefacts, an inherent problem in the discreet reconstruction when using a finite number of independent projections. It should be noted, however, that despite these issues, the quality
3.2. Forced flames The phase-averaged evolution of the volumetric flame structure is shown over six normalized phase angles (time steps) in figure 7. To aid in interpreting the data, it is instructive to also consult the first column of figure 10 alongside the volumetric plots. Over the first few phase angles, the velocity oscillation causes the shear layers and hence the flame to roll up into two co-annular counter-rotating vortex rings. As the flame sheet is primarily stabilized on the bluffbody shear layer, only the roll-up of the inner annular vortex can be observed in figure 7(b). Note that in figures 7(b) and (c), there is also a merged flame structure linking the two flames along the y = 0 mm axis. This is the top of the vortex pair that is clearly seen in figure 10 and results from the pinch-off of the large-scale vortex structures from the previous cycle (Worth and Dawson 2012). At θ = 120◦ , the outer vortex rings begin to exert an influence on the flame sheet, wrapping it outwards away from the bluff-body centres, increasing the visible flame surface in figure 7(c). Between θ = 120◦ and 240◦ , merging occurs creating a complex asymmetric flame structure in the merging region. When θ = 180◦ , both the inner and outer rings are visible simultaneously, with the inner rings collapsing into the bluff-body wakes to form a long tubular flame structure whose axes are aligned with the bluff-body centrelines whilst the outer rings wrapping the 8
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Figure 10. Phase-averaged forced flame, volume slices in the x–z plane, y = 0 mm. LHS column: volumetric OH∗ ; centre column: FSD (OH-PLIF); RHS column: OH-PLIF. Cycle position by row from 0◦ (top) to 300◦ (bottom) in 60◦ increments.
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of the reconstruction is still more than sufficient to capture the complex vortex–flame dynamics. Figure 9 shows cross-sections of the phase-averaged flame structure in the y–z plane at x = 0 mm, i.e. where the two flames merge. Here, we can see a large modulation in not only the amount of flame present but also the spreading rate over the oscillation cycle. The flame reaches a maximum width of approximately 40 mm at θ = 300◦ which is when the shear layers merge and separate in the wake of the vortex rings. This is best seen in the last two phase angles in figure 10. Compared with the unforced case, forcing significantly enhances the spreading rate of the flame in the y-direction over the forcing cycle. It was previously shown that the central interacting region dominates the heat release response (Worth and Dawson 2012); however, the widening of this region demonstrates that this behaviour is not simply limited to the central plane, and instead extends significantly in the y-direction. Therefore, the significance of the 3D structure of heat release in the merged region is large and highlights the need for 3D measurements of this nature. Finally, it is worthwhile comparing centreline slices in the x–z plane of the phase-averaged volumetric OH∗ , FSD and OH-PLIF which are plotted in figure 10. As mentioned in section 3.1, there is a good level of agreement between the volumetric OH∗ and the FSD, with both techniques capturing the vortex roll-up, and merging of the inner annular rings, in addition to the merged flame dynamics in the central interacting region (Worth and Dawson 2012). However, there are a number of differences: as before, the low OH∗ signal intensity near the flame base results in a poor description of the flame below around z = 10 mm; OH fluorescence in the combustion products results in significant FSD downstream due to the pinched-off vortex pair. The OH∗ signal during the evolution of the vortex pair in the interaction region falls to low levels at around z = 55 mm. Overall however, the volumetric reconstruction OH∗ chemiluminescence shows agreement with the FSD and OH-PLIF data. The 2D evolution of the vortex–flame structure is well captured and with the additional 3D information, it provides much greater insights into the large-scale unsteady flame dynamics.
The availability of volumetric measurements permits unique insights into the underlying flow-field physics, in this case a scalar field which is a surrogate of the heat release in premixed flames. Previous investigations by the authors have highlighted the importance of 3D interactions, which dominates the unsteady heat release response. This study demonstrates for the first time that interactions between turbulent flames that are closely spaced, as they would be in a gas turbine combustor for example, give rise to complex, 3D, large-scale interactions which need to be taken into account, particularly for studies in thermoacoustics. These results demonstrate that using a single camera and an intensifier is a powerful method to reconstruct quantities of interest in unsteady periodic flows. Moreover, important physical insights can be gained at low cost in comparison with more complex multi-camera setups.
Acknowledgments This work was funded by the EPSRC and is part of the SAMULET Project 2 Combustion Systems for Low Environmental Impact. Dr J R Dawson is funded by the EPSRC under the Advanced Research Fellowship scheme.
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4. Conclusions A single camera and an intensifier were used to capture the OH∗ chemiluminescence of two interacting bluff-body stabilized flames, under both steady and acoustically forced conditions, capturing for the first time the complex 3D vortex– flame interaction occurring between impinging turbulent flames in an industrially relevant configuration. Images were taken from 15 independent angles, and through 3D camera calibration, mean and phase-averaged volumetric OH∗ distributions were reconstructed using the computed tomography of chemiluminescence technique. Novel qualitative comparisons were made with OH-PLIF and FSD indicating that despite subtle differences, the volumetric distribution accurately captures the mean flame structure and phase-averaged evolution of the unsteady flame dynamics when the flames were subjected to acoustic forcing. 10
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