volumetric flame measurements in a lifted turbulent jet...

17th International Symposium on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2014

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Volumetric flame measurements in a lifted turbulent jet flame using

tomographic reconstruction of chemiluminescence

J. Weinkauff1, J. Köser1, D. Michaelis2, B. Peterson1, A. Dreizler1, B. Böhm3*

1: Fachgebiet Reaktive Strömungen und Messtechnik, Technische Universität Darmstadt

Jovanka-Bontschits-Str. 2, D – 64287 Darmstadt, Germany 2: LaVision GmbH, Anna-Vandenhoeck-Ring 19, D – 37081 Göttingen, Germany 3: Fachgebiet Energie- und Kraftwerkstechnik, Technische Universität Darmstadt

Jovanka-Bontschits-Str. 2, D – 64287 Darmstadt, Germany

* correspondent author: [email protected] Abstract This work aims to track the three-dimensional flame base of a lifted turbulent jet flame operated at a moderate Reynolds number of 5000. Tomographic reconstruction of chemiluminescence images was performed to provide information on the (spatially filtered) instantaneous flame structure in 3D space. The experimental setup consisted of eight sCMOS cameras to collect chemiluminescence of the electronically excited CH radical. For volume reconstruction a SMART (Simultaneous Multiplicative Algebraic Reconstruction Technique) algorithm was used providing a volume of interest of 34x34x35 mm³. The present work-in-progress provides some insights into limitations of spatial and temporal resolutions based on a setup of eight cameras for this specific flame. The signal quality is explored and the averaged flame base height was compared to planar measurements of Mie-scattering off oil droplets. Oil droplets evaporate in the vicinity of the flame and are therefore regularly used to track flame fronts. The presence of these droplets was additionally used for a first evaluation of their impact on the measurements in order to prepare simultaneous particle image velocimetry and tomographic chemiluminescence measurements. The spatial resolution of the presented configuration was estimated to be approximately 2 mm. Due to the low signal intensities of chemiluminescence long exposure times in the order of milliseconds were required. This caused an additional coupling of spatial and temporal resolution due to flame movement during camera exposition. Several open issues were identified to further improve the tomographic chemiluminescence signal quality. 1. Introduction Turbulent lifted flames are a class of flames found in almost every industrial burner. The lifted flame base has been subject of many investigations focusing on the flame stabilization mechanism (Lyons 2007; Lawn 2009). The role of large-scale vortices and the degree of premixing was of particular interest. Lyons (2007) points out in his review that “a definite picture of flame stabilization is not in hand”. He summarizes inconsistencies in the flame stabilization theories regarding the role of large-scale structures, local flame extinction, turbulent velocity fluctuations and the physical mechanisms leading to upstream flame propagation. A detailed analysis of the aforementioned processes requires the detection of the flame position. The flame front is often marked by flame radicals measured by laser induced fluorescence within planes (PLIF) illuminated by a laser-sheet providing cross-sections of the three-dimensional flame. Schefer (1997) addressed the 3D nature of these flames using CH-PLIF capturing cross-sections parallel and perpendicular to the jets axis in individual snapshots. The flame was observed to be significantly contorted and flame discontinuities were found in the flame surface especially in circumferential cross-sections at an axial distance close to the flame lift-off height. These flame discontinuities appearing as “flame holes” or “flame islands” were frequently observed in flame imaging experiments using CH-PLIF (Lyons et al. 2005; Schefer 1997) or OH-PLIF (Boxx et al. 2009; Gordon et al. 2012; Hult et al. 2005). Possible mechanism causing these discontinuities might be local extinction or out-of-plane motion. Because the volumetric information of the flame was not yet available, Boxx et al. (2009), for example, addressed the effect of out-of-plane motion by simultaneous time-resolved stereoscopic particle image velocimetry (PIV) and OH-PLIF measurements and concluded that appearing flame islands were coupled to out-of-plane motion. In a continuing work (Gordon et al. 2012), the flame contour velocity at the flame base relative to the fluid flow was found to be


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up to three times the laminar flame speed even though events with significant out-of-plane motion were already excluded from this analysis. It was therefore speculated that the flame needs to be steeply angled to the imaging plane to account for the measured high flame contour velocities. To overcome these ambiguities in interpreting planar images, three-dimensional instantaneous measurements of the flame base are of great interest. Imaging of chemiluminescence is widely used as a tool for flame visualization. The spatial resolution of such measurements is limited if only a single view from one camera is used due to the line-of-sight characteristic of such measurement. By the utilization of multiple independent views the three-dimensional flame can be reconstructed. The tomographic reconstruction of chemiluminescence has been applied, for example, to opposed jet flames (Floyd et al. 2011) and jet flames (Worth und Dawson 2013; Ishino und Ohiwa 2005) to receive the flame structure within an entire volume. Worth and Dawson (2013) have reported upon two periodic forced interacting turbulent flames using a single camera which was rotated around the flames providing useful insights into the large scale 3D structure from phase-averaged flame visualizations. Floyd et al. (2011) captured instantaneous images by the use of multiple cameras and an ART (Algebraic Reconstruction Technique) algorithm for reconstruction in opposed jet flames. A good reconstruction requires many observation angles. Additionally intensity signals from chemiluminescence are typically low. Therefore large apertures are required that are in conflict with the requirement of a large depth-of-field. To study turbulent flames, ideally both the turbulent flow and the flame should be captured simultaneously in a volumetric manner. In a previous paper of the authors (Weinkauff et al. 2013), the feasibility of volumetric flow measurements was demonstrated using tomographic PIV in the same lifted flame facility as presented here. In future tomographic PIV might be supplemented by volumetric chemiluminescence imaging. The focus of this work is on instantaneous volumetric measurements of the flame position. Therefore a tomographic reconstruction of chemiluminescence is utilized using a SMART (Simultaneous Multiplicative Algebraic Reconstruction Technique) algorithm for reconstruction. The measurements are discussed in terms of spatial and temporal resolution for the used experimental setup and data quality is assessed in terms of signal-to-noise ratios and reconstruction artifacts. Additionally seeding particles were introduced to characterize their effect on the reconstruction to provide information on the feasibility of simultaneous PIV and tomographic chemiluminescence measurements. Oil droplets were used as seeding particles which evaporate in the vicinity of the flame and therefore can be used to mark burned gas regions. By the use of an additional laser sheet, the flame base height was measured from Mie-scattering off the oil droplets to assess the flame base height determined by tomographic chemiluminescence on a statistical base. 2. Experimental setup 2.1 Turbulent jet burner The experiments were performed in a non-premixed lifted turbulent methane jet flame. The burner consists of a 350 mm long vertical tube with an inner diameter of 8 mm, sufficiently long to provide fully developed pipe flow at the exit. The nozzle exit was tapered to a sharp edge to minimize recirculation. Pure methane was provided by the jet surrounded by a coflow with an inner diameter of 130 mm providing air with an exit velocity of 0.2 m/s. The central jet Reynolds number based on the inner tube diameter was 5000. The flame stabilizes at an average lift-off height of 28 mm and is the same as presented in (Weinkauff et al. 2013). To test the applicability of tomographic chemiluminescence imaging together with simultaneous PIV both flows, the jet and the coflow, were seeded by individual seeders (AFG 10.0 Palas) providing oil droplets with a mean diameter of ~1 µm.


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Fig. 1 Photograph of the experimental setup. 2.2 Tomographic chemiluminescence For the chemiluminescence measurements eight state-of-the-art 16-bit sCMOS cameras (LaVision Imager cSMOS) with 2560x2160 pixels were setup in two horizontal planes in a half-circular shape around the centerline of the jet in a distance of 280-330 mm such that each camera projection provided independent line-of-sight information of the observed volume (see Figure 1). Each camera was mounted on a geared three-way camera head (Manfrotto 410) to allow for a better adjustment of the field-of-view. The optical setup comprised six Sigma 105 mm f/2.8 macro and two Zeiss 100 mm f/2 (with 12 mm macro extension ring) lenses. This results in pixel resolutions of ~20 µm/pixel. Each camera was equipped with a band pass filter with transmission at 400-700 nm (IF 468 Schneider Kreuznach) to collect primarily chemiluminescence of the electronically excited radical CH (termed CH*). The exposure time was set to 1 and 8 ms with f# of 4 and 8, respectively. The parameter variations are summarized in table 1.

Table 1 Parameter settings and averaged intensities extracted from the regions identified by the boxes in Figure 2 (top left).

Recording Exposure

time [µs] f# Seeding Images Cam 3

Background (1)[counts]

Cam 3 Center (2) [counts]

Cam 3 Foot (3) [counts]

Cam 3 Peak [counts]

R1 8000 8 No 500 3.4 110 224 250 R2 8000 8 Oil 400 4.3 162 324 1100 R3 1000 4 No 220 2.9 26 57 70 R4 1000 4 Oil 120 3.3 56 107 320

2.3 Planar Flame imaging by Mie-scattering For the Mie-scattering measurements a Nd:YAG laser was used to provide a light-sheet aligned with the jet’s centerline. Oil droplets, which evaporate in the preheat zone of the flame, were seeded into the jet and the surrounding co-flow. For an unstrained flame front the resulting gap is approximately 0.5 mm. The scattered light was captured by a CMOS camera (Phantom v711). The camera was calibrated to physical space by the same two-level spatial target (LaVision Type 058-5), as used for the tomographic reconstruction of chemiluminescence, using the pinhole model. Note that the Mie-scattering measurements were not taken simultaneously with chemiluminescence and were therefore only compared on a statistical base.


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2.4 Data processing for tomographic chemiluminescence reconstruction Figure 2 shows averaged chemiluminescence intensity distributions for camera 3 for the four different recordings specified in table 1. The images represent collected light from CH* along the line-of-sight. At its base the mean flame structure appears as a tube with no heat release and chemiluminescence in its interior. Its diameter increases from bottom to top. The mean flame base stabilizes around 28 mm above the jet nozzle exit. Low-intensity luminosity was observed due to diffuse reflections of the flame even though the cameras faced a black wall in a distance of ~1 m. A background correction was therefore performed to reject any remaining background luminosity. Due to temporal variations of flame luminosity the flame-induced background was estimated for each individual image. Therefore the flame was masked out first. Any light in the remaining outer regions to the left and right was assumed to result from the background. A second order polynomial was then fitted to every row of the image. This was subtracted from the individual flame image. The effectiveness of this background correction is highlighted from the marked region 1 in Figure 2 (top left) and is given for the 4 recordings shown in this work in table 1. The remaining background is in the order of 3-4 counts.

Fig. 2 Average chemiluminescence signal (camera 3) for all cases (R1-R4) summarized in table 1. The red boxes (top left) marks the regions from which intensities were extracted in table 1. The insert (top right)

demonstrates the change of flame luminosity due to the presence of seeding particles from an instantaneous snapshot. Color scales give intensities in arbitrary units; note the different color scaling.

For processing, the commercial software Davis 8 (LaVision) was used. The individual cameras were calibrated by a two-level spatial target (LaVision Type 058-5) using the pinhole model. The cameras were aligned circularly around the target, such that the inner cameras (camera 4 and 5 had a nearly perpendicular view on the planar calibration target, while the outer cameras (camera 1 and 8) had a viewing angle of about +/- 60°. Because of this large viewing angle and the low f# used, the calibration target was not completely in focus for the outer cameras. As a consequence, target marks on the calibration image became more


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defocused (i.e. blurred) for marks furthest from the vertical centerline. Therefore, the remaining calibration errors were largest for the outmost positioned cameras (camera 1 and 8). The maximum remaining calibration error was 0.4-2.1 pixel. A Gaussian filter was applied for noise reduction with a 5x5 kernel for 8 ms exposure time and a 7x7 kernel for 1 ms. The larger filter kernel was applied to the latter, because the signal-to-noise ratio was lower. For volume reconstruction each voxel was set to 1.0 for initialization followed by 10 SMART (Simultaneous Multiplicative Algebraic Reconstruction Technique) iterations (Atkinson und Soria 2009). Between the individual iterations, a 3x3x3 voxel average filter was applied for smoothing. The resulting volume of interest was 34x34x35 mm³ resolved by 424x437x424 voxel (8 ms exposure time: 80 µm per voxel) and 335x344x335 voxel (1 ms exposure time: 100 µm per voxel). Because of the higher noise level, a larger voxel size was used for the latter. 2.5 Impact of oil droplets To assess the applicability of simultaneous PIV and tomographic chemiluminescence test measurements were performed with oil droplets as seeding material (cases R2 and R4). Oil droplets were used for this test because reflections off the particles were considered to cause problems for the tomographic reconstruction and the evaporating droplets could be used additionally to validate the flame base height as measured by chemiluminescence imaging. The right images in Figure 2 show the averaged chemiluminescence intensity distribution for the flame with the presence of oil droplets. Diffuse reflections of chemiluminescence off the particles were negligible. With the applied background correction the remaining background determined from region 1 was less than one count above the cases without particles (R1 and R3). In contrast the intensities within the flame region increased significantly and were observed to increase additionally from bottom to top showing a significant change of the flame due to the presence of oil droplets. Oil droplets are commonly used to measure the flow field in the unburned gas regions and the evaporation of the droplets in the flame can be used to capture the position of the flame front. The addition of oil, which is fuel as well, is generally argued to be negligible since the fraction of oil is low compared to the original fuel (i.e. methane). This argument might hold for global quantities but is problematic if one is interested in the local flame structure. Here, the presence of oil changes the signal of interest significantly. The increase of intensities is generally an advantage but unfortunately the combustion of the oil droplets broadens the visible flame sheet beyond the flame thickness of the original methane flame. This is highlighted by an instantaneous snapshot of chemiluminescence with the presence of oil droplets shown in the insert in the top right image in Figure 2. Brigth streaks can be seen clearly in flow direction caused by the oil droplets. These are not seen when seeding is not present. The oil droplets evaporate in the vicinity of the flame and due to short residence times oil vapor streaks are generated which burn much brighter and require more time for a complete burn-out than the methane. Solid particles as MgO or TiO2 can be an alternative but they might glow when they are heated up in the flame. This depends on the specific flame temperature and seeding material. Therefore oil droplets as well as solid particles can be used if the detection of the flame base tip is of interest only. Further downstream the application of oil droplets at least will be problematic. 3. Assessment of the 3D flame reconstruction For a detailed investigation of the flame base of a turbulent lifted flame, as envisioned in this work, instantaneous 3D snapshots of the flame are needed. Following issues need to be addressed first to see if this technique provides the required information: (1) Is the spatial and temporal resolution sufficient to resolve the flame structure at the flame base? (2) Does the reconstruction represent the 3D flame reliably? To address these questions an estimation of spatial resolution was performed and the temporal resolution was compared to the time scales of the process. Measurement quality will be discussed in terms of signal-to-noise-ratio. 3.1 Spatial and temporal resolution The spatial resolution strongly depends on the specific setup because it is a function of the number of views and the resolution of these views. In planar measurements well known targets (as for example the 1951 USAF resolution test chart) are placed into the measurement volume and imaged by the specific setup. This allows determining the smallest resolvable structures and with this spatial resolution. To follow this method


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a 3D target would be required with well-known structure sizes which needs to emit light and to be translucence. Because such target is not available yet, a theoretical approach is followed as used by Floyd et al. (2011) for an estimation of spatial resolution and the reconstructed thickness of the CH* layer is compared to a typical thickness of such layers known from literature. Each camera can be assumed to provide one view or in other words one projection angle of the considered volume. Eight projection angles are provided in this setup with each camera imaging a field-of-view of 40x50 mm with a pixel resolution of ~20 µm. The number of projection angles is much lower than the number of pixels, even though the real resolution is reduced due to the optical setup (typically 50-100 µm for the cameras with the lenses used in this setup), and therefore the number of projection angles is the limiting factor (Floyd et al. 2011). Thus the further discussion focuses on the influence of the number of projection angles on spatial resolution. The ART/SMART algorithm reconstructs an image from a series of angular projections. This corresponds to a system of linear equations which are solved iteratively with 𝑁! unknown object values from 𝑁! projections. For a unique solution of the equation system 𝑁! has to equal 𝑁!. With this a wave number of 𝑁!/(2 2) can be resolved on a grid of 𝑁!x 𝑁! (for the reconstruction of one layer) by assuming the Nyquist theorem along the grid diagonal (Floyd et al. 2011). The resulting spatial resolution for this setup would then be ~10 mm. If the fields are smooth very good reconstructions can be achieved even for significant under-determined equation systems as shown by Frieder und Herman (1971) for an ART algorithm and supported by phantom studies (Floyd et al. 2011). The resolvable wavelength can then be estimated by the equation 𝜆!"# = 𝐷/𝑁! with the domain size 𝐷 which is here the diagonal of the reconstructed volume (Frieder und Herman 1971). An overview of previous experiments accessible from literature was provided by Floyd et al. (2011) and shows that many experiments achieved even better resolutions than this. From this a theoretical conservative estimation of the spatial resolution would be < 5 mm. Another approach to estimate the spatial resolution is to determine a filter size required to smear the thin CH* layer, which is typically only a few 100 µm wide, to the reconstructed CH* layer width which is on average ~2 mm. Because no a-priori assumptions are used in the reconstruction (assumption of steep gradients for example) the transfer function is assumed to be Gaussian. One standard deviation of this filter is ~2 mm. Thus the smallest resolvable structures can be assumed to be in the order of 2 mm. The temporal resolution is given directly by the exposure time of the cameras which is 1 and 8 ms, respectively. For an instantaneous snapshot the temporal resolution needs to be higher than the time scales of the underlying processes. These can be estimated by the movement of the flame due to convection and chemical reaction. For a laminar stationary flame the convection velocity opposes flame speed with same amplitudes resulting in no flame movement in physical space. For the turbulent case local convection velocity typically differs from flame speed resulting in local flame movement. The convection velocity for this flame can be extracted from previous flow field measurements by the authors (Weinkauff et al. 2013) obtained from tomographic PIV. The mean velocity at the average flame base location is ~0.5 m/s (with turbulence levels of ~20 %) and the laminar flame speed for stoichiometric methane/air mixtures under atmospheric conditions is 0.35 m/s (Tahtouh et al. 2009). Thus the flame movement during the measurement is less than 0.2 mm for 1 ms exposure time and less than 1.6 mm for 8 ms exposure time, respectively. 3.2 Signal quality First the average intensity levels of the 2D chemiluminescence images are presented from camera 3. The intensity distribution reveals 3 distinct regions for which averaged intensities are calculated from the regions marked by the boxes in Figure 2 (top left). The dark region gives the background (region 1). The high intensity vertical stripes are due to integrated light along the line-of-sight as a large portion of the flame is oriented with the line-of-sight (region 3); the region in between results mainly from the two flame-sheets in the line-of-sight (region 2). To resolve flame structures, the intensity of a single flame-sheet needs to be sufficiently above the background; therefore the signal from region 2 is divided by 2. The ratio of the resulting intensity levels and the background provides an estimation of the signal-to-noise ratio (SNR) of the measurement. For 1 ms and 8 ms exposure (both without seeding) SNR was 4.5 and 16, respectively.


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Fig. 3 Cross section of averaged reconstructed chemiluminescence signal (center plane z = 0 mm) for

exposure time 8 ms (left) and 1 ms (right). Color scale gives intensities in arbitrary units.

Figure 3 shows a cross-section of the averaged reconstructed images intersecting the centerline. The bright vertical stripes now represent slices of the flame and are used to estimate signal strength. Even though turbulence leads to local deformations of the “flame tube” and the flame base, turbulence levels are sufficiently below the regime where flame holes due to extinction are commonly found as shown, for example, by Hult et al. (2005) from planar simultaneous laser induced fluorescence of OH and PIV. Thus the flame can be assumed to burn as a connected sheet with no flame islands and no flame pockets present inside this tube. With this a-priori knowledge on the flame shape, the high intensity levels inside the tube can be attributed to reconstruction artifacts. For useful measurements the signal needs to be significantly above the background caused by these artifacts. In PIV processing the peak-height-ratio of the maximum correlation peak to the second peak is an established signal quality criterion. This criterion is applied to the images shown in Figure 3. The signal-to-background (or artifacts) ratio was estimated to be ~6 for 8 ms (R1) and ~4 for 1 ms exposure (R3). Even though signal intensities were ~ 4 higher for R1 compared to R3 no significant increase of signal-to-background ratio was observed. This possibly might be a result of small regions of remaining background which are seen by individual cameras only. Figure 4 shows background corrected averaged chemiluminescence images from camera 3 (left) which collects light from the flame only and for camera 7 (right) which views under a slight angle downwards and sees additional reflected light (highlighted by the red box) possibly from parts of the traversing system. This camera now collects locally more light than the remaining cameras which leads to artefacts in the reconstructed flame. With increasing exposure time the artifacts increase as well resulting in this poor signal-to-background ratio. Therefore great care needs to be taken to exclude any background which implies that the application of this technique in enclosed flames is questionable.

Fig. 4 Average chemiluminescence signal for camera 3 (left) and 7 (right) for 8 ms exposure time. The red

box highlights regions of remaining background reflections. Color scale gives intensities in arbitrary units. 4. Results


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Figure 5 shows a snapshot of line-of-sight integrated chemiluminescence captured by a single camera. The outer contour on both sides as well as the flame base at the bottom reveals a remarkable good contrast showing distinct distortions of the flame base with characteristic bulges and finger-like topologies which are in correspondence with findings of Boxx et al. (2009). The structure size can already be estimated from the 2D images to be in the order of a few millimeters. The flame stabilizes in the outer shear layer between the methane jet (bulk velocity of ~11 m/s) and the low velocity coflowing air (0.2 m/s) because flammability limits of methane/air mixtures are within ~5-17 % i.e. much more air than methane is needed. The flame stabilizes therefore in regions of low turbulence. In combination with a relatively modest Reynolds number, no small scale flame wrinkling is expected, which enables measurements that are spatially and temporally sufficiently resolved with respect to the flow scales at the flame base, although both the spatial and temporal resolution are close to the limits of required resolution (see section 3.1). The corresponding reconstructed 3D flame is shown in Figure 5 (right) represented by iso-surfaces of chemiluminescence intensities demonstrating the overall flame shape. The flame appears as a closed hollow-cone shaped volume. No local extinction was observed within the flame, which would appear as holes in the iso-surface. The flame base distortions as already discussed from the 2D image are captured remarkably well even though a smoothening of the sharp tongue edges is observed. The observed structures are typical for these flames and are possibly a result from large scale vortices (Lyons 2007).

Fig. 5 Instantaneous snapshot of the line-of-sight intensity distribution of chemiluminescence obtained from a single camera (left) and the corresponding reconstructed flame, represented by different iso-contour levels

(right). The intensity levels are given in arbitrary units. For a further statistical analysis the 3D flame base was extracted from 120 reconstructed 3D flame images. To determine the flame base the lowest position of the flame was extracted along the circumference in steps of 0.2 degrees. The unwrapped flame base is shown in Figure 6 as a function of the peripheral angle. This processing neglects back tapers of the flame which would result in discontinuities of the unwrapped flame base which were not observed for the flame investigated. Figure 6 (left) shows the unwrapped flame base for the reconstructed flame given in Figure 5 and Figure 6 (right) for all 120 individual snapshots. The latter provides the averaged flame base height at 28 mm. The averaged flame base height was also determined from the planar measurements of Mie-scattering off the oil droplets and was at 28.2 mm. This very good agreement supports that the 3D flame was reliably reconstructed by the tomographic chemiluminescence measurements. The turbulent flame brush thickness was 5.2 mm which is given as twice the root mean square of the axial position fluctuations relative to its mean position. The flame brush thickness is often used to globally characterize the impact of turbulence on premixed flames. According to mixture fraction measurements conditioned on the flame base of lifted hydrogen flames (Tacke et al. 1998), the flame base burns in a premixed mode close to stoichiometry. Instantaneous mixture fraction values upstream of the flame base might differ in stoichiometry due to turbulence. At this low stabilization region at a height of x/D~3 where the flame stabilizes, the large scale


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vortices formed in the shear layer are not fully developed and with this streaks of pure air into the jet region as typically found in the jet far field of non-reacting flows (Dahm and Dimotakis 1987) are not expected to be found. Therefore it is speculated that a premixed mixture is found in front of the flame base. This is supported by the apparently blue flame, typical for premixed combustion, which turns into a bright orange flame for x/D>10. For premixed flames the ratio of the turbulent flame speed sT and the laminar flame speed sL can be written as sT / sL = AT / A (Poinsot and Veynante 2012) with A being a cross section of a control volume and the flame surface of the turbulent flame AT. Here, the average axial position of the flame base provides a mean flame position. A laminar flame imagined to burn on this mean flame position provides the shortest length of the jet flames circumference and was used to estimate A (even though not a flame area but a flame length is used here) while the increased length of the flame base caused by the underlying turbulence is used to estimate AT. Note that in a first step of this analysis the flame wrinkling in axial direction but not yet in radial direction was considered. This concept forms the base of the flame surface density model (Peters 2000) which is widely used in flame simulations to weight the laminar flame speed by a factor to approximate the turbulent flame speed if the flame surface of the wrinkled flame is not fully resolved.

Fig. 6 Un-wrapped flame base of the individual snapshot shown in figure 5 (left) and for 120 snapshots

representing the flame brush at the lifted flame base (right). The average flame base height is given by the horizontal grey line and flame brush by the red dashed lines.

The distribution of the ratio AT / A was calculated for all 120 snapshots and is shown in Figure 7. The average value of AT / A is 1.4. This ratio in connection with a laminar flame speed of atmospheric stoichiometric methane/air flames of 0.35 m/s (Tahtouh et al. 2009) yields a turbulent flame speed of sT = 0.5 m/s. Flame stabilization of premixed flames is determined by the interplay between local flow field and flame displacement due to thermo-diffusive transport. Therefore the mean flow velocity at the averaged flame position was determined from PIV measurements (Weinkauff et al. 2013) and was found to be in the order of 0.5 m/s as well. This supports speculations by Gordon et al. (2012) that local flame speeds were observed to be larger than laminar flame speeds to stem from steep flame angles between the flame and the observation plane (typically the laser-sheet). Flame displacement due to chemical reaction is perpendicular to the flame surface. For a steeply angled flame, flame displacements appear to be significantly larger within the laser-sheet. Additional out-of-plane flow velocities can further increase these apparent flame motions. This estimation might be biased by several processes as for example the local mixture fraction field which might differ from stoichiometry leading to variations of flame speed or variations of the instantaneous local flow field. To determine these uncertainties simultaneous measurements of the flow field, mixture fraction and flame position would be required. However, this finding provides some confidence that the resolution of these measurements is sufficient for this specific configuration to address the mechanism of flame stabilization of lifted jet flames at moderate Reynolds numbers.


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Fig. 7 Histogram of the ratio AT / A for all 120 snapshots shown in Figure 6.

5. Conclusions Tomographic reconstruction of chemiluminescence images of the base of lifted jet flames yields information on the (spatially filtered) instantaneous flame structure in 3D space. Therefore it is a very useful complementary measurement technique to laser induced fluorescence measurements where high spatial resolutions can be achieved only within a single plane. The spatial resolution of tomographic reconstructions strongly depends on the number of views and the observed domain. Due to the low signal intensities of chemiluminescence long exposure times in the order of milliseconds were required leading to an additional coupling of spatial and temporal resolution since flame movement during the exposure time cannot be neglected anymore. In this work eight views were provided by eight individual cameras with a field-of-view of ~50 mm. Due to the smoothness of the intensity distribution field the resulting spatial resolution was better than theoretical predictions and was estimated to be in the order of ~2 mm. The applicability of this technique simultaneously with PIV seeding particles was tested. For these moderate Reynolds number flows this technique provides potential in combination with simultaneous tomographic PIV measurements to investigate stabilization effects in turbulent lifted jet flames avoiding the shortcomings of 2D measurements. A number of handles and open issues were identified to further improve the tomographic chemiluminescence signal quality. As most of these quantities are coupled an improvement of any of these quantities might already lead to an overall improvement. One major identified issue was noise due to background reflections. While diffuse background reflections due to flame luminosity were possible to be reduced quite efficiently in a post-processing step reflections which are observed by single views only lead to significant artifacts as one camera records more light along its line-of-sight which is not recorded by the other cameras resulting in poorer SNR. Therefore a best practice guideline for any setup is to take great care of background reflections and to reduce them as far as possible. Additionally de-noising strategies can be applied to further increase SNR as for example the wavelet-de-noising strategy which was demonstrated to reduce camera noise very effectively (Weinkauff 2014). To increase spatial resolution the number of views needs to be maximized which is obviously limited by the availability of detectors and the space around the measurement volume. A possible strategy to face this problem was demonstrated by Anikin et al. (2012) who developed fiber-coupled telescopes imaged onto a single camera. Additionally the observation domain needs to be minimized because all cameras need to collect all light from the same observation volume. Furthermore the determination of the real spatial resolution of the specific setup requires a strategy to measure the spatial resolution from a known translucent target similar to the flame as for example a laminar flame with well-known CH* layer thickness. Acknowledgments Financial support by Deutsche Forschungsgemeinschaft (KA 3483) is acknowledged. The authors gratefully acknowledge the help of Elias Baum and Max Greifenstein who provided useful discussions and valuable post-processing guidelines. References Anikin NB, Suntz R, Bockhorn H (2012) Tomographic reconstruction of 2D-OH*-chemiluminescence

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volumetric flame measurements in a lifted turbulent jet...

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