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Experimental study of the effect of turbulence on the structure and dynamics of a bluff-body stabilized lean premixed flame
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Experimental study of the effect of turbulence on the structure and dynamics of a bluff-body stabilized lean premixed flame Bikram Roy Chowdhury, Jason A. Wagner, Baki M. Cetegen∗ Department of Mechanical Engineering, University of Connecticut, 191 Auditorium Rd, U-3139, Storrs, CT 06269, USA Received 3 December 2015; accepted 28 July 2016 Available online xxx
Abstract The structure of an unconfined, lean propane–air flame stabilized on an axisymmetric bluff body is studied for different levels of turbulence intensity ranging from 4 to 30% in the approach flow. Simultaneous planar laser induced fluorescence imaging of hydroxyl radical (OH) and formaldehyde (CH2 O) was performed to determine the effects of turbulence on local flame structure. In an accompanying experiment high speed particle image velocimetry at 5 kHz was used to obtain the time resolved velocity field and evaluate flame surface density, brush thickness and probability density functions of curvature and strain rates conditioned on the flame front. The low turbulence intensity flame featured wrinkling and mostly symmetric flame structures with thin regions of heat release along the flame front. For moderate turbulence intensity (14%), pronounced formation of cusps and unburnt mixture fingers was observed with continuous heat release regions. However, the local flame structure was observed to be strongly altered for the 30% turbulence intensity. Localized extinction occurred along the flame surface creating isolated pockets of OH with heat release occurring along the boundary. Pockets of preheated reactants along with fresh reactants were observed inside the flame envelope. The overall flame appearance was observed to intermittently switch from varicose (symmetric) to sinuous (asymmetric) type (vortex shedding mode). The curvature and strain rates pdfs broadened with increasing levels of turbulence intensity. The flame brush thickness showed an initial increase with increasing turbulence levels but saturated beyond 24% turbulence intensity. Measurements showed reduction in flame surface density with increasing levels of turbulence intensity. © 2016 by The Combustion Institute. Published by Elsevier Inc. Keywords: Simultaneous OH–CH2 O PLIF; Turbulence–flame interaction; Lean premixed propane flames; Bluff-body stabilized flames
1. Introduction ∗
Corresponding author. Fax: +1 860 486 5088. E-mail address: [email protected] (B.M. Cetegen).
Turbulent premixed combustion has been a subject of extensive research for the last few decades owing to its practical application in spark ignition engines, gas turbines and lean premixed and
http://dx.doi.org/10.1016/j.proci.2016.07.125 1540-7489 © 2016 by The Combustion Institute. Published by Elsevier Inc.
Please cite this article as: B. Roy Chowdhury et al., Experimental study of the effect of turbulence on the structure and dynamics of a bluff-body stabilized lean premixed flame, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.07.125
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prevaporized combustors. High level of turbulence intensities (∼50%) are encountered in many practical applications [1]. To investigate the effects of turbulence on premixed flames, experimental studies have been performed in low swirl flames, Bunsen burner flames, jet flames and V-shaped flames [2–11]. The effect of turbulence in modifying the flame front structure has been demonstrated in these works. Broadening of the preheat region due to penetration of small scale eddies into the preheat zone have been shown in [3,5]. Localized extinctions as well as fragmentation of the flame due to high stretch rates coupled with heat loss effects have been shown in [4,5,8]. Several studies have focused on the characteristics of V-shaped flames subject to different levels of turbulence intensity. Under moderate levels of turbulence intensity (∼10%), the flame front has been shown to feature two straight branches near the flame holder and wrinkling away from the flame holder [6–8]. For higher turbulence intensities (∼17%), strong levels of wrinkling along with formation of cusps, localized quenching and pockets of reactants have been reported using laser tomography technique by Kheirkhah and Gülder [8]. The flame brush thickness experimentally investigated by Kheirkhah and Gülder [8,9] and Namazian et al. [10] show that it is dependent on turbulence intensity, distance from the flame holder and mixture equivalence ratio. They had shown that the brush thickness is proportional to the root mean square of the flame front position [8]. Estimations of flame surface density for V-shaped flames have been performed in [9,11] which show a parabolic variation with mean progress variable c¯. The distributions have been shown to be skewed towards c¯ greater than 0.5, which has been attributed to the formation of cusps [11]. In these studies, integral length scales associated with the turbulent flow were much smaller than those present in practical combustors and the turbulence intensities were limited to 17%. Estimation of heat release by simultaneous PLIF imaging of OH and CH2 O was first demonstrated by Paul and Najm [12] in premixed laminar flames. Application of this diagnostic technique to measure the heat release by pixel-by-pixel product of OH and CH2 O PLIF data in reacting turbulent flows have gained significant interest in the recent years. The effect of flame front structure, curvature and strain on the heat release region have been studied by this technique in [4,5,13]. In this work, characteristics of a lean propane– air flame stabilized on a bluff body, subjected to low (4%), moderate (14%) and intense (24% and 30%) levels of turbulence is reported. Detailed characterization of the reaction zone structure has been done by simultaneous PLIF imaging of OH and CH2 O with high spatial resolution. Additionally time resolved particle image velocimetry at 5 kHz has been carried out to study the turbulent flame-flow interactions. Flame surface density, brush thickness
and probability density functions of curvature and strain rates have been calculated for each condition. 2. Experimental 2.1. Experimental setup and characterization of the turbulent flow field A conical burner with an exit diameter of 40 mm and a diameter contraction ratio of 3.2:1 was used in the present study. A disk shaped flame holder with a diameter of 10 mm was mounted on a 7 mm diameter sting, centered at the burner exit. Propane–air mixture was prepared in a mixing chamber containing a series of baffles and perforated plates and then fed into the burner. The mass flow rate of air was regulated using a bank of critical flow orifices. Instrument-grade propane was metered using a set of mass flow controllers (Porter Instruments, Model 202). Detailed description of the burner configuration can be found in our previously reported studies [14,15] where it was used to study the blowoff dynamics of premixed flames. The turbulence intensity level of the incoming premixed mixture of fuel and air was varied from 4% to 30% by employing a combination of perforated plates, slotted plates, mesh screens and impinging jets as shown in Fig. 1. The use of slotted plates and impinging jets to generate high levels of turbulence intensity has been demonstrated for slotted contraction devices by Skiba et al. [5]. A fine mesh screen of 1 mm opening was placed 25 mm upstream of the bluff body, for all the cases to produce uniform turbulence characteristics. Low turbulence intensity of ∼4% was generated by placing a perforated plate (P.P-1) at 70 mm upstream of the bluff body. The perforated plate (P.P-1) had a hole diameter of 4 mm and a pitch distance of 5.9 mm between the centers of neighboring holes. For moderate (∼14 %) and higher levels (∼24% and 30%) of turbulence intensity, a perforated plate (P.P-2) with a blockage ratio of 87% was placed about 130 mm upstream of the bluff body, inside the conical burner. Slotted plates of different widths and having blockage ratios of 54%, 67% and 78% were placed 70 mm upstream of the bluff body (replacing P.P-1) to produce turbulence intensities of 14%, 24% and 30%, respectively. In addition, for the higher turbulence conditions, four impinging jets, each having a 4 mm diameter, were symmetrically placed between the slotted plate and mesh screen. The impinging jets were supplied with the same premixed mixture drawn from the mixing chamber at about 10% of the main flow. The velocity measurements were taken with a hot film anemometer probe (TSI 1210-20) and IFA-100 signal processing system in the plane of the bluff body to characterize the flow field. The probe was operated at a sampling frequency of 80 kHz and low pass filtered at 40 kHz to prevent aliasing.
Please cite this article as: B. Roy Chowdhury et al., Experimental study of the effect of turbulence on the structure and dynamics of a bluff-body stabilized lean premixed flame, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.07.125
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Fig. 1. Schematics of the turbulence generating arrangement (left) and the PLIF laser setup (right). Table 1 Properties of the turbulent flow field in the experiments. u’/Um is the turbulence intensity, lo is the integral length scale and Ret is the Turbulence Reynolds number (u’ lo /ν) based on the integral length scales estimated by using hot wire anemometer. u’/Um
lo (mm) (H.W.A)
lo (mm) (P.I.V)
Ret
0.04 0.14 0.24 0.30
2.8 7.8 12.8 13.6
2.84 7.62 12.44 13.2
87 844 2176 2890
Radially uniform turbulence characteristics were obtained except for the slight skewedness near the wall of the burner, due to boundary layer growth. The integral turbulent length scales (lo ) for different conditions were determined by processing the hot film data using the approach of Coppola and Gomez [16] in which the turbulent power density spectra were fit into a correlation expression. The integral length scale measurements were verified by estimating the autocorrelation functions from the time resolved PIV results of non-reacting flow and integrating them up to the first zero crossing. These values were found to be within 4% of those estimated from the hot film data. The values are summarized in Table 1. The flame structure was studied for a mean flow velocity of 10 m/s and stably burning flame (far from blowoff) at an equivalence ratio of 0.85. 2.2. Time resolved PIV Measurement of the velocity field for each condition was performed at 5 kHz using a high speed PIV system. The system consisted of a Nd:YLF diode laser from Litron Lasers (Model LDY304PIV) providing about 10 mJ per pulse. Using a combination of lenses, the beam was focused into a sheet of approximately 0.3 mm thick. The Mie scattering images were captured by a high speed CMOS
camera (Vision Research Phantom Model v710) having a resolution of 1008 × 696 pixels. The field of view was 56 × 39 mm (2 mm above the plane of the bluff body), providing a resolution of 0.056 mm per pixel. The timing of the PIV system was controlled by Dynamic Studio v.3.41 software. The time between laser pulses was chosen as 22 μs. Micron sized silicone oil droplets generated by a set of nebulizers, were used for seeding the flow. The silicone oil (polydimethylsiloxane) had a viscosity of 50 centistokes and a density of 970 kg/m3 . LaVision DaVis 7.2 software was used to process the Mie scattering images to obtain vector fields. Multi-pass cross-correlation algorithm starting with a 64 × 64 interrogation window size and finally reducing to a final interrogation window size of 16 × 16 with 50% overlap was used. The Mie scattering images were filtered using a nonlinear anisotropic diffusion filter [17] and binarized using the level set approach [14]. Flame front was determined from the binarized images using the contour algorithm in MATLAB. 2000 images were processed to estimate the statistics in this paper. 2.3. Simultaneous PLIF of OH and CH2 O Figure 1 shows the schematics of the laser diagnostics setup used to perform simultaneous PLIF imaging of OH and CH2 O. The field of view for PLIF imaging was 40 × 19 mm. The OH PLIF system consisted of a Nd:YAG laser (Continuum Surelite I) pumping a dye laser (Sirah Precision Dye Laser) containing Rhodamine 590 dye, which generated a 566 nm beam. The laser beam was then passed through a doubling crystal to create a beam at a wavelength of 282.67 nm, centered on the Q1 (5) ro-vibrational transition in the A2 + ← X2 (1,0) band of OH. Fluorescence collection was performed using an image intensified PI-Max CCD camera, with 1024 × 1024 pixel resolution. The Semrock Bright Line filter (FF01-320/40-25) with a central wavelength of 320 nm and a width
Please cite this article as: B. Roy Chowdhury et al., Experimental study of the effect of turbulence on the structure and dynamics of a bluff-body stabilized lean premixed flame, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.07.125
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of 40 nm was fitted on the camera lens. A gate width of 250 ns was used and 500 images were obtained. The OH PLIF images had a spatial resolution 39 × 39 μm. The CH2 O PLIF was carried out using the Spectra Physics Pro-230 Nd:YAG laser operating at 355 nm with pulse energies of approximately 300 mJ. The 355 nm beam was used to excite the A-X 41 0 (15, 5) transition of CH2 O. The CH2 O fluorescence was collected using a PI-Max II intensified camera having a 1024 × 256 pixel resolution and fitted with an Omega Optical Inc. (450DF70D) filter that has greater than 60% transmission for wavelengths between 425 and 485 nm and near zero transmission outside of the 410–490 nm range. The intensifier was gated for 150 ns. The CH2 O PLIF images had a spatial resolution 75 × 75 μm. The laser pulses were separated by 300 ns to prevent cross talk between the PLIF measurements. The two beams were combined using a dichroic mirror which reflected 355 nm and transmitted the 282.67 nm beam. Using a combination of lenses, the beams were expanded into a thin sheet of 20 mm height. The sheet was formed 5 mm above the bluff body to prevent laser scatter. The sheet thicknesses of the OH and CH2 O beam were approximately 190 μm and 280 μm respectively. The beam profiles were evaluated by passing the OH and CH2 O laser sheets through a cuvette filled with ethanol and imaging the corresponding fluorescence. A calibration plate having a grid pattern was placed along the measurement plane and imaged simultaneously by both cameras. Identical reference points were selected on the plate. Similar to the process performed for PIV analysis, LaVision DaVis 7.2 software was used to perform calibration for each camera using the calibration plate image and the selected reference point. After calibration, OH PLIF data were interpolated onto a grid matching the resolution to that of CH2 O grid. Following background subtraction and correction for beam profile variation, a nonlinear diffusion filter [17] was applied to each grid corrected image to reduce noise and enhance the contours. The heat release was then estimated by pixel-by-pixel multiplication of the OH and CH2 O image. The overall resolution of the heat release imaging was 75 × 75 μm. 3. Results and discussion 3.1. Simultaneous OH and CH2 O PLIF results Typical instantaneous images obtained from the simultaneous PLIF of OH and CH2 O is shown in Fig. 2. The edges of the OH PLIF image separate the burnt and unburnt gas regions while CH2 O PLIF regions are observed near the flame front on the unburnt side. The thin region of overlap of OH and CH2 O delineate the heat release region. This
Fig. 2. Instantaneous PLIF images for flames corresponding to (a) TI: 04% and (b) TI: 14 %.
Fig. 3. Instantaneous PLIF images for flames corresponding to TI: 30%. (a) Localized extinctions. (b) Pocket formation and (c) asymmetric structure.
region is shown by thick blue lines superimposed on the corresponding OH image. For the experimental condition of 4% turbulence intensity, the PLIF images show a weakly wrinkled and fairly symmetric (with respect to the vertical axis) flame front, as shown in Fig. 2(a). The thin heat release region is continuous. As the turbulence intensity is increased to 14%, the flame front topology is modified as shown in Fig. 2(b). The wrinkling of the flame front increases with pronounced formation of cusps and unburned mixture fingers (UMF). The occurrence of UMFs has been reported for turbulent premixed flame studies in low swirl flames [2] as well as Bunsen flames [3]. From the PLIF images, these regions of UMF are observed to be filled with preheated reactants shown by the presence of CH2 O. However, no localized extinction is observed from the PLIF images and a continuous, thin region of heat release is observed along the edges of UMFs. The flame front topology is highly distorted with significant change in the structure, both spatially and temporally as the turbulence intensity level of the incoming reactant stream is increased to 30%, as shown in Fig. 3. At this condition, the flame front experiences higher strain rates which modifies the surface properties and leads to localized extinctions in the regions where it exceeds
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the extinction strain rates, κ ext [15,18]. This is shown Fig. 3(a) where isolated regions of OH are observed. As the flame holes develop, it provides a possibility for the transfer of mass and energy between the incoming cold reactants and products. Thus, the fresh stream of reactants can penetrate through the locally extinguished regions of the flame and mix with the hot combustion products resulting in formation of preheated reactant regions, marked by the presence of CH2 O along the edges of isolated OH filled islands as shown in Fig. 3(a). Localized pockets void of OH and CH2 O are also observed, which indicate that cold reactants do penetrate through these flame holes. As discussed in Shanbhogue et al. [18], increase in local burning rates adjacent to flame holes due to dilution results in re-ignition and stabilization of the flame. Fresh reactants entrained through the locally quenched regions of the flame lead to the formation of pockets in the product region as shown in (i) and (ii) of Fig. 3(b). The observation of CH2 O filled regions (partly in (ii)) indicates the presence of preheated reactants in these pockets that are surrounded by OH filled region. Thin regions of overlap of OH with these islands of CH2 O indicate the occurrence of reaction along the boundaries of these pockets. However, in the inner core of (ii), a region void of both OH and CH2 O is observed, which suggests that it may contain reactants. In the intense turbulent conditions, the general shape of the stably burning flame is observed to intermittently change from varicose to sinuous mode, as shown in Fig. 3(c). The physics behind the occurrence of this phenomenon for bluff body stabilized flames has been studied by Emerson et al. [19] and Mehta and Soteriou [20]. Entrainment of cold reactants into the flame at high turbulence conditions reduces the global density ratio between the flame region and its surroundings, and causes the bulk flame motions to switch from a varicose to sinuous mode. This effect is associated with the lessening of the dilatation and baroclinic effects on vorticity, which play an important role in the development of the von Karman instability. This effect clearly becomes more pronounced at high levels of turbulence. The average PLIF of OH, CH2 O and heat release for the low (4%) and intense (30%) turbulence conditions is shown in Fig. 4. For 4% condition, no localized extinctions along the flame were observed and this leads to narrow distribution of mean CH2 O profile and heat release. However, in the 30% condition, the average CH2 O and heat release distributions become much wider due to the high level of turbulent fluctuations. 3.2. Flame front statistics The statistical quantities showing the effect of different levels of turbulence intensities on the
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Fig. 4. Averaged OH PLIF, CH2 O PLIF and heat release images for (a) TI: 04 % (b) TI: 30 %.
Fig. 5. Probability distribution functions of curvature for the different conditions.
flame was obtained by processing the Mie scattering images from the PIV experiments. Curvature along flame front was estimated by first parameterizing the instantaneous Cartesian coordinates of the flame front by the length parameter(s), measured from a fixed reference point on the edge. Then, the local curvature was evaluated using the equation: C=
x˙ y¨ − y˙ x¨ (x˙ 2 + y˙2 )3/2
(1)
where x˙ and x¨ denote the first and second derivatives with respect to length parameter (s), respectively. By convention, the curvature is defined positive if the flame front is convex towards the reactants. The PIV laser sheet thickness was approximately 0.3 mm which limited the resolution to approximate ( ± )3.3 mm−1 . The pdf of the curvature for the different conditions is shown in Fig. 5. The mean value of the curvature for all the conditions was zero, which is in agreement with the previous experimental results associated with bluff body stabilized flames [14,15]. As expected, the curvature distribution broadens with increasing turbulence intensity. The level of wrinkling of the flame surfaces is governed by the vortical structures present in the flow. With increase in turbulence intensity, the integral length scales were found to increase. Thus, for a higher level of turbulence intensity, larger eddies interact with the flame causing greater extent of wrinkling, as observed also from the PLIF images.
Please cite this article as: B. Roy Chowdhury et al., Experimental study of the effect of turbulence on the structure and dynamics of a bluff-body stabilized lean premixed flame, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.07.125
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Fig. 6. Pdfs of strain rate for the different turbulent intensity conditions.
Fig. 7. Variation of flame brush thickness for the different conditions.
The cause for localized extinctions at high turbulent intensities can be examined by evaluating the strain rates along the flame surface. The twodimensional strain rate was computed using the equation [15]: ∂u ∂u ∂v + 1 − nx 2 κa = −nx ny + ∂y ∂x ∂x 2 ∂v + 1 − ny (2) ∂y where u and v are the velocity components in the x and y directions, respectively and nx and ny are x and y components of the flame surface normal vector, n. This is the aerodynamic component of the overall stretch. Additional effects of flame curvature on stretch are not included here. Figure 6 shows the pdf of the aerodynamic strain rate exerted on the left wing of the flame for the different test conditions. The pdfs are observed to become broader which show that flames stabilized in intensely turbulent conditions experience higher strain rates and are more likely to have localized extinction along the flame sheet. For the turbulence intensity conditions of 4% and 14%, the distributions are observed to be approximately Gaussian and are symmetric about κ a = 0. However, in the higher turbulence cases, the pdfs are skewed towards positive values which contribute to generation of larger flame surface area. Similar trend is observed for the right wing of the flame as well. Flame brush thickness is an important parameter used in the study of turbulent flames. For the tested conditions present in this work, brush thickness was estimated at different heights from the flame holder by using the equation [8]: δt = 1/max(|dc¯ /dx| ),
(3)
where c¯ is the mean progress variable which was evaluated by averaging the corresponding binarized Mie scattering images. Figure 7 shows the variation of the brush thickness for the tested conditions with respect to the distance from the bluff body. The brush thickness is observed to depend strongly on the turbulence conditions of the flow. At a certain height from
Fig. 8. Measurements of FSD evaluated for the different conditions.
the flame holder, the brush thickness for the intensely turbulent conditions is much higher relative to low and moderate turbulence intensities. This trend can be attributed to the pronounced wrinkling, spread and modification of the flame structures with increasing turbulence intensities. The increasing trend of brush thickness with u’/U (for 4%, 14% and 24%) is in agreement with the previously reported work [8]. However, the brush thickness is seen to saturate beyond 24% turbulence intensity with no further increase. Flame surface density (FSD) was estimated from the Mie scattering images using the method demonstrated in [11]. It is given by the relation: (c¯) = L(c¯)/A(c¯) n f ,
(4)
where L(c¯) and A(c¯) are the flame length and flame zone area as a function of the mean progress variable and nf is the number of images. The width of the mean progress variable intervals was selected to be 0.05, as in [9]. Following Kariuki et al. [21], to estimate L(c¯) the instantaneous flame edges were divided into equal segments of 1 pixel and superimposed on the mean c map. A c¯ value was assigned to each segment and then the frequency distribution was obtained. This was divided by the total number of images (nf ). From the frequency distribution of c¯ values from the mean c map and the area of each pixel, A(c¯) was computed. Figure 8 shows the distribution of FSD for the different conditions. The surface density is observed to decrease with increasing turbulence
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intensity. This variation is in agreement with the results presented in [9]. No significant difference is observed in the flame surface densities of the intensely turbulent conditions, as observed similarly for the flame brush thickness. The observed behavior of brush thickness and flame surface density for the intensely turbulent conditions can be attributed to the increased frequency of localized extinction and flamelet merging. 4. Conclusions The study of local flame structure by simultaneous PLIF of OH and CH2 O and high speed PIV has been performed to determine the change in the structure of a lean premixed propane flame stabilized on an axisymmetric bluff body. Formation of cusps and unburnt mixture fingers were observed as the turbulence intensity was increased from 4% to 14% but, the heat release region remained continuous. For turbulent conditions higher than 14%, localized extinction was observed along the flame surface creating isolated islands of products surrounded by preheated reactants. Pockets of reactants were observed inside the flame area with heat release occurring along its boundaries. The overall flame shape was observed to change from symmetric (varicose) to asymmetric (sinuous) mode with increasing turbulence intensity. Broadening of the curvature pdfs occurred with increase in turbulence levels showing greater extent of wrinkling. The strain rate pdfs showed that flame under highly turbulent conditions experienced larger strain rates which lead to localized extinction zones along the flame surface. The brush thickness increased with increasing turbulence intensity but saturated beyond 24% while the flame surface density decreased. References [1] P. Koutmas, J.J. Mcguirk, Exp. Fluids 7 (1989) 344–354.
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[2] R.K. Cheng, I.G. Shepherd, B. Bédat, L. Talbot, Combust. Sci. Technol. 174 (2002) 3–26. [3] Y.-C. Chen, N. Peters, G.A. Schneemann, N. Wruck, U. Renz, M.S. Mansour, Combust. Flame 107 (1996) 223–244. [4] Z.S. Li, B. Li, Z.W. Sun, X.S. Bai, M. Alden, Combust. Flame 157 (2010) 1087–1096. [5] A.W. Skiba, T.M. Wabel, J.E. Temme, J.F. Driscoll, Experimental assessment of premixed flames subjected to extreme turbulence, American Institute of Aeronautics and Astronautics, Inc., 2016. [6] P. Goix, P. Paranthoen, M. Trinite, Combust. Flame 81 (1990) 229–241. [7] F.C. Gouldin, S.M. Hilton, T. Lamb, Proc. Combust. Inst. 22 (1) (1989) 541–550. [8] S. Kheirkhah, Ö.L. Gülder, Flow Turbul. Combust. 93 (2014) 439–459. [9] S. Kheirkhah, Ö.L. Gülder, Combust. Flame 162 (2015) 1422–1439. [10] M. Namazian, I.G. Shepherd, L. Talbot, Combust. Flame 64 (1986) 299–308. [11] I.G. Shepherd, Proc. Combust. Inst. 26 (1) (1996) 373–379. [12] P.H. Paul, H.N. Najm, Proc. Combust. Inst. 27 (1998) 43–50. [13] J. Kariuki, A. Dowlut, R. Yuan, R. Balachandran, E. Mastorakos, Proc. Combust. Inst. 35 (2) (2015) 1443–1450. [14] S. Biswas, K. Kopp-Vaughn, M.W. Renfro, B.M. Cetegen, Combust. Flame 160 (12) (2013) 2843–2855. [15] S. Chaudhuri, S. Kostka, M.W. Renfro, B.M. Cetegen, Combust. Flame 157 (2010) 790–802. [16] G. Coppola, A. Gomez, Exp. Therm. Fluid Sci. 33 (7) (2009) 1037–1048. [17] H. Malm, G. Sparr, J. Hult, C. Kaminski, J. Opt. Soc. Am. 17 (12) (2000) 2148–2156. [18] S.J. Shanbhogue, S. Hussain, T. Lieuwen, Prog. Energy Combust. Sci. 35 (1) (2009) 98–120. [19] B. Emerson, J.O’ Connor, M. Juniper, T. Lieuwen, J. Fluid Mech. 706 (2012) 219–250. [20] P.G. Mehta, M.C. Soteriou, Combustion heat release effects on the dynamics of bluff body stabilized premixed reacting flows, AIAA, 2003, p. 0835. [21] J. Kariuki, J.R. Dawson, E. Mastorakos, Combust. Flame 159 (2012) 2589–2607.
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Experimental study of the effect of turbulence on the structure and dynamics of a bluff-body stabilized lean premixed flame Bikram Roy Chowdhury, Jason A. Wagner, Baki M. Cetegen∗ Department of Mechanical Engineering, University of Connecticut, 191 Auditorium Rd, U-3139, Storrs, CT 06269, USA Received 3 December 2015; accepted 28 July 2016 Available online xxx
Abstract The structure of an unconfined, lean propane–air flame stabilized on an axisymmetric bluff body is studied for different levels of turbulence intensity ranging from 4 to 30% in the approach flow. Simultaneous planar laser induced fluorescence imaging of hydroxyl radical (OH) and formaldehyde (CH2 O) was performed to determine the effects of turbulence on local flame structure. In an accompanying experiment high speed particle image velocimetry at 5 kHz was used to obtain the time resolved velocity field and evaluate flame surface density, brush thickness and probability density functions of curvature and strain rates conditioned on the flame front. The low turbulence intensity flame featured wrinkling and mostly symmetric flame structures with thin regions of heat release along the flame front. For moderate turbulence intensity (14%), pronounced formation of cusps and unburnt mixture fingers was observed with continuous heat release regions. However, the local flame structure was observed to be strongly altered for the 30% turbulence intensity. Localized extinction occurred along the flame surface creating isolated pockets of OH with heat release occurring along the boundary. Pockets of preheated reactants along with fresh reactants were observed inside the flame envelope. The overall flame appearance was observed to intermittently switch from varicose (symmetric) to sinuous (asymmetric) type (vortex shedding mode). The curvature and strain rates pdfs broadened with increasing levels of turbulence intensity. The flame brush thickness showed an initial increase with increasing turbulence levels but saturated beyond 24% turbulence intensity. Measurements showed reduction in flame surface density with increasing levels of turbulence intensity. © 2016 by The Combustion Institute. Published by Elsevier Inc. Keywords: Simultaneous OH–CH2 O PLIF; Turbulence–flame interaction; Lean premixed propane flames; Bluff-body stabilized flames
1. Introduction ∗
Corresponding author. Fax: +1 860 486 5088. E-mail address: [email protected] (B.M. Cetegen).
Turbulent premixed combustion has been a subject of extensive research for the last few decades owing to its practical application in spark ignition engines, gas turbines and lean premixed and
http://dx.doi.org/10.1016/j.proci.2016.07.125 1540-7489 © 2016 by The Combustion Institute. Published by Elsevier Inc.
Please cite this article as: B. Roy Chowdhury et al., Experimental study of the effect of turbulence on the structure and dynamics of a bluff-body stabilized lean premixed flame, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.07.125
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prevaporized combustors. High level of turbulence intensities (∼50%) are encountered in many practical applications [1]. To investigate the effects of turbulence on premixed flames, experimental studies have been performed in low swirl flames, Bunsen burner flames, jet flames and V-shaped flames [2–11]. The effect of turbulence in modifying the flame front structure has been demonstrated in these works. Broadening of the preheat region due to penetration of small scale eddies into the preheat zone have been shown in [3,5]. Localized extinctions as well as fragmentation of the flame due to high stretch rates coupled with heat loss effects have been shown in [4,5,8]. Several studies have focused on the characteristics of V-shaped flames subject to different levels of turbulence intensity. Under moderate levels of turbulence intensity (∼10%), the flame front has been shown to feature two straight branches near the flame holder and wrinkling away from the flame holder [6–8]. For higher turbulence intensities (∼17%), strong levels of wrinkling along with formation of cusps, localized quenching and pockets of reactants have been reported using laser tomography technique by Kheirkhah and Gülder [8]. The flame brush thickness experimentally investigated by Kheirkhah and Gülder [8,9] and Namazian et al. [10] show that it is dependent on turbulence intensity, distance from the flame holder and mixture equivalence ratio. They had shown that the brush thickness is proportional to the root mean square of the flame front position [8]. Estimations of flame surface density for V-shaped flames have been performed in [9,11] which show a parabolic variation with mean progress variable c¯. The distributions have been shown to be skewed towards c¯ greater than 0.5, which has been attributed to the formation of cusps [11]. In these studies, integral length scales associated with the turbulent flow were much smaller than those present in practical combustors and the turbulence intensities were limited to 17%. Estimation of heat release by simultaneous PLIF imaging of OH and CH2 O was first demonstrated by Paul and Najm [12] in premixed laminar flames. Application of this diagnostic technique to measure the heat release by pixel-by-pixel product of OH and CH2 O PLIF data in reacting turbulent flows have gained significant interest in the recent years. The effect of flame front structure, curvature and strain on the heat release region have been studied by this technique in [4,5,13]. In this work, characteristics of a lean propane– air flame stabilized on a bluff body, subjected to low (4%), moderate (14%) and intense (24% and 30%) levels of turbulence is reported. Detailed characterization of the reaction zone structure has been done by simultaneous PLIF imaging of OH and CH2 O with high spatial resolution. Additionally time resolved particle image velocimetry at 5 kHz has been carried out to study the turbulent flame-flow interactions. Flame surface density, brush thickness
and probability density functions of curvature and strain rates have been calculated for each condition. 2. Experimental 2.1. Experimental setup and characterization of the turbulent flow field A conical burner with an exit diameter of 40 mm and a diameter contraction ratio of 3.2:1 was used in the present study. A disk shaped flame holder with a diameter of 10 mm was mounted on a 7 mm diameter sting, centered at the burner exit. Propane–air mixture was prepared in a mixing chamber containing a series of baffles and perforated plates and then fed into the burner. The mass flow rate of air was regulated using a bank of critical flow orifices. Instrument-grade propane was metered using a set of mass flow controllers (Porter Instruments, Model 202). Detailed description of the burner configuration can be found in our previously reported studies [14,15] where it was used to study the blowoff dynamics of premixed flames. The turbulence intensity level of the incoming premixed mixture of fuel and air was varied from 4% to 30% by employing a combination of perforated plates, slotted plates, mesh screens and impinging jets as shown in Fig. 1. The use of slotted plates and impinging jets to generate high levels of turbulence intensity has been demonstrated for slotted contraction devices by Skiba et al. [5]. A fine mesh screen of 1 mm opening was placed 25 mm upstream of the bluff body, for all the cases to produce uniform turbulence characteristics. Low turbulence intensity of ∼4% was generated by placing a perforated plate (P.P-1) at 70 mm upstream of the bluff body. The perforated plate (P.P-1) had a hole diameter of 4 mm and a pitch distance of 5.9 mm between the centers of neighboring holes. For moderate (∼14 %) and higher levels (∼24% and 30%) of turbulence intensity, a perforated plate (P.P-2) with a blockage ratio of 87% was placed about 130 mm upstream of the bluff body, inside the conical burner. Slotted plates of different widths and having blockage ratios of 54%, 67% and 78% were placed 70 mm upstream of the bluff body (replacing P.P-1) to produce turbulence intensities of 14%, 24% and 30%, respectively. In addition, for the higher turbulence conditions, four impinging jets, each having a 4 mm diameter, were symmetrically placed between the slotted plate and mesh screen. The impinging jets were supplied with the same premixed mixture drawn from the mixing chamber at about 10% of the main flow. The velocity measurements were taken with a hot film anemometer probe (TSI 1210-20) and IFA-100 signal processing system in the plane of the bluff body to characterize the flow field. The probe was operated at a sampling frequency of 80 kHz and low pass filtered at 40 kHz to prevent aliasing.
Please cite this article as: B. Roy Chowdhury et al., Experimental study of the effect of turbulence on the structure and dynamics of a bluff-body stabilized lean premixed flame, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.07.125
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Fig. 1. Schematics of the turbulence generating arrangement (left) and the PLIF laser setup (right). Table 1 Properties of the turbulent flow field in the experiments. u’/Um is the turbulence intensity, lo is the integral length scale and Ret is the Turbulence Reynolds number (u’ lo /ν) based on the integral length scales estimated by using hot wire anemometer. u’/Um
lo (mm) (H.W.A)
lo (mm) (P.I.V)
Ret
0.04 0.14 0.24 0.30
2.8 7.8 12.8 13.6
2.84 7.62 12.44 13.2
87 844 2176 2890
Radially uniform turbulence characteristics were obtained except for the slight skewedness near the wall of the burner, due to boundary layer growth. The integral turbulent length scales (lo ) for different conditions were determined by processing the hot film data using the approach of Coppola and Gomez [16] in which the turbulent power density spectra were fit into a correlation expression. The integral length scale measurements were verified by estimating the autocorrelation functions from the time resolved PIV results of non-reacting flow and integrating them up to the first zero crossing. These values were found to be within 4% of those estimated from the hot film data. The values are summarized in Table 1. The flame structure was studied for a mean flow velocity of 10 m/s and stably burning flame (far from blowoff) at an equivalence ratio of 0.85. 2.2. Time resolved PIV Measurement of the velocity field for each condition was performed at 5 kHz using a high speed PIV system. The system consisted of a Nd:YLF diode laser from Litron Lasers (Model LDY304PIV) providing about 10 mJ per pulse. Using a combination of lenses, the beam was focused into a sheet of approximately 0.3 mm thick. The Mie scattering images were captured by a high speed CMOS
camera (Vision Research Phantom Model v710) having a resolution of 1008 × 696 pixels. The field of view was 56 × 39 mm (2 mm above the plane of the bluff body), providing a resolution of 0.056 mm per pixel. The timing of the PIV system was controlled by Dynamic Studio v.3.41 software. The time between laser pulses was chosen as 22 μs. Micron sized silicone oil droplets generated by a set of nebulizers, were used for seeding the flow. The silicone oil (polydimethylsiloxane) had a viscosity of 50 centistokes and a density of 970 kg/m3 . LaVision DaVis 7.2 software was used to process the Mie scattering images to obtain vector fields. Multi-pass cross-correlation algorithm starting with a 64 × 64 interrogation window size and finally reducing to a final interrogation window size of 16 × 16 with 50% overlap was used. The Mie scattering images were filtered using a nonlinear anisotropic diffusion filter [17] and binarized using the level set approach [14]. Flame front was determined from the binarized images using the contour algorithm in MATLAB. 2000 images were processed to estimate the statistics in this paper. 2.3. Simultaneous PLIF of OH and CH2 O Figure 1 shows the schematics of the laser diagnostics setup used to perform simultaneous PLIF imaging of OH and CH2 O. The field of view for PLIF imaging was 40 × 19 mm. The OH PLIF system consisted of a Nd:YAG laser (Continuum Surelite I) pumping a dye laser (Sirah Precision Dye Laser) containing Rhodamine 590 dye, which generated a 566 nm beam. The laser beam was then passed through a doubling crystal to create a beam at a wavelength of 282.67 nm, centered on the Q1 (5) ro-vibrational transition in the A2 + ← X2 (1,0) band of OH. Fluorescence collection was performed using an image intensified PI-Max CCD camera, with 1024 × 1024 pixel resolution. The Semrock Bright Line filter (FF01-320/40-25) with a central wavelength of 320 nm and a width
Please cite this article as: B. Roy Chowdhury et al., Experimental study of the effect of turbulence on the structure and dynamics of a bluff-body stabilized lean premixed flame, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.07.125
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of 40 nm was fitted on the camera lens. A gate width of 250 ns was used and 500 images were obtained. The OH PLIF images had a spatial resolution 39 × 39 μm. The CH2 O PLIF was carried out using the Spectra Physics Pro-230 Nd:YAG laser operating at 355 nm with pulse energies of approximately 300 mJ. The 355 nm beam was used to excite the A-X 41 0 (15, 5) transition of CH2 O. The CH2 O fluorescence was collected using a PI-Max II intensified camera having a 1024 × 256 pixel resolution and fitted with an Omega Optical Inc. (450DF70D) filter that has greater than 60% transmission for wavelengths between 425 and 485 nm and near zero transmission outside of the 410–490 nm range. The intensifier was gated for 150 ns. The CH2 O PLIF images had a spatial resolution 75 × 75 μm. The laser pulses were separated by 300 ns to prevent cross talk between the PLIF measurements. The two beams were combined using a dichroic mirror which reflected 355 nm and transmitted the 282.67 nm beam. Using a combination of lenses, the beams were expanded into a thin sheet of 20 mm height. The sheet was formed 5 mm above the bluff body to prevent laser scatter. The sheet thicknesses of the OH and CH2 O beam were approximately 190 μm and 280 μm respectively. The beam profiles were evaluated by passing the OH and CH2 O laser sheets through a cuvette filled with ethanol and imaging the corresponding fluorescence. A calibration plate having a grid pattern was placed along the measurement plane and imaged simultaneously by both cameras. Identical reference points were selected on the plate. Similar to the process performed for PIV analysis, LaVision DaVis 7.2 software was used to perform calibration for each camera using the calibration plate image and the selected reference point. After calibration, OH PLIF data were interpolated onto a grid matching the resolution to that of CH2 O grid. Following background subtraction and correction for beam profile variation, a nonlinear diffusion filter [17] was applied to each grid corrected image to reduce noise and enhance the contours. The heat release was then estimated by pixel-by-pixel multiplication of the OH and CH2 O image. The overall resolution of the heat release imaging was 75 × 75 μm. 3. Results and discussion 3.1. Simultaneous OH and CH2 O PLIF results Typical instantaneous images obtained from the simultaneous PLIF of OH and CH2 O is shown in Fig. 2. The edges of the OH PLIF image separate the burnt and unburnt gas regions while CH2 O PLIF regions are observed near the flame front on the unburnt side. The thin region of overlap of OH and CH2 O delineate the heat release region. This
Fig. 2. Instantaneous PLIF images for flames corresponding to (a) TI: 04% and (b) TI: 14 %.
Fig. 3. Instantaneous PLIF images for flames corresponding to TI: 30%. (a) Localized extinctions. (b) Pocket formation and (c) asymmetric structure.
region is shown by thick blue lines superimposed on the corresponding OH image. For the experimental condition of 4% turbulence intensity, the PLIF images show a weakly wrinkled and fairly symmetric (with respect to the vertical axis) flame front, as shown in Fig. 2(a). The thin heat release region is continuous. As the turbulence intensity is increased to 14%, the flame front topology is modified as shown in Fig. 2(b). The wrinkling of the flame front increases with pronounced formation of cusps and unburned mixture fingers (UMF). The occurrence of UMFs has been reported for turbulent premixed flame studies in low swirl flames [2] as well as Bunsen flames [3]. From the PLIF images, these regions of UMF are observed to be filled with preheated reactants shown by the presence of CH2 O. However, no localized extinction is observed from the PLIF images and a continuous, thin region of heat release is observed along the edges of UMFs. The flame front topology is highly distorted with significant change in the structure, both spatially and temporally as the turbulence intensity level of the incoming reactant stream is increased to 30%, as shown in Fig. 3. At this condition, the flame front experiences higher strain rates which modifies the surface properties and leads to localized extinctions in the regions where it exceeds
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the extinction strain rates, κ ext [15,18]. This is shown Fig. 3(a) where isolated regions of OH are observed. As the flame holes develop, it provides a possibility for the transfer of mass and energy between the incoming cold reactants and products. Thus, the fresh stream of reactants can penetrate through the locally extinguished regions of the flame and mix with the hot combustion products resulting in formation of preheated reactant regions, marked by the presence of CH2 O along the edges of isolated OH filled islands as shown in Fig. 3(a). Localized pockets void of OH and CH2 O are also observed, which indicate that cold reactants do penetrate through these flame holes. As discussed in Shanbhogue et al. [18], increase in local burning rates adjacent to flame holes due to dilution results in re-ignition and stabilization of the flame. Fresh reactants entrained through the locally quenched regions of the flame lead to the formation of pockets in the product region as shown in (i) and (ii) of Fig. 3(b). The observation of CH2 O filled regions (partly in (ii)) indicates the presence of preheated reactants in these pockets that are surrounded by OH filled region. Thin regions of overlap of OH with these islands of CH2 O indicate the occurrence of reaction along the boundaries of these pockets. However, in the inner core of (ii), a region void of both OH and CH2 O is observed, which suggests that it may contain reactants. In the intense turbulent conditions, the general shape of the stably burning flame is observed to intermittently change from varicose to sinuous mode, as shown in Fig. 3(c). The physics behind the occurrence of this phenomenon for bluff body stabilized flames has been studied by Emerson et al. [19] and Mehta and Soteriou [20]. Entrainment of cold reactants into the flame at high turbulence conditions reduces the global density ratio between the flame region and its surroundings, and causes the bulk flame motions to switch from a varicose to sinuous mode. This effect is associated with the lessening of the dilatation and baroclinic effects on vorticity, which play an important role in the development of the von Karman instability. This effect clearly becomes more pronounced at high levels of turbulence. The average PLIF of OH, CH2 O and heat release for the low (4%) and intense (30%) turbulence conditions is shown in Fig. 4. For 4% condition, no localized extinctions along the flame were observed and this leads to narrow distribution of mean CH2 O profile and heat release. However, in the 30% condition, the average CH2 O and heat release distributions become much wider due to the high level of turbulent fluctuations. 3.2. Flame front statistics The statistical quantities showing the effect of different levels of turbulence intensities on the
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Fig. 4. Averaged OH PLIF, CH2 O PLIF and heat release images for (a) TI: 04 % (b) TI: 30 %.
Fig. 5. Probability distribution functions of curvature for the different conditions.
flame was obtained by processing the Mie scattering images from the PIV experiments. Curvature along flame front was estimated by first parameterizing the instantaneous Cartesian coordinates of the flame front by the length parameter(s), measured from a fixed reference point on the edge. Then, the local curvature was evaluated using the equation: C=
x˙ y¨ − y˙ x¨ (x˙ 2 + y˙2 )3/2
(1)
where x˙ and x¨ denote the first and second derivatives with respect to length parameter (s), respectively. By convention, the curvature is defined positive if the flame front is convex towards the reactants. The PIV laser sheet thickness was approximately 0.3 mm which limited the resolution to approximate ( ± )3.3 mm−1 . The pdf of the curvature for the different conditions is shown in Fig. 5. The mean value of the curvature for all the conditions was zero, which is in agreement with the previous experimental results associated with bluff body stabilized flames [14,15]. As expected, the curvature distribution broadens with increasing turbulence intensity. The level of wrinkling of the flame surfaces is governed by the vortical structures present in the flow. With increase in turbulence intensity, the integral length scales were found to increase. Thus, for a higher level of turbulence intensity, larger eddies interact with the flame causing greater extent of wrinkling, as observed also from the PLIF images.
Please cite this article as: B. Roy Chowdhury et al., Experimental study of the effect of turbulence on the structure and dynamics of a bluff-body stabilized lean premixed flame, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.07.125
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Fig. 6. Pdfs of strain rate for the different turbulent intensity conditions.
Fig. 7. Variation of flame brush thickness for the different conditions.
The cause for localized extinctions at high turbulent intensities can be examined by evaluating the strain rates along the flame surface. The twodimensional strain rate was computed using the equation [15]: ∂u ∂u ∂v + 1 − nx 2 κa = −nx ny + ∂y ∂x ∂x 2 ∂v + 1 − ny (2) ∂y where u and v are the velocity components in the x and y directions, respectively and nx and ny are x and y components of the flame surface normal vector, n. This is the aerodynamic component of the overall stretch. Additional effects of flame curvature on stretch are not included here. Figure 6 shows the pdf of the aerodynamic strain rate exerted on the left wing of the flame for the different test conditions. The pdfs are observed to become broader which show that flames stabilized in intensely turbulent conditions experience higher strain rates and are more likely to have localized extinction along the flame sheet. For the turbulence intensity conditions of 4% and 14%, the distributions are observed to be approximately Gaussian and are symmetric about κ a = 0. However, in the higher turbulence cases, the pdfs are skewed towards positive values which contribute to generation of larger flame surface area. Similar trend is observed for the right wing of the flame as well. Flame brush thickness is an important parameter used in the study of turbulent flames. For the tested conditions present in this work, brush thickness was estimated at different heights from the flame holder by using the equation [8]: δt = 1/max(|dc¯ /dx| ),
(3)
where c¯ is the mean progress variable which was evaluated by averaging the corresponding binarized Mie scattering images. Figure 7 shows the variation of the brush thickness for the tested conditions with respect to the distance from the bluff body. The brush thickness is observed to depend strongly on the turbulence conditions of the flow. At a certain height from
Fig. 8. Measurements of FSD evaluated for the different conditions.
the flame holder, the brush thickness for the intensely turbulent conditions is much higher relative to low and moderate turbulence intensities. This trend can be attributed to the pronounced wrinkling, spread and modification of the flame structures with increasing turbulence intensities. The increasing trend of brush thickness with u’/U (for 4%, 14% and 24%) is in agreement with the previously reported work [8]. However, the brush thickness is seen to saturate beyond 24% turbulence intensity with no further increase. Flame surface density (FSD) was estimated from the Mie scattering images using the method demonstrated in [11]. It is given by the relation: (c¯) = L(c¯)/A(c¯) n f ,
(4)
where L(c¯) and A(c¯) are the flame length and flame zone area as a function of the mean progress variable and nf is the number of images. The width of the mean progress variable intervals was selected to be 0.05, as in [9]. Following Kariuki et al. [21], to estimate L(c¯) the instantaneous flame edges were divided into equal segments of 1 pixel and superimposed on the mean c map. A c¯ value was assigned to each segment and then the frequency distribution was obtained. This was divided by the total number of images (nf ). From the frequency distribution of c¯ values from the mean c map and the area of each pixel, A(c¯) was computed. Figure 8 shows the distribution of FSD for the different conditions. The surface density is observed to decrease with increasing turbulence
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intensity. This variation is in agreement with the results presented in [9]. No significant difference is observed in the flame surface densities of the intensely turbulent conditions, as observed similarly for the flame brush thickness. The observed behavior of brush thickness and flame surface density for the intensely turbulent conditions can be attributed to the increased frequency of localized extinction and flamelet merging. 4. Conclusions The study of local flame structure by simultaneous PLIF of OH and CH2 O and high speed PIV has been performed to determine the change in the structure of a lean premixed propane flame stabilized on an axisymmetric bluff body. Formation of cusps and unburnt mixture fingers were observed as the turbulence intensity was increased from 4% to 14% but, the heat release region remained continuous. For turbulent conditions higher than 14%, localized extinction was observed along the flame surface creating isolated islands of products surrounded by preheated reactants. Pockets of reactants were observed inside the flame area with heat release occurring along its boundaries. The overall flame shape was observed to change from symmetric (varicose) to asymmetric (sinuous) mode with increasing turbulence intensity. Broadening of the curvature pdfs occurred with increase in turbulence levels showing greater extent of wrinkling. The strain rate pdfs showed that flame under highly turbulent conditions experienced larger strain rates which lead to localized extinction zones along the flame surface. The brush thickness increased with increasing turbulence intensity but saturated beyond 24% while the flame surface density decreased. References [1] P. Koutmas, J.J. Mcguirk, Exp. Fluids 7 (1989) 344–354.
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[2] R.K. Cheng, I.G. Shepherd, B. Bédat, L. Talbot, Combust. Sci. Technol. 174 (2002) 3–26. [3] Y.-C. Chen, N. Peters, G.A. Schneemann, N. Wruck, U. Renz, M.S. Mansour, Combust. Flame 107 (1996) 223–244. [4] Z.S. Li, B. Li, Z.W. Sun, X.S. Bai, M. Alden, Combust. Flame 157 (2010) 1087–1096. [5] A.W. Skiba, T.M. Wabel, J.E. Temme, J.F. Driscoll, Experimental assessment of premixed flames subjected to extreme turbulence, American Institute of Aeronautics and Astronautics, Inc., 2016. [6] P. Goix, P. Paranthoen, M. Trinite, Combust. Flame 81 (1990) 229–241. [7] F.C. Gouldin, S.M. Hilton, T. Lamb, Proc. Combust. Inst. 22 (1) (1989) 541–550. [8] S. Kheirkhah, Ö.L. Gülder, Flow Turbul. Combust. 93 (2014) 439–459. [9] S. Kheirkhah, Ö.L. Gülder, Combust. Flame 162 (2015) 1422–1439. [10] M. Namazian, I.G. Shepherd, L. Talbot, Combust. Flame 64 (1986) 299–308. [11] I.G. Shepherd, Proc. Combust. Inst. 26 (1) (1996) 373–379. [12] P.H. Paul, H.N. Najm, Proc. Combust. Inst. 27 (1998) 43–50. [13] J. Kariuki, A. Dowlut, R. Yuan, R. Balachandran, E. Mastorakos, Proc. Combust. Inst. 35 (2) (2015) 1443–1450. [14] S. Biswas, K. Kopp-Vaughn, M.W. Renfro, B.M. Cetegen, Combust. Flame 160 (12) (2013) 2843–2855. [15] S. Chaudhuri, S. Kostka, M.W. Renfro, B.M. Cetegen, Combust. Flame 157 (2010) 790–802. [16] G. Coppola, A. Gomez, Exp. Therm. Fluid Sci. 33 (7) (2009) 1037–1048. [17] H. Malm, G. Sparr, J. Hult, C. Kaminski, J. Opt. Soc. Am. 17 (12) (2000) 2148–2156. [18] S.J. Shanbhogue, S. Hussain, T. Lieuwen, Prog. Energy Combust. Sci. 35 (1) (2009) 98–120. [19] B. Emerson, J.O’ Connor, M. Juniper, T. Lieuwen, J. Fluid Mech. 706 (2012) 219–250. [20] P.G. Mehta, M.C. Soteriou, Combustion heat release effects on the dynamics of bluff body stabilized premixed reacting flows, AIAA, 2003, p. 0835. [21] J. Kariuki, J.R. Dawson, E. Mastorakos, Combust. Flame 159 (2012) 2589–2607.
Please cite this article as: B. Roy Chowdhury et al., Experimental study of the effect of turbulence on the structure and dynamics of a bluff-body stabilized lean premixed flame, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.07.125