- Email: [email protected]
Dynamic lifted flame in centerbody burner
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Proceedings of the
Proceedings of the Combustion Institute 33 (2011) 1187–1194
Combustion Institute www.elsevier.com/locate/proci
Dynamic lifted flame in centerbody burner Viswanath R. Katta a,⇑, William M. Roquemore b, Scott Stouffer c, David Blunck d a
Innovative Scientific Solutions Inc., 2766 Indian Ripple Road, Dayton, OH 45440, United States b Air Force Research Laboratory, Wright-Patterson Air Force Base, OH 45433, United States c University of Dayton Research Institute, Dayton, OH 45469, United States d Purdue University, West Lafayette, IN 47907, United States Available online 24 September 2010
Abstract During some exploratory experiments performed on a centerbody burner it was observed that the sooting behavior of the burner could be altered dramatically without changing the fluid dynamics. One of the interesting operating regimes, in which the flame lifts off and forms a column of soot, was identified when oxygen in the annular flow was sufficiently reduced. More interestingly, within a narrow window of flow conditions, an unusual toroidal flame was observed near the base of the lifted flame. This paper describes the numerical and experimental studies performed for understanding this peculiar toroidal flame tube. A time-dependent, axisymmetric, detailed-chemistry CFD model (UNICORN) is used. Combustion and PAH formation are modeled using Wang–Frenklach mechanism and soot is simulated using a two-equation model of Lindstedt. Calculations have accurately predicted the steady lifted flame that is anchored to the outer edge of the recirculation zone. Lift-off height of the computed flame matched well with that of the experiment. A dynamic lifted flame is then established through periodically oscillating the annular flow. The edge of the lifted flame is found to dance along the outer periphery of the recirculation zone while vortical structures establish downstream of it. However, none of the calculations made with varying flow conditions or perturbations yielded a toroidal-flame structure near the base of the lifted flame. Surprisingly, when time-averaging was performed for the CH-radical distributions of the dancing flame a toroidal flame-like structure, very similar to that observed in the experiment, appeared near the flame base. Based on these calculations and high-speed movies of the experimental flame it is concluded that the observed toroidal flame is an optical illusion created through the natural time-averaging process of the human eye. Detailed structures of the computed oscillating flame are compared with the thermal images of the flame obtained using an infrared camera. Ó 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Diffusion flame; Lifted flame; Recirculation zone; Soot; Radiation
1. Introduction The formation, growth, transport, and burnout of soot are perhaps the most complex and least ⇑ Corresponding author. Fax: +1 937 656 4110.
E-mail address: [email protected] (V.R. Katta).
understood processes in flames and combustion systems. Soot precursor particles containing several-thousand carbon atoms are formed in flames from simple fuel molecules within a few microseconds [1]. However, these precursor particles on a much longer time scale interact with the gas-phase molecules during the surface-growth process,
1540-7489/$ - see front matter Ó 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2010.06.153
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colloid with each other in the agglomeration process and react with oxygenated species in the oxidation process. All of these chemical and physical two-phase processes occur simultaneously in flames. In gas turbine combustors, these processes are further complicated by the burning of practical fuels, consisting of thousands of species, and the actions of turbulent flows and recirculation zones. There is a significant science base for understanding the soot processes; however, it is inadequate to provide accurate soot models that can aid in the design of future low sooting gas turbine combustors. Thus, there is a pressing need to expand the science base in ways that foster the development and evaluation of accurate CFD models for designing low sooting combustion systems. The Strategic Environmental Research and Development Program (SERDP) office has started a comprehensive fundamental soot research program involving a suite of burners [2]. Complexity of these burners progressed in a way that the effects of chemical kinetics, diffusion, flame stretch, recirculation, and turbulence on the formation of soot can be investigated systematically. The computational part of this effort is to use a state-of-theart, Navier–Stokes based CFD code (UNICORN) [3] to aid in designing experiments, predicting and interpreting results, and evaluating soot and chemistry models. The centerbody burner [2], one of the few designs selected for studying soot, consists of an annular oxidizer jet and a central fuel jet separated by a bluff body. The formation of recirculation zones in this burner [4] provides a mechanism for decoupling various soot processes and investigating them individually. Initial experiments performed by varying the amount of dilution with nitrogen in the fuel and/or annular jets [2] suggested that very different flames with filled, donutshaped, or ring-type soot layers could be obtained. Recent experiments have further revealed that a lifted flame with a column of soot extending into the relatively cold regions between the flame base and the face of the bluff body can also be obtained. For some flow conditions a very unusual toroidal flame tube formed near the lifted flame edge. Lifted flames established in the shear layers of coflowing fuel and air jets were studied by several investigators [5,6], mainly for understanding the flame stabilization mechanisms [7,8]. While some studies revealed that the edge of a lifted flame is sharp with a monobrachial structure [9,10], other studies suggested a tribrachial structure to the flame edge [11,12]. However, to the authors’ knowledge, lifted flame edge wrapping into a toroidal flame tube has never been identified. As the recirculation zones in the centerbody burner provide additional complexity to the stability of the lifted flame, the observed toroidal flame tube could be specific to the lifted flames anchored to recirculation zones. This paper describes the stud-
ies performed for understanding these flame-tubelike structures in a centerbody burner. High-speed visualizations of the lifted flames with and without the toroidal flame tube are obtained using standard and infrared cameras. Calculations for these lifted flames are performed using UNICORN code with detailed chemical kinetics for ethylene combustion. Dynamic flames are simulated through perturbing the annular flow sinusoidally at a frequency determined from the experiments. 2. Mathematical model A time-dependent, axisymmetric mathematical model known as UNICORN (Unsteady Ignition and Combustion using ReactioNs) [3,13] is used for the simulation of the unsteady combusting flows in the centerbody combustor. It solves for u- and v-momentum equations, continuity, and enthalpy- and species-conservation equations on a staggered-grid system. A detailed chemical-kinetics model developed by Wang and Frenklach [14] is incorporated into UNICORN for the investigation of ethylene flames. It consists of 99 species and 1066 elementary-reaction steps. Soot is simulated as a gaseous species and through the solution of two conservation equations—one for the soot volume fraction and the other for soot number density. Soot nucleation, agglomeration and oxidation processes are modeled following Lindstedt approach [15]. A simple radiation model for gaseous species, based on the optically thin media assumption, is incorporated into the energy equation [16]. Only radiation from CH4, CO, CO2, and H2O is considered in the present study. Radiation from soot is modeled assuming it as optically thin media [17]. The finite-difference forms of the momentum equations are obtained using an implicit QUICKEST scheme [13], and those of the species and energy equations are obtained using a hybrid scheme of upwind and central differencing. At every time step, the pressure field is accurately calculated by solving all the pressure Poisson equations simultaneously and using the LU (Lower and Upper diagonal) matrix-decomposition technique. The boundary conditions are treated in the same way as that reported in earlier papers [3,13,18]. This model has been extensively validated [3] by simulating various steady and unsteady counterflow [19,20] and coflow [21,22] jet diffusion flames and by comparing the results with experimental data. 3. Experimental apparatus and approach The geometry of the centerbody burner is described in detail in Ref. 2 and is similar to that used for studies of dynamic flames [23]. A schematic diagram of the burner is shown in Fig. 1. The centerbody is located in a vertical wind tunnel
V.R. Katta et al. / Proceedings of the Combustion Institute 33 (2011) 1187–1194
Fig. 1. Sketch of the centerbody burner designed for studying soot formation in flames stabilized with recirculation zones. Flow structure and typical flame location are shown in left half. Computational grid used for simulation of lifted flame is shown in right half.
designed to provide low turbulence inlet flows with a flat velocity profile. The centerbody is a 46 mm-diameter disk with a 7.6 mm-diameter fuel jet located at its center. It is symmetrically mounted in an 80-mm-ID quartz tube that extends vertically 30 cm from the face of the centerbody. The fuel, a mixture of ethylene and nitrogen, is injected through the center hole at a velocity of 1.25 m/s. Ethylene is used because detailed chemistry mechanisms for its combustion exist [14] and it has been widely used in soot studies [24]. The oxidizer, a mixture of air and nitrogen, is flowed through the annular gap, also at a velocity of 1.25 m/s. Two recirculation zones; namely, the outer recirculation zone (ORZ) and the inner recirculation zone (IRZ) establish in the gap between the fuel and oxidizer jets due to the viscosity of the gases as shown in Fig. 1. These recirculation zones play an important role in transporting fuel to the outer rim of the centerbody and in establishing a nonpremixed flame, as depicted in Fig. 1. These recirculation zones also influence the sooting characteristics of the burner. Three sooting structures; namely, filled, donut-shaped, and ring-shaped structures, were observed in previous studies [2]. Those different sooting structures were achieved by diluting the fuel and oxidizer jets with nitrogen while maintaining the ethylene + N2 and air + N2 flow rates constant, which mean that the average velocities at the exit of the central fuel and annular oxidizer jets (the inlet to the burner) are also held constant. This kept the fluid dynamics of the flame more or less the same, which greatly simplified the analysis of different sooting behaviors. Additional experiments are performed through further diluting the oxidizer jet and are reported in this work.
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Calculations for the centerbody flames are performed on a 451 251 grid system as shown in the right half of Fig. 1. Only every other grid line is plotted for clarity. Note that the computational domain in the axial direction is extended all the way to the end of the chimney (30-cm long) and is bounded between r = 0 and 40 mm in the radial direction. The resolution in the fine-mesh area is 0.1 mm in both the axial and radial directions. Calculations performed with different grid resolutions suggested that 0.1 mm resolution gives nearly mesh-independent results. Note that the fine-mesh region of the grid system shown in Fig. 1 is shifted upstream or downstream for capturing the leading edge of the flame. For the attached flames calculated in the previous work [2] this fine-mesh region was moved to the face of the centerbody. Gravitational force in the axial direction is included in the calculations for simulating vertically mounted burner. The Reynolds number (based on fuel-jet diameter) and Froude number at the top edge of the recirculation zone are 879 and 2.82, respectively. 4. Results and discussion As more and more nitrogen is added to the annular (oxidizer) jet, the flame sketched in Fig. 1 no longer stabilizes at the outer rim of the centerbody. Instead, it lifts off and stabilizes on the periphery of the outer recirculation zone. Experiments have revealed that a lifted flame with a column of soot establishes when oxygen concentration in the annular jet becomes sufficiently low. A direct photograph of the flame obtained with 13.14% oxygen in the annular jet is shown in Fig. 2. The fuel jet consisted of 82.4% ethylene and 16.6% nitrogen. Reflection of the visible radiation from the flame has illuminated the 46-mmdiameter centerbody. The flame has lifted off from the face of the centerbody and stabilized about 13 mm above it. Luminous radiation from CH and other ionized species that earmarks the reaction zone may be identified from the blue regions in Fig. 2. A column of soot that is extended all the way to the face of the centerbody can be identified from the orange1 color region. Although the flame in Fig. 2 is very stable, it is quite sensitive to the flow conditions and a small variation in flow velocity, nitrogen dilution, or chimney position could transition it into a totally different structure. Calculations for the lifted flame are performed using the 451 251 grid system shown in Fig. 1 and the results are superimposed on the experimental image in Fig. 2. Since the flame is surrounded with a chimney and a moderate-speed 1 For interpretation of color in Figs. 2-4, the reader is referred to the web version of this article.
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Fig. 2. Photograph of lifted flame obtained with central jet of nitrogen-diluted ethylene and annular flow of nitrogen-diluted air. Flame surface may be identified from blue emission from excited CH radicals and soot may be identified from yellow emission. Computed flame structure is overlaid on the photograph. CH contour is shown with thick red line and streamlines are shown in black.
coflowing air jet (1.25 m/s) buoyancy effects at the base of the flame are found to be negligible. Computed streamlines (lines with arrowheads) and isoconcentration contours (thick red lines) of CH radical are superimposed on the flame photograph in Fig. 2. Calculations predicted the flame lift-off height very well. The hook-type CH-concentration distribution near the flame base [25] in two dimensions matched well with the toroidal-type flame ring captured in the photograph. Both the outer and inner recirculation zones can be identified from the streamlines. The flame anchors to the periphery of the outer recirculation zone. It was observed in the experiment that a small perturbation to the chimney position resulted in a different lifted flame as shown in Fig. 3. Columntype sooting behavior recognized based on this direct photograph did not change much from the lifted flame shown in Fig. 2, except that more soot seems to accumulate in the recirculation zone in the latter case. On the other hand, the flame base looks quite different. A hollow blue-colored toroidal flame tube formed at the edge of the lifted flame (cf. Fig. 3). A first impression suggested that the flame edge rolled into a tubular flame structure. However, such conclusion raised a plausible question—how can fuel enter into the hollow core of the flame tube and provide continuous burning? Several calculations made with up to 5% higher or lower flow velocities and nitrogen concentrations and parabolic and flat velocity profiles failed to transition the flame in Fig. 2 to that shown in Fig. 3. Close observation of the flame shape in Fig. 3 reveals that the soot column initially converges as
Fig. 3. Second mode of lifted flame obtained with central and annular flows that are slightly different from those used in Fig. 2. A toroidal flame tube (blue color) surrounds a column of soot (yellow color) that is extends to burner face.
it moves downstream of the burner face and then starts to diverge at about one centerbody diameter, which is different from that observed in the flame in Fig. 2. Usually such necking occurs at the onset of turbulence (either with or without large-scale vortices). This led us to consider that the flame in Fig. 3 could actually be unsteady and that the transition from a steady to unsteady flame is causing the generation of toroidal flame tube. High-speed video and short-exposure photographs are taken for the flame shown in Fig. 3 for identifying the unsteady characteristics. Flame images obtained at two instants are shown in Fig. 4a and b. Soot columns in these photographs look quite different from that in Fig. 3. Vortical structures comparable to the jet diameter are rolling along the soot column, causing necking and bulging in the column. In fact, the base of the flame is also oscillating. It is determined based on the convective velocity of the vortical structures that a new vortex is being generated at a frequency of 50 Hz. Due to short exposure time (2 ms) the blue flame is not as clear in these high-speed photographs. However, it should be noted that the toroidal flame tube seen in Fig. 3 is not appearing in the high-speed photographs. For understanding the processes leading to the formation of flame tube in the dynamic lifted flame, calculations are repeated for the steadystate flame by increasing or decreasing the flow rates slightly (up to 5%). None of such efforts led to a dynamic flame. Finer grid systems and different boundary conditions for the centerbody also
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Fig. 4. High-speed images of the flame shown in Fig. 3 obtained with exposure time of 2 ms at two instants (a and b). The flame is dynamic. However, there is no evidence of the flame tube that was seen in longexposure photographs.
did not result in an oscillating flame. Assuming that there could be some system resonance in the experiment that is leading to such 50 Hz-flame oscillations, calculations are performed by sinusoidally perturbing the annular flow. The jet velocity was modified at a frequency of 50 Hz with amplitudes up to 10% of the mean flow of 1.25 m/s. Instantaneous solutions obtained with 10% perturbation are shown in Fig. 5a and b. The phase difference between these two images is 10 ms. In each image, iso-contours of OH concentration are superimposed on color distribution of temperature on the left side and color distribution of CH radical is overlaid on the color distribution of soot volume fraction on the right side. Dancing flame edges can easily be noted from these plots and is more pronounced in CH distributions. Nevertheless, these instantaneous images did not show the formation of toroidal flame tubes at the edges as seen in Fig. 3. Computed instantaneous solutions of the unsteady lifted flame are time averaged over a period of 100 ms, which represents five perturbation cycles. Temperature, soot volume fraction, OH and CH distributions of this time-averaged data are plotted in Fig. 6a using the notation employed in Fig. 5. Surprisingly, a hollow ring showed up in the CH distribution near the flame edge. This CH ring looks similar to that observed in the experiment (Fig. 3). It is important to realize that the photograph in Fig. 3 is the planar view of a
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three-dimensional flame, whereas simulation results in Fig. 6a represent a slice of the threedimensional flame. The wrinkles in the temperature field have disappeared in time-averaged visualization. On the other hand, the soot column is not affected much due to perturbation (instantaneous and time-averaged data look nearly the same). For understanding the impact of perturbation amplitude on flame dynamics, calculations are repeated by modifying the oxidizer jet with 5% amplitude. Time-averaged results are shown in Fig. 6b. The hollow ring in CH distribution near the flame base (Fig. 6b) got smaller with the decrease in perturbation amplitude. Interestingly, comparison of soot concentrations in Fig. 6a and b suggests that the amount of soot in the recirculation zone increased with the perturbation amplitude, which follows the observation made in the experiment between the steady state (Fig. 2) and unsteady (Fig. 3) lifted flames. Absence of a CH ring in the instantaneous flame solutions and presence of that in the timeaveraged data suggests that the blue toroidal flame tube observed in the experiment (Fig. 3) is an optical illusion caused by the long time response of human eye, similar to the long-exposure time of the photograph in Fig. 3. Nevertheless, to the authors’ knowledge, such tubular flame edge was never observed before in either steady state or dynamic flames. Interestingly, distribution plots made for various time-averaged species suggested that only CH possessed a hollow ring structure near the flame edge. The direct photographs (Fig. 4) of the dynamic flame show the necking and bulging of the soot column (yellow color) and flame surface (blue color), however they do not show the vortical structures that are actually causing such necking and bulging. The hot product gases advected away from the flame by the vortical structures can be visualized using thermal imaging [26]. Computed flame structure is compared with an infrared image of the flame in Fig. 7. The instantaneous image shown in Fig. 7b was obtained using an FLIR SC4000 infrared camera mounted with a 25 mm lens. The angle of divergence from the center of the flame to the camera is small; therefore the planar images provide essentially line-of-sight (LOS) measurements through the flame. The computed flame shown in Fig. 7a was obtained with 10% amplitude (flame in Fig. 5) and through qualitatively matching the phase of the thermal image in Fig. 7b. Instantaneous locations of the particles injected from the edge of the centerbody are superimposed on the left side of the color temperature plot (Fig. 7a). Computed vortical structures matched very well with those visualized in the thermal image. The decaying of computed vortical structures in the downstream locations is due to the coarse mesh used in those regions. Particle
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Fig. 5. Dynamic flame computed through driving the annular flow in Fig. 2 sinusoidally at 50 Hz and amplitude of 10% of mean flow. (a) Flame at instant t0 and (b) flame at instant t0 + 10 ms. Iso-contours of OH radical are superimposed on temperature distribution in left half and CH-concentration distribution is overlaid on soot volume fraction distribution in right half of each image.
Fig. 6. Time-averaged visualization of dynamic flames computed through driving the annular flow at (a) 10% and (b) 5% of mean flow. Iso-contours of time-averaged OH radical are superimposed on time-averaged temperature distribution in left half, and time-averaged CH-concentration distribution is overlaid on time-averaged soot volume fraction distribution in right half of each image.
distributions near the flame surface suggest that the vortices in upstream locations (z < 90 mm) are rolling inward from top (clockwise on the left side) and are rolling outward from bottom (counterclockwise on the left side) in downstream locations (z > 90 mm). The shape of the thermal image of the flame (Fig. 7b) confirms this change in the direction of vortex roll, which was also observed by the authors in buoyant jet diffusion flames [27]. Such change in vortex roll results from
the buoyant acceleration of hot gasses along the flame surface. The effects of perturbation on trailing jet flames are shown in Fig. 8a and b. Radial distributions of several variables at a height of 40-mm above the centerbody (in the necking region of Fig. 2) are shown for steady state and dynamic lifted flames in this figure. Temperature and soot volume fraction are shown in Fig. 8a and concentrations of CH and OH are shown in Fig. 8b.
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the perturbation amplitude (Fig. 8a). However, the low-temperature region widens with perturbation. Note that flame thickness based on either CH or OH concentration increased significantly with perturbation. 5. Summary and conclusions
Fig. 7. Comparison of computed instantaneous flame structure with infrared image of experimental flame in Figs. 3 and 4. (a) Temperature distribution at instant t0 + 16 ms. Instantaneous locations of particles injected from the edge of centerbody are superimposed in left half. (b) Thermal image of flame obtained with infrared camera with 1-ms exposure time.
Fig. 8. Radial distributions of (a) temperature and soot volume fraction and (b) CH and OH concentrations at z = 40 mm in dynamic and steady-state flames. Timeaveraged data is used for dynamic flames.
Time-averaged data are used for the dynamic flames. Interestingly, calculations predict that the width of the soot column and high-temperature region decreases in the flame-necking region with
Recirculation zones established between the fuel and oxidizer jets in a centerbody burner play a vital role in transporting fuel toward and burnt products away from the flame surface. Consequently, the sooting behavior of the centerbody burner could be modified dramatically without changing the fluid dynamics by diluting the fuel and oxidizer jets with nitrogen. Previous studies have revealed recirculation zones with filled, donut-shaped, and ring-shaped soot regions. It was found experimentally that the flame lifts off and a column of soot is formed when oxygen in the annular flow was sufficiently reduced. Interestingly, with a small shift in chimney position, a toroidal flame was dramatically established near the edge of the lifted flame. An experimental and numerical study was performed for understanding the structure of such toroidal flame tube. A time-dependent, axisymmetric, detailedchemistry CFD model (UNICORN) was used for the simulation of steady state and dynamic lifted flames in the centerbody burner. Combustion was modeled using Wang–Frenklach (99 species and 1066 reactions) mechanism. Soot was simulated using a two-equation model of Lindstedt. Calculations have predicted the structure of the steady lifted flame reasonably well. Liftoff height of the computed flame matched well with that of the experiment. Several attempts made through changing the boundary conditions did not result in the experimentally observed toroidal flame tube at the edge of the lifted flame. Calculations made after perturbing the annular flow periodically at 50 Hz resulted in a dancing edge flame and vortical structures along the trailing jet flame. However, none of these calculations yielded a toroidal-flame structure near the base of the lifted flame. Surprisingly, when time-averaging was performed for the CH-radical distributions of the dancing flame a toroidal flame-like structure, very similar to that observed in the experiment, appeared near the flame base. Such toroidal structures did not develop in temperature or other species distributions. Based on these simulations and high-speed video of the experimental flame it is concluded that the experimentally observed toroidal flame near the edge of a lifted flame is an optical illusion caused by the low time response of the human eye. Detailed structures of the computed oscillating flame matched well with the thermal images of the flame obtained with an infrared camera.
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Acknowledgement Financial support for this work was provided by the Strategic Environmental Research and Development Program (SERDP, Bruce Sartwell). References [1] I.M. Kennedy, Prog. Energy Combust. Sci. 23 (1997) 95. [2] W.M. Roquemore, V. Katta, S. Stouffer, et al., Proc. Combust. Inst. 32 (2009) 729–736. [3] W.M. Roquemore, V.R. Katta, J. Visual. 2 (2000) 257. [4] V.R. Katta, W.M. Roquemore, Predictions on Sooting Behavior of Recirculation-Zone-Supported Flames, D37, Proceedings of the Fifth US Combustion Meeting, San Diego, March 2007, pp. 25–28. [5] Y.S. Ko, S.H. Chung, G.S. Kim, S.W. Kim, Combust. Flame 123 (2000) 430. [6] X. Qin, C.W. Choi, A. Mukhopadhyay, I.K. Puri, S.K. Aggarwal, V.R. Katta, Combust. Theor. Model. 8 (2004) 293–314. [7] K. Robson, M.J.G. Wilson, Combust. Flame 13 (1969) 626–634. [8] F. Takahashi, W.J. Schmoll, V.R. Katta, Proc. Combust. Inst. 27 (1998) 675–684. [9] P.N. Kıoni, B. Rogg, K.N.C. Bray, A. Linan, Combust. Flame 95 (1993) 276–290. [10] F. Takahashi, V.R. Katta, Proc. Combust. Inst. 30 (2005) 375–382. [11] J. Buckmaster, M. Matalon, Proc. Combust. Inst. 22 (1988) 1527–1535. [12] S.H. Chung, Proc. Combust. Inst. 31 (2006) 877– 892.
[13] V.R. Katta, L.P. Goss, W.M. Roquemore, AIAA J. 32 (1) (1994) 84. [14] H. Wang, M. Frenklach, Combust. Flame 110 (1997) 173. [15] R.P. Lindstedt, in: H. Bockhorn (Ed.), Soot Formation in Combustion: Mechanisms and Models, Springer-Verlag, Heidelberg, 1994, pp. 417–439. [16] Anon., Computational Submodels, International Workshop on Measurement and Computation of Turbulent Nonpremixed Flames, 2001. http:// www.ca.sandia.gov/TNF/rdiation.html. [17] H. Guo, F. Liu, G.J. Smallwood, Combust. Theor. Model. 8 (2004) 475–489. [18] V.R. Katta, L.P. Goss, W.M. Roquemore, Int. J. Numer. Methods Heat Fluid Flow 4 (1994) 413. [19] V.R. Katta, C.D. Carter, G.J. Fiechtner, W.M. Roquemore, J.R. Gord, J.C. Rolon, Proc. Combust. Inst. 27 (1998) 587–594. [20] V.R. Katta, T.R. Meyer, J.R. Gord, W.M. Roquemore, Combust. Flame 132 (2003) 639. [21] V.R. Katta, W.M. Roquemore, AIAA J. 36 (1998) 2044. [22] F. Grisch, B. Attal-Tretout, P. Bouchardy, V.R. Katta, W.M. Roquemore, J. Nonlinear Opt. Phys. Mater. 5 (1996) 505. [23] W.M. Roquemore et al., Exp. Fluids 4 (1986) 205– 213. [24] M.D. Smooke et al., Combust. Flame 143 (2005) 613–628. [25] F. Takahashi, V.R. Katta, Proc. Combust. Inst. 29 (2002) 2509–2518. [26] D. Blunck, S. Basu, Y. Zheng, V. Katta, J. Gore, Proc. Combust. Inst. 32 (2009) 2527–2534. [27] V.R. Katta, L.P. Goss, W.M. Roquemore, Combust. Flame 92 (1993) 274–278.
Proceedings of the
Proceedings of the Combustion Institute 33 (2011) 1187–1194
Combustion Institute www.elsevier.com/locate/proci
Dynamic lifted flame in centerbody burner Viswanath R. Katta a,⇑, William M. Roquemore b, Scott Stouffer c, David Blunck d a
Innovative Scientific Solutions Inc., 2766 Indian Ripple Road, Dayton, OH 45440, United States b Air Force Research Laboratory, Wright-Patterson Air Force Base, OH 45433, United States c University of Dayton Research Institute, Dayton, OH 45469, United States d Purdue University, West Lafayette, IN 47907, United States Available online 24 September 2010
Abstract During some exploratory experiments performed on a centerbody burner it was observed that the sooting behavior of the burner could be altered dramatically without changing the fluid dynamics. One of the interesting operating regimes, in which the flame lifts off and forms a column of soot, was identified when oxygen in the annular flow was sufficiently reduced. More interestingly, within a narrow window of flow conditions, an unusual toroidal flame was observed near the base of the lifted flame. This paper describes the numerical and experimental studies performed for understanding this peculiar toroidal flame tube. A time-dependent, axisymmetric, detailed-chemistry CFD model (UNICORN) is used. Combustion and PAH formation are modeled using Wang–Frenklach mechanism and soot is simulated using a two-equation model of Lindstedt. Calculations have accurately predicted the steady lifted flame that is anchored to the outer edge of the recirculation zone. Lift-off height of the computed flame matched well with that of the experiment. A dynamic lifted flame is then established through periodically oscillating the annular flow. The edge of the lifted flame is found to dance along the outer periphery of the recirculation zone while vortical structures establish downstream of it. However, none of the calculations made with varying flow conditions or perturbations yielded a toroidal-flame structure near the base of the lifted flame. Surprisingly, when time-averaging was performed for the CH-radical distributions of the dancing flame a toroidal flame-like structure, very similar to that observed in the experiment, appeared near the flame base. Based on these calculations and high-speed movies of the experimental flame it is concluded that the observed toroidal flame is an optical illusion created through the natural time-averaging process of the human eye. Detailed structures of the computed oscillating flame are compared with the thermal images of the flame obtained using an infrared camera. Ó 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Diffusion flame; Lifted flame; Recirculation zone; Soot; Radiation
1. Introduction The formation, growth, transport, and burnout of soot are perhaps the most complex and least ⇑ Corresponding author. Fax: +1 937 656 4110.
E-mail address: [email protected] (V.R. Katta).
understood processes in flames and combustion systems. Soot precursor particles containing several-thousand carbon atoms are formed in flames from simple fuel molecules within a few microseconds [1]. However, these precursor particles on a much longer time scale interact with the gas-phase molecules during the surface-growth process,
1540-7489/$ - see front matter Ó 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2010.06.153
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colloid with each other in the agglomeration process and react with oxygenated species in the oxidation process. All of these chemical and physical two-phase processes occur simultaneously in flames. In gas turbine combustors, these processes are further complicated by the burning of practical fuels, consisting of thousands of species, and the actions of turbulent flows and recirculation zones. There is a significant science base for understanding the soot processes; however, it is inadequate to provide accurate soot models that can aid in the design of future low sooting gas turbine combustors. Thus, there is a pressing need to expand the science base in ways that foster the development and evaluation of accurate CFD models for designing low sooting combustion systems. The Strategic Environmental Research and Development Program (SERDP) office has started a comprehensive fundamental soot research program involving a suite of burners [2]. Complexity of these burners progressed in a way that the effects of chemical kinetics, diffusion, flame stretch, recirculation, and turbulence on the formation of soot can be investigated systematically. The computational part of this effort is to use a state-of-theart, Navier–Stokes based CFD code (UNICORN) [3] to aid in designing experiments, predicting and interpreting results, and evaluating soot and chemistry models. The centerbody burner [2], one of the few designs selected for studying soot, consists of an annular oxidizer jet and a central fuel jet separated by a bluff body. The formation of recirculation zones in this burner [4] provides a mechanism for decoupling various soot processes and investigating them individually. Initial experiments performed by varying the amount of dilution with nitrogen in the fuel and/or annular jets [2] suggested that very different flames with filled, donutshaped, or ring-type soot layers could be obtained. Recent experiments have further revealed that a lifted flame with a column of soot extending into the relatively cold regions between the flame base and the face of the bluff body can also be obtained. For some flow conditions a very unusual toroidal flame tube formed near the lifted flame edge. Lifted flames established in the shear layers of coflowing fuel and air jets were studied by several investigators [5,6], mainly for understanding the flame stabilization mechanisms [7,8]. While some studies revealed that the edge of a lifted flame is sharp with a monobrachial structure [9,10], other studies suggested a tribrachial structure to the flame edge [11,12]. However, to the authors’ knowledge, lifted flame edge wrapping into a toroidal flame tube has never been identified. As the recirculation zones in the centerbody burner provide additional complexity to the stability of the lifted flame, the observed toroidal flame tube could be specific to the lifted flames anchored to recirculation zones. This paper describes the stud-
ies performed for understanding these flame-tubelike structures in a centerbody burner. High-speed visualizations of the lifted flames with and without the toroidal flame tube are obtained using standard and infrared cameras. Calculations for these lifted flames are performed using UNICORN code with detailed chemical kinetics for ethylene combustion. Dynamic flames are simulated through perturbing the annular flow sinusoidally at a frequency determined from the experiments. 2. Mathematical model A time-dependent, axisymmetric mathematical model known as UNICORN (Unsteady Ignition and Combustion using ReactioNs) [3,13] is used for the simulation of the unsteady combusting flows in the centerbody combustor. It solves for u- and v-momentum equations, continuity, and enthalpy- and species-conservation equations on a staggered-grid system. A detailed chemical-kinetics model developed by Wang and Frenklach [14] is incorporated into UNICORN for the investigation of ethylene flames. It consists of 99 species and 1066 elementary-reaction steps. Soot is simulated as a gaseous species and through the solution of two conservation equations—one for the soot volume fraction and the other for soot number density. Soot nucleation, agglomeration and oxidation processes are modeled following Lindstedt approach [15]. A simple radiation model for gaseous species, based on the optically thin media assumption, is incorporated into the energy equation [16]. Only radiation from CH4, CO, CO2, and H2O is considered in the present study. Radiation from soot is modeled assuming it as optically thin media [17]. The finite-difference forms of the momentum equations are obtained using an implicit QUICKEST scheme [13], and those of the species and energy equations are obtained using a hybrid scheme of upwind and central differencing. At every time step, the pressure field is accurately calculated by solving all the pressure Poisson equations simultaneously and using the LU (Lower and Upper diagonal) matrix-decomposition technique. The boundary conditions are treated in the same way as that reported in earlier papers [3,13,18]. This model has been extensively validated [3] by simulating various steady and unsteady counterflow [19,20] and coflow [21,22] jet diffusion flames and by comparing the results with experimental data. 3. Experimental apparatus and approach The geometry of the centerbody burner is described in detail in Ref. 2 and is similar to that used for studies of dynamic flames [23]. A schematic diagram of the burner is shown in Fig. 1. The centerbody is located in a vertical wind tunnel
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Fig. 1. Sketch of the centerbody burner designed for studying soot formation in flames stabilized with recirculation zones. Flow structure and typical flame location are shown in left half. Computational grid used for simulation of lifted flame is shown in right half.
designed to provide low turbulence inlet flows with a flat velocity profile. The centerbody is a 46 mm-diameter disk with a 7.6 mm-diameter fuel jet located at its center. It is symmetrically mounted in an 80-mm-ID quartz tube that extends vertically 30 cm from the face of the centerbody. The fuel, a mixture of ethylene and nitrogen, is injected through the center hole at a velocity of 1.25 m/s. Ethylene is used because detailed chemistry mechanisms for its combustion exist [14] and it has been widely used in soot studies [24]. The oxidizer, a mixture of air and nitrogen, is flowed through the annular gap, also at a velocity of 1.25 m/s. Two recirculation zones; namely, the outer recirculation zone (ORZ) and the inner recirculation zone (IRZ) establish in the gap between the fuel and oxidizer jets due to the viscosity of the gases as shown in Fig. 1. These recirculation zones play an important role in transporting fuel to the outer rim of the centerbody and in establishing a nonpremixed flame, as depicted in Fig. 1. These recirculation zones also influence the sooting characteristics of the burner. Three sooting structures; namely, filled, donut-shaped, and ring-shaped structures, were observed in previous studies [2]. Those different sooting structures were achieved by diluting the fuel and oxidizer jets with nitrogen while maintaining the ethylene + N2 and air + N2 flow rates constant, which mean that the average velocities at the exit of the central fuel and annular oxidizer jets (the inlet to the burner) are also held constant. This kept the fluid dynamics of the flame more or less the same, which greatly simplified the analysis of different sooting behaviors. Additional experiments are performed through further diluting the oxidizer jet and are reported in this work.
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Calculations for the centerbody flames are performed on a 451 251 grid system as shown in the right half of Fig. 1. Only every other grid line is plotted for clarity. Note that the computational domain in the axial direction is extended all the way to the end of the chimney (30-cm long) and is bounded between r = 0 and 40 mm in the radial direction. The resolution in the fine-mesh area is 0.1 mm in both the axial and radial directions. Calculations performed with different grid resolutions suggested that 0.1 mm resolution gives nearly mesh-independent results. Note that the fine-mesh region of the grid system shown in Fig. 1 is shifted upstream or downstream for capturing the leading edge of the flame. For the attached flames calculated in the previous work [2] this fine-mesh region was moved to the face of the centerbody. Gravitational force in the axial direction is included in the calculations for simulating vertically mounted burner. The Reynolds number (based on fuel-jet diameter) and Froude number at the top edge of the recirculation zone are 879 and 2.82, respectively. 4. Results and discussion As more and more nitrogen is added to the annular (oxidizer) jet, the flame sketched in Fig. 1 no longer stabilizes at the outer rim of the centerbody. Instead, it lifts off and stabilizes on the periphery of the outer recirculation zone. Experiments have revealed that a lifted flame with a column of soot establishes when oxygen concentration in the annular jet becomes sufficiently low. A direct photograph of the flame obtained with 13.14% oxygen in the annular jet is shown in Fig. 2. The fuel jet consisted of 82.4% ethylene and 16.6% nitrogen. Reflection of the visible radiation from the flame has illuminated the 46-mmdiameter centerbody. The flame has lifted off from the face of the centerbody and stabilized about 13 mm above it. Luminous radiation from CH and other ionized species that earmarks the reaction zone may be identified from the blue regions in Fig. 2. A column of soot that is extended all the way to the face of the centerbody can be identified from the orange1 color region. Although the flame in Fig. 2 is very stable, it is quite sensitive to the flow conditions and a small variation in flow velocity, nitrogen dilution, or chimney position could transition it into a totally different structure. Calculations for the lifted flame are performed using the 451 251 grid system shown in Fig. 1 and the results are superimposed on the experimental image in Fig. 2. Since the flame is surrounded with a chimney and a moderate-speed 1 For interpretation of color in Figs. 2-4, the reader is referred to the web version of this article.
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Fig. 2. Photograph of lifted flame obtained with central jet of nitrogen-diluted ethylene and annular flow of nitrogen-diluted air. Flame surface may be identified from blue emission from excited CH radicals and soot may be identified from yellow emission. Computed flame structure is overlaid on the photograph. CH contour is shown with thick red line and streamlines are shown in black.
coflowing air jet (1.25 m/s) buoyancy effects at the base of the flame are found to be negligible. Computed streamlines (lines with arrowheads) and isoconcentration contours (thick red lines) of CH radical are superimposed on the flame photograph in Fig. 2. Calculations predicted the flame lift-off height very well. The hook-type CH-concentration distribution near the flame base [25] in two dimensions matched well with the toroidal-type flame ring captured in the photograph. Both the outer and inner recirculation zones can be identified from the streamlines. The flame anchors to the periphery of the outer recirculation zone. It was observed in the experiment that a small perturbation to the chimney position resulted in a different lifted flame as shown in Fig. 3. Columntype sooting behavior recognized based on this direct photograph did not change much from the lifted flame shown in Fig. 2, except that more soot seems to accumulate in the recirculation zone in the latter case. On the other hand, the flame base looks quite different. A hollow blue-colored toroidal flame tube formed at the edge of the lifted flame (cf. Fig. 3). A first impression suggested that the flame edge rolled into a tubular flame structure. However, such conclusion raised a plausible question—how can fuel enter into the hollow core of the flame tube and provide continuous burning? Several calculations made with up to 5% higher or lower flow velocities and nitrogen concentrations and parabolic and flat velocity profiles failed to transition the flame in Fig. 2 to that shown in Fig. 3. Close observation of the flame shape in Fig. 3 reveals that the soot column initially converges as
Fig. 3. Second mode of lifted flame obtained with central and annular flows that are slightly different from those used in Fig. 2. A toroidal flame tube (blue color) surrounds a column of soot (yellow color) that is extends to burner face.
it moves downstream of the burner face and then starts to diverge at about one centerbody diameter, which is different from that observed in the flame in Fig. 2. Usually such necking occurs at the onset of turbulence (either with or without large-scale vortices). This led us to consider that the flame in Fig. 3 could actually be unsteady and that the transition from a steady to unsteady flame is causing the generation of toroidal flame tube. High-speed video and short-exposure photographs are taken for the flame shown in Fig. 3 for identifying the unsteady characteristics. Flame images obtained at two instants are shown in Fig. 4a and b. Soot columns in these photographs look quite different from that in Fig. 3. Vortical structures comparable to the jet diameter are rolling along the soot column, causing necking and bulging in the column. In fact, the base of the flame is also oscillating. It is determined based on the convective velocity of the vortical structures that a new vortex is being generated at a frequency of 50 Hz. Due to short exposure time (2 ms) the blue flame is not as clear in these high-speed photographs. However, it should be noted that the toroidal flame tube seen in Fig. 3 is not appearing in the high-speed photographs. For understanding the processes leading to the formation of flame tube in the dynamic lifted flame, calculations are repeated for the steadystate flame by increasing or decreasing the flow rates slightly (up to 5%). None of such efforts led to a dynamic flame. Finer grid systems and different boundary conditions for the centerbody also
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Fig. 4. High-speed images of the flame shown in Fig. 3 obtained with exposure time of 2 ms at two instants (a and b). The flame is dynamic. However, there is no evidence of the flame tube that was seen in longexposure photographs.
did not result in an oscillating flame. Assuming that there could be some system resonance in the experiment that is leading to such 50 Hz-flame oscillations, calculations are performed by sinusoidally perturbing the annular flow. The jet velocity was modified at a frequency of 50 Hz with amplitudes up to 10% of the mean flow of 1.25 m/s. Instantaneous solutions obtained with 10% perturbation are shown in Fig. 5a and b. The phase difference between these two images is 10 ms. In each image, iso-contours of OH concentration are superimposed on color distribution of temperature on the left side and color distribution of CH radical is overlaid on the color distribution of soot volume fraction on the right side. Dancing flame edges can easily be noted from these plots and is more pronounced in CH distributions. Nevertheless, these instantaneous images did not show the formation of toroidal flame tubes at the edges as seen in Fig. 3. Computed instantaneous solutions of the unsteady lifted flame are time averaged over a period of 100 ms, which represents five perturbation cycles. Temperature, soot volume fraction, OH and CH distributions of this time-averaged data are plotted in Fig. 6a using the notation employed in Fig. 5. Surprisingly, a hollow ring showed up in the CH distribution near the flame edge. This CH ring looks similar to that observed in the experiment (Fig. 3). It is important to realize that the photograph in Fig. 3 is the planar view of a
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three-dimensional flame, whereas simulation results in Fig. 6a represent a slice of the threedimensional flame. The wrinkles in the temperature field have disappeared in time-averaged visualization. On the other hand, the soot column is not affected much due to perturbation (instantaneous and time-averaged data look nearly the same). For understanding the impact of perturbation amplitude on flame dynamics, calculations are repeated by modifying the oxidizer jet with 5% amplitude. Time-averaged results are shown in Fig. 6b. The hollow ring in CH distribution near the flame base (Fig. 6b) got smaller with the decrease in perturbation amplitude. Interestingly, comparison of soot concentrations in Fig. 6a and b suggests that the amount of soot in the recirculation zone increased with the perturbation amplitude, which follows the observation made in the experiment between the steady state (Fig. 2) and unsteady (Fig. 3) lifted flames. Absence of a CH ring in the instantaneous flame solutions and presence of that in the timeaveraged data suggests that the blue toroidal flame tube observed in the experiment (Fig. 3) is an optical illusion caused by the long time response of human eye, similar to the long-exposure time of the photograph in Fig. 3. Nevertheless, to the authors’ knowledge, such tubular flame edge was never observed before in either steady state or dynamic flames. Interestingly, distribution plots made for various time-averaged species suggested that only CH possessed a hollow ring structure near the flame edge. The direct photographs (Fig. 4) of the dynamic flame show the necking and bulging of the soot column (yellow color) and flame surface (blue color), however they do not show the vortical structures that are actually causing such necking and bulging. The hot product gases advected away from the flame by the vortical structures can be visualized using thermal imaging [26]. Computed flame structure is compared with an infrared image of the flame in Fig. 7. The instantaneous image shown in Fig. 7b was obtained using an FLIR SC4000 infrared camera mounted with a 25 mm lens. The angle of divergence from the center of the flame to the camera is small; therefore the planar images provide essentially line-of-sight (LOS) measurements through the flame. The computed flame shown in Fig. 7a was obtained with 10% amplitude (flame in Fig. 5) and through qualitatively matching the phase of the thermal image in Fig. 7b. Instantaneous locations of the particles injected from the edge of the centerbody are superimposed on the left side of the color temperature plot (Fig. 7a). Computed vortical structures matched very well with those visualized in the thermal image. The decaying of computed vortical structures in the downstream locations is due to the coarse mesh used in those regions. Particle
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Fig. 5. Dynamic flame computed through driving the annular flow in Fig. 2 sinusoidally at 50 Hz and amplitude of 10% of mean flow. (a) Flame at instant t0 and (b) flame at instant t0 + 10 ms. Iso-contours of OH radical are superimposed on temperature distribution in left half and CH-concentration distribution is overlaid on soot volume fraction distribution in right half of each image.
Fig. 6. Time-averaged visualization of dynamic flames computed through driving the annular flow at (a) 10% and (b) 5% of mean flow. Iso-contours of time-averaged OH radical are superimposed on time-averaged temperature distribution in left half, and time-averaged CH-concentration distribution is overlaid on time-averaged soot volume fraction distribution in right half of each image.
distributions near the flame surface suggest that the vortices in upstream locations (z < 90 mm) are rolling inward from top (clockwise on the left side) and are rolling outward from bottom (counterclockwise on the left side) in downstream locations (z > 90 mm). The shape of the thermal image of the flame (Fig. 7b) confirms this change in the direction of vortex roll, which was also observed by the authors in buoyant jet diffusion flames [27]. Such change in vortex roll results from
the buoyant acceleration of hot gasses along the flame surface. The effects of perturbation on trailing jet flames are shown in Fig. 8a and b. Radial distributions of several variables at a height of 40-mm above the centerbody (in the necking region of Fig. 2) are shown for steady state and dynamic lifted flames in this figure. Temperature and soot volume fraction are shown in Fig. 8a and concentrations of CH and OH are shown in Fig. 8b.
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the perturbation amplitude (Fig. 8a). However, the low-temperature region widens with perturbation. Note that flame thickness based on either CH or OH concentration increased significantly with perturbation. 5. Summary and conclusions
Fig. 7. Comparison of computed instantaneous flame structure with infrared image of experimental flame in Figs. 3 and 4. (a) Temperature distribution at instant t0 + 16 ms. Instantaneous locations of particles injected from the edge of centerbody are superimposed in left half. (b) Thermal image of flame obtained with infrared camera with 1-ms exposure time.
Fig. 8. Radial distributions of (a) temperature and soot volume fraction and (b) CH and OH concentrations at z = 40 mm in dynamic and steady-state flames. Timeaveraged data is used for dynamic flames.
Time-averaged data are used for the dynamic flames. Interestingly, calculations predict that the width of the soot column and high-temperature region decreases in the flame-necking region with
Recirculation zones established between the fuel and oxidizer jets in a centerbody burner play a vital role in transporting fuel toward and burnt products away from the flame surface. Consequently, the sooting behavior of the centerbody burner could be modified dramatically without changing the fluid dynamics by diluting the fuel and oxidizer jets with nitrogen. Previous studies have revealed recirculation zones with filled, donut-shaped, and ring-shaped soot regions. It was found experimentally that the flame lifts off and a column of soot is formed when oxygen in the annular flow was sufficiently reduced. Interestingly, with a small shift in chimney position, a toroidal flame was dramatically established near the edge of the lifted flame. An experimental and numerical study was performed for understanding the structure of such toroidal flame tube. A time-dependent, axisymmetric, detailedchemistry CFD model (UNICORN) was used for the simulation of steady state and dynamic lifted flames in the centerbody burner. Combustion was modeled using Wang–Frenklach (99 species and 1066 reactions) mechanism. Soot was simulated using a two-equation model of Lindstedt. Calculations have predicted the structure of the steady lifted flame reasonably well. Liftoff height of the computed flame matched well with that of the experiment. Several attempts made through changing the boundary conditions did not result in the experimentally observed toroidal flame tube at the edge of the lifted flame. Calculations made after perturbing the annular flow periodically at 50 Hz resulted in a dancing edge flame and vortical structures along the trailing jet flame. However, none of these calculations yielded a toroidal-flame structure near the base of the lifted flame. Surprisingly, when time-averaging was performed for the CH-radical distributions of the dancing flame a toroidal flame-like structure, very similar to that observed in the experiment, appeared near the flame base. Such toroidal structures did not develop in temperature or other species distributions. Based on these simulations and high-speed video of the experimental flame it is concluded that the experimentally observed toroidal flame near the edge of a lifted flame is an optical illusion caused by the low time response of the human eye. Detailed structures of the computed oscillating flame matched well with the thermal images of the flame obtained with an infrared camera.
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