- Email: [email protected]
air flames under sooting conditions
Proceedings of the
Proceedings of the Combustion Institute 31 (2007) 1047–1054
Combustion Institute www.elsevier.com/locate/proci
Experimental characterization of premixed spherical ethylene/air flames under sooting conditions Alfonso F. Ibarreta a b
a,1
, Chih-Jen Sung
a,*
, Hai Wang
b
Department of Mechanical and Aerospace Engineering, Case Western Reserve University, Cleveland, OH 44106, USA Department of Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, CA 90089, USA
Abstract Purely curved, premixed, sooting ethylene/air flames were studied using a spherical porous burner under microgravity. These weak, fuel-rich flames were shown to be stabilized without the complications of hydrodynamic straining and conductive heat loss to the burner. Using the rainbow Schlieren deflectometry (RSD) optical system, the flames were imaged and the sooting flame speeds and flame thicknesses of ethylene/air mixtures were determined in the equivalence ratio range of 3.0–4.5, and for selected equivalence ratios, with additional nitrogen dilution. Numerical results obtained with detailed reaction kinetics and transport properties, but without considering soot chemistry and radiation, were shown to over-predict the flame intensity. Ó 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Burning velocity; Ethylene combustion; Microgravity combustion; Soot; Rainbow Schlieren deflectometry
1. Introduction Laminar premixed flames and their propagation rates have been studied traditionally in the non-sooting regime. Experimentally, it is far more difficult to obtain reliable, unambiguous flame propagation data for sooting flames. Computationally, soot chemistry has been (and still is) lesser known than the main flame chemistry of chain branching, propagation, and termination. The recent advances in our understanding of soot chemistry have somewhat changed this situation. In particular, the novel development of detailed soot models (e.g. [1–4]) has significantly increased *
Corresponding author. Fax: +1 216 368 6445. E-mail address: [email protected] (C.-J. Sung). 1 Exponent Failure Analysis Associates, Natick, MA 01760, USA.
our predictive capabilities. Recent studies show that when coupled with efficient solvers of aerosol dynamics, such soot models are able to simulate the particle size distribution function and thus the surface area of soot formed in burner-stabilized flames with a reasonable degree of accuracy [5–7]. These advances have raised the question of whether the soot chemistry should be viewed as an integral part of flame chemistry. There have also been notable advances in our ability to model the structure and properties of non-premixed sooting flames (e.g. [4]). These advances were accomplished by considering detailed reaction kinetics, soot chemistry, and mass/heat transport. Parallel efforts, that consider soot chemistry in the property and dynamics of premixed flames, however, have not yet been made. To this end, we note that the sooting flame speed embeds several important and strongly coupled properties of significant fundamental
1540-7489/$ - see front matter Ó 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2006.08.029
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interests. These properties include the rate of fuel pyrolysis and oxidation, the sooting tendency of the fuel, heat release rates, and heat dissipation through radiation. Advances in this area will have to first rely on the availability of experimental methodologies to characterize the propagation and extinction of premixed sooting flames. The study of premixed sooting flame speeds has been hampered by a large number of problems, including flame stretch, finite residence times, and conductive heat losses. Burner-stabilized flat flames are not suitable for the measurement of sooting flame speeds because of the small flame standoff distance and the resulting radical destruction and conductive heat loss at the burner surface. In our previous work [8], we also demonstrated that the well-known counterflow configuration is unsuitable for premixed sooting flames, due to the dependence of soot layer characteristics on the stretch rate and the influence of buoyancy at low stretch rates. An alternate experimental method was recently proposed, utilizing a quasi-one-dimensional spherical flame geometry to establish a premixed freely propagating sooting flame [8]. As part of that study, stationary spherically symmetric sooting flames were established under microgravity conditions. The flames were then closely studied using the non-intrusive rainbow Schlieren deflectometry (RSD) optical method. It was demonstrated that the spherical geometry is capable of stabilizing flames with negligible conductive heat loss to the burner, and that the method was successfully used to generate the sooting flame speed for an ethylene/air mixture at atmospheric pressure and an equivalence ratio of 3.5 [8]. In the current work, we report experimental measurements for the laminar flame speeds of ethylene/air mixtures over the equivalence ratio range of 3.0–4.5 and for selected equivalence ratios, with additional nitrogen dilution. 2. Theoretical background The spherical burner geometry—in the absence of gravity—can stabilize a premixed flame with negligible conductive heat loss and flame stretch. Theoretically, stationary, burner-generated cylindrical and spherical premixed flames were studied using asymptotics with one-step reaction kinetics and constant properties [9,10]. It is well accepted that because the flow direction is always normal to the flame surface, these ideally-curved, stationery flames are not subject to aerodynamic straining, and that the flame standoff distance can be varied by simply adjusting the mixture flow rate. Obviously, ideally curved flames must be established in a microgravity environment in order to eliminate buoyancy-induced flame asymmetry. Indeed, nearly symmetric, non-sooting cylindrical
flames were experimentally demonstrated in that environment [9]. The burning velocity of a stationary spherical flame can be unambiguously defined, if the heat release zone is thin compared to the flame standoff distance. The burning velocity may be obtained by determining the mass flux through the surface of maximum heat release. Since the flow field is symmetric and at steady-state, the local mass flow rate is the same for all radii, making the local mass flux a function only of the radial position. If the mass flow rate through the burner surface, m_ s , is known, the burning flux, f, of a spherical flame surface of radius, rf, is ð1Þ f ¼ m_ S = 4pr2f : A spherical premixed flame may be stabilized by either flow divergence or conductive heat loss to the burner surface, depending on the injection velocity. Upstream conductive heat loss may be made negligible if the flame standoff distance is sufficiently large, allowing the flame to be stabilized purely through flow divergence. This characteristic of the spherical geometry enables the establishment of a freely propagating flame at a finite injection flow rate of the unburned mixture. To substantiate the above statements we carried out detailed calculations for spherical, steady-state ethylene/air premixed flames using PREMIX [11] with detailed reaction kinetics [12] (without soot). The cross-section area of the spherical configuration was explicitly specified as a function of the radial distance in the PREMIX code. Both non-radiative and optically thin radiation models were employed and compared, in order to assess the influence of geometry on the radiative flame solution. Radiative species considered in the calculations included CH4, H2O, CO, and CO2. A small burner radius (0.2 cm) was used here to emphasize the effect of curvature on flame propagation. Figure 1 shows the burning velocity computed for an ethylene/air mixture with equivalence ratio U = 3.5, unburned gas temperature equal to 298 K, as a function of normalized burner flow rate. The normalized burner flow rate is defined herein as: m_ s =ð4pR2 f o Þ, where R is the burner radius, (4pR2) is the burner surface area, and f o is the burning mass flux of the unstretched adiabatic planar flame. Thus, when the normalized value is unity, the mass flux at the burner surface is equal to that of the freely propagating unstretched planar flame. As shown in Eq. (1), the critical parameter in determining the flame propagation velocity is the flame radius rf, defined herein as the location of the maximum heat release rate. Since the burner mass flow rate m_ s is prescribed, the mass flux through the flame surface, f, is readily obtained from Eq. (1) with a known rf; and the burning velocity is subsequently determined by dividing f
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Fig. 1. Numerically-predicted burning velocities of stable spherical flames for an ethylene/air mixture (U = 3.5), with (dashed line) and without (solid line) radiative heat losses. Solutions were obtained using a revised version of the PREMIX code [11] and detailed reaction kinetics [12] (without soot). The radiative case was obtained with an optically thin radiation model.
by the density of unburned mixture qu. Figure 1 clearly shows that for small normalized burning flow rate (<4), the burning velocity is reduced due to conductive heat loss to the burner. At large flow rates, the burning velocity approaches a constant value, as expected, for both non-radiative and optically thin radiative flames. At the limit of large flow rates, flow divergence is the only mechanism of flame stabilization and the burning velocity becomes independent of the flow rate. In this limit for the non-radiative case, the burning flux f becomes the fundamental laminar burning flux f o [9,10], and the laminar flame speed can be obtained as S ou ¼ f o =qu . It is also interesting to note that for the current optically thin calculation radiative heat losses have a negligible effect on the burning velocity, so long as a large flow rate is used. 3. Experimental methodology The experimental setup employed in this study is described in detail in Ref. [8]. A microgravity rig (shown in Fig. 2a) was used to perform experiments at the 2.2-Second Drop Tower Facility at NASA Glenn Research Center. The experimental rig contained the combustion chamber (center of rig), along with the required gas-flow and power systems (bottom shelf) and optical diagnostics (middle shelf). The compact optical setup (shown in Fig. 2b) and diagnostics are also described in detail elsewhere [13]. During a microgravity experiment, a spherical flame was stabilized around a porous spherical burner by introducing a fuel/air mixture through a small tube, located at the bottom of the burner sphere. Results reported herein were obtained using a spherical porous brass burner of 0.92 cm
Fig. 2. Illustration showing: (a) rig layout and (b) setup of RSD and chemiluminescence imaging systems.
in diameter and with a pore size of 8 lm. The fuel/air mixtures were ignited in a slightly oxidizing atmosphere (10% O2 and 90% N2, by volume) about a fraction of a second prior to each drop. We used the non-intrusive RSD optical technique [14,15] to determine the position of the reaction zone, the flame thickness, and the temperature distribution [8,13]. The present RSD setup is similar to that of Ref. [16]. The technique was chosen over other methods because of its simplicity and its ability to provide quantitative information under sooting conditions. The RSD measures the angular deflection of light as it passes through the test section. The deflection of light rays traveling through the flame is an integral of the local deflections along the beam path. The radial distribution of index of refraction can therefore be obtained via a deconvolution of the raw data. Once the index of refraction field is determined, a correlation function between the index of refraction and the temperature is needed to finally obtain the radial thermal profiles. The local index of refraction depends mostly on the gas density (and therefore temperature), and to a lesser extent, on the gas composition. In this work, the species concentration profiles were computed in a similar manner as that described in Section 2. The local refractivities were then calculated using the computed gas density and species concentrations and the database of Ref. [17]. Sample radial temperature profiles obtained using the RSD method in sooting spherical flames can be found in Ref. [8]. These profiles
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are not included here. The flame location was defined as the point in the temperature profile corresponding to the maximum heat release. It is estimated that the uncertainty is of the order of ±0.3 mm for the position of maximum heat release [8]. Figure 3 shows the time evolution of a sample flame (U = 3.5 and m_ s =0.046 g/s) resulting from a sequence of direct imaging (top) and RSD imaging (bottom). The flame was ignited at the top of the tower prior to the drop (t = –0.5 s). The direct imaging shows the expansion of an outer diffusion flame. The RSD sequence in Fig. 3 illustrates that the inner premixed flame very quickly stabilizes around the burner (within 0.5–0.8 s after the rig is released). The advantage of RSD is clearly demonstrated, as it is able to image the inner layer of quite heavily sooting flames. Although the soot particles can create a shade in the RSD image when the soot loading is very high, in the cases studied herein the soot concentration is low enough to allow most of the rays to penetrate the soot layer unhindered. This allows for the use of the RSD technique even under moderately sooting conditions, where direct imaging of the reaction zone would otherwise not be possible. As discussed earlier, the spherical geometry allows the burning velocity to be obtained from the stabilized flame by accurately measuring its radial location using RSD. This optical technique is also capable of obtaining the temperature field
even under heavily sooting conditions, as demonstrated in Ref. [13]. The sooting tendency of the stable premixed flame is shown as a function of equivalence ratio in Fig. 4. These images were collected near the end of each drop. The decrease in the apparent soot luminosity with an increase in the equivalence ratio may be attributed to a reduced radiation intensity as well as a reduced soot production, both of which are the result of lowered flame temperature. It is seen that the resolution of RSD was not limited by the soot luminosity. With an increase in the equivalence ratio, the RSD contours become more diffuse, indicating an increase of the flame thickness. For U P 5.0 the results show that the inner premixed flame had not stabilized within the drop time. The finite 2.2 s duration and our current experimental setup prevent the study of mixtures with burning velocities less than 2 cm/s, but it is expected that the actual flammability limit lies close to this point. An additional limitation of the present experiment may be also inferred by examining the computational results shown in Fig. 1. The fact that the mass flux through the burner surface must be several times that of the burning mass flux (>4) to eliminate conductive heat loss, coupled with the property of the porous material used for the spherical burner, limited our measurements to mixtures with burning velocities <7 cm/s. On the other hand, the current experimental method is ideal to study very weak flames where other methods are unsuitable.
Fig. 3. Sample time sequence of a microgravity drop (U = 3.5 and m_ s ¼ 0:046 g=sÞ. Top panels: direct imaging; bottom panels: RSD imaging. Time t = 0 corresponds to the release of the drop package. Ignition occurred at –0.5 s before the drop. The retractable igniter is seen in the first frame of the RSD image. The minor asymmetry on the left side of the RSD image is caused by the intrusion of the thermocouple.
Fig. 4. Collage showing the spherical premixed flames prior to the end of the drop, as a function of equivalence ratio. Top panels: direct imaging; bottom panels: RSD imaging.
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4. Results and discussion Images of ethylene/air flames with U = 3.0 are shown in Fig. 5 for several representative burner injection flow rates. It is clear from these images that the soot luminosity was small at small injection rates (60.065 g/s), where conductive heat loss lowers the flame temperature and thus soot production and luminosity. At larger injection rates (P0.075 g/s) the soot luminosity becomes stronger and visually indistinguishable at varying injection rates. The RSD images illustrate how the flame standoff distance increases with increasing burner flow rate. Figure 6 shows the experimentally determined burning mass flux versus the normalized burner flow rate over a range of equivalence ratios. Qualitatively, the experimental results shown here are similar to that calculated in Fig. 1. That is, as the burner flow rate increases, the measured burning flux initially increases due to a decrease in the conductive heat loss to the burner. Once the burner flow rate has increased beyond a critical value, the flame burning flux becomes independent of the injection flow rate. This constant burning flux value was defined herein to be the fundamental burn-
Fig. 5. Collage showing the spherical premixed flames (U = 3.0) prior to the end of the drop, as a function of burner flow rate. Top panels: direct imaging; bottom panels: RSD imaging.
Fig. 6. Measured burning mass flux versus normalized burner flow rate.
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ing mass flux f o. We should note that in plotting Fig. 6, the value of fowas used to calculate the normalized burner flow rate, m_ s =ð4pR2 f o Þ. Thus an iterative procedure was employed in order to obtain Fig. 6. To further illustrate the validity of data shown in Fig. 6 and their interpretation, we plot in Fig. 7 the measured flame radius as a function of burner flow rate for several representative U values. The theoretical radii of a spherical flame propagating at a constant burning velocity, S ou , denoted by lines, are also included for comparison, showing that without conductive heat loss the flame radius generally increases with an increase in the mass flow rate. It is seen from the experimental results that the flame initially approaches the burner surface (located at a radius of 0.46 cm) as the flow rate is decreased. A minimum standoff distance is eventually established, where the conductive heat loss to the burner starts to become significant. A further reduction in the burner flow rate results in a reduction of the burning velocity and a subsequent increase in the standoff distance. This is in direct contrast to variation of the flame radii predicted using constant f o values of Fig. 6, where changes due to conductive heat losses are neglected. Figure 8 shows the measured laminar flame speeds of ethylene/air mixtures determined over the equivalence range of 3.0–4.5. The uncertainty in the flame speed data arise mainly from the uncertainty in the measurement of flame position. For comparison, we also plotted the flame speed values reported for non-sooting flames [18,19]. The figure illustrates that with the experimental technique reported herein we were able to extend the laminar flame speed measurements to a much richer equivalence ratios. Figure 8 also shows
Fig. 7. Spherical flame radius versus mass flow rate of reactants. Experimental data are given by the symbols. The solid lines indicate the expected result for spherical flames propagating at a constant velocity (obtained from Fig. 6), assuming no conductive heat loss to the burner.
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Fig. 8. Experimental (symbols) and computed (lines) laminar flame speeds of atmospheric ethylene/air mixtures. The open and filled circles are taken from previous counterflow results of [18,19] for non-sooting flames, respectively. The squares are from the current experimental results. The computed values (solid line) were obtained using PREMIX for non-radiative planar flame. The radiative solution (dotted line) is also shown but lies almost on top of the non-radiative one.
burning velocities computed for adiabatic (solid line) and optically thin (dashed line) planar flames using the mechanism of Ref. [12], in which soot formation was not considered. It is observed that the radiative solution is nearly indistinguishable form the adiabatic counterpart, but since soot chemistry is not included the effect of radiation is severely under-estimated. Here the radiative species considered were only CH4, H2O, CO, and CO2. For this reason, the gas-phase reaction model predicted the non-sooting flame speeds [18,19] quite well, but it grossly over-predicted the propagation rates of sooting flames. This discrepancy was expected since the dominant losses due to soot radiation and the chemistry effect of soot formation were not considered here. The discrepancy becomes larger at higher equivalence ratios, again because of more intense soot chemistry. In addition, the thermal thicknesses measured using RSD were twice of those of the numerical calculations. Although the precise extinction condition cannot be readily determined using the current experimental method, we observed that flames with burning velocities <2 cm/s could not be stabilized within the 2.2-s drop duration. For these mixtures (U P 5), the premixed spherical flame continuously expanded during the drop and became dimmer and more diffuse. This blowoff point, indicative of near-extinction conditions, actually occurred far from the flammability limit predicted using the optically thin radiative model that considered only gas-phase chemistry. This discrepancy can be further explored by observing the response of the burning velocity to nitrogen dilution. By adding nitrogen to the premixture at a fixed equiv-
Fig. 9. Experimental and computed laminar flame speeds of sooting ethylene/O2/N2 mixtures as a function of nitrogen dilution percentage. The experimentally observed blowoff is shown by the double line, while the computed radiative extinction solution is marked by the turning point.
alence ratio, flame extinction can be brought about with increasing amount of dilution. Figure 9 shows the effect of nitrogen dilution on the experimental and numerical burning velocities for sooting ethylene flames (U = 3.0 and 3.5). The extent of dilution was characterized by the molar percentage of nitrogen (equal to 0.79 for normal air) in the oxidizer mixture (N2 + O2). The experimental flame speeds are compared to numerical results obtained with planar premixed flame calculation using an optically thin radiation model. Again the effect of soot formation was not considered herein. The turning point at the dilution limit was determined using the flame controlling method of Nishioka et al. [20]. The results show that the experimental burning velocities were much lower than the calculated ones. The nitrogen dilutions needed to reach extinction were also much smaller than the computational counterpart. The numerical simulation indicated that to reach flame extinction the oxidizer needed to be composed of about 89% nitrogen, compared to about 81–82% in the experiments. We should note that these results correspond to replacing about 50% of the air with nitrogen in the calculations, versus replacing only 12–15% in the experiment. Again, these differences are expected since the soot chemistry and radiative heat losses due to soot would inevitably influence the extinction of fuel-rich flames. 5. Conclusions Using a spherical premixed flame configuration, we examined the propagation rates of premixed ethylene/air mixtures over the equivalence range of 3.0–4.5. The spherical flame was stabilized in
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experiments performed under microgravity. The resulting flames and their temperature profiles were studied in detail via the use of direct and rainbow Schlieren deflectometry (RSD) imaging. Such measurements have not previously been demonstrated, particularly for sooting flames and close to the fuel-rich flammability limit. It was shown that at small flow injection rates, the flame is dominated by conductive heat loss to the burner. As the flow rate, and thus the flame standoff distance, is increased, it was demonstrated that the flame is stabilized purely due to flow divergence. Under such conditions, it is possible to determine the laminar flame speed of the unstretched flame with reasonable accuracy. Laminar flame speed measurements were made for ethylene/air mixtures in the equivalence range of 3.0 to 4.5 (S ou values in the range of 2–7 cm/s), with and without further nitrogen dilution. The S ou values of sooting flames are about half of those predicted using adiabatic and optically thin models. Similarly, the experimental thermal thicknesses are twice of the calculated values. Nitrogen dilution of sooting spherical flames reduces their propagation rate in a non-linear fashion, with very little change in flame speeds observed for low dilution levels. As expected, numerical simulation with detailed reaction kinetics without considering soot formation and radiative losses due to soot over-predicts the laminar flame speed. In addition, the experimental flame extinction limits due to nitrogen dilution were substantially smaller than those predicted. The experimental methods and results documented herein illustrate that it is now possible to examine the rate of laminar flame propagation across the entire range of equivalence ratios, including sooting mixtures. This work also demonstrates an additional experimental tool that can be employed to examine the propagation and inherent soot chemistry in laminar premixed flames in a consistent fashion.
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References [1] H. Bockhorn (Ed.), Soot Formation in Combustion—Mechanisms and Models, Springer-Verlag, Berlin, 1994, p. 165. [2] M. Frenklach, H. Wang, Proc. Combust. Inst. 23 (1990) 1559–1566. [3] M.D. Smooke, C.S. McEnally, L.D. Pfefferle, R.J. Hall, M.B. Colket, Combust. Flame 117 (1999) 117– 139. [4] H. Richter, J.B. Howard, Prog. Energy Combust. Sci. 26 (2000) 565–608. [5] B. Zhao, Z. Yang, M.V. Johnston, H. Wang, A.S. Wexler, M. Balthasar, M. Kraft, Combust. Flame 133 (2003) 173–188. [6] J. Singh, R.I.A. Patterson, M. Kraft, H. Wang, Combust. Flame, 145 (2006) 117–127. [7] A. D’anna, A. Violi, A. D’alessio, A.F. Sarofim, Combust. Flame 127 (2001) 1995–2003. [8] A.F. Ibarreta, C.J. Sung, T. Hirasawa, H. Wang, Combust. Flame 140 (2005) 93–102. [9] J.A. Eng, D.L. Zhu, C.K. Law, Proc. Combust. Inst. 25 (1994) 1711–1718. [10] J. Qian, J.K. Bechtold, C.K. Law, Combust. Flame 110 (1997) 78–91. [11] R.J. Kee, J.F. Grcar, M.D. Smooke, J.A. Miller, Sandia Report SAND85-8240, Sandia National Laboratories, Livermore, California, 1985. [12] H. Wang, A. Laskin, available at
Acknowledgment This work was supported by the NASA Microgravity Combustion Program with Dr. Merrill King as contract monitor.
Comment L.P.H. de Goey, TU Eindhoven, Netherlands, The burning velocity and structure of curved (but unstructured) flames is different from flat flames. J.A. van Oijen et al. [1] Proceedings Combustion Institute 2005 showed
that SL is not dependent on curvature only at the inner layer position. This is in agreement with theoretical analysis. This means that SL should be derived at Vi being the radius of the inner layer. If the unburned flame
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boundary is used this leads to an error. Is this effect taken corrected in the results?
Reference [1] J.A. van Oijen, G.R.A. Groot, R.J.M. Bastiaans, L.P.H. de Goey, Proc. Combust. Inst. 30 (2005) 657.
Reply. In this work, we define the isotherm associated with the location of maximum heat release as the flame location. As such, the burning flux determined from this reference location is essentially independent of the curvature effect. This is akin to measuring the burning velocity at the inner layer as commented.
Proceedings of the Combustion Institute 31 (2007) 1047–1054
Combustion Institute www.elsevier.com/locate/proci
Experimental characterization of premixed spherical ethylene/air flames under sooting conditions Alfonso F. Ibarreta a b
a,1
, Chih-Jen Sung
a,*
, Hai Wang
b
Department of Mechanical and Aerospace Engineering, Case Western Reserve University, Cleveland, OH 44106, USA Department of Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, CA 90089, USA
Abstract Purely curved, premixed, sooting ethylene/air flames were studied using a spherical porous burner under microgravity. These weak, fuel-rich flames were shown to be stabilized without the complications of hydrodynamic straining and conductive heat loss to the burner. Using the rainbow Schlieren deflectometry (RSD) optical system, the flames were imaged and the sooting flame speeds and flame thicknesses of ethylene/air mixtures were determined in the equivalence ratio range of 3.0–4.5, and for selected equivalence ratios, with additional nitrogen dilution. Numerical results obtained with detailed reaction kinetics and transport properties, but without considering soot chemistry and radiation, were shown to over-predict the flame intensity. Ó 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Burning velocity; Ethylene combustion; Microgravity combustion; Soot; Rainbow Schlieren deflectometry
1. Introduction Laminar premixed flames and their propagation rates have been studied traditionally in the non-sooting regime. Experimentally, it is far more difficult to obtain reliable, unambiguous flame propagation data for sooting flames. Computationally, soot chemistry has been (and still is) lesser known than the main flame chemistry of chain branching, propagation, and termination. The recent advances in our understanding of soot chemistry have somewhat changed this situation. In particular, the novel development of detailed soot models (e.g. [1–4]) has significantly increased *
Corresponding author. Fax: +1 216 368 6445. E-mail address: [email protected] (C.-J. Sung). 1 Exponent Failure Analysis Associates, Natick, MA 01760, USA.
our predictive capabilities. Recent studies show that when coupled with efficient solvers of aerosol dynamics, such soot models are able to simulate the particle size distribution function and thus the surface area of soot formed in burner-stabilized flames with a reasonable degree of accuracy [5–7]. These advances have raised the question of whether the soot chemistry should be viewed as an integral part of flame chemistry. There have also been notable advances in our ability to model the structure and properties of non-premixed sooting flames (e.g. [4]). These advances were accomplished by considering detailed reaction kinetics, soot chemistry, and mass/heat transport. Parallel efforts, that consider soot chemistry in the property and dynamics of premixed flames, however, have not yet been made. To this end, we note that the sooting flame speed embeds several important and strongly coupled properties of significant fundamental
1540-7489/$ - see front matter Ó 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2006.08.029
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interests. These properties include the rate of fuel pyrolysis and oxidation, the sooting tendency of the fuel, heat release rates, and heat dissipation through radiation. Advances in this area will have to first rely on the availability of experimental methodologies to characterize the propagation and extinction of premixed sooting flames. The study of premixed sooting flame speeds has been hampered by a large number of problems, including flame stretch, finite residence times, and conductive heat losses. Burner-stabilized flat flames are not suitable for the measurement of sooting flame speeds because of the small flame standoff distance and the resulting radical destruction and conductive heat loss at the burner surface. In our previous work [8], we also demonstrated that the well-known counterflow configuration is unsuitable for premixed sooting flames, due to the dependence of soot layer characteristics on the stretch rate and the influence of buoyancy at low stretch rates. An alternate experimental method was recently proposed, utilizing a quasi-one-dimensional spherical flame geometry to establish a premixed freely propagating sooting flame [8]. As part of that study, stationary spherically symmetric sooting flames were established under microgravity conditions. The flames were then closely studied using the non-intrusive rainbow Schlieren deflectometry (RSD) optical method. It was demonstrated that the spherical geometry is capable of stabilizing flames with negligible conductive heat loss to the burner, and that the method was successfully used to generate the sooting flame speed for an ethylene/air mixture at atmospheric pressure and an equivalence ratio of 3.5 [8]. In the current work, we report experimental measurements for the laminar flame speeds of ethylene/air mixtures over the equivalence ratio range of 3.0–4.5 and for selected equivalence ratios, with additional nitrogen dilution. 2. Theoretical background The spherical burner geometry—in the absence of gravity—can stabilize a premixed flame with negligible conductive heat loss and flame stretch. Theoretically, stationary, burner-generated cylindrical and spherical premixed flames were studied using asymptotics with one-step reaction kinetics and constant properties [9,10]. It is well accepted that because the flow direction is always normal to the flame surface, these ideally-curved, stationery flames are not subject to aerodynamic straining, and that the flame standoff distance can be varied by simply adjusting the mixture flow rate. Obviously, ideally curved flames must be established in a microgravity environment in order to eliminate buoyancy-induced flame asymmetry. Indeed, nearly symmetric, non-sooting cylindrical
flames were experimentally demonstrated in that environment [9]. The burning velocity of a stationary spherical flame can be unambiguously defined, if the heat release zone is thin compared to the flame standoff distance. The burning velocity may be obtained by determining the mass flux through the surface of maximum heat release. Since the flow field is symmetric and at steady-state, the local mass flow rate is the same for all radii, making the local mass flux a function only of the radial position. If the mass flow rate through the burner surface, m_ s , is known, the burning flux, f, of a spherical flame surface of radius, rf, is ð1Þ f ¼ m_ S = 4pr2f : A spherical premixed flame may be stabilized by either flow divergence or conductive heat loss to the burner surface, depending on the injection velocity. Upstream conductive heat loss may be made negligible if the flame standoff distance is sufficiently large, allowing the flame to be stabilized purely through flow divergence. This characteristic of the spherical geometry enables the establishment of a freely propagating flame at a finite injection flow rate of the unburned mixture. To substantiate the above statements we carried out detailed calculations for spherical, steady-state ethylene/air premixed flames using PREMIX [11] with detailed reaction kinetics [12] (without soot). The cross-section area of the spherical configuration was explicitly specified as a function of the radial distance in the PREMIX code. Both non-radiative and optically thin radiation models were employed and compared, in order to assess the influence of geometry on the radiative flame solution. Radiative species considered in the calculations included CH4, H2O, CO, and CO2. A small burner radius (0.2 cm) was used here to emphasize the effect of curvature on flame propagation. Figure 1 shows the burning velocity computed for an ethylene/air mixture with equivalence ratio U = 3.5, unburned gas temperature equal to 298 K, as a function of normalized burner flow rate. The normalized burner flow rate is defined herein as: m_ s =ð4pR2 f o Þ, where R is the burner radius, (4pR2) is the burner surface area, and f o is the burning mass flux of the unstretched adiabatic planar flame. Thus, when the normalized value is unity, the mass flux at the burner surface is equal to that of the freely propagating unstretched planar flame. As shown in Eq. (1), the critical parameter in determining the flame propagation velocity is the flame radius rf, defined herein as the location of the maximum heat release rate. Since the burner mass flow rate m_ s is prescribed, the mass flux through the flame surface, f, is readily obtained from Eq. (1) with a known rf; and the burning velocity is subsequently determined by dividing f
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Fig. 1. Numerically-predicted burning velocities of stable spherical flames for an ethylene/air mixture (U = 3.5), with (dashed line) and without (solid line) radiative heat losses. Solutions were obtained using a revised version of the PREMIX code [11] and detailed reaction kinetics [12] (without soot). The radiative case was obtained with an optically thin radiation model.
by the density of unburned mixture qu. Figure 1 clearly shows that for small normalized burning flow rate (<4), the burning velocity is reduced due to conductive heat loss to the burner. At large flow rates, the burning velocity approaches a constant value, as expected, for both non-radiative and optically thin radiative flames. At the limit of large flow rates, flow divergence is the only mechanism of flame stabilization and the burning velocity becomes independent of the flow rate. In this limit for the non-radiative case, the burning flux f becomes the fundamental laminar burning flux f o [9,10], and the laminar flame speed can be obtained as S ou ¼ f o =qu . It is also interesting to note that for the current optically thin calculation radiative heat losses have a negligible effect on the burning velocity, so long as a large flow rate is used. 3. Experimental methodology The experimental setup employed in this study is described in detail in Ref. [8]. A microgravity rig (shown in Fig. 2a) was used to perform experiments at the 2.2-Second Drop Tower Facility at NASA Glenn Research Center. The experimental rig contained the combustion chamber (center of rig), along with the required gas-flow and power systems (bottom shelf) and optical diagnostics (middle shelf). The compact optical setup (shown in Fig. 2b) and diagnostics are also described in detail elsewhere [13]. During a microgravity experiment, a spherical flame was stabilized around a porous spherical burner by introducing a fuel/air mixture through a small tube, located at the bottom of the burner sphere. Results reported herein were obtained using a spherical porous brass burner of 0.92 cm
Fig. 2. Illustration showing: (a) rig layout and (b) setup of RSD and chemiluminescence imaging systems.
in diameter and with a pore size of 8 lm. The fuel/air mixtures were ignited in a slightly oxidizing atmosphere (10% O2 and 90% N2, by volume) about a fraction of a second prior to each drop. We used the non-intrusive RSD optical technique [14,15] to determine the position of the reaction zone, the flame thickness, and the temperature distribution [8,13]. The present RSD setup is similar to that of Ref. [16]. The technique was chosen over other methods because of its simplicity and its ability to provide quantitative information under sooting conditions. The RSD measures the angular deflection of light as it passes through the test section. The deflection of light rays traveling through the flame is an integral of the local deflections along the beam path. The radial distribution of index of refraction can therefore be obtained via a deconvolution of the raw data. Once the index of refraction field is determined, a correlation function between the index of refraction and the temperature is needed to finally obtain the radial thermal profiles. The local index of refraction depends mostly on the gas density (and therefore temperature), and to a lesser extent, on the gas composition. In this work, the species concentration profiles were computed in a similar manner as that described in Section 2. The local refractivities were then calculated using the computed gas density and species concentrations and the database of Ref. [17]. Sample radial temperature profiles obtained using the RSD method in sooting spherical flames can be found in Ref. [8]. These profiles
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are not included here. The flame location was defined as the point in the temperature profile corresponding to the maximum heat release. It is estimated that the uncertainty is of the order of ±0.3 mm for the position of maximum heat release [8]. Figure 3 shows the time evolution of a sample flame (U = 3.5 and m_ s =0.046 g/s) resulting from a sequence of direct imaging (top) and RSD imaging (bottom). The flame was ignited at the top of the tower prior to the drop (t = –0.5 s). The direct imaging shows the expansion of an outer diffusion flame. The RSD sequence in Fig. 3 illustrates that the inner premixed flame very quickly stabilizes around the burner (within 0.5–0.8 s after the rig is released). The advantage of RSD is clearly demonstrated, as it is able to image the inner layer of quite heavily sooting flames. Although the soot particles can create a shade in the RSD image when the soot loading is very high, in the cases studied herein the soot concentration is low enough to allow most of the rays to penetrate the soot layer unhindered. This allows for the use of the RSD technique even under moderately sooting conditions, where direct imaging of the reaction zone would otherwise not be possible. As discussed earlier, the spherical geometry allows the burning velocity to be obtained from the stabilized flame by accurately measuring its radial location using RSD. This optical technique is also capable of obtaining the temperature field
even under heavily sooting conditions, as demonstrated in Ref. [13]. The sooting tendency of the stable premixed flame is shown as a function of equivalence ratio in Fig. 4. These images were collected near the end of each drop. The decrease in the apparent soot luminosity with an increase in the equivalence ratio may be attributed to a reduced radiation intensity as well as a reduced soot production, both of which are the result of lowered flame temperature. It is seen that the resolution of RSD was not limited by the soot luminosity. With an increase in the equivalence ratio, the RSD contours become more diffuse, indicating an increase of the flame thickness. For U P 5.0 the results show that the inner premixed flame had not stabilized within the drop time. The finite 2.2 s duration and our current experimental setup prevent the study of mixtures with burning velocities less than 2 cm/s, but it is expected that the actual flammability limit lies close to this point. An additional limitation of the present experiment may be also inferred by examining the computational results shown in Fig. 1. The fact that the mass flux through the burner surface must be several times that of the burning mass flux (>4) to eliminate conductive heat loss, coupled with the property of the porous material used for the spherical burner, limited our measurements to mixtures with burning velocities <7 cm/s. On the other hand, the current experimental method is ideal to study very weak flames where other methods are unsuitable.
Fig. 3. Sample time sequence of a microgravity drop (U = 3.5 and m_ s ¼ 0:046 g=sÞ. Top panels: direct imaging; bottom panels: RSD imaging. Time t = 0 corresponds to the release of the drop package. Ignition occurred at –0.5 s before the drop. The retractable igniter is seen in the first frame of the RSD image. The minor asymmetry on the left side of the RSD image is caused by the intrusion of the thermocouple.
Fig. 4. Collage showing the spherical premixed flames prior to the end of the drop, as a function of equivalence ratio. Top panels: direct imaging; bottom panels: RSD imaging.
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4. Results and discussion Images of ethylene/air flames with U = 3.0 are shown in Fig. 5 for several representative burner injection flow rates. It is clear from these images that the soot luminosity was small at small injection rates (60.065 g/s), where conductive heat loss lowers the flame temperature and thus soot production and luminosity. At larger injection rates (P0.075 g/s) the soot luminosity becomes stronger and visually indistinguishable at varying injection rates. The RSD images illustrate how the flame standoff distance increases with increasing burner flow rate. Figure 6 shows the experimentally determined burning mass flux versus the normalized burner flow rate over a range of equivalence ratios. Qualitatively, the experimental results shown here are similar to that calculated in Fig. 1. That is, as the burner flow rate increases, the measured burning flux initially increases due to a decrease in the conductive heat loss to the burner. Once the burner flow rate has increased beyond a critical value, the flame burning flux becomes independent of the injection flow rate. This constant burning flux value was defined herein to be the fundamental burn-
Fig. 5. Collage showing the spherical premixed flames (U = 3.0) prior to the end of the drop, as a function of burner flow rate. Top panels: direct imaging; bottom panels: RSD imaging.
Fig. 6. Measured burning mass flux versus normalized burner flow rate.
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ing mass flux f o. We should note that in plotting Fig. 6, the value of fowas used to calculate the normalized burner flow rate, m_ s =ð4pR2 f o Þ. Thus an iterative procedure was employed in order to obtain Fig. 6. To further illustrate the validity of data shown in Fig. 6 and their interpretation, we plot in Fig. 7 the measured flame radius as a function of burner flow rate for several representative U values. The theoretical radii of a spherical flame propagating at a constant burning velocity, S ou , denoted by lines, are also included for comparison, showing that without conductive heat loss the flame radius generally increases with an increase in the mass flow rate. It is seen from the experimental results that the flame initially approaches the burner surface (located at a radius of 0.46 cm) as the flow rate is decreased. A minimum standoff distance is eventually established, where the conductive heat loss to the burner starts to become significant. A further reduction in the burner flow rate results in a reduction of the burning velocity and a subsequent increase in the standoff distance. This is in direct contrast to variation of the flame radii predicted using constant f o values of Fig. 6, where changes due to conductive heat losses are neglected. Figure 8 shows the measured laminar flame speeds of ethylene/air mixtures determined over the equivalence range of 3.0–4.5. The uncertainty in the flame speed data arise mainly from the uncertainty in the measurement of flame position. For comparison, we also plotted the flame speed values reported for non-sooting flames [18,19]. The figure illustrates that with the experimental technique reported herein we were able to extend the laminar flame speed measurements to a much richer equivalence ratios. Figure 8 also shows
Fig. 7. Spherical flame radius versus mass flow rate of reactants. Experimental data are given by the symbols. The solid lines indicate the expected result for spherical flames propagating at a constant velocity (obtained from Fig. 6), assuming no conductive heat loss to the burner.
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Fig. 8. Experimental (symbols) and computed (lines) laminar flame speeds of atmospheric ethylene/air mixtures. The open and filled circles are taken from previous counterflow results of [18,19] for non-sooting flames, respectively. The squares are from the current experimental results. The computed values (solid line) were obtained using PREMIX for non-radiative planar flame. The radiative solution (dotted line) is also shown but lies almost on top of the non-radiative one.
burning velocities computed for adiabatic (solid line) and optically thin (dashed line) planar flames using the mechanism of Ref. [12], in which soot formation was not considered. It is observed that the radiative solution is nearly indistinguishable form the adiabatic counterpart, but since soot chemistry is not included the effect of radiation is severely under-estimated. Here the radiative species considered were only CH4, H2O, CO, and CO2. For this reason, the gas-phase reaction model predicted the non-sooting flame speeds [18,19] quite well, but it grossly over-predicted the propagation rates of sooting flames. This discrepancy was expected since the dominant losses due to soot radiation and the chemistry effect of soot formation were not considered here. The discrepancy becomes larger at higher equivalence ratios, again because of more intense soot chemistry. In addition, the thermal thicknesses measured using RSD were twice of those of the numerical calculations. Although the precise extinction condition cannot be readily determined using the current experimental method, we observed that flames with burning velocities <2 cm/s could not be stabilized within the 2.2-s drop duration. For these mixtures (U P 5), the premixed spherical flame continuously expanded during the drop and became dimmer and more diffuse. This blowoff point, indicative of near-extinction conditions, actually occurred far from the flammability limit predicted using the optically thin radiative model that considered only gas-phase chemistry. This discrepancy can be further explored by observing the response of the burning velocity to nitrogen dilution. By adding nitrogen to the premixture at a fixed equiv-
Fig. 9. Experimental and computed laminar flame speeds of sooting ethylene/O2/N2 mixtures as a function of nitrogen dilution percentage. The experimentally observed blowoff is shown by the double line, while the computed radiative extinction solution is marked by the turning point.
alence ratio, flame extinction can be brought about with increasing amount of dilution. Figure 9 shows the effect of nitrogen dilution on the experimental and numerical burning velocities for sooting ethylene flames (U = 3.0 and 3.5). The extent of dilution was characterized by the molar percentage of nitrogen (equal to 0.79 for normal air) in the oxidizer mixture (N2 + O2). The experimental flame speeds are compared to numerical results obtained with planar premixed flame calculation using an optically thin radiation model. Again the effect of soot formation was not considered herein. The turning point at the dilution limit was determined using the flame controlling method of Nishioka et al. [20]. The results show that the experimental burning velocities were much lower than the calculated ones. The nitrogen dilutions needed to reach extinction were also much smaller than the computational counterpart. The numerical simulation indicated that to reach flame extinction the oxidizer needed to be composed of about 89% nitrogen, compared to about 81–82% in the experiments. We should note that these results correspond to replacing about 50% of the air with nitrogen in the calculations, versus replacing only 12–15% in the experiment. Again, these differences are expected since the soot chemistry and radiative heat losses due to soot would inevitably influence the extinction of fuel-rich flames. 5. Conclusions Using a spherical premixed flame configuration, we examined the propagation rates of premixed ethylene/air mixtures over the equivalence range of 3.0–4.5. The spherical flame was stabilized in
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experiments performed under microgravity. The resulting flames and their temperature profiles were studied in detail via the use of direct and rainbow Schlieren deflectometry (RSD) imaging. Such measurements have not previously been demonstrated, particularly for sooting flames and close to the fuel-rich flammability limit. It was shown that at small flow injection rates, the flame is dominated by conductive heat loss to the burner. As the flow rate, and thus the flame standoff distance, is increased, it was demonstrated that the flame is stabilized purely due to flow divergence. Under such conditions, it is possible to determine the laminar flame speed of the unstretched flame with reasonable accuracy. Laminar flame speed measurements were made for ethylene/air mixtures in the equivalence range of 3.0 to 4.5 (S ou values in the range of 2–7 cm/s), with and without further nitrogen dilution. The S ou values of sooting flames are about half of those predicted using adiabatic and optically thin models. Similarly, the experimental thermal thicknesses are twice of the calculated values. Nitrogen dilution of sooting spherical flames reduces their propagation rate in a non-linear fashion, with very little change in flame speeds observed for low dilution levels. As expected, numerical simulation with detailed reaction kinetics without considering soot formation and radiative losses due to soot over-predicts the laminar flame speed. In addition, the experimental flame extinction limits due to nitrogen dilution were substantially smaller than those predicted. The experimental methods and results documented herein illustrate that it is now possible to examine the rate of laminar flame propagation across the entire range of equivalence ratios, including sooting mixtures. This work also demonstrates an additional experimental tool that can be employed to examine the propagation and inherent soot chemistry in laminar premixed flames in a consistent fashion.
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References [1] H. Bockhorn (Ed.), Soot Formation in Combustion—Mechanisms and Models, Springer-Verlag, Berlin, 1994, p. 165. [2] M. Frenklach, H. Wang, Proc. Combust. Inst. 23 (1990) 1559–1566. [3] M.D. Smooke, C.S. McEnally, L.D. Pfefferle, R.J. Hall, M.B. Colket, Combust. Flame 117 (1999) 117– 139. [4] H. Richter, J.B. Howard, Prog. Energy Combust. Sci. 26 (2000) 565–608. [5] B. Zhao, Z. Yang, M.V. Johnston, H. Wang, A.S. Wexler, M. Balthasar, M. Kraft, Combust. Flame 133 (2003) 173–188. [6] J. Singh, R.I.A. Patterson, M. Kraft, H. Wang, Combust. Flame, 145 (2006) 117–127. [7] A. D’anna, A. Violi, A. D’alessio, A.F. Sarofim, Combust. Flame 127 (2001) 1995–2003. [8] A.F. Ibarreta, C.J. Sung, T. Hirasawa, H. Wang, Combust. Flame 140 (2005) 93–102. [9] J.A. Eng, D.L. Zhu, C.K. Law, Proc. Combust. Inst. 25 (1994) 1711–1718. [10] J. Qian, J.K. Bechtold, C.K. Law, Combust. Flame 110 (1997) 78–91. [11] R.J. Kee, J.F. Grcar, M.D. Smooke, J.A. Miller, Sandia Report SAND85-8240, Sandia National Laboratories, Livermore, California, 1985. [12] H. Wang, A. Laskin, available at
Acknowledgment This work was supported by the NASA Microgravity Combustion Program with Dr. Merrill King as contract monitor.
Comment L.P.H. de Goey, TU Eindhoven, Netherlands, The burning velocity and structure of curved (but unstructured) flames is different from flat flames. J.A. van Oijen et al. [1] Proceedings Combustion Institute 2005 showed
that SL is not dependent on curvature only at the inner layer position. This is in agreement with theoretical analysis. This means that SL should be derived at Vi being the radius of the inner layer. If the unburned flame
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boundary is used this leads to an error. Is this effect taken corrected in the results?
Reference [1] J.A. van Oijen, G.R.A. Groot, R.J.M. Bastiaans, L.P.H. de Goey, Proc. Combust. Inst. 30 (2005) 657.
Reply. In this work, we define the isotherm associated with the location of maximum heat release as the flame location. As such, the burning flux determined from this reference location is essentially independent of the curvature effect. This is akin to measuring the burning velocity at the inner layer as commented.