Studies on Combustion Characteristics and Flame Length of Turbulent

Energy & Fuels 2007, 21, 1459-1467

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Studies on Combustion Characteristics and Flame Length of Turbulent Oxy-Fuel Flames Ho Keun Kim and Yongmo Kim* Department of Mechanical Engineering, Hanyang UniVersity 17, Haengdang-Dong, Seongdong-Ku, Seoul, 133-792, Korea

Sang Min Lee and Kook Young Ahn Eco-Machinery Engineering Department, Korea Institute of Machinery & Materials, Jang-Dong 171, Yusung-Ku Taejon, 305-660, Korea ReceiVed July 29, 2006. ReVised Manuscript ReceiVed January 6, 2007

Combustion characteristics, flame structure, and length in an oxy-fuel combustor have been experimentally investigated for a wide range of fuel nozzle diameters and fuel and oxidizer velocities. Measurements are made for the model combustor (Combustor I) as well as for the actual oxy-fuel combustor (Combustor II). The experimental results for Combustor I clearly reveal basic features of the air-fuel and oxy-fuel flames in terms of flame length and structure. It was found that the flame length of the oxy-fuel flame in Combustor II decreases with increased fuel velocity or oxygen velocity due to greater turbulent mixing and entrainment. On the basis of the measured oxy-fuel flame lengths, a modified correlation between dimensionless flame lengths and a fire Froude number has been proposed for the two types of oxy-fuel combustors.

1. Introduction Oxy-fuel combustion is gradually gaining popularity in the industrial production of glass, aluminum, iron, and steel because of its inherent advantages. These include high combustion efficiency, low volumes of exhaust gas, low fuel consumption, high melting capacity, and low NOx emission. Energy efficiency can be greatly enhanced by utilizing oxy-fuel combustion because the unnecessary heating of nitrogen in air is eliminated. In pure oxygen combustion, the volume of exhaust gas significantly decreases and zero NOx emission can theoretically be achieved. Moreover, oxy-fuel combustion technology has been widely recognized as one of the most effective ways to capture CO2 from exhaust gas as well as to comply with future international CO2 regulations agreed upon at the 1997 Kyoto Protocol. Because exhaust gas contains only CO2 and H2O, CO2 gas is easily captured from exhaust gas by a condensing process.1-2 As the progress of oxygen separation technologies may possibly lead to continuous cuts in the cost of oxygen production, oxy-fuel combustion could be increasingly popular in hightemperature industrial combustion systems. Although detailed information on oxy-fuel flame characteristics is essential to the design and development of an oxy-fuel combustor, experimental data on flame structure and combustion processes are quite limited. Emission characteristics of the 0.03 MW oxy-fuel combustor had been experimentally investigated under a wide operating range of velocities and quarl angles by Kim et al.3 They showed * Corresponding author tel.: +82-2-2220-0428; fax: +82-2-2297-0339; e-mail: [email protected]. (1) Baukal, C. E. Industrial Burners Handbook; CRC Press: Boca Raton, FL, 2004. (2) Baukal, C. E. Oxygen-Enriched Combustion; CRC Press: Boca Raton, FL, 1998.

that the flame length decreases as either the fuel or oxidizer velocity increases because of the increased turbulent intensity. In the case of a relatively low fuel velocity (Uf < 40 m/s), the flame length substantially decreases with an increasing oxidizer velocity. However, in the case of very high fuel velocity (Uf > 80 m/s), the flame length slightly decreases with an increasing oxidizer velocity up to 40 m/s. Ditaranto et al.4-6 demonstrated that oxy-fuel combustion has great potential to increase thermal efficiency as well as to reduce NOx emission. They also showed that NOx emission levels are quite sensitive to the amount of air leaked into the combustion chamber. Baukal and Gebhart7 showed a thermal radiation increase with an enriched oxygen level in the oxidizer as well as with an increased firing rate. They also found that oxygen enhancement increases the flame temperature up to nearly 3000 K and that NOx formation in the oxygen-enhanced flame is mainly controlled by thermal mechanisms. In the soot formation process of an oxygen-enhanced flame, the previous studies8-11 revealed (3) Kim, H. K.; Kim, Y.; Lee, S. M.; Ahn, K. Y. Energy Fuels 2006, 20, 2125-2130. (4) Ditaranto, M.; Sautet, J. C.; Samaniego, J. M. Exp. Fluids 2001, 30, 253-261. (5) Sautet, J. C.; Ditaranto, M. J.; Samaniego, M.; Charon, O. Int. Commun. Heat Mass Transfer 1999, 26, 647-656. (6) Sautet, J. C.; Salentey, L.; Ditaranto, M. Int. Commun. Heat Mass Transfer 2001, 28, 277-287. (7) Baukal, C. E.; Gebhart, B. Int. J. Heat Mass Transfer 1997, 40, 25392547. (8) Beltrame, A.; Porshnev, P.; Merchan, W. M.; Saveliev, A.; Fridman, A.; Kennedy, L. A.; Petrova, O.; Zhdanok, S.; Amouri, F.; Charon, O. Combust. Flame 2001, 124, 295-310. (9) Lee, K. O.; Megaridis, C. M.; Zelepouga, S.; Saveliev, A. V.; Kennedy, L. A.; Charon, O.; Ammouri, F. Combust. Flame 2000, 121, 323333. (10) Naik, S. V.; Laurendeau, N. M.; Cooke, J. A.; Smooke, M. D. Combust. Flame 2003, 134, 425-431.

10.1021/ef060346g CCC: $37.00 © 2007 American Chemical Society Published on Web 04/06/2007

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Kim et al.

that the soot formation rate and the soot particle agglomeration rate progressively increase as the oxygen content of the oxidizer increases. In the semi-industrial oxy-fuel furnace, Hedley et al.12 measured detailed profiles for temperature and species concentration including NOx, and they made a comparison of experimental data and numerical results. The OXYFLAME project13-15 of the International Flame Research Foundation provided useful in-flame data collected in oxy-fuel flames of thermal inputs from 0.8 to 1 MW, which include the profiles of axial velocity, temperature, species concentrations, total radiance, and total radiative fluxes at the furnace wall. In their study, the effects of nitrogen concentration and momentum injection strength on the NOx emission level are also experimentally analyzed. However, those previous studies on oxy-fuel flames were conducted under limited conditions. The design of the oxy-fuel combustor requires detailed information about the oxy-flame structure as well as more comprehensive research efforts that include sequences of trial-and-error testing, combustion measurements, and analysis. Moreover, in terms of the basic flame properties, the effects of fuel and oxidizer velocity on the flame length of oxy-fuel combustors are still relatively unexplored. Hence, the main motivation of the present study was to experimentally investigate the flame length of oxy-fuel combustors for a wide range of fuel and oxidizer velocities. In this study, measurements are carried out for the model combustor (Combustor I) as well as the actual oxy-fuel combustor (Combustor II) under various operating conditions. On the basis of the oxy-fuel flame lengths measured, the modified correlation between dimensionless flame length and a fire Froude number has been proposed for the two types of oxy-fuel combustor. Figure 1. Experimental apparatus and schematics for two combustors.

2. Experimental Setup Figure 1a shows the experimental apparatus of Combustor I. In Combustor I, gaseous fuel is injected through the inner nozzle and the oxidizer at a low injection velocity, supplied through the wide annular nozzle installed around the fuel nozzle. A stainless fuel nozzle is placed at the center and protrudes 50 mm from the burner inlet. In order to create a uniform oxidizer velocity, a glass layer (50-mm-long) and a honeycomb (100-mm-long) are installed. The relatively long, 800 mm fuel nozzle is designed to obtain a fully developed fuel stream, and the diameter of the annular oxidizer nozzle is 300 mm. In order to visualize the flame, a quartz tube 300 mm in diameter and 1200-mm-long is installed. A chemically pure grade (>99.9%) methane is used as the fuel, and oxygen (>99.9%) or air is used as the oxidizer. The flow rate of the oxidizer is 400 L/min, and its corresponding oxidizer velocity is 0.095 m/s. The flame length characteristics are experimentally investigated by varying fuel nozzle diameters over a wide range (1.6, 2.7, 4.4, and 7.7 mm). The flame length versus fuel injection velocity is measured by utilizing a video camera (Sony, DCR-TRV 30) that obtains one flame image with 1/60 s at F 2.0. The flame length was defined as (11) Naik, S. V.; Laurendeau, N. M. Combust. Flame 2002, 129, 112119. (12) Hedley, J. T.; Pourkashanian, M.; Williams, A. Combust. Sci. Technol. 1995, 108, 311-322. (13) Lallemant, N.; Breussin, F.; Weber, R.; Ekman, T.; Dugue, J.; Samaniego, J. M.; Charon, O.; Van Den, A. J.; Van Der, J.; Fujisaki, W.; Imanari, T.; Nakamura, T.; Iino, K. J. Inst. Energy 2000, 73, 169-182. (14) Lallemant, N.; Dugue, J.; Weber, R. Analysis of the Experimental Data Collected during the Oxyflame-1 and Oxyflame-2 Experiments; IFRF Doc: 1997, F85/y/4 Part 1; International Flame Research Foundation: Livorno, Italy, 1997. (15) Bollettini, U.; Breussin, F.; Lallemant, N.; Weber, R. Mathematical Modeling of Oxy-Natural Gas Flames; IFRF Doc: 1997, F85/y/6; International Flame Research Foundation: Livorno, Italy, 1997.

the time-averaged value of the recorded flame length for a sufficient time interval (2 min). Figure 1b shows the schematic diagram and experimental apparatus for an actual oxy-fuel combustor (Combustor II). In this combustor, the fuel is supplied through the inner central nozzle and the oxidizer is fed through the outer annular nozzle. Unlike Combustor I, Combustor II has a sudden expansion section because the diameter of the annular oxidizer nozzle is much smaller than that of the combustion chamber. The inlet flow velocities of the fuel and oxidizer can be varied by adjusting the diameters of the central and annular nozzles. Detailed dimensions of the combustor geometry and inlet conditions are listed in Table 1. The overall equivalence ratio for the combustion conditions of Combustor II is 0.98. The flow rates of methane and oxygen are fixed at 50 L/min (0.03 MW) and 101.6 L/min (3% dry O2 at exhaust), respectively. For Combustor II, the flame length is measured by varying fuel and oxygen velocities.

3. Results and Discussion In order to systematically investigate the combustion characteristics of the air-fuel and oxy-fuel flames, in terms of flame length and structure, this study begins with measurements for the model oxy-fuel combustor (Combustor I). The flame images of Combustor I for air-fuel and oxy-fuel flames are displayed in Figure 2. The structure and characteristics of the two flames are visualized for the range of the fuel injection velocities with a fuel nozzle diameter of 2.7 mm. On the basis of the Reynolds number at the fuel nozzle exit, the flames are classified as laminar flame (Re < 2000), transition flame (2000 e Re < 4000), or turbulent flame (Re g 4000).

Turbulent Oxy-Fuel Flames

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Table 1. Dimensions and Inlet Conditions of Combustor II diameter [mm] df

do (in)

2.7

4.7

3.65

5.65

4.21

6.21

5.16

7.16

7.29

9.29

velocity [m/s]

do (out)

uo

Reo

9.52 10.8 12.98 17.89 9.52 10.8 12.98 17.89 9.52 10.8 12.98 17.89 9.52 10.47 10.8 12.98 17.89 10.8 12.98 17.89

34.2 24.8 16.0 7.9 39.9 27.7 17.2 8.1 45.0 30.0 18.0 8.3 59.6 40.2 35.9 20.0 8.7 77.3 28.5 10.0

10 412 9553 8375 6554 9760 9000 7948 6290 9413 8704 7716 6177 8877 8398 8244 7352 5911 7370 6648 5447

uf

Ref

Frf

146.2

25 391

31.52

79.9

18 782

14.82

2.0

16 284

10.38

146.9

13 286

6.24

279.1

9404

2.64

As illustrated in Figure 2, at fuel injection velocities higher than 20 m/s, the turbulent air-fuel flames are lifted. Around the flame base of turbulent non-premixed lifted flames, the partially premixed zone appears by entraining oxidizer in the fuel jet zone in the upstream, unburned zone of the lifted flame.16 As a result, flame luminosity is substantially reduced for this turbulent non-premixed lifted flame. Since the laminar flame speed (320 cm/s) of the oxy-fuel flame is much higher than that (37 cm/s) of the air-fuel flame,17 the former is more stable. In the case of the air-fuel flame, the flame length is clearly reduced in the transition flame. However, in the same range (2000 e Re < 4000) of Reynolds numbers, the oxy-fuel flame yields laminar flame characteristics such that its flame length is increased with injection velocity. Before we discuss this distinctly different combustion characteristic of air-fuel and oxy-fuel flames, it is worthwhile to analyze the detailed structure of the laminar diffusion flamelets.18 Figure 3 shows the molecular viscosity versus the equivalence ratio, the temperature, and the mass fraction of CO and H2 in the mixture fraction space for air-fuel (CH4) and oxyfuel (CH4) equilibrium diffusion flamelets. GRI-3.019 is used as the chemical mechanism for methane-oxygen combustion. As shown in Figure 3a, compared to the air-fuel flame, the molecular viscosity of the oxy-fuel flame is 1.2 times larger at stoichiometric conditions and up to 1.5 times larger at the hottemperature lean or rich side. This distinctly different trend is mainly caused by the higher molecular viscosity, which corresponds to the higher flame temperature of the oxy-fuel flame. This implies that the transitional Reynolds number of the oxyfuel flame must be much higher than that of the air-fuel flame. Thus, the transitional regime of the oxy-fuel flame could be (16) Pitts, W. M. 22nd Symposium on Combustion, Univeristy of Washington, Seattle, WA, August 14-19, 1988; The Combustion Institute: Pittsburgh, PA, 1989; pp 809-816. (17) Kee, R. J.; Grcar, J. F.; Smooke, M. D.; Miller, J. A. A Fortran Program for Modeling Steady Laminar One-Dimensional Premixed Flames; Sandia National Laboratories Report SAND85-8240; Sandia National Laboratories: Livermore, CA, 1985. (18) Lutz, A. E.; Kee, R. J.; Grcar, J. F.; Rupley, F. M. Oppdif: A Fortran Program for Computing Opposed-Flow Diffusion Flames; Sandia National Laboratories Report SAND96-8243; Sandia National Laboratories: Livermore, CA, 1999. (19) Smith, G. P.; Golden, D. M.; Frenklach, M.; Moriarty, N. W.; Eiteneer, B.; Goldenber, M.; Bowman, C. T.; Hanson, R. K.; Song, S.; Gardiner, W. C.; Lissianski, V. V.; Qin, Z. GRI-Mech. http://www.me.berkeley.edu/gri-mech (accessed Jan 2006).

shifted to a much higher Reynolds-number range. However, as shown in Figure 2b, in the Reynolds-number range of 4000 e Re, the flame length of the oxy-fuel flame is nearly constant. These experimental results suggest that the transitional flame characteristics are not quite apparent for the oxy-fuel flame. Moreover, compared to the air-fuel flame, the oxy-fuel flame yields a much higher and broader distribution for temperature. This indicates that the chemical reaction of the oxy-fuel flame occurs within a much broader zone of the mixture fraction space. As shown in Figure 3b, extremely high CO and H2 levels exist in the rich region of the oxy-fuel diffusion flamelet, and peak CO (0.54) and H2 (0.06) levels are 10 times higher than those of the air-fuel flame, due to the fairly strong chemical dissociation of the high-temperature oxy-fuel flame. In turbulent oxyfuel flames, it is quite possible that the highly distributed CO and H2 in the upstream fuel-rich region are transported to the downstream regions, increasing the flame length through CO and H2 oxidation in the downstream region, which is slightly leaner than the center-line stoichiometric location. Figure 4a shows the measured flame length versus fuel velocity for a relatively small nozzle size (d ) 2.7 mm). Compared to the air-fuel flame, the flame length of the oxyfuel flame at the same injection velocity is approximately 40% shorter in the laminar flame regime and 10% shorter in the turbulent flame regime. In the present study, the flame length was defined as being from the fuel nozzle exit to the flame tip for all flames including lifted flames. Figure 4b and c display the flame width versus fuel velocity for a fuel nozzle diameter of 2.7 mm. At fuel velocities lower than 15 m/s, the oxy-fuel flame with higher molecular viscosity yields a slightly thicker flame in the laminar upstream zone (x ) 150 mm) compared to the air-fuel flame, while it generates a much thinner flame at the turbulent downstream zone due to the suppression of turbulent fluctuations in the highly viscous, lean side. On the other hand, at fuel velocities larger than 15 m/s where the air-fuel flame is lifted, the corresponding airfuel flame width is sharply increased around the upstream zone (x ) 150 mm) owing to flow redirection in the flame stabilization region, while it abruptly decreases in the downstream zone because of the decreased concentration fluctuation in the partially premixed mixture. Figure 5a shows the air-fuel flame length versus the fuel velocity for three nozzle diameters, excluding the smallest nozzle diameter (1.6 mm). The experimental data for the smallest nozzle diameter (1.6 mm) cannot be obtained because of the flame instability of this fuel injection condition. In the case of the largest nozzle diameter (7.7 mm), the flame length is measured up to 1000 mm. In the laminar flame region, flame length is increased by increasing the fuel velocity. The largest nozzle diameter yields the highest flame length, followed by the intermediate nozzle diameter (4.4 mm) and the small nozzle diameter (2.7 mm). In the transition flame region of these airfuel flames, the flame length is decreased by increasing fuel velocity, and this trend is quite apparent for the intermediate nozzle diameter. In the turbulent flame region, the flame length remains nearly invariable with increasing fuel velocity for the small nozzle diameter (2.7 mm) or gradually increases with the decreasing rate for the relatively large nozzle diameter (4.4 and 7.7 mm). Figure 5b shows the air-fuel flame length versus the fuel flow rate for three different nozzle sizes. In this figure, several important features of diffusion flames are identified. In the laminar flame region corresponding to low flow rates, the flame length is nearly independent of the initial jet diameter and


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Figure 2. Flame images versus fuel velocity for air-fuel and oxy-fuel flames.

depends highly on the flow rate. In the transitional flame region, the enhanced turbulence intensity increases with the flow rate, resulting directly in a decrease of the flame length. As the flow rate is further increased, flame lengths either remain almost constant with the small-diameter nozzle (2.7 mm) or increase with a decreasing flow rate with the relatively large-diameter nozzles (4.4 and 7.7 mm). This is a consequence of enhanced air entrainment and mixing rate, which are more or less proportional to the fuel flow rate. In the turbulent flame region, flame length is also observed to be strongly dependent on the nozzle diameter. These results are similar to Wohl et al.’s.20 Figure 6a presents the oxy-fuel flame length versus the fuel velocity for four nozzle diameters. In the laminar flame region, flame length is increased by increasing the fuel velocity. It is also found that the larger nozzle diameter yields a higher flame length. Unlike the air-fuel flame, the transition flame region is (20) Wohl, K.; Gazley, C.; Kapp, N. Third Symposium on Combustion and Flame and Explosion Phenomena, University of Wisconsin, Madison, WI, 1948; The Combustion Institute: Pittsburgh, PA, 1949; pp 288-299.

not clearly identified. As discussed above, in the same range of Reynolds numbers (2000 e Re < 4000), the oxy-fuel flame length gradually increases with the fuel velocity, and this trend is progressively more apparent for the larger nozzle diameter. In the turbulent flame region, increasing the fuel velocity results in flame lengths staying nearly constant for the three nozzle diameters (1.6, 2.7, and 4.4 mm), while decreasing the rate at the largest diameter nozzle (7.7 mm) gradually increases the flame length. Figure 6b shows the oxy-fuel flame length versus the fuel flow rate for four nozzle sizes. In the laminar flame region, the oxy-fuel flame has a trend similar to that of the air-fuel flame. The flame length is nearly independent of the initial jet diameter and highly dependent on the flow rate. The transition flame region is not clearly identified, which is quite different from the air-fuel flame. In the turbulent flame region, with an increasing fuel flow rate, the flame length is strongly dependent on the nozzle diameter but nearly independent of the fuel flow rate. These experimental results indicate that the flame length



Figure 3. Viscosity, temperature, and mass fraction for air-fuel and oxy-fuel flames.

characteristics of the oxy-fuel flame are qualitatively similar to those of the air-fuel flame in the laminar and turbulent regimes. This suggests that a relationship might exist between the flame length and the oxy-fuel flame, similar to the relationship of the air-fuel flame. On the basis of dimensional arguments of physics for turbulent diffusion flames, Delichatsios21 analyzed the transition characteristics from momentum to buoyancy-controlled turbulent jet diffusion flames. Thus, a correlation between characteristics of dimensionless flame lengths, including a fire Froude number and a burning fuel parameter for buoyancy and momentumdominant jets, has been expressed in the following equations, eqs 1 and 2, respectively.22

Frf )

Uf fs3/2 (Ff/F∞)1/4(∆T/T∞gdf)1/2

(1)

where ∆T/T∞ ) (Tad - T∞)/T∞ [Tad ) 2200 K (air), 3050 K

L* )

L f fs df(Ff/F∞)1/2

(2)

(O2); T∞ ) 300 K) is the characteristic temperature rise resulting from combustion. Here, Uf is the fuel velocity, Ff/F∞ is the density ratio, df is the fuel diameter, and fs is the stoichiometry. The fire Froude number (Frf) physically represents the ratio of initial jet momentum-induced force to buoyancy-induced force, as shown in eq 1. The dimensionless flame length in eq 2 is expressed in terms of the stoichiometric mixture fraction, the ratio of the nozzle fuel density to the ambient gas density, and the initial jet diameter. On the basis of these dimensionless parameters, two regimes can be defined. In a buoyancy(21) Delichatsios, M. A. Combust. Flame 1993, 92, 349-364. (22) Turns, S. R. An Introduction to Combustion; McGraw-Hill: New York, 2000.

dominated regime, the dimensionless flame length is expressed by the following correlation:

L* ) 13.5Frf2/5 for Frf < 5

(3)

On the other hand, in a momentum-dominated regime, the dimensionless flame length L* is constant.

L* ) 23 for Frf g 5

(4)

Therefore, the relationship of the flame length to the oxy-fuel flame could be expressed by using eqs 1 and 2 associated with Delichatsios’s relation. Figure 7 shows the length of jet flames correlated with flame Froude numbers. In this figure, the only turbulent flame data are plotted using Delichatsios’s flame Froude number and the dimensionless flame length parameter, because most industrial combustors are operated in the turbulent regime. The experimental data for hydrogen, propane, and methane labeled in Figure 7 correspond to measurements by Kalghatgi23 of the air-fuel flame. As shown in Figure 7, the measured dimensionless flame lengths of the air-fuel flame are well-correlated with Delichatsios’s relation expressed in eqs 3 and 4. However, the measured dimensionless flame length data of the oxy-fuel flame are considerably higher than those estimated by the original Delichatsios’s correlation. This discrepancy is attributed to the shortcomings of the original Delichatsios’s correlation based on the mathematical similarity solution and experimental results of air-fuel flames. As mentioned in Figure 3, unlike air-fuel flames, the highly enriched CO and H2 in the upstream, fuelrich region of the turbulent oxy-fuel flames are mainly transported in a streamwise direction, and a certain amount of the transported CO and H2 is available for oxidation in the (23) Kalghatgi, G. T. The 9th International Colloquium on Gas dynamics of Explosion and Reactive Systems, Poiters, France, 1983.


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Figure 5. Flame lengths for three nozzle diameters of the air-fuel combustor.

Figure 4. Flame lengths and widths at a fuel nozzle 2.7 mm in diameter.

slightly downstream region of the axial stoichiometric location. The process of CO and H2 oxidation might possibly increase the flame length in the post flame zone. To realistically estimate the oxy-fuel flame lengths, it is necessary to modify the original Delichatsios’s correlation. Therefore, we propose the modified Delichatsios’s correlation described below. In a buoyancy-dominated regime, the dimensionless oxyflame length is correlated as

L* ) 22.5Frf0.4 for Frf < 10

(5)

The expression for the momentum-dominated regime is given as

L* ) 56.7 for Frf g10

(6)

The correlation data displayed in Figure 7 indicate that the flame lengths measured for the oxy-fuel flames have excellent conformity with the modified Delichatsios’s correlation. Such results confirm that oxy-fuel flames exist within the momentumdominated regime. Thus, these oxygen-methane turbulent jet flames are classified as the momentum-controlled flame. This implies that momentum-controlled oxygen-methane flames can be easily formed, while it is almost impossible to produce momentum-controlled air-methane flames due to flame stability limits. Figure 8 shows the flame image versus oxygen velocity at fuel velocities of 20 and 146.2 m/s for Combustor II. The lower fuel and oxygen velocity conditions yield a longer flame length and higher soot-induced yellowish luminosity due to insufficient turbulent mixing and lower turbulent intensity.4 By increasing the oxygen velocity while keeping the fuel velocity fixed, the flame length and flame volume decrease, especially for relatively low fuel velocity conditions. The flame length is also found to decrease by increasing the fuel velocity due to higher turbulent mixing and entrainment. The highest fuel velocity condition (146.2 m/s) yields the shortest flames and nonsooting flames at the four oxygen velocities (7.9, 16, 24.8, and 34.2 m/s). This trend is directly caused by the substantial increase in turbulent mixing and entrainment, especially at very high fuel velocities. As shown in Figure 8, unlike the air-fuel flames, the oxy-fuel flames are easily stabilized even at the highest fuel velocity



Figure 6. Flame lengths for four nozzle diameters of the oxy-fuel combustor.

Figure 8. Flame image versus oxygen velocity at two fuel velocities: (a) 20 m/s and (b) 146.2 m/s.

Figure 7. Dimensionless flame length correlated with flame Froude number.

(146.2 m/s) without swirling the flow. This stabilization characteristic of the oxy-fuel flame is mainly due to the much higher flame speed as well as the much wider flammability limits (5-14% in volume for the air-CH4 flame and 5-50% for the oxy-fuel flame). Figure 9 shows the flame length versus the oxygen velocity at five different fuel velocities (20, 40, 60, 80, and 146 m/s)

for Combustor II. As expected, the flame length is decreased by increasing the oxygen velocity for each of the five different fuel velocities. It is also observed that a higher fuel velocity yields a shorter flame length for a fixed oxygen velocity. These experimental results suggest that the flame length decreases by increasing the fuel velocity or oxygen velocity due to increased turbulent mixing and entrainment. Figure 10 shows the correlation between the jet flames’ dimensionless flame length and flame Froude number. In Combustor II, there exists the recirculation zone at the outer region of the coaxial oxygen jet. This might slightly increase the temperature and the CO2 level in the ambient mixture. Compared to the pure oxygen ambient condition, the increased temperature decreases the flame length while the elevated CO2 level possibly increases the flame height. Since these two effects are roughly counterbalanced with each other, it is expected that the flame length in Combustor II could be marginally influenced by the flow recirculation. Thus, the flame length in Combustor II is controlled mainly by the inlet conditions of the central fuel jet and coaxial oxygen jet. In this study, the ambient density


Kim et al.

fuel flame lengths for Combustor II. Since the oxygen velocity of Combustor II is higher than that of Combustor I (0.095 m/s), the turbulent mixing is much stronger for Combustor II, and its resulting flame length must be shorter than that of Combustor I at the same fuel injection rate. In order to develop a reliable correlation for the flame length, it is necessary to account for these additional turbulence intensities on the oxygen side of Combustor II. As indicated in Figure 9, flame lengths are decreased by increasing the oxygen velocity due to enhanced turbulent mixing. This implies that the flame length is considerably influenced by the turbulent kinetic energy of the oxygen flow. Therefore, as illustrated in Figure 11, the ratio of experimental oxy-fuel flame data to the corresponding, predicted results of eqs 5 and 6, versus the turbulent kinetic energy of the oxygen velocity, is displayed. The kinetic energy equation of the oxygen flow is estimated as2424 Figure 9. Flame length versus oxygen velocity at five different fuel velocities.

3 k ) (Uogto)2 2

(7)

Here, Uo is the oxygen velocity and to is the turbulent intensity, assumed to be 10%. These data plotted in Figure 11 reveal that a certain correlation exists between the kinetic energy of oxygen flow and L*/L(th). For the oxy-fuel flame length of Combustor II, the following correlation has been proposed:

If k < 30

L e* L(th)

) -0.0121k + 0.7857

(8)

Here, Le* is the dimensionless flame length of the experimental

If k > 30

Le* L(th)

) 0.42

(9)

data and L(th) represents the value predicted by eqs 5 and 6. As shown in Figure 11, this modified relationship is reasonably well-correlated with the experimental data for Combustor II. Figure 10. Dimensionless flame length of jet flames correlated with flame Froude number.

Figure 11. Ratio of measured to predicted flame length versus kinetic energy of injected oxygen.

and temperature are determined from the pure oxygen conditions. In Figure 10, the solid line denotes the modified Delichatsios’s correlation based on the oxy-fuel flame data of Combustor I. As illustrated in Figure 10, the flame data measured for Combustor II do not conform well to the modified Delichatsios correlation. These results indicate that the modified Delichatsios correlation could represent the upper bound of measured oxy-

4. Conclusions The flame structure and length characteristics of the oxyfuel flame have been experimentally investigated for two types of combustor: the model oxy-fuel combustor (Combustor I) and the actual oxy-fuel combustor (Combustor II). On the basis of experimental results for the two combustors, the following conclusions can be drawn: Compared to the air-fuel flame, the molecular viscosity of the oxy-fuel flame is 1.2 times larger at stoichiometric conditions and up to 1.5 times larger at the high-temperature lean or rich side. Thus, the transitional regime of the oxy-fuel flame could be shifted to a much higher range of Reynolds numbers. But experimental results suggest that the transitional flame characteristics of the oxy-fuel flame are not quite apparent. At fuel velocities lower than 15 m/s, the oxy-fuel flame, with the higher molecular viscosity, yields a much thicker flame at the laminar upstream zone (x ) 150 mm) than the air-fuel flame, while it generates a much thinner flame at the turbulent downstream zone due to the suppression of turbulent fluctuations in the highly viscous thin side. When the air-fuel flame is lifted, the corresponding flame width is sharply increased in the upstream zone (x ) 150 mm) owing to flow redirection in the flame stabilization region, while it is abruptly decreased downstream because of a decreased concentration fluctuation in the partially premixed mixture. (24) Tennekes, H.; Lumley, J. L. A First Course in Turbulence; MIT Press: Cambridge, MA.


The dimensionless flame lengths measured for the air-fuel flame are well-correlated with Delichatsios’s relation. However, the measured dimensionless flame length data of the oxy-fuel flame are considerably longer than those estimated by the original Delichatsios’s correlation. This discrepancy is attributed to the shortcomings of the original Delichatsios’s correlation based on the mathematical similarity solution and experimental results of air-fuel flames. The dimensionless flame lengths measured for the oxy-fuel flames show excellent conformity with the modified Delichatsios’s correlation. The flame length of Combustor II was found to decrease with increased fuel and oxygen velocities due to higher turbulent mixing and entrainment. The highest fuel velocity condition yields the shortest flames and nonsooting flames. This trend is directly caused by the substantial increase in turbulent mixing and entrainment, especially at very high fuel velocities.


The flame length data measured for Combustor II does not conform well to the modified Delichatsios’s correlation. This indicates that the modified Delichatsios’s correlation could be the upper bound of measured oxy-fuel flame lengths for Combustor II. Therefore, the modified relation that accounts for the turbulent kinetic energy of oxygen correlates reasonably well with the experimental data for Combustor II. Acknowledgment. This research was supported by a grant (AE2-101-1-0-1) from Carbon Dioxide Reduction & Sequestration Research Center, one of the 21st Century Frontier Programs funded by the Ministry of Science and Technology of Korean government. EF060346G

Studies on Combustion Characteristics and Flame Length of Turbulent

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