Experimental Study on the Characteristics of Impinging Reaction

A high-spatial-resolution UV imaging system is applied for the experimental study of opposed impinging diffusion flames for the first time, with speci...
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Experimental Study on the Characteristics of Impinging Reaction Region with OH* Chemiluminescence in Opposed Impinging Diffusion Flames Ting Zhang, Guangsuo Yu,* Qinghua Guo, and Fuchen Wang Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, East China University of Science and Technology, Shanghai 200237, China ABSTRACT: A high-spatial-resolution UV imaging system is applied for the experimental study of opposed impinging diffusion flames for the first time, with special emphasis on the characteristics of impinging reaction regions with detailed spatial profiles of OH* emission intensity. Two horizontal opposed concentric nozzles supplying both fuel and oxidizer are used. The effects of velocity and O/C equivalence ratio ([O/C]e) on the generation of impinging reaction region are discussed. Velocity has less effect on the reactions of impinging region compared with [O/C]e and has no influence on the production of reaction core area in impinging region. [O/C]e has a remarkable influence on the generation of reaction core area in impinging region due to the change of reaction region in single-jet diffusion flames. The heights of impinging upward and downward streams (H), namely, the distance from the edge of streams to the opposing center, rise with the increase of [O/C]e. The buoyancy and momentum are the two main factors on heights, and the effect of buoyancy reduces with the increase of [O/C]e. When [O/C]e is more than 1.10, both the two heights do not vary with [O/C]e, because momentum has more influence on the flame characteristics than buoyancy.

1. INTRODUCTION

In one study, OH* chemiluminescence signals at 310 nm were monitored to obtain data regarding the flame structure.14 Marchese et al.15 suggested that OH* distribution should yield a rational indication of flame-front position. The maximum OH* emission was near the maximum flame temperature, which was meaningful for defining the “flame position”. Tinaut et al.16 studied the OH* chemiluminescence emitted during the combustion of different primary fuels at different initial conditions, and they found the maximum of OH* chemiluminescence coincided with the highest rate of heat release ́ inside the combustion bomb. Verissimo et al.17 examined the emission characteristics of a small-scale combustor, and the OH* images revealed that the structure of the main reaction zone changed with the excess air coefficient. The structure of flame kernels was characterized first by the OH* radical radiation. Kojima et al.18 obtained the spatial profiles of OH* intensity of laminar methane-air premixed flames with a Cassegrain-type optical probe, from which the burned and unburned flame regions were distinguished clearly. Compared to the studies on the premixed flames, less work has been done with diffusion flames. Selim et al.19 examined the flame chemistry in hydrogen sulfide-based diffusion flames with the emission spectra of excited species, among which the major was OH*. Ikeda and Beduneau20 investigated the chemiluminescence along the centerline of the laminar diffusion flame using the cylindrical Bunsen burner, finding that the chemiluminescence spectra below the blue flame region were very similar to those measured in premixed flames. Zhang et al.21 measured the distribution characteristics of OH*, CH*, and C2* in CH4/

1,2

Due to the rapid heat-transfers and ease of application, impinging flames have been widely used for industrial processes, including coal gasification,3 mixing,4 liquid−liquid extraction,5 etc. The strengthened micromixing caused by impinging streams is beneficial for the combustion of gas, liquid, or solid fuels. The earliest application of impinging streams to the combustion field can be dated back to the 1950s, when the Koppers-Totzek pulverized coal gasifier was developed.6 Two of the patented advantages of the Opposed Multi-Burner (OMB) gasifier are enhancing the mass and heat transfer and prolonging the particle residence time via the formation of an impinging flow field.3 As important characteristics of flames, spectral emissions attributed to the fuel molecules and particles participating in combustion contain continuous and discontinuous spectra.7 When the fuel reacts with oxygen at high temperature, a number of excited radicals are produced, causing light emission in characteristic spectral bands. OH* is one of the major excited radicals in hydrogen/hydrocarbon flames, which has been extensively studied. Higgins et al.8 determined the feasibility of using OH* chemiluminescence as an activecontrol parameter for high-pressure, premixed-flames. Ballester et al.9 used chemiluminescence signals (OH*, CH*, C2*) as the only input in control tests to assess their applicability for advanced optimization strategies. OH is also able to reflect the structure of flame. Bechtel and Teets10 measured the OH concentration profiles by laser-induced fluorescence in several atmospheric-pressure premixed laminar flames. Compared with the laser-based flame diagnostics, the optical diagnostics based on spontaneous flame emission have the advantages of being inexpensive.11−13 © 2013 American Chemical Society

Received: June 27, 2013 Revised: October 14, 2013 Published: October 21, 2013 7023

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flow rate. The experimental conditions are shown in Tables 1 and 2, respectively. Large-separation conditions (L/Dj > 20) have been commonly used26 in large-scale industrial reactors of coal combustion or gasification with opposed nozzles. Here, L is the distance between these two nozzles, Dj is the equivalent nozzle diameter. In this study, L is 80 mm, which is about 40 times the length of Dj. Dj can be calculated using the following equation (for two-stream concentric jets system):27

O2 coflow jet diffusion flames, and the result showed that OH*/CH* intensity ratio exhibited an exponential increase with [O/C]e, which was in agreement with the result observed in the premixed flames. Due to the similarity between diffusion flames and premixed flames, OH* can also be used as an indicator of reaction regions for diffusion flames. Although OH* emission offers a lot of information on flame structure and chemistry, investigations on impinging flames by OH* chemiluminescence are rarely reported, and most of them are focusing on the flame structure, impingement heating, and combustion enhancement.22−25 The present work focuses on the application of OH* chemiluminescence in opposed impinging diffusion flames. The specific objective is to carry out a fundamental study and a feasible optical experimental approach for exploring the effects of different velocities and [O/C]e equivalence ratio on the generation of impinging reaction core area based on opposed impinging diffusion flames.

Dj = 2(mc + ma )/[πρ ̅ (Gc + Ga)]1/2

where m is the gas mass flow rate, G is the momentum flux, and ρ̅ is the average density of fuel and oxidizer. The subscripts c and a refer to the central tube and the annular tube, respectively. The average equivalent nozzle diameter for all the experimental conditions (Table 1 and 2) is 2.22 mm. 2.2. OH* Measurement Approach. The OH* band between 280 and 350 nm has the highest intensity in the UV region and is essentially isolated from all the other emission bands produced during the combustion process. The most intense OH* emission occurs at 309 nm,19 as shown in Figure 2, obtained by a grating spectrometer (QE6500, Ocean Optics Inc.). In this study, the OH* emission is investigated by using a high-spatial-resolution UV imaging system. An achromatic ultraviolet quartz lens with a 50-mm focal length (f/ 3.5) was used to focus the luminescence coming from the flame. Particular wavelength region corresponding to OH* chemiluminescence was then extracted through a 10-nm-wide bandpass filter centered on 310 nm, covering most of the OH* emission (A2∑+− X2Π). The emission information is recorded by a UV camera, equipped with a cooling system that keeps the CCD sensor and associated electronic components down to −10 °C, so as to reduce the dark current. The measured intensity at each point is an integration of the intensity within the wavelength range of 300−320 nm. The CCD panel is composed of 1024 × 255 pixels, with a dynamic range of 14 bit, and the image spatial resolution is 0.255 mm in both axial and radial directions. The signal is integrated over the line of sight, and each image is acquired with the exposure time of 3000 ms. The highspatial resolution can provide a good solution on the division of flame boundary. In the impinging area of the impinging flame, the flame structure is asymmetric and chaotic. As a result, the Abel inversion, a widely used method, may not be suitable for this study since it is mainly used for symmetric flame.28 It is well-known that the chemiluminescence emissions are superimposed on the dark current background emission. Hence, the dark current background is also collected for each experimental condition by blocking the natural flame luminescence emission, and subtracted from the OH* luminous image. The background emission due to other species (mainly nonlinear background associated with CO2* emissions) is comparatively weak in UV region for gaseous combustion6 and is minimized by the bandpass filter in a certain extent, which could be neglected. The OH* luminous image captured by the UV camera is shown in Figure 3. The experimental data presented in previous work was OH* relative intensity, not the absolute value.29 The relative intensity data collected by the UV imaging system is subject to errors including (1) losses in the intensity when the light passing through the ultraviolet quartz lens; (2) attenuation losses in the bandpass filter; and (3) losses caused by the CCD sensor. In order to provide more accurate data, the detection

2. EXPERIMENTAL APPARATUS 2.1. Burner and Operating Conditions. Impinging diffusion flames of CH4/O2 were stabilized on opposed jet nozzles under atmospheric pressure. A sketch of the burner is shown in Figure 1,

Figure 1. Schematic diagram of the experimental setup. which consists of two horizontal opposed nozzles supplying both fuel and oxidizer, while the flames are isolated from the environment by two concentric curtains of nitrogen. The inner diameter of the fuel tube is 0.8 mm with a wall thickness of 0.3 mm. The inner and outer diameters of the oxygen tube are 1.4 mm and 3.0 mm, respectively. The inner and outer diameters of the nitrogen tube are 10.0 mm and 12.0 mm, respectively. Pure methane (>99.9%) is used as fuel, and pure oxygen (>99.9%) is used as oxidizer. Four velocity conditions are chosen for both fuelrich combustion and fuel-lean combustion to study the influence of velocity on the flame impinging reaction region. The O/C equivalence ratio ([O/C]e) varies in the range 0.70−1.30, defined by the following equation:

[O/C]e = [O/C]a /[O/C]s

(2)

(1)

where [O/C]a is the actual O/C molar ratio calculated from the feeding flows of fuel and oxygen and [O/C]s is the stoichiometric O/ C molar ratio. The equivalence ratio is adjusted by the change of O2

Table 1. Experimental Conditions for Different Velocities [O/C]e vCH4 (L/min) uCH4 (m/s) vO2 (L/min) uO2 (m/s) Re

0.70 0.50 15.79 0.70 2.15 1602

0.60 18.84 0.84 2.58 1920

0.70 21.98 0.98 3.01 2241

0.80 25.12 1.12 3.44 2561 7024

1.10 0.50 15.79 1.10 3.37 2076

0.60 18.84 1.32 4.05 2485

0.70 21.98 1.54 4.73 2898

0.80 25.12 1.76 5.47 3297

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Table 2. Experimental Conditions for Different [O/C]e vCH4 (L/min) uCH4 (m/s) vO2 (L/min) uO2 (m/s) [O/C]e Re

1

2

3

4

5

6

7

8

9

10

11

12

13

0.50 15.79 0.70 2.15 0.70 1615

0.75 2.30 0.75 1685

0.80 2.46 0.80 1751

0.85 2.62 0.85 1813

0.90 2.77 0.90 1874

0.95 2.92 0.95 1932

1.00 3.07 1.00 1988

1.05 3.23 1.05 2039

1.10 3.37 1.10 2093

1.15 3.53 1.15 2139

1.20 3.69 1.20 2184

1.25 3.84 1.25 2230

1.30 3.98 1.30 2276

Figure 2. UV spectrum of CH4/O2 diffusion flame.

Figure 4. OH* axial distributions with different velocities ([O/C]e = 0.70).

axial distribution of impinging flame is similar to that of jet flame at fuel-rich conditions, as shown in Figure 5. The major reaction region exists near the nozzle exit. Figure 3. OH* luminous image ([O/C]e = 1.00). system used to measure OH*chemiluminescence has to be calibrated at different spectral bands. This calibration was performed using a spectrometer and an integrating sphere light source. The spectrometer (QE6500, Ocean Optics, Inc.) was calibrated in the National Measurement Laboratory beforehand and could provide the standard value. By measuring the same light source with the calibrated spectrometer and the UV imaging system at the same position, the calibration factor was designed according to the ratio of the standard value (measured by the spectrometer) to the measured value (measured by the UV imaging system). OH* absolute emission intensity was obtained via multiplying the measured value by the calibration factor. Figure 5. Comparison of OH* distributions between impinging flames and single-jet flames.

3. RESULTS AND DISCUSSION The two-dimensional profiles of OH* absolute emission intensity are obtained from the calibrated OH* luminous images. The intensity data used to analyze the OH* axial and radial distributions is the average emission intensity of every four pixels which compose a square dot in luminous images. 3.1. Effect of Velocity on Impinging Reaction Region. In order to explore the influence of flow velocity on the flame impinging reaction region under the same [O/C]e, four velocity conditions (shown in Table 1) were chosen for fuel-rich combustion ([O/C]e = 0.70) and fuel-lean combustion ([O/ C]e = 1.10), respectively. Figure 4 shows the OH* axial distributions under fuel-rich combustion condition with different velocities. There is no OH* emission in the impinging region as the velocity increases (the emission can be regarded as the background emission when the intensity is less than 0.001 mW·Sr1−·m−2 in the emission region). It is indicated that no reaction core area is generated in this region. However, the region without OH* emission becomes narrow with the increasing velocity as the OH* extends downstream. The OH*

A ternary-peak appears for fuel-lean state with sufficient oxygen supplied (Figure 6), and a clear reaction core area is generated in the impinging region. This presents the effect of impingement on reactions is promoted due to the mixing of fuel and oxidizer. With increasing velocity, the OH* emissions enhance significantly and approximate to the peak intensity of the major reaction region. In the impinging area, the OH* emission of the two nozzles may integrate, and the influence of impinging is more important than that of integration; thus, the peak intensity is higher. At the same [O/C]e, the OH* axial distributions with different velocities are in substantial agreement, while the OH* emissions enhance with increasing velocity. It is indicated that velocity has certain effect on the reactions of the impinging region. Comparing Figure 4 with Figure 6, it can be found that the OH*distributions of these two [O/C]e differ greatly, and 7025

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When the [O/C]e achieves 0.90, the OH* axial distribution from the nozzle exit to the nearby impinging region has no change nearly. The luminescence of unburned carbon particles is rather weak in the impinging region (Figure 8), while a new OH* peak appears. It indicates the existence of the new reaction core area (Figure 9). The higher fluctuation will

Figure 6. OH* axial distributions with different velocities ([O/C]e = 1.10).

[O/C]e would be a major factor for judging weather reactions of impinging region exist or not. 3.2. Effect of [O/C]e on Impinging Reaction Region. The experimental conditions for different [O/C]e are shown in Table 2. The [O/C]e changes in the range 0.70−1.30. All flames are in laminar conditions with different [O/C]e because the critical Reynolds number is 3000 for methane, and the jet velocity is not kept constant as the O/C equivalence ratio increases, but the effect of velocity can be ignored.12,14 Under the relatively low [O/C]e conditions ([O/C]e < 0.90) with inadequate oxygen supplied (compared to stoichiometry), the OH* distributions are very similar to each other (Figure 7).

Figure 9. OH* axial distribution ([O/C]e = 0.90).

increase the fuel/oxygen mixing rate, the kinetic energy and the reaction rate,24 therefore inducing the new reaction core area. The impinging reaction core area can be seen in the OH* twodimensional profile (Figure 10). Because of the lack of oxygen

Figure 10. OH* 2-D profile ([O/C]e = 0.90).

and the influence of buoyancy, there is a clear contour of impinging upward stream and a blurry downward stream in the impinging area. The OH* peak intensity of the impinging center is relatively weak, which is less than 30% of the maximum in the main reaction region. With increasing [O/C]e ([O/C]e > 1.05), intensity of the new OH* peak is enhanced (Figure 11), achieving half of the OH* emission maximum in the main reaction region for the highest [O/C]e. The contour of impinging center becomes more distinct, and a clearer downward stream appears (Figure 12). Sun et al.30 measured the flow velocities of two largeseparation opposed nozzles (L/Dj > 20) on a cold model by Dual PDA, revealing that the range of the impinging region is three times the nozzle diameter. When the impinging flame forms, the velocity and pressure gradients are amplified, causing the expansion of impinging region (L/Dj > 10), as shown in Figures 9 and 11. According to the analysis above, [O/C]e has remarkable influence on the new reaction core area generated in the impinging region for large-separation conditions: (1) for fuel-

Figure 7. OH* axial distributions with different [O/C]e ([O/C]e < 0.90).

As shown in Figure 8, the flame becomes fluctuant due to the impingement in central region. Moreover, the yellow luminosity is produced by unburned carbon particles under the relatively low [O/C]e conditions. However, the main reaction regions appear near the nozzle exit and no other reaction core areas are generated.

Figure 8. Color images of impinging flames (exposure time of 1/40 s). 7026

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region (nozzle exit to the OH * peak position). The extent of the primary reaction region moves downstream when the [O/ C]e increases. The OH* decreases slowly along the spread of flame at the fuel-lean condition, as a result, a “shoulder” in curve forms up. For low [O/C]e conditions ([O/C]e < 0.90), most reactions occur near the nozzle exit (about 2 times the equivalent nozzle diameter away from the nozzle) where the main reaction region appears. The unburned fuel and carbon particles in the downstream region cannot be further oxidized due to insufficient oxygen. When the impingement occurs in the flame downstream region, where the reactions are very weak, no reaction core area will be generated. The increase of [O/C]e intensifies the reactions in the flame downstream region, which will be further promoted by impingement. Thus, a new reaction core area is formed in the impinging region. 3.3. Effect of [O/C]e on the Heights of Upward and Downward Streams. The flame-front position can be obtained by OH* measurement,15 which can be regarded as a good indicator for the boundary of flame reaction region. The upward and downward streams with chemical reactions appear in the impinging region. According to the radial OH* distribution in the impinging reaction core area (defined by the OH* axial distribution), the heights of these two streams can be obtained. When [O/C]e < 0.90, no impinging reaction core area is generated. The upward stream could be recognized with a height of 7.65 mm (Figure 14a) when [O/C]e reaches 0.90. A blurry downward stream emerges at [O/C]e = 1.00 (Figure 14b), and the heights of the upward and downward streams are 8.67 mm and 3.06 mm, respectively, illustrating that most of the unburned fuel carried by gas flow moves upward and is further oxidized. The impinging reaction core area becomes more prominent with increasing [O/C]e, where the OH* emission intensity is higher than that of the flame downstream region without impingement. The downward stream is clearer with a height of 5.61 mm, about half of the upward stream height (11.73 mm) (Figure 15). The momentum of fuel and oxidizer increases with the increase of O/C equivalence ratio, which intensifies the flow radial fluctuation. The fluctuation impels more fuel moving downward and forming a clear downward stream. The OH* peak intensity in the impinging reaction core area increases with increasing [O/C]e ([O/C]e > 1.10), while the heights of the upward and downward streams tend toward stability with fixed values of 11.73 mm and 6.63 mm, respectively (Figure 16), and the reaction area of the impinging region becomes stable (Figure 17). Figure 18 shows the comparison of the relative heights of upward and downward streams (H/Dj) with different [O/C]e (≥0.90). For 0.90≤[O/C]e ≤ 0.95, the height of upward stream increases slowly with increasing [O/C]e (H/Dj < 4), while there is no downward stream. When 0.95 0.95, because most of the unburned fuel is carried by the gas that flows upward due to the buoyancy effect. It is necessary to

Figure 11. OH* axial distribution ([O/C]e = 1.10).

Figure 12. OH* 2-D profile ([O/C]e = 1.10).

rich condition with insufficient oxygen ([O/C]e < 0.90), no reaction core area appears in the impinging region, and the major reaction region exists near the nozzle exit; (2) when the [O/C]e reaches 0.90, a new reaction core area is generated in the impinging region with a clear upward stream, though it is still a fuel-rich flame; (3) for fuel-lean condition ([O/C]e > 1.05), reactions in the impinging center are violent, and the contour of impinging center becomes more distinct with clearer upward and downward streams. The difference of the effect of [O/C]e on the impinging reaction core area results from the change of reaction region in single-jet diffusion flames with various [O/C]e. Figure 13 shows the OH* axial distributions for single-jet diffusion flames. With the [O/C]e increasing, the change of OH* distribution in flame downstream region (OH* peak to the end of the flame propagation) is much more significant than that of upstream

Figure 13. OH* axial distributions with different [O/C]e for single-jet diffusion flames. 7027

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Figure 14. OH* radial distribution ([O/C]e = 0.90, 1.00).

oxidant and fuel and ambient density respectively, ΔTf and T∞ are the combustion adiabatic temperature and ambient temperature (the adiabatic temperature is calculated by thermodynamic calculation for each experimental condition) respectively, and Dj is the nozzle equivalent diameter. As shown in Figure 19, the length of the single jet flame (vertical flame) becomes longer with increasing Frf until the [O/C]e reaches 1.05. It indicates that the flame dominant regime converts from buoyancy to momentum, where Frf is about 9. Schefer et al.32 found the boundary point between buoyancy domination and momentum domination is Frf = 5 for hydrogen, which differs from methane. Comparing the impinging flame with the single jet diffusion flame, it can be found that the heights of upward and downward streams do not vary with the [O/C]e substantially when the [O/C]e is more than 1.10, showing that the effect of buoyancy becomes weak under this condition, which is accordant with the result of the single jet flame. In conclusion, for relatively low [O/C]e ([O/C]e < 1.05), the buoyancy effect is significant on the single jet flame and lengthens the flame length continuously until [O/C]e = 1.05, which directly causes the uneven changes of upward stream and downward stream heights. With the increasing [O/C]e, the flame dominant regime changes from buoyancy domination to momentum domination, and momentum has more influence on the flame characteristics than buoyancy. The heights of upward and downward streams do not vary with the [O/C]e, as shown in Figure.18.

Figure 15. OH* radial distribution ([O/C]e = 1.10).

discuss the buoyancy effect in the impinging region, which is associated with the buoyancy effect on single jet diffusion flames. The influence of buoyancy is mainly concentrated in the far field of single jet flame, and the flame Froude number (Frf) is introduced to characterize the flame dominant regime (momentum domination or buoyancy domination). The physical meaning of Frf is the ratio of the flame initial momentum to the buoyancy.31 For our particular experiment system, Frf can be calculated as follows:32 Fr f = uf̅ s3/2 [(ρ ̅ /ρ∞)1/4 (ΔTf /T∞gD1/2 j )]

(3)

where u̅ is the average velocity of central flow and annular flow, fs is the mixture fraction, ρ̅ and ρ∞ are the average density of

Figure 16. OH* radial distribution ([O/C]e = 1.20, 1.30). 7028

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Figure 17. OH* 2-D profile ([O/C]e = 1.20, 1.30).

Figure 18. The comparison of the relative heights of upward and downward streams with different [O/C]e.

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when the [O/C]e reaches 0.90, and the contour of impinging region becomes more distinct with clearer upward and downward streams under fuel-lean condition ([O/C]e > 1.05). 3. The downward stream arises in the impinging reaction region as [O/C]e > 0.95, because the stronger turbulence in the impinging region intensifies the flow radial fluctuation, impelling more fuel moving downward. The heights of upward and downward streams increase with increasing [O/C]e, and the upward stream reaches its maximum height when [O/C]e = 1.05. As the [O/C]e is larger than 1.10, both the heights do not vary with the [O/C]e, and the reaction area of the impinging region tends toward stability. 4. The buoyancy effect is significant on the single jet flame when [O/C]e < 1.05, causing the uneven changes of upward stream and downward stream heights for the opposed impinging diffusion flame. The heights of the upward and downward streams do not vary at larger [O/ C]e because momentum has more influence on the flame characteristics than buoyancy.

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-21-64252974. Fax: +86-21-64251312. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Figure 19. The relationship between Frf and single-jet flame length.

ACKNOWLEDGMENTS This work is financially supported by the National Nature Science Foundation of China (21176078), the National Key State Basic Research Development Program of China (973 Program, 2010CB227004), and the Fundamental Research Funds for the Central Universities.

4. CONCLUSIONS The radial distributions and two-dimensional profiles of OH* emission intensity are obtained by the high-spatial-resolution UV imaging system. The results indicate that the optical experimental approach is capable for exploring the effects of different velocities and [O/C]e on the generation of impinging reaction core area for opposed impinging diffusion flames under large-separation condition, which can be summarized as follows: 1. Velocity has little effect on the reactions of impinging region at the same [O/C]e and has less influence on the production of reaction core area in the impinging region than [O/C]e. 2. The influence of [O/C]e on the generation of reaction core area in the impinging region is significant due to the change of reaction region in single-jet diffusion flames. The reaction core area appears in the impinging region

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