Experimental Study of Effects of Axi-asymmetric Combustion Air

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Experimental Study of Effects of Axi-asymmetric Combustion Air Supply in Horizontally Oil-fired Burner and Furnace Young Gun Go,† Sangmin Choi,*,† and Won Yang‡ School of Mechanical, Aerospace and Systems Engineering, Korea AdVanced Institute of Science and Technology, 335 Gwahangno, Yuseong-gu, Daejeon, 305-701, Republic of Korea and Korea Institute of Industrial Technology ReceiVed March 26, 2009. ReVised Manuscript ReceiVed June 15, 2009

Effects of axi-asymmetric combustion conditions were studied in a horizontally oil-fired and 3-staged swirl burner. Axi-asymmetric conditions were created by supplying combustion air through six different air inlets from the back panel of the burner. Experimental conditions were decided by varying the degree of axi-asymmetry and changing the location of the minimum flow rate value in unimodal and bimodal air distribution. Temperature profiles inside a furnace were mapped using R-type thermocouples and flame images were captured by a CCD camera, to investigate the intensity of visible flames, focusing on the effects of asymmetrically supplied combustion air. Experimental results show three distinct phenomena. The unimodal air distribution deformed the flame shape more axi-asymmetrically (inclined toward the minimum flow region) than the bimodal case. The flame shape was affected by buoyancy and by the location of the minimum flow rate in air distribution. Especially, when combustion air was discharged less in the ascending region of the swirl flow, the flame shape was more deformed toward where the air was supplied less. The critical values in the degree of axiasymmetric combustion air supply were identified to form an inclined straight shape of the radial temperature profile rather than a parabolic one.

1. Introduction In a conventional power plant where a multiburner system is usually used, it is generally intended for each of the burners to behave in a uniform manner. But, in reality, differences of mass flow distribution among each burner are reported to vary up to (17% of the average mass flow.1 This is due to the complex shape of the combustion air supplying system, which is usually called a windbox. Although the main design concept of a windbox is to supply combustion air to each burner evenly, there exists unwanted nonuniform air supply through the windbox. As shown in Figure 1, a conventional windbox is a large and complex duct-shaped structure and is divided by splitting plates to supply combustion air to each burner. The shape of the cross section changes sharply along the flow path with sharply bended corners of 90°. This complex shape of a windbox makes the flow pattern inside the windbox be nonuniformly distributed and this flow pattern continues toward the burner entrances, which are attached at the wall of a boiler furnace. Baukal et al.1 and Go et al.2 studied this nonuniform flow in the windbox and reported that the uneven mass flow rates among each burner and noted uneven peripheral (axi-asymmetric) flow distribution around the burner exit. The present study focuses on the latter; uneven peripheral air flow distribution around the burner that breaks the axi-symmetric combustion conditions in a furnace. * Corresponding author. Phone: 82-42-350-3030; fax: 82-42-862-1284; e-mail: [email protected]. † Korea Advanced Institute of Science and Technology. ‡ Korea Institute of Industrial Technology. (1) Go, Y. G.; Choi, S. M.; Kim, Y. Z. J. Kor. Soc. Combust. 2006, 11, 1–10. (2) Baukal, JR. C. E.; Schwartz, R. E. The John Zink Combustion Handbook; John Zink Company: OK, 2001, 562-567.

Figure 1. Typical windbox in a power plant.

Many studies have been reported in literature on the effects of the variation of air supply on combustion in a burner and furnace, but most focus on the overall uneven flow inside a furnace caused by changing the operating conditions, such as the overfire air (OFA), secondary air stream. Literature on axiasymmetric combustion conditions caused by axi-asymmetric air distribution is scarce. For example, He et al.3 simulated the temperature and velocity distribution to retrofit the reheat panel overheating of the power plant by controlling the OFA and secondary air stream. He et al.4 also studied the counter-flow mode of air jets in a tangentially fired furnace using CFD method. Byun et al.5 studied the air flow distribution inside a (3) He, B.; Zhu, L.; Wang, J.; Liu, S.; Liu, B.; Cui, Y.; Wang, L.; Wei, G. Comput. Fluids 2007, 36, 435–444. (4) He, B.; Chen, M.; Yu, Q.; Liu, S.; Fan, L.; Sun, S.; Xu, J.; Pan, W. Comput. Fluids 2004, 33, 1201–1223. (5) Byun, Y. C.; Kim, E. K.; Park, S. H.; Hwang, J. H.; Lee, J. M.; Kim, J. S. Kor. Energy Eng. J. 2002, 11, 224–229.

10.1021/ef900264h CCC: $40.75  2009 American Chemical Society Published on Web 07/13/2009

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circulating fluidized bed combustion boiler by changing the shape and the arrangement of nozzles at a distributor and predicted the formation of clinker. Tucakovic et al.6 calculated the boundary conditions of mill gases and hot air distribution between burner sections for three-dimensional (3D) computational calculations in a utility boiler through simultaneous calculations of material and heat balances for the fan mill and corresponding air tracts. Li et al.7 investigated the influence of a vent air opening on ignition in a down-fired pulverized-coal utility boiler by measuring the temperature distribution and gas components such as O2, CO, CO2, and NOx in the furnace. Vijiapurapu et al.8 simulated the unbalanced coal/air flow in the pipe systems of coal-fired power plants to balance coal/air flow by sizing the orifices to prevent nonuniform combustion inside the furnace. Kalisz et al.9 studied the nonuniform flow distribution and influence of this flow pattern on the overall heat transfer in the convective bundle of a circulating fluidized bed boiler employing an isothermal test stand and naphthalene heat/mass transfer analogy technique. Kim et al.10 studied the velocity distribution and uniformity in a gas-boiler combustion chamber influenced by the number of holes and location at the air distribution plate. Buoyancy is also one of the major factors that make the combustion conditions in a furnace not be axi-symmetric, if a horizontally fired burner is used. In most experimental and computational researches on flames concerning the effects of buoyancy, vertically issued flames were introduced to remove the axi-asymmetric flame shape.11-15 These papers investigated the buoyancy effects in vertically issued flames where the overall flame shape is circular and axi-symmetric. For a horizontally fired flame, however, the flame bends in a vertical direction and the flame shape is no longer axi-symmetric due to the interaction between the flame jet momentum and the buoyancy. Choudhuri and Gollahalli16 divided the horizontally issued hydrogen jet flames into momentum-dominated near-burner region, buoyancy-dominated far-burner region, and midflame transitional region, according to the centerline trajectories of flames. Smith et al.17 studied the relative effects of buoyancy and momentum on the characteristics of horizontally oriented circular and elliptic burner flames. However, Choudhuri and Smith did not consider the swirling flow. In this paper, the effects of axi-asymmetric combustion in a horizontally oil-fired and 3-staged swirl burner were studied by supplying uneven peripheral air flow (axi-asymmetric) distribution around the burner exit. Axi-asymmetric combustion air distributions were classified into two different modes as unimodal and bimodal distribution, based on the flow simulation (6) Tucakovic, D.; Zivanovic, T.; Stevanovic, V.; Belosevic, S.; Galic, R. Appl. Therm. Eng. 2008, 28, 2178–2186. (7) Li, Z.; Ren, F.; Zhang, J.; Zhang, X.; Chen, Z.; Chen, L. Fuel 2007, 86, 2457–2462. (8) Vijiapurapu, S.; Cui, J.; Munukutla, S. Appl. Math. Modell. 2006, 30, 854–866. (9) Kalisz, S.; Pronobis, M. Heat Mass Transfer 2005, 41, 981–990. (10) Kim, J. J.; Son, Y. G.; Chang, S. W.; Ryu, D. S. Proc. Kor. Soc. Mech. Eng. Conf. 2000, 859–864. (11) Takhar, H. S.; Halim, M. A. Int. J. Numer. Methods Fluids 1984, 4, 1165–1184. (12) Mitsotakis, K.; Zauner, E.; Schneider, W. Ingenieur-ArchiV 1988, 58, 161–170. (13) Bedat, B; Cheng, R. K. Combust. Flame 1996, 107, 13–26. (14) Cheng, R. K.; Bedat, B. Combust. Flame 1999, 116, 360–375. (15) Agrawal, A.; Albers, B.; Alammar, K. Combust. Sci. Technol. 2005, 177, 305–322. (16) Choudhuri, A. R.; Gollahalli, S. R. Int. J. Hydrogen Energy 2000, 25, 1107–1118. (17) Smith, T.; Periasamy, C.; Baird, B.; Gollahalli, S. R. Trans. ASME 2006, 128, 300–310.

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Figure 2. Outline of a considered burner: (a) viewed from burner exit, (b) isometric view.

in the windbox.1 Temperature distribution profiles inside the furnace were measured in axial and radial direction, and flame images were captured from the windows. 2. Experimental Details 2.1. Experimental Setup. An oil-fired and 3-staged swirl burner with 470 kW (400 000 kcal/h) of maximum capacity was remodeled to supply combustion air through six separate inlets from the back panel of the burner as shown in Figure 2. It consisted of a central oil nozzle, swirler, spark igniter, and windbox. The flow stream discharged from the burner consisted of the following three staged coaxial air streams; primary air from the inner axial tube, secondary air passing through the swirler, and a tertiary stream flowing through the space between the outer rim of the burner exit and the swirler. The air flow ratios were 5% for the primary, 52% for the secondary, and 43% for the tertiary air stream. The internal space of the windbox of the burner was divided into six different spaces by installing flat plates inside the windbox, and combustion air supplied from six separate inlets were not mixed with each other until they were discharged at the burner exit. Air flow rates of 6 inlets were controlled individually by a combustion air distributor that consisted of a main chamber, six separate air flow paths, venturi-type flow meters, gate valves, and pressure gages. Axi-asymmetric combustion air conditions were induced by controlling the flow rates of six paths individually. Total air flow rate to the main chamber was measured by a vortex flow meter and calibrated with temperature and pressure measured before the entrance of the main chamber. Figure 3 shows the pilot furnace used in this study that had a horizontal cylindrical combustion chamber. It was equipped with a water-cooled tube between the inner and the outer walls, covered by refractory cement. The temperature difference between the inlet and outlet of the cooling tube was maintained within 20 °C or less. The internal diameter of the furnace was 800 mm, and the length

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Figure 3. Schematic diagram of experimental apparatus.

was 2090 mm. It had 7 measuring ports on both sides and on the top of the furnace wall (total ports: 21). The first port on each side was 450 mm away from the burner exit, and the other ports were spaced by 200 mm each. A round-shaped observation window is on the wall of the furnace exit, where a CCD camera was installed toward the burner. Measurement apparatus consisted of R-type thermocouples for temperature profiles in the furnace, the CCD camera for capturing the flame shape, gas-analyzer, and data acquisition system. In this paper, the results of measured temperature profiles and CCD images of flame shapes were presented to analyze the effects of axiasymmetric combustion air distribution. Temperature profiles were measured in radial and axial direction by inserting and sliding the R-type thermocouple probe set with into six measuring ports on both side walls of the furnace. Temperatures on 6 points in the axial direction and 13 points in the radial direction (total measured points: 78) were measured in each experimental case. Temperature values were acquired by an Agilent 34970A Switch Unit and transferred to a personal computer, and fluctuating data were typically averaged over 1 min. 2.2. Image Processing Procedure. Overall procedure of the flame image processing is similar to the method used in the work by Ohm et al.18 Flame images were captured by a CCD camera (IK-537, Toshiba) that has a resolution of 640 × 480 pixels and gray 8-bit resolution. At every experimental case, 20 images were captured to produce one average image. To compare and analyze the images quantitatively, image pixel data were normalized by the highest value in each of the averaged image, to reduce the effects of the contamination on the quartz window. To pick out the flame region and to eliminate the reflected light from the furnace wall, a zero value was assigned to pixels that had flame intensity lower than a specific value. Therefore, at every normalized and averaged image, the brightest pixels have the value of one while the pixels under the threshold have the value of zero. To represent the degree of the axi-asymmetry in flame images; mean translation direction (θc), mean translation distance (rc) from the axial axis to the center of flame, and flame area (Af) were estimated from the flame image data. Large value in rc means that flame is far apart from the axial axis and that flame shape is distorted to the inclination direction too much. It is appropriate to consider (18) Ohm, I. Y.; Park, C. J. Int. J. Automot. Technol. 2006, 7, 519– 526. (19) Meriam, J. L.; Kraige, L. G. Engineering Mechanics Statics; John Wiley & Sons: New York; Chichester, 2003; pp 229-230. (20) Brandt, S. Data Analysis Statistical and Computational Methods for Scientists and Engineers; Springer: New York, 1998; pp 21-22. (21) Kiemele, M. J.; Schmidt, S. R. Basic Statistics Tools for Continuous ImproVement; Air Academy Press: Colorado Springs, Co, 1993; pp 242243.

the intensity-weighted values because strong and weak values are distributed over the flame regions according to the following equations:19

jx )

∫ Ix dA/ ∫ I dA

(1)

jy )

∫ Iy dA/ ∫ I dA

(2)

rc ) √jx2 + jy2 /rb

(3)

( )

(4)

θc ) cos-1

jx /rb rc

where x, y, and I correspond to the axial coordinate, vertical coordinate, and the intensity value of flame image. jx and jy correspond to the axial and vertical coordinates of the flame center. The mean translation distance (rc) was normalized by the radius (rb) of burner exit. Flame area (Af) was calculated by summating the corresponding areas (dAp) of the pixels in image data according to the following equation:

(∫ dA )/A

Af )

p

(5)

b

where, the flame area was normalized by the area (Ab) of burner exit. This image processing procedure is represented in Figure 4. In the final flame image in Figure 4d, the dotted circle means the outline of the real burner exit and the solid line arrow is drawn to represent the degree of asymmetry in flame; the starting point of the arrow is the origin of burner exit, and the end point of arrow means the center of flame area. The length of arrow means the degree of axi-asymmetry. The values of rc, θc, and Af are printed on the bottom of the flame image. 2.3. Procedure for Analyzing the Temperature Profiles. To quantitatively represent the degree of asymmetry in temperature profiles measured in the radial direction, wall temperature difference (∆Tw) and skewness (γ) are calculated. If the flame shape is not symmetric, then temperature profiles downstream are also distributed asymmetrically. This results in an increase in the skewness of the temperature distribution and temperature difference between the side walls. Wall temperature difference (∆Tw) is calculated by simply subtracting the temperature measured at near the right side wall (ri /Rf ) 0.75) from near the left side wall (ri /Rf ) -0.75) according to the following equation.

∆Tw ) Tat ri/R )-0.75 - Tat ri/R )0.75 f

f

(6)

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Figure 4. Image processing procedure: (a) 20 raw images, (b) an averaged image is produced via pixel-by-pixel averaging, (c) flame image is normalized by the highest pixel value, (d) zero value is assigned to pixels having the less value of 0.2 and the degree of axi-asymmetry in flame is calculated in terms of mean translation distance (rc), direction (θc), and area (Af) of flame image.

Skewness is a general measure of the asymmetry of the probability distribution. If the distribution is concentrated on the right side of the distribution, then it is called left-skewed or negative skew and the value of skewness is negative; whereas it is called right-skewed or positive skew and the value of skewness is positive, if the distribution is concentrated on the left side. The skewness is calculated by dividing the third moment about the mean by standard deviation.20,21 Its calculation procedure is expressed in the following equations:

Xi ) ri /Rf

(7)

fi ) Ti at ri /Rf

(8)

m)

∑X f/∑f i i

(9)

i

∑ (X - m) f ∑f 3

µ3 )

i

i

(10)

i

σ)

(

∑ (X - m) f ∑f

i

i

γ)

µ3 σ3

)

1/2

2

i

(11)

(12)

where nondimensional radius (ri/Rf) and temperature (Ti) measured at ri /Rf are used as a variable (Xi) and a frequency (fi). m, µ3, and σ in the equations represent the mean, the third moment about the mean, and the standard deviation of the temperature distribution, respectively. For a symmetrical distribution, the value of skewness is zero; and as the distribution gets left-skewed or right-skewed more, the value of skewness increases as shown in Figure 5. Therefore, a high skewness of the temperature distribution means

Figure 5. Skewness values (γ) of the symmetric and asymmetric distributions.

that the temperature inside the furnace is distributed highly asymmetrically and will result in a large wall temperature difference. 2.4. Combustion Air Distribution and Experimental Cases. In the previous study of the authors’,1 the flow pattern of the combustion air in a test windbox of an oil-fired power plant shown in Figure 6 was investigated using numerical and experimental methods. From the air preheater, the combustion air is supplied to 24 burners on the furnace walls after passing through the windbox. The commercial CFD software package Fluent v6.0 was used for the numerical simulation of the isothermal flow inside the windbox with the standard κ-ε model adopted for turbulence. The experiments were carried out in a 1/20 scaled test rig, and the velocity distribution contours on the burner exits were mapped. The variation of the flow rates was reported to be about 30%. Details of the numerical and experimental studies of combustion air distribution in a windbox were reported elsewhere.1 In the numerical results of the above-mentioned study, combustion air distributions supplied to an oil-fired burner in a conventional power plant could be classified into two different modes; unimodal and bimodal, when the peripheral angle location was assigned to the abscissa and the flow rate was assigned to the ordinate as shown in Figure 7b. Distinct characteristics of unimodal distribution was



Figure 6. The considered windbox in a previous study:1 (a) computation domain in a numerical calculation, and (b) schematic diagram of test rig of a windbox.

Figure 7. Classification of the combustion air distribution modes into unimodal and bimodal distribution: (a) circumferential combustion air flow shape around a burner exit (results of the previous study),1 (b) representation of the flow shape as unimodal and bimodal distribution (1FBL and etc. represent burner locations in the windbox), and (c) simplified unimodal and bimodal distribution used in the present experiments.

that only one maximum and minimum values exist in the distribution and that flow shape around the exit of the burner is distorted to one direction. On the other hand, bimodal distribution has two maxima and minima values in the distribution, and flow shape is distorted into multiple directions as shown in Figure 7a. In the present paper, unimodal and bimodal distributions were simplified as follows. First, air flow rates were classified into two maximum, medium, and minimum values and assigned to six inlet ports of the burner. If one peak value is presented at two consecutive inlets,

this flow represents the unimodal distribution. On the other hand, the bimodal distribution refers to air supply with two separate peak values as shown in Figure 7c. Both the direction of deformation (asymmetry) of air flow shape and the degree of deformation (asymmetry) are highly dependent on the location of the burner in a windbox. To represent the asymmetric direction of air flow shape, experiments were done by changing the azimuthal location (θp), where the two minimum air flow rates exist, from 0 to 300° with an interval of 60°. To represent

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Table 1. Experimental Casesa

case name symb uni-000-0.272 uni-060-0.272 uni-120-0.272 uni-180-0.272 uni-240-0.272 uni-300-0.272 uni-240-0.071 uni-240-0.091 uni-240-0.106 uni-240-0.411 bi-120-0.272 bi-240-0.071 bi-240-0.117 bi-240-0.272

azimuthal standard air location (θp) of deviation (σp) of distribution mode minimum air flow air distribution symmetry unimodal unimodal unimodal unimodal unimodal unimodal unimodal unimodal unimodal unimodal bimodal bimodal bimodal bimodal

0° 60° 120° 180° 240° 300° 240° 240° 240° 240° 120° 240° 240° 240°

0.272 0.272 0.272 0.272 0.272 0.272 0.071 0.091 0.106 0.411 0.272 0.071 0.117 0.272

a Fuel: light fuel oil 30 L/h, excess air ratio: 1.2. b sym is the reference case: combustion air is supplied uniformly around the burner exit.

the degree of asymmetry, variation of the standard deviation (σp) of peripheral air flow distribution was used in the experiments. The fuel used during the experiments was light-fuel oil, and its feed and excess air ratio were fixed at 30 L/h and 1.2 respectively. All experimental cases are listed in Table 1. The meaning of case name is [Distribution type - location of minimum air flow - standard deviation].

3. Results and Discussion 3.1. Effect of the Azimuthal Location of Minimum Air Flow. In a horizontal flame, due to the buoyancy effect, the flame exhibits noticeable degree of upward bending and therefore is not symmetric about the horizontal plane. But flame is still supposed to be symmetric about the vertical plane.17 Figures 8 and 9 represent the transition of flame shape with respect to the change of the azimuthal location of minimum air flow from 0 to 300° with a 60° increment in unimodal air distribution. General effect of nonuniform peripheral air supply is that flame is spread out to the region of low air flow rates and the center of flame image moves to that region. But the degree of flame deformation and the translation of the flame center is not identical with each other as follows, although the degree of axi-asymmetry of combustion air distribution is identical. In the case of uniform air distribution (Figure 8a), flame shape is not exactly axi-symmetric, and the center of the flame moves slightly to the right side (reference case). In the case that the right side of the peripheral air flow is small (Figure 8c: θp ) 300°, Figure 8e: θp ) 240°), the center of the flame image moves further, and the flame is spread out to the right-hand side. In the cases that the left side of the peripheral air flow is small (Figure 8b: θp ) 60°, Figure 8d: θp ) 120°), the translation distance of the flame center is smaller than in the case of θp ) 300° and 240°. In the case that the top side of peripheral air flow is small (Figure 8g: θp ) 0°), the translation distance of the flame center is larger than in the case where the down side of peripheral air flow is small (Figure 8f: θp ) 180°). The outstanding point in Figures 8 and 9 is that overall flow is accelerated toward the right side and the acceleration of flow toward the left side is prevented. There is a certain force that induces this tendency of deformation of flame shape and translation of the flame center. This effect is caused by the simultaneous operation between swirl flow and buoyancy force in a furnace. The swirl flow is formed in a counter-clockwise direction by the secondary combustion air stream and makes descending flow in the left region of a furnace and ascending

Figure 8. Flame image with respect to azimuthal location (θp) of minimum air flow in unimodal distribution and standard deviation (σp) fixed at 0.272 (case: sym case and from uni-000-0.272 to uni300-0.272).

Figure 9. Mean translation distance (rc) of flame center and flame area (Af) with respect to azimuthal location (θp) of minimum air flow as a function of axial location in unimodal distribution and standard deviation (σp) fixed at 0.272 (case: sym case and from uni-000-0.272 to uni300-0.272).

flow in the right region, while buoyancy force produces the upward flow pattern throughout the inner space of the furnace. Therefore, the flows of opposite directions conflict with each other in the descending region and decelerate the flow field, increasing the pressure in the left side, while the flow in the



Figure 11. Temperature difference (∆Tw) and skewness (γ) of temperature distribution with respect to azimuthal location (θp) of minimum air flow in unimodal and standard deviation (σp) fixed at 0.272 (case: sym case and from uni-000-0.272 to uni-300-0.272): (a and b) temperature difference and skewness as a function of axial location, and (c) averaged temperature and skewness over the furnace.

Figure 10. Radial temperature distribution with respect to azimuthal location (θp) of minimum air flow in unimodal distribution and standard deviation (σp) fixed at 0.272 (case: sym case and from uni-000-0.272 to uni-300-0.272).

ascending region is accelerated by a buoyancy force acting in the upward direction decreasing the pressure in the right side. Due to the pressure difference between both sides, there exists the net force that is acting in the right direction (from the descending region to the ascending region). This net force makes the flame spread out to the rightward direction and prevents to the leftward direction. Therefore, when combustion air is discharged less in the ascending region of the swirl flow, flow is deformed and spread out to this direction as shown in Figures 8c (θp ) 300°) and 8e (θp ) 240°). But, when combustion air is discharged less in the descending region of swirl flow, flow deformation is resisted to deform in this direction (Figure 8b: θp ) 60°, 8d: θp ) 120°). Additionally, in the case of θp ) 60°, because of the portion of small flow in the upper region and the net force that resists the leftward deformation, flame is spread to the upward direction similar to the case of θp ) 0°, but the flame width becomes narrower. In the case of θp ) 300°, due to the net force influencing in the right direction and buoyancy influencing the upward direction, the flame is deformed the most. On the other hand, flame shape is almost axi-symmetric when θp is 120° because flame deformation

Figure 12. Flame image with respect to standard deviation (σp) of peripheral air distribution in unimodal distribution and azimuthal location (θp) of minimum air flow at 240° (case: from uni-240-0.071 to uni-240-0.411).

toward the left and down side is restricted by net force and buoyancy force. Similar results of interaction of buoyancy and swirl flow cannot be found in the research area of combustion, but it is found in the convection flows in a horizontal annulus with a heated rotating inner circular cylinder.22-28 In those numerical (22) Dyko, M. P.; Vafai, K. Int. J. Heat Mass Transfer 2007, 50, 348– 360. (23) Teamah, M. A. Int. J. Therm. Sci. 2007, 46, 637–648. (24) Al-Amiri, A. M.; Khanafer, K, M. Int. J. Therm. Sci. 2006, 45, 567–578. (25) Char, M.; Hsu, Y. Int. J. Heat Mass Transfer 1998, 41, 1633– 1643. (26) Kenjere s˘, S.; Hanjali c’, K. Int. J. Heat Fluid Flow 1995, 16, 429– 439.

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Figure 13. Mean translation distance (rc) of flame center and flame area (Af) with respect to standard deviation (σp) of peripheral air distribution in unimodal distribution and azimuthal location (θp) of minimum air flow at 240° (case: sym case and from uni-240-0.071 to uni-240-0.411).

researches, authors did not mention the net forces acting on the inner rotating cylinder because the cylinder is a rigid body and is fixed at the rotating axis, but they also found the result of flow acceleration in the ascending region and flow deceleration in the descending region, and this flow pattern significantly affects the temperature and velocity distribution between the concentric and eccentric cylinders. Radial temperature profiles in a furnace were plotted in Figure 10 as a function of nondimensional radius (r/Rf) at different

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axial location (x/Df). The first graph represents a temperature distribution of uniformly distributed air condition. The shape of overall temperature distributions is similar to the parabola forming the maximum temperature near the center (r/Rf ) 0.0) of a furnace and minimum near the furnace walls (r/Rf ) (0.75). As combustion gas flows to the downstream, the width of parabolic temperature distribution gets wider because of flow mixing and heat transfer from the center to the wall sides. When combustion air is discharged nonuniformly (axi-asymmetrically), the temperature distribution is concentrated toward the lessflow region. Axes of temperature distributions are translated from the center to the left in the case of θp ) 60° and 120°, whereas in the case of θp ) 300° and 240°, axes of temperature distribution are translated from the center to the right. Then in ideal cases, temperature distributions of 60° and 120° have to be similar to 300° and 240°, respectively, because of the geometric symmetry about the vertical plane. But, same as the reasons explained in the analysis of the flame images, because of the mutual interaction of buoyancy and swirl flow, asymmetric temperature distributions of Figure 10c (θp ) 300°) and Figure 10d (θp ) 240°) are more concentrated to the right side, and the shapes of temperature profiles changes from the parabola to almost a straight line forming the highest temperature on the right side and the lowest on the left side due to the net force acting in the right side.

Figure 14. Radial temperature distribution with respect to standard deviation (σp) of peripheral air distribution in unimodal and azimuthal location (θp) of minimum air flow at 240° (case: sym case and from uni-240-0.071 to uni-240-0.411).



Figure 15. Temperature difference (∆Tw) and skewness (γ) of temperature distribution with respect to standard deviation (σp) of peripheral air distribution as a function of axial location in unimodal and azimuthal location (θp) of minimum air flow at 240° (case: sym case and from uni-240-0.071 to uni240-0.411): (a) temperature difference, (b) skewness, and (c) average temperature and skewness over the furnace.

The degree of asymmetry in temperature distribution is well expressed in terms of skewness (γ) and wall temperature difference (∆Tw) between near regions to side walls of a furnace. Both values of skewness and wall temperature difference have the similar pattern throughout the furnace as shown in Figure 11. When temperature distribution in a furnace is concentrated on the left side, the skewness and wall temperature difference have the positive values; on the other hand, when temperature distribution is concentrated on the right side, the values are negative. The large value of skewness represents the highly asymmetric temperature distribution in the furnace. This means that the temperature on the one side wall is higher than that of the opposite side wall. Thus, the wall temperature difference is high if the skewness is large. In the cases where the degree of asymmetry in temperature distribution is not large (case: sym, θp ) 0, 180°), the values of skewness are very small and the wall temperature differences are not large throughout the furnace. In the cases of a large asymmetric temperature distribution (case: θp ) 60, 120, 240, 300°), both the skewness and the wall temperature differences are large. Particularly, in the upstream of combustion gas, the skewness is extremely high and the temperature difference rises up to about 250 °C or higher. This represents the highly asymmetric combustion near the burner exit (Figure 11, panels a and b). Figure 11c shows the averaged skewness and wall temperature difference throughout the furnace. The graph shows that the degree of asymmetry is largest in the cases that the combustion air is discharged less in the descending region (right-hand side, θp ) 240, 300°) as explained in the above paragraph, because the skewness and temperature distribution are highest in those cases. 3.2. Effect of the Degree of Axi-asymmetrical Combustion Air Supply. The degree of axi-symmetric air distribution (changing the standard deviation of peripheral air distribution) also affects the combustion characteristics in a furnace. The flame deformation resulting from varying the degree of asymmetry in unimodal distribution and azimuthal location of minimum air flow at 240° are shown in Figure 12. As the standard deviation of air flow distribution increases, the center

of flame image moves further to the right side, and translation distance is almost linear to the standard deviation, but flame area is not linear as shown in Figure 13. For the standard deviation (σp) is below 0.106, the deformation of flame image is almost similar to the symmetric case. Above the value, the flame spreads out further to the right side and flame shape becomes distinctly different from the former cases. Also, temperature distribution shape changes suddenly from a parabolic shape to almost straight line, forming maximum temperature on the right side and minimum value on the left side when standard deviation (σp) is 0.161 as shown in Figure 14. This means that the degree of axi-asymmetry in combustion is not linearly dependent on the degree of axi-asymmetric peripheral air distribution and there exists a distinct point enough to change the temperature distribution suddenly. The degree of axi-asymmetry in flame can be represented more precisely, if skewness (γ) and temperature difference (∆Tw) between regions near to side walls of a furnace are introduced as shown in Figure 15. There is a distinct difference in the pattern of skewness and temperature distribution between under 0.106 and over 0.161 in standard deviation of peripheral air distribution; in the case that σp is over 0.161, they are positioned far from the zero point (Figure 15, panels a and b), and there is a sudden increase as shown in Figure 15c compared with the values under 0.101. Additionally, in the case that axiasymmetry is too extreme, temperature difference goes up to about 350 °C in the upstream of combustion gas flow, and this can affect the characteristics of heat transfer to the furnace wall. 3.3. Results of Bimodal Peripheral Air Distribution. Flame shape and temperature distribution in bimodal air distribution cases are almost the same as in the unimodal distribution as shown in Figure 16. The translation of flame center and deformation toward the left side is restricted (Figure 16a). On the other hand, the translation toward the right side is accelerated and temperature distribution is concentrated more on the righthand direction (Figure 16c). Also, the asymmetry in combustion is not significant in the small σp (Figure 16b), but the amount of the deformation and translation of the flame center and the

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Figure 16. Flame image and radial temperature distribution in bimodal distribution: (a) θp ) 120°, σp ) 0.272; (b) θp ) 240°, σp ) 0.071; and (c) θp ) 240°, σp ) 0.272.

Figure 17. Comparison of the skewness (γ) of the temperature distribution in unimodal and bimodal distribution.

asymmetry of temperature distribution are smaller than in the unimodal distribution case. These differences result in the large skewness in unimodal cases compared with in bimodal cases as shown in Figure 17. When combustion air is supplied in unimodal, the combustion air spreads out to the uniform direction from the large air flow region to the small flow region. On the other hand, in bimodal, the combustion air spreads out to the dual direction from the large air flow region to the medium flow region and to the low flow rate region. These result in the less nonuniformity in the bimodal distribution cases, although the standard deviation of combustion air distribution is the same as in the unimodal distribution cases. 4. Conclusion In most researches on combustion in a burner, combustion air is assumed to be axi-symmetrically supplied. However, in a windbox of a real power plant, flow is developed nonuniformly and this nonuniform flow induces the axi-asymmetric peripheral distribution of combustion air around the burner exit. In this study, the deformation in the flame and temperature profiles in a furnace with the horizontally fired swirl burner was investigated experimentally for two modes of peripheral air distribution. The degree of asymmetric combustion, which is affected by nonuniform peripheral combustion air distribution, was (27) Lee, T. S. Comput. Fluids 1992, 21, 355–368. (28) Yoo, J. S.; Ha, D. H. Kor. Soc. Comput. Fluids Eng. 2001, 6, 1–9.

estimated in terms of flame deformation and temperature distribution in a furnace. To analyze the flame deformation, the definition of mean translation distance (rc) and direction (θc) of the flame center in flame images were introduced. And wall temperature difference (∆Tw) and skewness (γ) were used to express the asymmetry of temperature distribution. The results show that the asymmetric combustion characteristics in a furnace are significantly affected by axi-asymmetrically supplied combustion air. The flame shape and corresponding temperature distribution are deformed and spread out to the less discharged region of combustion air due to the momentum difference around the burner exit. However, the degree of flame and temperature deformation is also affected by the mutual interaction of the buoyancy and the swirl flow in the horizontally fired swirl burner. The key findings are as follows: (1) When the peripheral air was discharged less in the ascending region (right side) of the swirl flow, flame shape and temperature distribution are deformed much more toward this region. When peripheral air is discharged less in the descending region, those deformations are restricted, because of the net force acting to the right side direction, which is induced by the mutual interaction of buoyancy and swirl flow. This net force is produced by the pressure difference; pressure increase in the ascending region of swirl flow and pressure decrease in the descending region. (2) The transition of temperature distribution and flame shape is not significant within a certain degree (σp: 0.161) of axiasymmetry of peripheral air distribution. Over this threshold point, the shapes of temperature distribution and flame image show dramatic changes. (3) Unimodal peripheral air distribution affects the deformation of flame shape and temperature distribution more than bimodal distribution, because the degree of asymmetry in unimodal is larger although the standard deviation of air distribution is identical in both modes. Acknowledgment. This study was funded by the combustion Engineering Research Center (CERC), Brain Korea 21 (BK21) program and ECO21 in Korea. EF900264H

Experimental Study of Effects of Axi-asymmetric Combustion Air

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