Local Interactive Patterns of Dispersed and Swarm Particles in a

Oct 15, 1996 - The observed speed of ascending particles ranges from 0 to 3.0 m s-1 at a superficial gas velocity of 6.0 m s-1. Descending particles a...
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Ind. Eng. Chem. Res. 1996, 35, 4360-4365

Local Interactive Patterns of Dispersed and Swarm Particles in a Circulating Fluidized-Bed Riser Hiroyuki Hatano,*,† Satoru Matsuda,† Hiromi Takeuchi,‡ Alexander T. Pyatenko,‡ and Katsumi Tsuchiya§ Thermal Energy and Combustion Engineering Department, National Institute for Resources and Environment, 16-3 Onogawa, Tsukuba 305, Japan, Hokkaido National Industrial Research Institute, Sapporo, Japan, and Department of Chemical Science and Technology, The University of Tokushima, Tokushima, Japan

Motion of glass beads of 0.4 mm mean diameter is observed in the central region of a circulating fluidized-bed (CFB) riser by coupling a reflective-type particle image scope (RPIS) with a highspeed video system of 3000 frames s-1 recording rate. Transient changes in the particle velocity are measured under both dilute and dense conditions. The observed speed of ascending particles ranges from 0 to 3.0 m s-1 at a superficial gas velocity of 6.0 m s-1. Descending particles are also observed, and their speed ranges from 0 to 1.6 m s-1. The structure and motion of particle swarms are analyzed with a special emphasis on particle collision. It is confirmed that the speed, moving direction, and packed density of the particle swarms are subjected to persistent time variations. Various patterns of particle motions are identified in the CFB riser, induced by a diversity of collision mechanisms. These include momentum transfer from a colliding particle to other particles being contacted, grazing followed by particle rotation, roundabout motion of a particle around another, simultaneous collision involving more than three particles, and successive particle collisions. Introduction It has been recognized that particles in the riser of a circulating fluidized bed (CFB) flow in both dispersed and swarming states. The particle swarms exhibit fluctuations in size, packed density, and velocity; such a time-varying complexity of the swarm structure makes it difficult to elucidate the role of the swarms in such processes as combustion reactions. Optical fiber probe systems (Oki et al., 1980; Ishida et al., 1980) can be (and have been) used to measure the velocity of the swarm particles as well as the dispersed ones (Nowak et al., 1990). This technique, however, relies solely on a point probe that can only provide very local information, leading to less distinction between the two states of particle assemblage. It is also inadequate for observing transient characteristics of the particle flow. An alternative technique, laser sheet lighting, has been applied to visualizing the flow structure of a CFB; Kuroki et al. (1993) obtained pictures showing a three-dimensional network structure of U-shaped dense particle swarms. Note, however, that the solids concentrations examined were substantially low compared to those under the normal CFB operating conditions. As an extension of the former technique, Takeuchi and Hirama (1990) used a bore scope to obtain backlight images of particle groups in a fine particle CFB and discussed the inherent relation between the interception time distribution and the prevailing flow pattern. Li et al. (1990) applied microscopy with an inserted tube as the light guide to obtaining the averaged lateral holdup distribution in a CFB riser. Due to insufficient resolution, sensitivity, and recording rate, however, detailed motion of the particles was not revealed in those systems. * To whom correspondence should be addressed. Phone: +81-298-58-8223. Fax: +81-298-58-8209. E-mail: hatano@ nire.go.jp. † National Institute for Resources and Environment. ‡ Hokkaido National Industrial Research Institute. § The University of Tokushima.

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To circumvent these problems, particle image scopes, both reflective and penetrative types, have recently been developed (Hatano et al., 1994) to obtain images of individual particles as well as the structure, bulk velocity, and size of particle swarms. In this study, the reflective-type particle image scope (RPIS) is combined with a high-speed video system to extract clear images for a detailed analysis of the motion of solid particles in a CFB riser. The high-speed video system can provide frozen images of the whole particle projection; its multiple-playback-format function is useful in obtaining the speeds, directions, and diameters of moving particles. While three-dimensional motion analysis remains still in embryo at this stage, the RPIS enables us to analyze two-dimensional motion of the solid particles in detail. With this measuring system, the present analysis is extended to observing the motion of swarm particles and identifying the mode of particle collision patterns. Experimental Section Circulating Fluidized Bed and Operating Conditions. The test apparatus used in this study was a transparent CFB cold model, and its schematic diagram is shown in Figure 1. Both the riser and the downcomer were 60 mm in inner diameter and 4500 mm high. All the particle images in this study were taken near the centerline of the riser at the level 3250 mm from the bottom of the riser. Solids loading rate, GS, ranging from 10 to 35 kg m-2 s-1, was controlled by altering the aeration velocity through an H-valve which connected the riser and the downcomer at the bottom region. While the corresponding particle concentration in the riser was not measured systematically, limited measurements were made under selected conditions: 0.06 ( 0.01 and 0.09 ( 0.01 for GS ) 18 and 35 kg m-2 s-1, respectively. Glass beads of 0.40 mm mean diameter were used as the fluidized particles; their bulk density was about 1530 kg m-3 and the minimum fluidization velocity 0.20 m s-1. The superficial gas velocity, U0, ranged from 3 to 6 m s-1. © 1996 American Chemical Society

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Figure 3. Brightness and sharpness of a particle image as functions of distance from RPIS tip.

Figure 1. Schematic diagram of the circulating fluidized bed.

Figure 2. Arrangement of the RPIS and visualization system.

Reflective-Type Particle Image Scope. The RPIS consists of an image fiber of 3.0 mm diameter as the image transmitter and 23 optical fibers as the light projectors [for more information, refer to our previous papers (Hatano et al., 1993, 1994)]. The images from the RPIS are taken via a high-speed video system to monitor fine-scale motion of solid particles. The schematic structure of the RPIS is shown in Figure 2. Light emitted from the optical fibers scatters depending on the refractive-index ratio of the core/clad of the fibers. In the present system, the most brightly illuminated area was found to lie between 3 and 5 mm from the tip of the RPIS; the obtained images, while complicated to interpret, have some common basic features such as the following: (a) The RPIS can focus very sharply on areas very close to its tip (within 0-1.5 mm). Particles in the view appear relatively dark, although their central region is well illuminated. (b) For the particles passing at a distance between 1.5 and 3 mm from the RPIS tip, the resulting images are dark and blurred. (c) Bright images of the particles are obtained when they are passing at a distance between 3 and 5 mm from the tip, although the focus is not very sharp. (d) Brightness decreases with a further increase in the distance. Figure 3 shows a schematic representation of the above features. Parallax of the RPIS was also inspected in the range of 0-7 mm from the tip of the RPIS by measuring the

distance between the central axes of two optical fibers, which were kept parallel to the RPIS and 2.2 mm apart. The results indicate that the parallax was negligible within this range. Therefore, the velocity of particles passing this region was directly calculated from the distance traveled by the edge or the center of the particles whatever the sharpness and darkness of the particle images were. Arrangement of Visualization System. The devices used for the visualization were arranged as shown in Figure 2. The light source was connected to an optical fiber illuminating device. Reflected images were transferred through the central optical fiber bundle and introduced into a high-speed video camera. The highspeed video system used was Kodak Ektapro EM1012. The video camera had an image intensifier so that clear images were ensured even under insufficient lighting conditions. The resolution of the picture was 238(H) × 192(V) pixels. Each pixel had 1 byte so as to span 256 gray levels. While the maximum recording rate was 1000 frames s-1 for full frames, it could be increased up to 12 000 frame s-1 by splitting the frame. Exposure time could be selected from 0.01 to 5 ms by adjusting the electrical shutter speed. Images were stored on the memory of the video processor over a total recording time of 1.6 s for the 1000 frames s-1 recording. The stored images were then transferred to a video cassette recorder at a rate of 30 frame s-1, although they could be stored in a mass storage device directly. It was decided to do it this way since the latter way required a very long process time during each run. After the run, each image was converted to a digital image and transferred to a microcomputer for the analysis. All the pictures were processed using a photo-retouch software to improve the image quality. The number of frames splitting was determined as follows. If we assume that the particle velocity is 3 m s-1 and the diameter of the view field of the RPIS is 3 mm, the time required for the particle to pass the whole view field is estimated to be 1 ms. This means that the recording rate of at least 2000-3000 frames s-1 needs to be attained to calculate the particle velocity and to observe the particle motion. In this study, therefore, we chose split-frame mode 3, which secures a recording rate of 3000 frames s-1 at the sacrifice of the picture height of every exposure being reduced to 1/3 of the fullframe image. The recorded images were reproduced using the multiple-playback-format function so that the calculation of the particle velocity could be easily performed. Due to such limitations on the picture height, the high-speed video camera was tilted by 90°

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Figure 4. Typical images of individual particles obtained using split frame and multiple-playback-format function.

counterclockwise so as to effectively use the display area for achieving higher precision in the particle velocity measurement.

Figure 5. Successive images of two interactive particles.

Results and Discussion Consecutive Images of Particles in View. Single Particles. Figure 4 shows an example of three successive images of individual solid particles for a superficial gas velocity of 6.0 m s-1 and a solids loading rate of 18 kg m-2 s-1. In this figure (and the ensuing three figures), the consecutive images are so arranged that time advances from top to bottom at an interval of 0.33 ms with a common length scale shown on the top of each figure, thus providing necessary information on the particle size and velocity. In most runs, images were taken over an exposure of 0.1 ms to assure frozen images of solid particles. This exposure time causes blurring in an image by only 0.1-0.2 mm “shift” for objects (in this case particles) moving at 1-2 m s-1. Since the camera is tilted as mentioned in the previous section, the left-to-right side of the picture corresponds to the bottom-to-top direction of the CFB riser. One particle (denoted as a in Figure 4) can be identified clearly as a dark spot in the central area of the first frame. Its rise velocity is calculated to be 2.1 m s-1, its direction of movement is upward, and its size is 0.45 mm. Judged from its sharply focused image, this particle is considered to have passed an area within 1.5 mm from the tip of the RPIS. A couple of more particles can be identified, although not well focused (b and c in Figure 4). The rise velocity of particle b is also 2.1 m s-1, having closely followed particle a 0.5 mm apart vertically. From its image, particle b must have passed a region between 1 and 2 mm from the RPIS tip. In between these two particles, a bright, white spot is recognizable (see c); this spot can also be regarded as an image of an ascending particle, having passed in a range of 3-5 mm from the tip. Its rise velocity is estimated to be about 2.0 m s-1. Since all these particles are considered to be “isolated” and rise at the same velocity (≈2 m s-1), it can be concluded that the presence of the probe tip has a negligible effect on estimating the particle rise velocity provided the distance exceeds an order of 1 mm. Interacting Particles. Figure 5 shows two particles, of sizes 0.35 and 0.45 mm, having passed through an area very close to the tip of the RPIS. In the top and middle frames, the two particles appear to touch each other, signifying possible collision between the particles. Between the middle and bottom frames, the smaller particle denoted as s has accelerated from 2.0 to 3.2 m s-1, while the larger particle B has decelerated from 1.7 to 1.5 m s-1. This example of consecutive images implies that at least 1/3000 s plus some ad-

Figure 6. Successive images for a collision process possibly involving three particles.

Figure 7. Typical images of particles in a swarm.

ditional time is required for the completion of the pertaining collision process. Figure 6 gives six successive images showing a collision process involving three particles, although the involvement of particle b is not clear. The first particle of 0.45 mm diameter (denoted as a followed by numbers corresponding to the frame numbers) was initially ascending rather slowly at 0.6 m s-1 (see a1 to a3). The other two small particles, b and c, were approaching particle a at 1.2 m s-1. In the next succession, particle b disappeared from the view area; particle a rapidly accelerated to 1.2 m s-1 (a4 to a6); and particle c retarded to 0.6 m s-1 (c4 to c6). Swarm Particles. Figure 7 shows an example of consecutive images for a group of solid particles swarming near the central axis of the riser. In this example, all the particles appearing in sharp focus were descending at 0.15-0.3 m s-1. Note that the particles in the view are well within the core of the swarm; the images were taken about 40 ms after the detection of the leading edge of the swarm. Although the swarm appears to be quite dense, the images of each particle are clearly identified. A total of 7-10 particles is identifiable in the vicinity of the RPIS tip. The closest distance between the particles ranges from about 0.05 to 0.1 mm; the solids holdup can then roughly be estimated to be 0.4 or so, which is slightly lower than a typical solids holdup value in a loosely packed state of glass spheres. Although not shown in this sequence of images, most

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Figure 8. Time variation of particle velocity in a dispersed phase under dilute condition (U0 ) 6.0 m s-1; GS ) 18 kg m-2 s-1).

Figure 10. Time variation of particle velocity in a swarm under dilute condition (U0 ) 6.0 m s-1; GS ) 18 kg m-2 s-1).

Figure 9. Time variation of particle velocity in a dispersed phase under dense conditions (U0 ) 6.0 m s-1; GS ) 35 kg m-2 s-1).

Figure 11. Time variation of particle velocity in a swarm under dense condition (U0 ) 6.0 m s-1; GS ) 35 kg m-2 s-1).

particles disappeared from the view about 50 ms after this moment. In the meantime, some descending particles suddenly changed their moving direction to upward under the influence of ascending particles passing very nearby, possibly caused by particle-particle collision. Transient Particle Velocity. In Dispersed State. Time variation/fluctuation of the velocity of particles passing near the RPIS tip is shown in Figure 8 for the dispersed phase of a dilute flow condition, U0 ) 6.0 m s-1 and GS ) 18 kg m-2 s-1. The particle velocities range from -0.1 to 2.7 m s-1 over a given period of time, and the average passing frequency of the individual particles is estimated to be about 550 s-1. Note that the real passing frequency should be a little less than the apparent one since the particle velocities are plotted at intervals of 1 ms and in this manner it is possible to count the same particle twice (or more), especially for slow particles. Descending particles are less frequently observed, and their speed is less than 0.1 m s-1. Only three downward particles were identified in this particular period, giving an estimate of the passing frequency of the downward particles to be about 17 s-1. When the solids loading rate is increased to GS ) 35 kg m-2 s-1, the particle velocities for the dispersed phase range from -1.0 to 3.0 m s-1 as shown in Figure 9; the average passing frequency increases to 1340 s-1. The velocity distribution for the ascending particles having velocities greater than 0.5 m s-1 resembles that shown in Figure 8. The velocity distributions for the slower particles as well as the descending particles, on the other hand, are quite different. These particles are observed at disproportionately high frequencies for this higher solids loading rate; the passing frequency of the

downward particles, for instance, is estimated to be about 280 s-1. An increase in the number of the slower and descending particles reflects mainly the enhancement of particle-particle collision. In Swarming State. Occasionally, groups of densely packed particles or swarm particles were observed, as shown in Figure 7, which stalled temporarily near the RPIS tip. Figure 10 shows the time variation in the velocity of individual particles in a swarm measured under the dilute condition identical with that for the results shown in Figure 8. In the beginning of the measuring period, the particle velocities are as high as those shown in Figure 8. As the number of particles in the view increases, the particle velocity drastically decreases to zero or even negative values. Thus, for this particular particle swarm, most particles in the swarm descend at almost invariant speed (≈0.2 m s-1) if they arrive at the view area 30 ms after the front end/edge of the particle swarm; this period of “near-zero, constant descent velocity” continues for about 40 ms. Then, a few ascending particles are being observed near the RPIS tip; this situation continues for another 40 ms or so. It also should be mentioned that very fast, ascending particles are observed in areas 2-3 mm apart from the RPIS tip, which are considered to rise outside the swarm, during the pertaining period. Finally, the particle number in the relevant region decreases and the particle velocity increases sharply to positive values comparable to those for the dispersed particles. The corresponding results for the solids loading rate of 35 kg m-2 s-1 are plotted in Figure 11. Although the basic features of the velocity distribution are similar to those for the dilute condition, the period spanning the appearance of the particle swarm under this dense

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condition is appreciably shorter than that under the dilute condition. Particle Collision and Induced Motion. As mentioned in the above section, the descending flow of particles near the riser central axis is not frequently observed for the low solids loading rate, GS ) 18 kg m-2 s-1, at U0 ) 6 m s-1. While particle-particle collision may be responsible for this particle descending to some extent, other factors such as stall of a group of particles under the influence of locally weakened gas flow could be as important as the former factor. For the high GS ) 35 kg m-2 s-1 (and U0 ) 6 m s-1), the descending particle flow becomes more frequent. In this case, direct collision of nearby particles is believed to be the dominant factor causing the particle descending. In this section, we schematically show typical examples of particle collision mechanisms and the patterns of particle motion induced by the collision. Collision of Two Particles. Factors to be taken into account in describing two-particle collision include directions and speeds of particles translation, rotation rates of the particles, particle sizes and densities, and colliding spot on the particle surfaces. Depending on how these factors are prescribed, the basic collision patterns can be classified into the following four types. (1) Identical Directions and Axes of Collision (Rear-End Collision). This type of collision is realized due to the difference in the initial speed between two particles. The leading particle having a speed slower than that of the following particle is accelerated after the collision; the following particle decelerated. This most standard mechanism predominates under dilute flow conditions. (2) Opposite Directions but Identical Axes of Collision (Head-On Collision). Due to the presence of frequently observed descending particles under dense flow conditions, this type of collision becomes dominant as the solids loading rate is increased. (3) Colliding Particles Approaching at Right Angle. Under the dense flow condition, some particles are moving (near) horizontally after having experienced collision(s) which have resulted in lateral rebound. The colliding counterpart, which may move either upward or downward, then meets such one particle at a right angle. (4) Collision Spot Not on the Central Axes of Particles Motion (Grazing). This type of collision leads, after the collision, to very complicated particle motions such as particle rotation, lateral rebounding, or one particle standing still at the collision site. If other particles are present near the collision site, even more complicated motions are expected (and indeed have been observed). Details of this latter respect are discussed later. Figure 12 shows schematically these collision mechanisms/patterns. The root of each arrow specifies the initial position of a particle and its head the final position of the particle. A circle represents a stationary particle. In collision types 1 and 2, the length of arrows roughly corresponds to the particle speed. In comparison to relatively simple outcomes expected/observed in the first three situations, the last situation depicts rather unpredictable behavior of the collided particles. Collision of Three or More Particles. When the solids loading rate is increased, collisions involving three or more particles are frequently observed. Figures 1315 reproduce schematically typical examples of the collision-induced motions of particles from the original

Figure 12. Schematic representation of two-particle collision patterns classified.

Figure 13. Schematic representation of a momentum-transferring collision process.

Figure 14. Schematic representation of simultaneous collision resulting in (a) particle scattering and (b) negligible disintegration of a particle group.

images. In each column consisting of successive frames, time starts from the top of the sequence with an increment equal to 1/3000 s. Specific features of the relevant motions and the underlining mechanisms are described as follows. (1) Momentum-Transferring Collision. Three particles can collide almost linearly and complete the whole collision process over a short period, i.e., within 0.3-0.5 ms. Two examples of such collision patterns are shown in Figure 13. While the middle particle (a) may move upward (Figure 13a) or downward (Figure

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1. The present measurement system is very effective in visualizing the solid particles behavior not only in the dispersed state but also in the swarming state. 2. The transient particle velocity, determined using the multiframe reproduction function of the high-speed video system, exhibits a distinctive difference in its distribution between the dispersed and swarming states; in the former state, the distribution largely scatters over the range -1 to 3 m s-1, and in the latter, it is confined in the range -0.5 to 0.2 m s-1. 3. The dense particle swarms identified through the present imaging technique have their packing density within their core slightly lower than that of loose packing. 4. The particle swarms observed in this study, while mostly descending slowly (at ≈0.2 m s-1), eventually change the direction of their bulk motion upward by successive collisions with ascending dispersed particles. Figure 15. Schematic representation of roundabout motion.

Nomenclature

13b), its behavior will not change appreciably before and after the collision. (2) Multiple Collision Counterparts. One particle can collide with a few particles simultaneously. Figure 14 shows two examples of such situations, more typical of which is observed when an isolated particle attacks a group of particles from the outside to end up with scattering of all the particles (Figure 14a). Less frequent though, a particle sometimes approaches a particle group with less impact, resulting in the particle’s “squeezing-into” the group (Figure 14b). (3) Roundabout Motion/Collision. When two particles collide with strong grazing, one of them (at least) often experiences an induced rotational motion. As shown in Figure 15, if another particle (a) is present very close to the rotating particle (c), the latter particle can exhibit a roundabout motion along the surface of the former particle. The rotating particle soon departs from the third one. This process has been observed only occasionally, but it certainly is an interesting phenomenon. (4) Successive Collisions. If a particle enters at a very high speed into a group of particles having proper space between each, two-particle collision can be triggered successively within a very short time. These successive collisions, however, have not been clearly confirmed in the present high-speed imaging. There might be some other possible interactions between the particles and the resulting motions not mentioned in this section. The observed motions of particles described in this study are noted to be quite similar to the motionssthough two-dimensionalsof balls observed in the billiard game. It could, therefore, be helpful to seek techniques of the billiard game to interpret the motion of particles in a CFB riser.

GS ) solids loading rate [kg m-2 s-1] U0 ) superficial gas velocity [m s-1] Vp ) particle velocity [m s-1]

Conclusions The reflective-type particle image scope and the highspeed video system are combined to analyze the gassolid flow in a circulating fluidized-bed riser. Magnified images provide a distinct appearance of both the individual and the swarm particles passing near the tip of the particle image scope. The following conclusions are obtained:

Literature Cited Hatano, H.; Suzuki, Y.; Kido, N. Flow Structure in Circulating Fluidized Bed Combustors. Proceedings of the IVth International Conference on Circulating Fluidized Bed, Pittsburgh, 1993; pp 123-130. Hatano, H.; Kido, N.; Takeuchi, H. Microscope Visualization of Solid Particles in Circulating Fluidized Beds. Powder Technol. 1994, 78, 115-119. Ishida, M.; Shirai, T.; Nishiwaki, A. Measurement of the Velocity and Direction of Flow of Solid Particles in a Fluidized Bed. Powder Technol. 1980, 27, 1-6. Kuroki, H.; Ogasawara, M.; Kamiya, H.; Horio, M. Visualization of the Suspension Flow in a Three-Dimensional Circulating Fluidized Bed. Proceedings of VIth SCEJ Symposium on Circulating Fluidized Beds; Tokyo, Japan, 1993; pp 75-82. Li, H.; Xia, Y.; Tung, Y.; Kwauk, M. Micro-Visualization of TwoPhase Structure in a Fast Fluidized Bed. In Circulating Fluidized Bed Technology III; Basu, P., Horio, M., Hasatani, M., Eds.; Pergamon: Oxford, 1990; pp 183-188. Nowak, W.; Mineo, H.; Yamazaki, R.; Yoshida, K. Behavior of Particles in a Circulating Fluidized Bed of a Mixture of Two Different Sized Particles. In Circulating Fluidized Bed Technology III; Basu, P., Horio, M., Hasatani, M., Eds.; Pergamon: Oxford, 1990; pp 219-224. Oki, K.; Ishida, M.; Shirai, T. The Behavior of Jets and Particles near the Gas Distributor Grid in a Three-Dimensional Fluidized Bed. In Fluidization; Grace, J. R., Matsen, J. M., Eds.; Plenum Press: New York, 1980; pp 421-429. Takeuchi, H.; Hirama, T. Flow Visualization in the Riser of a Circulating Fluidized Bed. In Circulating Fluidized Bed Technology III; Basu, P., Horio, M., Hasatani, M., Eds.; Pergamon: Oxford, 1990; pp 177-182.

Received for review April 22, 1996 Revised manuscript received August 16, 1996 Accepted August 19, 1996X IE960224J

X Abstract published in Advance ACS Abstracts, October 15, 1996.