Experimental investigation on flow asymmetry in solid entrance region of a square circulating fluidized bed

Experimental investigation on flow asymmetry in solid entrance region of a square circulating fluidized bed

Particuology 7 (2009) 483–490 Contents lists available at ScienceDirect Particuology journal homepage: www.elsevier.com/locate/partic Experimental ...

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Particuology 7 (2009) 483–490

Contents lists available at ScienceDirect

Particuology journal homepage: www.elsevier.com/locate/partic

Experimental investigation on flow asymmetry in solid entrance region of a square circulating fluidized bed Zhengyang Wang, Shaozeng Sun ∗ , Hao Chen, Qigang Deng, Guangbo Zhao, Shaohua Wu Combustion Engineering Research Institute, Harbin Institute of Technology, Harbin 150001, China

a r t i c l e

i n f o

Article history: Received 9 February 2009 Received in revised form 20 May 2009 Accepted 8 July 2009 Keywords: Square circulating fluidized bed Solid entrance region Flow asymmetry Experimental investigation

a b s t r a c t To study the influence of back feeding particles on gas–solid flow in the riser, this paper investigated the flow asymmetry in the solid entrance region of a fluidized bed by particle concentration/velocity measurements in a cold square circulating fluidized beds (CFB). The pressure drop distribution along the riser and the saturation carrying capacity of gas for Geldart-B type particles were first analyzed. Under the condition of u0 = 4 m/s and Gs = 21 kg/(m2 s), the back feeding particles were found to penetrate the lean gas–solid flow near the entrance (rear) wall before reaching the opposite (front) wall, thus leading to a relatively denser region near the front wall in the bottom bed. Higher solid circulation rate (u0 = 4 m/s, Gs = 33 kg/(m2 s)) resulted in a higher particle concentration in the riser. However the back feeding particles with higher momentum increased the asymmetry of the particle concentration/velocity profile in the solid entrance region. Lower air velocity (u0 = 3.2 m/s) and Gs = 21 kg/(m2 s), beyond the saturation carrying capacity of gas, induced an S-shaped axial solid distribution with a denser bottom zone. This limited the penetration of the back feeding particles and forced the fluidizing air to flow in the central region, thus leading to a higher solid holdup near the rear wall. Under the conditions of u0 = 4 m/s and Gs = 21 kg/(m2 s), addition of coarse particles (dp = 1145 ␮m) into the bed made the radial distribution of solids more symmetrical. © 2009 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

1. Introduction Circulating fluidized beds (CFB) have been successfully used in fluid catalytic cracking (FCC), fossil fuel combustion and gasification, and flue gas (SO2 , NOx ) adsorption processes. In combustion and gasification, the CFB technology offers significant advantages such as fuel flexibility, in-bed sulfur capture, and relatively low NOx emissions with high combustion efficiencies (Leckner, 1998). One determinant for the reactions inside a circulating fluidized bed is the mixing of solids in the bottom zone. The back feeding particles have a limited lateral dispersion rate, which is determined by the main gas–solid flow in the riser and the flow momentum of back feeding particles itself. The flow structures in the entrance region are critical to the overall particle distribution in the riser (Cheng, Wei, Yang, & Jin, 1998), which affects the reaction and the heat transfer. You, Zhu, Du, and Fan (2008) reported that the heterogeneous structure in the bottom bed extends to the acceleration region and fully developed region. Cheng et al. (1998) investigated the effects of solid inlet structure types on the

∗ Corresponding author. Tel.: +86 451 86412238. E-mail address: [email protected] (S. Sun).

restriction to the flow of back feeding particles and the average axial distribution of solid concentration. Yan, Parssinen, and Zhu (2003) studied the flow properties in the entrance regions of a high-flux CFB riser (Gs = 550 kg/(m2 s)). de Wilde, van Engelandt, Heynderickx, and Marin (2005) gave the gas–solid mixing properties in the entrance regions of a dilute CFB riser (Gs = 3 kg/(m2 s)) by 3D LDA measurement and numerical simulation. The asymmetrical solids distribution extends from the inlet region to the exit. All above studies were performed on the circular risers and did not evaluate the effects of solid circulation rate and air velocity on the flow asymmetry of the solid entrance region. Square or rectangular cross-section is common for CFB combustors. Friction of walls makes a denser region in the corner of a cross-section (van der Meer, Thorpe, & Davidson, 2000; Wang, Lu, & Li, 2008; Zhou et al., 1994). This results in a different solid distribution profile from circular risers. Adding some coarse particles into a fine-particle group will change the solid holdup in the riser (Bai, Nakagawa, Shibuya, Kinoshita, & Kato, 1994). According to the coarse particle fraction in the flow of outlet solids, Bai defined three binary-solid mixing types, including complete segregation, partial segregation and complete mixing. Bai and Kato (1995) and Bi, Jiang, Jean, and Fan (1992) gave the axial distribution of solid concentration in binary

1674-2001/$ – see front matter © 2009 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

doi:10.1016/j.partic.2009.07.004

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Nomenclature Ar dp Fr Gs Gs∗ Gsm Nd Nu u0 ut Um Udi Uui V Xco

Archimedes number, Ar = dp3 g g(p − g )/2 particle mean diameter (m) Froude number, Fr = u0 /(gdp )0.5 external solid circulation rate (kg/(m2 s)) saturation carrying rate (kg/(m2 s)) mean solid flux rate of a point (kg/(m2 s)) effective series number of downwards velocity effective series number of upwards velocity superficial gas velocity (m/s) terminal gas velocity (m/s) mean particle velocity of a point (m/s) mean particle downwards velocity of series i (m/s) mean particle upwards velocity of series i (m/s) signal voltage (V) original coarse particle fraction with respect to total particle amount

Greek letters εm mean solid holdup of a point εs solid holdup mean solid holdup of effective downwards velocity εs-di series i εs-ui mean solid holdup of effective upwards velocity series i density of gas (kg/m3 ) g s density of solid (kg/m3 )

solids CFB using pressure measurements. Until recently, there were few studies of the effects of coarse particle fraction on the radial solid distribution. In this study, the effects of solid circulation rate (11–42 kg/(m2 s)), air velocity (3.2–4.4 m/s) and the coarse particle fraction in the original total particles (0–0.25) on the flow properties in the entrance region were examined. The information obtained from this study could be beneficial for design and operation of CFB reactors. 2. Experimental setup 2.1. Cold circulating fluidized bed model The circulating fluidized bed cold model (Fig. 1) has a riser with a square cross-section of 0.25 m × 0.25 m and a height of 6.07 m. A smooth exit is used to reduce the exit effects on the gas–solid flow in the riser. Two stage cyclones collect most of the quartz sand particles with a density of 2550 kg/m3 , a mean size of 276 ␮m for fine particles and 1145 ␮m for coarse particles. The particle size distribution is shown in Table 1. The fluidizing air is supplied into the riser from a Roots blower through an air distributor with four layers of 38 ␮m steel sieves and two layers of 2000 ␮m steel sieves. The circulating solids return back to the riser from the downer through a U-type loop seal. The solid circulation rate (Gs ) is changed by adjusting the loosing air (about 30–35 m3 /h). An inclined solid return duct (50◦ , ID = 70 mm, length = 838 mm) is located on the rear wall (R-W) and the height of the outlet center to the air distributor is 0.357 m. Ten pressure taps are located along the riser to measure the axial pressure profile by differential pressure sensors. The solid circulation rate (Gs ) is measured through stop-watch way by shuting the flapper valve quickly in the return line. This procedure was repeated four times during an experimental run to get a mean Gs value.

Fig. 1. Experimental CFB cold model.

2.2. Reflective optical fiber particle concentration/velocity measurement system The particle concentration/velocity measurement system (PV6A, Institute of Process Engineering, CAS, China) is composed of two types of optical fiber probes, a control system, an A/D converter and a PC equipped with necessary hardware and software for data acquisition. A reflective-type optical fiber probe is used to measure the local particle concentration. Similar to that described by Zhou et al. (1994) and Zhang, Johoston, Zhu, de Lasa, and Bergougnou (1998), the probe is composed of a light source (LED), an optical probe with a diameter of 4 mm and a photodiode. The active area of the probe is 2 mm × 2 mm, consisting of about 5400 emitting and receiving optical fibers (the diameter of each fiber is 20 ␮m). This system was calibrated in a downer (similar to Zhang’s et al., 1998) to obtain the functional relationship (Fig. 2) between the optical fiber system signal voltage V and the particle concentration εs , as shown below V = 1.4087 ln(εs + 0.02121) + 5.37443.

(1)

Table 1 Particle-size distribution. Fine particles

Coarse particles

Size (␮m)

wt (%)

Size (␮m)

wt (%)

>600 400–600 355–400 300–355 250–300 200–250 125–200 75–125 <75

0.000 1.034 17.600 27.970 19.921 16.176 8.033 7.461 1.805

>1600 1400–1600 1180–1400 1000–1180 800–1000 600–800 <600

0.000 13.523 33.232 26.137 19.284 7.823 0.000

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Fig. 3. Measurement positions of the cross-section. Fig. 2. Relationship between signal voltage and particle concentration.

Particle velocity measurements were also conducted with a reflective-type optical fiber probe. Unlike the particle concentration measurement probe, the optical probe for velocity measurement consists of two sensors with a central distance of 2.04 mm. Each sensor sends the emitted light and receives the light reflected on the particles. Both sensor tips are 1 mm × 1 mm and consist of about 1400 emitting and receiving optical fibers (each diameter is 20 ␮m). As described by Nieuwland, Meijer, Kuipers, and van Swaaij (1996) and E, Lu, Xu, Gao, and Shi (2003), moving particles pass both sensors, causing the two sensors to produce two particle concentration signals x(t) and y(t) with a time delay. The velocity of the particles moving in front of the probe tips is obtained by dividing the distance between the sensor tips (2.04 mm) by the time delay obtained via cross-correlating both signals. The cross correlation between the two signals gives a degree of similarity between x(t) and y(t). It should be noted that the probe cannot be used in the gas–solid system with big particles (>1 mm) because of the limited probe tip active area. In the case of binary particle system, only particle concentration was measured. The measurement system was calibrated by a rotating black disk which is covered with a thin layer of white reflective material. The tangential velocity can be calculated by the angular velocity and the distance to the axis for correcting the error of measurement results. The mean velocity is expressed by E et al. (2003) as follows: Um =

Gsm = εsm s

Nd Nu U ε − Udi εs-di i=1 Nuui s-ui Ni=1 . d ε i=1 s-ui

+

ε i=1 s-di

(2)

In a CFB riser, the local velocity is a time dependent variable with fluctuations around its mean value. So the mean particle velocity should be calculated by taking into account the series of positive and negative instantaneous velocities measured. The function in Eq. (2) also considers the fluctuating local solid concentration εs . This concentration is added as a weighting factor in each series calculation. In the calibration process, it was found that the measurement error decreases with increasing sampling frequency. However, higher sampling frequency will acquire more noise signal which will increase the particle concentration error. Therefore, for the present case, the optimal conditions are as follows: Data sampling frequency: 10 kHz. Number of series: 320 time series per run. Number of samples: 1024 samples per series. Correlation coefficient: ≥0.7.

For the particle velocity optical probe, the measured voltage signals V and the particle concentration εs have a relationship as below: V = 1.4664 ln(εs + 0.02382) + 5.39604.

(3)

Measurements were taken at a height of 0.672 m above the distributor (0.315 m above the solid entrance center). Because of the symmetrical structure in the x direction, only half of the cross-section was measured. At each level, measurements were performed along the centerline and diagonal line marked “x+”, “y+”, “y−”, “D+” and “D−” respectively (Fig. 3). The measurement results of D− and y− stand for the flow properties in the region of the rear wall side, while those of y+ and D+ for the front wall side. Eleven radial positions (l/(L/2) = 0, 0.2, 0.36, 0.52, 0.68, 0.76, 0.84,0.88, 0.92, 0.96 and 1) were chosen for each line. In this study, L stands for two lengths, i.e. the centerline length (0.25 m) for x+, y+ and y−, and the diagonal length (0.354 m) for D+ and D−. 3. Results and discussion 3.1. Axial pressure drop of riser Pressure drop of a certain height of the riser is directly proportional to the particle concentration in the section (Schlichthaerle & Werther, 1999). The variations of axial pressure drop profiles with u0 and Gs are shown in Figs. 4 and 5, respectively. At a fixed solids circulating rate (Gs = 21 kg/(m2 s)), pressure drop along the riser decreases with increasing air velocity u0 due to the increase of particle entrainment rate. A dense zone is formed in the bottom bed especially at the low air velocity. On the other hand, at a given u0 , pressure drop increases with increasing Gs , possibly due to the clustering effects and the increase of solid back-mixing. At low Gs (11 kg/(m2 s)), the axial pressure drop in the riser is almost uniform. As the Gs is increased to 21 kg/(m2 s), the bottom bed pressure drop increases slightly. Increasing Gs to 33 kg/(m2 s) leads to a relatively dense bottom bed which forms an exponential distribution along the riser. A further increase of Gs from 37 to 40 kg/(m2 s) results in a significant increase of pressure drop in the bottom bed. After that, the bottom bed pressure drop changes little with the increase of Gs and a S-shaped pressure drop starts to form. Gs = 40 kg/(m2 s) is then taken as the saturation carrying rate Gs∗ for u0 = 4 m/s (Bai & Kato, 1995). By similar process, the Gs∗ for other three gas velocities were obtained (Fig. 6). Since the u0 = 2 m/s is closed to the calculated terminal velocity of particle (ut = 1.67 m/s),

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Fig. 4. Effect of u0 on pressure drop profile. Fig. 7. Solid holdup profile (u0 = 4 m/s, Gs = 21 kg/(m2 s)).

the range for Gs at which exponential pressure drop distribution can be kept is small. For the Gs∗ calculation, Bai and Kato (1995) summarize a correlation based on the data of their own and other researchers, with most of the particles being of Geldart-A type: Gs∗ dp = 0.125Fr 1.85 Ar 0.63 



p − g g

−0.44

,

(4)

where the Archimedes number Ar ranges from 4.7 to 1019, and the Froude number Fr varies from 41 to 226. For the GeldartB type particle used in our work, the calculated Ar = 2017 and Fr = 34.6, which are not in the range of the correlation data source. Therefore, more work should be done to get a more accurate correlation of Gs∗ for Geldart-B type particles, in which ut should be considered. 3.2. Radial flow asymmetry in solid entrance region

Fig. 5. Effect of Gs on pressure drop profile.

Fig. 6. Saturation carrying rate vs. u0 .

Fig. 7 shows the solid holdup profile at a height of 0.672 m under the condition of u0 = 4 m/s and Gs = 21 kg/(m2 s). There is a lean core surrounded by a denser annulus zone. Both the solid distribution profile and visual observation demonstrated that it was denser in the region near the front wall. A relatively higher solid holdup exists in the corner of the rear wall side (D−). However, in the front wall side, the particle concentration at the central line (y+) is higher than that at the diagonal line (D+). When the recycling particles are entrained by the air from the loop seal to the inclined return duct, the duct inlet has a relatively even solids distribution. However, under the effects of gravity, solids are separated from air and only move along the bottom section at the lower part of the return duct. After acceleration through the inclined return duct by the gas entrainment and the gravitational effects, back feeding particles usually accumulate an amount of horizontal and vertical momentum components when they enter into the bottom bed. This is different from the horizontal return duct that is often used with the L-valve, which has only horizontal momentum determined by entrainment capacity of loosing air and the push force of solid seal. In this operating condition, solid holdup is so lean that the flow of back feeding particles penetrates the gas–solid flow in the riser and gets to the opposite wall (front wall, F-W). In the penetration process, some solids are entrained by the upward gas–solid flow. Meanwhile others flow further down to the core and to the region near the front wall. Visual observation

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Fig. 8. Particle velocity profile (u0 = 4 m/s, Gs = 21 kg/(m2 s)).

of the front wall revealed that the back feeding particles that get to the front wall collect the falling solids and make a denser zone. Lateral profiles of the mean vertical particle velocities are shown in Fig. 8. Particles near the wall are seen moving downwards, while other particles are conveyed upwards in the core. In the corner, downwards particles (D+, D−) move faster than the particles near the facing wall according to centerline measurement (x+, y+, y−). The maximum particle velocity in the corner region does not exist at the corner point (l/(L/2) = 1), but instead exists between l/(L/2) = 0.88 and l/(L/2) = 0.92 due to the wall friction effects. Similar experimental results have been obtained by Zhou, Grace, Lim, and Brereton (1995). There is no symmetry of the particle velocity profile or the particle distribution (Fig. 8). A thicker falling particle region exists in the front wall side (y+, D+). At the centerline of y−, the particle velocity falls, but increase in the region from l/(L/2) = −0.52 to l/(L/2) = −0.76 and then decrease near the wall. One possible reason is the effect of the loosing air from the loop seal. This part of air enters into the riser with a low velocity of about 2–3 m/s, which cannot penetrate and rises in the vicinity of the rear wall. At y+, the particle velocity falls monotonically from the center to the wall. The velocity at y+ in the central region (from l/(L/2) = 0 to l/(L/2) = 0.36) is a bit lower than that at y−. As analyzed above, there exists a larger fall in the solid flux near the front wall in the bottom bed. This solid flow forced some fluidizing air to turn to the rear wall side (Fig. 9), which makes the solids in the rear wall side move faster. Comparing other points of the rear wall region, the particle velocity falls smoothly from the center to the front wall and has a bigger value in the region between l/(L/2) = 0.52 and l/(L/2) = 0.68. The escaped particles from the flow of back feeding particles may have some remaining momentum before they are accelerated upwards, similar to the analysis of Yan et al. (2003).

Fig. 9. Flow of back feeding particles in bottom zone of riser (u0 = 4 m/s, Gs = 21 kg/(m2 s)).

The particle velocity profile (Fig. 11) shows that higher solid circulation rate leads to lower rising velocities in the central region than the case of Gs = 21 kg/(m2 s). It is possible that the bigger clusters formed by the increasing solid holdup can not get enough acceleration from the bottom bed to reach the measurement level. Higher particle velocity is found in the core zone of the rear wall side. Visual observation of the side walls shows that the falling solids flow near the bottom front wall force more fluidizing air from the air distributor to the rear wall side. At the centerline y−, the particle velocity decreases monotonically towards the wall, unlike in the condition of lower Gs . It is possible that denser solid holdup in the riser lessens the effects of loosing air from the loop seal. The bias phenomenon of fluidizing air flow which can be seen on the side walls, weakens greatly when Gs is increased to 37 kg/(m2 s). And further increase in Gs to more than 40 kg/(m2 s) leads to steady

3.3. Flow asymmetry in solid entrance region for high solid circulation rate Higher solid circulation rate (u0 = 4 m/s, Gs = 33 kg/(m2 s)) leads to a higher solid holdup in the riser (Fig. 4). But the solid flow with higher momentum increases the asymmetry of the particle concentration profile in the entrance region (Fig. 10). In contrast to the condition of lower Gs (u0 = 4 m/s, Gs = 21 kg/(m2 s)), the corner has a denser solid holdup than the region near the centerline of the front wall.

Fig. 10. Solid holdup profile (u0 = 4 m/s, Gs = 33 kg/(m2 s)).

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Fig. 11. Particle velocity profile (u0 = 4 m/s, Gs = 33 kg/(m2 s)).

Fig. 13. Particle velocity profile (u0 = 3.2 m/s, Gs = 21 kg/(m2 s)).

dense bottom bed which extends to a higher position when Gs increases further. No lateral solid flow impinged on the front wall and no bias of fluidizing air could be seen.

wall, while others flow further into the central region of the riser. The measurements show that the solid holdup difference between the diagonal line and the centerline increases, so does that between the annulus zone and core zone. Although the superficial gas velocity in this condition (u0 = 3.2 m/s, Gs = 21 kg/(m2 s)) is lower than that in the first condition (u0 = 4 m/s, Gs = 21 kg/(m2 s)), the particle in the center region at the height of 0.672 m has a higher velocity (about 3.3 m/s) (Fig. 13), which is also higher than the superficial air velocity across the section. A much higher solid holdup in the riser reduces the net flow area for the rising air and forces more air to flow through the center region with higher velocity, which also resists the flow of back feeding particles moving deeper into the bed. This makes the particles in the region from l/(L/2) = 0.68 to l/(L/2) = 0.84 rise with a lower velocity. Comparing with that under the two conditions above with higher u0 , the velocity profile in this condition is more symmetrical. There is no distinct air distribution bias in the bottom bed from the side walls observation, which is different from the higher u0 conditions.

3.4. Flow asymmetry in solid entrance region under condition of lower air velocity With constant Gs = 21 kg/(m2 s), when u0 is decreased from 4 to 3.6 m/s, the bias phenomenon in the bottom bed doesn’t change. A further decrease of u0 to 3.2 m/s while Gs = 21 kg/(m2 s) causes the system to exceed the saturation carrying capacity of the gas. As a result, a relatively apparent S-shaped pressure drop profile forms with a denser bottom zone (Fig. 6), which limits the flow penetration of back feeding particles and force the fluidizing air flow in the central region. This also results in a little higher solid holdup in the rear wall side (Fig. 12). Visual observation showed that in the return duct, the flow of back feeding particles was resisted by the dense solid holdup near the duct outlet. Sometimes the solids in the bed flow back into the inclined duct. When solids enter into the bed, many of them are carried by the falling solids near the rear

3.5. Effect of coarse particle addition on solid distribution in solid entrance region

Fig. 12. Solid holdup profile (u0 = 3.2 m/s, Gs = 21 kg/(m2 s)).

When coarse particles (>600 ␮m, dp = 1145 ␮m) of different masses were added to the fine particles (dp = 276 ␮m, <600 ␮m), four Xco (original coarse particle mass fraction with respect to total particle amount) values (0.1, 0.15, 0.2, 0.25) were obtained. The experiments are performed with same solids height of 0.4 m in the downer side of the loop seal. The coarse particles change the solid holdup in the riser (Fig. 14). More solids accumulate in the riser with increasing coarse particle fraction, especially in the bottom zone. The coarse particle fraction of the external circulating particles increases with increasing original coarse particle fraction (Fig. 15). But in all cases, it failed to reach the original fraction demonstrating that only some of the coarse particles can be entrained into the recycling system. According to the Bai’s definition (1994) that is partial segregation. More coarse particles accumulate in the bottom bed and change the solid distribution across the section. At a lower Xco (0.15), the solid holdup profile is similar to the ones with no coarse particle case except for the higher values (Fig. 16). That is probably due to the inertial effects of the coarse particles. Adding more coarse

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Fig. 14. Effect of original coarse particle fraction Xco on pressure drop profile (u0 = 4 m/s, Gs = 21 kg/(m2 s)). Fig. 17. Solid holdup profile (u0 = 4 m/s, Gs = 21 kg/(m2 s), Xco = 0.25).

particle into the CFB system (Xco = 0.25) makes the solid holdup so high that it can limit the flow penetration of back feeding particles, making the solid holdup profile more symmetrical (Fig. 17). 4. Conclusions

Fig. 15. Effect of Xco on coarse particle fraction of external circulating particles.

1. The flow of back feeding particles usually has some momentum after acceleration through the inclined return duct due to the gas entrainment and gravitational effects. 2. The saturation carrying capacity of gas for the experimental particles (Geldart-B type) can be determined when the S-shaped pressure drop distribution starts to form. 3. Under the condition of u0 = 4 m/s and Gs = 21 kg/(m2 s), the back feeding particles penetrate the lean gas–solid flow in the riser and get to the opposite wall. This movement results in a relatively denser region near the front wall. A higher solid circulation rate (Gs = 33 kg/(m2 s)) makes a relatively denser bottom bed in the riser and the solid flow with higher momentum increases the asymmetry of the particle concentration/velocity profile in the entrance region. 4. Lower superficial air velocity (u0 = 3.2 m/s) makes a denser bottom zone when the Gs is beyond the saturation carrying capacity of gas, which limits the recycling solid flow’s penetration and results in a slightly higher solid holdup near the rear wall. 5. Under the condition of u0 = 4 m/s, Gs = 21 kg/(m2 s), the addition of coarse particle (dp = 1145 ␮m, Xco = 0.25) into the bed material causes the radial distribution of solids to becomes more symmetrical. Acknowledgement The present study is supported financially by the Ministry of Science of China under the National Key Technology R&D Program of China (Contract No.: 2006BAA03B01-07). References

Fig. 16. Solid holdup profile (u0 = 4 m/s, Gs = 21 kg/(m2 s), Xco = 0.15).

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