Voidage and particles velocity profiles in magnetically fluidized beds

• Main Authors: Yung-Chung Lin, 林永忠
• Other Authors: Lii-Ping Leu
• Format: Others
• Language: zh-TW
• Published: 2001
• Online Access: http://ndltd.ncl.edu.tw/handle/25737558326796659474

博士 === 國立臺灣大學 === 化學工程學研究所 === 89 === Abstract Keywords:fluidized bed, magnetic fluidized bed, voidage, optical fiber probe Voidage and Particle motion in a fluidized bed of iron particles (230 µm) were investigated under the influence of an external uniform axial magnetic field. Passing a direct current through five solenoids generated uniform magnetic field. The five solenoids were arranged elaborately to get larger uniform magnetic space than that generated by Helmholtz electromagnet coils. A sensitive Y-type optical probe measuring system, based on detection of light reflected by particles, was used to measure local voidage in both dense and phases; and compared the cross-sectionally averaged voidages determined using optical fiber probe with those values measured by pressure probe and shutter plates. A parallel-type optical probe measuring system, based on detection of light reflected by particles, was used to measure particle velocity in dense phase. Local voidage was measured as a function of superficial fluidizing air velocity, magnetic field intensity and the position in the bed. At a given magnetic field intensity and at the same position in the bed, the voidage was constant for a low air velocity range (in a fixed bed). The local voidage changed irregularly with increasing air velocity for an intermediate air velocity range (in a magnetically stabilized fluidized bed, MSFB). The local voidage changed linearly with increasing air velocity for a slightly high air velocity range (in a magnetized bubbling fluidized bed, MBFB). A general correlation was developed to predict the local solids fraction at the arbitrary position in the bed: (1—ε) = (1—ε)c + [(1—ε)w — (1—ε)c ](r/R)B where (1—ε), (1—ε)c and (1—ε)w represented the local solids fraction at arbitrary position in the bed, at the bed center and on the bed wall; and B, (1—ε)c and (1—ε)w were the function of air velocity, distance from the distributor and magnetic field intensity. Particle velocity was measured as a function of superficial fluidizing air velocity, magnetic field intensity and the position in the bed. The experimental results revealed that the particle velocity was zero in a magnetically stabilized fluidized bed (MSFB). Particle velocity in a magnetized bubbling fluidized bed (MBFB) decreased with increasing magnetic field intensity, increasing distance from the axis of bed column, decreasing gas velocity, and decreasing distance from the distributor. There were three flow patterns of particles in MBFB whether the magnetic field intensity was large or not: (1) Particles up flow at the bed center and down flow at the wall when bubble broke at bed center. (2) Particles up flow between the bed center and bed wall and down flow at both bed center and bed wall when bubble broke between the bed center and bed wall. (3) Particles up flow at the bed wall and down flow at bed center when bubble broke at bed wall. The magnitude and direction of particle velocity was calculated by using the below equation: Vp = Lef / ( τr2 + τz2 )1/2 and θ= tan-1 [ τz / τr ]. A general correlation was developed to predict the axial transit time (τz): 1/τz = 1/τzc— [1/τzc— 1/τzw] (r/R)B where τzc, τzw and B represented the axial transit time at the bed center and at the bed wall; and they were the function of air velocity, distance from the distributor and magnetic field intensity, they had similar form: C1(z/hs)C2(Us/Umf)C3{exp[C4(z/hs) + C5(Us/Umf)]}(H/1000)C6. The evolution of bubble was be found from the simulation by commercial software. Gas velocity increased with decreasing magnetic field intensity and formation of bubble. Particle concentration increased with increasing magnetic field intensity, increasing distance from the axis of bed column and decreasing gas velocity. Particle velocity decreased with increasing magnetic field intensity, increasing distance from the axis of bed column and decreasing gas velocity. The solids fraction profiles computed from simulation were larger than those from experiments, but the standard deviation was smaller than 5 % at Us/Umf =1.42. The particle velocity profiles computed from simulation were similar to those from experiments, the standard deviation was smaller than 12 % at Us/Umf =2.22.