The study of axial dispersion behavior of non-Newtonian fluid at various flow regimes in a liquid-solid magnetofluidized bed
|關鍵字:||液固磁流體化床;軸向分散係數;liquid-solid magnetofluidized bed;axial dispersion coefficient||公開日期:||2006||摘要:||本論文在探討有外加磁場作用下，液、固流體化床之流動模式、分散行為與傳統液、固流體化床之差異。流體化床採用內徑0.05 m高約0.75 m壓克力管，固相以平均粒徑274 μm之鐵粒子為磁性粒子，液相為水及濃度分別為0.03 wt%、0.1 wt%與0.2 wt%之CMC(carboxymethyl cellulose)溶液。床體外部利用漆包線纏繞成的兩螺線管來產生穩定的軸向磁場，使其作用於床體內部之粒子。利用壓力轉換器測量所得之單點壓力降對流體速度之曲線圖，來界定各流態間轉變之速度。並且利用追蹤劑技術分析不同流態區域下的軸向分散情況。
單點壓降實驗中，藉由Leva法所得鐵砂粒子之最小流體化速度Umf(L)並不會受磁場強度影響；而以Davidson and Harrison法所得之最小流體化速度Umf(DH)則會隨磁場強度上升而增加。磁滯現象的形成即可決定移轉速度Ut(P)，且隨著磁場強度上升而增加。
In this study, the difference of hydrodynamic and dispersion behavior between the liquid-solid magneto-fluidized (0.05 m i.d. × 0.075 m height) bed and the liquid-solid traditional fluidized bed was investigated. The sphere iron particles of 274 μm average diameter were used as the solid phase, water and CMC (carboxymethyl cellulose) (0.03 wt%, 0.1 wt% and 0.2 wt%) solution were used as the liquid phase. A magnetic solenoid coiled by the magnet wire was applied to generate axially magnetic field. The dependency of pressure drop on the velocity was used to establish transition velocities between various regimes. The axial dispersion of various regimes was analyzed by the tracer input-response technique. By Leva’s method, the minimum fluidization velocity Umf(L) was not affect by magnetic force. However, by Davidson and Harrison’s method, the minimum fluidization velocity Umf(DH) was increased with increasing magnetic field intensity. Further more, the transition velocity Ut(P) determined by the hysteresis phenomenon was increased with increasing magnetic field intensity. The experimental results show that there were four flow regimes in a magneto-fluidized bed: fixed bed, transition, magnetically stabilized and unstable regime. The boundaries were minimum fluidization velocity Umf(L), minimum fluidization velocity Umf(DH) and transition velocity Ut(P). When the concentration of the CMC solution increased, the range of fixed bed, transition and magnetically stabilized regime was narrowed down, the unstable regime was enlarged. Both in the magnetic and the traditional fluidized beds, the axial dispersion coefficient was increased with the enhancing fluid velocity. The axial dispersion coefficient of the magnetic fluidized bed was lower than the traditional fluidized bed. The dispersion coefficient of the water (Newtonian) and the CMC solutions (non-Newtonian) both were decreased with the enhancing magnetic field intensity. Besides, the effect of magnetic field on the dispersion coefficient was decreased with the increasing concentration of CMC solution.
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