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Two-dimension Experimental Study of Flow Dynamics in a Gas-Solids Turbulent Fluidized Bed

Author: ChengKai
Tutor: ZengTao
School: Sichuan Institute of Technology
Course: Mechanical Design and Theory
Keywords: Turbulent fluidized bed Pressure fluctuation Flow characterization Globalflow structure Solids concentration
CLC: TB126
Type: Master's thesis
Year: 2012
Downloads: 75
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Abstract


In recent years, with the rapid development of traditional industries, the advantages of turbulent fluidization are increasing significantly over bubbling fluidization, and thus its application becomes more widely, arousing the interest of many experts and scholars. The turbulent fluidization is a flow regime between bubbling and fast fluidization. Compared with the bubbling fluidized bed, turbulent fluidized bed has a relatively low pressure pulsation amplitude and, gas contacts fully with solid,the uniform temperature within the bed, high efficiency of heat transfer and mass transfer; And due to the intensified consolidation and fragmentation of the bubbles, turbulent fluidized bed operation is smooth, no large air bubbles. However, because of the different experimental conditions and different experimental methods, there is considerable controversy on the findings of the flow characteristics of turbulent fluidized bed in recent years. With the development of computer numerical simulation, experts start to research on two-phase fluidization with method of CFD numerical simulation.Experimental research is carried out in the two-dimensional gas-solid fluidized bed of cold model with cross-section of411×60×1770mm. Distribution of pressure and the local solids concentration were dealt with through a detailed test and analysis, using the differential pressure transducers and fiber optic probe of solids concentration, and other high-precision equipments.The results show that the variation of standard deviation of differential pressure fluctuation in the two-dimensional fluidized bed with the superficial gas velocity still showed a significant turning point in the two-dimensional fluidized bed, and the transition velocity Uc from the bubbling to the turbulent flow regime is obtained:Transition velocity Uc of each cross-section in the axis direction of the bed decreases gradually from the bottom to the top of the bed which indicated that transformation from bubbling to turbulent fluidization is occurred from top to bottom of the bed, rather than the transient fluid behavior of the entire bed. At the same time transition velocity is influenced to a large extend by the static bed height.The analysis result of local solids concentration shows that bed expansion is enhanced with the increase of superficial gas velocity, and the distribution of average solids holdups of each axial cross-section becomes uneven. Top of global flow structure is sparse and the bottom is dense in the bed. The average behavior of the fluidization shows great heterogeneity in the axial direction.At last, the comparative analysis of the experimental conclusion and the numerical simulation result on dynamics behavior of two phase fluid of gas-solid of this fluidized bed with standard k-ε turbulence model of CFD was accomplished. The same trend of the experimental results and simulation results on transition process from the bubbling to the turbulent flow regime and the distributions of local solids concentration, etc, was found in the bed.

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