Knowledge Management System Of Institute of process engineering,CAS
循环流化床是一种气固加工的高效反应器，在现代工业过程中具有广泛应用。由于循环流化床具有复杂的非均匀流动流动结构，因此利用CFD软件模拟循环流化床已成为一种预测其流体力学性能的有效手段。曳力是模拟流化床关键因素之一，传统的曳力模型没有考虑流动结构的影响，模拟精度较差，因此发展结构曳力模型成为研究热点，其核心是结构参数的模型化求解。课题组前期发展了通过经验方程封闭求解结构参数的方法，并成功应用于鼓泡床和湍动床中，但尚未应用于循环床。另外，结构曳力模型往往包含众多的“结构参数”，这些参数对曳力的影响规律尚不清楚，影响模型的优化。针对上述问题，本文采用聚团性质方程封闭了循环床提升管反应器结构参数模型，建立了基于传递系数的结构参数方法，并进行了详细的参数敏感性分析，简化了模型，并将该方法推广到鼓泡床和湍动床中。提出了利用多粒径输送床（MPTB）验证多粒径循环床的方法。最后将提升管建模方法应用到下行床中。本论文取得的主要研究结果和结论如下所示：（1）利用聚团性质方程（聚团直径和空隙率）对于提升管的非线性方程组进行了封闭求解，建立了结构参数模型。利用该模型准确的计算了提升管的曳力系数。模拟的固体循环量、径向和轴向时均空隙率分布与实验值吻合良好，能够捕捉到提升管典型的上稀下浓和环核结构。同时，在模拟中能够清晰的观察到聚团的时空波动。（2）开发了基于传递系数的结构参数(Transfer Coefficient-based Structure Parameters，TC-SP)方法，该方法能够确定循环床必要的结构参数，从而简化结构参数模型。通过对循环床TC-SP模型进行参数敏感性分析发现，由于聚团相曳力远大于分散相曳力，曳力对聚团相结构参数更为敏感，从而得到了基于聚团相简化的循环床TC-SP模型。通过与其它结构曳力模型的模拟结果对比发现，尽管简化的TC-SP模型仅具有其它模型一半的参数，却可以获得相同甚至更好的模拟结果，表明聚团相曳力才是计算总曳力的关键。（3）将循环床TC-SP方法推广至鼓泡床和湍动床，建立了相应的结构参数模型，通过参数敏感性分析发现，对于鼓泡床乳化相曳力占主导，对于湍动床聚团相曳力占主导。将简化的TC-SP模型与其它多参数结构曳力模型对比，结果表明简化的模型同样可以获得相同甚至更好的模拟结果。因此对于三种床型，密相（乳化相、聚团相）曳力才是计算总曳力的关键。（4）本文建立了拟均相模型，并利用该模型和简化的循环床TC-SP模型模拟了MPTB中的粗颗粒终端速度和细颗粒体积分数，验证了模型的准确性的同时，也说明了利用简单且准确的MPTB实验数据验证多粒径CFB曳力模型的有效性。（5）最后，借鉴了提升管建模方法，建立了下行床的结构曳力模型和结构参数模型。前人所建立的模型由于没有考虑曳力的影响，仅限于模拟下行床的径向流动结构。基于本文建立的结构曳力模型，模拟结果与实验结果在径向和轴向上都取得了良好的一致性。通过改变聚团性质方程中聚团直径和空隙率大小，考察了聚团对曳力的影响。结果发现相对于提升管，下行床中的聚团直径和空隙率对于曳力影响很小。下行床独特的结构被观测到，包括浓环结构、轴向速度分布、轴向压力分布。 ;A Circulating fluidized bed (CFB), which has the high flow rate of gas-solid and intensive gas-solid contacting, has been widely used in various industrial processes. Characterized by the complex heterogeneous meso-scale structures in the CFB, computational fluid dynamics (CFD) has been a valuable tool to predict the fluid dynamics. The gas-solid drag model is believed to be one of the key factors to capture the characteristics of fluidized beds. Conventional drag models could not correctly simulate fluidized beds because meso-scale structures are neglected. Therefore, developing structure-based drag models has received much attention, and the key to resolve the drag model is to resolve structural parameters. Our group has developed the method where incorporates with reliable classical equations to close the insufficient solving equations. The method has been successfully applied for the bubbling fluidized bed (BFB) and turbulent fluidized bed (TFB), but not used for the CFB yet. Besides, structure-based drag models contain many structural parameters, but the effect of various structural parameters on the drag force is unclear. This weakens the modification of the drag model.Aiming at these problems, firstly, the structural parameter model of the riser is established incorporating with the equations of clusters properties. Secondly, the transfer coefficient-based structure parameters (TC-SP) method is proposed, and the sensitivity of structural parameters is analyzed. Then, this method is applied for the BFB and TFB. Besides, this thesis presents an approach where validates the drag model of the multisolid CFB by the multisolid pneumatic fluidized bed (MPTB). Lastly, the method of establishing the drag model of the riser is used for the downer reactor. The principal results and major conclusions are as follows:i) Equations of clusters properties, including the cluster voidage and diameter, are firstly adopted to close the insufficient solving equations, and a new structural parameter model of risers is established. The model can correctly resolve the drag force coefficient of risers. The simulated solid mass flux, radial and axial voidage profiles are in reasonable agreement with the experimental data. The dilute-top/dense-bottom and the core-annular flow structure are also captured. Moreover, the spatiotemporal fluctuation of clusters can be observed from those simulations.ii) By analyzing the averaged drag coefficient of the CFB, the transfer coefficient-based structure parameters (TC-SP) method is proposed. Based on the TC-SP method, a quantitative sensitivity analysis of the structural parameters is performed. It is found that since the drag force in the cluster phase is extremely larger than that in the dispersed phase, the drag coefficient is more sensitive to the parameters in the cluster phase than those in the dispersed phase. Therefore, the structural parameter model and drag coefficient based on the original TC-SP method can be further simplified which relies mainly on the cluster phase. The simplified TC-SP model and other structure-based drag model are compared. Interestingly, despite with half number of parameters, the simplified TC-SP model can achieve same or even better simulation results than other structure-based drag model with more parameters. It indicates that the drag force in the cluster plays a dominant role in the total drag force.iii) The TC-SP method for the CFB is applied for the BFB and CFB. By analyzing the sensitivity of the structural parameters, it is found that the drag force in the emulsion phase for the BFB and the cluster phase for the TFB play a dominant role in the total drag force, respectively. Then, the simplified TC-SP models are compared with other structure-based drag models, and the simulated results also show that the simplified TC-SP models can achieve same or even better simulation results than other structure-based drag models with more parameters. Therefore, for the three typical fluidized beds (BFB, TFB and CFB), the drag coefficient in the dense phase (emulsion phase or cluster phase) is the key to resolve the total drag coefficient.iv) This thesis uses a new pseudo-homogeneous method and the simplified TC-SP model of the CFB to simulate the coarse particle terminal velocity and fine particle holdup in the MPTB. These results demonstrate that the multisolid pneumatic transport system can serve as a simple binary system, with its coarse particle terminal velocity and fine particle holdup being used for the verification and validation of binary-particle CFB models.v) A structure-based drag model for downers is established by the similar method for establishing the model of the riser reactor. Meanwhile, the effects of clusters on the drag force are investigated. By analyzing the effects of variations of the cluster voidage and diameter on the drag force, it is found that, unlike risers, the cluster voidage and diameter have a negligible effect on the drag in downers. Previous studies are limited to simulate the radial performance in downers because of using the conventional drag model. The simulation results of the new structure-based drag model show reasonable agreements with the experimental data in the radial and axial direction. The unique trends of downers are also obtained, e.g., the dense-ring flow structure, axial velocity flow structure and pressure distribution.
|刘文明. 循环流化床模拟和基于传递系数结构参数方法研究[D]. 中国科学院研究生院,2017.|
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