基于EMMS结构的多尺度传质反应模拟
发布时间：2018-09-11 13:57
【摘要】：气固循环流化床反应器是一个流动、传热／传质和反应多尺度时空耦合的复杂系统,其中介尺度流动结构(如团聚物或气泡)起着关键性的作用。基于均匀假设,忽略了亚网格介尺度结构影响的传统双流体模型(two-fluid model,TFM)对于模拟气固非均匀流动反应体系是不合适的。需要在合理物理简化的基础上建立介尺度模型,才可准确描述流化床中的“三传一反”行为。介尺度EMMS曳力模型在循环流化床流动模拟中已经得到了很好的应用,但其网格无关性还需进一步的验证。同时,介尺度流动结构对于传质和反应的影响也需进行考察。鉴于此,论文第二章首先研究了不同固相体积分率下的双周期微元区域内,应用微元EMMS曳力和均匀曳力预测的滑移速度和传质系数随网格分辨率的变化。对于A类颗粒,微元EMMS曳力在不同固相体积分率下都表现出了更好的网格无关性。两种曳力模型预测的流动结构随网格细化都捕捉到更合理的非均匀流动结构,造成更大的传质阻力,使得有效传质系数下降,并在网格尺寸为10个颗粒直径时趋于收敛。对于B类颗粒的传质研究也发现了类似现象。细网格预测的有效传质因子和基于EMMS结构的传质非均匀因子随固相体积分率的变化趋势相同且在同一量级上。为了考虑介尺度非均匀结构对于传质、反应的影响,论文第三章和第四章提出了基于EMMS结构的多流体传质和反应模型,此模型在局部平衡或网格内没有非均匀结构的假设时,可以退化为TFM框架下的传质和反应模型。应用该模型分别在基于团聚物或气泡的流动结构下分析传质、反应过程,定义了反应和传质的非均匀因子,以修正TFM传质反应模型。论文通过二维和三维构体下的臭氧催化分解反应模拟,对模型进行了初步的验证,模拟结果与文献结果相符。反应速率越快,非均匀流动结构对于传质和反应的影响越大。论文第五章通过虚拟实验来考察计算中在线调节机械阀门和改变提升管悬浮段长度对于宏尺度非均匀流动行为的影响。首次实现了带可调节机械阀的、三维全循环的循环流化床模拟,模拟结果和实验描述相符。第六章对本论文进行了总结,提出了主要的结论和创新点,并对未来的研究进行了展望。
[Abstract]:Gas-solid circulating fluidized bed reactor (CFB) is a complex system with flow, heat / mass transfer and reaction multi-scale space-time coupling. Its mesoscale flow structure (such as agglomerates or bubbles) plays a key role. Based on the homogeneous hypothesis, the traditional two-fluid model (two-fluid model,TFM), which ignores the influence of mesoscale structure on subgrids, is not suitable for simulating gas-solid nonuniform flow systems. It is necessary to establish mesoscale model on the basis of reasonable physical simplification in order to accurately describe the behavior of "three to one inverse" in fluidized bed. Mesoscale EMMS drag model has been well applied in CFB flow simulation, but its mesh independence needs further verification. At the same time, the effect of mesoscale flow structure on mass transfer and reaction also needs to be investigated. In the second chapter, we first study the variation of slip velocity and mass transfer coefficient with grid resolution in the two-period microelement region with different volume fraction of solid phase, using differential EMMS drag and uniform drag force to predict slippage velocity and mass transfer coefficient. For A particles, the EMMS drag of microelement shows better mesh independence under different solid volume fraction. The flow structures predicted by the two drag models both capture more reasonable non-uniform flow structures with mesh refinement, resulting in greater mass transfer resistance, lower effective mass transfer coefficient, and convergence when the mesh size is 10 particle diameters. A similar phenomenon was found for the mass transfer of B particles. The effective mass transfer factor predicted by fine mesh and the non-uniform mass transfer factor based on EMMS structure have the same trend with solid volume fraction and are of the same order of magnitude. In order to consider the effect of mesoscale nonuniform structure on mass transfer and reaction, a multi-fluid mass transfer and reaction model based on EMMS structure is proposed in chapter 3 and chapter 4 in this paper. It can degenerate into mass transfer and reaction model under TFM framework. The model is used to analyze the mass transfer and reaction process under the flow structure of agglomerates or bubbles, and the heterogeneous factors of reaction and mass transfer are defined to modify the TFM mass transfer reaction model. In this paper, the ozone catalytic decomposition reaction is simulated under two and three dimensional structures, and the model is preliminarily verified. The simulation results are in agreement with the literature results. The faster the reaction rate, the greater the effect of heterogeneous flow structure on mass transfer and reaction. In the fifth chapter, the effect of on-line adjusting mechanical valve and changing the length of hoisting pipe on the macro-scale non-uniform flow behavior is investigated by virtual experiment. For the first time, a three-dimensional circulating fluidized bed simulation with adjustable mechanical valve is realized. The simulation results are in agreement with the experimental results. The sixth chapter summarizes the thesis, puts forward the main conclusions and innovations, and looks forward to the future research.
【学位授予单位】：中国科学院研究生院(过程工程研究所)
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TQ031
[Abstract]:Gas-solid circulating fluidized bed reactor (CFB) is a complex system with flow, heat / mass transfer and reaction multi-scale space-time coupling. Its mesoscale flow structure (such as agglomerates or bubbles) plays a key role. Based on the homogeneous hypothesis, the traditional two-fluid model (two-fluid model,TFM), which ignores the influence of mesoscale structure on subgrids, is not suitable for simulating gas-solid nonuniform flow systems. It is necessary to establish mesoscale model on the basis of reasonable physical simplification in order to accurately describe the behavior of "three to one inverse" in fluidized bed. Mesoscale EMMS drag model has been well applied in CFB flow simulation, but its mesh independence needs further verification. At the same time, the effect of mesoscale flow structure on mass transfer and reaction also needs to be investigated. In the second chapter, we first study the variation of slip velocity and mass transfer coefficient with grid resolution in the two-period microelement region with different volume fraction of solid phase, using differential EMMS drag and uniform drag force to predict slippage velocity and mass transfer coefficient. For A particles, the EMMS drag of microelement shows better mesh independence under different solid volume fraction. The flow structures predicted by the two drag models both capture more reasonable non-uniform flow structures with mesh refinement, resulting in greater mass transfer resistance, lower effective mass transfer coefficient, and convergence when the mesh size is 10 particle diameters. A similar phenomenon was found for the mass transfer of B particles. The effective mass transfer factor predicted by fine mesh and the non-uniform mass transfer factor based on EMMS structure have the same trend with solid volume fraction and are of the same order of magnitude. In order to consider the effect of mesoscale nonuniform structure on mass transfer and reaction, a multi-fluid mass transfer and reaction model based on EMMS structure is proposed in chapter 3 and chapter 4 in this paper. It can degenerate into mass transfer and reaction model under TFM framework. The model is used to analyze the mass transfer and reaction process under the flow structure of agglomerates or bubbles, and the heterogeneous factors of reaction and mass transfer are defined to modify the TFM mass transfer reaction model. In this paper, the ozone catalytic decomposition reaction is simulated under two and three dimensional structures, and the model is preliminarily verified. The simulation results are in agreement with the literature results. The faster the reaction rate, the greater the effect of heterogeneous flow structure on mass transfer and reaction. In the fifth chapter, the effect of on-line adjusting mechanical valve and changing the length of hoisting pipe on the macro-scale non-uniform flow behavior is investigated by virtual experiment. For the first time, a three-dimensional circulating fluidized bed simulation with adjustable mechanical valve is realized. The simulation results are in agreement with the experimental results. The sixth chapter summarizes the thesis, puts forward the main conclusions and innovations, and looks forward to the future research.
【学位授予单位】：中国科学院研究生院(过程工程研究所)
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TQ031
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