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B类颗粒气固鼓泡流化床内两相流模拟的新曳力模型
其他题名A new drag model for TFM simulation of gas-solid bubbling fluidized beds with Geldart-B particles
王英策
学位类型硕士
导师李洪钟 ; 朱庆山
2013-04-01
学位授予单位中国科学院研究生院
学位专业化学工程
关键词鼓泡床   两相流模型   曳力模型   b类颗粒   介尺度
摘要气固鼓泡流化床已经广泛应用于工业过程中,如煤气化,石油催化裂化,干燥和乙烯聚合等。由于较大的气固相接触面 积和充分的气固两相混合,与传统反应器相比,鼓泡床具有处理量大、传质传热系数高、温度分布较均一等优点。近些 年,随着计算机硬件的飞速发展,商用流化床的CFD模拟已成为可能。已有研究表明,使用Fluent软件进行气固鼓泡流 化床的两相流模拟时,气固两相曳力模型的选择是关键。气固流化床是典型的非线性非平衡系统,呈现出复杂的时空多 尺度结构。其中气泡和团聚物是以气体聚集(气泡相,稀相)和颗粒聚集(乳化相,密相)共存为代表的两种典型多尺度结 构。已有文献中的曳力模型,大多都是均匀结构曳力模型,而没有将介尺度结构的影响考虑在内。本论文,针对低气速 下Geldart B类颗粒提出一个新的介尺度结构曳力模型,将新的曳力模型耦合到Fluent软件中模拟实验。将模拟结果与 实验结果对比以验证模型的合理性。 论文第二章首先介绍了实验装置和测定方法。其次,罗列了实验物料的物性, 并汇总了实验结果。论文第三章首先将 气固鼓泡流化床分解为气泡相、乳化相和相间相,针对每相总共提出七个结构参数,它们分别是乳化相中表观气速,乳 化相中表观颗粒速度,气泡体积分数,气泡相中相对气体速度,气泡上升速度,气泡直径和乳化相空隙率。其次,建立 封闭方程组,以求解上述结构参数,封闭方程组包括气泡群力平衡方程,气体质量守恒方程,单位体积乳化相中颗粒受 力平衡方程,颗粒质量守恒方程,气泡速度的经验方程,平均空隙率方程和乳化相中表观气速方程。 论文第四章在综合气固两相受力平衡方程和曳力的定义的基础上,提出了新的包含第三章已经提出的七个结构参数的曳 力模型。通过求解封闭方程组,将每个气速下的新曳力结果求出,并通过新定义的非均匀因子表达出来。 论文第五章罗列了Fluent软件模拟时的控制方程。论文第六章首先考察了二维模拟过程中的主要影响因素,确定了合适 的步长和网格大小。然后,分别采用传统曳力模型和新曳力模型模拟三个气速下的流动状态,通过模拟结果与实验结果 的对比,得出结论,对于Geldart B类颗粒在低气速下的CFD模拟,采用Gidaspow曳力模型模拟流化过程低估了曳力值, 而新模型的模拟结果与实验较为吻合。
其他摘要Gas-solid bubbling fluidized beds have been used extensively in industrial process, such as coal gasification, petroleum catalytic cracking, drying and polymerization of ethylene and so on. Compared to traditional reactors, because of large gas-solid contact area and sufficient mixture of the two phases, they have many merits, such as large capacity, high heat and mass transfer coefficient and uniform temperature distribution etc. Recently, with the rapid development of computer hardware, the CFD simulation of the commercial fluidized beds becomes possible. Studies have shown that when using the Fluent to execute the TFM simulation of the bubbling fluidized beds, the selection of the drag model is of vital importance. Gas-solid fluidized bed is a typical non-equalibrium and non-linear system, displaying complex spatio-temporal multiscale structures. Therein, bubbles and clusters are two typical forms of heterogeneous structure which manifests coexistence of a gas-rich dilute phase (bubble phase) and a particle-rich dense phase (emulsion phase). In this thesis, a new meso-structure drag model for Geldart B particles under low gas velocity was proposed. We used the software of Fluent, coupled with the new drag model, to simulate the experiments. Compare the simulation results with experimental results to verify the rationality of the new model. In chapter 2, the experimental set-up and the method of measurement were firstly introduced. And then, the physical properties of material and the experimental results were set out. In chapter 3, we firstly resolved the system into three sub-phases, i.e., bubble phase, emulsion phase and their inter-phase, and seven structure parameters, i.e., the superficial gas velocity in the emulsion phase, the superficial solids velocity in the emulsion phase, the volume fraction of bubbles, the relative velocity of the gas in the bubble phase, the rising velocity of bubbles, the diameter of bubble and the voidage of emulsion phase, were proposed. Then we built an equation set, including seven equations, to solve the value of the structure parameters and the seven equations were respectively the bubbles force balance equation, gas mass conservation equation, the force balance equation for particles in the emulsion in unit volume, the mass conservation equation of particles, the empirical equation of bubble velocity, the equation of average voidage, and the equation of the superficial velocity of gas in emulsion phase. In chapter 4, we proposed a new drag model, based on the force balance equation of gas and solid phase and the definition of the drag, which incorporated the seven structure parameters proposed in chapter 3. By solving the equation set, we got the calculated results with the new drag model under different gas velocities and expressed them by a new definition (heterogeneous index). In chapter 5, we set out the governing equations. In chapter 6, we firstly investigated the main influencing factors during 2D simulation and determined the suitable time step and the size of the grid. And then, we adopted traditional drag model and the new drag model respectively to simulate the bubbling fluidized bed under three different gas velocities. By the comparison between the simulation results and the experimental results, we came to the conclusion that for Geldart B particles under low gas velocity, the traditional drag model underestimated the drag and the simulation results of the new model agreed well with the experimental results.
页数79
语种中文
文献类型学位论文
条目标识符http://ir.ipe.ac.cn/handle/122111/8349
专题研究所(批量导入)
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王英策. B类颗粒气固鼓泡流化床内两相流模拟的新曳力模型[D]. 中国科学院研究生院,2013.
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