基于结构参数响应的内循环流化床流体特性优化数值模拟

流化床是一种结构复杂而能量转化高效的反应器型式,为了实现系统化的内循环流化床优化设计,利用欧拉-欧拉双流体模型构建了不同结构的CFD流化床模型,在分别改变高径比、导流筒与反应器的直径比和底隙高度3个结构因素的情况下,考察流化床内全流场、局部流场、液相运动速度及气含率等气液两相流的响应特性,分析结构因素及操作因素对流体运动的内在影响,阐明工程优化设计的方向.数值模拟结果表明:高径比主要影响气液流动型态,低高径比时容易出现漩涡和返混,工程中应采用导流内构件并重视气体分布装置的合理设计;导流筒与反应器的直径比主要影响液体循环速度,存在一个合理的区间,考虑流动速度与气含率,流化床直径比可取0.6～0.8,最佳取值为0.7;底隙高度影响流化床底泥区的流体运动,应约等于流化床下降区的缝隙长度.CFD模拟可作为污染控制技术工业放大和优化设计的辅助工具.

Numerical optimization on hydrodynamic characteristics of internal-loop fluidized bed based upon structure parameter response relationships

Fluidized bed reactor is a reactor with complex internal structure and high energy transformation efficiency. A computational fluid dynamics (CFD) model for different structures by using Eulerian-Eulerian double-fluids models was established to help achieve the optimal design of internal-loop fluidized beds. With the aid of this model, the two-phase hydrodynamic characteristics of the flow field, gas holdup distribution, and liquid velocity distribution of the fluidized bed were investigated under different ratios of height to diameter, different ratios of diameters of draft tube to reactor, and height of bottom clearance and operation factors. The internal effects between structure parameters and operation factors were analyzed. The simulation results showed that the ratio of height to diameter can affect the flow pattern of liquid and gas significantly and the low ratio of height to diameter may lead to vortex and backmixing easily, suggesting that the internal draft tube should be implanted and spargers should be designed properly in engineering applications. Diameter ratio of draft tube to reactor has a large effect on liquid recirculation velocity, and thus has a reasonable range. Considering the liquid circulation velocity and gas holdup, the appropriate range of diameter ratio of draft tube to reactor is from 0.6 to 0.8, with 0.7 the optimal. The height of bottom clearance can affect local flow structure in bottom zone, and its height should be equal to the length of the downcomer. This study indicates that the CFD simulation can be used as a supportive approach for optimizing design and pilot test of reactors in pollution control field.