非定常工作条件下流化床内粒子分布的实时测量及动力学分析

Recently, it has been estimated that around 15% of the global NOx and 5-8% of the global SOx emissions are attributable to ships. In some regions, depending on the season, the emission contribution from shipping may reach much higher levels than the global average. Therefore, an exhaust gas treatment system in a ship is very important for energy conservation and environmental protection. .This report put forward a proposal of applying circulating fluidized bed (CFB) into the exhaust gas treatment system to restrict the shipping emissions and improve the heat recovery rate. Circulating fluidized bed is popular devices that can provide excellent heat transfer and reaction property due to its ability to promote high levels of contact between gases and solids. Due to the potential applications and advantages, the study of circulating fluidized bed has received considerable attention in the past three decades and these studies provide detailed information of the flow dynamics characteristics. However, in these studies the circulating fluidized bed is always considered as a motionless land plant. And there are almost no studies about the flow properties in the circulating fluidized bed under swing motion can be found. Thus, the application of circulating fluidized bed in the shipping is still very limited now. Because, for the application of circulating fluidized bed in the shipping, the swing motion of the ship should be considered due to its potential influences to the flow characteristics and the performance of the circulating fluidized bed. Therefore, clarification of dynamic properties in the circulating fluidized bed with swing motion is very necessary for the application of circulating fluidized bed in the shipping..The present study focused on the particle distribution behaviors in the circulating fluidized bed with swing motion. Because the particle distribution behaviors is a key factor of the dynamic property in the ordinary circulating fluidized bed due to its direct effects on gas-solid contact efficiency, heat and mass transfer rates, and chemical reaction performance. The electrical capacitance tomography (ECT) technique, which has been investigated as a non-intrusive, comprehensive measurement technique for multiphase flow , is used for the visualization of the particle distribution properties in the circulating fluidized bed with swing motion. Moreover, the CFD-DEM (combined computational fluid dynamics and discrete element method) model, which has been proved to be effective in the study of particle-fluid flow systems, as briefly reviewed by various investigators, is proposed to validate the effect of swing motion on particle distribution behaviors visualized by ECT sensor. The numerical model applied the continuum-discrete approach on a microscopic level devoted to the physical interactions between particles, that is, an Eulerian-Lagrangian model, to simulate the interested dynamics of particle distribution.

节能减排是我国应对全球气候变化的国策之一，循环流化床作为一种高反应效率以及热传导效率的化学反应式废气处理系统，在节能减排中起到重要的作用。本课题以非定常工作条件下循环流化床内的多相流的动力学特性研究为目的，采用电容层析成像这种先进可视化技术， 获取不同试验参数条件下各相分布图像信息， 并以此为依据分析循环粒子大小， 密度，床体摆动强度，频率等重要参数对床内流体动力学特性的影响，找出影响各相分布的均匀性及稳定性的主要因素，提出非定常条件下床内流体动力学性能的优化方案，进行实验验证。另外，建立连续相以及离散相的流体数学模型，利用数值仿真并结合实验研究的结果，揭示影响各相分布的均匀性以及稳定性的主要因素及其变化规律。该项目的研究将填补目前对循环流化床内流体在摇摆或定倾斜等不安定工作条件下的动力学性能研究的缺乏。为循环流化床在船舶废气处理系统等复杂工作条件下的高效应用提供理论及技术

引擎排出废气的热能回收利用是船舶节能化的焦点。循环流化床作为一种具有高反应效率以及热传导效率的化学反应装置，能同时实现废气的脱硫及热能回收，可在船舶的节能减排中发挥重要作用。. 本项目以阐明风、浪、流等海上非定常工作条件的综合作用下循环流化床内的气固两相流的动力学特性及其主要影响因素为目的，利用电容层析成像技术和CFD-DEM仿真模型实现了非定常工作条件下循环流化床内颗粒分布状态及其动态演化过程的可视化测量及数值解析，旨在量化工作条件对颗粒分布特性的影响规律，为基于循环流化床反应器的船用废气处理系统的设计开发和高效应用提供理论及实践基础。项目取得的重要结果及其科学意义主要有：.1.由电容层析成像技术的原理出发，综合考虑床内流动状态及流体性质，传感器工作环境，电场敏感阵的影响因素，图像重构算法适应性以及测量精度要求等各方面因素，确定了传感器的最佳设计方案，构建了稳定的数据采集系统，找出了颗粒分布图像的最佳重构算法。在静态试验下，获得了测量体积分数误差10%以内，测量体中心位置平均误差1个像素（2mm）以内的较高精度，以及每秒100帧图像数据的采集处理速率。.2.从粒子的斯托克斯数这一表征粒子弛豫时间与气相的时间特征常数比值的决定性参数为出发点，以不同粒子大小、密度、摆动幅度、频率等参数条件下的颗粒分布图像为基础，通过数据的精细反演和分析，分别提取了颗粒相在时间和空间维度的分布特性，并以气固两相的动力学理论模型为基础，分析了影响颗粒相分布均匀性和稳定性的主要影响因素及其影响机理。.3.考虑船用流化床的非定常工作状态，将计算流体动力学与离散单元法相结合建立了在运动坐标系（非惯性系）下的CFD-DEM仿真模型，分析了静电场力，范德华力等远程交互作用力对颗粒分布特性的影响，并根据各相速度分布、粒子与粒子以及粒子与气相之间的相互作用力等动力学参数的仿真数据分析，量化了摆动幅度和周期等参数对颗粒分布均匀性及稳定性的影响规律。