【作者】 李飞；

【导师】 徐旭常；

【作者基本信息】 清华大学 ， 动力工程及工程热物理， 2009， 博士

【摘要】 为了揭示流态化气固两相流以及在此基础上发生的中温烟气脱硫反应过程的规律,实现性能预测和过程优化,本文首先分别从脉动流和平均流两个层面研究了气固两相的湍流相互作用。气固两相在脉动流层面的相互作用,称为“湍流变动”。采用PDA方法测量了垂直突扩圆管内下行稀疏气固两相流场和湍流特性。提出研究湍流变动时,应首先解决流场比较的基准问题——消除流场拉伸效应和入口效应。研究证实了小颗粒削弱湍流的特性;在未充分发展的突扩流动中,湍流变动会不断累积;流场的强剪切可强化湍流变动。提出将气相脉动速度与平均流速度梯度之间的线性关系作为一个重要判据,用于判定流动的强剪切性,以及在强剪切流中是否发生了湍流变动。针对气固两相在平均流层面的相互作用,研究了非均匀流态化稠密气固两相流动的曳力问题。运用流态化多尺度最小能量原理(EMMS),与欧拉框架下的计算流体力学(CFD)相结合,发展出了不含经验系数的QL-EMMS非均匀曳力模型。通过与曳力实验数据和循环流化床实验数据的双重对比,证明了该曳力模型的准确性和普遍适用性。又结合O-S模型的合理内核发展了更加符合物理真实的QL-EMMSn模型,但仍需进一步数值检验。为实现中温烟气脱硫的数值模拟,建立了脱硫反应产物的质量守恒方程和适用于欧拉框架的T-T吸收剂中温固硫模型。发现了作为反应状态参数的钙转化率与固硫反应速率之间的分段平方线性关系,据此改进了固硫模型表达式。实现了化学反应方程与欧拉双流体气固两相流模型的成功耦合。分别采用O-S和QL-EMMS曳力模型数值研究了流化床中温烟气脱硫过程。通过非均匀系数分析,发现吸收剂颗粒浓度的非均匀度在浓、稀相过渡区达到最大;SO2浓度的非均匀度与颗粒浓度及其非均匀度的乘积存在一一对应的关系;证明了颗粒浓度的均匀分布可有效提高脱硫效率的推断;发现高风速会强化颗粒的均匀分布,但同时也会缩短气固有效反应时间。提出了基于撞击流原理的新型反应器结构,初步研究表明,新结构有强化气固两相传质、大幅度提高脱硫效率的作用,为后续研究奠定了重要的基础。更多还原

【Abstract】 For the optimization and prediction of the moderate temperature flue gas desulfurization (FGD) process in the fluidized bed reactor, an investigation was conducted on the turbulent gas-solid two-phase interactions from the fluctuation level and mean flow level.To study the two-phase interaction occurs at fluctuation level, which is the so called“turbulence modulation”, PDA was used to measure the turbulent gas-solid two-phase flow field. A criterion that is to eliminate the elongation effects and inlet effects was first proposed to unify the basis for comparison of flow field variables. It is approved that fine particles attenuate turbulence. And in the under developing flows, the turbulence modulation will be accumulated. The intense shear can strengthen the turbulence modulation. The linear relationship between the gas fluctuation velocity and velocity gradient can be used as an important criterion to distinguish whether there is intense shear in the flow and further, whether the turbulence modulation occurs.The drag force in the heterogeneous dense fluidized flows, as a key problem of interactions in the mean flow level, was studied with theoretical analysis. The Energy Minimization Multi-scale (EMMS) theory was combined with Computational Fluid Dynamics (CFD) to develop an auto-adaptive drag force model, which is called QL-EMMS model. Comparison between the simulation and the experiment data approves the advances of the model. The model again was combined with the reasonable kernel of the O-S model to develop the QL-EMMSn model which was proved to be more close to physical reality. However, the new model is still need to be verified by numerical method.In the numerical study of dry FGD process at moderate temperature in CFB, an improved sulfur capture model for T-T sorbent is first obtained based on the finding that the reaction rate is proportional to the square of calcium conversion rate in different ranges.The FGD process in CFB was simulated by using the O-S model and QL-EMMS model respectively. The results show that the heterogeneity of the particle volume fraction reaches a maximum at the transition region with a small peak near the outlet beneath the hat chamber structure. The sulfating reaction rate as well as the desulfurization efficiency increase as the heterogeneity of the particle volume fraction distribution decreases. Higher inlet gas velocity will intensify the uniform distribution of particles; however, it will also reduce the effective reaction time.A new reactor structure was proposed based on the impinging stream theory. It was found that, the new reactor can intensify two-phase mass transfer thus to improve the desulfuration efficiency. The present investigation provides an important potential for further study.更多还原