CAS OpenIR  > 研究所(批量导入)
基于EMMS的介尺度模型及其在气固两相流模拟中的应用
其他题名EMMS-based Meso-Scale Model and Its Application in Simulating Gas-Solid Two-Phase Flows
鲁波娜
学位类型博士
导师李静海
2009-11-13
学位授予单位中国科学院过程工程研究所
学位授予地点过程工程研究所
学位专业化学工程
关键词气固两相流 双流体模型 细网格模拟 介尺度模型 Emms 曳力 曲面拟合 团聚物直径 噎塞 提升管高度
摘要气固两相流是个典型的非线性非平衡系统,呈现出复杂的时空多尺度结构。有关它的连续介质模拟方法,当前主流观点认为:在网格足够细的情况下,不考虑结构影响的双流体模型(Two-Fluid Model, TFM)也称为一般双流体模型(General Two-Fluid Model, GTFM),能准确预测非均匀流动的行为。而我们认为,必须将结构因素考虑到双流体模型中,建立介尺度结构模型,实现对曳力系数的修正,才能准确预测非均匀两相流动行为。上述争论实际上涉及两相流模拟的基本问题,有必要对它进行明确回答。 针对上述基本问题,我们开展了系统的数值实验(论文第二章)。首先针对与粗网格模拟中单个网格尺寸相当的双周期边界Geldart A类颗粒系统,采用GTFM模拟,发现计算结果随着网格的减小逐渐变化,约在10个颗粒直径尺寸时趋于稳定。将此结果应用于实际的提升管体系,不断细化网格直至网格足够小,发现模拟结果仍与实验值相差甚远;对于Gelart B类颗粒系统,也发现上述类似现象。相比较而言,采用基于EMMS的介尺度模型(EMMS/Matrix模型),针对周期边界系统的模拟表明计算结果与网格基本无关。而模拟实际的提升管系统时,发现即使采用较粗网格(只需保证满足一定的差分精度要求),预测的颗粒通量、轴径向浓度分布、径向速度分布以及介观结构变化都与实验观察吻合较好。基于上述结果可知,单纯细化网格并不可行,对于复杂的非均匀气固两相流动,必须建立合理的介尺度模型,而其中,EMMS/Matrix模型则是目前可达到网格无关的较理想模型。 论文第三章进一步简化了原EMMS/Matrix介尺度模型。基于严格的推导,非均匀结构因子HD的三个自变量(ug, us, εg)被简化成两个变量(uslip, εg), 由此基础上,提出二步拟合法,对HD曲面进行了拟合。采用拟合公式(3%相对误差)取代原来的矩阵插值法,简化了与双流体模型的耦合。 EMMS/Matrix模型可以成功预测A类和B类颗粒的提升管系统的复杂流动,但却不适用于模拟典型的循环流化床锅炉(Circulating Fluidized Bed Bolier, CFBB),其主要问题在于团聚物方程。论文第四章分析了原团聚物方程的适用范围以及失效原因。通过调整输入能量的定义,使得改进后的团聚物方程也能适用于CFBB系统的计算,而原方程则是它的一个特例。方程预测的团聚物直径在固定通量下随着气速的增加而减小,在某个临界值后,又随着气速的增加而增大,体现了颗粒流体协调(particle-fluid compromise, PFC)和气体控制(fluid dominating, FD)的两种极值趋势的交替。采用新团聚物方程的模拟表明,出口通量、流型状态、各区浓度分布等主要监测数据都与实验值吻合较好。 对于循环流化床中典型的状态多值性行为―噎塞,论文第五章利用双流体模型结合基于EMMS的介尺度模型成功预测了实验室和工业尺度反应器中的A类颗粒流化系统中的这种典型现象。在此基础上,论文利用该模拟方法考察了提升管高度对噎塞的影响。模拟表明,噎塞区域随着提升管高度的增加而逐渐增大,而饱和夹带量和噎塞平台起点则维持不变。 论文第六章总结了本论文获得的主要成果,展望了本文的气固介尺度模型的前景以及进一步开展研究的方向。
其他摘要Gas-solid two-phase flow is a typical non-equilibrium and non-linear system, displaying spatio-temporal multiscale structures. As to its related simulations with continuum models, it is believed, to most researchers, that two-fluid model (TFM) without considering the effects of heterogeneous structures (general two-fluid model, GTFM) can successfully predict the hydrodynamics of gas-solid two-phase flows if the grids are fine enough. However, we consider that the effects of meso-scale structures should be taken into account for TFM and a reasonable meso-scale model is required for modifying the constitutive relations, especially for the drag closure. The above argument is crucial for simulations of gas-solid two-phase flows, requiring a definite answer. In chapter 2, a series of numerical experiments are carried out to tackle the above problem. Firstly, simulations are performed for gas-solid flows with Geldart A particles in a periodic domain whose size is comparable to the grid size used in typical coarse-grid simulation. It is found that the GTFM simulation can converge to its asymptotic solution when the grid size is as small as 10 times the particle diameter. However, for riser flows it fails to capture the typical S-shaped axial voidage profile and highly over-predicts the solids flux even using fine-enough grids. Similar findings are discovered for simulations of flows with Gelart B particles. In comparison, the TFM simulation with EMMS-based meso-scale model (EMMS/Matrix model) in a periodic domain seems grid-independent for flows with both Gelart A and Geldart B particles. Furthermore, for riser flows it shows a good prediction in solids flux, axial and radial voidage profiles, radial velocity profiles as well as dynamics of meso-scale structures, even using relatively coarse grids. Therefore, fine-grid GTFM simulation is inadequate for predicting gas-solid riser flows. We need meso-scale modeling of heterogeneous structures, and at present, EMMS/Matrix model is a good choice. In chapter 3, EMMS/Matrix model is further simplified. Based on rigorous derivation, three independent variables (ug, us, εg) of the heterogeneity index HD are reduced to two variables (uslip, εg). Then, two-step fitting method is proposed for surface fitting with reasonable relative errors. The fitting formulae which substitute the matrix of HD and its related interpolation scheme facilitate the combination of EMMS/Matrix model with commercial CFD software (e.g. FLUENT). EMMS/Matrix model successfully predicted riser flows with Geldart A and Geldart B particles, but failed to simulate gas-solid flows in circulating fluidized bed boiler (CFBB) under typical operating conditions. This failure comes from unreasonable formulation of the cluster diameter. In chapter 4, the original cluster diameter equation is re-examined, and then a new correlation is proposed through modifying the input energy. The predicted cluster diameter decreases with increase of gas velocity at fixed solids flux, then reverses its trend after a critical value, manifesting an alternation between particle-fluid compromise and gas-domination. EMMS/Matrix model with this new cluster equation shows a good ability for prediction of typical CFBB flows. For the typical choking behaviors in CFB, TFM simulations with EMMS-based drag model successfully capture those phenomena with Geldart A particles both in lab-scale and industrial-scale reactors (chapter 5). On this basis, the effect of riser height on choking is investiaged through simulations. It is found that the choking region extends as the riser height increases, while the saturation carrying capacity K* as well as the starting point of choking is independent of the riser height. Finally, chapter 6 summarizes the main achievements in this thesis, and presents future works on EMMS-based model.
页数160
语种中文
文献类型学位论文
条目标识符http://ir.ipe.ac.cn/handle/122111/985
专题研究所(批量导入)
推荐引用方式
GB/T 7714
鲁波娜. 基于EMMS的介尺度模型及其在气固两相流模拟中的应用[D]. 过程工程研究所. 中国科学院过程工程研究所,2009.
条目包含的文件
文件名称/大小 文献类型 版本类型 开放类型 使用许可
10001_20061800413300(4397KB) 开放获取CC BY-NC-SA浏览 请求全文
个性服务
推荐该条目
保存到收藏夹
查看访问统计
导出为Endnote文件
谷歌学术
谷歌学术中相似的文章
[鲁波娜]的文章
百度学术
百度学术中相似的文章
[鲁波娜]的文章
必应学术
必应学术中相似的文章
[鲁波娜]的文章
相关权益政策
暂无数据
收藏/分享
文件名: 10001_200618004133005鲁波娜_paper.pdf
格式: Adobe PDF
此文件暂不支持浏览
所有评论 (0)
暂无评论
 

除非特别说明,本系统中所有内容都受版权保护,并保留所有权利。