非均一颗粒在浓相流化床系统中的流动行为研究

发布时间：2018-03-12 23:24

本文选题：非均一颗粒　切入点：气固流动　出处：《清华大学》2015年博士论文　论文类型：学位论文

【摘要】：非均一颗粒在气固流化床中的流动行为是流态化工程领域研究的热点之一,也是数值模拟研究的难点。本文针对非均一颗粒的浓相气固流动建立数值模型,研究颗粒的流动、扩散和分层行为,并从微观上剖析这类多相流动的气固动力学特征和颗粒的运动机制。这类流动涉及气相、介质颗粒和具有不同属性的外加颗粒。鉴于该过程的复杂性,建模首先从只包含气相和介质颗粒的基础气固流动入手,分析对比现有的数值方法在模拟浓相气固流动上的特征,研究其应用于多相气固流动的可行性。研究表明,目前的数值模拟方法可以在宏观上准确描述气相、介质颗粒的浓相气固流态化过程,但同时考虑具有不同属性的外加颗粒比较困难。因此,本文提出采用一种新的模型——TFM-DEM混合模型,来描述外加颗粒和介质颗粒之间具有较大的密度和粒度差异的多元浓相气固流动系统的流动行为。其中,采用TFM模型描述气相和介质颗粒的基础气固流动,而外加颗粒用DEM方法描述。通过实验研究了外加颗粒在浓相流化床中的分层行为,并验证了TFM-DEM混合模型的准确性。以此为基础,采用TFM-DEM混合模型模拟研究了非均一颗粒在浓相流化床内的流动行为,考察外加颗粒密度和粒度、以及一系列操作参数对颗粒流动行为的影响规律。研究得到的主要结论有:(1)外加颗粒在气相-介质颗粒浓相流化床内按密度差异发生分层现象,但随着外加颗粒粒度减小,外加颗粒的密度分层程度逐渐变差。当外加颗粒粒度小至3 mm时,平均颗粒密度在1.9~2.2 g/cm3之间分布,此时流态化分选效果不佳。(2)通过颗粒受力分析发现,外加颗粒悬浮主要依赖于床层静压梯度力,该静压梯度力大于轻组分颗粒的重力而小于重质组分颗粒的重力,此差异推动了轻重颗粒的分离和分层。外加颗粒受到的介质颗粒相曳力是它们无规则流动和混合主要动因。随着外加颗粒粒度减小,气相曳力和介质颗粒相曳力所占的比例增加,受这些曳力的影响,细颗粒的流动轨迹与介质相流动接近,因此分离很困难。(3)操作参数影响外加颗粒的分层质量,其中介质颗粒特性和流化风速改变了床内的流态化行为。床内气泡运动和介质颗粒的返混是造成外加颗粒混合的主要原因,它们比床层密度变化对分层结果的影响更大。床内的外加颗粒量和床高对外加颗粒的分层行为有类似的影响,提高床内外加颗粒的数密度,可以改善外加颗粒的群体分层行为。
[Abstract]:The flow behavior of heterogeneous particles in a gas-solid fluidized bed is one of the hotspots in fluidization engineering and a difficult point in numerical simulation. In this paper, a numerical model of dense gas-solid flow of heterogeneous particles is established to study the flow of particles. Diffusion and stratification behavior, and microcosmic analysis of the gas-solid dynamics and the motion mechanism of such multiphase flows. These flows involve gaseous phase, medium particles and additional particles with different properties. Given the complexity of the process, The modeling begins with the basic gas-solid flow with only gaseous phase and medium particle, analyzes and compares the characteristics of the existing numerical methods in simulating the gas-solid flow of the dense phase, and studies the feasibility of its application to the multiphase gas-solid flow. The present numerical simulation method can accurately describe the gas-solid fluidization process of gas phase and medium particle macroscopically, but it is difficult to consider the addition particles with different properties at the same time. In this paper, a new model, TFM-Dem mixed model, is proposed to describe the flow behavior of a multicomponent dense phase gas-solid flow system with large density and particle size difference between the applied particles and the medium particles. The TFM model is used to describe the basic gas-solid flow of gas phase and medium particles, while the DEM method is used to describe the applied particles. The delamination behavior of the added particles in a dense phase fluidized bed is studied experimentally. The accuracy of TFM-DEM mixing model was verified. Based on this, the flow behavior of heterogeneous particles in a dense phase fluidized bed was simulated by TFM-DEM mixing model, and the particle density and particle size were investigated. The main conclusions of the study are as follows: (1) the separation of added particles in the gas-medium particle dense phase fluidized bed occurs according to the density difference, but with the decrease of the particle size, When the particle size is small to 3 mm, the average particle density is distributed in the range of 1.92g / cm ~ 3, and the fluidized separation effect is not good. The suspension of applied particles mainly depends on the static pressure gradient force of the bed, which is larger than the gravity of the light component particle but less than the gravity of the heavy component particle. This difference promotes the separation and stratification of heavy and heavy particles. The drag force of the medium particles subjected to the applied particles is the main cause of their irregular flow and mixing. The proportion of gas phase drag force and medium particle drag force increases. Due to these drag forces, the flow path of fine particle is close to that of medium phase flow, so it is very difficult to separate. The characteristics of medium particles and fluidization wind speed change the fluidization behavior in the bed. The bubble movement in the bed and the back mixing of the medium particles are the main reasons for the external particle mixing. The effect of the change of bed density on the stratification results is greater than that of the bed density, and the amount of particles in the bed and the bed height have similar effects on the stratification behavior of the added particles. Increasing the number density of the particles added inside and outside the bed can improve the population stratification behavior of the added particles.
【学位授予单位】：清华大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TQ051.13

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