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Geldart-B类颗粒磁场流化床流体力学行为研究
朱全红
学位类型博士
导师李洪钟
2016-07
学位授予单位中国科学院研究生院
学位授予地点北京
学位专业化学工程
关键词磁场流化床 操作模式 流域转变 气泡 颗粒分级
其他摘要

Geldart-B类颗粒的典型流化行为是当操作气速(Ug)超过最小流化速度(Umf0)后,多余的气体(Ug–Umf0)并不进入颗粒群去均匀增大颗粒间的距离,而是以气泡的形式快速通过床层。气泡的形成致使流化质量恶化,气固两相之间的传质效率降低。磁场常用来改善Geldart-B类磁性颗粒及其与Geldart-B类非磁性颗粒组成混合物料的流化质量,形成著名的磁场流化床;本论文特别将前者称为纯磁性物料磁场流化床,而将后者称为混合物料磁场流化床。相比于其它改善颗粒流化质量的方法,磁场可以从外部起作用,因而不受流化床反应器内苛刻反应条件的影响。目前,文献针对Geldart-B类颗粒磁场流化床流体力学行为的研究主要集中在“先加磁场”操作模式下形成的磁稳定流域(UmfUmb)。另一方面,文献对“后加磁场”操作模式的研究也很不充分:纯磁性物料磁场流化床在此操作模式下的流域转变规律仍存在争议,混合物料磁场流化床在此操作模式下的流域转变规律至今仍不清楚。针对上述问题,本论文详细研究了磁控流化流域内的气固流动规律。对于磁场流化床在两种操作模式下的流域转变规律,本论文也进行了详细的研究和比较。针对混合物料磁场流化床,本论文还从实验测量和数学模型两个角度研究了磁场诱导颗粒分级的规律。本论文取得的主要研究结果和结论如下:(1)传统的磁控流化流域还可以进一步划分为两个具有不同流体力学性质的流域。对于纯磁性物料磁场流化床,这两个流域为沟流鼓泡流域和磁控鼓泡流域;对于混合物料磁场流化床,这两个流域为分级鼓泡流域和磁控鼓泡流域。在沟流鼓泡和分级鼓泡流域内,由于沟流或者颗粒分级的出现,磁场并不能改善Geldart-B类颗粒的流化质量。只有在磁控鼓泡流域内,磁场才可以显著减小气泡尺寸,起到改善流化质量的作用。(2)相比于气泡长度,轴向磁场对气泡宽度的减小作用更加显著,导致气泡的形状由近似球形演变为椭球形。在两种磁场流化床中,磁场减小气泡尺寸的机理存在较大差异。在纯磁性物料磁场流化床中,磁场对气泡尺寸的减小作用主要源自其强化了磁性颗粒之间的相互作用力,致使固相的“表面张力”增大,最终导致气泡的生成和长大(亦即气固相界面面积的增加)都变得更加困难。而在混合物料磁场流化床中,磁场对气泡尺寸的减小作用主要源自磁链在床中形成一个“浮游内构件”,该内构件可以有效地破碎大气泡并阻碍小气泡的聚并长大。(3)在“后加磁场”操作模式下,两种磁场流化床均呈现出四个具有不同流体力学性质的流域。对于纯磁性物料磁场流化床,这四个流域为固定床、磁控鼓泡、沟流鼓泡和磁凝床;对于混合物料磁场流化床,这四个流域为固定床、磁控鼓泡、部分分级鼓泡和完全分级鼓泡。“后加磁场”操作模式下,磁场流化床的流域转变规律与“先加磁场”操作模式下不同:不存在著名的磁稳定流域。(4)两种操作模式下流域转变规律的比较表明磁场流化床具有四个操作区域:I、II、III和IV。在操作区域II内,床层状态不仅与H(磁场强度)和Ug的大小相关,还与两者的施加顺序(亦即操作模式)密切相关。这种相关性从本质上讲是指床层状态与所经历的路径相关。床层状态的这一路径相关性源自在操作区域II内磁场流化床本来就可以具有两个平衡态,不同的路径会分别到达这两个平衡态。进一步的研究表明:磁场流化床的磁稳定状态是一种亚稳定平衡态,经历稍大扰动便不可恢复。在操作区域II内,“先加磁场”操作模式下新相生成的困难最终导致了磁稳定这一亚稳定平衡态的形成。(5)混合物料磁场流化床中常会出现颗粒分级现象:磁性颗粒作为沉积组分,而非磁性颗粒作为浮升组分。外磁场作用下,磁性颗粒以磁链的形式存在,因此和非磁性颗粒共流化的并不是原来的磁性颗粒,而是磁链。在“后加磁场”操作模式下,颗粒分级的发生主要源自磁链尺寸的长大。随着首先施加的Ug增大,开始分级和完全分级时对应的磁场强度(Hms和Hts)都呈现出增大的趋势。较高的气速意味着磁链和非磁性颗粒之间具有较大的混合推动力,因此磁链需要长大到更大的尺寸才能沉积至床层下部,相应地,所需的磁场强度也就更大。(6)针对混合物料磁场流化床中的颗粒分级现象,本论文还建立了初步的数学模型,亦即Hms-Ug函数关系。模型计算结果与实验测量结果可以较好地吻合,说明该模型可以较为准确地预测混合物料磁场流化时的颗粒分级规律,可以为磁场诱导颗粒分级的避免或者利用提供一定的理论依据。

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The typical gas-fluidization behavior of Geldart-B particles is that bubbles rise in the bed immediately after the superficial gas velocity (Ug) exceeds the minimum fluidization velocity (Umf0). The excess gas (Ug–Umf0) does not disperse uniformly among particles; instead it passes quickly through the bed in the form of bubbles. The formation of gas bubbles deteriorates the fluidization quality and the gas-solid mass transfer.The magnetic field is often used to improve the fluidization quality of Geldart-B magnetizable particles as well as their admixture with Geldart-B nonmagnetizable particles, thus creating the magnetized fluidized bed (MFB). Compared with other techniques of improving the fluidization quality, the magnetic field has the primary advantage of not being affected by the harsh condition inside the fluidized bed reactor since it can work from the outside.Thus far, the research on the MFB with Geldart-B particles mainly focused on the famous magnetic stabilization flow regime (UmfUmb). Besides, the research on ‘Magnetization LAST’ operation mode is far from sufficient: as for the MFB with purely Geldart-B magnetizable particles (termed magnetizable bed here), the flow regime transition under this operation mode remains a controversy; as for the MFB with admixtures of Geldart-B magnetizable and nonmagnetizable particles (termed admixture bed here), the flow regime transition is still unknown.Aiming at these shortcomings, this thesis presents a detailed study on the gas-solid flow hydrodynamics in the magnetized fluidization flow regime. Besides, the flow regime transition of the MFB under the two operation modes is carefully studied and compared. For the admixture bed, the particle segregation behavior is quantitatively investigated from both the experimental and mathematical perspectives. The principal results and major conclusions are as follows:(1) The traditional magnetized fluidization flow regime could be further divided into two distinguishable sub-regimes. For the magnetizable bed, the two sub-regimes are channel-bubbling and magnetized-bubbling; for the admixture bed, the two sub-regimes are segregation-bubbling and magnetized-bubbling. In the channel-bubbling and segregation-bubbling sub-regimes, due to the appearance of channels or segregation the magnetic field could hardly improve the fluidization quality. Only in the magnetized-bubbling sub-regime could the magnetic field effectively reduce the bubble size and improve the fluidization quality.(2) The axial magnetic field has a much stronger reduction effect on the bubble width than on the bubble length, causing the bubble shape to change from nearly spherical in the absence of a magnetic field to elliptical in the presence of a magnetic field. The mechanism of the bubble size reduction is different in the two types of magnetized fluidized beds. In the magnetizable bed the bubble size reduction arises mainly from that the magnetic field significantly enhances the cohesive force between magnetizable particles and increases the surface tension of the solid phase (a pseudo-fluid phase). According to the knowledge from phase interfacial physical chemistry, this would eventually make the bubble formation and growth up (i.e., the area increase of the phase interface) become more difficult. On the other hand, in the admixture bed the bubble size reduction results mainly from that the magnetic chains form a floating internal inside the bed which could effectively break up large gas bubbles as well as hinder the coalescence of small bubbles.(3) Under ‘Magnetization LAST’ operation mode, both the magnetizable and admixture beds exhibit four distinguishable flow regimes. For the magnetizable bed the four regimes are fixed, magnetized-bubbling, channel-bubbling, and magnetically condensed; for the admixture bed the four regimes are fixed, magnetized-bubbling, partial segregation-bubbling, and complete segregation-bubbling. Note particularly that under ‘Magnetization LAST’ operation mode the magnetic stabilization flow regime could not be obtained.(4) Comparison of the flow regime transition between the two operation modes indicates that there exist four operation zones for the MFB: I, II, III, and IV. It is found that in operation zone II, the bed state depends not only on the values of H (magnetic field intensity) and Ug but also on their application sequence (i.e., operation mode). Such dependence in essence is the dependence of the bed state on the path taken in order to achieve it. The reason why in operation zone II the bed state is path-dependent lies in that in this zone the MFB could have two different equilibrium states and different paths will probably lead to the two different equilibrium states, respectively. Experimental study demonstrates that the famous magnetic stabilization state is a metastable equilibrium state, which could not recover after a slightly larger perturbation is introduced. In operation zone II, the reason why there is a metastable equilibrium state of magnetic stabilization aside from the stable equilibrium state lies in the difficulty in forming a new phase under ‘Magnetization FIRST’ operation mode.(5) In the admixture bed particle segregation would often occur: the magnetizable particles play the role of jetsam while the nonmagnetizable particles work as flotsam. Under the action of the magnetic field the magnetizable particles exist in the form of magnetic chains. As a result, it is the magnetic chains rather than the original magnetizable particles that co-fluidize with the nonmagnetizable particles. Under ‘Magnetization LAST’ operation mode, the reason for the occurrence of particle segregation lies in the size growth of magnetic chains with increasing H. With the increase of the first introduced Ug, both the magnetic field intensities at which particle segregation begins (Hms) and completes (Hts) increase. A higher Ug means a stronger driving force for the mixing of the magnetic chains and nonmagnetizable particles, and therefore the magnetic chains need to grow even larger to sink to the bottom bed. Accordingly, the magnetic field intensity (Hms) needed to arouse the segregation is larger.(6) A preliminary mathematical model is established to calculate the particle segregation in the admixture bed. The calculated Hms-Ug relationship shows a good agreement with the experimental result, indicating that the magnetized segregation model established here could be used to predict the segregation behavior in the admixture bed and further to provide a theoretical direction on how to avoid or utilize such magnetized segregation.

语种中文
文献类型学位论文
条目标识符http://ir.ipe.ac.cn/handle/122111/22911
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朱全红. Geldart-B类颗粒磁场流化床流体力学行为研究[D]. 北京. 中国科学院研究生院,2016.
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