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题名:
流体粘度对液固流态化非均匀性的影响
作者: 邱欧
学位类别: 博士
答辩日期: 2004
授予单位: 中国科学院过程工程研究所
授予地点: 中国科学院过程工程研究所
导师: 李洪钟
关键词: 流体粘度 ; 临界颗粒聚团理论 ; 最佳粘度模型 ; 流态化非均匀性 ; 液固流态化
其他题名: Effect of Fluid Viscosity on the Heterogeneity of Liquid-Solid Fluidized Bed
学位专业: 化学工艺
中文摘要: 流体粘度对流态化质量的影响这一问题长期以来为研究者所忽视。在实际的和潜在的工业应用中,气固流化床的数量都远远多于液固流化床,因此人们对于流体粘度对流态化质量的影响的研究,不象对于颗粒直径,颗粒/流体密度差,温度和压力对流态化质量影响的研究那样重视。但是,随着一些应用液固流化床新过程的开发,流体粘度因素在反应器设计中所占的比重在日益增加,例如,在生物制药所应用的流化床中,其流体的粘度要比水高出几百倍至几千倍。在这样高的粘度下,流化床的流体动力学有可能与常态下大相径庭。目前的研究现状显然不能满足工业设计的需要。为此,本文通过理论分析和实验研究,系统地研究了流体粘度对液固流态化非均匀性的影响这一问题,希望能够澄清关于流体粘度对流态化质量的影响这一问题的不同认识。本文首先从理论上分析了流体粘度对液固流态化非均匀性的影响。理论分析表明,液固流态化非均匀性的形成机理在低粘度区和高粘度区是不同的,因此,在不同的粘度区流体粘度的变化会对液固流态化的非均匀性产生不同的影响。在低粘度区,液固流态化的非均匀性受流体鼓泡控制。在这个区域内提高流体粘度,可以有效地减小流体鼓泡的直径,从而降低了流态化的非均匀性,改善了流态化的质量。在这一区域,流体粘度对液固流态化非均匀性影响的作用机理可用Harrison气泡稳定性模型方程予以阐明,即:在高粘度区,流体鼓泡受到了充分的抑制,液固流态化的非均匀性受颗粒聚团控制。在这个区域内降低流体粘度,可以有效地削弱颗粒聚团的倾向,从而降低了流态化的非均匀性,改善了流态化的质量。本文通过分析对高粘度区颗粒聚团的形成机理提出了临界聚团理论。颗粒在高粘度流体中流态化时所形成的聚团是由于颗粒表面的滞流层彼此粘附所致。若令颗粒表面滞流层的最小厚度为δ1,则δ1可定义为颗粒的终端雷诺数Re1的函数,即δ1=f1(Re1);若令两个彼此相邻的颗粒之间的最大距离为δ2,则δ2也可以表达为颗粒的终端雷诺数Ret的函数,即δ2=f2(Rel)。当δ2≤2δ1时,两个颗粒表面的滞流层就会彼此粘附。由此可知,当时,颗粒聚团就会形成。式(3.3)中的等号代表聚团临界形成的条件。显然,聚团临界形成时,Rel应为常数。在上述理论分析的基础上,本文预测在低粘度区和高粘度区之间,必然存在一个临界粘度值。当流体的粘度等于临界粘度值时,流体鼓泡和颗粒聚团都受到了有效抑制,此时流态化的非均匀性最低,流态化质量最好,正好满足临界聚团理论所表示的临界聚团条件。这个临界粘度值可定义为最佳粘度μopl。从临界聚团理论可以推断,对应于最佳粘度μopl,颗粒的终端雷诺数应为一常数,定义为临界聚团雷诺数Re1。从单颗粒在流体中的受力分析出发,并引入了多颗粒系统中的曳力系数修正公式之后,本文得到了关联Rel和μopl的数学方程为了检验上述的理论分析,本文用六种不同密度和大小的颗粒,在九种不同粘度(982-7cP)的甘油水溶液中进行了流态化实验。用光纤探头测量了床层局部颗粒浓度波动的信号,以此来评价流体粘度对液固流态化非均匀性的影响。通过计算不同流体粘度下波动信号的标准偏差发现:每种颗粒都存在一个最佳粘度产μopl。当流体粘度高于产。l时,降低流体的粘度可以减小波动信号的标准偏差,这就是说,降低流体粘度可以减小床层局部两相结构的差异,从而降低了流态化的非均匀性;当流体粘度低于μopl时,提高流体的粘度可以减小波动信号的标准偏差,这就是说,提高流体粘度可以减小床层局部两相结构的差异,从而降低了流态化的非均匀性。这些实验观察和本文的理论分析是一致的。实验研究了不同因素对最佳粘度μopl的影响,发现最佳粘度群。,是颗粒与流体物性的一个本征参数,它不受流速的影响,但受颗粒密度和大小及流体密度的影响。当颗粒大小相同时,密度高的颗粒对应的最佳粘度μopl的值高,密度低的颗粒对应的最佳粘度群。,的值低。当颗粒密度相同时,直径大的颗粒对应的最佳粘度刀。,的值高,直径小的颗粒对应的最佳粘度μopl的值低。实验还发现,在高粘度区域,提高颗粒的密度,增大颗粒的直径可以降低流态化的非均匀性,而在低粘度区域,降低颗粒密度,减小颗粒直径可以降低流态化的非均匀性。这些实验事实说明,在高粘度区,密度小和直径小的颗粒更倾向形成聚团,而在低粘度区,密度大和直径大的颗粒更易于造成鼓泡。这些实验观察和本文的理论分析是一致的。 根据最佳粘度刀。,的实验测定值,可以从式(3.11)中回归算出各种颗粒的临界聚团雷诺数Rel。实验数据的回归结果表明,在实验误差允许的范围内,各种颗粒的Rel值都近似等于0.004,即:Rel=0.004,这和本文所提出的临界聚团理论的预测是一致的。在此基础上,推导出最佳粘度ont的计算公式由于现有文献中缺乏高粘度流态化的运动规律的报道,在正式实验之前,本文还进行了一系列探索性的实验,以确定实验的方法和操作的条件。这些探索性实验发现了高粘度流态化的一些特殊性,包括(1)高粘度下床层整体结构变化是一个缓慢的过程;(2)高粘度下床层局部结构变化也是一个缓慢的过程;(3)高粘度下床层不同高度的非均匀性不同;(4)流化床在极高粘度下会出现沟流造成测量信号失真;(5)床层在流化过程中要经历一个节涌一活塞解体一颗粒沉降一完全流化等过程。这些特殊性的发现对于保证实验设计的有效性和准确性有重要意义。
英文摘要: Researchers have ignored the effect of fluid viscosity on fluidization quality for a long time because actual and potential industrial applications of gas fluidized beds are vastly more numerous than those of liquid beds. While the effect of particle diameter, particle/fluid density difference, temperature, and pressure on fluidization has been quantitatively determined, the precise knowledge of the dependence of fluidization quality on fluid viscosity is far from being a matter of consensus. Recently, however, new processes which are being studied involving the utilization of liquid fluidized bed in the fields of hydrometallurgy, food technology, biochemical processing, water treatment, etc., together with the rapidly increasing industrial application of gas-liquid-solid fluidized bed, have attracted more interest of researchers and investors. In such systems, fluid is not ambient water, but high-viscosity liquid. So it is time to pay attention to studying the effect of fluid viscosity on fluidization quality. The object of this work is to clarity how fluid viscosity affects on the local bed heterogeneity of liquid fluidized bed. A theoretical model has been constructed in this work in order to understand how the two-phase structure of liquid fluidized bed varies with changing fluid viscosity. It is showed by the theoretical analysis that there exist two viscosity zones: the low-viscosity zone and the high-viscosity zone, in which two different fluidization behaviors could be observed. In low viscosity, it is found that fluid bubbles dominate the local bed heterogeneity, and the hydrodynamics in this zone can be described by the following equation based on the Harrison's bubble stability theory: Equation (2.32) indicates that increasing fluid viscosity could minish bubble size, which results in weakening the local bed heterogeneity and improving the fluidization quality of liquid fluidized bed. In the high-viscosity zone, it is found by some experiments that particle clusters dominate the local bed heterogeneity. But the mechanism of particle aggregation in this zone is known little so far. A physical model for the critical aggregation of particles in the high-viscosity zone is proposed in this work, which supposed that aggregation of particles in viscous fluid is formed by adhering the laminar layer surrounding a particle to that of another particle. It is well known that the minimum thickness of the laminar layer of a particle, Sl, is the function of terminal Reynolds number of the particle, i.e., 5X = fx (Re(); and the maximum distance between two particles, S2, is also the function of the terminal Reynolds number of the particle, i.e., S2 = f2 (Re,). Therefore a cluster of two particles will be formed when S2 < 2SX, i.e., Equation (3.3) implies that the terminal Reynolds number at the critical particle aggregation point,Re*, is a constant, and in the high viscosity zone, decreasing fluid viscosity could suppress particle aggregation, which results in weakening the local bed heterogeneity of liquid fluidized bed. Based on the theoretical analysis, it is predicted that there should exist a critical value of fluid viscosity between these two viscosity zones. At this critical point, fluid bubbles and particle clustershave been suppressed effectively, so the local bed heterogeneity of liquid fluidized bed is the smallest at the critical value of fluid viscosity, which is defined as the optimal viscosity /Jnpl. Moreover, this model implies that the terminal Reynolds number, Re*, is corroding to the optimal viscosity juopl. The forces balance for a solid particle in a high-viscosity liquid flow gives the related equation between Re* and nopl as follows:
语种: 中文
内容类型: 学位论文
URI标识: http://ir.ipe.ac.cn/handle/122111/1415
Appears in Collections:研究所(批量导入)_学位论文

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Recommended Citation:
邱欧. 流体粘度对液固流态化非均匀性的影响[D]. 中国科学院过程工程研究所. 中国科学院过程工程研究所. 2004.
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