移动床中固体颗粒运动与传热的研究
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摘要
颗粒材料在自然界中广泛存在,在各种工业过程中也有广泛的应用,以化学工业为例,其产品的一半、原材料的3/4为颗粒状物质,现代化工中若干前沿发展也与颗粒学有关,例如材料、制药、生化和环境等。但是人们对颗粒系统中的许多现象认识还不深,据估计在相关的工业部门,单由输送颗粒材料遭遇的问题所带来的工业设备利用能力的浪费就高达40%,远远达不到优化设计和节能的要求。因此研究颗粒系统中的各种传递过程是化学工程研究领域的前沿,极富有挑战性。本论文以颗粒随机运动模型(PKM)、离散元模型(DEM)和颗粒接触传热模型为研究手段,以颗粒移动床为研究对象,从不同尺度、不同角度研究了移动床中的动量传递和热量传递过程,拓展了原有模型的应用范围,提出了新的概念和理论。研究具有重要的理论意义,对移动床的优化、设计也具有重要的实用价值。
移动床在散体物料储存、多相反应、物质分离等领域有着广泛的应用,要有效的设计移动床就需要对颗粒物料间的各种传递过程的特性有深入的了解。传统的实验方法可以获得颗粒系统的一些特性,得到相应的经验关联式,但是要深入的了解颗粒系统的本质特征所需要的实验费用十分昂贵。近年来随着计算机成本的不断降低,越来越多的研究可以通过计算机模拟来完成,在保证结果准确性的同时,大大降低了研究成本。本论文采用了计算流体力学的方法,研究了移动床中的动量和热量传递过程,主要开展了以下几个方面的工作:
1.颗粒随机运动模型(PKM)是从纯运动学的角度考察颗粒的运动,其形式简单,只有一个模型参数——运动常数B,而且B只与颗粒的大小有关,因此十分便于工程的实际应用。本论文建立了一个2-D实验移动床,通过与实验结果的对比,确定运动常数B的最优值为颗粒直径的2.5倍。传统的PKM模型未考虑移动床壁面对颗粒运动的影响,只适用于中心卸料过程。本论文利用Lie群变换的方法求取了模型方程相似解的一般形式,将相似解对x求一阶导数后发现颗粒运动的最大速度分布在x=0的线上,因此本论文采用移动坐标的方法来解决偏心卸料问题。按照该思想对PKM模型进行修正,模拟结果与实验结果吻合的很好,拓展了该模型的应用范围。
2.PKM模型虽然形式简单,但是对形状复杂的流场很难得到准确的结果,并且只能模拟稳态过程的问题。DEM方法是在颗粒角度上考察各种量的变化,其主要优点是可以基于基本的数据来模拟复杂系统而不必采用过分的简化假设,能够更好地反映一些过程的本质,并且与PKM模型相比,模拟得到的颗粒运动轨迹即为颗粒的实际运动历程,同时可以很容易地将时间因素考虑进来,因此有
Granular materials are widely used in the chemical industry with particulate reaction engineering, design of powders, storage of grain flows, separation and granulation. Roughly one-half of the products and about three-quarters of the raw materials of chemical industry are in form of granular materials. Considerable importance has been given in industries these days for the handling of granular materials. But the understanding of the behaviors in granular materials is not very established. It has been estimated in interrelated industries that about 40% of equipment capacity is wasted in conveying granular materials. It is far from the aim of optimization design and saving energy. Therefore, it is the frontal and full of challenge in chemical engineering research. The thesis focused on the behaviors of granular moving bed, and investigated the solid flow and heat transfer by means of particle kinematic model (PKM), discrete element model (DEM), and particle contact heat transfer model (PCHM). Not only the novel conceptions and theories were established, but also discover many new phenomena and then theory established.The experimental cost becomes more and more expensive, while the cost of computer decreases rapidly. More investigations have been completed by computer simulation with high accuracy and low expense. The thesis investigated the transfer phenomena in granular materials by computational fluid dynamics (CFD). The creative works can be summarized as following:1. PKM considers that the granular flow velocities are determined by pure kinematic effects. Only one coefficient is included in PKM and is only related to particle diameter, so it is very convenient to engineering application. In the thesis PKM was used to simulate the particles flow. For center discharging the simulation results accorded with experimental one very well, while for eccentric discharging the simulation couldn't show the effects of boundaries on particles flow. An analytic solution of kinematic model was obtained by Lie transformation group. It showed that the velocity of particles flow was maximum when x=0. Through localization of max particles velocity distributing lines and making sure that the value of x on the lines always equal 0, the modified kinematic model was proposed and could simulate the eccentric discharging well.
2. The main advantage of DEM is that highly complex systems can be modeled with basic data without oversimplifying assumptions. DEM can obtain the real trajectory of particle motion, so it can reflect intrinsic characters of granular materials. In the thesis, DEM was used to investigate the flow pattern of the particles discharging and continuously flowing in two-dimensional silos (flat-bottomed silo and wedge-shaped hopper silo). The accuracy was confirmed by comparing calculated results with experimental one. The effects of key parameters such as the shape of silos, the width of outlet, and the shape of inserts on particle flow were clarified. The particles flowed as "slug flow" in the top part of any silos, while "funnel flow" in the low part. In batch discharging, the particles on the surface of materials in silos flowed together to the silo center. 3 kinds of inserts were used to change the flow field of silos. The placement of inserts improves the flowing performance. Comparing with traditional conical insert, the 2# insert (inverse hopper insert) invented in the thesis could improve granular flow better.3. There are many factors can effect granular motion, such as particle diameter, density, shape, angle of repose, character of particle group (diameter distribution, void fraction and so on), discharging rate. Three granular materials (Polyethylene, Polystyrene, Colophony) flow characteristics ware studied experimentally. The results showed that discharging rate has little effects on flow pattern, while the repose angle, diameter and shape of particles have great effects on flow pattern. The time line becomes more sharp-angled with increasing the repose angle, and the dead zoon becomes greater. With the increasing of sphericity degree and decreasing of particle diameter, the flow performance becomes better and particles mix more easily.Four kinds of diameter distribution (random distribution, normal distribution, binary mixing and uniform diameter) were studied by DEM. The spatial distribution of velocity, void fraction and stress were obtained, while probability distribution of these variables by Mathematical Statistics. The results showed that:a) The distribution of particle diameter has obvious effects on particle flow, and the mixing of different particle diameter is propitious to particles flow. The effects were more remarkable in flat-bottom silo than in wedge-shaped hopper silo.b) Linear equations can be used to describe the relationship between discharge rate and orifice size by G215 vs. Do for the same distribution of particles diameter. Do has least effects on discharge rate for uniform diameter distribution, while greatest effects for normal distribution. The effects become little in hopper silo.
c) The force structures of particles in hopper are spatially non-uniform. There are arched stress chains above orifice in flat-bottomed silo, but the arched stress chains are not evident in hopper silo.d) The stresses between particles and bed walls are spatially non-uniform too. The shape of silo, the shape of inserts and the installation position of inserts all can influence the stresses distribution.e) The probability distributions of each variables are influenced by diameter distribution, shape of silo, insert and so on. The effects of diameter distribution are greater in flat-bottomed silo than in hopper silo.4. The heat transfer in granular materials can be found in many industry processes. But the understanding for it is very poor. In the thesis, a particle contact heat transfer model (PCHM) was established. PCHM includes four independent mechanisms: ?thermal conduction through the solid, ?thermal conduction through the contact area between particles, ?thermal conduction through the fluid film near the contact surface, (Dheat transfer by convection solid-fluid-solid. The model combined with DEM was used to simulate heat transfer between particles and heating wall. At the same heat transfer temperature difference, the relationship of each mechanism was: Q4 , and Q2 has greatest effect on Qeff. The keyfactors effecting on ￡>eff include: the number of particles contacting with heating wall, the contact time, and the contact area. When the average diameter is same, the diameter distribution has little effects on heat transfer. When the average diameter is different, the effective heat transfer coefficient decreases with the increasing of particle diameter. The quantity of heat transfer is spatially non-uniform.
移动床在散体物料储存、多相反应、物质分离等领域有着广泛的应用,要有效的设计移动床就需要对颗粒物料间的各种传递过程的特性有深入的了解。传统的实验方法可以获得颗粒系统的一些特性,得到相应的经验关联式,但是要深入的了解颗粒系统的本质特征所需要的实验费用十分昂贵。近年来随着计算机成本的不断降低,越来越多的研究可以通过计算机模拟来完成,在保证结果准确性的同时,大大降低了研究成本。本论文采用了计算流体力学的方法,研究了移动床中的动量和热量传递过程,主要开展了以下几个方面的工作:
1.颗粒随机运动模型(PKM)是从纯运动学的角度考察颗粒的运动,其形式简单,只有一个模型参数——运动常数B,而且B只与颗粒的大小有关,因此十分便于工程的实际应用。本论文建立了一个2-D实验移动床,通过与实验结果的对比,确定运动常数B的最优值为颗粒直径的2.5倍。传统的PKM模型未考虑移动床壁面对颗粒运动的影响,只适用于中心卸料过程。本论文利用Lie群变换的方法求取了模型方程相似解的一般形式,将相似解对x求一阶导数后发现颗粒运动的最大速度分布在x=0的线上,因此本论文采用移动坐标的方法来解决偏心卸料问题。按照该思想对PKM模型进行修正,模拟结果与实验结果吻合的很好,拓展了该模型的应用范围。
2.PKM模型虽然形式简单,但是对形状复杂的流场很难得到准确的结果,并且只能模拟稳态过程的问题。DEM方法是在颗粒角度上考察各种量的变化,其主要优点是可以基于基本的数据来模拟复杂系统而不必采用过分的简化假设,能够更好地反映一些过程的本质,并且与PKM模型相比,模拟得到的颗粒运动轨迹即为颗粒的实际运动历程,同时可以很容易地将时间因素考虑进来,因此有
Granular materials are widely used in the chemical industry with particulate reaction engineering, design of powders, storage of grain flows, separation and granulation. Roughly one-half of the products and about three-quarters of the raw materials of chemical industry are in form of granular materials. Considerable importance has been given in industries these days for the handling of granular materials. But the understanding of the behaviors in granular materials is not very established. It has been estimated in interrelated industries that about 40% of equipment capacity is wasted in conveying granular materials. It is far from the aim of optimization design and saving energy. Therefore, it is the frontal and full of challenge in chemical engineering research. The thesis focused on the behaviors of granular moving bed, and investigated the solid flow and heat transfer by means of particle kinematic model (PKM), discrete element model (DEM), and particle contact heat transfer model (PCHM). Not only the novel conceptions and theories were established, but also discover many new phenomena and then theory established.The experimental cost becomes more and more expensive, while the cost of computer decreases rapidly. More investigations have been completed by computer simulation with high accuracy and low expense. The thesis investigated the transfer phenomena in granular materials by computational fluid dynamics (CFD). The creative works can be summarized as following:1. PKM considers that the granular flow velocities are determined by pure kinematic effects. Only one coefficient is included in PKM and is only related to particle diameter, so it is very convenient to engineering application. In the thesis PKM was used to simulate the particles flow. For center discharging the simulation results accorded with experimental one very well, while for eccentric discharging the simulation couldn't show the effects of boundaries on particles flow. An analytic solution of kinematic model was obtained by Lie transformation group. It showed that the velocity of particles flow was maximum when x=0. Through localization of max particles velocity distributing lines and making sure that the value of x on the lines always equal 0, the modified kinematic model was proposed and could simulate the eccentric discharging well.
2. The main advantage of DEM is that highly complex systems can be modeled with basic data without oversimplifying assumptions. DEM can obtain the real trajectory of particle motion, so it can reflect intrinsic characters of granular materials. In the thesis, DEM was used to investigate the flow pattern of the particles discharging and continuously flowing in two-dimensional silos (flat-bottomed silo and wedge-shaped hopper silo). The accuracy was confirmed by comparing calculated results with experimental one. The effects of key parameters such as the shape of silos, the width of outlet, and the shape of inserts on particle flow were clarified. The particles flowed as "slug flow" in the top part of any silos, while "funnel flow" in the low part. In batch discharging, the particles on the surface of materials in silos flowed together to the silo center. 3 kinds of inserts were used to change the flow field of silos. The placement of inserts improves the flowing performance. Comparing with traditional conical insert, the 2# insert (inverse hopper insert) invented in the thesis could improve granular flow better.3. There are many factors can effect granular motion, such as particle diameter, density, shape, angle of repose, character of particle group (diameter distribution, void fraction and so on), discharging rate. Three granular materials (Polyethylene, Polystyrene, Colophony) flow characteristics ware studied experimentally. The results showed that discharging rate has little effects on flow pattern, while the repose angle, diameter and shape of particles have great effects on flow pattern. The time line becomes more sharp-angled with increasing the repose angle, and the dead zoon becomes greater. With the increasing of sphericity degree and decreasing of particle diameter, the flow performance becomes better and particles mix more easily.Four kinds of diameter distribution (random distribution, normal distribution, binary mixing and uniform diameter) were studied by DEM. The spatial distribution of velocity, void fraction and stress were obtained, while probability distribution of these variables by Mathematical Statistics. The results showed that:a) The distribution of particle diameter has obvious effects on particle flow, and the mixing of different particle diameter is propitious to particles flow. The effects were more remarkable in flat-bottom silo than in wedge-shaped hopper silo.b) Linear equations can be used to describe the relationship between discharge rate and orifice size by G215 vs. Do for the same distribution of particles diameter. Do has least effects on discharge rate for uniform diameter distribution, while greatest effects for normal distribution. The effects become little in hopper silo.
c) The force structures of particles in hopper are spatially non-uniform. There are arched stress chains above orifice in flat-bottomed silo, but the arched stress chains are not evident in hopper silo.d) The stresses between particles and bed walls are spatially non-uniform too. The shape of silo, the shape of inserts and the installation position of inserts all can influence the stresses distribution.e) The probability distributions of each variables are influenced by diameter distribution, shape of silo, insert and so on. The effects of diameter distribution are greater in flat-bottomed silo than in hopper silo.4. The heat transfer in granular materials can be found in many industry processes. But the understanding for it is very poor. In the thesis, a particle contact heat transfer model (PCHM) was established. PCHM includes four independent mechanisms: ?thermal conduction through the solid, ?thermal conduction through the contact area between particles, ?thermal conduction through the fluid film near the contact surface, (Dheat transfer by convection solid-fluid-solid. The model combined with DEM was used to simulate heat transfer between particles and heating wall. At the same heat transfer temperature difference, the relationship of each mechanism was: Q4
引文
1. Bernhard Peters, Algis Dziugys, Numerical simulation of the motion of granular material using object-oriented techniques, Comput. Methods Appl. Mech. Engrg., 2002, 191 (17-18): 1983-2007.
2. Yang S C, Hsiau S S, The simulation and experimental study of granular materials discharged from a silo with the placement of inserts, Powder Technology, 2001, 120 (3): 224-255.
3.PG 热纳,J 巴杜.软物质与硬科学.湖南教育出版社,长沙,2000.
4.毛炳权.我国聚烯烃工业的回顾与展望。http://www.zshg.com/wenzhang/hongguanshidian/e3/wgix-3.htm.
5.毛炳权.发展我国聚烯烃技术的一些建议.石油炼制与化工,2001,32(12):1-6.
6.威廉·巴克·布罗德,比利·杰克·加纳.在脱气容器中处理聚烯烃树脂的方法[P].88102601,1993.
7.尹哗东,王运东,费维扬.计算流体力学(CFD)D 化学工程中的应用.石化技术,2000,7(3):166-169.
2. Yang S C, Hsiau S S, The simulation and experimental study of granular materials discharged from a silo with the placement of inserts, Powder Technology, 2001, 120 (3): 224-255.
3.PG 热纳,J 巴杜.软物质与硬科学.湖南教育出版社,长沙,2000.
4.毛炳权.我国聚烯烃工业的回顾与展望。http://www.zshg.com/wenzhang/hongguanshidian/e3/wgix-3.htm.
5.毛炳权.发展我国聚烯烃技术的一些建议.石油炼制与化工,2001,32(12):1-6.
6.威廉·巴克·布罗德,比利·杰克·加纳.在脱气容器中处理聚烯烃树脂的方法[P].88102601,1993.
7.尹哗东,王运东,费维扬.计算流体力学(CFD)D 化学工程中的应用.石化技术,2000,7(3):166-169.
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