层流条件下无黏性均匀颗粒起动规律研究
发布时间：2018-09-12 11:30
【摘要】：颗粒起动是颗粒运动的基本问题之一,掌握层流条件下的颗粒起动规律对研究层流边界层及高黏度流体中的颗粒运动有重要的意义,同时也可丰富层流区颗粒起动实测资料。以起动拖曳力曲线(Shields曲线)的离散分布为切入点,综合利用试验观测、理论分析及数值模拟相结合的方法,对无黏性均匀颗粒的起动规律进行了研究。其中试验观测采用PIV(Particle Image Velocimetry,粒子图像测速)技术、RIM(Rrefractive Index Matching,折射率匹配)技术及CCD(Coupled Charged Device,电荷耦合组件)摄影技术对无黏性均匀颗粒床的表层过渡段流场进行了研究,并分别在矩形有压管及锥-板环形水槽中对无黏性均匀颗粒的起动进行了系统观测。理论分析上,根据观测结果,综合考虑颗粒床的表面结构特性,采用滚动起动力学模型对无黏性均匀颗粒的起动拖曳力公式进行了推求。数值模型方面,采用Fluent对不同简化床面结构下的颗粒受力进行了模拟。主要研究结果如下:(1)颗粒床和主流之间的过渡段垂线流速分布服从指数分布规律。颗粒床对边界流场的影响主要集中在表层颗粒的影响上,随着主流雷诺数增大过渡段表层的滑移流速增大,但过渡段厚度不随雷诺数改变且和表层颗粒粒径有关,厚度约和表层颗粒粒径相等,表层颗粒以下流动微弱仍然服从Darcy渗流规律。(2)在矩形有压管中,采用颗粒床表面冲刷停止的临界状态作为颗粒起动的临界状态,观测发现层流区范围内希尔兹数呈规则的带状分布,流体作用引起颗粒床表面粗化并导致床面颗粒突起减小,使颗粒起动拖曳力增大1倍以上。同时,通过改变颗粒形状及底坡,均能够观测到床面的粗化现象。进一步采用粒径不同的两种均匀球形颗粒按照不同体积混合比混合进行试验,观测发现当其中粗颗粒的混合体积比大于70%时,临界状态下颗粒床表面的形态变化主要是由于该颗粒的自身结构调整引起。综合表明起动拖曳力的离散分布是由无黏性均匀颗粒床的自身结构调整引起,对于某些级配不良的颗粒床在考虑级配引起的粗化同时,也应当考虑颗粒床表面结构自身调整引起的粗化。(3)锥-板环形水槽中的起动观测试验表明长期剪切作用下颗粒的起动为连续的起动过程,存在2个不同的临界状态:初始临界状态,颗粒从静止转为运动,但运动状态不稳定,一段时间后颗粒运动停止,初始临界状态下的起动拖曳力和矩形有压管起动试验观测结果基本一致;稳定临界状态时,颗粒运动并开始形成稳定的颗粒流动,稳定临界状态下的起动拖曳力高于矩形有压管中的试验观测结果。但整体曲线的分布形式和矩形有压管中试验结果分布形式一致,呈现规则的带状分布,因此综合表明流体剪切力作用下颗粒床表面结构存在自身调整的特性,且在剪切力长期作用下颗粒的起动拖曳力增大显著,该现象和矩形有压管中观测到的床面粗化现象一致且直接对颗粒的起动过程进行了描述。(4)在矩形有压管试验及锥-板环形水槽试验的基础上,结合过渡段的流场特性及颗粒床表层的结构特征,建立了颗粒的滚动起动力学模型。模型计算表明颗粒突起不同时,无量纲起动拖曳力不同,起动拖曳力曲线的形式也不同。实际起动过程中,由于床面粗化,颗粒突起在不断减小,多数试验数据分布在颗粒突起从0.2至1的范围内,当颗粒突起小于0.2时,已不能观测到颗粒的起动。以往的研究中并没有注意到床面结构的影响问题,纯粹建立在试验数据基础上的经验公式不能从本质上揭示层流区无黏性均匀颗粒的起动规律。(5)在试验结果及理论模型的基础上,采用Fluent数值模拟,考虑不同床面结构对目标颗粒的影响,对目标颗粒受到的拖曳力及升力进行了分析。分析得到由于周围颗粒的遮挡,目标颗粒的迎流面减小,从而使得拖曳力减小,且同一床面结构,迎流面的方向改变也会使得目标颗粒受到的拖曳力改变。但对于升力而言,在本文的模拟条件下,周围颗粒的遮挡反而会使得升力增大。床面结构直接影响到颗粒的受力大小,在起动临界状态,当床面结构发生粗化时,目标颗粒所需起动拖曳力将发生变化。该结果与试验观测及理论模型计算一致,表明床面结构的改变是导致无黏性均匀颗粒起动拖曳力离散分布的主要原因。以上研究综合表明颗粒床表面结构性状直接影响着颗粒起动拖曳力的大小,由于颗粒床表面粗化,在层流区无量纲起动拖曳力曲线(Shields曲线)具有带状分布特性。
[Abstract]:Particle start-up is one of the basic problems of particle motion. It is very important to understand the law of particle start-up in laminar boundary layer and high viscosity fluid. It can also enrich the measured data of particle start-up in laminar flow region. The starting law of non-cohesive uniform particles is studied by means of experimental observation, theoretical analysis and numerical simulation. PIV (Particle Image Velocimetry), RIM (Refractive Index Matching) and CCD (Coupled Charged Device) are used in the experimental observation. (2) The flow field in the surface transition zone of the non-cohesive uniform granular bed was studied by photography, and the incipient motion of the non-cohesive uniform granular bed was systematically observed in the rectangular pressurized tube and the cone-plate annular flume respectively. In the numerical model, Fluent was used to simulate the forces acting on the particles under different simplified bed structures. The main results are as follows: (1) The vertical velocity distribution between the particle bed and the main stream obeys the exponential distribution law. Concentrating on the influence of surface particles, the slip velocity increases with the increase of Reynolds number, but the thickness of the transition section does not change with Reynolds number and is related to the particle size of the surface layer. The thickness of the transition section is about the same as the particle size of the surface layer. The flow below the surface particles still obeys Darcy seepage law. (2) In a rectangular pressurized tube, particles are used. The critical state of the particle bed surface scouring stops is regarded as the critical state of the particle start-up. It is found that the Hiltz number in the laminar flow region distributes regularly in a band. Fluid action causes the surface of the particle bed to coarsen and the particle protrusion on the bed surface to decrease, which makes the particle start-up drag force increase more than one time. The coarsening of the bed surface can be observed. Two uniform spherical particles with different particle sizes are mixed according to different volume mixing ratios. It is found that the morphological changes of the bed surface in critical state are mainly caused by the structural adjustment of the particles when the volume ratio of coarse particles is greater than 70%. It is concluded that the discrete distribution of the starting drag force is caused by the self-structural adjustment of the non-cohesive uniform granular bed, and the coarsening caused by the self-structural adjustment of the granular bed surface should also be taken into account when considering the coarsening caused by the gradation of some non-cohesive uniform granular beds. (3) The start-up test in a cone-plate annular flume shows that the long-term shearing is necessary. There are two different critical states: the initial critical state, in which the particles change from static state to motion, but the motion state is unstable. After a period of time, the particle motion stops, and the initial critical drag force and the rectangular pressurized tube start-up test results are basically consistent. The starting drag force in the stable critical state is higher than that in the rectangular pressurized tube, but the distribution of the whole curve is consistent with that of the experimental results in the rectangular pressurized tube, showing a regular band-like distribution, so it shows that the granular bed under the action of the fluid shear force. The surface structure has the characteristic of self-adjusting, and the starting drag force of particles increases remarkably under the long-term shear force. This phenomenon is consistent with the bed coarsening observed in rectangular pressurized tube and directly describes the starting process of particles. (4) Based on the rectangular pressurized tube test and cone-plate annular flume test, the starting drag force of particles increases remarkably. The mechanical model of particle rolling starting is established based on the characteristics of flow field and the structure of particle bed surface in the crossing section.The results show that when the particle protrusion is different,the dimensionless starting drag force is different,and the form of starting drag force curve is different.In the actual starting process,the particle protrusion is decreasing because of the coarsening of bed surface,and most test data are divided into two parts. In the range from 0.2 to 1, the starting of particles can not be observed when the particle protrusion is less than 0.2. Previous studies have not paid attention to the influence of bed structure. Empirical formulas based solely on experimental data can not reveal the starting law of non-viscous uniform particles in laminar flow region. Based on the experimental results and the theoretical model, the drag force and lift of the target particles are analyzed by using Fluent numerical simulation considering the influence of different bed structures on the target particles. But for the lift, the shielding of the surrounding particles will increase the lift. The bed structure directly affects the force on the particles. In the critical state of starting, when the bed structure coarsens, the target particles need to start the drag force. The results are in agreement with experimental observations and theoretical model calculations. It is shown that the change of bed structure is the main reason for the dispersion of the starting drag force of non-cohesive uniform particles. The dimensionless starting drag force curve (Shields curve) in the flow area has a zonal distribution characteristic.
【学位授予单位】：西北农林科技大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TV149
本文编号：2238869
[Abstract]:Particle start-up is one of the basic problems of particle motion. It is very important to understand the law of particle start-up in laminar boundary layer and high viscosity fluid. It can also enrich the measured data of particle start-up in laminar flow region. The starting law of non-cohesive uniform particles is studied by means of experimental observation, theoretical analysis and numerical simulation. PIV (Particle Image Velocimetry), RIM (Refractive Index Matching) and CCD (Coupled Charged Device) are used in the experimental observation. (2) The flow field in the surface transition zone of the non-cohesive uniform granular bed was studied by photography, and the incipient motion of the non-cohesive uniform granular bed was systematically observed in the rectangular pressurized tube and the cone-plate annular flume respectively. In the numerical model, Fluent was used to simulate the forces acting on the particles under different simplified bed structures. The main results are as follows: (1) The vertical velocity distribution between the particle bed and the main stream obeys the exponential distribution law. Concentrating on the influence of surface particles, the slip velocity increases with the increase of Reynolds number, but the thickness of the transition section does not change with Reynolds number and is related to the particle size of the surface layer. The thickness of the transition section is about the same as the particle size of the surface layer. The flow below the surface particles still obeys Darcy seepage law. (2) In a rectangular pressurized tube, particles are used. The critical state of the particle bed surface scouring stops is regarded as the critical state of the particle start-up. It is found that the Hiltz number in the laminar flow region distributes regularly in a band. Fluid action causes the surface of the particle bed to coarsen and the particle protrusion on the bed surface to decrease, which makes the particle start-up drag force increase more than one time. The coarsening of the bed surface can be observed. Two uniform spherical particles with different particle sizes are mixed according to different volume mixing ratios. It is found that the morphological changes of the bed surface in critical state are mainly caused by the structural adjustment of the particles when the volume ratio of coarse particles is greater than 70%. It is concluded that the discrete distribution of the starting drag force is caused by the self-structural adjustment of the non-cohesive uniform granular bed, and the coarsening caused by the self-structural adjustment of the granular bed surface should also be taken into account when considering the coarsening caused by the gradation of some non-cohesive uniform granular beds. (3) The start-up test in a cone-plate annular flume shows that the long-term shearing is necessary. There are two different critical states: the initial critical state, in which the particles change from static state to motion, but the motion state is unstable. After a period of time, the particle motion stops, and the initial critical drag force and the rectangular pressurized tube start-up test results are basically consistent. The starting drag force in the stable critical state is higher than that in the rectangular pressurized tube, but the distribution of the whole curve is consistent with that of the experimental results in the rectangular pressurized tube, showing a regular band-like distribution, so it shows that the granular bed under the action of the fluid shear force. The surface structure has the characteristic of self-adjusting, and the starting drag force of particles increases remarkably under the long-term shear force. This phenomenon is consistent with the bed coarsening observed in rectangular pressurized tube and directly describes the starting process of particles. (4) Based on the rectangular pressurized tube test and cone-plate annular flume test, the starting drag force of particles increases remarkably. The mechanical model of particle rolling starting is established based on the characteristics of flow field and the structure of particle bed surface in the crossing section.The results show that when the particle protrusion is different,the dimensionless starting drag force is different,and the form of starting drag force curve is different.In the actual starting process,the particle protrusion is decreasing because of the coarsening of bed surface,and most test data are divided into two parts. In the range from 0.2 to 1, the starting of particles can not be observed when the particle protrusion is less than 0.2. Previous studies have not paid attention to the influence of bed structure. Empirical formulas based solely on experimental data can not reveal the starting law of non-viscous uniform particles in laminar flow region. Based on the experimental results and the theoretical model, the drag force and lift of the target particles are analyzed by using Fluent numerical simulation considering the influence of different bed structures on the target particles. But for the lift, the shielding of the surrounding particles will increase the lift. The bed structure directly affects the force on the particles. In the critical state of starting, when the bed structure coarsens, the target particles need to start the drag force. The results are in agreement with experimental observations and theoretical model calculations. It is shown that the change of bed structure is the main reason for the dispersion of the starting drag force of non-cohesive uniform particles. The dimensionless starting drag force curve (Shields curve) in the flow area has a zonal distribution characteristic.
【学位授予单位】：西北农林科技大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TV149
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