准二维垂直振动楔形颗粒床中颗粒对流现象的研究

发布时间：2018-05-19 00:02

本文选题：颗粒对流 + 准二维垂直振动楔形颗粒床　；参考：《北京理工大学》2015年硕士论文

【摘要】：本文采用实验和离散元模拟方法研究不同侧壁倾角下准二维垂直振动楔形颗粒床的对流模式，及其随侧壁倾角和振动加速度的变化。发现侧壁倾斜颗粒床在振动作用下发生了反向对流，即颗粒沿侧壁向上流动，在表面汇聚到中心，然后沿中心下降通道运动到底部，最后沿底部向两侧分流，形成左右两个对流卷。同等振动强度下65°侧壁倾角颗粒床的对流速度最大，倾角偏离该角度越大，对流速度越小。将对流速度随倾角变化曲线外推至对流速度为零，发现反向对流终止时对应的临界倾角为86.5°。保持侧壁倾角不变，对流速度随加速度大致呈线性增长。但65°侧壁倾角时，加速度从10g增加到12g时，对流速度增加了117%，远大于其它倾角平均增长10%。对流卷中心也随着振动加速度的增加而缓慢下降。以上实验观察可以和离散元模拟结果相吻合。颗粒对流主要发生在颗粒床两侧翼在重力波作用下的横向膨胀阶段。当膨胀后的颗粒床侧翼边缘颗粒与侧壁在更高的位置碰撞并向上运动时，仍然保持横向运动分量的侧翼内侧颗粒填补到沿侧壁向上流动颗粒的下方，形成对流卷的底部。颗粒床侧翼横向膨胀程度与此区域内颗粒床上抛最初阶段的横向力链强度呈正相关。当颗粒床底部离开底壁时，底部力链迅速消失，而侧壁与颗粒床在短时间内仍保持接触，侧翼区域内力链得以维持。随着颗粒床上抛距离的增大，颗粒床侧翼与侧壁分离，横向力链的松弛导致侧翼开始膨胀。但速度的变化较力链存在滞后性，在颗粒侧翼反向下落时，，侧翼的膨胀才变得明显。侧翼膨胀程度相同时，侧壁倾角更大的侧翼颗粒将更早与侧壁碰撞，导致对流速度增大。但上抛最初阶段侧翼颗粒与侧壁接触时间随倾角增大而延长，导致侧翼颗粒受到侧壁向下摩擦更强，上抛速度减小，在空中飞行时间缩短，以至于侧翼还未充分膨胀就与侧壁碰撞。在这两种竞争效应的作用下，65°倾角颗粒床对流最强。小于此角度，削弱了横向膨胀程度；大于此角度，侧壁摩擦效应会明显抑制反向对流。
[Abstract]:In this paper, experimental and discrete element simulation methods are used to study the convection model of quasi two-dimensional vertical vibrational wedge granular bed with different lateral wall inclination, and its variation with the lateral wall inclination and vibration acceleration. It is found that reverse convection occurs in the lateral inclined granular bed under the action of vibration, that is, the particles flow upward along the lateral wall, converge to the center on the surface, then move along the central descending channel to the bottom, and then shunt along the bottom to the two sides. Form two convection rolls on the left and right. At the same vibration intensity, the convection velocity of 65 掳sidewall obliquity granular bed is the largest, and the larger the inclination angle is, the smaller the convection velocity is. By extrapolating the curve of convection velocity with inclination to zero, it is found that the critical inclination angle at the end of reverse convection is 86.5 掳. The velocity of convection increases linearly with acceleration. However, when the slope angle of the lateral wall is 65 掳and the acceleration increases from 10 g to 12 g, the convection velocity increases by 117, which is much larger than the average increase of the other inclination angles by 10%. The convection coil center also decreases slowly with the increase of vibration acceleration. The above experimental observations agree well with the results of discrete element simulation. Particle convection mainly occurs in the lateral expansion stage of the wings on both sides of the granular bed under the action of gravity waves. When the expanded particle bed flanking edge particles collide with the lateral wall at a higher position and move upward, the inner flanking particles that still maintain the lateral motion component are filled under the flowing particles upward along the lateral wall, forming the bottom of the convection roll. The lateral expansion of the flank of the granular bed is positively correlated with the strength of the transverse force chain in the initial stage of the pellet throwing in this region. When the bottom of the granular bed leaves the bottom wall, the bottom force chain disappears rapidly, while the lateral wall and the granular bed remain in contact with each other for a short period of time, and the internal force chain in the flanking region can be maintained. With the increase of the throwing distance on the granular bed, the flank of the granular bed is separated from the lateral wall, and the relaxation of the transverse force chain leads to the expansion of the flank. However, the variation of velocity is more lag than that of the force chain, and the expansion of the flank becomes obvious when the particle flanks fall in reverse direction. With the same expansion degree of the flank, the flank particles with larger sidewall inclination angle will collide with the lateral wall earlier, resulting in the increase of convection velocity. However, in the initial stage of upper throwing, the contact time between the flank particles and the side wall increases with the increase of the inclination angle, which results in stronger friction between the flank particles and the downward friction on the side wall, a decrease in the throwing velocity, and a shortening of the flying time in the air. The flank collides with the side wall before it is fully inflated. Under these two competitive effects, the convection of 65 掳tilted granular bed is strongest. Smaller than this angle, the lateral expansion degree is weakened, and the lateral wall friction effect obviously inhibits the reverse convection when the angle is larger than this angle.
【学位授予单位】：北京理工大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TQ021.1

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