振动颗粒分离与床层内低压区的关系

Vibration-induced Segregation and Effects of the Low-pressure Region in a Granular Bed

近年来,人们对振动颗粒分离的现象进行了许多研究,提出了不少机理解释巴西果效应。到目前为止,振动颗粒分离的实验都是在床底封闭,振动过程中颗粒床内压力梯度发生周期性变化的情况下进行的。在受垂直振动激励的二维颗粒床(床层颗粒为平均粒径2 mm的分子筛)中通入空气,以保持振动过程中压力梯度方向不变,研究了气体对9种不同的大颗粒运动的影响。大颗粒是由有机玻璃制成的圆柱,与床层颗粒的密度比为0.33～3.62。在床内一直为负压力梯度的情况下观察到了颗粒体系发生"巴西果效应"。实验结果表明振动过程中床层内形成的低压区或沿床高周期性产生的压力梯度方向变化不能作为小颗粒填充效应的原因。认为振动过程中由于激振频率与颗粒层振动频率的不同导致颗粒之间相位角的差异引起了颗粒分离。

A significant amount of attention in scientific literature has been given to the Brazil nut effect.These studies have revealed several different mechanisms behind the segregation.Many research support that＂void filling＂due to low-pressure when the granular subjected to vibration leads to the large particles at the top.Until now,all of the studies on these effects have been conducted under the condition that the bottom of the container was closed. The low-pressure region exists in the bed and the pressure gradient direction changes periodically under this condition. We blew air from through a perforated distributor to the granular bed in the study so as to ensure low-pressure region did not exist in the bed.The segregation of different disk-shaped intruders in a pseudo-2D granular bed consisting of 2mm molecular sieve beads under vertical vibration and blowing air was studied.The intruders used were polymethyl methacrylate.The diameter ratio ranged from 2.0 to 7.0,and the relative density from 0.33 to 3.62.We observed the Brazil nut effect successively when low-pressure region did not exist in the bed.The experimental results revealed that the low-pressure region in each shaking cycle could not be the main reason for the Brazil nut effect.The phase difference between the intruder and the bed beads due to different frequency between vibration exciter and the granular is the mechanism of the vibration-induced segregation.