锥-板环形水槽的研制及其在颗粒起动试验中的应用

在颗粒起动观测试验中,传统长直型水槽及环形水槽试验无法观测流体长期作用下的颗粒运动,依据锥-板转子流变仪原理,设计了一种锥-板环形水槽装置;采用无量纲拖曳力作为起动时的流体力学指标,结合CCD(电荷耦合组件,charge coupled device)成像技术及PIV(粒子图像测速,particle image velocimetry)技术,运用该装置对层流时无黏性均匀颗粒在流体长期作用下的运动形态及起动规律进行了观测,观测发现流体长期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,分别为初始临界状态和饱和临界状态,初始临界状态下的起动拖曳力和经典Shields曲线的取值基本一致,饱和临界状态下的起动拖曳力明显高于Shields曲线的取值,表明锥-板环形水槽是一种合理有效的颗粒起动观测装置,特别适用于流体长期作用下的颗粒起动规律研究。

Development of cone-plate annular flume and its application on onset motion of particles

Abstract: For experiments about the onset motion of particles, the rheological property of granular bed under long-term fluid shearing cannot be observed in traditional long flume and annular flume since the long flume is limited in length and velocity distribution in the annular flume is not even. In order to investigate the long term evolution of the granular bed under fluid shearing, according to the principle of cone and plate rheometer, a cone and plate annular flume is developed. Compare to cone and plate rheometer, the size of cone and plate annular flume is larger, which make the system easier to combine with imaging technology and to observe solid properties. Also by adding torque-meter, the torque on the bottom of the flume can be read, making it possible to test the force directly. There are three types of flow region in this equipment: laminar regime, transition regime and turbulence regime. The flow field in each regime is unique. To test and verify this equipment, the tangential velocity distribution in this setup is compared with the tangential velocity distribution in traditional annular flume. It shows lower discrepancy in the cone and plate flume since the velocity difference is less than 5%, while in traditional annular flume, the difference can reach 30%. Further, choosing non-dimensional drag force as the onset criterion, the onset motion of uniform non-cohesive particles under long term fluid shearing and the torque on the flume bottom are studied in cone and plate annular flume in laminar regime, combining with CCD imaging technology and PIV technology. Compare the particle tracking results with torque test results, two thresholds can be noticed for the onset in this experiment: initial threshold and saturated threshold. At initial threshold, the movements of particles are not stable and cease after running for a while. At the saturated threshold, stable particle movements can be observed. Compare to the classic Shields curve, the test results are consistent with Shields curve at initial threshold; while much higher than Shields curve at the saturated threshold. This phenomenon can be explained by that at the very beginning the bed is loosely packed, as the particle start moving, it can be trapped in the pore of the vicinal particles. As this process going on, the granular surface will finally reach a stable packing state and particles form a constant flow rate. Consequently, the drag force will increase. The scatter in pervious drag force experiments is induced by different granular surface structure. But this is still reasonable when the result is in-between the low and high boundary that we found. So it is necessary to consider rheological property for onset of granular bed. The cone and plate annular flume gives a new method of particle onset motion experiments. In this equipment, both the particle movements and torque can be observed. And the experiments show that cone and plate annular flume is reasonable for particles' onset motion observation, especially for the long term evolution of granular material under fluid shearing. And further investigation should be extended to the turbulence regime.