基于颗粒流理论的微灌砂滤层反冲洗过程砂粒速度场模拟

砂颗粒流在石英砂滤层反冲洗流场中的速度分布,对滤层流化状态的稳定性和反冲洗效果起决定性作用。为了对滤层反冲洗过程砂颗粒的速度场进行分析,并确定最佳反冲洗速度,该文以厚度为400 mm,粒径范围为1.0～1.18 mm的石英砂滤层为研究对象,基于颗粒流运动理论,采用Eulerian-Eulerian模型对滤层反冲洗过程砂粒的速度场进行3维动态模拟。为了验证模拟结果的准确性,作者开展了室内模型试验,并将模拟结果与试验结果进行对比,结果显示,滤层膨胀高度的最大模拟误差为9.8%,能够控制在10%以内,说明数值模拟结果是可信的。在此基础上,分析了反冲洗流化倍数为1.3、1.5、1.7和1.9时,滤层高度分别为15、25和35 cm 3个横截面上,在不同的反冲洗时间,砂粒的轴向速度沿横坐标的分布规律。根据砂粒在3个横截面上运动速度的大小和方向,判断砂滤层是否达到完全流化;根据砂粒在3个横截面上运动趋势是否一致,砂粒的上升区是否保持稳定,判断滤层流化状态是否稳定。结果显示,当反冲洗流化倍数不小于1.7时,滤层才能达到稳定的流化状态,从而达到比较理想的反冲洗效果,并得出滤层最佳反冲洗流化倍数为1.7。研究结论为砂过滤器的反冲洗研究提供了理论基础和技术支撑,为反冲洗性能参数的确定提供了参考。

Numerical simulation of velocity field of sand grains in backwashing process of sand filter layer in micro-irrigation based on granular flows theory

The velocity distribution of sand grains in the backwashing flow field is key to the backwashing performance of sand filter layer, such as the expansion height, distribution uniformity and the stability of fluidized state. To analyze the velocity distribution of sand grains in the backwashing process and find out the optimal backwashing speed, numerical simulation was used in this paper. Moreover, a geometric model of sand filter was established and the mesh division of the geometric model was carried out through Gambit software. Because the backwashing process of quartz sand filter layer is a solid-liquid multiphase flow system composed of water and quartz sand, we can conclude that the Eulerian model is suitable for the numerical simulation of the velocity field of sand grains by comparing the applicability of the current multiphase flow numerical simulation models such as Eulerian model, Mixture model and VOF(volume of fluid) model. At the same time, because the backwashing process of quartz sand filter layer is both a dynamic and a stable process, the transient simulation solver was adopted. Additionally, the granular flow theory was used to seal the momentum equation of the model, because of the formation of granular flows in the backwashing process. The simulation objects was the quartz sand filter layer whose thickness was 400 mm, and the equivalent grain diameter was 1.06 mm. In order to verify the reliability of simulation results, laboratory experiments of backwashing were conducted in Farmland Irrigation Research Institute, Chinese Academy of Agricultural Sciences, Xinxiang, China. The parameters such as the backwashing speed and the total height of the filter layer were measured during the experiments. And the simulation results were compared with the experimental results. Comparison results showed that the maximum simulation error of the sand grains velocity was 9.8%. So the numerical simulation results were proved to be reliable. On this basis, three cross-sections, with the height of 15, 25 and 35 cm, were selected in the filter layer and the axial velocity distribution of sand grains was analyzed. The fluidization ratio of backwashing for this simulation was 1.3, 1.5, 1.7 and 1.9 respectively. Based on the magnitude and direction of the velocity of sand grains in the three cross-sections, we can figure out whether the sand filter layer is completely fluidized or not. The stability of the fluidization state of the filter layer can be estimated by the consistency of the movement trend of sand grains in the three cross-sections and the stability of the rising zone of granular flows. The results showed that the bigger the fluidization ratio of backwashing is, the less time needed for completely fluidizing the filter layer. As a consequence, only if the fluidization ratio of backwashing is not less than 1.7, the filter layer might reach a stable state of fluidization. In a stable flow, the rising zone and the descending zone formed a stable circulation in the filter layer. As the grains swarm moved along a relatively fixed path, the ideal backwashing effect was achieved. It can be seen from the above that the optimal fluidization ratio of backwashing of the filter layer is 1.7. The research provide not only a theoretical basis and technical support for the study of the sand filter but also a reference for the determination of performance parameters for the backwashing.