基于砂滤层内水体积分数瞬态模拟的反冲洗速度优选

Optimization of backwashing speed based on transient simulation of water volume fraction in sand filter layer

为了对石英砂滤层反冲洗过程水的体积分数波动规律进行分析,并确定合理的反冲洗速度范围,该文采用数值模拟手段对滤层反冲洗过程水的体积分数进行三维动态模拟,采用Gambit软件建立了石英砂过滤器的几何模型,并对几何模型进行了网格划分,以Mixture模型做为反冲洗过程水的体积分数的数值模拟模型。以当量粒径分别为1.06、1.2和1.5 mm的3种石英砂滤层为研究对象进行动态模型。为了验证模拟结果的准确性,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,水的体积分数的最大模拟误差为5.64%,说明数值模拟结果是可信的。在使用模拟数据进行流场分析时,为了得出更具普遍性的结论,引入了反冲洗流化倍数的概念,最小反冲洗流化速度的倍数称为反冲洗流化倍数。在此基础上,分别分析了反冲洗流化倍数为1.1、1.3、1.5、1.7和1.9时,滤层高度分别为15、25和35 cm共3个横截面上,反冲洗过程水的体积分数随时间的变化规律。计算了水的体积分数的均值和标准偏差,分析了水的体积分数的均值和标准偏差随随反冲洗流化倍数的变化规律。在3个截面上水的体积分数均值基本相同的情况下,根据标准偏差的大小,判定滤层反冲洗的稳定性。由此得出,使反冲洗水的体积分数波动保持稳定的反冲洗流化倍数的临界值为1.7。当反冲洗流化倍数范围为1~1.7时,标准偏差适中,反冲洗效果理想。结果表明,对于均质石英砂滤层,反冲洗效果是否理想,决定因素是反冲洗流化倍数。该文可为砂过滤器的反冲洗运行机理提供参考。

The volume fraction of water is an important parameter which affects the backwashing effect of quartz sand filter layer.In order to analyze flow field of the volume fraction of water and to determine the reasonable range of backwashing speed in the backwashing process of quartz sand filter layer,numerical simulation method was used in this paper to simulate the dynamic process of the volume fraction of water in the filter layer.For this,the geometric model of quartz 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 mixture model is suitable for the numerical simulation of the volume fraction of water 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.The simulation objects were 3 kinds of quartz sand filter layers whose thickness was all 400 mm,and the equivalent particle diameter was 1.06,1.2 and 1.5 mm respectively.In order to verify the reliability of simulation results,laboratory experiments of backwashing were conducted with the 3 different quartz sand filter layers in Farmland Irrigation Research Institute,Chinese Academy of Agricultural Sciences,which is located in Xinxiang City,Henan Province,China.The parameters such as the backwashing speed and the total height of the filter layers 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 volume fraction of water was 5.64%.It was proved that the numerical simulation results were reliable.When the flow field of the volume fraction of water was analyzed with the simulation data,in order to draw a more general conclusion,the concept of fluidization ratio of backwashing was introduced.On this basis,3 cross-sections,whose heights were 15,25 and 35 cm respectively,were selected in each filter layer and the fluctuation rule of the volume fraction of water on the sections with time was analyzed when the fluidization ratio of backwashing was 1.1,1.3,1.5,1.7 and 1.9 respectively.Then the mean and the standard deviation of the volume fraction of water were calculated.And their variation trend with the backwashing speed of quartz sand filter layer was analyzed.In the condition that the volume fraction of water in the 3 cross-sections is basically the same,the stability of filter layer can be determined according to the standard deviation.Therefore,it was concluded that the critical value of the fluidization ratio of backwashing was 1.7 for these 3 filter layers.It is said that the standard deviation is modest and the backwashing effect is ideal when the range of the fluidization ratio of backwashing is 1-1.7.The results showed that the fluidization ratio of backwashing decided whether the backwashing effect was ideal.The research results above provide not only a theoretical basis but also a technical support for the operation of the sand filter in the process of backwashing.