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

Abstract: 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.