| Quartz sand filters are among the most widely used filtering equipments in micro-irrigation so far.Based on its unique way of three-dimensional filtering it can filter the organic and inorganic impuritiesin the water effectively, which makes it having great interception of particles. Researches on filteringand backwashing parameters have been carried out for many years, but so far, many of the phenomenaand the nature of filtering and backwashing haven’t been revealed. Therefore, to find new researchmethods and study further have very important significance.(1) To reduce the impact of the outflow boundary, the filter beds were set with a thickness of80cm.related factors were muddy water impurity particles mass fraction, impurity particle sizes and filteringspeed. We found that the pressure value in the flow field correlated with its height in the filter bed—itreduced while its height decreased. Dividing the bed into eight segments from the top down, each layerhas a thickness of10cm. Among the eight layers, the surface layer generated the maximum pressuredrop while other layers generated a smaller pressure drop, and the last layer generated the smallest one.In all of the experiments carried out, the pressure drop of the upper one quarter of the filter bed accountsfor34%~40%of the total pressure drop, which indicates homogeneous quartz sand filter media stillcome up with slight surface filtration. Of the three factors, filtering rate has the most importantinfluence on filters’ pressure drop while influence of muddy water turbidity mass fraction and particlesize is relatively not obvious. Total pressure drop has a positive correlation with filtration rate. Thegreater the filtration rate causes the greater total pressure drop, and pressure drop is a quadratic functionof filtration rate.(2) Porous model of Fluent software was adopted to simulate the flow field, and parameters likepore size, porosity of the filter bed, viscous drag coefficient and inertial resistance were calculated. Wefound that the size of the single pore was closely related to the size of filer sand’s diameter, the greaterof it, the larger the pore.(3) A suitable fluid dynamic theory was found to descript filtration process in the quartz sand filterfiltration. Setting a two-dimensional planar type for numerical calculation and the results shown that theupper of the filter bed has a high pressure value, and it decreases near the bottom. However, at the sameheight of the filter, the pressure value is not a constant. It can approximately be seen as a constant at theinterface of quartz sand and water. When turning to the lower height level, it changes. Pressuredistribution of the sideline is larger than that of the center line. Numerical simulation of the pressure isin highly consistent with the measured value by piezometric tube when the filtration rate is equal to orbigger than0.025m/s, that is, the hierarchical error of the two drops below6%, and the error of totalpressure drop is below14%.(4) A three-dimensional filter model was set based on the two-phase flow theory to calculate theprocess of backwashing. Results shown that phases fraction drops while increasing the backwashingrate, which reflects the expansion of the filter bed. By analyzing conclusions were drawn as follows: the height of expansion is in a positive linear correlation with the backwashing rate. Expansion height valueis larger with large filter bed thickness than that with smaller filter bed thickness, but its expansion ratiois smaller. It has greater expansion height value and expansion ratio if the filter bed has smaller filterdiameter. All simulated results agree with the measured values. |
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