Fractal Resistance Model Of Clean Pressure Drop And Numerical Simulation Of Backwashing Process Of Quartz Sand Filter Layer In Micro-Irrigation

Quartz sand filter in Micro irrigation is one of the most commonly used filtration equipment.It can effectively remove organic and inorganic compounds in water. This paper mainly carried out the following research:In this paper, research about the filtration process of 3 kinds of quartz sand filter layer with different particle diameters in micro irrigation were carried out with the porous media model and the fractal model. The non-dimension formula of the Ergun equation was given. Based on the formula, the flow pattern zone was studied combined with a filtration test. Meantime, the fractal model of the clean pressure drop was established. In the fractal model, the fractal dimension of the curve DT and the fractal dimension of cross section Df were undetermined parameters. In order to determine the values of the fractal dimension of the curve DT and the fractal dimension of cross section Df, Comparing the Ergun type equation with the fractal model, the fractal expression of the empirical coefficient A and B of Ergun equation were obtained. The values of Df and DT were determined by A and B. Firstly, the empirical coefficient A and B of turbulent flow region were fitted by the experimental data. And according to the fractal expression of A and B, the parameters of the the fractal model were determined, such as the fractal dimension of the curve DT、the fractal dimension of cross section Df. And the expression of fractal model of turbulent flow region was obtained.And then, using the values of Df and DT in turbulent flow region, the fractal dimension of the curve DT and the fractal dimension of cross section Df in Forchheimer flow region were determined. And the expression of fractal model of Forchheimer flow region was obtained. And based on the studies, Filter layer?s filter characteristics were analyzed. Firstly,according to the variation of DT in Forchheimer flow region, the conclusion of the maturity of the filter layer in the Forchheimer flow region was obtained. Secondly, the calculation method of the best filtration rate and the best clean pressure drop were given.The filter function of quartz sand filter was constructed, and the best filter speed and the best clean pressure drop were calculated with the filter function.In order to realize the numerical simulation of the backwashing process of quartz sand filter, the geometric model of quartz sand filter was established through Gambit software, and the mesh division of the geometric model was carried out. Because the backwashing process of quartz sand filter layer is a solid-liquid multiphase flow system interacted by both water and quartz sand, in which quartz sand are only concentrated in a part of the region during the process of backwashing, not being carried away to the outside of the filter by the stream, and in which quartz sand accounted for a larger proportion of the whole volume, the Eulerian model was adopted as a model for the numerical simulation of the backwashing. At the same time, because the backwashing process of quartz sand filter layer is a dynamically stable process, the transient simulation solver was adopted. In order to verify the reliability of simulation results, experiments of backwashing were conducted. The parameters such as the backwashing speed, the pressure drop and the expansion height of the filter layer were measured. Thesimulation results were compared with the experimental results. Comparison showed that the maximum simulation error of the whole pressure drop of the filter layer was 7.03% and the maximum simulation error of filter layer density was 1.93%. It was proved that the numerical simulation results were reliable.On this basis, the fluctuation trend of pressure drop with time and the variation trend of the mean value and the standard deviation of the pressure drop with the speed of the backwashing in the process of backwashing of quartz sand filter layer were analyzed. If the standard deviation of pressure drop is large,it shows that the fluctuation of pressure drop is unstable, and the unstable fluctuation of pressure drop will affect the effect of backwashing. This result can determine the reasonable range of the backwashing speed which can keep the pressure drop stable. Then, the distribution rule of the density of the filter layer with time, the change trend of the mean value and the standard deviation of the density with the change of the backwashing speed were analyzed. When the density is close to the static density of the mixture of water and quartz sand, the effect of backwashing is poor. When the standard deviation of density is large, the filter layer will appear local accumulation phenomenon, so as to influence the effect of the backwashing. Thus, the reasonable range of the backwashing speed was determined which can keep the pressure density distribution stable. Finally, the dynamic behavior of backwashing process is analyzed.