Study On Flow Resistance Distribution Of Sand Filter And Filtration Characteristics Of Different Granular Filter Media

Sand media filters are often considered one of the standard equipments in micro-irrigation system.Due to its unique filtration mechanism,sand filter performs better than other filters with the advantage of wide adaptability,large pollution capacities,easy to realize automatic control,especially for many kinds of especial water resources with complex water quality.The research mainly focused on the hydraulic performance optimization and high efficiency filtration of the sand filter for micro-irrigation.The flow resistance distribution,pore structure characteristics and the characteristics of transportation and deposition of suspended particles in filter layers were investigated with laboratory model experiments,numerical simulation,combined with the CT scanning technology.The results provided well guidance and reference for sand filter structure optimization and filter layer parameters configuration.The main findings are as follows:（1）Firstly,the pressure drop under filtration and backwashing was measured with the prototype test.The results showed that the pressure drop of the evaluated sand filter（without media layer）in the backwash direction was significantly greater than that in the filtration direction.The backwashing pressure drop curves as a function of the velocity had larger slopes than filtration process,indicating that the backwash pressure drop was more affected by velocity.In the case of a filter operating with media layer,the layer produced the larger pressure drop than the other components which reached 52.2%±7.1%of the total pressure drop,followed by the underdrain,backwash valve and lastly,the diffuser.At low filtration velocity（30-60m/h）,the relationship between pressure drop and the velocity was approximately linear,but when the filtration velocity exceeded 60m/h,the relationship presented a nonlinear exponential function.Therefore,the velocity should be limited in the linear region thereby achieving a better energy use efficiency.The regression equations of clean pressure drop across the four different granular media were established by using multiple stepwise linear regression method.The equations showed that the velocity had a quadratic effect on pressure drop.The hydraulic behavior of three backflush valves were compared,one was developed by our team and the other two were imported valves common used in practice.The treatments were based on evaluation of resistance pressure drop,minimum starting pressure and switching time.The results provide data reference for the selection and optimization design of backflush valve in application.（2）Taking nozzle-type underdrain as the research object,an experimental study and a numerical simulation with a scaled commercial filter were conducted for a wide range of surface velocity between0.005～0.040 m/s.The flow resistance characteristics around the underdrain and the effects of underdrain structure on flow behaviours were analyzed.Simulated and experimental pressure drop values exhibited a very good agreement in all cases with a relative error 3.54%-6.53%.The percentage of pressure drop produced by underdrain tended to increase drastically with the filtration velocity increasing.When the filtration velocity was 0.040m/s,the percentage of pressure drop generated by underdrain reached 72.3%.Therefore,it is necessary to pay attention to the selection of the underdrain type and avoid overloaded operation which would lead to larger energy consumption.Furthermore,the core roller and outlet diameter were the main factors on the pressure drop of the filter underdrain.The pressure drop produced by core roller was much higher than that produced by the slots.Thus,the pressure drop could be greatly reduced by optimizing the structure of support rod and enlarging the outlet diameter of core roller.（3）The flow resistance characteristics and flow field distribution of four different underdrains（T1、C2、R3 and O4）under filtering and backwash process were analyzed using CFD model ANSYS-Fluent（19.0）.The simulated results indicated that the pressure drop followed the order T1>C2>O4>R3 in filtering process and C2>T1>O4>R3 in backwashing process,respectively.Furthermore,both the slots distribution and the core roller structure highly influenced the overall pressure drop of the underdrain,but the total slots open area was the most important influencing factor.The effect of velocity increasing was more significant on pressure drop of underdrain than that of filter layer.R3 and O4 were more suitable for high filtering velocity condition.There were significant differences in the streamlines distribution,vortex size and position characteristics for different underdrains under backwashing condition,which were directly related to the core roller structure and slots position.In view of the above results,a new design of rotating underdrain was proposed,which adopted the internal and external double slots layout and arc flow channel.The CFD simulations with the new design showed significantly reduction in filtering pressure drop and improvement in the filter layer disturbance,thereby achieving a better energy and water consumption efficiency.（4）CT scanning experiments of four granular filter layers were conducted and the three-dimensional pore structure of the layers were reconstructed with the CT images.Subsequently,the parameters such as porosity,pore size distribution,pore shape and fractal dimension of pore structure were quantitatively characterized.Moreover,the typical hydraulic parameters（permeability,pore tortuosity,specific surface area）of filter layers were analyzed by VG Studio software.Based on the CT reconstruction model,numerical simulation was conducted to evaluate the pressure drop and streamline distribution characteristics of each filter layer.In addition,the fractal geometry of pore structure in filter layer and the seepage simulation was studied.The results showed that the apparent porosity of each filter layer decreased with the increase of the layer depth,and the filter layer with lower sphericity exhibited dispersed apparent porosity and larger volume porosity.Pore size distribution characteristic showed the pore diameter of the four filter layers was between 70-1050μm and small pores（70μm-150μm）occupied the majority.For filter layers with greater angularity,the proportion of large pore size（150μm-1000μm）was relatively higher,and the trends for roundness R and the flattening value A_R were similar.The binarized CT slice images were further analyzed with the box-counting method and the fractal dimensions which described the pore structure fractal characteristics of four layers were 1.621（QS1）,1.566（SS2）,1.661（MG3）,and 1.446（GB4）,respectively.The greater the fractal dimension was,the more complex the internal pore structure was.Meanwhile,the fractal dimension was negatively related with the porosity of the layers.（5）An experimental study using turbid water was conducted with four different granular media,three filtering velocities and three particle sizes combinations.The total and stratified amount of sediment deposited in filter layers,the particle size distribution and the particle fractional removal efficiency were investigated with the stratified sampling method.The results showed that the decrease of deposition amount along the depth direction followed negative exponential law.The surface deposition phenomenon of QS1 and SS2 was more remarkable than MG3 and GB4,where as MG3 layer exhibited slightly higher total interception than that of QS1 and SS2,and GB4 layer interception was significantly lower than the other three media.Under the condition of low filtration velocities and fine particle size,the intercepted particles in four filter layers had similar particle size distribution.With the increase of filtration velocity,the proportion of fine particles trapped in MG3 filter layer,such as clay particles and powder particles,decreases significantly compared with QS1 and SS2,and GB4 filter media had poor interception effect on fine particles.With the increase of the filter depth,the media particle size D₅₀ of the trapped particles decreased gradually,and the Span value of the particle size distribution also decreased correspondingly,indicating that the trapped particles at the bottom were smaller and more concentrated.The results showed that MG3 and GB4 filters can effectively reduce the removal efficiency of fine particles when the sediment size was less than 50μm,which was conducive to the realization of selective filtration.