Numerical simulation of local clogging in screen filter based on coupled DEM-CFD

Abstract: Due to low cost and long operational life span, screen filter is widely used in chemical industry, pharmaceutical engineering, and agricultural irrigation. In order to study the motion and spatial distribution of particles in the filter and reduce the harm of local clogging，in this paper we simulated the particles' motion under different flow rates and diameters by the method of CFD-DEM, and analyzed the effects on the particles' spatial distribution caused by flow rate and particle size. The correctness of theoretical analysis and the validity of methods were verified in experimental filtration with both clean water and muddy water under different inlet pressure. According to the velocity vector distribution and streamlines, there is a jet-flow and backflow, causing the uneven flow field in the filter. The uneven flow field in screen filter leads to local clogging whose distribution is influenced by flow rate, particle size, streamlines and etc. Pressure and velocity distribution directly reflects the resistance characteristics of mesh which results in 77 percentage of total pressure drop. The charts about the flow rate on different mesh sections, with stepped distribution characteristics, show that flux differences are obvious on account of filter shells. The lowest flow rate is located in the middle of screen close to inlet while the highest in the upper of screen close to outlet, and the former is as high as 5.9 times that of the latter. Therefore, it is necessary to optimize the filter shell for better performance. Particles' motion and distribution shows sieve effect of mesh on the movement and distribution of sediment. Although particles which can pass through the screen will not attach themselves to the mesh surface, they will aggravate clogging when the porous medium forms on mesh surface. The higher the flow rate is, the more concentrative the particles pass through the screen and the more serious the local clogging is. For the clogged particles which are the essential factor of clogging on mesh surface, bigger ratios of filter pore size to particle size cause the particles cycling in the housing, while the smaller firmly adhere to mesh surface and are more heavily agglomerated on the mesh with the rise of flow rate. So the smaller particles can lead more serious clogging and higher pressure drop than the bigger one under the same mass. The motion of both passing particles and smaller clogging particles is closely related to streamlines and surface velocity on mesh. Along streamline, particles can easily adhere to mesh surface or pass through mesh pore in lower surface velocity. When particles move with a high-velocity or the flow field speed up by the increase of inlet velocity, the kinetic energy of particles increase and the collision becomes more frequent. This decreases the possibility through the screen and weakens particles' stability. Particles prefer to adhere to the mesh in low velocity, but rarely adhere to the mesh where the streamline does not skim over regardless of the surface velocity. When the overall velocity rises, a great deal of particles amass on the section lying on end of streamlines because of the lower velocity. In most cases, adopting high inlet velocity can increase the work efficiency, but when the ratios of filter pore size to particle size is slightly greater than 1, high inlet velocity will sharply increase the harm of local clogging. In this situation, reducing the inlet velocity can increase the uniformity of particle distribution, enhance the auxiliary effect of filter cake, prolong the effective time and the operational life span of filter, and reduce the difficulty of flushing at the cost of efficiency loss. So, in practice, the filtering accuracy and inlet pressure of the filter should be adjusted according to the particle size of irrigation water for better perform.