Experimental Research On Laws Of Particle Transportation And Clogging In Porous Media Under Seepage

Stormwater treatment based on porous media,which is a kind of technology to manage the quality and quantity of urban runoff,can effectively reduce the negative effects of stormwater runoff on the environment.However,clogging of porous media under seepage may cause the reduction of permeability and thus affect its lifespan.Physical clogging caused by particulate matter is the most common type and can be observed first.Studying the characteristics of particle transportation and clogging in porous media under seepage is significant for the understanding of particle movement laws and thus helps to avoid clogging.In this paper,the infiltration experiment of a packed column was carried out with X-ray computed tomography(X-CT)technology to study the profile of particles in porous media.The influencing factors and mechanisms of particle transportation and clogging in porous media was analyzed,and the threshold curve for clogging under bed-to-grain ratio(ratio of grain diameter to particle diameter)and average pore velocity(ratio of seepage velocity to porosity)was obtained.For the conditions where particles occurred at the bottom of the porous media,i.e.penetrated,the transportation speed and its influencing factors were discussed.The relationship between the non-dimensional parameters was explored through dimensional analysis,and the empirical formula of penetration fraction was obtained.The conclusions are as follows:(1)The transportation and clogging of particles in porous media is affected by the bed-to-grain ratio and average pore velocity under seepage.The transportation capacity of particles is effectively improved through larger bed-to-grain ratio and larger average pore velocity,thus larger penetration fraction and smaller surface residual fraction are presented.The relationship between the penetration fraction and the bed-to-grain ratio can be fitted linearly,the relationship between the penetration fraction and the average pore velocity can be fitted by a polynomial function,and relationship between the surface residual fraction and the average pore velocity can be fitted linearly.According to the result,the threshold curve for clogging of particles in porous media is a set of dividing lines including the upper threshold line and the lower threshold line,which are functions of bed-to-grain ratio and average pore velocity.Particles can be clogged below the lower threshold line,and can be penetrated above the upper threshold line.There is uncertainty in the clogging or penetration of particles in the area between the upper and lower threshold lines.(2)For the conditions where particles occurred at the bottom of the porous media,i.e.penetrated,this paper further studied the effect of bed-to-grain ratio and average pore velocity on the particle transportation speed.It is found that less initial penetration time is required for particles under larger bed-to-grain ratio.And the average pore velocity plays a similar role.The penetration rate of particles can increase with the increase in bed-to-grain ratio or average pore velocity.Based on the qualitative research on the laws of particle transportation,the quantitative relationship between particle penetration fraction and non-dimensional parameters is discussed by the dimensional analysis,and the results show that the correlation of penetration fraction and bed-to-grain ratio(D/d)is acceptable,the correlation of penetration fraction and Reynolds number(Re)is poor,and the correlation of penetration fraction and(D/d)2.5Re2.1 is the best.Based on the results above,the optimized empirical formula can be obtained.