| The transport and deposition of suspended solids is widely recognized to occur in both nature and industry applications. Especially in recent years, with an intense concerns about our own living environment, the subject has increasingly been given attentions, for example the study on movement characteristics of suspended solid particles in the liquid, which is an important and significant issue for many applications such as geothermal reinjection, drinking water supply, soil and water conservation, pollutant purification of underground structures, petroleum exploration, filtration systems, internal erosion of hydraulic structures, nuclear waste disposal and so on. The paper here mainly took the physical clogging problem resulting from waste water reinjection in geothermal resources as the research background. In this study, the process of particles transport and deposition in saturated porous media was conducted. The aim is to investigate the characteristics of particles transport and deposition laws in porous media. On the one hand, examining the influences of particle diameter, concentration of suspended solids, and particle diameter of the porous media on the retention of suspended particles both at the entrance interface and in the porous media system; on the other hand, understanding better how the deposited particles affect the hydrological properties with time in the porous media. Since that the in situ field experiments inevitably contend with many specific hydrogeological settings, the results obtained are less general in application, the laboratory experiment has been taken in this paper by using a pipe filling with spherical glass beads to simulate the underground aquifer.By using a limited diameter pipe to simulate an infinite space of underground aquifer, the wall effect of non-uniform porosity distribution near the wall in the parked porous medium channel must exist. Therefore, the wall effect was examined experimentally at first, which includes the variation of the ratio of the tube diameter to the filling particle diameter, and compared with those in the previous literature. An extreme case for the porosity distribution having a step change in the cross-section of a porous media channel was then studied theoretically. Considering the channel with up and down two free fluid regions having the porosities of one and a porous region in the middle, a linear stability analysis of a Poiseuille flow was performed based on the base flow. After that, a reinjection test system in laboratory scale was established, and numerous experiments were conducted by changing the porous test section and the water suspended with variable concentrated particles, in order to investigate the characteristics of fluid carrying microparticles flowing through the porous media. Especially, the effects of particle diameter and concentration on the transport and deposition in the porous media were investigated in detail. As a numerical approach, the motion and deposition behaviors of particles in fluid passing various square pores were simulated by using Lattice Boltzmann Method.The detailed description of the research contents are as follows:First of all, to know if the prepared porous media test section can stand for the porous media in a large space, we investigated the wall effect resulting from the anisotropy near the impermeable wall after the circular tube was filled with the spherical particles. The correlations obtained in the previous literature regarding the average porosities (s) with the ratio of the tube diameter to the filling particle diameter (Dt/Dp) were summarized. It can be concluded that:for the diameter ratio Dt/Dp<10, the confining wall effect is very obvious; for 10<Dt/Dp<40, the effect of the wall is weak; for Dt/Dp>40, the wall effect is negligible.In addition, considering that the fluid flow in underground aquifer is in a stable state, we theoretically studied a linear stability analysis of a flow through a porous media channel in which porosity of the cross-section having an uneven distribution, i.e. a step change distribution. For that case, the channel was divided into three layers with up and down two free fluid layers (ε=1) and a porous layer in the middle. The stability problem of that coupled flow through the channel was investigated. Based on the fluid flow models in porous media region, free fluid region and the momentum transfer model at the interface between the adjacent free fluid region and porous media region, the perturbation equations were derived by ignoring the inertia effects. The Chebyshev spectral collocation technique was adopted to solve the fully coupled Orr-Sommerfeld linear stability equations. The effects of Reynolds number (Re), permeability (σ), porosity (ε), wave number (α) and the porous filling ratio (Ψ) on the critical Reynolds number (Rec), and still the characteristics of the eigenvalues and velocity perturbation were investigated. According to the above analysis, we can further validate that in the prepared porous media test section the fluid flow is stable under the present experiment conditions.On this basis, a linear stability of Poiseuille flow in a channel filled with multiple porous medium layers was performed. In order to compare the results with those in a channel inserting with only one porous layer, two ways for increasing the number of porous layers have been considered, one is constant Re condition, and the other is constant Ψcondition. The general characteristics of the wall modes in Orr-Sommerfeld spectrums at constant Re condition were emphasis studied. The results are of great theoretical significance in guiding the instability problem for groundwater flow in multilayer fracture aquifer. It is shown that:porous wall can decrease the stability of channel flows with respect to channel flows with impermeable walls. The channel flows with inserting one porous layer uniformly at the middle in a channel is more stable than the channel flows with two porous walls at up and down, and also is more stable than the flows with inserting more than one porous layer. Though that both ways for increasing the number of porous layers can result in a decrease of Rec, the decreasing degree differs:at the same number of porous layers np, the flow for constant Ψ condition in a channel are much less stable than the situation of constant Re condition.The above stability analyses were performed by ignoring the inertial effects in the porous layers and at the interfaces. To understand the inertial effects on the base flow, a method to solve the coupled flow problem in a complex channel structure was studied. By taking the Brinkman viscous dissipation term and Forchheimer nonlinear inertia term into account, we proposed a new and more general nondimensionalized governing equation. According to the boundary layer theory, the porous region can be divided into two regions, the core region without the influencing of the interface, and the near interface region with the influencing of the interface. The Runge-Kutta-Gill numerical differentiation method, combined with the first derivative guess and shooting method were then employed to resolve the base flow equations which including the inertial effects. The solution of the basic flow can be obtained without using the given physical parameters limitation curve as proposed in the previous literature.By improving the porous test section many times, a reinjection experimental system in laboratory scale was established. Numerous experiments were conducted in order to elucidate the effects of microparticle diameter and concentration of suspended solids on their transport and deposition in the porous media cylinder made by filling with of spherical particle with different diameters. On the one hand, presenting how the deposited particles affect the degree of clogging or relative permeability kt/ko in porous media with time; on the other hand, investigating the accumulated and instantaneous deposition characteristics of the suspended particles during the process of reinjection time. At last, the retentions of suspended particles both at the interface of entrance and in the porous media system were tested and analysed. The observations by these experiments are:the deposition of suspended particles in porous media does not always reduce the value of relative permeability kt/ko, which may lead further clogging of porous media as expected by people and concluded by some previous researchers. Conversely, for a certain case, the value of kt/ko will increase, a wave crest can occur and it appears earlier if the distance from the entrance is shorter. This phenomenon very much likes the propagations of a dynamic wave, and its happenence mechanism can be explained as the difference of friction factor of a corrugated pipe from that of a smooth pipe. A lower entrance concentration and a smaller size suspended particle, the later the time for the wave crest occurring, and the larger increasing range for k1/k0. However, at a larger entrance concentration, there will not be a wave crest, and the ratio of kt/ko decreases monotonously with time at any locations.Fine particles dispersion and deposition in a sudden expansion and attraction square pore was investigated by using the lattice Boltzmann method (LBM). The simulation was undertaken with the assumption of that particles were completely absorbed by the wall once contacting the wall, regardless of the particles accumulation effect and the forces in flow field acted by the particles. In addition to the drag force, the Saffman force and the Gravity, the Brownian force had been taken into account due to the small size of particles. First, under the condition of forced convection, the particle trajectories in vorticity field and deposition on each wall for different Stokes numbers (Stk) in three different square pore structures were studied. Second, since the temperature of waste water is usually lower than the temperature of the underground aquifer during reinjection project, fine particles dispersion and deposition in a single pore under mixed convection condition, particularly at different intensity of natural convection (Ra) was simulated. The effects of Stk and Rayleigh number Ra on the particles transport and deposition were researched. The simulated results are:for a same Re number, with increasing the Stk number, the distribution of particles in the vortex field is a process that first to concentrate around the periphery of the vortices, then gradually hardly affect by the vortices. An intense natural convection could result in high deposition efficiency, especially for the case of low Stokes number and high Rayleigh number, which is true in many realistic reinjection tests in geothermal uses. If the inertial force and the gravity are dominant during the movement of particles, the deposition efficiencies tend to be the same at whatever a larger Stk.Finaly, in order to know deeply about the characteristics of particle transport and deposition in the porous media, some proposals and the improvements needed for the future study are given according to the problems and the insufficiencies found in this study. |
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