Direct Numerical Simulation Of Particulate Flows With The FD/SI Method

Particulate flows are widespread in nature and industry. Better understanding of the mechanism of particulate flows is helpful to the optimal design of the relevant engineering devices. In recent years, the direct numerical simulation (DNS), which can obtain accurately both microscopic and macroscopic information of particulate flows, attracts the attention of more and more researchers. So far, there have been a variety of the DNS works on general particulate flows. However, there have been only limited works on the DNS of complex particulate flows coupled with the electric field or the temperature field. In the present thesis, a new DNS method for the complex particulate flows coupled with the electric field or the temperature field is proposed, and then applied to various problems.The new method is a combination of the fictitious domain method (FD) and the sharp interface method (SI), and thus is referred to as the FD/SI method. The flow field is solved by the FD method, and the electric field or the temperature field is solved by the SI method. The feature of the method is the use of a fixed Cartesian grid. The FD/SI method is employed to simulate the two-dimensional and three-dimensional dielectrophoresis of particles, particulate flows with heat transfer and the melting of particles.For the dielectrophoresis of particles, this thesis proposes an SI/MST method to compute the dielectrophoretic force exerting on particles. This method can capture and keep the discontinuity of the electric field across the surface of particles, and as a result, is highly accurate. The comparison of the SI/MST method with the traditional PD approximation shows that when the particle is much smaller than the size of the non-uniformity of the electric field, the dielectrophoretic forces obtained from these two methods are largely the same. However, when the electric field is strongly non-uniform around the particle, which occurs when the particle is close to the electrode or other particles, the PD approximation is not applicable. This thesis applies the above-mentioned method to accurately simulate the conventional dielectrophoresis and the travelling-wave dielectrophoresis of particles. The results showed that particles with different dielectrophoretic parameters can be captured and separated by the cavity capturer and can be transported directionally by the travelling-wave dielectrophoresis. For the conventional dielectrophoresis of a group of particles, the particles form chains and columns in the direction of the electric field. If the electric field is non-uniform, particles with different dielectrophoretic parameters can be separated by the positive and negative dielectrophoresis.Regarding the particulate flows with heat transfer, this thesis is mainly concerned with the effects of particles on the heat transfer characteristic of natural convection in a cavity and the Couette flow, respectively. When the natural convection in the cavity is weak, the solid-fluid thermal conductivity ratio plays a positive role in the heat transfer mainly through the thermal diffusion effect. When the natural convection in the cavity is strong enough, the particle’s solid-fluid thermal conductivity ratio affects adversely the heat transfer mainly through the thermal convection effect. The heat transfer rate of the Couette flow increases with the volume fraction of solid particles, Reynolds number, Prandtl number and the solid-fluid thermal conductivity ratio, and decreases with the solid-fluid specific heat ratio.For the melting of particles immersed in heated fluid, this thesis firstly adjusts the distribution of the pseudo body-force, and then applies the FD/SI method to simulate the melting of two-dimensional particles, with focus on the melting characteristic of circular and elliptic particles. The numerical results show that in the range of parameters studied, the dimensionless melting time of a fixed circular particle under the forced convection is approximately0.4589Re0.5976Pr0.6561Ste-0.9332. For the elliptic particle with the same area, the slender one with its major axis parallel to the incoming flow melts faster.