Direct Numerical Simulation Of The Particulate Flows In Duct

Particle-laden flows are wildly encountered in nature, e.g. the suspended sediment transport in rivers. And there are extensive applications involving industrial and agricultural productions, such as the fluidized bed, the oil transportation in the pipe, cell separation and, etc. Although numerous works in both experiment and numerical simulation have been dedicated to the study of the flow, for the complexity of the interaction between particles and fluid, the flow characteristics and the mechanism still remain unclear.A parallel direct-forcing fictitious domain method (DF/FD) was employed to perform the direct numerical simulations (DNS) of the inertial migration of neutrally buoyant particles (pr=1) and the effects of particles on the second flow and turbulence in a square duct respectively.At the bulk Reynolds number (Re) ranging from100to1500, the migration trajectories and equilibrium positions were investigated. Our results indicate that particles migrate to two main different kinds of equilibrium positions:one at the diagonal line near the corner and the other at center line. With the increasing of the Re, on one hand, the equilibrium position at the diagonal line becomes closer to the corner, besides the smaller the particle becomes the closer to the corner; on the other hand, the equilibrium position at the center line tends to the wall first and then leaves the wall tending to the center while the Re increased to around800, especially for the large particle. And the final equilibrium positions are similar to the tube’s inner equilibrium position. Comparatively the smaller the particle is, the closer to the wall the equilibrium position is. Within the long duct length (L=4H), the center line equilibrium position lose its stability when Re is low while the corner equilibrium position becomes unstable when Re is high especially for small particles. For the short duct, the particle chain is enhanced and the induced vortex existed all through the duct that both of the two positions can stay stable. Besides, the flow will earlier turn to turbulence with high Re in the long duct. The behaviors of the multi-particles are similar to the single particle’s and their equilibrium positions can form particle chains along the flow. However, the flow may turn to weak turbulence that the equilibrium positions disappear when Re is a little high.At the friction Reynolds number of150, the influence of the particles on the duct turbulence flow as well as the second flow was investigated. The neutrally buoyant particles with low volume fraction (0.105%) increase the average flow-direction velocity on the middle plane while the flow drag in the whole duct is increased. For the mean second flow, the concentration of particles plays a key role in changing the vortex’s center which moves along a nearly straight line heading towards the center. With the same concentration, the effect of the small particle is large than large particle. For the area near the wall, the existence of particles reduces the rms of flow-direction velocity fluctuation and increases rms of the other two direction velocity fluctuation. For the log law region, the effect of the velocity fluctuation rms decreasing appears mainly in the spanwise and transverse direction. The influence on the probability density function of the fluctuating velocity is similar to the channel flow. The probability of small fluctuating velocity in the flow-direction and large fluctuating velocity in the other two directions is increased while the probability of large fluctuating velocity in the flow-direction and large fluctuating velocity in the other two directions is reduced resulting the above rms’ changing. At the same time, the skewness of the PDF of the streamwise fluctuating velocity on the area near the wall is reduced. Besides, the addition of the particle reduces the Reynolds stress. The above effects rise with the increasing of the particle concentration.