Effect of turbulence-driven secondary flow on particle preferential concentration and clustering in turbulent square duct flows

In this study,the preferential concentration and clustering of inertial particles in fully developed turbulent square duct flows are studied using large eddy simulations combined with Lagrangian approach,where the Reynolds number is equal to Reτ=600(based on the mean friction velocity and duct full height),and the particle Stokes number ranges from 0.0007 to 1.16.The results obtained for duct flows are compared with those for channel flows under the same working conditions.Then,the effect of the secondary flow on the particle concentration in duct flows is investigated.The equation of particle motion is governed by the drag force,lift force,added mass force,pressure gradient force,and gravity.The inter-phase interaction that was considered includes one-way and two-way coupling.The simulations of a single phase are verified and in good agreement with the available literature data.For the discrete phase,particles in the duct flow are found to be more dispersed in the vertical direction compared with the channel flow.In near-wall regions,a small fraction of particles tends to accumulate in duct corners,forming stable particle streaks under the effect of the secondary flow.Meanwhile,most particles are likely to reside preferentially in the low-speed flow regions and form elongated particle streaks steadily in the middle region of duct or channel floors.The Voronoi diagram analysis shows that the near-wall secondary flows in the square duct could cause particle clusters to transfer from regions of high to low concentration,and this trend increases with particle size.In addition,two-way coupling is found to enhance the near-wall particle accumulation and to promote particles to form more elongated streaks than one-way coupling.Finally,the mechanism responsible for the particle preferential concentration in turbulent square duct flows is determined.