Numerical Simulation On Mechanisms Of Particle Motions In Two-phase Turbulent Square Duct Flows Based On LES/DNS

Particle-laden flow has always been a key research topic in the field of international multiphase flow due to its wide engineering application background.In the present study,Large eddy simulation（LES）and Direct numerical simulation（DNS）combined with Lagrangian particle tracking（LPT）technique are applied to investigate the transport mechanisms of different inertial particles in two-phase turbulent square duct flows at multi-scale Reynolds number.The main purpose is to fully uncover the interaction mechanism between secondary flow,turbulent coherent structures and particle behaviors in turbulent square duct flows.Firstly,one-way coupled LES-LPT is adopted to investigate the mechanisms of deposition and resuspension for particles with different size（St⁺=0.0007-2415）in turbulent square duct flows with a wide range of bulk Reynolds number(Re_2h=10k-250k)considered.The results indicate that the deposition of small particles on duct walls is relatively uniform,with the deposition rate increasing with Reynolds number.For larger particles,the effect of secondary flow and gravity is important.Influenced by the two factors,mid-inertia particles tend to deposit in the central regions of the bottom wall,while high-inertia particles tend to deposit in duct corners.Meanwhile,under the effect of transverse secondary flow,particles located in the central regions and duct corners of the bottom wall are also prone to resuspend,and the resuspension rate increases with Reynolds number.Secondly,the mechanism of particle dispersion in a turbulent square duct flow with a top free surface at the Reynolds number of Re_2h=83k,250k are discussed by using the same numerical method.The results show that particle distribution in the lower half of the duct is similar to that o f the closed square duct.However,the change of the secondary flow topology caused by the free surface is found to inhibit the overall dispersion of inertial particles in the whole duct,and the extent of inhibition gradually increases with particle size.In addition,it is also observed that particle concentration decreases in the region near the free surface where the secondary flow moves vertically downward along the surface centerline.Thirdly,one-and two-way coupled LES-LPT is used to further investigate the particle aggregation mechanism in the near-wall regions of turbulent square duct at Re_2h=10k.It is shown that under the synthetic effect of secondary flow and near-wall coherent structures,particles tend to accumulate in the bottom corners and the low-streamwise-velocity streaks in the middle regions of the bottom wall,where they tend to form slender streamwise-aligned streaky patterns.The Voronoi diagram analysis demonstrates that the near-wall secondary flow could cause particle clusters to transfer from regions of high to low concentration,thus weakening the particle aggregation behaviors to some extent.On the contrary,the two-way coupling can enhance the aggregation of particles in the near-wall region.Then,the transport mechanism of neutral suspended particles（St⁺=0.31～260）in turbulent square duct flow at Re_2h=10k is studied by conducting DNS-LPT of one-and two-way coupling without the consideration of particle gravity and buoyancy.Results obtained indicate that the cross-stream secondary flows encourage particles to accumulate preferentially in the duct corners,where the maximum of the cross-sectional particle concentration occurs.The extent of accumulation here is strongly dependent on the Stokes number,with the greatest accumulation found at St⁺=25.The topology of particle secondary motion is similar to that of fluid,but its intensity significantly relies on St⁺.A region-based correlation analysis between the particle spatial distribution and local flow topology shows that the particle motion is basically decoupled from the coherent structures in the viscous sublayer,but still retains part of the topology characteristics from the buffer layer.For particles located in the buffer layer,their motions are mainly dominated by Q2 events near the duct corners,and affected by both Q2 and Q4 events in the central regions of the duct walls.Finally,the interaction mechanism between particles and turbulence in square duct flow of Re_2h=10k with a higher particle volume fraction(φ_V=10^-3)is further explored by employing DNS-LPT of two-way and four-way coupling.The results show that for the fluid phase,the modulation of particles can reduce the mean fluid streamwise velocity,increase the average secondary flow intensity and shift the rotation center of the secondary flow closer to the center of the duct cross section,but can restrain the fluctuation velocitites,destroy the structure of large-scale coherent vortices near the wall and simultaneously increase the number of small-scale vortices.For the particle phase,the two-way coupling can increase the secondary velocity along the corner bisector and promote the particles to aggregate near the walls.Whereas the inter-particle collisions have exactly the opposite effect,which can greatly enhance the particle fluctuation velocities and Reynolds shear stress in the cross section,and promote the particles with higher inertia to gather in the central regions of the duct cross section.