PIV Measurements And Direct Numerical Simulation Of Turbulence Modification In A Horizontal Particle-laden Channel Flow

Explore the mechanism of turbulence modification is very important for dredging waterway, improving transportation efficiency of raw materials, reducing air pollution, and so on; so it has been a hot spot of multiphase flow. Although numerous inverstigations have been done and some consensuses have been reached, many aspects of turbulence modifications have not yet been understood, especially for the low-mass-loading case. This paper will study the mechanism of turbulence modifications in a horizontal channel flow at low mass loadings, both from experiment and simulation, and special attentions will be paid to the modulation of coherent structures due to particles in the boundary layer.In the first part of this paper, a simultaneous two-phase PIV measurement technique with high-resolution is adopted to acquire the turbulent statistics quantities and to examine the coherent structures in the lower boundary layer at a wall Reynolds number of Reτ=430. Polythene beads with diameters of 60μm and 110μm are used as dispersed phases; for each size, there are three mass loadings varying from 2.5×10-4 to 5×10-3. The results show that the presence of the particles suppresses the coherent structures, with shorter streamwise extent, lower burst frequency and weaker burst strength; and then, the wall-normal velocity fluctuations and shear Reynolds stresses are both decreased in the near-core region. In addition, as a result of the particle wake, the turbulence intensity and shear Reynolds stress both increase in the vicinity of the wall. Due to the drag effects of the particles on the gas, the streamwise velocity gradients decrease in the outer region and increase in the viscous sublayer, meanwhile the thickness of the viscous sublayer also decreases. These results cause the peak values of the streamwise velocity fluctuations adjacent to the wall to increase, and the peak positions shift to the wall. This is the reason for decreasing the near-wall region and increasing the near-core region of the streamwise velocity fluctuations m appearance.In the second part of this paper, direct numerical simulation (DNS) and Lagrangian particle tracking are performed to investigate the interaction of small heavy solid particles with turbulence of channel flow at a wall Reynolds number of Reτ=150. First of all, on the condition of one-way coupling, the turbulent diffusion behaviors of three kinds of particles (with Stokes number of 1,5 and 25) show that the preferential concentration of particles in wall-bounded turbulence is different from the case in isotropic and homogeneous turbulence or free shear turbulence; when the particle Stokes number is between 10 and 100, the segregation into the low-speed fluid regions is the highest. Subsequently, on the condition of two-way coupling, the turbulence modifications due to copper beads (St=200) both in horizontal and vertical channel flow indicate that gravity is a very important factor, which leads to some discrepancy of turbulence modulations between horizontal and vertical flow. Moreover, the preferential collection of particles in the low-speed fluid regions is more pronounced in vertical channel relative to that in horizontal channel. Finally, the modifications of turbulence structures were examined by detecting coherent structures from instantaneous flow fields using the vortex definition of Jeong et al. The addition of particles results in the diameter and streamwise extent of the mean structures decreasing in horizontal channel flow, but increasing in vertical channel flow. By examining velocity fluctuations, shear Reynolds stresses, vorticities and pressure fluctuations around the mean vortex at x+=0 plane, it is found that the presence of particles inhibits the coherent structures and the degree of suppression due to particles is more notable in horizontal channel flow.The results of this study demonstrate the influence of particles on the near-wall quasistreamwise vortices is a important mechanism of turbulence modification at low mass loading. In gas-particle channel flows, the particle behavior is mainly governed by inertia, gravity and particle-wall collision, therefore the saltation mechanism plays a major role in the particle velocity fluctuations and leads to the strong trajectory-crossing effect. There are some discrepancies between DNS and PIV measurement in the near-wall region as a result of point-particle method using in the two-way coupling, this fact shows that paritlce volume is a very important factor in wall-bounded turbulence modifications and can not be neglected.