Experimental Investigation On The Particle Distribution Of Gas-solids Turbulent Flow And The Characteristics Of Particle Wake In A Horizontal Channel Flow

Turbulent gas-solids flows are important phenomena in a number of technically relevant engineering and environmental applications. And the distribution and motion of the solid particles in the boundary layer directly affect the flow resistance and the wear of the particles to the wall. To clean and efficient use of energy, optimize design of industrial production facilities, it is necessary to research the particle distribution and its movement rule in the turbulent boundary layer. In this paper, both numerical simulation and experimental study are carried out, the results indicate the feasibility of digital in-line holography to measure particle size and 3D position with different particles size and concentration distribution in multiple-phase flow. The measurement system of a horizontal channel flow is developed. By using digital holography and particle image velocimetry (PIV), the particle distribution of turbulent gas-solids boundary layer and the structure of gas-phase flow are studied, and the rotational speed of the moving coal particles is measured. Through the nearest neighboring matching algorithm, the 3D velocity of particles are obtained. Using particle image velocimetry (PIV), the characteristics of sphere particle wake in a horizontal channel flow is experimentally investigated under different Reynolds numbers. The main contents are as follow:1. Gas-solids flow measurement by digital in-line holographyBy simulation and experimental research, the particle size and 3D position in multiple-phase flow are obtained under different particle size and concentration. The results indicate the feasibility of digital holography to measure particle size in the range of several micron to tens. It is found that the existence of large particle can affect the small particle size measurement and its Z axis location accuracy, when we use microholography to measure small particle. Meanwhile, as the decrease of fringe visibility, the particle measurement error increases and even particle cannot be identified. Analysis of the experiment results shows that the sample particles should be evenly dispersed, which is important part of obtaining the precise particle size distribution and spatial location.2. Experimental investigation on the particle distribution and the flow structure of gas-solids turbulent flow in the boundary layerBy the three-dimensional and non-contact optical technique digital holography, the particle distribution of turbulent gas-solids boundary layer is studied in fully developed horizontal channel. The results show that flow rate has a significant effect on the particle size distribution. When the channel flow rate is low (Ua=1.50 m/s), the ability of gas to entrain particles is greatly weakened, and the particles will be deposited on the lower wall. As the increasing of flow velocity, the peak of particle distribution occurs in logarithmic region. Meanwhile, with the increase of particle size, the peak shifts toward the wall. As the velocity increases, the peak shifts toward the channel center. Under the influence of the velocity gradient, the irregular particle shape and the particle-wall collision, the particle rotation speed reaches several hundred revolutions per second and the speed increases with the flow velocity. The gravity and sweeps has influence on the particle motion toward the wall; particle-wall collisions, ejections and the Magnus lift force have influence on the particle motion toward the channel center. Statistical analysis of the streamwise velocity of particles shows that the velocity of downward moving particles is higher than the upward moving. The former mostly dragged to accelerate by the fluid for a long time, while the latter usually rebound after the particle-wall collision. This caused momentum loss of particle, resulting in the decreased streamwise velocity.3. Experimental investigation on the characteristics of spherical particle wake by PIVThe characteristics of sphere particle wake in a horizontal channel flow is experimentally investigated under different Reynolds numbers, using particle image velocimetry (PIV). Time-averaged velocity vectors, patterns of streamlines and vorticity are examined from the point of flow physics for different sphere locations in the range of 0≤G/D≤1.0. It was found that for the case of the sphere touching the wall at G/D=0, a large circulation region formed and covered the whole wake flow region, whose size was about 1.5 times of particle diameter. With the gap ratios of G/D increasing, a recirculation zone appear at the trailing edge of particles, the vortex above the recirculation zone is along clockwise rotation while the bottom is counter clockwise and the influence of the wall on the particle wake structure is gradually weakened. At the gap ratios of G/D=0.25 and 0.5, there is a tendency of the wake to move upward and the two recirculation regions become asymmetric about the sphere centerline. With the increased gap ratio, the length of the recirculation zone decreased firstly, and then increased. As the gap increased to a value of G/D=1.0, the high velocity gradient shear layer formed around the particle, and the two rows of vortex structure occur detached from the upper and lower parts of the particles, then encounter at the end of the particle. At the same time, analysis the distribution of the particle wake velocity at the near wall, it is found that the boundary layer state is not changed by the shear layer of the the particle bottom and the farther distance from particles, the smaller of particles effect. Analysis of particle tail vorticity field shows that both above and below particle tail where have vortex shedding, and with the increased particle Reynolds number, the peak of vorticity increased significantly. With the ratio reduced, the existence of the wall surface makes the bottom of the vorticity value decrease, meanwhile the vorticity value of wall shear layer caused by the vortex shedding also decreased. At G/D=0, the vortex shedding below the particle is completely suppressed, from the perspective of the vorticity size, the strength of the shedding vortex is reduced obviously.