Numerical Simulation Of Dust Emission Characteristics Of Open Storage Pile And Optimization Design For Porous Fence

The fugitive particulate emitted from stockpiles in the open storage yards of industrial sites constituted the significant part of air pollution. Presently, the physical mechanism of dust emissions has been clearly understood. Most wind tunnel simulation and numerical research have focused on the relationship between dust emission and wind speed, the direction of wind speed was not considered yet. But as a matter of fact, when the wind flowed around the storage piles, its speed and direction were not in a regular way, and the reason lied in that the pile inserted atmospheric boundary layer would alter wind flow structure of the ground layer. Therefore, it was very essential to study the dust emission mechanism and the optimization design for porous fences by analysing and understanding the relevance of the velocity vector and dust emission characteristics and rules. In this paper, the air flow around the open storage pile and the physical mechanism of dust emissions were analyzed. On this basis, a three-dimensional mathematical and physical model of static flow fields was established and the distribution of velocity and surface shear stress around the pile behind the fences with different porosity were researched by using the standard k-ε turbulence model. The numerical simulation results have been validated by experimental measurements. The following conclusions have been drawn.The wind speed profile on the windward and leeward slope of the pile surface deviated from the original logarithmic distribution on the level surface because the pile changed velocity distribution of the atmospheric turbulence boundary layer of the Earth. Since the absolute value of the wind speed alone couldn’t embody fluid dynamics rule, the shear stress of the pile surface was suggested to be a better indicator to study the dust emission from storage yards. When the thickness of the shear layer decreases and the velocity of the shear layer increases with the increasing of the pile height, wind profile deviates from the original logarithmic distribution. The shear layer of the first half of the flat-top surface was thin, and the air speed of the second half agreed to the logarithmic distribution, where the dust erosion weakened. Then air flowed down along the leeward slope and performs reverse vortex under two-thirds the height of the surface, but the dust emission was much lighter than from windward slope and flat-top surface because of the lower stress speed.For the oncoming wind, change of the wind speed from 3m/s to lOm/s had almost no effect on the velocity profiles on the surfaces of the pile. The shear stress gradually increased from the bottom of the slope to the two-thirds height of the pile, and then sharply increased to the top. Shear force on the flat-top surface first decreased dramatically, and then turned smoothly. Velocity vector fields showed the shear stress on the leeward above two-thirds the height of the pile decreased along the surface and then induced a vortex in the downstream, and allowing particles to be emitted from the piles.Porosity was the most important factor determining the efficiency of the porous fence, and the turbulence characteristics of flow fields around fences of small porosity considerably differed from those around fences of large porosity. When the porosity was smaller than 0.3, a recirculating flow in the region between the fence and the pile appeared because bleed flow passing through the fence holes was far less than the airflow over the fence. When the porosity was 0, the intensity of the vortex with a center higher than the top of the pile reached the maximum, so reverse flow performed above the flat-top and the particle materials move opposite to the oncoming flow. Moreover, when the porosity was 0.2, the intensity of the vortex and the vortex diameter were smaller than when the porosity was 0, and its center located at two-thirds the height of the pile, then air flowed down along the flat-top surface. The shear stress increased from the foot of the windward slope, reaching a maximum dust emission at two-thirds the height of the pile, and decreasing with increasing height as the shear force acted downward along the slope surface.The shear stress on the windward side increased firstly and then decreased with increasing height; and the maximum dust emission occurred at two-thirds the height of the windward side, unlike in unfenced conditions, the maximum dust emission did not occur at the windward surface crest. The leeward surface was always in the recirculation region and the shear stress changed gently with the increasing porosity. With high porosities, bleed flow through the fence was increased and the air pressure differential between both sides was reduced; and the particle was elevated to the air, maximal dusting occurring on the top of the windward side of the pile. When the porosity was 0.6, the flow structure behind the porous fence was similar to that of unfenced condition.The flow structures with the porosity of 0.225,0.25 and 0.275 were studied and results showed that porosity 0.25 was the turning point above which attached flow dominated and below which vertex flow became significant. On that evidence,0.25 was defined as the critical porosity, and the shear stress at this point was the least. The study of the aerodynamic characteristics around the pile can manifest the microscopic properties of dust emission well and this research will provide a new idea for the optimization design of porous fence.The influence of discrete scheme, porous jump boundary conditon, and turbulence model on the numerical results was analyzed. The accuracy and reliability of the established physical and mathematical models have been validated by experimental results.