Numerical Simulation Of Critical Porosity Of Porous Fence In Open Storage Piles

Particulate pollution caused by dust emissions from open storage piles is an significant aspect of air pollution. Then porous fences as efficient devices has been successfully applied to reduce dust suppression. Porosity is the most important parameter that determined the fences efficiency, and it is the key to optimize the design of porous fences. However, according to present studies, microscopic dynamics analysis of factors on the pile behind porous fences was insufficient and urgently needed.In this study, the airflow field around the storage piles and porous fences were numerically simulated using the software of computational fluid dynamics and the microscopic dynamics and kinematics characteristics on the surfaces of the pile was analyzed. Based on the distributions of the velocity vector and shear stress on each surface, the critical porosity and its variation with wind velocity and shapes of the pile was determined. The main work of the paper was as follows:(1)A three-dimensional mathematical model was established and calculation was carried out based on the numerical simulation of the airflow field using the FLUENT software. The distributions of wind related velocity over the sections of the flow field, surfaces of the piles, the velocity vectors, pressure, pressure coefficient, turbulence and shear stress with different porosities and velocities was analyzed. The surface shear stresses directly reflected the sheltering effect, on this basics, the significantly differences between high porosities(ε≥0.3)and low porosities(ε≤0.2) indicated the existence of the critical porosity. Thus the distribution of velocity vector around the stockpile and shear stress on each surface with porosities of 0.225,0.24,0.25,0.26 and 0.275 varied with height and width was researched. The result showed that total shear force of the pile decreased firstly, then increased with increasing porosity indicating a minimum corresponding to the critical porosity. According to present study, the critical porosity was determined to be 0.25 with wind velocity 6.7m/s using the prismatic model.(2)Based on the numerical results around the pile with different velocities(2.5m/s,4.4m/s,6.7m/s,9.4m/s,12.3m/s,15.5m/s) effect of wind speed on the critical porosity was considered. Though the comparative analysis of flow characteristics between the pile and the material and shear force distribution cover each surface, the velocity field characters were basically identical but the porosity related to flow regime altering slightly changed. The critical porosity was decided by the minimum shear stress of the storage pile. When the velocity less than or equal to 6.7m/s, the critical porosity was almost invariant but gradually decreased with the increasing velocity when the speed exceeded 9.4m/s. Within these speeds, the critical porosity changed in the range of 0.225~0.25.(3) As the stock pile shapes affected the distribution of flow field, three prisms approved by scholars and the flow field around the pile was simulated to explore the influence of the pile shapes on the critical porosity comparing to previous model with six speeds and eleven porosities. The flow field and force distribution showed that surface stress of three models changed consistently when the oncoming velocity was 12.3m/s. That mean shear stress increased firstly then decreased with increasing porosity lead to a critical porosity. The critical porosities of the fences sheltering previous quadrangular prism and triangular prism had not appear to much different, 0.23 and 0.24 respectively. As to the cone prisms windward and leeward area fell sharply with the height of the pile, the flow structure differed greatly with the two previous prisms under the same porosity. Therefore the critical porosity carried out to be 0.325 with the cone prism in the same operating conditions.In this paper, characteristics of flow field and the distributions of surface stress had been researched by means of microanalysis to explore the critical porosity between the high porosities and less porosities. For the first time, the influence of wind speeds and the pile shapes on the critical porosity was investigated providing data support for the optical design of porous fences. The method used in this paper also provided a new idea for the following studies of porous fences.