Study on Dust Retention Characteristics of Vegetation Barriers and Bionic Design of Porous Fence
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【Author in Chinese】 赵冬森；
【Author's Information】 吉林大学， 农业机械化工程， 2020， 硕士
【Abstract】 With the rapid development of economy,the pollution of particulate matters（PM）from open source becomes more and more serious.Escaped atmospheric particles can cause smog,waste of piled materials,negative effects on human health,and restrict economic development.One of the most effective management strategies to reduce particulate matter from open source is to set vegetation barrier or artificial porous barrier to block and intercept the particulate matter during the diffusion process.Vegetation barrier（VB）as a porous medium,can trap particles with different particle sizes in the canopy and adjust the porosity by swinging branches and leaves to adapt to wind changes.But the growth of vegetation is affected by water resources,soil and seasons.However,artificial porous barriers have no such limitations.Currently,commercial artificial porous barriers,including rigid and flexible windbreak fence,can reduce the dispersion of particles by reducing the wind speed.However,these commercial artificial fences are poor at trapping fine particles.In addition,there is a contradiction between the wind speed reduction and the wind load acting on the barrier.Moreover,the opening form of the hole on the porous barrier is fixed leading to fixed porosity of the fence.Therefore,it is of great significance to study the vegetation barrier as the bionic prototype to optimize the artificial porous barrier and improve the wind environment and retain fine particles from open sources.In this study,the typical tree species in northeast China were studied,including Padusvirginiana‘Canada Red’,Pinussylvestris var.mongolicaLitv.,Malusbaccata（L.）Borkh.andAcer buergerianumMiq.Powder particles were milled from the collected dust sources in a laying hen house to simulate the dispersion of PM from the laying hen house.The ability of different tree species blocking particles was investigated by establishing natural barriers and conducting tests on the four vegetation barriers.The effects of VBs on the attenuation efficiency of wind speed and PM concentration as well as the influcing factors,such as ambient temperature,relative humidity,and characteristics of vegetation leaves and barriersn,on particle blocking ability were analyzed.The macroscopic mechanism of vegetation barriers blocking and adsorbing particles was identified.The results showed that the simulation test of particle diffusion retardation designed in this study can be used,and the four kinds of vegetation barrier can effectively block fine particles.There were differences in the blocking efficiency of different vegetation barriers on particles,with the order of Padusvirginiana‘Canada Red’>Pinussylvestris var.mongolicaLitv.>Malusbaccata（L.）Borkh.>Acer buergerianumMiq..The blocking efficiency of Padusvirginiana‘Canada Red’on PM1,PM2.5 and PM10 was 73.80±15.61%,73.73±15.80%and67.35±19.55%,respectively.The order of attenuation efficiency of vegetation barriers to wind speed was Padusvirginiana‘Canada Red’>Acer buergerianumMiq.>Pinussylvestris var.mongolicaLitv.>Malusbaccata（L.）Borkh..The attenuation efficiency of Padusvirginiana‘Canada Red’to wind speed was 41.14±6.91%.The above two orders were not the same,indicating that the ability of the vegetation barrier to block particles was also affected by other factors.Therefore,the grey correlation analysis was used in this study.It was found that the wind speed attenuation rate of vegetation barrier had the greatest influence on the efficiency of vegetation blocking particles,followed by the relative humidity and the temperature.The inspiration in bionic fence design is that the collision of particle in the airflow with vegetation barrier can improve the particle blocking efficiency of vegetation barriers.In addition,the microstructure of vegetation leaves was also a key factor affecting their dust retention ability.After particle blocking test,the test of scanning electron microscopy（SEM）was conducted to observe microstructure of four kinds of leaves,and the test of X-ray energy spectrometer（EDX）was conducted to determine the element composition of particles retained on the leaves surface.The surface roughness of clean leaves was also measured by super-high magnification lens zoom3D microscope.The results showed that the high roughness of vegetation leaf surface was beneficial to the dust retention.And the roughness of the near-axis surface of the four kinds of leaves was higher than that of the far-axis surface,which can better retain the naturally settled particles.There were more particles attached to the leaf surface of Padusvirginiana‘Canada Red’leaves.The closer to the main leaf vein of the leaves,the more particles were deposited.The structures conducive to the adhesion of particles were"ridge-like"stripes and large stripes composed of pinstripes.The structure which was not conducive to the attachment of particles was the wax tube around the stomata.Both sides of the needles of Pinussylvestris var.mongolicaLitv were intersected longitudinally by stomata belt and long flaky stripe belt.There were more particles in the flaky stripe space than that in the stomatal area.The structure favorable for particle retention was the stripe structure,and the surface roughness（11.35±3.41μm）of this sampling position was higher than that at other positions.For Malusbaccata（L.）Borkh.,there were particles attached between the stripes,around the stomata,and under the cilia.The surface roughness of the leaf stripe was lower than that of the Padusvirginiana‘Canada Red’leaf,which was not conducive to particle retention.The structures of stomata and cilia in the far-axial surface were favorable for particle retention.The Acer buergerianumMiq.leaves near the vein had more particles.The structures favorable for particle retention were grooves and reticular cilia.The stomatal structure on the far-axial surface of the leaves had fewer particles.The particles on the surface of the tested leaves contained all the elements of the laying hen house particle:C,N,O,F,Na,Mg,Al,Si,P,S,Cl,K,Ca,Fe,Ti and Mn.Finally,inspired by the arrangement of plant branches and leaves,the dust retention microstructure of the leaves and the wind-adjusted porosity of vegetation,five bionic porous barriers were designed and compared with commercial artificial porous barriers.The shielding capacity and particle blocking efficiency of these porous barriers were evaluated in wind tunnels with different wind speed.The results showed that all bionic porous barriers were more effective in reducing wind speed and particle concentration than traditional porous barriers.The bionic porous barriers were more effective in capturing PM10.Among them,the bionic porous barrier of B-4L-n had an retardation rate of 51.87%±4.09%,56.74±1.82%and 52.03±2.30%for PM10when the inlet wind speed was 1 m·s-1,2 m·s-1 and 3 m·s-1,respectively.At the same wind speed level,the bionic porous barrier had the smallest reduction in PM2.5.Among the 5 kinds of bionic porous barriers,the non-woven cloth material with four-leave opening presented the best capability both in reducing wind speed and PM.The arrangement of bionic blades and the adjustable openings increased the turbulence of the air,thus increasing the interaction area between particles and the porous barriers,ultimately improving the shielding capacity of the porous barrier and its retardation rate for PM.The bionic porous barriers proposed in this study can be further developed and studied to create a porous barrier which can adapt to different wind speeds and has a stronger ability to capture PM2.5.