The Capture Efficiency Of Aerosol Particles By The Branch Scale Vegetation In A Wind Tunnel

Because particulate matter affects the global climate system,it has become a major research topic in meteorological science and air pollution research,so people’s interest in particulate matter is exponentially increased.In addition,adverse effects on health,such as respiratory disease and cardiovascular disease aggravation are also associated with particulate matter.An important feature of particulate matter is that they can be removed by vegetation,and trees were planted as a windbreak around the world.In the environment of northern Chinese cities,leaves of evergreen plants can effectively adsorb dust.The spatial distribution of branch scale leaves is much simpler than in ecosystems,and this scale still provides a sufficient number of leaves to collect aerosol particles,so understanding the intermediate-scale deposition of particles between the leaf and the ecosystem is necessary,while wind tunnel experiments are a convenient way to study the deposition of aerosol particles on branch scale vegetation.While this wind tunnel setup does not address all the complexities encountered in aerosol particles deposition onto ecosystems,it does zoom onto a key process common to all particles deposition-Brownian diffusion,and provide reference for the current management of respirable particulate matter.In order to obtain the flow resistance of vegetation and the deposition velocity of aerosol particles,three branch scale vegetation were studied in this paper,The relationship between the leaf area index and the packing density was studied by experiment.The relation expression of flow resistance and the deposition velocity of aerosol particles is proposed through experiment.The main research work and conclusions are as follows:(1)In order to obtain the branch scale vegetation capture efficiency and flow resistance,by building a wind tunnel experiment,the capture efficiency of aerosol particles by the branch scale vegetation branches were measured under different conditions.The relationship between the capture efficiency of aerosol particles and wind speed,packing density,the diameter of aerosol particles was obtained.The correlations were validated with the experimental data of other groups under the same operating conditions,and then the integrated correlation of the three kinds of vegetation to the aerosol particulate capture efficiency was proposed.The experiment results show a tendency for the capture efficiency of aerosol particles by the branch scale vegetation in a wind tunnel to first decrease with increasing wind speed,and then increase as wind speed is further increased.The results showed that the maximum penetrating particle size of aerosol particles was 1μm.When the particle size is less than 1μm,the main trapping mechanism is diffusion.When the particle size is larger than 1μm,the main trapping mechanism is intercept and impaction.At the same time,the relationship between the resistance coefficient and the leaf area index of the branch scale vegetation is given.The correlations were validated with the experimental data of other groups under the same operating conditions,and then the integrated correlation of the resistance coefficient was proposed.(2)In order to obtain the deposition rate of aerosol particles on branch scale vegetation,the capture efficiency values of the whole leaf under a given leaf area index were transformed into the deposition velocity,and then the experimental values of deposition velocity were calculated.And the relationship between the deposition velocity of aerosol particles and particle size,wind speed and leaf area index on three branch scale vegetation was proposed.In addition,the theoretical values of the deposition velocity of the particles on the leaf surface were calculated based on the theory of the trapping of the homogeneous fiber trapping layer.It is found that the experimental results are in agreement with the theoretical results.The experimental value and the calculated value of the deposition velocity Vd both first decrease with increasing aerosol particle size,and then increase as aerosol particle size is further increased.(3)In order to calculate the capture efficiency of aerosol particles by the typical shrub model,a typical bush model was set up,and the wind tunnel experiments were carried out under different working conditions to calculate the capture efficiency of aerosol particles.The correlations between capture efficiency and its influencing parameters are proposed,and nonsample data are used to verify them.At the same time,the DPM model in CFD is used to simulate the model.The simulation results show that there are some errors between the simulated value and the experimental value.The reason of the error is that the size difference between the geometric grid size and particle size is too large,How to reduce the error between the simulation value and the experimental value needs to be further studied.