| In recent years,ecological environment protection and food safety have been widely concerned at home and abroad.The extensive use of abamectin and other pesticides has brought certain threats to the ecological environment and human health.As an efficient means to degrade pesticide residues,atmospheric dielectric barrier discharge non-thermal plasma is characterized by airflow,which is a key parameter affecting its discharge characteristics and practical effect.However,systematic studies on dielectric barrier discharge characteristics and active product yield from the perspective of gas flow characteristics are relatively scarce.Moreover,the effect of the production of active products on the degradation path and degradation products of avermectin is still insufficient.In this thesis,a dielectric barrier discharge low-temperature plasma generator which can flexibly change the gas flow rate and gas flow path is designed,and the experimental platform of dielectric barrier discharge parameter diagnosis is built.The electrical parameters,discharge images,emission spectrum,electron temperature and reactive species content were diagnosed under different argon flow velocity(0.5L/min-5.0L/min)and different argon flow paths,and the influence mechanism of argon flow characteristics in turbulent numerical simulation was analyzed.The degradation rate of avermectin was tested under different argon flow velocity and flow path to obtain the optimal argon flow velocity and flow path of avermectin degradation in this experimental facility.The molecular dynamics simulation parameters were set reasonably according to the diagnostic results of the output of reactive species,so as to study the influence of the type and quantity of active products on the degradation path and degradation products of abamectin.This study provides theoretical support for the airflow regulation characteristics of dielectric barrier discharge and the degradation mechanism of avermectin.The main research content of this thesis is as follows:(1)The experimental platform of dielectric barrier discharge parameter diagnosis and degradation of avermectin,turbulence numerical calculation model and molecular dynamics calculation model are proposed.The experimental platform is composed of discharge device,air flow control system and diagnosis system.The reaction device is a dielectric barrier discharge generating device with 8 air inlet and outlet.The air flow control system is composed of flow control meter.The k-ε standard turbulence model was used for numerical simulation of turbulence.In molecular dynamics simulation,avermectin Bla was selected for molecular modeling and calculated under ReaxFF reaction force field.(2)Turbulence numerical simulation of gas flow characteristics in dielectric barrier discharge device is carried out under different argon flow velocity and flow conditions,and diagnostic experiments are carried out on electrical parameters,discharge images,emission spectrum,electron temperature and reactive species of dielectric barrier discharge lowtemperature plasma.Combined with turbulence numerical simulation results,the variation and influence mechanism of dielectric barrier discharge parameters under different argon flow velocity and different argon flow path were explained.Firstly,the turbulence numerical simulation results show that the argon flow mode in the discharge space is simple laminar flow.At low velocity(0.5L/min-2.0L/min)and high velocity(4.0L/min-5.0L/min),the argon velocity and argon atom distribution in the discharge space are more uniform.At the intermediate velocity(2.0L/min-4.0L/min),the argon velocity and argon atom distribution are not uniform.When the total flow rate of argon was kept at 1.5L/min,the changes of argon velocity and argon atom distribution in discharge space were not significant with the addition of transverse airflow,opposite airflow and laminar flow mode.When the transverse airflow and opposite airflow were added,the argon annular flow or vortex appeared in discharge space.The experimental results of changing the argon flow velocity under unidirectional flow condition show that the current is uniform at low and high flow velocity,and the weak discharge zone appears at the middle flow velocity,but the discharge mode is filament discharge.The difference of discharge power with argon velocity is not obvious,and it stays in the range of 9-11W.The spectral lines of Ar,N,O and OH were observed in the emission spectrum,and the overall spectral intensity was significantly stronger at low and high velocity than at medium velocity.The electron temperature is relatively high at the medium flow velocity,and remains in the range of 1-2eV at low and high flow velocity.The diagnostic results of pH value,solution conductivity and hydroxyl concentration showed that the content of reactive species in solution increased first,then decreased and then increased with the increase of argon flow velocity,and reached the maximum value at 1.5L/min.On the other hand,when the argon flow rate is kept at 1.5L/min and the argon flow path is changed,the difference of discharge form,electrical parameters and electron temperature is not obvious compared with that in unidirectional flow.The spectral line intensity of Ar was significantly enhanced by changing the laminar flow mode,but the content of active components in solution was significantly increased by increasing the opposite flow and transverse flow mode,and the change was not obvious when changing the laminar flow mode.The Ar spectral line was enhanced most significantly when the laminar flow path-Ⅳwas changed,and the active component content was increased most significantly when the opposite flow path-Ⅲ was increased.(3)The degradation rate of avermectin treated by dielectric barrier discharge lowtemperature plasma was tested under different argon flow velocity and flow path,and the mechanism of degradation rate change was explained based on the conclusions of Chapter 3.At the same time,combined with the detection results of reactive species content in Chapter 3,a molecular dynamics calculation model was established to explore the effects of the types and amounts of O and OH on the molecular degradation pathway and degradation products of avermectin.Firstly,the detection results of avermectin degradation rate showed that the degradation rate of avermectin first increased,then decreased and then increased with the increase of argon flow rate in unidirectional flow.When the argon flow rate was 1.5L/min,the degradation rate reached the maximum 77.8%after 30s treatment.The degradation rate of avermectin was not significantly affected by the change of laminar flow mode.The degradation rate of avermectin was significantly increased by the addition of transverse airflow and contraairflow.The degradation rate of avermectin increased the most under the condition of convection-Ⅲ,and the degradation rate was 90.4%after 30s treatment.Further,according to the molecular dynamics simulation results,O and OH destroy the molecular structure of avermectin mainly through the hydrogen capture reaction and adsorption reaction,and the destructive power of O on the structure of avermectin is stronger than that of OH.When the number of O and avermectin molecules reached 30:1,new damage sites appeared,and when O and OH acted together,new damage sites also appeared.Under the combined action of O and OH,the avermectin molecule will eventually degrade into five different products.This thesis puts forward a research idea to explore the influence mechanism of airflow control on dielectric barrier discharge characteristics and the actual effect of low temperature plasma from the perspective of gas flow characteristics,which makes an effective supplement to the research of airflow control dielectric barrier discharge and provides a new research direction.At the same time,the key role of the type and quantity of reactive species on the degradation path and degradation products of avermectin was explained,which provided theoretical guidance for the more efficient application of low temperature plasma degradation of pesticide residues by dielectric barrier discharge.The next step is to carry out verification experiments on gas flow characteristics and avermectin degradation products. |
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