Research On The Mechanism Of Corona Discharge Promoting Water Vapor Condensation And Droplet Growth

Rainfall enhancement catalysed by charged particles is a new artificial precipitation technology used to develop atmospheric water resources.The core mechanism and goal of catalyzed rainfall is to accelerate the condensation of water vapor and the growth of droplets in the process of natural rainfall.Due to the advantages of economy,high efficiency and so on,the technique of generating charged particles catalyzed rainfall by corona discharge has been initially used in the field catalyzed rainfall experiment and made positive progress in foreign countries.In China,Huazhong University of Science and Technology,as one of the few research institutions in this field,has carried out research on the key technology of precipitation catalyzed by charged particles generated by corona discharge.In the research process,it was found that corona discharge can promote water vapor condensation and droplet growth in the surrounding air when releasing charged particles.Because this phenomenon is highly likely to affect the growth and evolution of droplet in the subsequent catalytic process,it is crucial to successfully achieve artificial catalyzed rainfall.However,at present,the research on water vapor condensation and droplet growth under corona discharge is very scarce,and there is a lack of systematic study on this phenomenon and the explanation of its internal mechanism.Therefore,in view of the key problem of how corona discharge promotes water vapor condensation and droplet growth,the mechanism of the coupling effect of charge and electric field on water vapor condensation and droplet growth under discharge environment is analyzed theoretically and experimentally.Firstly,aiming at the micro mechanism of corona discharge promoting water vapor condensation,Molecular Dynamics Simulation method was adopted to simulate the condensation process of water molecules under the action of ions and electric field.The effects of water vapor concentration,ion species and electric field intensity on water molecules condensation at different stages were studied respectively.The results show that ions can effectively promote the condensation of water molecules,reduce the critical supersaturation,and induce the condensation of water molecules under the condition of undersaturation.The order of the three typical corona discharge ions promoting the condensation of water molecules is SO₄^2-,NO₃^-and NH₄⁺.The strong electric field（>1×10⁶V/m）inhibited the ion induced coagulation in the early stage of nucleation.The effect of ions and electric fields on the coagulation in the later stage of nucleation is related to the electric field intensity.When the electric field intensity is greater than 1×10⁶ V/m,the electric field plays a promoting role in the condensation of charged droplets,and the promoting effect increases with the increase of electric field intensity.Secondly,based on the statistics of the Mean-Square Displacement of molecular motion and the proposed analysis method of the frequency distribution of the distance between the ion and the centroid of droplet,the mechanism of the effect of the electric field on the ion-induced condensation was revealed.In the early stage of nucleation,water molecules move in a directional manner under the action of electric field,which reduces the probability of being trapped by ions and leads to slower ion-induced condensation.In the late stage of nucleation,the electric field makes ions closer to the surface of the droplets,thus weakening the shielding effect of liquid water molecules on the electrostatic field of ions,restoring the interaction between ions and free water molecules,and finally enhancing the condensation and growth of nanoscale charged droplets in the late stage of nucleation.The molecular simulation results were verified by the water condensation experiment on the surface of needle electrode and the discharge induced water vapor nucleation experiment.The experimental results show that corona discharge can accelerate the process of water vapor from gas state to liquid state and form nanoscale liquid droplets,accelerating the water vapor liquefaction process.Thirdly,based on thermodynamic theory and considering the interaction of charge and water molecular dipole moment,the Kelvin-Thomson equation and the formula of charged droplet condensation growth are modified.Based on the condensational growth time curve of charged droplet and the relationship between the size of charged droplet and relative humidity under equilibrium conditions,the mechanism of corona discharge promoting droplet growth under over-saturation and under-saturation conditions was analyzed.The results show that the single-charged ions and double-charged ions can induce the condensation growth of water vapor to 4 nm and 8 nm in the critical saturation environment（S=1）.In the supersaturated environment,whether the droplet is charged or not has almost no effect on the droplet condensation growth.The corona discharge promotes the droplet condensation because the ions released from the discharge induce water vapor condensation to form condensation nuclei,which further induces the droplet condensation.The supersaturated water vapor condition plays a decisive role in the droplet growth.In the undersaturated environment,the size of droplet growth induced by charge is restricted by the relative humidity of the environment.The single effect of charge is not enough to induce droplet growth to tens of microns.The droplet growth induced by corona discharge is attributed to the synergistic effect of charge and electric field to enhance droplet condensation and collision.Then,in view of corona discharge promoting droplet collision and growth process,based on fluid mechanics theory,through the analysis of the critical distance of droplet collision and the equivalent average distance of static droplets in space,the effect of image electric field on droplet coalescence is obtained,and the collision strength of droplet coalescence growth is characterized by introducing the droplet collision kernel function.The results show that the single action of image electric field force between droplets is not enough to trigger the collision and growth of droplets,and the background electric field plays a dominant role in droplet colliding and growth.The calculation results of collision kernel function show that,the background electric field and electric charge have a significant effect on the droplet collision and growth.Taking the charged droplet with a radius of 1μm and the neutral droplet with a radius of 0.1μm as an example,the electric field of 1×10⁶ V/m and the corresponding field-charged charge can enhance the droplet collision nucleus by at least 7 orders of magnitude compared with the two neutral droplet pairs without electric field and electric charge.Finally,aiming at the phenomenon of corona discharge promoting droplet growth,the experimental platforms of cloud chamber with different functions and sizes are designed and built.Based on a series of experiments,the macroscopic law,mechanism and related boundary conditions of droplet growth induced by corona discharge were explored.The results show that in the measuring range of the instrument,the discharge can promote the droplet growth to 32μm in the undersaturated environment with relative humidity greater than 85%.By comparing the relationship between the equilibrium charge Q_b of the charged droplet and the average charge of the droplet by experiment,and by analyzing the droplet motion around the needle electrode and the microscopic imaging of the ion wind field,the mechanism of synergetic enhancement of droplet condensation and coalescence growth by charge and electric field is verified;By measuring the ion concentration in the discharge environment under different humidity and voltage levels,it was found that the concentration of negative ions in the discharge environment should be more than 10⁶/cm³.The research results of this paper are of great significance for understanding the role of corona discharge in catalytic rainfall process and for further developing the technology of corona discharge generating charged particles to catalyze rainfall.