Study On Microscopic Physical Processes Of AC-DC Corona Discharge

In solving the ion-flow field problem,severe numerical wiggles ordinarily derived from the downstream boundary where the variables change rapidly,which is usually due to inadequate tessellations of finite elements on the boundary layer.Therefore,whether precluded wiggles are the use of upwind scheme or mesh refinement should not be determined by the appearance of the convection-dominated equation,but rather should be based on the existence of boundary layer formation mechanism.On the other hand,the existing mathematical methods to solve the problem of ion-flow field are mainly through finite element method and finite volume method;however,there are still many problems in the calculation accuracy,computational efficiency and stability for these two methods,which also only reach the first-order in the time-domain.Therefore,how to construct a new mathematical method to solve these problems has become a challenge to ion-flow field research.The fluid model of the gas discharge requires the input of transport coefficients and rate coefficients dependent on the electron energy distribution function.These coefficients are usually obtained from the collision cross section data by solving the electronic Boltzmann integro-differential equation.The Boltzmann equation is extremely complex in form,and it describes the simplest uniform electric field,uniform growth,or exponentially growing electron density.Therefore,how to simplify the Boltzmann equation without affecting its physical description accuracy and calculation accuracy,while making it possible to describe the more general condition of the fluid discharge model has become a focus for the study of gas discharges.Corona discharges as a self-sustaining discharge,which involves a series microscopic physical process,such as impact ionization,electron attachment,and secondary electron emission.However,the corona plasma region as the occurrence of these micro-physical processes,since the duration of microscopic physical process of corona discharge in the corona plasma layer is extremely short(nanosecond),as well as unpredictable propagation paths and low radiation from the streamer,it is difficult to obtain microscopic discharge parameters by experimental testing,thus these processes are often overlooked.How to build a mathematical physics model to accurately describe these physical processes has been widely concerned for the corona discharge research.In order to deal with the above-mentioned series of problems,this paper has done the following work in the establishment of mathematical discontinuous method and corona discharge microscopic physical model:(1)This paper introduces the absorption boundary condition and relaxes the continuity requirement at interelement boundaries,thus formulating the boundary conditions according to the actual physical process,and finally the numerical wiggle is well solved.In this paper,we use the nodal discontinuous finite element method,which uses the basis function of the same order as the finite element method,but its coefficient matrix is more sparse than the finite element method,which leads to a faster solution process.The discretization of the interelement boundary is achieved by adding the mathematical operator to ensure that the computational grid is continuous in the computational domain.Without increasing the number of grids,the computational accuracy is improved by using the ?-optimized node set and the multi-order polynomial base.For the time-domain discretization,the low-order version of the fourth-order five-step Runge-Kutta method is introduced,which greatly improves the computational efficiency under conditions that require only one additional memory register;at the same time,the calculation of an additional stage is offset by the use of a larger stable time step.(2)This paper expands the electron distribution function by the expansion of the spherical harmonic function,and solve the Boltzmann equation by using the two-term approximation without affecting the accuracy and improving the computational efficiency.By introducing the generalization of the coefficients of the function of the reduced electric field E/N or the average electron energy,while paying attention to the definition of the coefficients,we ensure that whenever the fluid model is used for the simple conditions assumed by the Boltzmann equation solving method,it will produce exactly the same average velocity and average energy as the complete solution,so we obtain the maximum consistency between the fluid model and Boltzmann equation.On the basis of the above method,the expressions of the electron transport coefficient and the rate coefficient are deduced.Compared with the experimental data,it is proved that the proposed method is effective in a wide range of electric fields.(3)For the AC-DC corona discharge microscopic physical mathematics model,in this paper,the distribution of charged particles is divided into two parts: corona plasma region and unipolar drift region.Secondly,two-dimensional numerical modeling of the secondary process(photoionization and photoemission)for maintaining the positive and negative corona self-sustained discharge is carried out.In addition,the effect of the ion generated in the previous half cycle during the study of AC corona discharge is also considered in numerical modeling.The final AC-DC corona discharge physical model utilizes the same governing equation based on the hydrodynamic convection-diffusion model,and the boundary conditions and the solution process are set separately.At the same time,the new calculation formula of corona current of AC corona discharge is given based on the theory of current induced by charge motion.The method proposed in this paper shows good performance by comparing with the experimental results reported in the literature.Furthermore,on the basis of numerical simulation results,the mathematical expression for the thickness of the ionization layer is fitted.Finally,the ion-flow distribution of the AC-DC hybrid field is studied by the physical model of DC ion-flow field proposed in this paper,and the general rules of various configurations are summarized,which provides theoretical guidance for engineering design and optimization.