Research On The Image Classification Methods And Influences Of The Uniformity Of Dielectric Barrier Discharge

Dielectric barrier discharge (DBD) is a typical non-equilium plasma source at atmospheric pressure, which has been extensively used for various industrial applications, such as ozone production, high power lasers and the plasma flow control. The uniformity of discharge has been a research focus due to the needs of industrial applications. Now the so-called uniform discharge only refers to the absolute uniform discharge without any microdischarges in a short timescale (ns). But in fact the uniform discharge used in industrial applications does not need to be limited to the absolute uniform discharge. On the contrary, the relative uniform discharge consisted by a large number of microdischarges in a long timescale may meet the needs of more industrial applications. Therefore, the problems on how to evaluate this discharge uniformity quantitatively, how to produce the discharge with different uniformity, how to improve discharge uniformity, and the physical mechanisms of uniform and absolutely uniform discharge are raised. This paper studied the uniformity of DBD by using digital image processing technology, electrical measurements and approximate analytical calculation method, the research results are as follows:A new method-gray level histogram (GLH) based on the digital image processing technology was proposed to classify the uniformity of DBD quantitatively. The disadvantages of complex, cumbersome and expensive can well be overcome by using the GLH method instead of the conventional method. So it can be used to classify the filamentary and uniform discharge and evaluate the discharge uniformity quantitatively in a simple and effective way. In addition, the effectiveness of the GLH method in classifying the different discharge modes was validated by the experiments under low pressure which can produce absolutely uniform discharge and the distinguishing method of current waveform which commonly used by researchers. Moreover, the gray level probability model of filamentary and uniform discharge were established by obtaining the parameters of double Gaussian and single Gaussian probability model, which were obtained by solving nonlinear least squares problems using trust region algorithm. Furthermore, the model was applied to study the discharge characteristics of DBD.Fourier energy spectrum (FES), autocorrelation function (ACF) and gray level co-occurrence matrix (GLCM) method based on the digital image processing technology were proposed to identified quantitatively the spatial structure of the discharge image in DBD. The spatial structure of filamentary, periodic and homogeneous discharge were identified effectively by using thev ACF method for the obvious advantages of suppressing the noise, high object recognition ability and requiring not any preprocessing. The effectiveness of the ACF method was validated by identifying the spatial structure of patterns in DBD.The influences of mesh electrodes on the uniformity of DBD were studied.(1) The uniform discharge was produced by using the mesh electrodes with a characteristic of spatial period, and the discharge uniformity produced by some mesh electrodes with certain characteristic lengths was better than plate electrode. If the aperture of the mesh electrode was long enough (L≥1.5mm), those periodic discharge spots will be produced on every grid node of the mesh electrode. If the aperture was slightly smaller (1.25mm6≥L≥0.6mm), those periodic discharge spots will not be produced on every grid node, but on alternate grid nodes. If L≤0.5mm, the discharge spots will be distributed randomly and even more densely than when produced by a planar electrode.The reason that these periodic spots were not produced on every grid node should be related to the size of an avalanche. The experimental results mentioned above were validated by analysis of the FES and ACF.(2) A coefficient of variation (CV) of discharge image was proposed to evaluate the uniformity of the discharge qualitatively. The smaller the CV, the more uniform the discharge will be. The research results show that the CV decreases gradually as the aperture of the electrode decreases to a certain extent, thus the uniformity increases.(3) A new dimensionless scale invariant (h) was introduced to analyze the physical mechanisms leading to the relatively uniform DBD, which includes the combined effect of the aperture and the normalized change rate of the field strength of the avalanche head. The research results show that the variations of CV and h for the different electrodes have almost the same trends. This means that the physical mechanisms leading to the uniform discharge in mesh electrode can be well explained by using the h.The influences of gaseous species on the uniformity DBD were studied.(1) It was found that the discharge uniformity in different types of gases can be classified by using the GLH method. The research results show that the GLH and the dependence of the CV on applied voltage for non-noble gases (Air, N2) were much different from the noble gas (He, Ar).(2) The addition of Ar to N2can help to improve the discharge uniformity. The calculated results of the CV for different gases show that the CV decreases in the order Air, N2, He, N2/Ar gas mixture and Ar, thus the discharge uniformity increases successively.(3) The calculation results of the equivalent electrical model of microdischarge expand in DBD show that the gas with smaller breakdown voltage can make the microdischarge easier to expand the entire electrode surface and form uniform discharge. Moreover, the energy of atomic metastables in argon was significantly higher than the energy of molecules metastable in nitrogen, so more seed electrons can be produced through penning ionization when the addition of Ar to N2, which was beneficial to forming uniform discharge in N2/Ar gas mixture.The physical mechanisms leading to the uniform DBD were studied.(1) A multiple electron avalanches radial expansion dynamic model was established by considering the effects of electron diffusion and electrostatic repulsion in the avalanche head. The minimum required preionization level for the formation of multiple electron avalanches coupling was found to be dependent on electric field strength, gas pressure, electron temperature, the heterogeneity of preionization level and threshold value. Moreover, the effectiveness of the model was validated by a relevant application example.(2) The approximate analytical calculation on the development of electron avalanches in pure gas dielectric and DBD were carried out. The results show that the expansion mechanisms in the avalanche head will transit from free electron diffusion to electrostatic repulsion when the electron avalanche develops to a certain stage. Moreover, reducing the initial spacing between electron avalanches by improving the seed electron is beneficial to coupling the adjacent electron avalanches, thus improving the discharge uniformity. The above results validate the self-consistency of the electron avalanche radial expansion dynamic model, which can help us better understand the physical mechanisms leading to the uniform DBD.