Research On Diagnostics Of Gas Temperature Field Of Atmospheric Pressure DC Microplasmas

Atmospheric pressure glow discharge is an effective technology to generate plasmas at atmospheric pressure,and have arouse wide attention in field of plasma science and technology.Due to its excellent non-equilibrium property(with low gas temperature and highly physical and chemical activity),it has great application potentials in various field such as pollution treatment,biomedicine and advanced material science.However,people lack basic understanding of plasma production and treatment mechanism by now,and this makes many applications stay at the stage in laboratory.In this dissertation,comprehensive investigations are carried out around a basic parameter of atmospheric pressure glow discharge,i.e.,the gas temperature,and the details are presented as follows:1.Investigations of tomography algorithms under axisymmetric condition.Most of common used optical method are line-of-sight techniques measuring the line integral of light intensity(optical emission spectroscopy)or refractive index gradient(schlieren).Therefore,deconvolution process is needed to reconstruct the local distribution.(1)To deal with above two types of measurements,several techniques to realize deconvolution is covered including its principle and basic features,respectively.For optical emission spectroscopy,matrix method,projection-interpolation and fourier method are introduced.For schlieren measurement,simpson's 1/3rd rule,two-point formula,adaptive fourier–hankel method are introduced.(2)For the comparison of tomography algorithms applied in schlieren measurement,numerical experiment is carried out to choice the technique of best performance and determine its corresponding parameters.Detailed procedures are as followed:first according to Abel transform and assumed temperature distribution,the refraction angle distribution is obtained.Then,based on different techniques,computed temperature distribution is reconstructed respectively and compared with assumed distribution to evaluate the performance.With numerical experiment carried out under the noise-free and noise-included condition,adaptive fourier–hankel method is proved to have the best performance.2.Gas temperature is an important basic parameter for both fundamental research and applications of plasmas.In this part,efforts were made to visualize the full spatial field of gas temperature(T_g)in a microdischarge with sharp T_g gradients by a method of calibrated schlieren(CS)photography.Compared to other two typical diagnostic approaches,optical emission spectroscopy(OES)and Rayleigh scattering,the proposed CS method exhibits the ability to capture the whole field of gas temperature using a single schlieren image,even the discharge is of non-luminous zones like Faraday dark space(FDS).The image shows that the T_g field in the studied micro-glow air discharge expands quickly with the increase of discharge currents,especially in the cathode region.The two-dimensional maps of gas temperature display a‘W-shape'with sharp gradients in both areas of negative and positive glows,slightly arched distributions in the positive column,and cooling zones in the FDS.The obtained T_g fields show similar patterns to that of the discharge luminance.With an increase in discharge currents,more electric energy is dissipated,heating air gas and inducing constriction of the low-temperature FDS.Except in the vicinities of electrode boundaries,due to the interference from optical diffraction,the estimated gas temperature distributions are of acceptable accuracy,confirmed by the approaches of OES and UV Rayleigh scattering.3.The humidity effect on thermal field of an atmospheric pressure DC glow discharge generated in Ar laminar flow with water vapor is investigated.The gas temperature T_g distribution is determined axially and radially by calibrated schlieren(CS)photography and space-resolved optical emission spectroscopy(SR-OES).The three-dimensional T_g mapping obtained by two methods were in good agreement and presented like a cone with a‘W-shape'in its axial distribution.However,it is shown that SR-OES methods only characterize the luminous pattern corresponding to the core region of discharge.And the luminous pattern depend on the dynamics of radiative exited species and vary with different discharge conditions,especially the vapor content.Instead,with CS method,more complete T_g field spatial distribution could be determined robustly even for the dimmer discharge structure like Faraday dark space.Humidity effect on the distribution of gas temperature and heat dissipated of whole thermal field is determined quantitatively.It is revealed that,heat is significantly underestimated by SR-OES method compared to CS.