Experimental Study On Propagation And Jet Characteristics Of Cold Plasmas At Atmospheric Pressure

Due to the great potentials in environmental,material processing,biomedical and energy fields,atmospheric pressure cold plasma jet(APCPJ)has become a hot research issue in plasma science and technology.As a plasma generated method coupled with gas flow and gas discharge,much more attention has been paid to the correlation between the optical and electrical parameters and atmospheric pressure plasma characteristics,whereas the impact of the gas flow character is less investigated.However,from a view of gas flow and heat transfer,the propagation of atmospheric pressure cold plasma jet could be regarded as the pipeline flow of plasma state substance(i.e.ionized gas)as well.During the propagation of ionized gas,therefore,the switch of flow regime,energy exchange between the inner fluid and the external environment are worthy of an in-depth research.In this dissertation,based on a new perspective of flow and heat transfer,comprehensive investigations are carried out as follows:Through using a self-designed plasma generator,the effect of the gas flow in a tube on the propagating behavior of cold plasma jet was studied.Contrasting the characteristics between upstream and downstream plasmas by the change of the argon flow rate,the effect of the transport of active particles induced by gas flow on the plasma behavior was demonstrated.In addition,the effects of gas flow and concentration fields on the length of plasma jet were explored by varying the concentration of ambient gas.The result revealed that the flow field of the working gas controls the propagating direction,while the concentration field of the working gas dominates the propagating distance of the plasma jet.The effects of the tube material and wall temperature on the propagation characteristics of cold plasma jet were studied through using a single-electrode configuration.Comparing the electrical and physical behaviors of the plasma between the ceramic and glass tubes,it was found that the volume of the plasma jet was significantly altered while the discharge current and jet length are almost constant with the variation of gas flow rate.The differences of the thermal conductivity,and the amount of the ’potential trough’ between two materials are mostly responsible for this.Moreover,once the wall temperature was heated,the jet length has a significant enhancement in laminar,transition and part of the turbulent regions.Also,the transition region remarkably expanded over a large range of argon flow rates.The increase in the gas flow velocity and the enhancement of the thermal diffusion are the main causes of the expansion of the transition region to the low-velocity region.The expansion to the high-velocity region is dominantly induced by a change in the viscosity.By changing the ambient dielectric surrounding the capillary tube,The effect of the ambient dielectric on the propagation characteristics of cold plasma jet was studied.When the ambient dielectric was replaced by distilled water,ethanol,and carbon tetrachloride,the discharge current and bullet velocity of the plasma jet are obviously increased,and the molecular polarity of ambient dielectrics was responsible for this.Meanwhile,the involved mechanism of the molecules with different polarity on the plasma characteristics is distinct.For nonpolar dielectric of carbon tetrachloride,this was mainly resulted from the electron polarization in the built-in electric field.For polar dielectrics of ethanol and distilled water,in addition to the electron polarization,orientation polarization was the main cause for the further increase in discharge current and bullet velocity.Besides that,with the presence of distilled water and a ground electrode,a propagating discontinuity is observed in a cold argon plasma jet at atmospheric pressure.Results showed that the strong polarization of the distilled water,the constraint from the electric field(or a sufficiently high electric field),and the transport of the active species are in combination responsible for the propagating plasma discontinuity.A novel plasma assisted combustion device with needle-needle floating electrode configuration was proposed.The effect of the jet plasma on CH4/Air premixed flame and CH4 diffusion flame were investigated respectively,and its involved mechanism was revealed.For the laminar premixed combustion,the injection of plasma can effectively increase the flame propagation speed,and the combustion could obtain more benefits at a higher argon flow rate under the same applied voltage.But for the hysteresis in a turbulent diffusion flame,plasma injection tended to keep the flame in an attached state,and there was always an optimal value of applied voltage and injected argon flow rate for the stabilization of the flame.However,no matter for the premixed or the diffusion flame,a much greater impact of the plasma jet was present when the flame stayed in the non-steady or critical state.Thus the plasma always produces a more pronounced effect in the flame stabilization instead of the combustion enhancement.