Study On The Expansion Characteristics Of Plasma Jet In Air

The expansion characteristics of pulsed plasma jet in air are researched with both experiment and numerical simulation on the engineering background of electrothermal chemical launch.The main research contents and results are as follows:(1)Experimental study on propagation characteristics of pulsed plasma jet in airAn experimental simulation platform for the expansion of plasma jet in air is designed,in which a piezoelectric pressure sensor is used to measure the pressure change characteristics of the polyethylene capillary and a digital high-speed video system is used to record the evolution of the plasma jet in the atmosphere.And the effects of the discharge voltage and the injection breaking pressure on the propagation characteristics of plasma jet are investigated.The results indicate that:when the pulsed plasma jet expands in air,its shape is gradually extended from the initial shape like cone to a conical head and an elongated cylindrical tail.Extended volume reveals a single peak distribution with time.The main body of the jet is gradually broken and fractured,and the brightness changes from strong to weak.When discharge voltage or injection breaking pressure is increased,the crush of the jet body intensified and the extended displacement increases,and the maximum expansion volume increases significantly.(2)Numerical simulation of the expansion parameters of plasma jet in airBased on the experimental results,a two-dimensional axisymmetric unsteady model for plasma jet expansion in air is established,and the numerical simulation of plasma jet propagation in the atmosphere under transient conditions is carried out by Fluent.The evolution characteristics of plasma-air interface and the spatiotemporal distributions of density,pressure,velocity and temperature in the flow field are obtained.The results show that the Mach disk appears in the flow field near the nozzle when the plasma jet expands in the atmosphere.The temperature in the downstream zone of Mach disk is high and the velocity is low.There is bifurcated part in the head of plasma jet,which gradually merges to the center with the expansion of the jet.The density,pressure,velocity and temperature change greatly in the flow field near the nozzle while the change in the far field is small,and the parameters have pulsation along the axial direction.The simulation values of axial expansion displacement of plasma jet are in good agreement with the experimental results.(3)Influence of variation of characteristic parameters on the expansion characteristics of plasma jetIn order to analyze the effect of the change of the characteristic parameters on the expansion characteristics of the plasma jet,the Fluent is used to numerically simulate the expansion process of the plasma jet in the range of 1.5?3.5MPa injection pressure and 3mm?5mm nozzle diameter.The evolution characteristics of the two-phase interface and the variation law of the parameters in the flow during the plasma jet expanding process under different working conditions are obtained.The results show that the axial and radial displacement of the plasma jet,the size of the bifurcated part and the expansion volume of the plasma jet increase with the increase of injection pressure and nozzle diameter,but the amplitude decreases.As a whole,the density,pressure,velocity and temperature of the jet are also increased.The size of Maher disk decreases with the decrease of injection pressure and nozzle diameter.And when the injection pressure or nozzle diameter decreases to a certain critical value,it will not appear Maher disk.(4)Study on expansion and coalescing characteristics of twin plasma jets in airA three-dimensional unsteady mathematical model is established,and the expansion processes of twin plasma jets in air are numerically simulated by Fluent.The evolution law of coalescence and separation process of twin plasma jets and the spatiotemporal distributions of pressure,velocity and temperature are obtained.At the same time,it is discussed that the expansion characteristics of twin plasma jets at two injection pressures of 2.5MPa and 3.0MPa under two dual-orifice intervals(18mm and 22mm,respectively).The results show that the two jets are bent inward when the plasma jet is expanded in the atmosphere,and the jet head gradually converges and then gradually separates.The pressure,velocity and density distributions of the twin jets are similar to those of the single jet,but in the later stage of the expansion,the alternating phenomenon of high and low pressure waves disappears.The higher is the injection pressure,the earlier is the coalescence time of twin plasma jets,and longer is the duration of the coalescence,and the turbulence effect of twin plasma jets is very intense.As the injection pressure increases,the pressure,velocity and temperature of the jet increase and the distribution along the radial direction becomes more concentrated toward the center.The effect of dual-orifice interval on the expansion characteristics of twin plasma jets is approximately contrary to that of the injection pressure.