Study On The Expansion Characteristics Of Plasma Jet In Liquid Medium

The liquid-propellant electrothermal chemical gun(LPETCG)is a promising new concept hypervelocity gun.Our research has been carried out on one of the critical physical issues of LPETCG interior ballistic which is the interaction between the plasma j et and the liquid medium.The research goals are to reveal the plasma jet expansion characteristics and mechanism in the bulk-loaded liquid medium and to explore reasonable and feasible methods for the control of the interior ballistic stability.Research results are of great significance to the LPETCG interior ballistic design and control in the future.The main research contents and results are as follows:(1)Experimental study on the expansion of plasma jet in liquidAccording to the requirements of the small-caliber LPETCG,an experimental device and platform of the plasma jet expanding in liquid are designed and built to investigate the plasma expansion characteristics for various working conditions.Results show that there are irregular fluctuations around the plasma jet boundary due to interphase turbulence,and the plasma brightness is non-monotonously distributed in space and time.Besides,the plasma jet’s axial length is increased exponentially,volume and surface area are both increased in volatility,and the moving speed of the front face is gradually decreased at the same time.Further,the steppedwall simulated combustion chamber can effectively control the Taylor-Helmholtz instability during the plasma jet expansion process.Moreover,decreasing the structure factor of the stepped-wall chamber(the ratio of the step radius increment to the step height)or increasing the nozzle diameter or the discharge voltage enhances the expansion ability of the plasma jet in liquid.Finally,based on the experimental results,a calculation formula for the surface area of the plasma jet is proposed,which provides a theoretical reference and basis for further studies about the liquid propellant combustion ignited by the plasma jet.(2)Numerical study on the expansion of plasma jet in liquidFirst,the plasma jet is assumed as the high-temperature gas jet,and then the expansion of the plasma jet in liquid is theoretically analyzed starting from the Navier-Stokes equation and the continuity equation.Some physical phenomena during the plasma jet expansion are theoretically explained such as the reduction of interface speed and the phenomenon of plasma jet retraction.Meanwhile,deriving from the energy conservation equation,we theoretically prove that the axial expansion ability of the plasma jet in the stepped-wall simulated combustion chamber is weaker than that in the cylindrical chamber.Second,a zero-dimensional transient model of high-temperature plasma jet generated by capillary ablation and a two-dimensional unsteady multiphase flow model of the plasma-liquid interaction are both established in this paper.The calculation results are in good agreement with the experiment results,which indicates that our numerical models are reasonable and feasible.After that,the expansion processes of the plasma jet in the cylindrical and stepped-wall simulated combustion chamber are numerical analyzed,respectively,and the results are as follows:In the cylindrical chamber:the arc-shaped pressure wave and a local high-pressure region are formed near the head region of the plasma j et,in which the arc-shaped pressure wave finally evolves into the plane-shaped wave,and the local high-pressure region gradually moves downstream with the pressure decrease;the flow field structure near the nozzle exit is complex and volatile with the liquid backflow entrainment and the jet necking phenomena;the plasma jet boundary fluctuates irregularly,and the physical parameters such as the temperature and the velocity have greater gradients here.In addition,increasing the injection pressure not only enhances the plasma jet expansion in liquid but also aggravates the Taylor-Helmholtz instability during the plasma-liquid interaction.Moreover,the liquid density is the dominant effect factor on the expansion intensity of the plasma jet,while the influence of the specific heat capacity is small on the thermal diffusion in the flow field.The Taylor-Helmholtz instability of the plasma jet expanding in water,OTTO-Ⅱ,and LP1845 simulated liquid medium are decreased successively.In the stepped-wall chamber:according to the formation mechanism and location,the vortices in the flow field can be divided into four types that are main vortices,corner vortices,secondary vortices,and small vortices.The corner vortices enhance the radial expansion and turbulence of the plasma jet,which leads to a decrease in the axial expansion and random fluctuations.At the same time,the radial fluctuation of the plasma jet is also limited by the chamber wall,and thus the total expansion instability of the plasma jet can be controlled by the stepped-wall chamber.The truncated cone-shaped chamber with the ability to enhance the radial jet expansion can weaken the Taylor-Helmholtz instability to a certain extent,but its control effect is not as good as the stepped-wall chamber.In addition,increasing the radius increment of the stepped-wall chamber and decreasing the injection pressure both improve the expansion stability of the plasma jet in liquid.In summary,there is intensive interphase turbulence and random fluctuations when the plasma jet expands in liquid,and the stepped-wall chamber can effectively control the instability of the plasma jet expansion.The reasonable design of the chamber structure and appropriate selection of the injection pressure and liquid type can maximize the control effect of the stepped-wall chamber on the Taylor-Helmholtz instability during the plasma-liquid interaction.(3)Numerical study on the combustion propulsion process of the bulk-loaded energetic liquidA two-dimensional two-phase reaction flow numerical model with a movable boundary is established for the combustion propulsion process of the bulk-loaded energetic liquid.The interior ballistic of a 60 mm caliber LPETCG is calculated,and the calculation results agree well with the experiment results.After that,the combustion propulsion process of energetic liquid in a 30 mm LPETCG with a stepped-wall combustion chamber is predicted by using such a numerical model.The mixture of plasma and combustion gas forms a typical Taylor cavity,and the calculation results show that the expansion characteristics of the Taylor cavity in the stepped-wall combustion chamber are similar to that of the plasma jet in the stepped-wall chamber.The appearance of the corner vortexes in the flow field and the radial induced effect of the chamber structure are both confirmed by this numerical analysis.According to the stability control mechanism of the stepped-wall simulated combustion chamber revealed before,it can be concluded that the stepped-wall combustion chamber can reduce the expansion instability of the Taylor cavity,and thus realize the stability control of the bulk-loaded liquid combustion.