Numerical Study On The Structures Of Shock Wave System In The New Ram Accelerator Tube

The ram accelerator is a new concept propulsion device that uses the chemical energy of mixed gas propellant to accelerate the projectile to hypersonic speed.The flame front will move to the projectile precursor and pass over the shoulder of the projectile when the reaction rate and heat release of the propellant are too high,which will cause resistance to the projectile and cause unstart problem.When there is a shock wave reflection point on the surface of the projectile’s shoulder,the high-pressure area behind the shock wave has a certain blocking effect on the propagation of the flame,which can expand the reaction rate range of the propellant.According to this idea,the effects of the projectile velocity and the size of the accelerating tube on the shock wave system structure and thrust performance in the traditional ram accelerator were numerically investigated based on the SST k-ωturbulence model,the Finite-Rate/Eddy-Dissipation model,and the methane-oxygen single-step chemical reaction mechanism with assumption of viscous ideal gas.The research results show that when the height of the accelerator tube h=14 mm and the projectile Mach number Ma<5,the ram accelerator cannot operate;when h=14 mm,Ma=5～6,h=16 mm,Ma=4～6 and h=18 mm,Ma=4～4.5,the shoulder surface of the projectile can generate a shock wave that intersects the tube wall;in this paper,under the condition of the propellant reaction rate is fixed(the activation energy Ea=1.9×10⁸ J·kg^-1·K^-1),when h=16～22 mm and Ma<5,the calculation examples will occur unstart.When Ma≥5,h=14～16 mm,it can start successfully and generate thrust,as the speed of the projectile increases,the thrust performance will also improve.In order to solve the unstart problem of traditional ram accelerator at low Mach and high reaction rate(Ma<5,Ea=1.9×10⁸ J·kg^-1·K^-1),a new projectile for ram accelerator was designed.The front body of the new projectile is the same as the ordinary projectile.The rear body is of equal straight configuration.The height of the rear body is smaller than the maximum height of the front body,forming a shoulder of the projectile with a baffle effect.The same numerical method is used to verify that the new projectile solves the unstart problem of the ram accelerator.The impact of the size of the projectile’s rear body on the the shock wave system structure in the acceleration tube and thrust performance was also investigated.The results show that under the same working conditions,when the new projectile moves in the acceleration tube,two temperature extremes will be formed at the shoulder and the bottom of the projectile.The temperature extremes are higher than that of the traditional ram accelerator,and the back wedge of the new projectile reflects excitation.The wave intensity is higher,and the pressure and temperature of the gas behind the shock wave are also higher.The new ram accelerator is easier to start.Under the condition of Ma=4,when the height of the projectile rear body l=2.5 mm and3.5 mm,the flame front in the new punch accelerator can be stabilized behind the projectile shoulder,the new projectile has the ability to solve the unstart problem,the size of the projectile’s rear body does not have a significant impact on the thrust performance,and when the height of the projectile’s rear body l≥4.5 mm,the unstart problem will occur.