Research On Unsteady Combustion Characteristics Of Igniter For Base Bleed Unit

The BBU(base bleed unit)experiences a strong transient depressurization disturbance when a BBP(base bleed projectile)is flying out of the muzzle,causing the oscillation combustion and even extinguishment of base bleed propellant,whereas the igniter continues to ignite the propellant.However,the inconsistency of ignition delay time leads to a large dispersion of BBP,which seriously affects the long-range strike capability of large-caliber guns.To improve the ignition consistency of base bleed propellant,it is necessary to clearly understand the intrinsic physical mechanism of the ignition process of base bleed propellant by ignitier combustion flame in the rapid depressurization process of BBU and the influencing factors of inconsistent ignition delay time.The experimental and theoretical researches on the unsteady combustion characteristics of base bleed igniter are carried out at multiple angles,focusing the igniting process of base bleed propellant which is instantly extinguished under rapid depressurization.The main research contents and results are as follows:(1)Experimental study on combustion characteristics of base bleed igniter in different working environmentsThe experimental study on the steady combustion characteristics of 7 kinds and three types of igniters with different apertures and pyrotechnics in the atmosphere is carried out using high-speed camera.Then an experimental platform of fast depressurization is built,and the unsteady combustion characteristics of different pyrotechnic igniters are observed.Thus multi-angle observation results are obtained.The results show that under normal pressure,the MT and Ba(NO₃)₂ igniters burn as a high-temperature gas flow,which mainly ignite the propellant by thermal convection,and the propellant combustion meets the"parallel layer"combustion law.However,the Zr H₂/Pb O₂ igniter burns as a condensed-phase particle flow,which ignites the propellant by heat conduction,and will seriously damage the"parallel layer"combustion law.During fast depressurization,the MT and Ba(NO₃)₂igniter flame fluctuation is small,whereas the working continuation capacity of MT igniters is stronger than that of Ba(NO₃)₂ igniters.However,the combustion jet of Zr H₂/Pb O₂ igniter firstly is the condensed-phase particle flow,and becomes gas-phase flame after depressurization.Also Zr H₂/Pb O₂ igniters have weak work sustainability.(2)Numerical study on steady combustion characteristics of base bleed igniterBased on experiments,the numerical researches on the steady combustion characteristics of 2D MT pyrotechnic colume and 3D MT six-hole igniter are carried out.The distribution rule of characteristic parameters of the combustion field of MT pyrotechnics is revealed.The results show that when the MT pyrotechnic column burns,as the pressure increase,the kinetic reaction rate increases.The CF₂ distribution core and the reaction core of Mg+CF₂=Mg F₂+C move downstream.After the cores of C distribution and C-C binding reaction are split by a central reaction core,a pair of small reaction cores are formed on both sides of the reaction zone.When the MT six-hole igniter burns,the 3D combustion field consists of a converging region and a combined region in axial direction,and a potential core exists above each nozzle hole in converging region.The temperature above the potential core of six combustion jet is the highest.The velocity on the potential core side is the largest.In converging region,momentum,energy,and components are transmitted from each jet to the center.In combined region,the velocity,temperature and mass fraction of each component are the largest on the central axis,and the parameter distributions of different transverse profile show similarity.The jet is characterized by a single free jet.(3)Numerical simulation of unsteady combustion characteristics of base bleed igniter under the condition of rapid depressurizationFor the experimental device simulating the reignition of base bleed propellant during rapid depressurization,the shape evolution of exhaust plume is obtained numerically,which is in good agreement with experimental observations,verifing the feasibility of numerical model.The transient combustion characteristics of igniter under different initial NPRs(nozzle pressure ratios)are analyzed.The results show that during rapid depressurization,in the initial stage,the igniter flame is suppressed at the end face of igniter,and the gun propellant gas forms a supersonic underexpanded exhaust plume after exiting the nozzle.In the middle stage,the igniter flame gradually becomes an erected“?”shape,and the supersonic underexpanded plume of gun propellant gas becomes a mixing-gas supersonic underexpanded plume.As the igniter flame spreads downstream,the flame changes into a conical shape by degree.Downstream of the igniter jet,the temperature decays more slowly in radial direction,and the thermal convection and diffusion are stronger than upstream.The periodic diamond flame string are formed in the exhaust plume.In the final stage,the mixing-gas supersonic underexpanded plume gradually changes into the subsonic plume of the igniter gas.(4)Numerical study on unsteady combustion characteristics of igniter during rapid depressurization of base bleed unitFor stationary actual BBU,the coupling characteristics of igniter and propellant gases in rapid depressurization are numerically studied.The equivalent constant convective heat flux is proposed to estimate reignition delay time.The effects of initial NPR,particle size and mass ratio of MT on the igniter combustion characteristics are discussed.The results show that when the depressurization in stationary BBU begins,the pressure drop disturbance is transmitted from nozzle to the upstream of combustion chamber,and gradually decreases toward upstream.As the time goes,the pressure in BBU gradually decreases with similar depressurization rate.The speed difference between igniter gas and peripheral propellant gas causes Kelvin–Helmholtz instability.With the decrease of initial NPR,the increase of PTFE particle size,the decrease of Mg particle size,and the increase of Mg content,the gas temperature on the surface of base bleed propellant becomes higher,and the secondary ignition delay time is shorter.(5)Numerical study on unsteady combustion characteristics of igniter in the after-effect period of base bleed projectile flying out of muzzleFor the launch condition of a 155mm BBP,the transient combustion characteristics of igniter after the BBU exits the gun muzzle are studied numerically under the initial gun-propellant temperature conditions of low,normal and high temperature.The results show that during the depressurization process of BBU,the ignition jet is repeatedly stretched and contracted three times,which appears Kelvin–Helmholtz instability.Shortly after the start of depressurization,as the igniter flame shrinks,the axial velocity difference between the igniter gas upstream and downstream of the combustion chamber forms a contact discontinuity which disappears as the igniter flame spreads.After 1.5ms,the contact discontinuity is always present,and moves downstream until the nozzle in the stretching process of ignition jet flame.During the convection heating process of the high-temperature igniter combustion-gas on base bleed propellant,the maximum heat flux density and its position will oscillate and fluctuate.At the end of fast depressurization,the maximum heat flux density is stable at around 1200 W/cm²,and its position is stabilized upstream of the combustion chamber.