| When the base-bleed projectile is launched from the muzzle,the gunpowder gas is ejected from the base-bleed combustion chamber.At this time,the base-bleed combustion chamber is in a state of high-speed rotation and transient depressurization,resulting in ammonium perchlorate/hydroxyl terminated polybutadiene(AP/HTPB)propellant and magnesium/polytetrafluoroethylene(MTV)igniter combustion instability or even extinguishment.Futhermore,the wake flow field fluctuates sharply and post combustion flame pulsates strongly,which reduce the consistency of the base drag and increases the range dispersion.Based on this engineering background,the unsteady reaction flow model of the base-bleed equipment under the dual stress of high-speed rotation and transient depressurization is established,and the wake flow field evolution and its influence mechanism on the base drag are revealed.The main research contents and results are as follows:(1)Theoretical model of wake unsteady reaction flow for the base-bleed equipmentAiming at the unsteady combustion characteristics of AP/HTPB propellant,an 8-specises and 12-steps elementary reaction model of the post combustion for the base-bleed gas was established.Subsequently,the 3-steps reaction model of MTV igniter gas was considerated,and the unsteady combustible flow model under the dual stress of high-speed rotation and transient depressurization was established.Finally,the models were verified through the experimental results of the semi-closed explosive device and the literature results.The results show that the two results are in good agreement,which verifies the rationality of the numerical model.(2)The effect of post combustion on the unsteady reaction flow field at the wake of basebleed equipmentAiming at the transient depressurization procress of the base-bleed equipment,the influence mechanism of post combustion on the unsteady wake flow field was investigated,the temporal and spatial distributions of characteristic parameters such as flow field temperature,pressure and Mach number were analyzed.Moreover,the effect of the initial temperature in the base-bleed combustion chamber on the wake flow field and base drag was discussed.The results show that,the gas temperature is improved near the base due to the post combustion,the base pressure and the temperature fluctuation increase.In the last stage of depressurization procress,the primary recirculation region is pushed downstream by the wake reaction flow,causing the shear layer to rise and the base expansion wave to be weaken.Moreover,the base pressure increases and the base drag decreases.With the increase of the initial temperature in the base-bleed combustion chamber,the transient depressurization process is accelerated,the post combustion is enhanced,and the drag reduction rate increases,but the initial temperature has little effect on the wake flow field at the last stage of the depressurization procress.(3)Comparative analysis of the unsteady reaction wake flow field of the base-bleed equipment with two kind of nozzle structureAiming at the new bleed structure of annular nozzle,the flow field structure and characteristic parameter variation of base bleed equipment with the annular nozzle and circular orifice nozzle in the transient depressurization process were investigated,and the evolution characteristics of wake flow field under different nozzle sizes were compared.The results show that the high-pressure gas jet of annular nozzle squeezes each other near the central axis to be a central jet,forming a complex nested under-expanded jet structure.In the last stage of depressurization,the central jet gradually disappeares,and the gas jet developes into a subsonic annular flow.And then the annular flow squeezes the shear layer near the nozzle to weaken the base expansion wave and reduce the base drag.For the traditional structure of circular orifice nozzle,the depressurization process is that the highly under-expanded circular jet decays to a subsonic flow.The circular orifice nozzle with same bleed area has greater heat loss than the annular nozzle,and the base temperature decreases.However,the backflow effect is obvious resulting the fluctuation of the wake flow field increases,the base pressure decreases.In general,the higher bleed area means littler fluctuation of base pressure and base temperature,and the bleed area ratio has little effect on the pressure and temperature at the last stage of depressurization for the same nozzle structure.(4)Wake flow field characteristics of the base-bleed equipment under the dual environmental stress of transient depressurization and high-speed rotationDuring the 155 mm base-bleed equipment is ejected out of the muzzle,the combustion and flow process of AP/HTPB base-bleed propellant gas under the dual stress effect of high-speed rotation and transient depressurization was investigated,and the effect of rotation on the flow field inside and outside the base-bleed combustion chamber was researched.The results show that in the interal flow field of the base-bleed equipemnt,the propellant gas gradually releases from the combustion surface,and develops into a quasi-Rankine combined vortex under the effect of high-speed rotation in the last stage of depressurization.The tangential velocity in the vortex core increases linearly along the radial direction,while the turbulent kinetic energy increases and the pressure decreases.In the external of the vortex core,the tangential velocity decreases and then increases in the radial direction,while the pressure changes little.The combined vortex in the combustion chamber gradually dissipates along the axial,and then develops into a conical swirling flow near the nozzle.After the swirling flow enters the wake flow field,which increases the tangential disturbance in the recirculation zone.The higher the rotation speed of the base-bleed equipment,the greater the tangential velocity disturbance and the stronger the combined vortex turbulence.The high-speed rotation significantly improves the outlet mass flow rate and the base pressure,and then the base drag is reduced.(5)Wake flow field characteristics of the base-bleed equipment considering the igniter under dual environmental stressAiming at the phenomenon of oscillating combustion and reignition of the base-bleed propellant during the launch procress of a 155 mm base-bleed projectile,the interaction effect mechanism of MTV igniter on the AP/HTPB propellant under the dual stress of high-speed rotation and transient depressurization was investigated.The flow field evolution characteristics of the base-bleed equipment were analyzed under dual stress in the case of the un-reignition state and oscillating combustion state,and the effect of initial pressure in the base-bleed equipment on base drag were discussed in the case of the propellant oscillating combustion state.The results show that when the propellant is in un-reignition state under dual stress,the radial expansion speed of the igniter gas is much lower than the axial velocity,and it is ejected out of the combustion chamber quickly.There is a significant pressure oscillation process in the combustion chamber,and the igniter burns with highly unstable.Under propellant oscillating combustion condition,the pressure fluctuation in the combustion chamber is significantly weakened,and the base pressure increases.An obvious transition zone is formed between the propellant gas and the igniter gas,which contains a rotating vortex ring with a high tangential velocity and a series of small-scale vortices formed by Kelvin-Helmholtz instability.In the initial and middle stages of depressurization,pressure reduction disturbance is the dominant factor,and the higher the initial pressure cases have greater base pressure.In the last stage of depressurization,the effect of the initial pressure disappears,and the rotation disturbance gradually increases.The base-bleed equipment with higher rotation speed shows greater base pressure and lower base drag. |
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