| Energetic materials,as the core components of warhead charge,may ignite and explode when accidentally stimulated,which leads to accidents for the security.At the beginning,the conductive combustion happens after ignited.If conditions permitting,conduction combustion can be further developed into convective combustion.The local high pressure generated by convective combustion is sufficient to drive cracks to propagate at a high speed in charge.Furthermore,as the combustion specific surface area increases,the reaction may be transformed from low-intensity to high-intensity,or even detonation.Therefore,ignition-convection combustion is the key to the upgrade of the reaction intensity.It is necessary to clarify how the combustion reaction evolves in this stage and what factors and mechanisms may affect it.The establishment of corresponding simulation calculation and analysis methods for reaction evolution can study the process of reaction evolution effectively,which is of great significance for the further understanding of the security issues of explosives’accidental ignition,and it can also improve the technical support for the security of the weapon system.This paper focuses on the evolution of reaction intensity from non-shock ignition to convection combustion in PBX,which is carried out from two aspects:non-shock ignition and the post-ignition reaction evolution.Through the numerical simulation on non-shock ignition,experiment on ignition of charge,and the simulation calculation and analysis of the reaction evolution,the key information about the evolution of reaction have been obtained,and a simulation and analysis method for the reaction evolution after ignition of the charge have been established.The main research work and innovation results are as follows:Based on the discrete element method to simulate the temperature rise and ignition of explosives under the low impact,the force-thermal response of explosives at mesoscale with different degrees of binder content and different constrains is simulated,as well as the matrix with prefabricated cracks or a single hole under low-velocity impact.The results show that the coating of the explosive crystal with binder can significantly weaken the stress bridge effect,resulting in a more uniform temperature rise distribution.The ignition resulted by the temperature rise is more likely to occur without lateral constraint than with lateral constraint and crack friction causes obvious hot spots in local areas.Shear bands caused by hole collapse are the important reasons for the formation of hot spots near the holes.For the hot spot generation process caused by cracks,the shear bands spreading outward from the crack area trigger the formation and ignition of hot spots near the crack.Two experimental methods to study the reaction evolution of PBX after ignition are explored.Through the center ignition experiment,the reaction evolution process after PBX’s center ignition under the action of weak,medium and strong constraint was photographed.The process is divided into four stages:crack propagation and combustion flame propagation,the flame propagates along the constrained interface after arriving,fragmentation of the matrix and the rapid increase of combustion reaction area and intensity,and the disassembly of constraint and extinction of flame.Among them,when the crack and the combustion flame propagate,the constraint affects the internal stress state of the explosive and slows down the propagation speed of cracks and combustion reaction as the constraint strength increases.When the flame reaches the constrained interface and propagates along it,the weakly constraint makes the flame easier to enter into the confinement interface but the corresponding reaction intensity is lower.For the strong constraint,the flame entering the confinement interface is reduced.The fracture and fragmentation of the matrix are the key to make a further escalation of the reaction intensity.In the experiment on the dynamic propagation of crack,plexiglass and tempered glass windows were used as observation windows to photograph the reaction evolution process of the matrix after locally ignited and the propagation velocity of the flame-driven crack under the plexiglass window was measured,about 51m/s.In order to simulate the convective combustion after the charge’s ignition,a kind of combustion and gas-phase simulation method based on the discrete element method is developed.To describe the process of combustion,the relationship between the delay of combustion ignition and the loading temperature is calculated,as well as the relationship between pressure and burning rate based on the Ward-Son-Brewster(WSB)combustion model.Based on the simulation idea of equating gas to a special collection of discrete units,the Micro gas-phase element method and the Corpuscular Method(CPM)are developed respectively and they are respectively coupled with the discrete element to make a simulation of the reaction evolution process after the charge’s ignition.When the Micro gas-phase element and the discrete element coupling are used to simulate the reaction evolution process of the constrained charge with center after ignition,the simulation results reflect the convective combustion mechanism to a certain extent.The combustion and pressurization process of the pre-cracked charge is simulated by the coupling of CPM and discrete element.The calculated results are in good agreement with the experimental data,which verifies the feasibility of simulating convective combustion in the crack by this method.Furthermore,the reaction evolution process after the ignition at the center charge is simulated and the results show that the calculation of the small-sized samples are similar to the real convective combustion phenomenon while the results of the large-sized samples deviate from the real situation.Finally,the process of the reaction evolution after the ignition of the notched charge is simulated and the results show that the process of flame penetration charge and rapid crack pressurization,which is consistent with the experimental phenomenon.The analysis model of the gas flow in the process of crack combustion and pressurization-combustion reaction propagation are respectively constructed.The effects of viscous friction force on the crack wall surface,crack length-to-width ratio L/w,crack propagation,and initial fracture toughness of sample K_αon the combustion pressurization process are analyzed.Results have shown that the viscous frictional resistance of the crack wall has a very significant influence on the restriction of reaction propagation and rapid pressurization of local areas.Both the viscous friction coefficientλand the crack width w affect the gas flow rate,and the decrease of w is equivalent to the increase ofλ.The evolution process of internal pressure can be divided into three states according to the crack length-to-width ratio L/w from small to large:gradually decompress and reach a stable state,gradually pressurize to reach a stable,and rapid pressurized state.The increase of L/w caused by crack propagation may change the internal pressure evolution state of the crack.The crack initiation toughness K_αhas a certain influence on the evolution of the crack’s internal pressure and a lower K_αcontributes to the crack growth and leads to rapid pressurization.Further,it is proposed that the combination of the method for calculates gas flow inside a crack and the discrete element method,which is a new idea for simulating the evolution process of the convective combustion reaction.Hence,a evaluation and calculation method of convective combustion coupled with discrete element and gas flow between crack segments was established.Based on the establishment of the crack segment grid inside the discrete element system,the gas flow law and the corresponding calculation method between multiple crack segments are defined.Based on the above simulation method,the center ignition experiment is simulated,and the effectiveness of this method in the study of the reaction evolution process after ignition of the charge is verified. |
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