Research On The Reaction Behavior And Flow Behavior Of Energetic Materials Under Laser Ablation

The destructive effect of explosion is not conducive to the further study of experimental phenomenas and reaction behaviors in energetic materials.Pulsed laser ablation is one of the important micro-experimental methods to achieve rapid reaction and combustion in explosives,which has the advantages of high safety,high efficiency,less resource consumption and directional excitation,and it is considered to be one of the effective means to study the reaction behaviors of energetic materials.In this paper,based on the combination of laser ablation method,bioluminescence imaging,emission spectra and shadow imaging measurement method,the reaction behavior of aluminized RDX explosives and the flow behavior of nitromethane was studied.The main results are as follows:1.We have designed and improved the laser ablation experiment system,and studied the reaction behavior of pure RDX and aluminized RDX explosives.Experimental results show that,laser ablation of RDX powder mainly results in localized plasma formation,no obvious persistent reaction behavior was observed.The reaction behavior of aluminized explosive under laser ablation has typical secondary reaction characteristics,which also has obvious persistent combustion characteristics compared to pure RDX.The oxidation reaction of aluminum powder is the main reason leading to secondary reaction,AlO molecule is an important intermediate product of the oxidation reaction.2.In order to understand the process of laser ablation more clearly,we try to change the distance from the laser focusing point to the surface of the material,and study the influence of different laser energy deposition modes on the reaction development.The study finds that RDX based aluminized explosives show obvious secondary characteristics under different focusing distances.The geometric shape of combustion image is analyzed from the view of fluid dynamics,we have pointed out that the obvious vortex motion characteristics in combustion images at different distances should be the combined action of vorticity distribution and the airflow after the shock wave.The experimental study also shows that,the ejected particles in the shadow imaging are probably not the main substances involved in the combustion reaction,the main material of combustion reaction should be the explosive vapor produced by laser ablation and some very tiny particles,and they are easier to be affected by airflow in the combustion flow field and form the vortex motion images observed in the experiment.3.The measurement of transient temperature in explosive and combustion system is always difficult,we have calculated the temperature of the reaction region based on the emission spectra of Na elements and AIO molecular.The temperature measurement by Na emission spectra is unable to obtain reliable temperature information because of the small spectral energy difference in the spectra line of Na.The range of the calculated temperature based on the AIo molecular spectra is between 3000-5000 K,which is consistent with the results of many other researches.There is also a good correlation between the temperature change and the reaction development,the temperature shows a higher value in the early stage of the reaction,as the time increases,the temperature gradually decreased with the weaken of the light intensity and the reaction.4.In order to further understand the mechanism of aluminum oxidation reaction,the reaction behavior of RDX based aluminized explosives in argon atmosphere was also studied.The overall light intensity decreases monotonically with time in argon atmosphere.The change relation of light intensity can be approximated by two exponential decay functions(before 6?s/after 6 ?s),and this indicates that there is no secondary reaction phenomenon and increase in chemical reaction in argon environment.Its essence should be the dissipation process of two kinds of matter from the initial value to zero,the dissipation rate depends on the dominant material composition in each period.5.Combined with the results in argon environment,we have a new understanding of the reaction behavior of aluminized explosives in the air environment:the light intensity can be regarded as a typical exponential decay pattern in the early(before 5 ?s)and late stages(20-50?s),which corresponding to the process of dissipation and cooling of different elements and substances from the initial state.The light intensity presents parabola shape in the middle stage(5-20?s),it can be regarded as a typical feature of the secondary reaction and the growth of reaction,the main type of reaction is the reaction between aluminum powder and oxygen.In general,the improvement of the combustion performance of aluminized explosives in the air environment is mainly manifested in two aspects:one is the oxidation reaction between RDX and aluminum particles,generally occurs in the early stage;the other is the oxidation reaction between oxygen and aluminum particles,it mainly occurs in the middle stage,and belongs to the typical exothermic reaction,it is also the most important reason for the reaction growth and the secondary reaction of aluminized explosives.6.In this paper,the flow diffusion behaviors of nitromethane and water under laser ablation are studied.The images of typical processes such as shock wave propagation,cavitation expansion and collapse in nitromethane and water within the 40ns-100ms time range under laser ablation are obtained.The laser-induced breakdown in nitromethane produces some non-evaporative gas at the final stage,and this phenomenon is not obvious for the water case.The secondary shock wave was not observed in nitromethane;the existence of non-evaporative gases may make it difficult to compress the bubble content into a smaller volume and generate a high pressure collision to form a secondary shock wave.On the basis of the experiment and calculation,the bubble energy in nitromethane is more than twice as large as that in the water case at each incident laser energy.It is suggested that energy release from chemical reactions during the breakdown contributes the bubble energy.However,the confirmation of that needs more work on the determination of the shock wave energy.In this paper,an experimental platform for studying the reaction behavior of energetic materials under laser ablation is established and perfected.The understanding of the reaction behavior of aluminized explosives and nitromethane has been deepened.The whole work has extended the range of laser ablation technology to a certain extent.