The Transport And Reaction Characteristics Of Metal Micro-ejecta In Gas

The formation of micro-ejecta on the metal surface under dynamic loading and the subsequent transport of ejecta in gases widely exist in the process of shock compression.The mixing of ejecta and gas has an important influence on the ignition process,and is a key scientific problem to be solved urgently in the current weapon physics research.Microjets are essentially complex dynamic phenomena generated by the interaction of shock waves with metal free surfaces.The subsequent transport of micro-ejecta in the high-temperature and high-pressure gases may be accompanied by chemical reactions,ionization and so on.Predicting the transport and mixing process of micro-ejecta in gases with high accuracy has important practical application value for weapon physics.However,many disciplines such as high-pressure physics,metal phase transitions,interface instability and chemical reactions are involved in the whole process of shock compression,the source of ejecta and particle transport,which poses a higher challenge for numerical modeling of the reactive flow of microjets.In this thesis,based on molecular dynamics method,the formation,transport and reaction characteristics of metal microjets in gas environment are studied,including:1.The formation and development of tin microjets under different waveforms(supported wave and triangular/unsupported wave)are investigated.Simulation results show that the waveform has little effect on the velocity of the spike.By comparing the simulated data with theoretical and experimental results,it is found that the surface tension effect can significantly inhibit the development of the jet at nanoscale.The bubble still maintains a non-zero relative velocity in the later stage under unsupported shock loading,which is caused by the dragging effect of the unloading wave on the matrix near the bubble.This phenomenon is consistent with the results observed in the detonation loading experiment.By analyzing the time evolution of the total ejected mass,the inhibitory effect of unmelted metal on ejection under lower impact pressure is revealed.In addition,by comparing the relationship between the total ejected mass and shock pressure under different waveform loading,it is found that the critical pressure that causes melting of tin upon shock release is lower under unsupported shock.It successfully explains the phenomenon in the experiment,that is,compared with supported shock case,the ejected mass of unsupported shock case saturates at lower pressure.2.The transport and fragmentation process of aluminum jet in vacuum,inert neon and reactive oxygen are investigated.In inert gas,during the transformation of the micro-ejecta from the initial strip-shaped fragments to the final spherical particles,large fragments are broken into small particles due to the aerodynamic drag and shearing,accompanied by a large number of atomized atoms being stripped from the surface of the particles.In the reactive gas,the high temperature in mixing zone formed by the chemical reaction between particles and gases leads to the rapid evaporation of the ejecta,and finally the micro-ejecta of smaller size are formed.3.The combsution mechanism and mode of aluminum nano particles in oxygen are investigated.The influence of the structure change of oxide shell on the nanoparticle reaction rate was investigated,and a model for oxidation of aluminum nano particles at early stage was proposed,which is consistant with experimental trend.Moreover,the effects of different parameters,including particle size,oxide shell thickness,and oxygen concentration,on the oxidation modes of aluminum nano particles are investigated,and the phase diagrams for different reaction modes,including diffusion oxidation,crack-enhanced diffusion oxidation and micro-explosion oxidation are presented.The enhancement effect of nano-aluminum particle additives on the combustion of hydrocarbon fuels was investigated,and it is found that the early microexplosion oxidation of aluminum particles produced a large number of small molecules that could promote the decomposition of methane.The monoatomic oxygen accounts for more than 60%the decomposition of methane.