Study On Ignition,combustion,and Energy Release Characteristics Of Discrete Magnesium Diboride Particles

Magnesium diboride,which combines boron and magnesium atoms by chemical bonds,has the characteristics of high mass calorific value and volume calorific value from boron-containing fuel.At the same time,the addition of active metal magnesium improved the performance of boron-contained fuels in problems including ignition difficulty,burnout difficulty and low combustion efficiency.Among all kinds of boron-magnesium compounds,magnesium diboride has become a kind of high-energy boron-contained fuel with great application potential because of its low manufacturing cost and excellent ignition and combustion performance.In a solid rocket ramjet,the fuel particles are not stationary,but move and burn with the compressed air produced in the combustion chamber.Therefore,this paper takes magnesium diboride discrete particles in the state of motion as the main research object,focusing on its ignition,combustion and energy release characteristics,aiming at revealing the inherent mechanism of its ignition and combustion processes and promoting its practical engineering application as solid ramjet fuel.The stacking magnesium diboride particles ignition combustion experimental system and the magnesium diboride discrete particle in the state of motion ignition combustion experimental system were built up in this paper.The X-ray diffraction and laser particle size analysis were first applied to determine the main composition and particle size distribution of magnesium diboride particles.Then the experiments were carried out on the ignition and combustion experimental system of stacking magnesium diboride particles,and the combustion processes of stacking magnesium diboride particles were monitored online by analyzing the flame image and atomic emission spectrometry from the combustion process.The ignition and combustion characteristics of stacked magnesium diboride particles were obtained.By distinguishing the flame and spectral characteristic signals from the combustion process,the combustion process of stacking magnesium diboride particles can be divided into three stages:flame developing stage,stable combustion stage,and burnout stage.During the combustion process,the flame color was light white,which was close to magnesium combustion but different from boron,indicating that the combustion intensity of magnesium diboride was higher than that of boron.The comparison of the signal intensity of the magnesium oxide and BO₂ in the corresponding atomic emission spectrum also indicated this.The burnout stage,which was affected by thermal diffusion,had a continuous combustion time of only 30 ms.The ignition and combustion characteristics of magnesium diboride discrete particles in the state of motion were studied through combustion flame image analysis,combustion time analysis and combustion product analysis.According to the different characteristics of combustion flame images,the combustion modes of magnesium diboride discrete particles could be divided into three types:single-stage combustion,two-stage combustion and micro-explosion combustion.The single-stage combustion process was relatively uniform,and a secondary enhanced release of magnesium vapor occurred in the two-stage combustion process,while the particles disintegrate to produce fragments in the process of micro-explosion combustion.The flame characteristics of the combustion image confirm that there was a co-oxidation process of magnesium and boron-magnesium compounds in the combustion process of magnesium diboride,and the flame color was determined by the dominant reaction between them.The average combustion time of single-stage combustion and two-stage combustion were 12.1 ms and 12.3 ms,respectively,which shared a similar combustion time,while the average combustion time of micro-explosion combustion was greatly shortened（5.6 ms）due to the disintegration of particles and the decrease of particle size.Among the three combustion modes,the probability of two-stage combustion and micro-explosion combustion was relatively low（9.1%and 4.2%,respectively）.The main components of the combustion products were Mg B₂,Mg O,Mg,B₂O₃ and Mg B_x.There is a condensed boron oxide layer covering the surface of the particles to make it smoother.Regular grains could be distinguished under the boron oxide layer,while a large number of clustered magnesium oxide particles could be seen outside the boron oxide layer.There were two forms of magnesium oxide particles,which are floccule and spherical particles,respectively.With the help of graphic analysis,the average particle sizes of initial magnesium diboride grains,grains on the combustion product surface and magnesium oxide particles were obtained as 0.24μm,1.04μm and 0.18μm,respectively,indicating that the surface grains of combustion products came from the coagulation process of initial grains during combustion,while magnesium oxide particles came from the condensation of gaseous magnesium oxide.Based on the single-stage combustion mode,the combustion process of magnesium diboride discrete particles was analyzed.Taking the condensation process of magnesium oxide on the particle surface,the evaporation process of boron oxide on the particle surface and the effect of convection into account,a combustion model of magnesium diboride discrete particles in air was established.Combining with the existing model research results of boron particles and magnesium particles,the combustion process of magnesium diboride was appropriately simplified.The combustion time of magnesium discrete particles was calculated as 11.7 ms,and the proportion of the condensation part of magnesium oxide produced from magnesium vapor oxidation on the particle surface was about 35%.During the combustion process,the particle radius and the flame radius shared the same trend,and the flame radius was about 3 times of the particle radius.During the combustion process,the particle temperature was stable at approximately 1650K,while the flame surface temperature increased from 1100K and gradually exceeded the particle temperature to about 2000K.The particle combustion process reflected in the model fitted well with the existing experimental results.The effects of environmental pressure and temperature on the combustion characteristics of magnesium diboride discrete particles in air were numerically analyzed.The results showed that the changes of environmental conditions had different effects on the mass flow rate of magnesium vapor,particle temperature,flame temperature and combustion time.