Investigation On The Ignition And Combustion Of Aluminum/Aluminummagnesium Alloy Particle In Hot Gas

Aluminum powders are widely used as fuel additives in various solid propellants.Aluminum powder fuel additives can not only improve the energy density of solid propellant,but also reduce the high frequency unstable combustion in the rocket motor.In this work,the ignition and combustion characteristics of single aluminum particles in hot oxidizing gas are investigated by the methods of experimental research,theoretical analysis and numerical simulation.Due to the high ignition temperature and long ignition delay time,the aluminum particles will be melted into coarse agglomerations before they are ignited,which reduces the combustion efficiency of the aluminum powder.In order to solve this problem,the ignition and combustion process of single atomized Al-Mg alloy particles was also studied experimentally in this work.The experimental system and method of single metal particle ignition and combustion in hot gas are designed and built.Stable fluidization of single particles with diameter ranging from tens to hundreds of microns is realized by a metal particle generator.A premixed flat flame burner is employed to provide a series of high-temperature environments for the ignition and combustion of metal particles.The whole ignition and combustion process of single metal particles is recorded by the high-speed cameras and filtered photomultiplier tubes?PMTs?simultaneously.The particle size,initial shape,ignition delay time,and combustion time are measured directly from the images by using the in-house automated data processing routines.The characteristic particle temperature is measured using the method of two-color pyrometry.To investigate the morphology of metal particles during combustion,they are collected using the sampling device.Besides,a data processing method named“Grouping-Statistics-Fitting”has been proposed in this work,which can be used to solve the problems caused by the excessively concentrated or dispersed data.The influence of effective oxidizer mole fraction of hot gas on the ignition and combustion characteristics of single aluminum particles are investigated.Based on the bright-spot diameter profiles and the known respective reaction mechanisms,the total ignition and combustion process of aluminum particle can be divided into three stages,namely,pre-heating,ignition and combustion.The asymmetric flame around the burning aluminum particle is caused by local coverage of the oxide cap,and the microexplosion is the result of the surface oxidation products hindering the volatilization of aluminum.Both of the ignition delay time and combustion time of aluminum particles increase with the increase of particle diameter.Under the influence of oxide layer on particle surface,the increase of oxygen in hot gas will prolong the ignition delay time of small particles?40–100?m?and shorten the ignition delay time of large particles?120–160?m?.Different from the change trend of ignition delay time,shorter combustion time can be achieved in the environment with high oxygen content,which is caused by the high combustion temperature and the microexplosion.High ambient temperature can also reduce the ignition delay time largely,but has little effect on combustion time.The ignition probability of aluminum particles in oxidizing gas increases with the increase of ambient temperature,but decreases with the increase of oxygen content.The ignition probability of aluminum particles with diameters in the range of 40–170?m first increases and then decreases as the particle diameter increases,and there is a suitable particle size with the largest ignition probability.Moreover,the increase in particle unevenness level can improve the particle ignition probability,which should be resulted in the preignition on the raised part.Characteristic particle temperature of single aluminum particle increases with the increase of ambient temperature,but decreases with the increase of particle diameter.Lots of holes in the oxide layer of combustion product has been found,which should be the micro channels for the external oxidizing gas diffusing to the metal surface.The ignition and combustion models of aluminum particle in hot oxidizing gas are established.Based on the influence of oxide layer during ignition,the effective area of surface chemical reaction is modified.Considering the surface tension between molten aluminum and alumina,the calculation method of oxide cap dimensions is established in the combustion model.The model prediction errors of ignition delay time and combustion time of in typical conditions are within 20%,and the main sources of the errors are analyzed.The influence of environmental pressure on ignition delay time and combustion time of aluminum particles in typical conditions is also investigated.With the increase of environmental pressure,the surface chemical reaction rates increase and the ignition delay time decrease consequently.However,the combustion time does not change with the environmental pressure.The main reason is that the change trends of combustion time in O₂,H₂O and CO₂ with pressure are different,and these change trends can be traded off against each other in the mixture oxidizing gas.In order to improve the ignition and combustion characteristics of aluminum particle,the ignition and combustion processes of atomized Al-Mg alloy particles?Al90Mg10,Al60Mg40 and Al50Mg50?have also been studied in this work.Microexplosion can be found during the combustion of almost all the alloy particles in hot gas,which results from the inner elevated magnesium vapor pressure.Because of this phenomenon,the combustion times of alloy particles and the loss of specific impulse of rocket motor can be reduced.The ignition delay time and combustion time can be reduced by increasing the magnesium content in alloy particle.High ambient temperature can shorten the ignition delay time,but has little effect on the combustion time.High oxygen content in the gas is helpful for the alloy particle combustion.Compared with aluminum particles,the Al-Mg alloy particles can achieve much higher ignition probability,much lower ignition temperature and much shorter ignition delay time and combustion time.Therefore,the disadvantage of Al-Mg alloy particles in energy density is expected to be compensated by their outstanding ignition and combustion characteristics.