Study On Laser Ignition And Combustion Characteristics Of Nano-metal Aluminum Powder

In the past few decades,metal fuels have been widely used in solid propellants.Compared to conventional propellant fuels,metal fuels have the advantage in high combustion energy,such as Magnesium,Aluminum,Boron,Titanium.Nano-aluminum is widely used as an additive in propellants to improve the combustion performance of propellants for its high density,high heat release,low cost and relative safety.However,in practical applications,the problems like smoky and agglomeration are not be well solved because of the complexity of the combustion mechanism.Although there have been many researches on nano-aluminum,most of them have focused on the topic of composite materials containing aluminum.Therefore,this paper chooses pure nanoaluminum powder heap as the research object.Focusing on the influence of physical parameters of nano-aluminum powder on the oxidation process,including particle size,oxide film thickness and water content.And the coupling effects of these factors on the combustion of nano-aluminum are comprehensively analyzed by changing the laser power density.The ignition and combustion properties of nano-aluminum powder were tested by laser ignition.The results show that the ignition delay time of nano-aluminum powder is about 2ms,the melting temperature is 615K~952K,and the ignition temperature is 1369 K to 2083 K.This is much smaller than the melting temperature of aluminum oxide film(~2350K).It found that the combustion process of the nano Al has no obvious flame front,because of its surface burning.When under the same laser heating power,nano-aluminum powders with different particle sizes have different ignition and combustion characteristics.In the nano-scale range,the ignition delay time of aluminum powder decreases with increasing particle size,which is contrary to the rule that the ignition delay time of micron aluminum varies with particle size.Thus,within the transition size interval,it appears to be a critical value that optimizes the ignition performance of the nano-aluminum powder.Comparing the combustion of nano and micro aluminum,it is found that nano aluminum is obviously better than micro aluminum in the ignition stage.However,the flame intensity generated during the combustion process is not as strong as that of micron aluminum.This paper discussed the DOM and MDM mechanisms,and have the new understandings and conjectures by establishing a simple physical model.Further research found that the increase of the thickness of the oxide film will increase the reaction melting point and increase the melting temperature rise time.Then,this hinder the progress of the oxidation reaction,and increase the ignition delay of the sample.Therefore,we conclude that the thickness of the oxide film is the main reason affecting the oxidation mechanism of nano-aluminum.However,the influence of the oxide film on the combustion performance can be reduced by increasing the laser heating power.In addition,the moisture content in the nano-aluminum powder also has a great impact on its combustion performance.The results show that the increase of the moisture content does not affect the ignition delay time,but it will affect the combustion stability of the nano-aluminum powder,making the micro-explosion phenomenon more obvious.