Study On Ignition Combustion Characteristics And Mechanism Model Of Al And AlH₃

In solid propellant formulations,aluminum（Al）has been widely studied and applied due to its low oxygen consumption,low cost,non-toxicity,and high energy density.However,high aluminum powder content propellants produce agglomeration during combustion and may also cause nozzle erosion and two-phase flow loss.Aluminum hydride（AlH₃）has high energy density,high hydrogen storage capacity and high combustion heat.In solid propellants,replacing aluminum with AlH₃can greatly improve specific impulse.However,the decomposition and ignition combustion mechanism of AlH₃ under complex atmospheric and pressure conditions is not clear.In this paper,using a thermogravimetric experimental platform and a self-built laser ignition system,multiple combustion diagnostic techniques were used to compare AlH₃with Al of similar particle size in slow oxidation in carbon dioxide（CO₂）and air atmospheres under normal pressure CO₂/O₂ mixed atmosphere and ignition combustion characteristics under different pressure pure oxygen atmosphere.The main contents and conclusions of this paper are as follows:（1）TG-DSC experiments show that both AlH3 and Al have similar slow oxidation processes in air and CO₂ atmosphere.In their initial oxidation stage,the crystalline shape of alumina changed from amorphous toγ-Al₂O₃.When the external temperature reached the melting point of core aluminum,both of them produced the melting and oxidation stages of aluminum.In this process,the oxidation degree of both of them in CO₂ atmosphere was higher than that in air,which was due to the fact that the core aluminum was exposed more by the CO produced by the reaction.Different from aluminum,AlH₃ has a stage of hydrogen release and weightlessness at about 167℃.The decomposition of hydrogen leads to a gap in oxide layer,which increases the contact area between residual Al and oxidizing gas and accelerates its oxidation,which also leads to the reaction activation energy of AlH₃ in the same atmosphere is smaller than that of Al in the rapid oxidation stage.（2）In the atmospheric pressure laser ignition experiments,AlH₃ and Al only emit red light in CO₂ atmosphere.However,with the addition of oxygen,both were gradually ignited,and the reaction became violent with the increase of oxygen concentration,and the combustion time was gradually shortened.The ignition delay time did not have a large change due to the increase of oxygen concentration,and only fluctuated in the range of 1ms.It can be seen that the ignition and combustion phenomena of the two samples were relatively similar,but the ignition temperature of AlH₃ in the same atmosphere was due to the hydrogen release of AlH₃.Ignition delay time.Ignition and combustion performance such as burning time are better than Al,and the resulting agglomeration is also smaller than that of Aluminum.（3）High-pressure laser ignition experiments showed that as pressure increased combustion became more intense for both materials shortening burning time significantly but changes in their respective ignition delay times were not obvious.Under equal pressure conditions compared to Al,AlH₃had better properties being easier to ignite having shorter combustion times,higher flame intensity,higher peak temperatures during combustion.The high-pressure burning spectrum of AlH₃ was relatively simple basically being“three-finger peak”shape while Al’s high-pressure burning spectrum was more diversified with characteristic peaks observable at 486nm etc.In addition although there were also small jet explosions during Al particle pile combustion most AlH₃burned above original particle pile accompanied by dense smoke clouds greatly strengthening heterogeneous surface reactions greatly reducing sample’s own agglomeration.