Study On The Combustion Mechanism Of Aluminum Powder In Carbon Dioxide

The application of aluminum/carbon dioxide（Al/CO₂）as a high-energy fuel to Mars probes can make full use of Mars resources and save costs and increase the success rate of Mars exploration.However,although there are some reports on the combustion of Al and CO₂,the study on its combustion process and mechanism is insufficient.In this paper,CHEMKIN was used to study the detailed progress of the combustion reaction of nano-aluminum powder（nAl）in CO₂ atmosphere and the effect of water vapor（H₂O（g））on the nAl/CO₂ combustion process;Secondly,the effect of silicon dioxide（SiO₂）on the combustion process of nano aluminum powder（nAl）and submicron aluminum powder（1²μm Al）in CO₂ atmosphere and the flame propagation rate of nAl in microchannels were studied.In addition,the combustion of nAl in CO₂/H₂O（g）atmosphere was studied.Finally,the influence of SiO₂ on the flame propagation of suspended nAl in the CO₂ atmosphere was studied using a self-made suspension combustion device and the combustion products were analyzed.The main research results are as follows.（1）In the nAl（g）/CO₂ reaction system,the ignition reaction can take place when the initial temperature reaches to 800 K at least.Since the consumption process of nAl is not synchronous with the formation process of Al₂O₃,the combustion process of nAl in CO₂ exists two-stage ignition regime.As the initial temperature increases,the consumption of nAl tends to coincide with the formation of Al₂O₃ in the time dimension.And the two-stage ignition regime in the nAl（g）/CO₂ system gradually disappears.In the nAl（l）/CO₂ system,the ignition of nAl occurs only when the initial temperature reaches to 1800 K.The combustion reaction process also shows a two-stage ignition system.As the temperature rises,the two-stage ignition regime becomes more apparent which shows an opposite trend with the nAl（g）/CO₂ system.In addition,with an initial temperature of 2100 K,the one-dimensional laminar premixed flame propagation rate of nAl（l）/CO₂ is 32 m/s.（2）In the nAl（g）/CO₂/H₂O（g）system,the ignition and combustion reaction of nAl（g）in CO₂ still shows a two-stage ignition regime;the ignition reaction can be performed at an initial temperature of T₀=400 K.When the H₂O（g）was introduced,the system ignition energy decreased,but the evolution of temperature and product during the reaction was basically the same as the nAl（g）/CO₂ system.At the same time,the concentration of Al₂O₃（l）slightly increased due to the decrease of the system equilibrium temperature.（3）In the nAl/CO₂ reaction system,Al₂O₂ is the key factor for the formation of Al₂O₃（l）.The reaction（Al₂O₃=Al₂O₂+O）is the key reaction pathway for the formation of Al₂O₃,while the O atom is also mainly formed by the decomposition of Al₂O₂,so the production of Al₂O₃（l）is relatively small.Therefore,if there are more O atoms or the addition of combustion promoter containing O atoms in the system,the production of Al₂O₃（l）may increase and release more heat.（4）Adding SiO₂ can effectively promote the exothermic of nAl combustion and increase its maximum combustion temperature and maximum heating rate simultaneously;when the addition amount is 12.00 wt%,its maximum combustion temperature,maximum heating rate and combustion efficiency reach the maximum compared to other samples.However,the agglomeration degree of nAl and SiO₂increases during combustion and cause the decrease of the maximum combustion temperature,maximum heating rate,and combustion efficiency when the addition amount of SiO₂ reaches 18.00 wt%.In addition,although the addition of SiO₂increases the flame propagation rate of nAl in the microchannel,with the increase of SiO₂ contents,the flame propagation velocity of n Al combustion decreases.When the addition amount of SiO₂ is 6.00 wt%,the flame propagation rate of nAl combustion reaches maximum and it is 4.50 cm/s.（5）The combustion product components of nAl in CO₂ atmosphere mainly include Al and Al₂O₃,while the combustion product of nAl in CO₂ atmosphere after adding12.00 wt%SiO₂ mainly includes Al,Al₂O₃,Si,C,Al₂OC and Al₄SiC₄.The promotion reaction mechanism of SiO₂ for nAl combustion is 4Al+3SiO₂=2Al₂O₃+3Si.（6）After adding 12.00 wt%SiO₂ in 1²μm Al,its maximum combustion temperature,maximum heating rate and combustion efficiency all increased significantly,and the increase range was much greater than the increase of nAl related parameters after adding 12.00 wt%SiO₂.The ignition temperature of 1²μm Al with the addition of SiO₂ has a slightly decrease.The combustion product components of1²μm Al combustion in CO₂ atmosphere mainly contain Al and Al₂O₃.While 12.00wt%of SiO₂ is added in 1²μm Al,the combustion products mainly consisit of Al,Al₂O₃,Si,Al₂OC and Al₄SiC₄.（7）Compared with the combustion characteristics of nAl in CO₂,the ignition temperature and ignition delay time of nAl in the CO₂/H₂O（g）atmosphere are significantly reduced,and the combustion efficiency is improved.However,the maximum combustion temperature in the CO₂/H₂O（g）atmosphere decreased,and the combustion products contained Al and Al₂O₃ only,but no Al₂OC was found.（8）The nAl suspension combustion process in CO₂ atmosphere can be divided into four stages:nAl ignition,flame propagation,flame separation,and flame disintegration.However,after the addition of SiO₂,the height of the suspended combustion flames of nAl in CO₂ and the combustion propagation rate increased significantly,and the maximum height of flame propagation,the average propagation rate and the maximum propagation rate increased significantly with the increase of SiO₂ addition.