Mechanism Of Mg/CO₂ Ignition And Combustion And Suppress Of Carbon Deposition

The metal/CO₂ powder rocket engine for Mars exploration directly uses carbon dioxide in the Martian atmosphere as an oxidant,and only needs to carry metal powder fuel from the earth,which can greatly reduce the launch mass and significantly increase the effective specific impulse of the propellant.In addition,the engine can achieve thrust adjustment and multiple start functions by changing the flow of powder fuel and carbon dioxide,so that the probe can achieve complex and changeable tasks on Mars.It is an ideal propulsion system for Mars exploration.Magnesium powder is considered to be the most promising fuel for the power system due to its low ignition temperature,excellent combustion performance,low cost,non-toxic and pollution-free characteristics.However,the current basic research on the characteristics and mechanisms of ignition,combustion and carbon generation of magnesium powder in carbon dioxide is not deep enough,resulting in the lack of basic theoretical support for the engine structure design and combustion mode organization.This paper takes the micron-sized magnesium powder particles used in actual engines as the research object,and conducts detailed studies on their reaction characteristics and mechanisms in carbon dioxide from the perspectives of thermodynamics,kinetics and combustion.First of all,the final equilibrium state of the magnesium carbon dioxide reaction under different conditions is obtained through thermodynamic calculations,so as to establish a preliminary understanding of the reaction process and characteristics,combining thermal analysis kinetics and quantum chemical calculation methods to analyze the reaction process of Mg/CO₂ heterogeneous and homogeneous systems,and combining theoretical calculations to obtain corresponding kinetic parameters;Secondly,a dynamic ignition and combustion system was built,and the influence of particle size,ambient temperature,ambient pressure,and CO₂ concentration on the characteristics of Mg particles’ignition and combustion and carbon generation characteristics was studied,and through the combination of online shooting of combustion flame and offline detection of condensed phase products,the ignition and combustion mechanism were revealed;Then,combined with the previous research results,the ignition and combustion mechanism of magnesium particles in the carbon dioxide environment was deduced,and a detailed physical and mathematical model of particle ignition and combustion was constructed,according to the numerical simulation results,the heat and mass transfer characteristics of the magnesium particles during the entire combustion process were quantitatively described.Finally,based on model calculations and experimental results,feasible ideas and methods for reducing carbon deposition in actual engines were proposed.Through the research of this article,the main results and conclusions obtained are as follows:（1）When the concentration ratio of Mg to CO₂ reaches the stoichiometric ratio,the reaction adiabatic combustion temperature is close to 3000 K,while the temperature of magnesium particles is close to its boiling temperature（1363 K）,and the concentration of combustion products（Mg O and CO）is 14 times the concentration of reactants.Therefore,a thinner gas phase reaction front is likely to exist.There is almost no CO₂ in the area between the front face and the surface of the Mg particles,and the combustion products are mainly Mg O and C;while in the area between the front face and the environment,there is almost no Mg,and the combustion products are mainly Mg O and CO.（2）When the temperature of Mg particles is lower than 450°C,the oxidation rate is low and can be ignored.The heterogeneous reaction between Mg and CO₂/CO is controlled by the first-order reaction model（F1）,that is,linear kinetic control.The apparent activation energies are 132.4 k J·mol^-1 and 100.2 k J·mol^-1,respectively,and the pre-reference factors are1.376×10⁴ s^-1 and 238.599s^-1,respectively.The reaction rate constant of Mg and CO₂ is much higher than that of Mg and CO.In the temperature range between the boiling point of Mg and2000K,the former is 10-17 orders of magnitude higher than the latter.It shows that the gas phase reaction of Mg in CO₂ environment is mainly the reaction between Mg vapor and CO₂,while the reaction between Mg and CO mainly occurs on the surface of molten Mg droplets.（3）Mg particles will form a layer of uneven thickness on the surface of the particles during the ignition and heating process in a CO₂ environment.The accumulated Mg vapor in the shell damages the weakest position of the oxidation shell,and then it is ejected out to react with CO₂ in the gas phase and produce a bright flame,thereby achieving fire.Mg vapor will damage the weaker parts of the Mg shell,causing the Mg vapor to form a local spray state around the particles.Depending on the distribution of the broken holes,some Mg particles may behave as spin combustion,while the other part may behave as translational combustion.In addition,due to the structural reformation of the particle surface shell during the combustion process,the combustion mode of Mg particles may be transformed from spin combustion to translational combustion,which may cause five different combustion behavior modes in the combustion of Mg particles in CO₂.The condensed phase combustion products are hollow and nearly spherical particles with a diameter similar to that of the original magnesium sample particles,and there are many holes on the surface of the shell.（4）With the gradual increase of the ambient temperature,the ignition delay and combustion time of the samples of Mg particles with different particle sizes show a decreasing trend.The influence of ambient pressure and CO₂ concentration on the ignition delay and combustion time of the particles is relatively complicated,which mainly depends on the result of the battle between the two competing mechanisms,the increase in the thickness of the particle surface protective shell and the increase in the temperature rise rate of the particles.The ignition pressure limit of Mg particles in CO₂ is about 0.025 MPa.（5）Higher ambient temperature and smaller particle size are beneficial to increase the temperature rise rate of the particles,and higher temperature will facilitate the evaporation of Mg droplets,thereby increasing the proportion of Mg participating in the homogeneous reaction.Although the reduction of environmental pressure is beneficial to reduce the amount of carbon deposition,it will cause the ignition delay time of Mg particles to increase significantly.Therefore,it is necessary to comprehensively consider the impact of ambient pressure on the ignition and combustion characteristics of Mg particles and carbon generation characteristics,and make the optimal working parameter selection after weighing the pros and cons.The weight of Mg participating in heterogeneous reactions increases with the increase of CO concentration,and too high CO concentration will also cause the burning rate of Mg particles to decrease.Adding AP and Al to the small particle size Mg source powder can indirectly reduce the CO concentration on the surface of the Mg particles and increase the local temperature of the reaction system,thereby effectively reducing the amount of carbon deposition.（6）The combustion of magnesium particles in CO₂ can be roughly divided into three different stages,namely the low-temperature latency period,the high-temperature oxidation period and the combustion stage.The consumption of Mg nuclei is mainly composed of two parts:Mg vaporization and Mg/CO surface heterogeneous reaction.During the entire combustion process,the evaporation rate of magnesium nuclei is approximately 2 to 3 orders of magnitude higher than the surface reaction rate,indicating that the consumption of magnesium nuclei is mainly dominated by the evaporation of magnesium nuclei.The heat transfer forms of magnesium particles mainly include gas-phase reaction heating,convection heat transfer,surface reaction heating and radiant heat transfer.The magnitudes of them decrease in order,among which the gas-phase reaction heat is dominant,and its magnitude is about 2 orders of magnitude higher than other heat transfer forms.