Simulation Study On Hydrogen Release Performance Of Magnesium Metal Charge Column In Wet Air Environment

Hydroreactive metal fuel is a sort of high-energy propellant used in water ramjet engines.The mainly magnesium metal powder in propellant can react with water to release a large amount of hydrogen gas and heat.The higher content of magnesium metal fuel,the more significant the energy and speed advantages of water ramjet engines.Due to the active chemical properties of the magnesium metal in the magnesium metal grain,it is extremely easy to react with oxygen and water vapor in the air during transportation and storage,resulted in a decrease of its overall performance.Therefore,it has important guiding significance that studying its hydrogen release performance under different temperature and humidity environments for the transportation and storage of columns.In this paper,a core shrinking physical model for the reaction of magnesium metal grains and water vapor in a humid air environment has been established.Water vapor in the wet gas first reacts with the magnesium metal on the surface of the grain,forming a product layer,and then gradually undergoes a diffusion reaction inward.The unreacted area in the solid gradually shrinks until the reaction stops.Assume through the shrinking core model,simplify the control steps of gas film diffusion control,product layer diffusion control,and surface chemical reaction control,determine the model parameters and solve them,and simulate the hydrogen release amount at different temperatures and humidity.The simulation results show that in the process of gas film diffusion control and surface chemical reaction control,when the temperature is higher and the humidity is higher,the hydrogen release amount of the magnesium metal grain continuously increases in a very short time;During the diffusion control process of the product layer,when the humidity is higher and the temperature is higher,the hydrogen release amount of the magnesium metal charge column gradually increases with the increase of time at the initial stage,and then gradually tends to stabilize.However,the final hydrogen release amount of the magnesium metal charge column continuously increases with the increase of temperature,and the degree of reaction is also increasing.Under different humidity,the hydrogen release amount of magnesium metal propellant columns has little difference.Increasing temperature has a more significant effect on hydrogen release from magnesium metal propellant columns than increasing humidity.In order to validate the physical model of shrinking core,a device for analyzing the thermal oxidation performance of grain was built to investigate the hydrogen release laws of magnesium metal grain under different temperature and humidity conditions.The experimental results showed that the higher the humidity,the faster the hydrogen release rate of fuel grain;The higher the temperature,the greater the concentration of hydrogen released from the fuel grain.When the temperature is 20℃and the relative humidity is 60%,the time for the column reaction to reach equilibrium is about 5500 minutes,and the concentration of hydrogen in the stable state is 1.5 mg·m^-3;When the temperature is 50℃and the relative humidity is 60%,the time for the column reaction to reach equilibrium is 4500 minutes,and the concentration of hydrogen in the stable state is about 2.0 mg·m^-3.Adjust the temperature from 50℃to 70℃,and the column continues to release hydrogen.After 7000 minutes,it reaches equilibrium,with a hydrogen concentration of about 2.45 mg·m^-3.The thermal conductivity of the magnesium metal grain before and after the test is good,the XRD diffraction peak coincides with the standard diffraction peak of magnesium hydroxide,and the thermogravimetric curve changes in accordance with the experimental law,which proves that magnesium metal reacts with water vapor in the air to generate magnesium hydroxide.The comparison between simulation and experiment shows that the simplified treatment of product layer diffusion control as a control step in the shrinking core model is more consistent with the experimental results.The reaction temperature has a more significant impact on the hydrogen release amount of magnesium metal grains than the gas humidity.The higher the reaction temperature,the faster the rate of hydrogen release from the grain water reaction,the shorter the time required for the hydrogen concentration to stabilize,and the greater the difference in the amount of hydrogen released as the reaction reaches equilibrium under the same humidity as the temperature continues to rise.The higher the relative humidity,the faster the rate of hydrogen release from the magnesium metal grain water reaction,and the shorter the time required to reach equilibrium.At the same temperature,there is no significant difference in the amount of hydrogen released as the reaction reaches equilibrium with increasing humidity.