| Water ramjet as a new type of underwater propulsion device, using hydro-reactive metal fuel and seawater in the external environment as the main oxidant. The propulsion of the engine is generated by the metal fuels reacting with water, which has very high mass and volumetric energy densities. Comprehensive comparison of the energy density and the reactivity of various metal reacting with water, magnesium is the first choice for hydro-reactive metal fuel. Studying the Mg/H2O reaction mechanism is the basis to grasp the reaction mechanism of the metal fuel reacting with water, which is important for enhancing the combustion efficiency and energy conversion efficiency of the water ramjet. This paper mainly performance four areas of research, including the ignition and combustion mechanism and model of magnesium in steam, high temperature homogeneous chemical reaction mechanism between magnesium and water, and the combustion characteristics of magnesium-aluminum alloy in steam.This paper firstly studied the ignition mechanism and model of the magnesium in steam. The effects of heating rate, steam concentration and particle diameter on ignition temperature and ignition delay time of magnesium in steam were studied by tube reactor. Based on experimental study, the ignition energy equation of magnesium particles in steam was established, and control variables in the energy equation were analysed and simplified. According to the experimental results, the kinetic parameters of magnesium particles with different diameters were calculated, and the ignition temperature and time of magnesium particles in steam were predicted.Then the combustion mechanism and model of the magnesium in steam were studied. The surface morphology of the magnesium particle in combustion process was shot by high-speed camera in visualization xenon lamp heating burner, the emission spectrum of the combustion flame was detected. The burning time of magnesium particle was determined by characteristic lines of combustion flame, and the burning time of magnesium particles with different diameters was measured. Based on experimental study, the quasi-steady physical and mathematical model of the combustion of magnesium particles in steam was established, mainly affected by the diffusion-controlled evaporation combustion process, and the magnesium oxide condensation and deposition was also considered. The burning time was calculated by the combustion model. The results shows that it is approximate proportion to the2.5power of the diameter in atmospheric pressure and1.5power of the diameter in high pressure, and inversely proportion to the0.9power of steam concentration, which are consistent with the experimental results. The oxidation, ignition and combustion process of100μm magnesium particles at different ambient temperature was also simulated.Reaction mechanism of Mg with water was investigated by ab initio quantum chemical methods. The geometries and frequencies of all reactants, products, intermediates and transition states were calculated at the B3LYP/6-311G++(3df.2p) level. Higher-level energies were obtained at G2M (CC2) level using the B3LYP-optimized geometries. The Mg and water firstly formed an atom-molecule adduct Mg·OH2, and then either formed MgOH+H by a H-dissociation process or formed HMgOH by a H-migration process. The barrier heights of two processes are48.28kcal/mol and32.51kcal/mol. The rate constants were calculated by using the variational transition-state theory with the zero-curvature tunneling correction in a temperature range of1000-5000K, and the total rate constant of Mg/water is3.85×10-10×T0.39×exp(-17852/T). Then a detailed reaction mechanism of magnesium/water under high temperature and combustion mathematical model was established, and OPPDIF simulator in CHEMKIN program was used to analysis the structure of the combustion flame.Finally, the combustion characteristics of magnesium aluminum alloy in steam were studied. The physical and chemical properties and composition of magnesium, aluminum, and magnesium-aluminum alloy with aluminum content of40%，50%and 60%were analyzed. The combustion characteristics of magnesium-aluminum alloy was studied in high-temperature hydrogen-oxygen flame, and found that the combustion process is divided into two phases:the first stage is the combustion of magnesium with nearly the same burning time, and the second stage is combustion of aluminum with longer burning time when the aluminum content in alloy increased. The combustion products were collected and analyzed by XRD. The reaction pathway of magnesium-aluminum alloy with steam is also analyzed, found that the combustion products are mainly the MgO, and MgAl2O4when the magnesium content in the alloy is more than aluminum, and the combustion products Al2O3only appears when the aluminum content is higher. |
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