Study On Primary Combustion Characteristics Of Magnesium-based Hydro-reactive Metal Fuel Used For Water Ramjet

Water ramjet, a new type of water-breathing jet propulsion system, uses high-energy hydro-reactive metal fuel as fuel and seawater as oxidant, gives high specific impulse and thrust and will be the best propulsion system for the ultra-high-speed underwater weapons in the future. As the fuel of water ramjet, the magnesium-based hydro-reactive metal fuel is one of the key technologies of the water ramjet. The energy of the magnesium-based hydro-reactive metal fuel is released by two combustion process in the water ramjet. The primary combustion characteristics of the fuel play an important role in the energy release efficiency of reaction between magnesium and water vapor in the secondary combustion process and in the overall performance of the water ramjet. The high energy property of the fuel is dependent on the metal content. However the higher metal content is against to increase the primary burning rate and the primary ejection efficiency. Therefore, it is very important to study on the primary combustion characteristics and the burning-rate modification methods.In this dissertation, the characteristics of the condensed phase reaction and the steady primary combustion of magnesium-based hydro-reactive metal fuel are studied by means of theoretical analysis, numerical simulation and experiments. The action of magnesium powders and other formulation parameters on the condensed phase reaction, the steady primary combustion properties and the primary combustion wave structure is investigated. The mechanisms of thermal decomposition and the primary combustion of the fuel are deduced rationally. Then the primary combustion model of the fuel is established. The actions of the key formulation parameters on the primary burning rate of the fuel are numerically analyzed with the model. The above mentioned results will supply theoretical and experimental data for the combustion performance modification of magnesium-based hydro-reactive metal fuel and application of water ramjet.The magnesium-based hydro-reactive metal fuels developed by the author can combust steadily in the engine. The burning rates of the fuel with 50%~65% magnesium powder are in the range of 1.35 to 7.92mm·s-1 at 2.0MPa, and the burning-rate exponents are changed from 0.15 to 0.30 under 0.5MPa ~ 2.0MPa. The primary ejection efficiency of the primary combustion is higher than 88.3%, and the maximum primary ejection efficiency is up to 95.2%. The static combustion surface temperatures of the fuel are changed from 517℃to 591℃and the equilibrium flame temperatures are about 1182℃to 1443℃under 0.5MPa.The primary combustion process of magnesium-based hydro-reactive metal fuel includes condensed phased heating, condensed phase reaction and gas reaction. In condensed phased heating zone, there is no chemical reaction and the temperature increases caused by heat conduction from condensed phased reaction zone. In the condensed phase reaction zone, there are many thermal decomposition reactions, such as, decomposition of plasticizer, thermal decomposition of AP, and thermal decomposition of HTPB binder. Mg does not react and only absorbs heat in the condensed phase reaction zone. The ultimate thermal decomposition products in the zone include some gaseous products, such as N2, CO, CH4, CO2, C2H2, HCl and so on, and some condensed products like carbon residues and Mg powders. In the gas reaction zone, the all magnesium powders of the fuel take endothermic phase transition into liquid and gas and part of the magnesium powders combust and release heat. Because of the low content of AP, the magnesium oxidation competes with carbon oxidation and only part of Mg or C is oxidated. The ultimate primary combustion products of the fuel include some gaseous products, such as CO, CH4, N2, CO2, C2H2, H2, MgCl2, gaseous Mg and so on, and some condensed products included carbon residue, magnesium oxide and chloride.The primary burning rate of magnesium-based hydro-reactive metal fuel is affected by the condensed phase reaction, and especially controlled by the thermal decomposition rate of AP. The thermal decomposition rate of AP and the burning rates of the fuel enhance with the increase of combustion surface temperature and the temperature gradient in the condensed phase reaction zone. There are some methods to increase the primary burning rate of the fuel effectively, such as, the higher Fe catalyst content, the higher mass ratio of oxidizer and binder, the higher content of fine magnesium powder and fine AP powder, and lower content of Mg. The burning-rate exponent is mainly affected by the mass ratio of oxidizer and binder and the content of fine Mg powder.The primary ejection efficiency of the fuel is affected synthetically by the primary combustion products, the flame temperature, the combustion pressure and the primary burning rate. The primary ejection efficiency of the fuel is greater when the mass content of master gaseous combustion products increases, the flare temperature increases, pressure of the primary combustion room increases and the primary burning rate increases. There are some methods to increase the primary ejection efficiency of the fuel effectively, such as, lower content of Mg, the higher mass ratio of oxidizer and binder, the higher Fe catalyst content and the higher content of fine AP powder.The primary combustion of magnesium-based hydro-reactive metal fuel is not complete because of the high content magnesium and the low AP content. Therefore, the heat released from the gas reaction zone, the equilibrium flame temperature and the combustion surface temperature are low. Owing to the low content of AP, the number of combustion cell around AP powders decreases. So the bright flares on the AP powder are surrounded by the dark zone on the HTPB binder and Mg powders in the gas reaction zone. Because of the high magnesium content, the endothermic melting and gasification heat of magnesium leads to the temperature fluctuation in the gas reaction zone of the fuel.On the basis of systematic investigation on the primary combustion characteristics and mechanism of the fuel, a steady primary combustion model of magnesium-based hydro-reactive metal fuel has been established by modification of BDP multi-flame model of AP-based solid composite propellant. The model is based on the energy and mass conversation equations on the combustion surface. The model takes into account the influence of magnesium powder on the combustion surface structure and heat-conducting coefficient of gas flame. Meanwhile, the model also takes into account the competition between magnesium oxidation with carbon oxidation. Further more, the action on the heat effect in the gas phase zone and the heat feedback on the combustion surface by endothermic melt of Mg, part of Mg's endothermic gasification and exothermic combustion is accounted in the model. The calculated burning rates of the fuels are in good consistent with the experimental data, which means that the model can describe the primary combustion characteristics of magnesium-based hydro-reactive metal fuel well enough. The model can be used for numerical study and theoretical analysis of the primary combustion properties of magnesium-based hydro-reactive metal fuel.