| Water ramjet, an advanced type of propulsion system, uses fuel-rich metal-based hydro-reactive fuel and the outside water as main oxidizer. One of the key technologies of water ramjet is the reaction characteristics between metal-based hydro-reactive fuel and water. On the other hand, the reaction characteristics are the basic and core of efficient combustion and efficient energy transform. On the basis study of magnesium based, aluminum metal, to solve the propellant ignition and combustion difficult problems, and to improve the metal reaction rate and reaction degree, the magnesium-aluminum alloy was studied in this paper. High-energy ball-milling was used to improve activity of magnesium-aluminum alloy. And low melting point metal Bi as active promoter was added in the process of high-energy ball-milling. Combustion characteristics of Mg-Al based hydro-reactive metal fuel propellant added with active Mg-Al alloy were studied, and the combustion efficiency of Mg and Al in primary and secondary combustion was analyzed.First of all, the combustion process and mechanism of Mg-Al based hydro-reactive metal fuel propellant were studied. Burning rate and combustin heat of the propellant were tested, and the combustion products were characterized by using scanning electron microscopy(SEM) and X-ray diffraction(XRD). The residual Al content of secondary combustion products was measured by chemical means. The results show that the content of residual aluminum increases with the increasing of temperature of the tube furnace first, and then decreases. It was thought to be stepwise oxidation that combustion of Mg in Mg-Al based propellant occurs first and the combustion of Al happens later.Secondly, to improve activity of Mg-Al alloy, high-energy ball-milling was used to change microscopic morphology of Mg-Al alloy and low melting point metal Bi was added in the process of high-energy ball-milling. X-ray diffraction(XRD), thermogravimetric analysis(TGA), and Scanning electron microscopy(SEM) were used to characterize the alloy powders. Spherical Mg-Al alloy powders were broken by the impact of high-speed rotating steel balls. The results show that high-energy ball milling is an efficient method for improving the activity of Mg-Al alloy, and low melting point metal Bi can further improve the activity of Mg-Al alloy.And then, the high activity Mg-Al alloy powders were applied to hydro-reactive metal fuel propellants to study their combustion process and mechanism. Fluoride was added in the experiment formula to combustion efficiency of propellants. The results show that the propellants added with milled Mg-Al exhibit better performance in terms of the Mg and Al reaction efficiencies than the Mg-Al system, and the reaction efficiencies of propellants of Mg-Al milled with Bi were further improved. Fluoride can also improve combustion of Mg-Al alloy in propellants.Finally, physical properties of hydro-reactive metal fuel propellants directly affect their combustion efficiency, performance in enlarge experiment, and safty of transportation and storage, thus density, porosity, needle penetration determination and compressive strength of propellants were tested. Density and compression strength of propellants added with milled Mg-Al were larger than that of propellants added with Mg-Al and similar with that of propellants added with Mg-Al milled with Bi, while porosity of propellants added with Mg-Al was similar with that of propellants added with Mg-Al milled with Bi and was larger than that of propellants added with milled Mg-Al. With increasing of fluoride’s content, density of propellants increased, porosity of propellants decreased, and compression strength of propellants increased first but decreased then. For lubrication of fluoride, it can be used to improve mechanical properties of hydro-reactive metal fuel propellants. |
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