Research On Steady-state Combustion Mechanism In High-metal Magnesium-based Fuel Water Ramjet Engines

The performance of water ramjet engines increase with the metal content of their hydro-reactive fuels. It is significant to develop a steady-state combustion mechanism of hydro-reactive metal fuels for performance enhancement and combustion sustainment of high-metal magnesium-based fuel water ramjet engines.Approaches of experimental study, theoretical analysis and numerical simulation were applied for research on the steady-state combustion mechanism of high-metal magnesium-based fuel water ramjet engines.An experimental method for combustion mechanism research was developed. An experimental system was built and a combustor with functions of observation and measurement was designed.Magnesium-based hydro-reactive fuels of Type 60 and Type 73, in which magnesium contents were 60% and 73% respectively, were adopted in the study. The thermal decomposition courses of both fuels were analysed with DSC and TG-DTA method. It was observed that the thermal decomposition of the fuels consist of decompositions of their components, and the maximum decomposition temperature of AP in the Type 73 fuel was higher than that in Type 60.The combustion experiments of the two fuels were carried out in Argon and steam atmospheres respectively. For the Type 60 fuel, results show that the burning surface temperature is higher than the melting point of magnesium, and the magnesium droplets enter the gas zone, along with the decomposed gas products of AP and HTPB. The flame brightness in steam is stronger than that in Argon, and the content of MgO in combustion products is also higher in steam. For the Type 73 fuel, the burning surface temperature is still higher than magnesium melting point, while the magnesium droplets do not leave the burning surface but adhere to the carbon framework of HTPB.A model was built to describe the combustion of the Type 60 fuel. Magnesium particles melt and ignite in the surface temperature. The gas-phase combustion near the burning surface includes AP premixed flame, AP/HTPB primary and final diffusion flame. The magnesium droplets react with the decomposed products of AP prior to the reaction with the steam. Under the working pressure, the thermal feedback to burning surface mainly comes from the reaction of magnesium and AP. It was found that working pressure, concentration of steam and atmosphere temperature have positive effect on the burning rate and surface temperature.A model describing the combustion of the Type 73 fuel was also established. The flame consists of AP premixed flame, AP/HTPB primary and final diffusion flame forms near the burning surface. Under the surface temperature, magnesium particles melt, evaporate and then react with the oxidization products of AP. With the moving of AP/HTPB/Mg flame, water steam diffuses into the burning area and reacts with the remained magnesium. Under the working pressure of water ramjet engines, thermal feedback to the burning surface mainly comes from the combustion of magnesium. The burning rate and surface temperature increase with the working pressure, concentration of steam and temperature of atmosphere.A numerical model was built to simulate the combustion and flow of high-metal magnesium-based fuel water ramjet engines. Influences of the moving of the burning surface, water/fuel ratio, injection angle, atomize diameter and injection velocity of primary water-jet on combustion characteristic were studied. The variation rules of surface temperature and burning rate were obtained. Methods for optimizing the combustion characteristic were put forward.