Combustion of Magnesium Wires with Oxygen and Water Vapor

The combustion of magnesium with water is an attractive power source because of the high combustion enthalpies and safety of the materials. For space propulsion, wire-shaped magnesium, which is easy to be fed to the combustion chamber, is considered to be used as a fuel. This study aims to obtain the combustion characteristics of magnesium wire with water for use in spacecraft and oxygen for comparison to combustion behavior. We used a 0.5 mm diameter wire with a length of 40 or 50 mm as fuel. An electric current heated the wire in an enclosed combustion chamber pressurized at 10-100kPa by the oxidizers. The wires continued to be heated until they broke, and then the behaviors were divided into cases where they self-combusted and cases where combustion did not occur. This result revealed the critical conditions of sustainable combustion, which is the same as the ignition temperature in thermal ignition theory. In the case of water vapor, the wire breakage temperature was lower than that in the oxygen case, suggesting that the gaseous products altered the oxide film of the wire. The spectroscopic measurements obtained several excitation spectra and the combustion temperature, which is calculated from a continuous spectrum. In the reaction with water, the measured combustion temperature was lower than the melting point of MgO. As a result, the oxides were deposited on the surface and formed a characteristic fibrous shape rather than dispersed.