Temperature measurement on condensed combustion products of magnesium with oxygen and water vapor

This study investigated the behavior and temperature of condensed combustion products (CCPs) in a magnesium-water vapor flame. The experiments were conducted using magnesium plates with a surface area of 5 x 5 mm2 and an oxidizer pressure in the range of 10-100 kPa, in consideration of potential applications in microspacecraft. Compared with the air and oxygen cases, a substantial quantity of white residue remained around the fuel when the oxidizer was water vapor. Scanning electron microscopy revealed that this was deposited by submicron CCPs formed in the flame. Additionally, the laser light extinction method revealed the spatial distribution of the CCP. In the case of water vapor, the point of maximum CCP density was closer to the fuel than to the oxygen, which caused a difference in residue formation. Next, the temperature of the CCP in the combustion flame was calculated using Planck's law with a spectrometer. This study utilized six types of emissivity indices, ranging from 0 to -4, for power fitting. A more plausible temperature range was determined using the coefficient of determination, leading to an uncertainty of 20% in the temperature calculation using Planck's law. The observed temperature was approximately 600 K lower for water vapor than for oxygen, a discrepancy exceeding the difference predicted by the adiabatic flame temperatures. The two-dimensional temperature distribution was measured with CCD cameras using the two-color pyrometer method, and the distance to the point of maximum reaction temperature was shorter for water vapor than for oxygen. This study concluded that the decrease in the reaction distance in the water vapor case owing to the lower temperature significantly influenced the spatial distribution of CCPs within the flame. Novelty and significance statement: The present study is novel in that the condensed combustion products in a magnesium flame were identified, and their temperature was measured distinctly from the fuel. These observations explain why the oxide residue accumulated only in water vapor cases. Our research stands out, recognizing the limited experimental forays into magnesium combustion with pure water vapor. The combustion research on these materials, which are safe and obtainable from space bodies, has profound implications for advancing space propulsion and energy.