Investigations On Ignition And Combustion Process Of A Magnesium Particle In Water Vapor

Developments of water ramjet are being promoted by their application in advancedunderwater propulsion systems of high-speed torpedoes. At present, domestic researchesare mainly focused on high-metal magnesium-based fuel water ramjet. The performanceof the engine increases with the metal content in the fuel, but the combustionperformance of magnesium and water vapor, during the work process, determines, to alarge extent, that of the engine. Therefore, studies on the ignition and combustioncharacteristics in water vapor are critical for the investigations on both combustionoptimization methods and sustained and stable combustion.Measurement parameters during the ignition and combustion process of amagnesium particle in water vapor were determined and relevant measuring methodswere designed based on the summary of magnesium particle combustion in variousoxidizing gases. Particle temperature variation during ignition process was obtained bymicro thermocouples and flame temperature was achieved with an infrared thermometer.Particle surface morphology was recorded by camera and flame characteristics werecaptured with an additional optical filter and a neutral optical attenuator.Both theoretical and experimental studies on the ignition process of a magnesiumparticle in water vapor were conducted. Gas-solid reaction theory was adopted toanalyze the macro-process of oxidation reaction on magnesium particle surface anddescribe oxide layer thickness increasing law. Oxide layer cracking, surface morphologychanging and spark flashing were interpreted combined experimental phenomena withelastic mechanics. On the basis of oxide layer thickness increasing law and energyequilibrium, ignition model of a magnesium particle in water vapor was establishedwith lumped parameter method. In the ignition experiments of a magnesium particle,ignition temperature was determined and ignition mechanism was concludedpreliminarily.The combustion process of a magnesium particle in water vapor was studied boththeoretically and experimentally. The combustion model of a magnesium particle inwater vapor, in which the condensation of gas-phase magnesia on particle surface wasincluded and its influences on the combustion process were simply discussed, wasestablished. Combustion experiments were conducted. The condensation of gas-phasemagnesia on particle surface was confirmed and gas-phase combustion flamecharacteristics were observed. With the combustion model and experimental conditions,simulation of the combustion process was accomplished and the results were consistentwith experimental data.The influences of gas-phase magnesia condensation on particle surface andenvironmental factors on the combustion process of a magnesium particle in water vapor were briefly analyzed with numerical calculations. The results implicated thatgas-phase magnesia condensation on particle surface had different effects onmagnesium evaporation rate, particle temperature, flame temperature and flame radius,while environment temperature and pressure and water vapor concentration hadinfluences on magnesium evaporation rate, particle temperature and flame temperatureto varying degrees, and hardly affected flame radius.