Study on the impact of the atomized water droplet size on the performance of magnesium-based water ramjet engine

Due to the increasing demand for high-speed and long voyage propulsion systems for underwater vehicles, water ramjet engines (WREs) have been widely studied as they have significant advantages. This study develops a numerical model of magnesium-based WREs with internal flow field atomization combustion. This model is built based on the DPM atomization model and the combustion surface heat and mass transfer model. The engine performance for eight different atomized water droplets diameters in the range of 5e-3-1e-6 m is then analyzed. The obtained results show that the engine performance highly depends on the droplet size. Very large large droplets (e.g., 5e-3 m) cause localized water accumulation in the combustion chamber, which results in flameout. On the other hand, very small droplets (e.g., 1e-6 m) rapidly evaporate. However, this compromises the mixing effectiveness. Droplets of 5e-4 m diameter have high initial kinetic energy. They can penetrate the central combustion zone and mix thoroughly with high-temperature fuel gas, which results in significantly increasing the combustion chamber temperature. However, the increase of the droplet size (e.g., 1e-3 m) consumes more heat for vaporization, limiting the temperature increase. Smaller droplets of 5e-6 m diameter rapidly evaporate into gaseous working fluid, which significantly enhances the engine thrust and pressure. However, very small droplets reduce the thrust and the pressure due to decreased mixing effectiveness. In summary, a droplet diameter of 5e-4 m is optimal for the combustion efficiency and equilibrium temperature, while that of 5e-6 m is the most convenient for maximizing the thrust and pressure.