Single ammonia droplet combustion in a high-pressure environment in microgravity

Since ammonia is a carbon neutral fuel, it is expected to be widely utilized in the near future. Although there have been many researches on ammonia combustion, no research has been conducted on ammonia droplet combustion, which is a fundamental research on spray combustion, because ammonia is in a gaseous state at room temperature and atmospheric pressure. This study investigated the combustion characteristics of single ammonia droplets in microgravity for the first time. Single ammonia droplets were formed in high pressure air and were successfully ignited in microgravity. In an early stage of burning, the droplet vaporized with an almost constant vaporization rate of 0.87–1.0 mm/s. After such a quasi-steady period, the vaporization rate decreased over time. In accordance with the decrease in the vaporization rate, the flame standoff ratio also decreased over time. After that, the droplet exhibited unique phenomena such as droplet disruption, puffing, or droplet re-expansion although the fuel was initially pure. These types of unique behavior are caused by the diffusion of water vapor, a combustion product, to the droplet surface, and dissolution and accumulation of water on the droplet surface. Since water has a lower volatility than ammonia, the concentration of dissolved water at the droplet surface increases rapidly as the droplet diameter decreases. As a result, the vaporization of ammonia is suppressed and the flame standoff ratio decreases. Additionally, as the water concentration at the droplet surface increases, the boiling point at the droplet surface also increases, resulting in superheating of the liquid ammonia and homogeneous bubble nucleation. The growth of bubbles caused droplet disruption, puffing, and droplet re-expansion.