Effects of boundary conditions on flash boiling ammonia sprays from a twin-fluid atomizer

Ammonia is a carbon-free fuel with a high hydrogen capacity (17.8 wt%) and well-established production and distribution infrastructures, making it a viable option for decarbonized and dispatchable power. In addition, ammonia combustion in its liquid phase is preferred for simplified storage, higher energy density, and compatibility with high-pressure industrial conditions. However, ammonia fuel has drawbacks that must be addressed. This work aims to investigate experimentally the effect of the boundary conditions on the dynamics of an ammonia spray injected by a commercial twin-fluid atomizer in a chamber at room pressure and temperature. A flash boiling analysis was conducted to assess the thermodynamic conditions that promote this phenomenon and the accuracy of existing correlations to predict this transition to flash boiling. The result showed that low injection pressures and preheated atomizing flow promote the formation of bubbles and the subsequent collapse inside the atomizer, creating an unstable spray. Such instabilities are significantly reduced when the injection pressure is increased. Additionally, increasing the atomizing air flow, injection pressure, and atomizing flow temperature improves the fuel breakup, widening the spray angle. However, the spray angle decreases at a certain point, indicating that atomization is more sensitive to thermodynamic conditions than a mechanical breakup.