Ammonia Flashing

Description

This project presents a numerical simulation of the ammonia flashing that occurs when liquid ammonia is injected through a small orifice nozzle. Flash boiling is a rapid evaporation process triggered by a sudden pressure drop, which leaves the liquid in an unstable state and drives a very fast phase change. Using the VOF multiphase model in ANSYS Fluent, the interaction between the liquid ammonia, its vapor, and the surrounding gas is resolved in time. The mass transfer between the liquid and vapor phases is the heart of the study: the simulation captures the jet breakup, the liquid-to-vapor phase transition, and the subsequent development of the two-phase ammonia flow within the domain.

Geometry & Mesh

The geometry was created in ANSYS SpaceClaim as a 2D symmetric domain. The lower horizontal line acts as a symmetry boundary, so only half of the physical domain is modeled to reduce computational cost. The geometry consists of an inlet nozzle section and a larger downstream chamber where the flashing and mixing take place, with the inlet and the two outlets clearly defined on the schematic to specify the boundary conditions. Meshing was performed in ANSYS Meshing, producing approximately 95,000 high-quality structured elements to ensure the numerical stability and accuracy of the simulation.

Methodology

The simulation employs a transient, incompressible, pressure-based solver to resolve the unsteady behavior of the flow field. The k-ω SST turbulence model is used to accurately capture boundary-layer separation and the unstable vortical structures in the ammonia jet. For the multiphase treatment, the VOF model tracks the interfaces between air, liquid ammonia, and ammonia vapor. Crucially, the Lee evaporation-condensation model is coupled with VOF to represent the mass transfer — the phase change from liquid ammonia to ammonia vapor — within the flashing region.

Conclusion

The results show the injected liquid ammonia jet expanding into the larger chamber and rapidly converting into a two-phase mixture. The volume fraction contours clearly reveal the formation of a liquid film near the bottom wall, with the ammonia vapor fraction increasing downstream as the flashing and evaporation intensify. The velocity field shows a high-speed jet along the lower boundary and a large recirculation zone in the upper chamber, indicating strong mixing between the phases. The temperature contours confirm cooling in the liquid-rich region (around 270 K) and a warmer, vapor-dominated layer (approaching 300 K) — a direct consequence of the energy consumed during the phase change. Together, these results demonstrate how coupling the Lee model with VOF captures the mass transfer that governs ammonia flash boiling, providing a detailed picture of the evaporation-driven two-phase flow.