A numerical investigation into the in-nozzle flow and near-field structure of flash-boiling ammonia sprays

Ammonia is a zero-carbon fuel candidate, and flash-boiling atomisation is an enabling technology for efficient combustion. While flash boiling produces complex spray structures, physical mechanisms governing the ”flare-flashing” regime remain poorly understood. This study aims to identify the mechanism controlling spray structure and morphology in flare-flashing ammonia jets. Specifically, it investigates the role of compressibility and shock wave formation, which have not been previously identified as control elements. A two-dimensional simulation was established, coupled with the Homogeneous Relaxation Model (HRM) to simulate a non-equilibrium phase transition. The model was used to analyse the influence of nozzle geometry (L/D ratio, diameter) and operating conditions (pressure ratio, injection pressure) on in-nozzle flow and near-field spray development. The results revealed that the flow remains subsonic at the nozzle exit and is not choked at the orifice. It is ‘choked’ by a stationary, two-phase shock wave that forms in the near-field region downstream of the nozzle. This shock induces deceleration at the spray's core, creating a velocity deceleration to the periphery, which drives a recirculating vortex, providing a physical explanation for the “feather-shaped” morphology observed in the experiments. It is concluded that the downstream shock wave is a mechanism controlling the structure and mass flow of flare-flashing sprays. This finding establishes a paradigm for injector design, offering a pathway to optimise fuel-air mixing.