Modeling of ammonia under-expanded jets for application in advanced propulsion systems

Abstract

The adoption of ammonia as a fuel is gaining significance in various applications, including propulsion systems, gas turbines, and burners. In particular, the combustion of liquid ammonia presents an attractive option due to its low cost and its capacity to reduce local temperatures, thereby limiting thermal NOx emissions. In this context, investigating ammonia injection processes is relevant for optimizing and improving ammonia-fueled combustion systems. Thus, this work presents a combined experimental and numerical analysis of ammonia jets for applications in advanced propulsion systems, where the multiphase flow resulting from ammonia injection was investigated using a purposely developed CFD code characterized by low numerical dissipation and the adoption of real-fluid properties. The simulation methodology is validated against experiments performed at the Université d’Orléans, including optical measurements based on Schlieren and Diffuse Back-Illumination (DBI) extinction techniques, as well as local temperature measurements. This study provides insights into jet morphology and offers a quantitative assessment of parameters — including liquid penetration and local temperature — that are crucial for advancing the design of ammonia combustion systems. The outcomes offer an enhanced representation of jet structure, highlighting the presence of under-expanded jets and Mach discs when ammonia is injected at high pressures. They also demonstrate that under-expanded shock structures are strongly dependent on the temperature of liquid fuel injection