Detailed Numerical Approach for Liquid Ammonia Spray Combustion Accounting for Flash Boiling

Ammonia has garnered significant attention as a carbon-free alternative to conventional fuels for power generation, offering potential solutions to global energy challenges. However, its combustion faces inherent difficulties, including low flammability and high nitrogen oxide (NOx) emissions. Traditionally, gaseous ammonia (GNH3) injection into combustion chambers results in heat losses and requires intricate combustor designs. Liquid ammonia (LNH3) injection eliminates these inefficiencies but introduces new complexities, particularly flash boiling during atomization, which significantly influences spray characteristics such as droplet size, velocity, and temperature distribution. A key novelty of this study is the detailed simulation of the primary atomization process without making prior assumptions about droplet size distribution, which is commonly represented by approximations such as the Rosin-Rammler distribution [1]. Instead, the first step of the proposed two-step numerical workflow employs the Volume of Fluid (VOF) multiphase mode with heat and mass transfer mechanism, coupled with a VOF-to-DPM (Discrete Phase Model) transition tool, to predict the size distribution during primary atomization. Adaptive Mesh Refinement (AMR) is utilized to refine the liquid-gas interface while maintaining computational efficiency. The resulting droplet data, including size, velocity and temperature distributions, is directly used in subsequent combustion simulations, ensuring higher fidelity in the modelling process. The second step focuses on droplet evaporation and combustion, incorporating a custom User-Defined Function (UDF) to capture the interplay of heat and mass transfer during flash boiling. The workflow is validated against experimental measurements for LNH3 injection, demonstrating excellent agreement and robust predictive capabilities. Finally, the workflow is applied to simulate LNH3 spray combustion cofired with gaseous hydrogen (GH2) in a swirl combustor under high-pressure conditions (0.3 MPa). The combustion model integrates Large Eddy Simulation (LES) with the WALE subgrid model and a detailed Finite Rate Chemistry mechanism featuring 25 species and 175 reactions. Preheated airflow at 600 K and an equivalence ratio of 1.29 are used to stabilize the flame. This study introduces a novel approach to LNH3 spray combustion modelling by directly predicting primary atomization characteristics, avoiding conventional assumptions on droplet size distribution. The validated workflow provides critical insights for advancing ammonia as a sustainable fuel in practical gas turbine applications, bridging the gap between experimental observations and combustion simulations.