Spray and evaporation characteristics of high-pressure liquid ammonia injection under flash-boiling and evaporating conditions

• Spray and evaporation of liquid ammonia under flash-boiling and evaporating conditions. • The POD method is used to verify the repeatability and reliability of the test. • Large-nozzle liquid ammonia spray is more likely to form flash-boiling bubbles. • A post-supercooling residual region exists in the liquid ammonia spray head. • Liquid ammonia has greater axial penetration than diesel while less radial penetration. Ammonia, as an efficient hydrogen carrier, is considered a promising zero-carbon alternative fuel for energy systems. High-pressure liquid ammonia injection enables efficient and clean utilization of ammonia in energy devices. This study systematically investigates the spray and evaporation characteristics of high-pressure liquid ammonia spray with two different nozzle diameters (0.1 mm and 0.3 mm) over wide operating conditions by combining high-speed DBI and shadowgraph techniques. The results indicate that the expansion rate of the small-nozzle spray is greater than that of the large-nozzle spray at low superheat (1.4 ∼ 2). As the superheat exceeds 2, the spray expands significantly radially, with a larger expansion rate for the large-nozzle spray, suggesting the large-nozzle spray is more prone to forming flash-boiling bubbles. Under high temperature and pressure conditions with fixed ambient pressure, the liquid penetration increases and then decreases as the temperature increases, and the inflection point temperature of the large-nozzle spray is higher. Compared with diesel, the axial diffusion of the liquid ammonia is larger while the radial diffusion is smaller. There is a post-supercooling residual region affected by the pilot-evaporated ammonia in the capturing and recirculation regions, where the fuel cannot completely evaporate like the diesel. This characteristic should be considered when designing the liquid ammonia spray system.