Liquid ammonia in-nozzle flow: Effects of aspect ratio and surface roughness on the flash boiling

Liquid ammonia, a promising carbon-free fuel with a significantly high saturation pressure, frequently exhibits flash boiling during its spray processes. The nozzle configuration plays a crucial role in flash boiling by altering the in-nozzle flow states. To gain deeper insight into the underlying physics, this study conducts numerical simulations of liquid ammonia in-nozzle flow. Six cases are examined to assess the effects of nozzle aspect ratio and surface roughness, including L/D = 2.5, 5, 10 and k-type rib roughness. The distributions of liquid volume fraction α and pressure p reveal that the enhancing effects of high aspect ratios on flash boiling predominantly manifest in rough nozzles due to the linear loss of pressure. However, a non-linear behavior emerges from the competing effects of cavitation and flash boiling, resulting in the lowest α at the nozzle outlet for L/D = 5 rather than L/D = 10 condition. Additionally, the observed drop in local temperature and saturation pressure indicates that, in smooth nozzles, this enhancing effect is offset by the cooling effect caused by the large latent heat of liquid ammonia. Regarding surface roughness, this study finds that roughness-induced turbulent flow generates eddies of various scales. These eddies detach and convect into the central flow, promoting bubble development through energy dissipation. Furthermore, surface roughness increases both the local velocity and velocity fluctuations at the nozzle outlet. As a preliminary study, the results are anticipated to guide the practical applications of liquid ammonia injection.