Influence of liquid nitrogen droplet fragmentation and deformation on tracing in cryogenic wind tunnels

Cryogenic wind tunnels (CWT) replicate high-Reynolds-number environments by cooling the medium with liquid nitrogen, realizing critical research on aircraft aerodynamics under extreme conditions. The application of nitrogen droplets as tracer particles for noninvasive measurements in CWT shows significant potential based on prior research. Furthermore, nitrogen droplets should remain in a steady shape to ensure accurate measurements. However, the droplet fragmentation and deformation characteristics during tracing in CWT remain to be investigated, and the analysis regarding the shape stability of droplet tracing is lacking. Accordingly, direct numerical simulation was conducted to study the fragmentation and deformation of nitrogen droplets as tracer particles in CWT. The dual Gaussian distribution effectively characterizes post-breakup droplet size, resulting in a predictive formula for size distributions across varying Weber numbers (We). A critical threshold has been established, with a deformation coefficient remaining below 2 for We up to 17.8, which ensures effective tracing with minimal distortion. Even at We values reaching 23.2, transient deformation has negligible effects on accuracy. Additionally, droplets with an initial diameter below 1.3 μm maintain their structural integrity and consistent performance as tracers, while those exceeding 22 μm break apart into smaller droplets above 1 μm, which remain suitable for tracing applications. These findings provide insights into optimizing the use of nitrogen droplets for more accurate flow measurements in cryogenic conditions.