Liquid Nitrogen Spray Characterization in Ambient and Development of Correlations for Droplet Diameter and Velocity

Sprays are among the most complex fluid systems to model and design. This study presents detailed measurements of droplet diameter and velocity for liquid nitrogen (LN2) sprays discharged into ambient air. Using a high-resolution Phase Doppler Particle Analyzer (PDPA), both mean values and statistical distributions of droplet size and velocity were obtained. Five full-cone spray nozzles were tested across a range of injection pressures. PDPA data were used to develop new correlations for Sauter Mean Diameter (d32), Arithmetic Mean Diameter (d10), and Mean Droplet Velocity (uf). Results show that increasing injection pressure leads to higher uf but lower d32 values. Evaluation of existing d32 correlations—primarily developed for water sprays—revealed significant predictive errors when applied to LN2 data. To address this, new dimensionless correlations were derived for d32, d10, and uf, incorporating Reynolds and Weber numbers. These correlations demonstrated strong predictive performance when validated against the experimental data. The spray cone angle was found to be relatively insensitive to injection pressure but consistently smaller than the manufacturer’s specified angle for water sprays. This reduction in cone angle is attributed not only to differences in thermophysical properties—such as viscosity, surface tension, and latent heat of vaporization—but also to ambient heat transfer causing evaporation at the spray’s periphery, where droplet density is lowest. The newly developed LN2 correlations provide valuable predictive tools essential for the design and optimization of cryogenic spray systems, particularly in future space applications.