Wall-impact dynamics of liquid nitrogen droplets in cryogenic wind tunnels

The impact of liquid nitrogen droplets on superheated wall is a fundamental phenomenon in liquid nitrogen spray cooling of cryogenic wind tunnels. The impact characteristics of droplets affect the advancement of liquid nitrogen spray field and the cooling performance of gas flow. In this study, a visualization experimental platform was designed and established to explore the wall-impact of a single liquid nitrogen droplet with controllable size and impact velocity. Various dynamic behaviors and transition criteria of liquid nitrogen droplets impacting superheated wall were obtained. In different boiling modes, the effect of Weber number (We) on the maximum spreading coefficient of liquid nitrogen droplets impacting wall was investigated. During the generation of liquid nitrogen droplets, surface tension worked against gravity, resulting in three stages of accumulation, necking, and breakup. As the wall temperature increased, the droplets successively exhibited contact boiling, atomization boiling and film boiling upon impact. The two corresponding critical temperatures were not affected by the We. The droplet spreading process involved the conversion of impact kinetic energy to surface energy. With an increase in We, droplets transitioned from non-splashing to splashing during the spreading process, and the corresponding critical We was not affected by the wall temperature. Furthermore, the spreading characteristics of droplets were associated with the Leidenfrost temperature. Below the Leidenfrost temperature, droplets spread on the wall, and an increase in the wall temperature led to intensified boiling bubbles, causing the maximum spreading coefficient to decrease. Above the Leidenfrost temperature, droplets spread on a vapor film, and the maximum spreading coefficient was independent of the wall temperature. This study deepened the understanding of the dynamic characteristics of liquid nitrogen droplets impacting superheated surfaces, and provided a theoretical basis for improving the spray cooling performance of liquid nitrogen in cryogenic wind tunnels.