To investigate the melting characteristics and shape evolution of non-spherical ice crystals in hot airflow environment, a melting model for non-spherical ice crystals was developed based on a decoupling strategy for flow and phase-change heat transfer, enabling a solution to the complex heat transfer problem involving gas-liquid-solid coupling. The results demonstrate that the proposed model accurately predicts the shape evolution process during ice crystal melting, and the predicted melting time agrees well with experimental results, with a deviation of ±15%. The shape evolution of non-spherical ice crystals undergoes three distinct stages: an initial warming stage where the shape remains stable, a partial water coverage stage where liquid water partially covers the ice core surface, and a complete water coverage stage where the ice core is fully enveloped. The initial aspect ratio is identified as the key factor governing the shape evolution during melting. A larger initial aspect ratio results in a longer time required for the ice crystal to evolve into a spherical shape. Furthermore, an empirical correlation between dimensionless sphericity and the melting ratio was established, overcoming the limitations of the traditional linear approximation model for high-aspect-ratio (initial aspect ratio λ > 2) ice crystals and significantly improving computational accuracy.

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.