Asymmetric growth and fractal evolution of frost crystals on a single frozen droplet under forced convection

Frost accumulation severely restricts the safe and efficient operation of equipment. Frost layers fundamentally comprise crystals growing on frozen droplets; thus, clarifying these early-stage characteristics is crucial for revealing macroscale frosting mechanisms. This study investigates the effects of wind velocity, cold surface temperature, and droplet volume on crystal growth characteristics on a single frozen droplet under forced convection. Results indicate that compared to natural convection, forced convection yields shorter maximum crystal heights but significantly larger mean radial lengths (e.g., 2.78 mm at 1.62 m/s versus 2.25 mm at 0 m/s) and equivalent radii. Lower temperatures promote all growth parameters, while droplet volume primarily influences local surface temperature and leeward water vapor transport. Crucially, forced airflow not only accelerates vapor transport but promotes the reverse melting of tip frost crystals, fundamentally reshaping the morphology. Fractal analysis quantifies this evolution. The fractal dimension decreases with increasing wind velocity, from ∼1.25 under natural convection to 1.05–1.10 at 2.95 m/s, reflecting a smoother profile induced by enhanced reverse melting. Finally, based on the fractal dimension, a symmetric growth mode under natural convection and a distinct asymmetric growth mode under forced convection are proposed. The findings provide droplet-scale theoretical support for modifying macroscale frost layer porosity and density prediction models.