Frost microstructure during the frost layer growth

Frost formation on cold surfaces significantly impacts heat exchanger performance by increasing thermal resistance and reducing airflow efficiency. Understanding the microstructure of the frost layer during its growth phase is essential for predicting and mitigating its effects. This study investigates the frost layer formation process using X-ray µCT at SPring-8. Experiments were conducted on a 6 mm-diameter aluminum cooling surface, capturing the frost microstructure every two minutes over a 90 minute period. In the early stage of the frost formation, the crystal morphology was influenced by the size of ice droplets: larger droplets predominantly generated columnar crystals, while smaller droplets formed plate-like crystals. During the frost growth stage, the frost layer developed into three distinct layers: an ice droplet layer, an inner layer composed of plate-like crystals, and ice crystals in outer layer connected through columnar crystals. The heat transfer load in the outer layer caused the sublimation of columnar crystals connecting the outer layer to the ice droplet layer, thereby altering the frost layer structure. The frost density distribution in the thickness direction was evaluated. The density increase in the outer layer was related to frost layer growth, while that in the inner layer contributed to an increase in frost density. The respective mass fluxes were successfully quantified.