Observation of the Ice-Making Process by Radiative Cooling Part 2: Investigation of the Mechanism of Bubble Column Formation and Its Suppression Measures

Abstract

Transparent ice without cloudiness caused by air bubbles is considered a high-quality ice and has high commercial value in the beverage industry. However, they cannot be easily obtained using the existing technologies. When necessary, only the transparent portions were removed from large ice blocks. Upon observing ice produced using the radiative ice-making technology developed by Kamimura et al., two types of bubbles were identified: “microbubbles” that form near the freezing surface in the early stage of the freezing process and “bubble columns” that start forming from specific locations during the ice formation process. This study focuses on bubble columns to understand their formation mechanisms and explore suppression methods. It was found that as ice grew, the concentration of dissolved gases in the water increased, and bubble columns appeared when it reached saturation. Based on this, a theoretical formula was derived to estimate the initial bubble formation location as a function of the initial dissolved-gas concentration. Using oxygen as a representative dissolved gas, experiments were conducted by varying the initial dissolved oxygen concentration in the source water and the cooling conditions. The results indicate that the trend of the observed initial bubble formation locations closely matched the predicted values from the theoretical formula, where the predicted value was greater than 40 mm. Furthermore, experiments were conducted by changing the growth rate by varying the radiative-cooling surface temperature. It was observed that under conditions of high growth rate, the discrepancies between the predicted and actual measurements increased.