Thermophysical properties and convective heat transfer characteristic of alcohol-salt aqueous refrigerant in deep cryogenic artificial ground freezing

To address the low cooling capacity, slow freezing rate, and poor freezing effect of CaCl2 brine (> -30 ℃) used in conventional artificial ground freezing, an alcohol-salt aqueous refrigerant capable of achieving deep refrigeration in the range of -50 ~ -60 ℃ was developed. The refrigerant exhibited a freezing point as low as -65.8 ℃. When the refrigerant temperature reached -65 ℃, its viscosity, thermal conductivity, and specific heat capacity were 206.8 mPa·s, 1.359 kJ/(m·h·C), and 2.584 kJ/(kg·℃), respectively. Numerical analysis indicated that, at a flow velocity of 0.5 m/s, the freezing efficiency of -60 ℃ alcohol-salt aqueous refrigerant significantly outperforms -30 ℃ CaCl2 brine. After 12 h, the average soil temperature decreased by 8.5 ℃, and the frozen radius increased by 66.9%. Additionally, increasing the refrigerant flow velocity enhanced the heat transfer performance of the freezing pipe. As the velocity rose from 0.5 m/s to 1.5 m/s, the thickness of the thermal boundary layer decreased by 36.3%, and the convective heat transfer coefficient increased by 50.0%. The refrigerant developed in this study exhibits stable cooling transport capacity, providing a novel refrigerant material and a new pathway for deep cryogenic artificial ground freezing in water-rich strata.