A review of micro-scale trapped air bubble growth distribution characteristics and thermal mechanical effects in ice

Icing is a nonlinear, variable density liquid-solid phase change process coupled with thermal and mass transfer and flow, which occurs widely in nature and industry, often with negative consequences. Trace amounts of air dissolved in water converge and nucleate into larger bubbles during icing due to the extrusion of ice crystals. These bubbles then remain at the freezing front due to adhesion, ultimately forming microscale trapped air bubbles of varying sizes and distributions in ice. The formation of micro-scale trapped air bubbles in the icing process not only affects the later dynamic icing process by changing the internal structure, density distribution, thermal conductivity and freezing rate of ice, but also affects the overall thermal conductivity, thermal resistance distribution, compressive strength, stress distribution and other macroscopic thermal and mechanical properties of the ice body after the icing process concludes. To accurately predict and control the icing process, as well as to develop and optimize various types of anti-icing technologies, the study of the growth and distribution characteristics of microscale trapped air bubbles and the macro-thermal effects has attracted much attention in both academia and industry. Firstly, this paper takes micro-scale trapped air bubbles in ice as the research object, and reviews their nucleation mechanism, growth process, distribution characteristics and static stability from the micro and macro scales. The results show that the bubble shape is directly related to the freezing rate, and when the freezing rate exceeds 25 μm / s, egg-shaped trapped air bubbles with a length-to-width ratio smaller than 5 appear in ice. When the freezing rate is between 5 and 25 μm / s, needle-shaped trapped air bubbles with a length-to-width ratio larger than 5 appear in ice. No bubbles can be found in ice when the ice freezing rate is below 3 μm / s. Secondly, by reviewing and analyzing existing literature, the influencing factors of the whole life cycle of trapped air bubbles and their different influencing mechanisms on the thermal and mechanical characteristics during icing and after ice formation are summarized and explained. Trapped air bubbles in ice significantly reduce the effective thermal conductivity of the ice by lowering its the density and changing the internal ice crystal structure. In ice melting experiments, ice with a bubble volume fraction of 57% exhibits a delay of approximately 50% in the starting time of melting compared to clear ice without bubbles. Additionally, the ice with bubbles has a 36. 81% lower meting height within the same time frame. With the increase of bubble volume fraction, both the horizontal and vertical compressive strength of the ice decrease gradually. When the bubble volume fraction increases from 4% to 34%, the horizontal and vertical compressive strengths decrease to 8. 38% and 8. 10% of the original values, respectively. Finally, based on existing research on trapped air bubbles, the current research gaps and development trends are predicted and elaborated. This review is helpful for clarifying the complex characteristics of trapped air bubbles and enriching the mass transfer theory of icing process. It can also provide references and insights for the optimal design of existing anti-deicing technologies. © 2024 Harbin Institute of Technology. All rights reserved.