山西省宁武县万年冰洞持续存在机制数值模拟

 在年平均气温高于0℃的自然条件下存在少量常年保存有冰体的洞穴,这是一种十分罕见的现象.山西宁武“万年冰洞”是国内已发现的规模最大的冰洞,如何合理解释其内部冰体的形成和保存机制是一个科学难题.本文通过有限元暂态数值计算,模拟冰洞四季热传递过程认识到,冰体得以持续存在的控制因素有2个：(1) 冬季外部空气比冰洞内空气冷,外部冷空气通过冰洞口,与冰洞内空气发生自然热对流,使冰洞内部冷却;春、夏、秋季外部空气比冰洞内热,冷空气沉在洞底,效率很低的热传导不足以使冰洞内温度显著上升.(2) 由于冰-水相变潜热作用,即使夏季靠近洞口部分冰体融化,也需吸收大量热量,从而进一步阻止冰洞内部冰体的融化.计算结果也显示,如果一些冰洞开发为游览景点后在洞口设置密封门,切断冬季冷空气进入洞内自然对流冷却洞体的通道,冰洞内冰体在不到40年时间内将全部融化,“万年冰洞”将被人为破坏.

Numerical Simulation of Ningwu Ice Cave Preserved for Thousands of Years in Shanxi Province

Some ice caves exist in places where the yearly average ambient temperature is higher than zero centigrade. This phenomenon is quite rare. Ningwu Ice Cave in Shanxi Province is the largest ice cave of this kind in China. To explain the mechanism how the Ice Cave is preserved is a difficult scientific problem. In this paper, using the FEM (Finite Element Method) transient numerical calculation, the process of the heat transfer in the Ice Cave is simulated. We consider two controlling factors. Firstly, the external temperature is higher than the internal temperature of the Ice Cave in spring, summer or autumn. The air density increases in the gravity direction. So, there would be no convection in the Ningwu Ice Cave and the energy is transferred from the outside world to the Ice Cave by conduction. But in winter, the external temperature is lower than the internal temperature of the Ice Cave. The air density decreases in the gravity direction. Thus, the energy is transferred from the Ice Cave to the outside environment by convection. Secondly, the ice-water phase change prevents the external heat from being transferred to the internal Ice Cave. As the thermal convection is much more efficient than the thermal conduction, the heat inflow is much less than the outflow. The calculation results show that: (1) At a quasi steady state, the internal temperature of the Ice Cave is at the range of -3.9℃ to -2.9℃. That means that the ice body in the Ice Cave can be preserved. (2) If a sealed door is set at the Ice Cave entrance, the ice in the cave will melt within 40 years, because a sealed door can cut off the cold air into the Ice Cave in winter.