Question

传热学一个问题

It is known that on clear nights a thin layer of water on the ground will freeze before the air temperature drops below 0℃. Consider such a layer of water on a clear night for which the effective sky temperature is -30℃ and the convetion heat transfer coefficient due to wind motion is h=25 W/m2^k. The water maybe assumed to have an emissivity(发射率) of 1.0 and to be insulated from the ground(no conduction)（和地面之间没有热量交换）. Neglecting evaporation（忽略蒸发）, determine the lowest temperature taht air can have without the water freezing.

Answer 1

我们都知道，在明朗的夜晚，温度降到零度以下，地上的水面就会结一层薄冰。考虑下，天空有效温度为-30摄氏度的明朗夜晚，由于风的影响，水面对流传热系数是25 W/m2^k。这时水可以被认为发射率是1，并且其与地面隔绝（不导热）。忽略蒸发，确定水不结冰情况下，空气所能达到的最低温度。

求出 辐射放热量 还有 对流换热量。相加即可