温度与尺寸效应对冷却辐射膜热学性能的影响

Influence of temperature and size effect on thermal properties of radiative cooling film

冷却辐射膜是一种零能耗、零排放的辐射制冷薄膜,但尺寸效应和温度对其传热性能的影响机理尚不明晰。在动力学理论的基础上,采用数值计算方法提出金属平均自由程的计算公式,并通过引入温度变量改进宏观热输运模型;基于不可逆热力学计算微观热导率,对比分析修正后的宏观热导率,并利用玻尔兹曼声子散射理论阐释温度与尺度效应对热导率的影响机理。结果表明:宏观、微观状态下薄膜热导率随温度的变化趋势是相反的;随着温度的升高,微观热导率增大,出现明显尺寸效应的临界尺寸减小;当温度为40 K时,其临界尺寸约为50μm,而当温度为300 K时,其临界尺寸约为1μm,因此适当调控微纳结构尺寸和环境温度可以有效地提升冷却辐射膜的热学性能。

Radiative cooling film is a kind of film with zero energy consumption and zero emission,but the influence mechanism of size effect and temperature on its heat transfer performance is not clear.Based on the kinetic theory,proposes the calculation formula of metal mean free path by numerical calculation method,and modifies the macroscopic heat transport model by introducing the temperature variables.This paper also calculates microscopic thermal conductivity based on the irreversible thermodynamics,and compares it with the modified macroscopic thermal conductivity.Furthermore,this paper also elucidates the influence mechanism of temperature and size effect on the thermal conductivity based on the Boltzmann phonon scattering theory.The results show that the variation of thermal conductivity of the film with temperature is opposite between macroscopic and microscopic states.With the increase of temperature,the microscopic thermal conductivity increases,and the critical size with obvious size effect decreases;the critical size is about 50μm at the temperature 40 K and about 1μm at 300 K.Therefore,the thermal performance of the radiative cooling film can be effectively improved by properly adjusting the size of the micro-nano structure and the ambient temperature.