The effect of randomly-distributed droplets on thermal radiation of surfaces

A general method for simulating the thermal radiation characteristics of surfaces with randomly-distributed droplets is established to investigate the effect of droplets on the equivalent emittance of surfaces. This approach includes the generation of randomly-distributed water droplets, the estimation of the droplet influence on the surface apparent radiative properties, and the thermal radiance model. The randomly-distributed water droplets are generated by the developed FFT (Fast Fourier Transform) method while the estimation of droplet influence on the radiative properties of a surface is based on the geometric optics approximation, which is validated by experimental measurement. Meanwhile, the self-emitted radiation, solar radiation, sky radiation, and other background radiations are considered in the thermal radiance model to analyze the thermal radiation characteristics of the surfaces with randomly-distributed droplets. The results show that the absolute error of the simulated equivalent temperature of the surfaces with randomly-distributed droplets lies within 1.6 K compared to the experimental data, which proves the reliability of the radiance model. In addition, the effect of droplets on the surface equivalent emittance is analyzed. The results indicate that the water droplet coverage fraction has a greater impact than the water droplet mean height. Furthermore, the equivalent emittance almost linearly increases with the increment of the coverage fraction of water droplets.