Microstructure of Ice Accretions Grown on Aluminum Conductors

Abstract

In order to study the microstructure of glaze and rime deposits formed on the conductors of power lines, ice accretions are grown on a slowly rotating aluminum cylinder placed in the working section of a wind tunnel. The growth conditions cover both dry and wet regimes in the air temperature interval between ?2 and ?15°C and are close to those commonly prevailing in natural icing storms near the ground: liquid water contents vary from 0.4 to 0.8 g m?3 and wind speed from 4 to 20 m s?1; thew values are lower than those usually used in hail simulation studies. Four droplet spectra of 12, 22, 38 and 98 ?m mean volume diameter were used. The air bubble features of glaze and rime deposits grown in the abovementioned conditions show that the degree of transparency and the layering of the fine air bubbles strongly depend on the deposit temperatures and the intensity of accretion. On the other hand, crystal textures reveal that the mean width of ice crystals depends mainly on the ambient temperature and to a lesser degree on the wind velocity, particularly at low wind speeds. Mean droplet size and liquid water content seem to have little effect upon the crystal mean width. In addition, it is found that in the layer of ice closest to the aluminium cylinder, the crystal mean width increases with the radial distance from the conductor surface and this increase is more marked at low air temperatures. This effect cannot be attributed to the thermal conductivity of the substrate, but probably to the nucleation rate of ice crystals near the ice-conductor interface.