Ventilation coefficients for cylindrical collectors growing by riming as a function of the cloud droplet spectra

Laboratory measurements of the ventilation coefficient of ice particles growing by riming are presented in this work. The effect of the cloud droplet size spectrum after accretion on the ventilation coefficient was analyzed with droplets of mean volume diameter between 15 and 33 μm. The study was performed with cylindrical collectors of 2.8 and 4 mm diameter, the air temperature was varied from −5°C to −27°C, and three different velocities were used: 4.0, 7.0 and 8.5 m s−1. The results show a significant dependence of the ventilation coefficient on the droplet sizes; in particular it was found that for small droplets the coefficient is increased and it can be twice its predicted theoretical value. It is suggested that this effect is produced by the different surface structure formed on the collector as a consequence of the different sizes of water droplets. The influences and effects of the cloud droplet size spectrum on the surface temperature and ventilation coefficient are discussed as a function of the Stokes number, which could be a more appropriate parameter to describe or simulate heat and mass transfer processes to accreting surfaces.