Study of the in-cloud dry ice accretion on cylinders for low values of the Stokes number

Abstract

The study of the atmospheric ice accretion has received some attention from the scientific community in the past, with the available knowledge spanning from the works of the Langmuir and Blodgett (1946) on the Mt. Washington Observatory till the analytical parameterization of the Finstad et al. (1988), with the latter being the current analytical benchmark and the integral part of the ISO 12494 “Atmospheric Icing on Structures”, which serves as a current guideline for the analytical estimation of the ice loads on structures. One of the major limitations of the Finstad et al. parameterization is its applicability for the range of the overall collision efficiencies of 0.07 < E < 0.63, resulting from the experimental verification by Makkonen and Stallabrass (1987). Furthermore, ISO 12494 standard states that the current model underestimates the accreted ice masses for the collision efficiencies values below E < 0.10 and the Finstad et al. themselves postulate that they consider the lower limit of droplet inertia parameter being K = 0.25 in their model. Below this limit Finstad et al. advise to “recalculate the droplet trajectories using the appropriate drag coefficients for each droplet size in the spectra”. As evidenced by the available data from the test span measurements at the Ålvikfjellet test span in Norway, the majority of the extreme ice loads occur for the value of K below the critical value of 0.25. Thus, there is a need for a method which allows for better prediction and estimation of ice loads for such conditions, with one of the primary applications being modeling of extreme value loads on the overhead power lines for the purposes of the ice maps generation and ice load guidelines. However, the calculation of the “history” term, which is a non-steady state drag coefficient, which needs, ideally, to be taken into account in the modeling of the atmospheric ice accretion for the cases when K < 0.25 is rather challenging, with the issues pertaining to it and some possible solutions being reviewed within the scope of this thesis. Instead, the usage of the “idealized” Langmuir distributions is suggested, those originally proposed by Langmuir and Blodgett (1946) and Howe (1990). Those distributions have the same values of the Median Volume Diameter (MVD) as the typically postulated assumption of the monodispersed distribution from the ISO 12494, which makes them suitable under the framework.