Marine icing can arguably be considered to be the primogenitor of icing problems, having occasioned grief to mariners long before aircraft or electrical transmission lines were invented. Adopting an historical perspective, we elucidate the nature of the phenomenon and the scienti c and engineering approaches which have been embraced to mitigate it. With a view to encouraging other scientists to address this problem, we also identify the critical knowledge gaps which stand in the way of a nal solution, paying particular attention to recent developments and future trends in numerical modelling and wind-tunnel experimentation.
ABSTRACT.A hybrid analytical and random-walk model has been developed to predict the shape of ice accreted on a horizontal cylindrical insulator due to freezing rain. The freezing rain occurs with the temperature of the vertically falling raindrops above the freezing point and the air temperature below freezing. The analytical model calculates the angular distribution of the water-film temperature and the location where freezing begins. The random-walk model predicts the accretion shape. The two random-walk model parameters, the freezing probability and the shedding parameter, are expressed as functions of the atmospheric conditions. The model predicts a variety of realistic accretion shapes from cylindrical to icicle-like. Model verification based on comparisons with other models and with experimental results demonstrates quantitatively and qualitatively the credibility of this new modelling approach.
The influence of atmospheric conditions (specifically precipitation rate and external heat flux) on the freezing rain ice accretion forming on a non-rotating, horizontal cylinder is studied, using an analytical model based on a simple form of the equations for conservation of mass and heat balance. In keeping with the freezing rain application, but in order to simplify this first step, we have assumed vertical incidence of precipitation (no wind) and no dripping from the accretion (hence light to moderate precipitation rates with relatively low air temperatures). The initial ice accretion shape and the location of its center of mass are examined as a function of the ratio of the precipitation mass flux to the total heat flux lost from the ice surface. An increase in the flux ratio leads to a quantifiable downward displacement of the accretion center of mass. We complement this analysis with numerical simulations, using an improved, two-dimensional version of the Szilder-Lozowski morphogenetic model that predicts the evolution of the accretion shape. For the first time, the freezing probability, which is the critical model parameter, is expressed as a function of location and atmospheric conditions for an accretion shape evolving with time. Using the morphogenetic model, we examine the influence of atmospheric conditions on the accretion shape and ice load. In particular, we address the question of what gives rise to extreme ice loads by identifying the range of atmospheric conditions that tends to maximize (or minimize) the ice load for a given amount of precipitation. The results of this research are applicable to predicting ice formation on overhead transmission lines.
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