Vessel icing and spongy accretion modelling

This thesis develops an heuristic vessel spray icing model and an exploratory cylinder spray icing model. They are used to study the prediction of vessel spray icing severity.;The physics of vessel spray icing is reviewed in Chapter 1. The resulting overview suggests an elusive phenomenon with many aspects that are as yet understood only in qualitative ways. Two aspects are reviewed in detail: modelling of the distribution of ice accretion over vessels, and the problem of spongy ice growth. This review prepares the background for the model of vessel spray icing of Chapter 2. This model, configured around a non-traditional spray/air heat balance, portrays the overall growth of ice on a vessel in a new and unique way. It accounts for many of the sub-processes of vessel icing while predicting both ice thickness growth rate and overall mass accretion rate for vessels of different size. The model's versatility is illustrated by including an hypothesized nucleated spray icing mechanism that enhances the icing rate significantly (up to 350%) over the original supercooled spray icing mechanism. The problem of spongy ice growth is dealt with in Chapter 3 by modelling the spray icing of a vertical cylinder. In the model the ice has a laminated surficial structure overlain by a falling film of excess liquid flowing over a layer of growing ice called the freezing zone. This new model depends on an analogy between ice growth in the freezing zone and the growth of the tip of a freely-growing ice dendrite, along with traditional thermodynamical elements of spray icing prediction. Using plausible impinging spray temperatures, this model successfully predicts both icing rate and liquid entrapment (i.e. sponginess). It agrees well with icing data for horizontal rotating cylinders. The model's performance shows that spongy spray ice growth is driven by heat loss from the icing surface, and that it is very sensitive to conditions in the surficial liquid film and the freezing zone. The model's prediction of accretion sponginess is also sensitive to air temperature and spray impingement temperature, which in turn implies the importance of the thermal evolution of the spray in the airstream. Together, the heuristic vessel icing model and the exploratory, spongy spray icing model, demonstrate the importance of the aerodynamics and thermodynamics of spray, as well as the sponginess of the accreted ice, to the rate at which ice accumulates on a vessel under freezing spray conditions.