The mass of liquid remaining on a substrate following a drop impact is a crucial quantity for modelling of numerous phenomena, e.g. spray cooling, spray coating or aircraft icing. In the present study, a method to measure this residual mass after impact of liquid drops is introduced. This method is also applicable to supercooled drops, which may freeze upon impact on cold surfaces. Using the data obtained from extensive measurements in which the size, impact speed and temperature of the drops was varied, a modelling of the residual mass is formulated, following closely the theory of Riboux and Gordillo (Phys Rev Lett 113(2):024507, 2014. 10.1103/PhysRevLett.113.024507). A key adaptation of this model accounts for the deformation of drops immediately prior to impact. This modified theoretical model results in very good agreement with experiments, allowing prediction of residual mass for a given impact situation.
Graphical abstract
Icing, as a result from the impact of supercooled water drops, is a hazard for structures exposed to low temperatures, for instance aircraft and wind turbines. Despite very intensive study of the involved phenomena, the underlying physical processes are not yet entirely understood; hence, modelling of the conditions for ice accretion and prediction of the icing rate are presently not reliable. A major difference between solidification of a liquid drop below the melting temperature and a supercooled drop is a very fast dendritic solidification phase associated with the latter. In the present experimental study, the impact behaviour of such mushy, dendritic drops is investigated using a high-speed video system. The impact parameters and the initial liquid drop temperature are varied in the experiments in order to characterize particle deformation and its residual shape. The dynamics of impact of such particles have never been studied before. A theoretical impact model for the particle deformation is applied to this problem. The model is based on the assumption that the material properties of mushy particles can be approximated by rigid-perfectly plastic behaviour. Specifically the yield strength of such particles is estimated as a function of solid ice content, which is estimated for different initial temperatures of particles prior to freezing.
Ice accretion resulting from the impact of supercooled water drops is a hazard for structures exposed to low temperatures, for instance aircraft wings and wind turbine blades. Despite a multitude of studies devoted to the involved phenomena, the underlying physical processes are not yet entirely understood. Hence, modelling of the conditions for ice accretion and prediction of the ice accretion rate are presently not reliable. The research conducted in this study addresses these deficiencies in order to lend insight into the physical processes involved. While presenting an overview of results obtained during the first funding periods of this project, new results are also presented, relating to the impact of supercooled drops onto a cold surface in a cold air flow. The experiments are conducted in a dedicated icing wind tunnel and involve measuring the residual mass after impact of a liquid supercooled drop exhibiting corona splash as well as the impact of dendritic frozen drops onto a solid surface.
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