Abstract
Icing affects many technical systems, like aircraft or high-voltage power transmission and distribution in cold regions. Ice accretion is often initiated by ice nucleation in sessile supercooled water droplets and is influenced by several influencing factors, of which the impact of electric fields on ice nucleation is still not completely understood. Especially the influence of transient electric fields is rarely or not at all investigated, even though it is of great interest, e.g., for high-voltage transmission lines or for the food industry. In the present study the impact of transient electric fields on ice nucleation in supercooled sessile water droplets is experimentally investigated under well-defined conditions. A set of droplets is cooled down to a certain temperature and is subsequently exposed to electric fields generated from standard lightning or standard switching impulse voltages, which are commonly used for testing of high-voltage equipment. The nucleation behavior of individual droplets is captured using a high-speed camera and the effect of the transient electric field on ice nucleation is analyzed by considering both the singular and the stochastic nature of nucleation. While the singular nature of nucleation is referred to during analysis of the relative number of droplets remaining liquid long times after the impulse voltage, its stochastic nature is accounted for in the analysis of the temporal evolution of the relative number of frozen droplets. It is shown that low electric field strengths (E[over ̂]≤6.52kV/cm) only have a negligible impact on ice nucleation, independent of the supercooling. In contrast, high electric field strengths (E[over ̂]≥9.78kV/cm) promote significantly ice nucleation. It is also shown that depending on the supercooling, the freezing delay of the different droplets in the ensemble may vary over several magnitudes for the same conditions. It is demonstrated that the electric field appears to indirectly affect the nucleation rate by generating droplet oscillations, finally promoting ice nucleation. The experiments clearly demonstrate the possibility to actively force ice nucleation by applying transient electric fields. These results improve the understanding of ice accretion on high-voltage insulators and may also lend insight into freezing processes in food industry. We expect that these results will be a valuable contribution in formulating and/or validating new nucleation models.
Highlights
Ice accretion is a severe problem for many applications like aircraft [1], ships [2,3], wind farms [4], food industry [5,6,7], or power engineering [8,9,10]
It is expected that the electric field strength as well as the type of the impulse voltage and the shape, including the steepness and the duration of the impulse, have an impact on ice nucleation
The underlying physical mechanism for the impact of lightning impulse voltages on ice nucleation can be expected to be different from the impact of constant or alternating electric fields, whose characteristic times are much more comparable to the characteristic time of droplet oscillation
Summary
Ice accretion is a severe problem for many applications like aircraft [1], ships [2,3], wind farms [4], food industry [5,6,7], or power engineering [8,9,10]. An accreted ice layer can lead to serious safety risks or may significantly decrease operational performance of the application. For aircraft or ships a safe operation is of first priority and ice accretion should be avoided, since the ice alters the shape of the wings or the ship, introduces additional loads and might lead to a functional failure by, for example, reducing the lift of the aircraft. Ice accretion alters the shape of the insulators by bridging the space between the weather sheds and may lead to increased creeping currents on the surface or to a flash-over caused by the reduced insulation distance [13,14]. The understanding of the physical mechanisms involved in ice accretion is essential for an optimization of the application and to ensure a reliable and safe operation
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