Ouédraogo, Yun ; Gjonaj, Erion ; De Gersem, Herbert ; Schöps, Sebastian (2018)
Numerical Simulation of Droplet Dynamics in Strong AC Electric Fields.
12th European Fluid Mechanics Conference. Vienna, Austria (09.-13.09.2018)
Conference or Workshop Item, Bibliographie

Abstract

Fluid motion induced by strong electric fields affects many engineering applications.On the surface of insulators in surge arrestors, the presence of the rain water dropletslocally enhances the strength of the AC electric fields. This effect can be sufficient totrigger partial discharges on the surface of the insulator, leading to surface damage.Oscillations of the droplets due to the external field additionally modify the elec-tric field distribution around the droplets, increasing the risks of partial discharges.The effect if particularly relevant as mechanical resonances of typical millimetric raindroplets occur in the same frequency range as commercial power frequencies. Fur-thermore, the presence of surface charge on insulators can introduce free charges indroplets, affecting their motion. Such multiphase electrohydrodynamical problemsrequire an electroquasistatic field description in order to capture dynamical chargingeffects as well as net charge in liquids.We propose a method for numerically solving the strongly coupled electrohydro-dynamical problem of droplet oscillations on the surface of insulators subjected tostrong AC voltage. Fluid-fluid interfaces are represented using the Volume of Fluidmethod, applied to both the Navier-Stokes equations and the electroquasistatic fieldproblem, so that topology changes are implicitely considered. Dynamical charging ef-fects in liquids are accounted for by treating the free charge as a primary quantity andconsidering free charge transport in liquids in addition to ohmic conduction effects.Furthermore, special care is taken in modeling contact line dynamics, including pin-ning effects. Numerical simulations are performed using the Finite Volume Methodprovided by the OpenFOAM library.The applicability of the model is illustrated with the dynamics of droplets oscilla-tions on insulator surfaces1. The modes of single droplets are considered at frequenciesclose to those of mechanical resonances. Both charged and uncharged droplets areconsidered, in order to evaluate the impact of net charge on asymmetric oscillationmodes. Electric field induced merging of multiple droplets is also considered, andcompared to experimental results. Simulated electric fields are, in a second step, usedto evaluate the relevance of the different dynamics for partial discharge inceptionvoltage estimation.

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