Motion at the liquid-solid interface-developing design tools from fundamental models

Although it is well known that the enforced flow of dielectric liquids over a solid interface can give rise to charge separation with the resulting accumulation of static charges, the underlying physics is complex. The incidence of streaming electrification in large transformers (and other industrial equipment) has generated the need for both an understanding and mathematical description of the physics. Modelling of the phenomena is needed since the mechanisms do not readily scale so that small-scale laboratory experiments cannot be used to predict performance. This contribution chronicles the development of both steady-state and transient descriptions of the streaming electrification process. The nature of the phenomenon is such that appropriate assumptions and approximations need to be made. As a consequence, it is also important to include some experimental verification of the outcomes; at least in qualitative form. The entrainment of charge from the interface Helmholtz layer also intimately involves the flow of the liquid. A description of the flow regimes and a transient solution of the flow through a duct are thus an integral part of the modelling effort The utility of the steady-state model developed is demonstrated by application to an oil-cellulose system typical of a large core-form transformer. The method advocated takes advantage of the repeat duct arrangement in such units. The results presented allow the internal behaviour of such a unit to be studied in considerable depth, and provide insight into the complex interplay between generation and relaxation in circumstances when flow rates, temperature, external fields and influent charge are changed.