Computational fluid dynamics calibration for network modelling of transformer cooling oil flows - part I heat transfer in oil ducts

In the context of thermal performance and thermal lifetime, it is greatly important to predict the magnitude and location of the `hot-spot` temperature inside a transformer. Various calculation approaches have been developed in the attempt to accurately predict hot-spot, including so-called `network models`. In terms of the methodology used in network modelling, the complex pattern of oil ducts and passes inside a winding is reduced to a matrix of simple hydraulic channel approximations, where empirical analytical expressions are employed to hydraulically and thermally describe oil flow and heat transfer. These expressions are often obtained by a limited number of experimental cases of relatively simple flows. Their applicability should therefore be carefully evaluated and if necessary corrected, when being used in the wide range of conditions of transformer oil flow; this is the primary objective of this study. A detailed parametric study has been performed using `COMSOL` multi-physics software package for computational fluid dynamics (CFD), which offers a higher order of accuracy than network modelling. By processing the resulting data sets, a new set of heat transfer equations are proposed specifically for transformer cooling oil flow. Comparison is finally made between the newly proposed equations and the currently used `off-the-shelf` expressions.