Simulation of Convective Heat Transfer Around Oil-Cooled Electrical Windings Simulazione del Trasferimento di Calore Convettivo Intorno agli Avvolgimenti Elettrici Raffreddati ad Olio

Oil-immersed power transformers rely on conjugate heat transfer between winding discs and circulating oil to manage temperature and ensure reliable operation. Accurate prediction of the hot-spot temperature is essential, since it directly governs insulation ageing and transformer lifespan. This thesis addresses the numerical modelling of this problem using OpenFOAM v2306, an open-source computational fluid dynamics toolbox. The first part of the work ports an existing conjugate heat transfer simulation, originally developed in foam-extend 4.0, to OpenFOAM v2306, resolving multiple region-splitting, solver, and numerical stability issues encountered during the transition. The ported model is validated against the original foam-extend results on both a coarse and a refined mesh of a single-pass disc-type winding geometry, comparing oil temperature, velocity, and copper/paper temperature fields, and explicitly distinguishing physical results from numerical or solver-architecture artefacts. The second part extends the validated single-pass model to a two-pass winding geometry, following a case study from the literature, in order to assess the validity of the Boussinesq approximation for buoyancy-driven oil flow. Two simulations are performed on the two-pass domain: one using temperature-dependent (variable) thermophysical properties, and one using the Boussinesq approximation with properties evaluated at a reference temperature. The results are compared against each other and against analytical validity criteria for the Boussinesq approximation, to determine the conditions under which this common simplifying assumption remains appropriate for oil-cooled transformer winding simulations.