CFD-based analysis of oil circulation in ONAN-cooled power transformer radiators

Purpose This study aims to use computational fluid dynamics (CFD) simulations to investigate the thermofluid dynamics and oil circulation in power transformer radiators working in oil natural air natural mode. Design/methodology/approach Two numerical approaches are compared: a closed-loop buoyancy-driven flow model capturing the oil natural convection and a simplified model that imposes the oil mass flow rate at the upper collector of the radiator bank. First, a set of numerical simulations is conducted assuming a fixed power loss per fin for radiator configurations featuring 1, 5 and 15 fins, and the results are validated against a reduced semianalytical model. Findings The analysis reveals that the imposed mass flow rate boundary condition yields a higher average outlet temperature and approximately 5% lower dissipated power compared with the natural convection model. Furthermore, experimental data on radiator fin temperature and oil mass flow rate were compared with numerical simulations using both approaches. Although this discrepancy is acceptable from an engineering perspective, oil flow distribution within the fin channels is significantly modified by the type of boundary condition being used. The analysis leads to notable differences in the oil local velocity and temperature fields, affecting the fin surface temperature distribution. Originality/value The findings indicate that although the forced-circulation approach is useful to estimate the overall dissipated power, it is inadequate for detailed internal thermofluid dynamic analyses. The implications of this work are critical for the accurate design and performance evaluation of power transformer radiators.