This chapter discusses oil flow through vertical ducts in less detail. It considers transient heating, steady state is only reached gradually. A thermal model of an oil-cooled power transformer is presented here, along with details of the computer implementation and experimental verification. Any such model, particularly of such a complex system, is necessarily approximate. Thermal energy is also stored in the rest of the transformer. In order to estimate the temperature rises, which occur under overload conditions, approximate empirical methods have been developed, which make use of the m and n exponents. Although a detailed thermal model such as the one presented previously should make these approximations unnecessary, these exponents are widely used to obtain quick estimates of the overload temperatures for transformers in service. The thermal conductivity in the plane of the laminations is much higher than the thermal conductivity perpendicular to this plane or in the stacking direction.