The present article describes flow pattern of cooling oil inside the windings of a disc-type transformer. The geometrical model is established as an array of rectangular blocks arranged in-line and dissipating heat. The attention of the investigation is essentially focused on the parameters supposed to influence the flow pattern in order to get a one-dimensional correlation for the heat transfer. Since the convection is mixed, the parameter Re/Gr1/2 is used to characterise the flow. It has the range (0.1,…,1). The ranges of the other parameters are: 15<Re<120, 66<Pr<85. It has been observed that mass flow fluctuation occurs in vertical channels due to some flow through horizontal channels. This fluctuation is proved to be influenced by the above-mentioned parameters.