Investigation of Convective Heat Transfer Efficiency in the Horizontal Cooling Duct of a Disc-Type Transformer Winding

Thermal ageing of the insulation paper is predicted to be one of the transformer failure mechanisms when considering the large population of ageing assets in the power network. Disc-type winding structure is widely used in power transformers. The thermal behaviour of the horizontal cooling ducts is critical to determining the temperature in the winding. Analytical and numerical analyses of the heat transfer process in the horizontal cooling duct are performed in this paper. It is shown that the Nusselt number (Nu) is a function of the dimensionless distance x*, which is associated with duct geometry, flow condition, and coolant properties. The expressions for the Nu-x* relationship from analytical analyses and computational fluid dynamics (CFD) simulations are slightly different due to the fact that variations of the coolant properties with temperature are taken into account only in the CFD simulations. Therefore, the CFD results are deemed more accurate. The CFD simulations reveal that the influences of gravity and duct inlet velocity profile over the Nu-x* relationship at the duct outlet are negligible. Modification of the dimensionless distance x* is recommended before its application to take into account the distortion of the duct inlet temperature profile due to the existence of eddy circulation at the duct inlet, and the cooling effect of the vertical duct near the horizontal duct outlet.