Experiments and modeling of heat transfer in oil transformer winding with zigzag cooling ducts

In natural oil-cooled (ON) electric transformers, cooling fluid flow and heat transfer process depend on each other. A coupled thermal/hydraulic model has been developed to investigate 2-D axi-symmetric temperature field in solid heating components (transformer windings) and hydraulic and thermal fields in network of oil cooling ducts. The convective heat transfer at solid/fluid interface was taken as coupling element between the heat generating windings and the cooling oil flow. An experimental study has also been conducted to investigate oil and disk temperatures in ON transformer windings. The results have been used to validate and then calibrate the developed simulation model, with an implicit nonlinear optimization approach. Consequently two empirical correlations for local convection heat transfer coefficient have been developed, corresponding to cooling ducts with heating on one-side and both-side walls, respectively. The correlations are valid for Reynolds numbers ranging from 7.5 to 75.9 in both-side heating ducts and from 1.5 to 218.4 in one-side heating ducts. The improved thermal model is in good agreement with the experimental results. It is also found from the experimental study that the conductor temperature gradients in circumferential direction is small and can be neglected, and therefore a two-dimensional axi-symmetric model is sufficiently accurate for the thermal simulation of the winding disks.