| Summary: | 碩士 === 國立中央大學 === 機械工程研究所 === 89 === Knowledge of the temperature and position of the hot spot is very important for the design and operation of power transformers. The rate of deterioration of the winding insulation increases with the conductor temperature. Thus it is necessary to know the hottest conductor temperature in order to ensure a reasonable life of the insulation. Pumping working fluid through a set of ducts generally cools the windings of large modern transformers. However, compared with forced convection cooling, natural convection cooling offers minimal safety requirements to avoid burnout. In the present study, the natural convection heat transfer in a disc-type pole-top transformer is investigated. The study depicts flow pattern of cooling fluid inside the windings of a transformer and provides an estimate of the position of the hot spot. The geometrical model consists of two arrays of rectangular heat-dissipating blocks arranged in line in a transformer tank. A two-dimensional, steady and turbulent flow is simulated. The set of transport equations is solved numerically using the finite volume technique. Solutions are presented for the temperature distribution in the disc coils and cooling horizontal ducts of a transformer. The attention is focused on the parameters, such as the Prandtl number, aspect ratio of the tank, heat dissipation rate, thermophysical properties, configuration, number and block ratio of the heated coils.
The results show that the flow behavior has a primary clockwise circulating cell in the top region caused by buoyancy effects originating from the heat source. The bottom zone is almost stagnant and thermally stratified. The parametric study indicates that the block ratio, BR, and the aspect ratio, AR, have a significant influence. Increasing BR from 2 to 5, the maximum temperature drops by 20 percent. When aspect ratio increases from 0.257 to 0.4, the maximum temperature is lowered by 33 percent. The effects of other parameters are studied to arrive at qualitative suggestions that may improve the cooling design of the power transformers
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