對於許多電力設備的操作與設計者來說，能夠得知系統發生熱點的最大溫度值與其位置是非常重要的。電力變壓器的使用期限與其包裹導體之絕緣層劣化率密切相關。變壓器所用之絕緣紙或壓紙板等絕緣材料，隨溫度的升高，因熱劣化的關係，機械強度逐漸降低，最後只要略受震動，絕緣物就會碎裂，造成破壞，因此為了確保絕緣層之合理使用期限，預防不正常的劣化速度，如何正確預知導體最熱點的溫度是必要的。隨著能源節約及空間的需求越來越重要，很多實際的工程應用是利用自然對流達到冷卻。變壓器系統通常由油泵引導流體進入冷卻流道達到散熱的效果，相較於強制對流冷卻，自然對流冷卻的目的即是提供系統操作者一個安全的估算，當強制對流外力消失時，物件仍不致燒毀。本論文的目的係以一桿上型變壓器作為研究的系統，分析工作流體在流道內之自然對流行為與其熱傳特性，並預測其熱點發生的位置。其幾何模型為一密閉的圓筒型容器，內部中心擺置一圓柱體與層列式的線圈，解析二維穩態自然對流紊流問題。研究重點在於藉由不同的Ra、Pr、線圈與鐵心熱性質、容器徑高比、線圈數目、區塊比等參數，來探討系統之溫度分佈、線圈熱點值與位置及熱傳之間的變化情形。結果發現其流動型態主要在容器頂部區域，產生由內往外循環之渦流，而在容器底部則多為停滯的狀態且溫度呈現熱分層的現象。參數分析的結果顯示，線圈區塊比與容器徑高比有效影響系統熱點值，將區塊比從3提高至5，可降低最大溫度值20%，徑高比由0.25增大至0.4可降低33%。本文並研究不同的邊界條件如何影響其系統溫度分佈。期以此變壓器自然對流的分析模式，對未來許多採用自然對流散熱設計之設施分析上有所幫助。 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
