An experimental study of natural convection in a distribution transformer slice model

The aim of this work is to obtain experimental information on the thermal and fluid dynamics of a distribution transformer. The transformer of interest is of the oil natural - air natural type and our interest is centered in the natural circulation that is produced in the oil volume. An experimental device was built to simulate a representative slice of the transformer and a purpose built PIV setup was used to obtain the flow velocity field. The device overall dimensions and the refrigerating oil are the same as those of commercial transformers of 1000 KVA (Tubos Trans Electric S.A. 1000 KVA Standard Distribution Transformer) Natural circulation is produced by a heating element simulating the heat produced by copper losses in the transformer. A double glass window allows visual access for flow visualization and PIV capture. The slice comprises two fins and the air side is bounded by two isolating boards, forming three air channels. Two reference powers were tested for the experiment: The power corresponding to design operating conditions and a lower power to observe the sensibility to power variations. The result reported in this work shows features of the flow pattern that give important information on the sort of turbulent mixing to be expected. The transient evolution was measured and the thermal characteristic time of the system was estimated. It was found that the convection during the start-up transient is significantly stronger than during steady state operation.