Flow and temperature distributions in a disc type winding-Part II: Natural cooling modes

The liquid flow and temperature distributions in oil natural (ON) cooled power transformer windings are of great importance for thermal design, thermal rating and life estimation of ON power transformers. In this paper, both experimental and numerical investigations of liquid flow and temperature distributions in a disc-type winding model are conducted. The influences of representative operating conditions on liquid flow and temperature distributions are studied using Particle Image Velocimetry (PIV) and temperature measuring systems. Reverse liquid flows and consequential localised overheating at the top part of winding passes are recorded in the experiments. 2D and 3D CFD simulations are implemented and compared with the measurements. 3D CFD simulations are required for reliable predictions of temperature distribution when reverse flows occur, Dimensional analysis is adopted to interpret the measurement and CEO simulation results in order to provide simplicity, insight and universally applicable conclusions. The liquid flow and temperature distributions are found to be mainly controlled by the Richardson number (Ri). The higher Ri the more uneven the liquid flow and temperature distributions would be. At low Ri region flow and temperature distributions are relatively uniform with a low hotspot factor. Above the region, flow and temperature distributions become rapidly uneven and the hotspot factor increases steeply with the increase of Ri. Further increase of Ri results in the occurrence of reverse flows and fluctuation of the hotspot factor.