There are two possibilities to drive an oil flow in transformers:
Generally, in transformers both methods are applied, usually a forced oil convection with separate pumps is only used for special designs in the large power range. The natural convection is sufficient strong to transport the heat to the heat exchangers in most transformers.
The natural convection itself is driven exactly in the same way, as it is well known e.g. from ocean currents. Heated fluid has a lower density than cold fluid, and this causes a driving force in the warm fluid which is directed upwards in the gravitational field. After reaching the heat exchanger, the fluid cools down, its density rises, and it falls back downwards for becoming heated again.
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