Convection will carry energy from a hot part of a fluid to a cooler part. You may have heard someone say "Hot air rises". This is an example of convection and is based on the same principal as a hot air balloon.
Picture 7. A hot air balloon.
Expanding and floating
The pilot heats the air in a hot air balloon. As the air gets hotter, it expands and its density drops. It will want to float above the more dense cool air and it rises up into the sky.
The air expands and some of the air will be pushed out of the balloon. This reduces the mass of air in the balloon. Therefore the balloon will be less dense than the cool air outside.
The same thing happens if we heat the air without the balloon there. The hot air will become less dense than the surrounding cool air and will float up over it. As the hot air rises, it leaves a gap behind which will be filled by fresh cool air. This will then get heated and rise up, dragging in more cool air. This constant flow of air is a convection current.
Picture 8. A simple water cooling system.
Some cooling systems are driven by convection currents in liquids. For example, the cooling pipes in the surface of a blast furnace use convection currents. Cool water flows into the pipes at the bottom of the furnace. This is heated by the hot furnace and the hot water rises up the pipe. As it flows upwards, it has to drag in more cool water from below. So a continuous flow is maintained.
Although convection currents are part of the system in modern blast furnaces, they are supplemented by pumps as well.