Definition of convection
The process by which a fluid (gas or liquid) transfers heat through the macroscopic flow of its parts. Because the thermal conductivity of the fluid is small, the heat transferred through heat conduction is very small, and convection is the main heat transfer method of the fluid. Convection can be divided into natural convection and forced convection. The temperature gradient in the fluid will cause a density gradient. If the low-density fluid is down and the high-density fluid is up, it will naturally convect under the action of gravity. Indoor heating in winter is based on the natural convection of indoor air to transfer heat. Natural convection also exists in the atmosphere and the ocean. The external force makes the fluid circulate, so the heat transfer is forced convection.