In fluids (liquids and gases), on top of conductive transport there is a much more efficient one: mass transport. The hot fluid, being less dense, rises — by Archimedes’ principle — while the cold, denser fluid sinks. This forms circular currents called convection cells, which continuously mix the fluid, carrying heat where it’s needed.
Two regimes can be distinguished:
- Natural convection: circulation is driven only by the density difference due to temperature. Examples: household radiators, the circulation of the Earth’s atmosphere, ocean currents.
- Forced convection: a fan or pump actively pushes the fluid. Examples: fan cooling of processors, forced-air heating.
Why "you're better off standing" in a cold room
By natural convection, cold, denser air stagnates at the bottom, while warmer air rises towards the ceiling. In a cold room the feet are therefore the most exposed part: air stratification is convection at work.
Links
Topics: Termologia Concepts: Trasmissione del calore
Related exercises: Problem — Single-pane window · Problem — From graph to conductivity · Problem — Two fabrics