Thermal convection

Thermal convection is heat transfer by mass motion of a fluid, which can be either a liquid or, more typically, air. The movement of molecules within the convective medium of fluids takes place when there's a temperature gradient between a solid surface and the fluid surrounding it or an artificially induced convection current. Convection is dependent upon the mobility of the fluid and the active surface area exposed to the flowing fluid. The mobility of the fluid is expressed by the heat transfer coefficient which is the proportionality constant between the heat flux and the thermodynamic driving force for the flow of heat. When the fluid motion is not generated by external source but by means of the buoyancy force induced by thermal gradients between the fluid and solid, the heat transfer is considered to be natural convection. In other words, natural convection utilizes the buoyancy force induced by thermal gradients between the fluid and solid. A heat sink operating in this mode is referred to as a passive heat sink, which has a heat transfer coefficient of between 5-20 W/m2 K. If a flow is created by means of external forces, the heat transfer is forced convection. Using an external force generated by fans, heat pipes, synthetic jets, or liquid cooling elements to increase heat transfer is called active cooling. With forced convection, the velocity of the air moving through a heat sink can be drastically increased above that of natural convection, with heat transfer coefficients in the range of 25 to 250 W/m2 K for air and 100 to 20,000 W/m2 K for liquids.