Everyday convection

Temperature differences are unavoidable in everyday life due to a wide variety of mechanisms. Starting with the heating of surfaces by sunlight through to physical activity which causes us to sweat.

A majority of these temperature differences are ultimately equalised by convection. Heat conduction alone can "only" provide equalisation within solids or in motionless fluids. Effects of radiation can be ignored in many cases.

The air present everywhere in the environment is rarely at rest, such that this effect is rarely negligible. Depending on the application, the aim is to reduce or increase the heat transferred by convection.

Increase/reduce convection:

In principle there are three options to change heat capacity through convection:

1) Increase/reduce the size of the heat-dissipating area
2) Increase/reduce the heat transfer coefficient
3) Increase/reduce the temperature difference between the surface and the fluid

Examples:

The listed examples show how convection is made noticeable in everyday life and how it can be consciously counteracted.

Hot air balloon:

The hot air balloon rises due to buoyancy in the air. Its shell encases heated air. This rises due to the lower density and carries the balloon. The heated air has a temperature difference to the environment, but since the balloon is carried in the air there is no relative movement between the balloon and the surrounding air. Hence "only" free convection can occur.

House in cold environment:

Houses in colder climates are very different to houses in warmer climates. Additional insulation is widespread and keeps the heat in the house.

CPU fan:

Computer processors (CPU) must be cooled. Otherwise the heat destroys the processor (B). For this purpose we use convection by fans (A). The fan is placed directly on the processor (black arrows) and absorbs the heat practically at the place it arises (red arrow).

The heat is conducted near the point it arises by good heat conductors (low heat conduction losses) in a number of cooling fins (large surface area). These dissipate the heat to the surrounding air. Usually the cooling capacity is regulated by the air flow of the fan.

Combustion engine:

The generated heat must be dissipated. This is why the engine has a cooling circuit filled with water. The water is constantly circulated by a pump (not shown). The heat of the combustion engine is first transferred to the water of the cooling circuit, then from the cooling circuit to the ambient air.

In contrast to air, water heats much less at the same rate of thermal absorption. This makes the cooling of the internal combustion engine more efficient.

In contrast to this however, a much larger surface must be provided to cool the water so that the absorbed heat can be dissipated at a relatively lower temperature difference.