Convection: heat transmission in fluids

Convection it is a process of heat transmission which occurs by the internal movement of a fluid, like air or water. Convection arises when there is a temperature gradient in a fluid. In this way, the warmer regions of this, which are less dense than the rest, move, giving rise to upward convection currents. The rest of the fluid that is at a lower temperature tends to “go down”, since its density is a little higher.

Lookalso: Three Common Mistakes in Thermology

Convection heat transmission

Convection causes large atmospheric air displacements.
Convection causes large atmospheric air displacements.

THE convection, also known as convectionthermal, arises from the tendency that most fluids exhibit when heated: they become expand, becoming any lessdense. THE differenceindensity makes the fluid ascend thanks to the action of the force of buoyancy. The convective movement takes place until all the fluid is under it temperature.

In addition to the natural convection, which occurs thanks to buoyancy, there is the convectionforced. In this the cool air is pumped up, while the warm air moves down, in the same way as when we put a ceiling fan to cool a room.

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Unlike other heat transfer processes, in convection the mass transference, since heat is transmitted by the moving fluid. In addition to the convection process, there are two other heat transfer processes: conduction and radiation.

Thermal convection can only occur in the inside a fluid that is subject to the action of the gravity, such that the buoyancy be able to “pump” the less dense fluid upward. During its ascent, the fluid tends to give heat to its surroundings, so that its density gradually increases, causing it to sink again.

The convection process can be observed during the water heating and is also one of the main responsible for the displacement of air masses that occurs in the Earth's atmosphere.

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Formula for thermal convection

The formula used to calculate heat transfer by thermal convection is given by Newton's law of cooling. This law states that the heat transfer rate is directly proportional to the temperature difference between the body and its surroundings. Check out:

Q - heat (J)

t - time interval(s)

H - thermal transfer coefficient (W/m²K)

THE - heat transfer area (m²)

TSUP - fluid surface temperature (K)

TAMB - ambient temperature (K)

In the case of convection, the coefficientintransferthermal (h) is related to a large number of variables, such as fluid viscosity and thermal conductivity. However, for most cases, it is possible to assume that H is a function that directly depends on the fluid density difference, caused by its thermal expansion.

See too: What is thermal equilibrium?

convection examples

Clouds are formed by the convection of water vapor.
Clouds are formed by the convection of water vapor.

Check out some examples:

  • You heaters are installed close to the floor of the rooms as the density of the hot air is less than the density of cold air, causing the air leaving the heater to rise, raising the temperature of the whole the room.

  • O air conditioning it is installed at the top, so the cold air it injects into the room tends to descend, thanks to its density.

  • You exhaust fans, common in markets and warehouses, are used so that the hot air that rises can circulate to the outside of buildings, keeping them cool.

  • O magmatic material (lava) moves within the Earth's mantle and is expelled by volcanoes due to convection currents.

  • THE solar radiation evaporates water, this vapor rises and condenses when reaching high altitudes, giving rise to rain clouds.

See too: Superficial dilation: formula, experiment, exercises

conduction and radiation

Check out the main characteristics of the other heat transfer processes:

  • Driving: occurs when there is contact between the surfaces of different bodies and inside solids. In this process there is no mass transfer, but the “contact” between the interface molecules that exchange energy with each other. When we step on hot asphalt, or a ceramic floor, most of the heat is transferred by the conduction process.

  • Radiation: is the heat transfer process that occurs by the emission of electromagnetic waves, the main one being the infra-red. When very hot, some bodies become incandescent and start to emit visible light in frequencies close to red, yellow and even blue, as occurs during the burning of gas from kitchen.

By Me. Rafael Helerbrock

Would you like to reference this text in a school or academic work? Look:

HELERBROCK, Rafael. "Convection"; Brazil School. Available in: https://brasilescola.uol.com.br/fisica/conveccao.htm. Accessed on June 27, 2021.

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