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Convection is the transfer of heat by the motion of or within a fluid. It may arise from temperature differences either within the fluid or between the fluid and its boundary, or from the application of an external motive force. It is one of the three primary mechanisms of heat transfer, the others being conduction and radiation. Convection occurs in atmospheres, oceans, and planetary mantles, and chicken soup.
In heat transfer, a distinction is made between free and forced convection.
Free convection is convection in which motion of the fluid arises solely due to the temperature differences existing within the fluid. Example: hot air rising off the surface of a radiator.
The basic premise behind free convection is that heated matter becomes more buoyant and "rises"; while cooler material "sinks". Free convection occurs in any liquid or gas which expands or contracts in response to changing temperatures when it is exposed to multiple temperatures in an acceleration field such as gravity or a centrifuge. The local changes in density results in buoyancy forces that cause currents in the fluid.
Forced convection happens when motion of the fluid is imposed externally (such as by a pumpA pump is a mechanical device used to move liquids or gases. Also the heart is an essential pump in humans and animals to move the blood around. The earliest pump was described by Archimedes around 300 BC and is known as the Archimedes screw pump. Pumps w or fanA fan has two purposes. Firstly, to move air for human comfort or for ventilation. Secondly, to move air or gas from one location to another for industrial purposes. Fans have broad surfaces that usually revolve. Leaves or flat objects, waved to produce a). Example: a fan-powered heater, where a fan blows cool air past a heating element, heating the air. When a person blows on their food to cool it, she is using forced convection.
In both of the previous examples, an engineer would often be interested in the rate of heat transfer from the hot 'source' surface to the fluid medium.
The local convective heat flux of a fluid passing over a surface is expressed as
The total heat transfer over a surface is then calculated as the integral of q",
This then leads to a definition of average convection coefficient, h-bar, defined from
Studies of forced convection lead to a close inspection of the flow in the boundary layerThe boundary layer is the layer of fluid in the immediate vicinity of a bounding surface. In the atmosphere the boundary layer is the air layer near the ground affected by diurnal heat, moisture or momentum transfer to or from the surface. On an aircraft of the fluid.
In the case of EarthEarth also known as the Earth or Terra is the planet on which we live, the third planet outward from the Sun. It is the largest of the solar system's terrestrial planets, and the only planetary body that modern science confirms as harbouring life. The pla's atmosphereEarth's atmosphere is the layer of gases surrounding the planet Earth and retained by the Earth's gravity. It consists of nitrogen (78. 1% by volume) and oxygen (20. 9%), with small amounts of argon (0. 9%), carbon dioxide (variable, but around 0. 035%),, solar radiation heats the Earth's surface, and this heat is then transferred to the air by conduction. When a layer of air receives enough heat from the EarthEarth also known as the Earth or Terra is the planet on which we live, the third planet outward from the Sun. It is the largest of the solar system's terrestrial planets, and the only planetary body that modern science confirms as harbouring life. The pla's surface, it expands, becomes less dense and is pushed upward by buoyancy. Colder, heavier air sinks under it and is then warmed, expands, and rises. The warm rising air cools as it reaches the higher, cooler regions of the atmosphere and becomes more dense. Since it cannot sink through the rising air beneath it, it moves laterally and then begins to sink. When it reaches the surface again it is heated, and is drawn back into the original rising column. These convection currents cause local breezes, windFor the 1928 film, see The Wind. Wind in the most general sense, is the movement of air. It occurs at all scales, from local breezes generated by heating of land surfaces and lasting tens of minutes to global winds resulting from solar heating of the plans, thermals, cyclones and thunderstorms, and at a larger scale, produce the global atmospheric circulation features.
A single region of air with a rising and falling current is called a convection cell.
Heat is lost from the rising air through radiation into space.