Convection is the transfer of heat by the movement of a fluid (liquid or gas) between areas of different temperature. Warm air is less dense than cold air, and so convection currents can form in the presence of a temperature gradient. When currents are produced only by temperature-derived density differences in the fluid, it is known as natural convection. When the convection currents are due to an external factor such as a pump or fan, this is forced convection. The faster the fluid is moved, the faster the rate of convection. The exchange of heat by convection between a body and its environment depends on:
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The temperature gradient between the two (this determines the amount of heat absorbed or donated by a given mass of air that comes into contact with the skin).
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Relative movement of the fluid with which the body is in contact.
Despite the name, a radiator produces a good example of convective heat transfer: if a hand is placed some distance above the radiator, the hot air rising from it can still be felt. Within clothing, wind may enter the clothing system and warm air within the clothing may be replaced with cold air due to convective currents.
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General Principles
P.M. Schlosser, ... M. Medinsky, in Comprehensive Toxicology, 2010
1.04.1.8 Convection
Convection is a process common to particles, gases, and vapors. Convection occurs when a fluid is in motion and that motion carries with it a material of interest such as the particles or droplets of an aerosol. During inhalation, convection moves a material along with the air from the space surrounding an individual into the respiratory tract, bringing the material into proximity with the lining of the respiratory tract. The pattern of airflow in the respiratory tract is determined by the airway geometry which differs among species. Interspecies differences in airway geometry are most marked in the nose, but also exist in the lung. Once an aerosol particle or droplet comes into contact with the airway lining, it usually sticks there. Likewise, highly soluble/high mucosa:air partitioning gases are readily taken up by the lining once delivered to the surface and are unlikely to reenter the air stream. Therefore, convection which depends on airway geometry plays a large role in the site specificity of deposition of particles and highly soluble gases and vapors.
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Insulation
John Willoughby, in Plant Engineer's Reference Book (Second Edition), 2002
30.2.3 Convection
Convection occurs in liquids and gases. For any solid to lose or gain heat by convection it must be in contact with the fluid. Convection cannot occur in a vacuum. Convection results from a change in density in parts of the fluid, the density change being brought about by an alteration in temperature.
30.2.3.1 Convection in gases
If a hot body is surrounded by cooler air, heat is conducted to the air in immediate contact with the body. This air then becomes less dense than the colder air further away. The warmer, light air is thus displaced upwards and is replaced by colder, heavier air which, in turn, receives heat and is similarly displaced. There is thus developed a continuous flow of air or convection around the hot body removing heat from it. This process is similar but reversed if warm air surrounds a colder body, the air becoming colder on transfer of the heat to the body and displaced downwards.
30.2.3.2 Convection in liquids
Similar convection processes occur in liquids, though at a slower rate according to the viscosity of the liquid. However, it cannot be assumed that convection in a liquid results in the colder component sinking and the warmer one rising. It depends on the liquid and the temperatures concerned. Water achieves its greatest density at approximately 4°C. Hence in a column of water, initially at 4°C, any part to which heat is applied will rise to the top. Alternatively, if any part is cooled below 4°C it, too, will rise to the top and the relatively warmer w
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